3.

14 Vinyl Polymerization in Heterogeneous Systems

B Charleux, Université de Lyon, Lyon, France
M Cunningham, Queen’s University, Kingston, ON, Canada
JR Leiza, University of the Basque Country, San Sebastián, Spain
© 2012 Elsevier B.V. All rights reserved.

3.14.1 Introduction 463

3.14.2 Vinyl Polymerization in Aqueous Dispersed Systems 463
3.14.2.1 Conventional Radical Polymerization 463
3.14.2.1.1 Suspension polymerization 463
3.14.2.1.2 Emulsion polymerization 465
3.14.2.1.3 Miniemulsion polymerization 474
3.14.2.1.4 Microemulsion polymerization 478
3.14.2.1.5 Aqueous dispersion and precipitation polymerizations 479
3.14.2.2 Controlled Radical Polymerization 480
3.14.2.2.1 Overview of CRP in aqueous dispersed systems 480
3.14.2.2.2 Nitroxide-mediated radical polymerization 480
3.14.2.2.3 Atom transfer radical polymerization 483
3.14.2.2.4 Reversible addition–fragmentation chain transfer 485
3.14.2.2.5 Other CRP methods 488
3.14.2.3 Other Vinyl Polymerization Methods 488
3.14.2.3.1 ROMP in aqueous dispersed systems 488
3.14.2.3.2 Ionic polymerizations 490
3.14.2.3.3 Catalytic polymerization 490
3.14.3 Vinyl Polymerization in Nonaqueous Dispersed Systems 491
3.14.3.1 Conventional Radical Polymerization 491
3.14.3.1.1 Inverse emulsion, miniemulsion, and microemulsion polymerizations 491
3.14.3.1.2 Dispersion polymerization 492
3.14.3.2 Controlled Radical Polymerization 493
3.14.3.2.1 Dispersion polymerization in organic solvent 493
3.14.3.2.2 Dispersion polymerization in scCO2 494
3.14.3.3 Ionic Polymerization of Vinyl Monomers 494
3.14.4 Conclusion 494
References 494

3.14.1 Introduction 3.14.2 Vinyl Polymerization in Aqueous Dispersed

Systems
This chapter is devoted to the chain-growth polymerization of
vinyl monomers in dispersed systems, including aqueous and 3.14.2.1 Conventional Radical Polymerization
nonaqueous systems. It covers the various polymerization 3.14.2.1.1 Suspension polymerization
methods such as conventional and controlled/living free radi­ 3.14.2.1.1(i) General principles
cal polymerizations, anionic polymerization, cationic Free radical polymerization, when conducted in bulk and con­
polymerization, ring-opening metathesis polymerization verted to high conversions, yields a highly viscous mixture
(ROMP), and catalytic polymerization of olefins. The aqueous that poses various reaction engineering problems. Stirring
dispersed systems are mainly suspension, emulsion, minie­ the mixture during polymerization (which is essential for ade­
mulsion, microemulsion, and dispersion/precipitation. In quate removal of the enthalpy of polymerization), pumping
nonaqueous systems, the systems are inverse emulsion, mini- the mixture from the reactor through process lines and into
emulsion, and microemulsion for the polymerization of storage vessels, and monomer devolatilization are significant
hydrophilic monomers and mainly dispersion polymerization challenges when viscosity is high. If the process requires semi-
for a broader variety of monomers polymerized in various batch addition of other components during polymerization
dispersing solvents such as organic solvents, supercritical car­ (e.g., initiator, monomer, chain transfer agent), high viscosity
bon dioxide (scCO2), and ionic liquids. and the consequent poor stirring may preclude effective mixing

Polymer Science: A Comprehensive Reference, Volume 3 doi:10.1016/B978-0-444-53349-4.00073-X 463

464 Vinyl Polymerization in Heterogeneous Systems

of those components throughout the reaction mixture, particu­ stabilizer with suspension particles, often causing severe coa­
larly in large reactors. While solvent can be used to reduce gulation. Water-soluble inhibitors can be added to minimize
viscosity, solvent removal becomes an additional (and expen­ emulsion particle nucleation.
sive) process step. If it is desired to crosslink the polymer, even
lightly, bulk or solution polymerization often becomes 3.14.2.1.1(i)(b) Suspension stabilization Polymerizing
unfeasible. droplets are stabilized by either water-soluble polymers
The challenges described above can be readily overcome (including, e.g., poly(vinyl alcohol), hydroxyalkyl celluloses,
by suspending monomer droplets in an aqueous medium carboxymethyl cellulose, poly(acrylic acid) (PAA) and its
and then polymerizing the droplets. The final product in a salts, and water-soluble acrylic copolymers) or insoluble inor­
suspension polymerization is a low viscosity, two-phase ganic powders (usually salts of calcium, aluminum, or
mixture of solid polymer particles suspended in water. The magnesium). Polymeric stabilizers reduce the interfacial ten­
formulation of the monomer phase is similar to a bulk sion between the organic and aqueous phases, and provide a
polymerization, including the use of monomer-soluble protective layer on the droplet/particle surface that inhibits
initiators and chain transfer agents. Typical particle sizes coalescence. Surfactants such as sodium dodecyl sulfate (SDS)
are
50–1000 µm, large enough so that the particles settle may be added to further reduce the interfacial tension, thereby
quickly in the absence of stirring. Solids contents for indus­ facilitating smaller particle sizes. Inorganic powders such as
trial processes are
50 wt.%. Recovery of the particles can calcium phosphate preferentially locate at the droplet/particle
usually be done by filtration. Removal and recovery of interface with the aqueous phase to provide a physical barrier
residual monomer by techniques such as steam stripping against coalescence. Advantages include reduced reactor foul­
are facile, although it is desirable to attain the maximum ing and ease of removal from the particle surface after
possible monomer conversion in the polymerization polymerization, for example, with a dilute acid wash.
stage. For these reasons, suspension polymerization is Polymeric stabilizers, having numerous adsorption sites per
generally preferred over bulk or solution for most applica­ molecule, are difficult to effectively remove from the particles
tions, and is widely used industrially. Specific examples of after polymerization.
important industrial polymers made by suspension poly­
merization are given in Section 3.14.2.1.1(i). Several 3.14.2.1.1(i)(c) Particle size distribution Several factors
extensive reviews of suspension polymerizations are avail­ contribute to the final PSD in a suspension polymerization,
able.1–4 including the monomer type, stabilizer type and concentration,
solids content in the reactor, and very importantly, mixing in
3.14.2.1.1(i)(a) Kinetics of suspension polymerization the reactor. In the early stages of polymerization, the droplets/
versus bulk polymerization It has often been stated that particles are of low enough viscosity that both breakage and
suspension polymerization is analogous to conducting a poly­ coalescence occur – breakage in the vicinity of the impeller and
merization in miniature bulk reactors. While this is a coalescence in the quiescent regions of the reactor. The overall
reasonable approximation for some cases, the causes of possi­ droplet/PSD is determined by a dynamic equilibrium between
ble differences in the kinetics between bulk and suspension breakage and coalescence. As the viscosity in the particles
should be understood. If all components in the formulation increases, they will become too viscous to easily undergo break­
(e.g., monomer, initiator, chain transfer agent) have low or age but can still undergo coalescence. This phase, sometimes
negligible water solubility, and if the monomer droplet (or referred to as the ‘sticky region’, is where serious coagulation
polymer particle) size is large (≳ 50 µm), the kinetics of a problems are most likely to occur. At even higher viscosities,
suspension polymerization will likely resemble bulk polymer­ the particles become less prone to coalescence and suspension
ization. Primary factors contributing to deviation from stability improves considerably.
bulk-like kinetics are the following: (1) components of the The formulation largely determines the interfacial tension
formulation, especially one or more of the monomers, having and is readily reproduced during scale-up but the same cannot
moderate to high water solubility; and (2) small droplet/parti­ be said of mixing. One of the major challenges in scaling up
cle size. Significant monomer partitioning into the aqueous suspension polymerizations lies in scaling up the mixing pat­
phase can result in lower overall monomer conversions, lower tern. Larger reactors have inherently different mixing behavior
rates of polymerization, and changes in the copolymer compo­ than laboratory reactors,5 including lower average shear rates,
sition distribution compared to bulk. When the droplet/ higher maximum shear rates, and longer circulation times. A
particle size becomes quite small, the likelihood of having further challenge is that the mixing requirements change during
concurrent emulsion polymerization also becomes a concern. the course of polymerization. At low conversion, the droplets/
A population of very small droplets, especially if coupled with particles are usually less dense than the aqueous phase and are
moderately water-soluble monomer and initiator, create con­ prone to pooling on the surface, while higher conversion par­
ditions suitable for homogeneous/coagulative nucleation of ticles are usually denser than water, requiring the agitator to
emulsion polymer particles. (Suspension polymerization effectively suspend solids that are prone to settling on the
where the mean particle size is under
10 µm is often referred reactor bottom.
to as ‘microsuspension polymerization’.) Concurrent emulsion
polymerization is highly undesirable, since the emulsion par­ 3.14.2.1.1(ii) Industrial applications
ticles will have a different molar mass distribution (MMD), Several commercially important vinyl copolymers are
copolymer composition distribution, and particle size distribu­ manufactured using suspension polymerization, including poly­
tion (PSD) than the desired suspension particles. Of greater styrene (general purpose, expandable and high impact), poly
concern is that emulsion polymer particles compete for (methyl methacrylate), poly(vinyl acetate), styrene–acrylonitrile
 Vinyl Polymerization in Heterogeneous Systems 465

(SAN) copolymers, acrylonitrile–butadiene–styrene (ABS) copo­ 100%

lymers, and poly(vinyl chloride) (PVC). Crosslinked particles
(e.g., based on styrene and divinylbenzene) used as ion

Conversion
exchange resins, as beads for chromatographic separation, or as I II III
solid support for chemical reactions are also prepared by suspen­
sion polymerization. While for most monomer–polymer
systems the polymer is fully soluble in the monomer, for some
systems (vinyl chloride, acrylonitrile) the polymer is only
slightly soluble in the monomer. During polymerization, the
polymer begins precipitating at very low conversions. In suspen­ 0
sion polymerization, these systems are commonly referred to as Time
‘powder suspension polymerizations’, while the more common Figure 1 Typical conversion vs. time plot in emulsion polymerization,
systems where the monomer and polymer are mutually soluble showing the three intervals.
are called ‘bead suspension polymerization’. Examples of indus­
trial formulations and process diagrams are available in the
literature.4 and it exhibits an increasing conversion rate. Once the number
of particles has reached a constant value, the system enters the
interval II, which corresponds to the particle growth by propa­
3.14.2.1.2 Emulsion polymerization gation (it may extend from 5–10% to 30–70% conversion,
3.14.2.1.2(i) General principles depending on the monomer system). The monomer concen­
3.14.2.1.2(i)(a) Overall description Free radical polymer­ tration within the particles remains constant, as far as
ization of vinyl monomers in emulsion has been widely monomer droplets are present, because the latter supply the
studied in the past 60 years, and has still been the topic of a consumed monomer by molecular diffusion through the aqu­
multitude of books6–15 and review articles16–21 in the past eous phase. In consequence the conversion rate is also
10 years. This is mainly because of its huge industrial interest constant. When the emulsion polymerization is started with
as well as its intrinsic complexity. This polymerization process preformed polymer particles, it is called ‘seeded emulsion poly­
is indeed used in the large-scale production of a broad variety merization’ and begins directly with interval II. The interval III
of polymers as it offers many technical and environmental is the final stage, during which the polymerization takes place
advantages. Although mature, it remains very attractive and within the particles in the absence of monomer droplets, that
has undergone continuous improvement over the years, the is, at a decreasing local monomer concentration. It is thus
most recent advances being, for instance, in the domain of characterized by a reduction of the polymerization rate until a
controlled free radical polymerization. possible gel effect takes over, and the polymerization rate can
Emulsion polymerization is a way of polymerizing hydro­ start to increase again.
phobic, liquid monomers in water, in which they form an
emulsion at the initial stage, most generally in the presence of 3.14.2.1.2(i)(b) Nucleation (interval I) Formation of the
a surfactant. The monomer is then initially partitioned between particles follows complex nucleation mechanisms that strongly
different phases: the large monomer droplets (diameter >1 μm) depend on the surfactant concentration and the water solubi­
formed by stirring and stabilized by the surfactant adsorbed at lity of the monomer(s). Droplet nucleation remains a
the interface, the continuous water phase (saturation concen­ negligible event. In all cases, the polymerization starts in the
tration), and the micelles (when the surfactant concentration is aqueous phase with the introduction of a water-soluble radical
above its critical micelle concentration (CMC)). Upon poly­ initiator, which forms oligoradicals upon initiation and subse­
merization in the presence of a water-soluble radical initiator, quent polymerization with the dissolved monomer molecules.
the process leads to a latex, that is, an aqueous suspension of These oligoradicals may behave in different ways: either they
submicrometer polymer particles, which are stabilized against self-terminate in the aqueous phase to form water-soluble or
flocculation and coalescence by the surfactant, adsorbed at amphiphilic oligomers or they exclude themselves from the
their surface. aqueous phase upon decrease of their water solubility by
The number of particles per unit volume of latex (Np) chain extension. It is now well established that the particles
cannot be determined directly but is calculated from the experi­ originate from these oligoradicals when they reach a given
mental measurement of the particle diameter (D, by degree of polymerization, at which their physicochemical prop­
transmission electron microscopy, dynamic light scattering, erties and water solubility are strongly altered. Their fate is then
capillary hydrodynamic fractionation, to cite the most popular dictated by the presence or absence of surfactant micelles.
techniques) and the amount of polymer in the latex (τ), accord­ When the surfactant concentration is above the CMC and the
ing to the eqn [1], in which dp is the polymer density. monomer is rather hydrophobic, the oligoradicals are captured
by the monomer-swollen micelles as soon as they become
6τ
Np ¼ ½1 surface active (i.e., degree of polymerization = z) and generate
π D3 dp
particles by the so-called micellar nucleation mechanism
An ab initio emulsion polymerization can be divided into three (Figure 2(a)). The local monomer concentration is high
successive steps, each of them corresponding to a particular enough to allow for a fast chain growth and hence an irrever­
state of the system (Figure 1). The interval I is related to the sible entry process. Only part of the micelles is actually
nucleation, that is, the formation of particles. Its duration nucleated, while the others serve as surfactant reservoirs to
corresponds approximately to 2–10% monomer conversion, stabilize the created interfaces. Nucleation then ceases when
466 Vinyl Polymerization in Heterogeneous Systems

(a) SO4–•+ M

Propagation
Termination

z
Entry

Monomer-
swollen
Entry particle
Propagation/
in a micelle coagulation Micelles =
surfactant
reservoir
New particle

(b) SO4–•+ M

Propagation
Termination

Monomer-
z swollen
Entry
particle

Propagation
+ coagulation

Precipitation
New particle
jcrit

Figure 2 Schematic representation of (a) micellar nucleation and (b) homogeneous-coagulative nucleation, for an emulsion polymerization initiated by
sulfate radicals (i.e., persulfate initiator).

all micelles have been consumed. At this stage, and according 3.14.2.1.2(i)(c) Particle growth (interval II),
to the Smith and Ewart law,22 Np is theoretically proportional polymerization kinetics, and average molar masses During
to [initiator]2/5[surfactant]3/5. More generally, Np is propor­ interval II, the monomer-swollen polymer particles become the
tional to [surfactant]α with α being usually below 1. At main polymerization loci and this corresponds to their growth
surfactant concentration below the CMC or in the absence of by propagation, which is permitted by the presence of both
surfactant and for hydrophilic monomers such as methyl radicals and monomer. The radicals come from the capture of
methacrylate and vinyl acetate (VAc), formation of the particles oligoradicals generated in the aqueous phase, while monomer
follows a so-called homogeneous nucleation mechanism is continuously supplied from the droplets (which act only as
(Figure 2(b)). In that situation, the oligoradicals grow in the monomer reservoirs), by diffusion through the water phase.
water phase by addition of monomer units until they become For traditional monomers with sufficient water solubility (i.e.,
insoluble (critical degree of polymerization = jcrit) and precipi­ solubility equal or higher than that of styrene), the diffusion
tate to form primary nuclei. Their colloidal stability is ensured process is generally much faster than propagation, and hence
by the surfactant, when present, and the charged fragment of the monomer concentration within the particles is governed by
the initiator at the chain end. It might be enhanced by the thermodynamic equilibrium. The local monomer concentra­
limited coagulation of several nuclei in order to increase the tion [M]p remains, thus, constant as the result of the balance
charge surface density (homogeneous-coagulative nucleation). between opposing effects: reduction of the surface free energy
Nucleation ceases when capture of the oligoradicals by the by a decrease of the surface area (i.e., leading to a decrease of
already existing particles prevails. the particle volume and consequently of [M]p) and reduction
 Vinyl Polymerization in Heterogeneous Systems 467

of the free energy of mixing of polymer and monomer, which from bulk polymerization, due to a longer lifetime of the
contributes to increase in [M]p. As the particle number remains propagating radicals resulting from compartmentalization.
constant during the interval II, the polymerization rate Rp is For a 0-1 system with radical entry faster than radical transfer
then a constant too (eqn [2a]). Polymerization rate in interval II: ~ ¼ 0:5) and for negligible termination in the aqueous phase,
(n
the instantaneous Mn is given by eqn [2b].8 It is thus possible to
~
kp ½Mp Np n increase simultaneously the average molar mass and the poly-
Rp ¼ ½2a
NA merization rate by increasing Np, whereas in bulk or in
suspension polymerization, the molar mass decreases when
Instantaneous number-average molar mass, Mn, in interval II
the rate is increased (by an increase of the initiator concentra­
for a 0-1 system with radical entry faster than radical transfer
tion). In general, the MMD is broader than in homogeneous
~ ¼ 0:5):
(n
systems.
inst kp ½Mp Np
Mn ¼ MMm ½2b 3.14.2.1.2(ii) Particle stabilization
2 Ri NA
While the stability of the initial monomer-in-water emulsion is
In eqn [2], kp is the rate constant of propagation, n ~ the average not critical, the colloidal stability of the formed particle is in
number of radicals per particle, NA the Avogadro’s number, contrast of utmost importance. In most cases, it is ensured by
Ri the initiation rate in the aqueous phase, and MMm the low-molar-mass surfactants, typically anionic and nonionic
monomer molar mass. Propagation thus obeys a zeroth order amphiphilic molecules, which are adsorbed at the particle sur­
with respect to monomer concentration and not a first order as face in dynamic equilibrium with the small fraction dissolved
observed in homogeneous polymerizations and suspension in the aqueous phase. When ionic surfactants are used, they
polymerization. contribute to charge the particle surfaces and provide an elec­
The unique feature of kinetics in emulsion polymerization trostatic repulsion. The particles are surrounded by an electrical
results from the compartmentalization of the propagating radi­ double layer constituted by the counterions of opposite charge,
cals within separate particles. A direct consequence of the which screens the surface charges and induces the build up of
compartmentalization is a decreased overall termination rate, an osmotic pressure responsible for the repulsion. This layer is
that is, a longer radical lifetime, a larger overall concentration actually divided in two regions. Close to the interface, the ions
of propagating radicals, and thus a much faster polymerization are strongly bound to the surface (the so-called Stern layer),
rate than in homogeneous system. One of the most famous and whereas at a longer distance, they are more loosely bound and
useful theoretical description of the kinetics in emulsion hence more mobile. The latter region is named the diffuse layer,
polymerization was established by Smith and Ewart22 who con­ the thickness of which is the Debye screening length. It is larger
sidered several cases, including the very important pseudo-bulk when the ionic strength in the aqueous phase is lower.
and 0-1 systems. In the former case, the average number of Consequently, the electrostatic stabilization of latexes is
radicals per particle n ~ is well above 1. This occurs when the strongly dependent on the salt concentration and its valency.
particle size is sufficiently large or the viscosity inside the particle The repulsion is counterbalanced by attractive forces, namely
is sufficiently high (at high conversion), so that two or more van der Waals and hydrophobic interactions. The DLVO
radicals can coexist within a single particle without instanta­ (Derjaguin, Landau, Verwey, and Overbeek) theory23–26 gives
neous termination. This case cannot be distinguished from that an approximate description of the overall interaction energy
of the equivalent homogeneous system, and consequently the between the particles and allows prediction of latex stability.
polymerization rate is independent of the particle number (no In some situations, the latex particles are covered with
effective compartmentalization effect). In contrast, with particles adsorbed hydrophilic polymer chains (either ionic or nonio­
of sufficiently small size, the entry of an oligoradical into an nic), which are swollen with water and expand in the aqueous
active particle (i.e., a particle that already contains 1 radical) phase. These polymers contribute to the steric stabilization of
causes an instantaneous termination reaction. The rate of radical the particles through their osmotic pressure. In the absence of
consumption becomes, thus, governed by the rate of entry and adsorption of the hydrophilic polymer at the particle surface,
not by the termination reaction itself. The particles contain either an opposite effect may take place, known as depletion
1 or 0 radical and this leads to n ~ ¼ 0:5 when entry is fast and the attraction.
exit of radicals is negligible (negligible transfer reactions). This Besides the classical low-molar-mass surfactants, various
case is applicable to most emulsion polymerizations. When other possibilities have been studied to stabilize the latex par-
radicals easily escape from the particles (i.e., significant transfer ticles originating from emulsion polymerization: those can be
reaction to a small molecule) and undergo fast termination in either water-soluble comonomers and macromonomers, or
the aqueous phase, n ~ may become very small, below 0.5. In both reactive surfactants27–31 or even amphiphilic copolymers.32,33
situations, the polymerization rate shows a strong dependence Their advantage is to remain strongly bound to the polymer
on the number of particles, hence the surfactant and initiator constituting the particles, either through a covalent bond
concentrations are of critical importance and the compartmen- formed in the radical polymerization mechanism in the case
talization effect dictates the kinetics. When radicals easily escape of reactive molecules, or through anchorage of the hydropho­
from the particles but reentry into another particle is fast, n ~ is bic segment(s) in the case of copolymers. Water-soluble
also well below 0.5, but the polymerization rate is little affected comonomers react with the hydrophobic monomer(s) of the
by the number of particles, and the compartmentalization effect emulsion polymerization, in principle, during the first stages of
is actually not effective. the reaction and afford amphiphilic random copolymers,
The molar masses of the polymers obtained from emulsion which adsorb at the particle surface. In the case of ionic como­
polymerization are significantly larger than those obtained nomers, the stabilization is mainly electrostatic and possibly
468 Vinyl Polymerization in Heterogeneous Systems

steric, depending on the length of the hydrophilic segments. 3.14.2.1.2(iii)(a) Batch reactors A batch reactor is a closed
In the case of macromonomers (based predominantly on poly system in which the time is the only independent variable. The
(ethylene oxide) (PEO)), a graft copolymer is formed and batch operation can be used for some small production of
induces a steric stabilization. Similarly, the reactive surfactants homopolymers from monomers with a relatively low heat of
exhibit a functional group able to participate in one of the polymerization. However, the drawbacks associated with this
various steps of a radical polymerization, but they exhibit an type of operation limit its industrial use: (1) the control of the
intrinsic amphiphilicity due to the combination of polar and polymer properties is impracticable; (2) the productivity is low
nonpolar parts in their structure and behave as classical considering the load, unload, and cleaning times; (3) the heat
surface-active molecules in water. They are called ‘inisurfs’ generation rate is high and the control of the reactor tempera­
when they react as initiators (peroxide or diazoic group), ‘trans­ ture is very difficult because all of the monomer is initially
urfs’ when they react as chain transfer agents (through a thiol charged into the reactor; and (4) the system suffers from
group), or ‘surfmers’ when they react by copolymerization.27 batch-to-batch variations due to irreproducible particle nuclea­
The latter represent the most important class of reactive surfac­ tion that may jeopardize product consistency. In order to avoid
tants and are now commercial products, based, for instance, on this problem, seeded polymerization may be employed.
methacrylic or maleic esters. Differently, amphiphilic copoly­ Batch reactors are commonly used in research laboratories
mers do not react in the polymerization process but can be because of its simplicity and low cost of operation. The com­
used as stabilizers (the term surfactant may not be appropriate position of the copolymers produced in batch reactors will be
in the absence of surface-active properties) in replacement of dictated by the reactivity ratios of the comonomers and the
the classical surfactants. These copolymers can be prepared by ratio of their concentration in the polymer particles (Mayo–
(1) classical radical polymerization (mainly random copoly­ Lewis copolymer composition equation – see eqn [3] in Section
mers and alkali-soluble resins based, for instance, on styrene or 3.14.2.1.2(iv)). Most of the common monomers employed in
methyl methacrylate (MMA) and (meth)acrylic acid), (2) anio­ emulsion polymerization recipes present different reactivities,
nic polymerization (diblock or triblock copolymers composed, and a consequence of this is the compositional drift (noncon­
for instance, of polystyrene or polybutadiene as the hydropho­ stant copolymer composition) produced in batch operation.
bic block and of PEO or poly(methacrylic acid) as the
hydrophilic one), and (3) controlled/living radical polymeriza­ 3.14.2.1.2(iii)(b) Semibatch reactors In semibatch opera­
tion (CRP; i.e., radical polymerization operating through a tion mode, some fraction of reactants (initial charge) is initially
reversible deactivation of the propagating radicals – see charged into the reactor, and the rest of the formulation is
Section 3.14.2.2 for a broad variety of chemical structures). continuously fed over some period of time. Most commercial
Depending on the chemical structure of the hydrophilic seg­ products are manufactured in semibatch reactors. The main
ment(s), they will provide either a steric stabilization or an characteristic of this process is the great flexibility. Varying the
electrosteric one. With the block copolymers, the structure composition and amount of the initial charge, as well as the
and thickness of the hydrophilic layer are well defined and composition and flow rates of the feeds, both temperature and
the latex particles are often referred to as ‘hairy’ particles. The polymer quality may be controlled. A wide range of products
primary property of those stabilizers is the strong anchorage at are accessible using this technique that allows tailoring any
the particle surface, hence impeding their migration during polymer property, including copolymer composition, MMD,
film formation. They may also find advantages during the polymer architecture, particle morphology, and PSD. In addi­
nucleation step, when their self-assembled structure in water tion, a large portfolio of products can be produced with a single
is stable at the timescale of the polymerization. With the reactor. The main drawback of this operation mode is the
so-called frozen micelles, for instance, it was shown that all relatively low productivity, which is being compensated by
micelles are nucleated and the final number of particle is then using larger reactors.
dictated by the initial number of micelles present.32,34 In general, the initial charge contains a seed (i.e., preformed
latex particles, used principally to avoid the lack of reproduci­
3.14.2.1.2(iii) Emulsion polymerization processes bility of the nucleation stages when the seed is produced in situ
Batch, semicontinuous, and continuous reactors are used in and for scaling up issues), a fraction of water, surfactant, and
emulsion polymerization. Typically, these reactors are stirred initiator. Under some circumstances, certain amount of the
tank reactors, and the most common operation mode is the monomer(s) can also be present. The rest of the formulation
semicontinuous one because of its versatility. Because of their ingredients are added to the reactor at a constant flow rate (or
large heat transfer area/reactor volume ratio, tubular reactors following predefined trajectories in time that can be calculated
are an attractive alternative, but they are not often used in based on empirical knowledge of the process or on optimiza­
emulsion polymerization, principally due to the high risk of tion techniques from mathematical models).35,36
phase segregation, fouling, and pipe clogging. Loop reactors
(a tubular reactor with high recirculation rate) and pulsed 3.14.2.1.2(iii)(c) Continuous reactors In continuous
reactors have been used but the main drawback of these tubular operation mode, both inlet and outlet streams flow continu­
reactors is that recipes with high mechanical stability are ously. The main feature of a continuous stirred tank reactor
required to prevent shear-induced coagulation. Tubular micro- (CSTR) is the broad residence time distribution (RTD) that is
reactors have proven to provide some advantages in characterized by a decaying exponential function. Due to this
homogeneous polymerization (temperature control and poly­ broad RTD, it is not possible to obtain narrow PSD using a
mer microstructure) but they are still in a very preliminary stage single CSTR. In addition, CSTRs are prone to suffer intermittent
of investigation for heterogeneous systems such as emulsion nucleations that lead to multimodal PSDs. This may be alle­
polymerization. viated by using a seeding reactor (such as a tubular reactor)
 Vinyl Polymerization in Heterogeneous Systems 469

before the CSTR. In steady-state conditions, the properties of composition equation (so-called Mayo–Levis equation, eqn [3])
the polymer remain constant and hence it is ideal to produce for a terminal model of copolymerization also applies in emul­
high-tonnage polymers. The broad RTD together with the pro­ sion copolymerization, but the concentration of monomers are
blem of heat removal in large stirred tanks makes it difficult to now replaced by the concentration of monomer in the polymer­
achieve high conversions in a single tank. This drawback might ization loci, that is, in the polymer particles.
be overcome by arranging multiple stirred tanks in series that
r1 f 21 þ f 1 f 2 ½M1 p
allow a better heat removal and narrower RTD, which in turn F1 ¼ and f 1 ¼ ½3
leads to a narrower PSD. CSTRs in series are used for r 1 f 21 þ 2f 1 f 2 þ r 2 f 22 ½M1 p þ ½M2 p
high-tonnage productions such as styrene-butadiene rubber In eqn [3], F1 is the instantaneous copolymer composition
(SBR), but the production of specialty polymers is more chal­ referred to monomer 1, and r1 and r2 are the monomer
lenging because of the difficulties associated with grade reactivity ratios for monomer 1 and 2 (terminal model),
transitions. respectively. During intervals I and II, the concentration of
the monomers in the polymer particles are governed by the
3.14.2.1.2(iii)(d) Tubular reactors From a safety point of partitioning of the monomers among monomer droplets,
view, tubular reactors are advantageous because they have a polymer particles, and aqueous phase. In interval III, there
large area/volume ratio (the highest values being those are no droplets and the monomer is mostly located in the
obtained in tubular microreactors) and hence the heat removal polymer particles. The concentration of the monomers in the
capacity is higher than that of the CSTR. An important disad­ polymer particles depends on the relative values of mass
vantage of the tubular reactor is the inadequate mixing that can transfer and polymerization rates. Except for poorly emulsi­
lead to phase separation, reactor plugging, and wall fouling. fied, highly water-insoluble systems, mass transfer is much
Several modifications have been performed to improve radial faster than polymerization rate, and hence the concentration
mixing and minimize the associate problems, but to date tub­ of monomers in the different phases is given by the thermo­
ular reactors are not widely utilized for industrial production. dynamic equilibrium.
The most important modified tubular reactors include loop For a multimonomer system, the calculation of the concen­
reactor,37,38 pulsed flow reactor,39 wicker tube reactor,40 and trations of the monomers in the different phases involves the
Couette–Taylor flow reactor.41,42 simultaneous resolution of the thermodynamic equilibrium
equations and the material balance equations. Equilibrium
3.14.2.1.2(iv) Emulsion copolymerization equations based on the Morton–Flory–Huggins (MFH) equa­
In most of the cases, latex products are composed of more than tion43 or on partition coefficients44 can be used. For a
one monomer. In copolymerization, two or more monomers multimonomer system, the interaction parameters of the
are simultaneously built-in into the polymer chains. Emulsion MFH equation are not usually available; therefore, the use of
copolymerization allows the production of materials with the partition coefficients is easier and as accurate as the MFH
properties that cannot be obtained by homopolymer latex equations at high solids content (>50 wt.%).45 In the case
products or by blending homopolymers. The properties of the partition coefficients are used, the following system of algebraic
materials required are usually dictated by the market. equations must be solved (eqns [4] and [5]):
Nowadays, most of them are achieved by combination of i
j
j
more than two monomers in the copolymer product. Typical Equilibrium equation: Ki ¼
wi
industrial emulsion polymerization formulations are mixtures
j ¼ polymer particles; droplets ½4
of monomers giving hard polymers and monomers leading to
soft polymers. Styrene and MMA are examples of monomers Material balance equations:
giving hard polymers, that is, polymers with a high glass transi­ pp þ ∑ ip ¼ 1
tion temperature (Tg). Soft polymers, that is, polymers with a i

low Tg, are, for example, formed from n-butyl acrylate (BA). w
w þ ∑ i ¼ 1
w
i
The industrial emulsion polymerization formulations (see
below) also contain small amounts of functional and specialty ∑ di ¼1 ½5
i
monomers such as acrylic and methacrylic acid, or hydro­
Vp ip þ Vd id þ Vw w
i ¼ Vi
xyethyl methacrylate to impart improved or special
characteristics (functionalization) to the latex product. Vw w
w ¼ Vwater

Vp pp ¼ Vpol
3.14.2.1.2(iv)(a) Mechanism and kinetics The inclusion of
a second (or additional) monomer(s) in the formulation of a In eqns [4] and [5], Kji is the partition coefficient of monomer i
homogeneous free radical polymerization greatly complicates between the phase j and the aqueous phase; φji the volume
the reaction kinetics and brings additional requirements related fraction of monomer i in phase j; the superscripts w, p, and d
to the difference in reactivity of the monomers and its impact on denote aqueous phase, polymer particles, and monomer dro­
the copolymer composition and copolymer sequence distribu­ plets, respectively; Vp, Vd, and Vw are the volumes of
tion. In an emulsion copolymerization, the complexity is even monomer-swollen particles, monomer droplets, and aqueous
more profound because the heterophasic nature of the polymer­ phase, respectively; and Vi, Vpol, and Vwater are the volumes of
ization makes aspects such as the partition of the monomers monomer i, polymer, and water, respectively.
in the different phases to play a significant role in the instanta­
neous copolymer composition produced and hence in the 3.14.2.1.2(iv)(b) Structured particles Composite polymer
copolymer sequence distribution. Thus, the classical copolymer particles (particles made out of more than one phase) with
470 Vinyl Polymerization in Heterogeneous Systems

Time: 10 minues Time: 15 minues Time: 30 minues Time: 1 hour 30 minues

–5 –5 –5 –5
x10 x10 x10 x10
1.5 1.5 1.5 1.5

0 0 0 0
z

z
–1.5 –1.5 –1.5 –1.5
1.5 1.5 1.5 1.5 1.5 1.5
1.5 1.5
0 0 0 0 0 0 0 0
–5 –4 –5 –4 –5 –5 –5 –5
x10 x10 x10 x10 x10 x10 x10 x10
–1.5 –1.5 –1.5 –1.5 –1.5 –1.5 –1.5 –1.5
Y x Y x Y x Y x

Figure 3 Simulated evolution of the morphology during a seeded semicontinuous experiment. First-stage polymer in gray (spherical grid). Second-stage
polymer in green. From left to right, conversion of the second-stage monomer increases indicated by the time caption on top of each figure.

tailored physical properties are of great interest for many indus­ Equilibrium morphologies may be attained if the internal visc­
trial applications. Composite latex particles are mainly used as osity of the particle is low (low molar mass and low polymer
architectural and automotive coatings,46 impact modifiers in concentration in particles), the polymers are very incompatible
engineering plastics to improve toughness and impact strength,47 (high interfacial tensions resulting in high van der Waals
opacifiers,48 and in hybrid polymer–polymer49,50 and polymer– forces), and in very long process times. The equilibrium mor­
inorganic51 materials among other high added-value products. phology is the one that minimizes the interfacial energy of the
Composite latex particles are commonly produced by seeded system and depends on the polymer–polymer interfacial ten­
semicontinuous polymerization where the second-stage mono- sion (σ12) and polymer–water (σ13 and σ23 for a two-phase
mer(s) is(are) fed into the reactor in a given period of time system) interfacial tension. Modeling the equilibrium mor­
together with additional initiator, surfactant, and water (if neces­ phology of composite particles for two and three polymer
sary). The conditions are adjusted in such a way that the phases has been reported,56–58 and Figure 4 presents the poten­
polymerization is favored inside the existing particles. Figure 3 tial equilibrium morphologies for a two-phase system as a
illustrates the development of the morphology during the seeded function of the interfacial tensions. For a three-phase system,
semicontinuous reaction. the number of potential morphologies is too large,58 and it is
The position at which the polymer chain is formed depends not possible to draw a figure like Figure 4, and furthermore, it
on the radical concentration profile inside the particle. If the is very difficult to accurately compute the equilibrium
entering radicals are anchored to the surface of the particles, the morphologies without simplifying the potential morphologies.
newly formed polymer chains will be predominantly located in Most recently, a general method to predict the equilibrium
the shell layer. As the concentration of polymer increases, phase morphology of multiphase systems based on Monte Carlo
separation occurs, leading to the formation of clusters (green simulations has been presented.59,60 The proposed method
particles in Figure 3). Polymerization occurs in the clusters as reproduces well the equilibrium morphologies calculated by
well as in the polymer matrix; therefore, both the size of the the conventional methods mentioned above for two-phase
cluster and the number of clusters increase. The resulting sys­ systems. In addition, it allows the equilibrium morphology of
tem is not thermodynamically stable due to the large surface three or more polymer phase particles to be predicted (without
area associated with the large polymer–polymer interfacial any a priori assumption of the morphology). Figure 5 repro­
area. In order to minimize the free energy, the clusters migrate duces the three-dimensional equilibrium morphology of
toward the equilibrium morphology. During migration, the composite polymer particles composed by three polymer
size of the clusters increases due to (1) polymerization in the phases.
clusters, (2) diffusion of polymer into the cluster, and (3) coa­
gulation between clusters. The motion of the clusters is
governed by the balance between the van der Waals forces 10
and the repulsion and resistance to flow that arise from viscous |σ23-σ12| 1 2 3
drag. The van der Waals forces between the clusters are always σ13
Water
attractive whereas the van der Waals forces between cluster and
aqueous phase can be attractive (the cluster will be drawn to
the surface of the particle) or repulsive (the cluster will be
brought toward the center of the particle). In Figure 3, the 1
forces between the cluster (green particles) and the aqueous
phase are attractive. It is worth mentioning that the van der
Waals forces are proportional to the interfacial tensions. The
final morphology heavily depends on the kinetics of the cluster
migration.52–54 Metastable morphologies can be achieved by 0.1
working under starved conditions (high concentration of poly­ 0.1 1 σ13 10
σ23
mer in the particles and hence high viscosities), promoting
grafting reactions or producing block copolymers in situ Figure 4 Equilibrium morphology of biphasic composite polymer par­
(hence reducing the polymer–polymer interfacial tension).55 ticles (white, polymer 1; black, polymer 2; 3, water).
 Vinyl Polymerization in Heterogeneous Systems 471

(a) (b)

Figure 5 Equilibrium morphology of a three-phase waterborne polymer particle with a volume ratio V1/V2/V3 = 0.40:0.40:0.20 with interfacial tension
values σ1−w = 9.51, σ2−w = 11.57, σ3−w = 10.54, σ1−2 = 1.50, and σ1−3 = σ2−3 = 1.23 mN m−1. Polymers 1, 2, and 3 are represented in gray, light gray, and
black, respectively. The water is not shown for clarity. (a) Visualization of the three-dimensional (3D) structure cut at different planes and (b)
cross-sectional view of the particle.

3.14.2.1.2(iv)(c) Functionalized particles Some of the particles have potential applications in biomedical and biolo­
monomers used in emulsion polymerization formulations are gical applications (tracer, immunoassay, recognition, etc.),
so-called ‘functional monomers’ because in addition to bear a drug delivery (cancer treatment), bioelectronics, and biosen­
double bond (C=C) that covalently links to the polymer back­ sors to name a few. Many types of chemical functionalities can
bone, they contain a functional (reactive) group that might be incorporated onto the polymer particles: simple chemical
impart other properties to the polymer and to the colloidal groups or more complex molecular or macromolecular struc­
system. The most common functional monomers for latexes tures able to provide specific recognition of biomolecules or
produced in large tonnage are monomers with carboxylic acid living systems. The functionalization can be achieved by phy­
and amide functionality, and they are used in small amounts sical and chemical means.63 Physical adsorption of surfactant
(typically below 5 wt.%).61 Acrylic acid (AA), methacrylic acid, and polymers has been used to provide functionalization to the
fumaric acid, and itaconic acid are the most frequently used particles in addition to steric stability. An example is the func­
carboxylic acids. Acrylamide is also often used as a functional tionalization of preformed polymer particles with PEO groups
monomer. Due to the polarity of these monomers, they are in order to impart hydrophilicity to avoid adsorption of biolo­
mainly located at the surface of the particles. The COOH group gical compounds (proteins, peptides, etc.) to the particle
ionizes in water and the degree of ionization depends on the pH surface when the particles are used as nanocarriers in the blood­
of the aqueous phase. In the ionized form (COO−), the negative stream.64 Functionalization by chemical reaction is preferred
charge of the carboxyl moieties imparts extra stability to the because there are a large number of chemical groups that can be
dispersion. In other words, they act as surfactants (see Section easily incorporated at the surface of the particles. The most
3.14.2.1.2(ii)) with the advantage of being covalently linked to common way to achieve this goal is by seeded emulsion poly­
the polymer chains. In addition to the stability provided by these merization (core-shell particles) using a latex of hydrophobic
functional monomers, they also impart reactivity. For instance, polymer (likely polystyrene) in the first step. Then a shell of the
the presence of carboxyl groups at the surface allows crosslinking functional polymer is created with monomers bearing car­
reactions with urea-formaldehyde, phenol-formaldehyde, and boxylic acid, aldehyde, acetal, chloromethyl, amine, hydroxyl,
others of the like. However, the use of these functional mono­ epoxide, or protected thiol groups.63 Sometimes, the surface
mers might also bring problems such as an increase in the functionalization requires hairy layers or polymer brushes with
viscosity of the dispersion if excessive water-soluble high­ well-defined structure and molar mass along with narrow
molar-mass polymer is formed in the polymerization. The MMD. The incorporation of this type of structure at the surface
large partition of these monomers to the aqueous phase and of latex particles by conventional free radical polymerization is
their high reactivity (especially the acrylic ones) in radical poly­ not an easy task.65,66 The advent of CRP (see Section 3.14.2.2)
merization are the main reasons for the production of and the possibility to run this method in aqueous phase has
hydrosoluble material. allowed and simplified the synthesis of this type of complex
Functionalized polymer particles produced by emulsion particle morphology.67–70 ‘Click chemistry’ has also shown
polymerization are also very attractive for high added-value potential to produce functionalized polymer particles.71
applications (especially biotechnology) because of the out­
standing properties that surface-functionalized polymer 3.14.2.1.2(v) Properties and industrial applications
latexes offer when used as colloidal supports. Emulsion poly­ Emulsion polymers are produced by a complex heterogeneous
merization is to a great extent the polymerization technique of polymerization mechanism that has been described above.
choice to synthesize the support particles due to the long-term However, this inherent complexity enables the production of
experience and the basic understanding of the fundamental polymers with complex architectures and microstructures with
mechanisms controlling the polymerization. In addition, the a huge application potential. Application properties of latexes
versatility to carry on the polymerization (seeded batch or depend to a great extent on the polymer architecture that is
semibatch, ab initio, etc.) allows for the control of the PSD mostly defined during the polymerization process, namely, in
and particle surface chemistry.62 A great deal of work has the polymerization reactor. The microstructure of polymer
been done in the past two decades to incorporate functionality latex includes (see Figure 6) the copolymer composition,
to polymer particles, and it has been demonstrated that these monomer sequence distribution, MMD, polymer architecture
472 Vinyl Polymerization in Heterogeneous Systems

Particle morphology
Particle size distribution
0.025 Molar mass distribution
n(dp)
w(dp)
0.02

0.015 wGPC

0.01

0.005

0
0 100 200 300 400 logMw
dp(nm) Copolymer composition
Branching
Characteristics of polymer
dispersions
that affect final properties

Crosslinking and gel Sequence distribution

Surface composition

-COOH -CHO

Figure 6 Microstructural features of polymers and particles from emulsion polymerization.

(branching, grafting, crosslinking, and gel content), average properties of these materials. The PSD and particle surface
particle size and PSD, particle morphology, and particle surface functionality determine the rheology of the latex. Rheology is
functionality. critical during the polymerization because, to a large extent, it
Copolymer composition has a direct effect on the Tg of the controls mixing and heat transfer. In the synthesis of high
polymer, which determines the minimum film formation tem­ solids content latexes, the maximum solids content achievable
perature (MFFT) of the latex and the application. Copolymer is controlled by the rheology of the dispersion. Therefore, the
composition also affects properties such as resistance to hydro­ control of the PSD is crucial to increase the solids content of a
lysis and durability. MMD has a strong effect on application waterborne dispersion.76–78 The particle size and PSD also
properties. Thus, for adhesives, it is well known that an appro­ affect the film properties. The smaller the particle size, the
priate balance of low- and high-molar-mass polymer chains is better the quality of the film (i.e., gloss).79,80
necessary; low-molar-mass chains impart tack, resistance to Half of the polymer latexes synthesized by emulsion poly­
peel increases with intermediate-molar-mass chains, and resis­ merization are commercialized as waterborne dispersions and
tance to shear increases with high-molar-mass chains. Polymer the rest as dry polymer. The main polymer families produced
architecture defines several final properties too. For instance, are based on (1) styrene-butadiene, (2) acrylonitrile-butadiene,
gel content above certain values has shown to damage adhesive (3) chloroprene, (4) vinyl chloride, (5) VAc and its copolymers,
properties. Surface properties or nature of the functional and (6) acrylic (co)polymers.81
groups located at the surface of the particles allow many appli­ Styrene-butadiene, acrylonitrile-butadiene, chloroprene,
cation properties to be tailored, as discussed in the previous and vinyl chloride emulsion (co)polymers are mainly used in
section. Relatively small amounts (lower than 5 wt.% based on their dried form. Carboxylated SBR, VAc (co)polymers, acrylics,
the polymer) of carboxylic monomers are frequently used in and styrene-acrylic copolymers are used, on the other hand, as
the manufacture of latexes (carboxylated SBR latexes are a clear binders of formulation for several industrial applications in
example). The presence of the carboxylic groups imparts elec­ their dispersed form. Figure 7 shows the share of each of
trostatic stabilization upon neutralization. Another example of these families and the major industrial applications of these
surface functionalization is the incorporation of hydroxyl func­ latexes.11,61
tionality to the surface (using, for instance, hydroxyethyl Paper industry, paints and coatings, adhesive and sealants,
methacrylate monomer). This functionality allows crosslinking and carpet industry cover approximately 80% of the latexes.
with thermoset coatings by curing with melamine chemistry. Other industrial applications where latexes are directly applied
Particle morphology enlarges the applications of emulsion are printing inks, automotive coatings, nonwoven fabrics,
polymers. Core-shell particles composed of a rubber-like core leather industry, and asphalt modification to name a few.
and a hard shell are used as impact modifiers for commodity In the paper industry, the use of polymer dispersions is
plastics such as PVC and poly(methyl methacrylate). Hybrid restricted to surface sizing and paper coating.82 Surface sizing
polymer–polymer49,50,72 and polymer–inorganic latexes73–75 means hydrophobizing the surface of the paper sheet to reduce
are emerging as a new class of materials in which the morphol­ its absorbency. This is achieved using a formulation that
ogy of the particles is a key parameter that affects the potential includes preferentially starch and the sizing agents that are
 Vinyl Polymerization in Heterogeneous Systems 473

Others
Others Paper and
Carboxylated paper board
styrene-butadiene
22% 19% 23%
34%
Carpet
backing 9%

20%
24%
Acrylics and 25% Paints and
20%
styrene-acrylics coatings

Vinyl acetate and Adhesives and

copolymers sealants
Figure 7 Commercial share and main industrial applications of polymer dispersions.

composed of emulsion polymers. The most important emul­ performance than VAc homopolymers especially as far as
sion polymers employed are acrylic (co)polymers stabilized by hydrolytic stability and MFFT are regarded, and are predomi­
protective colloids. The polymer particles are core-shell type nantly used for interior paints. Styrene-acrylic copolymers
with a hydrophobic core made out of acrylic polymers and a (50:50) are more hydrophobic, more water resistant, and
hydrophilic shell formed by the protective colloid, which is have better barrier properties than the VAc copolymer latexes.
either cationic or anionic. Paper coating is the most important They are still used in interior paints because of their sensitivity
surface finishing process for the paper, and the amount of to ultraviolet (UV) light. All acrylic polymer dispersions (for
emulsion polymers employed in this process is significantly instance, MMA/BA = 50:50) are likely the best in terms of per­
higher than that used in the sizing process. The goal of the formance, and hence, they are preferred for exterior applications.
emulsion polymer is not only to bind the pigment particles, The other systems (VAc copolymers and styrene-acrylic copoly­
but also to secure them at the coating surface and anchor them mers) can also be used for exterior applications, but only for
to the base paper. The coated paper increases the homogeneity low-cost systems. All copolymer systems mentioned above also
of the surface and significantly improves the optical properties use specialty comonomers in much lower level than the main
such as gloss and brightness. The different emulsion polymers components, but they are frequently the ones that provide the
used for coating paper (or board) include SBR, poly(styrene-co­ performance features to the application. Typical functional
n-butyl acrylate) copolymers, poly(vinyl acetate), poly(acry­ monomers are acidic comonomers for stabilization and adhe­
lates), poly(ethylene-co-vinyl acetate) copolymers, and in most sion, amine-functionalized monomers for adhesion, n-butyl
of the cases those latexes include functional monomers such as methacrylate (BMA) in BA/MMA copolymers to improve dur­
acrylonitrile or monomers bearing acid or amide groups. ability, 2-ethylhexyl acrylate (2-EHA) to control hydrophobicity
Decorative and protective coatings (paints) employed in styrene-acrylic copolymers, and hydroxyethyl methacrylate to
about 3 billion liters emulsion polymers in the wet state, and provide functionality to acrylic resins.
the market is growing at a rate of 3–6% mainly due to the Adhesives represent the largest market for the emulsion poly­
advantages that waterborne polymers present with respect to mers. Pressure-sensitive adhesives (PSAs; including self-adhesive
the traditional solvent-borne polymers (environmentally labels and tapes), laminating adhesives, and construction adhe­
friendly, easy cleaning, and low toxicity of the solvents). The sives (including floor covering adhesives, subfloor and wall
main application areas are coatings for building, furniture, mastics, sealants and caulks, ceramic tiles adhesives, and
automobiles, and large industrial structures. Other less impor­ polymer-modified mortars) are the applications where most of
tant but common use includes removable coatings, and the emulsion polymers are used. All acrylic copolymers (with
coatings for optical fiber and electronic components. large amount of a low Tg acrylic monomer, that is, 2-EHA or BA,
The latexes are used as binder in complex formulations that and small amounts of a high Tg methacrylate (i.e., MMA))
comprise a pigment dispersion, the binder (latex), a thickener together with butadiene-rich SBR latexes are mainly used for
(rheology control), coalescent agents (which promote film PSA. The PSAs are formulated using in addition to the latex a
formation), surfactants (which promote stability), a biocide tackifying agent, plasticizers, wetting agents, defoamers, and
(which prevents microbial attack), defoamer, and neutralizing thickeners to adjust the adhesive to the prevailing coating
agents. The volume of solids in the coating formulation is conditions. The SBR latexes need more tackifying agents than
between 40–50% and the volume of polymer is c. 83% of the the acrylics for the same performance. For the laminating adhe­
total volume of the dried paint. The first polymers used for sives, polyurethane dispersions are predominant. Adhesives
coatings were the SBR latexes (with a styrene/butadiene ratio of for construction applications use in their formulations
65:35). However, nowadays they have been replaced by VAc non-carboxylated SBR, all acrylic, vinyl-acrylic, styrene-acrylic,
copolymers, styrene-acrylic copolymers, or pure acrylics. In the and VAc copolymers, depending on the required conditions.
case of VAc copolymers, the most used comonomers are BA Finally, carpet backing applications use carboxylated SBR
(VAc/BA = 80:20), Veova (vinyl ester of versatic acid), and ethy­ latexes with contents in styrene in the 60–70 wt.% range. The
lene (VAc/E = 90:10). These copolymer latexes exhibit better carboxylic acid monomers vary from company to company and
474 Vinyl Polymerization in Heterogeneous Systems

the amount is typically lower than 3–5 wt.%. AA, methacrylic Typically, the procedure to prepare the miniemulsion is as
acid, itaconic acid, fumaric acid, and acrylonitrile are the most follows: the surfactant system is dissolved in water, the costa­
common functional monomers used. bilizer is dissolved in the monomer(s) mixture, and both
solutions are brought together and mixed under magnetic agi­
3.14.2.1.3 Miniemulsion polymerization tation. The resulting coarse emulsion is converted into a
3.14.2.1.3(i) General principles ‘nanoemulsion’ by applying energy, generally from mechanical
As explained in Section 3.14.2.1.2, in emulsion polymerization devices (rotor–stator systems, sonifiers, and high-pressure
an oil-in-water emulsion stabilized by surfactant is polymer­ homogenizers are the most common ones)94 or based on the
ized using a free radical initiator. In this process, the nucleation chemical potential of the components (low energy emulsifica­
of polymer particles takes place by entry of radicals into tion methods such as phase inversion temperature).95
micelles (heterogeneous or micellar nucleation) or by precipi­ Among the mechanical devices, the high-pressure homoge­
tation of growing oligoradicals in the aqueous phase nizers (Manton–Gaulin homogenizer and microfluidizer) are
(homogeneous nucleation). Although droplet nucleation is the most efficient techniques in terms of achieving the smallest
possible (by entry of oligoradicals into the micrometer-sized droplet sizes.96 Both equipments have in common that the
droplets), this is very unlikely because of the large difference in coarse dispersions are pressurized using a positive displace­
surface area of the monomer-swollen micelles (5–20 nm) with ment pump, and flow through a narrow gap at high velocity.
respect to the monomer droplets (1–10 μm) that favors entry of A strong pressure drop also occurs. Figure 8 presents a sche­
the oligoradicals in micelles and hence micellar nucleation. matic of a high-pressure homogenizer.97
Once the particles are formed in emulsion polymerization, In the high-pressure homogenizer, the homogenization is
the polymer particles undergo substantial growth by polymer­ mainly due to extensional forces (shear) with some contribu­
ization. The monomer required for the polymerization must be tion from cavitation and impact forces. Cavitation occurs
transported from the monomer droplets by diffusion through because of the strong pressure decrease that makes the vapor
the aqueous phase. This represents, in many cases, a limitation pressure of the liquid to exceed the local pressure causing vapor
of the emulsion polymerization technique because it is very bubbles. When these bubbles implode, shock waves are gener­
difficult to incorporate very hydrophobic monomers into the ated in the liquid that break up the droplets. The impact forces
polymer particles due to their limited or negligible diffusion with the walls are not strong because the decrease in the velo­
ability. The need for mass transport of monomer through the city is considerable at the outlet of the valve. The average
aqueous phase would be greatly reduced if all (or a large droplet size decreases and the droplet size distribution (DSD)
fraction) of the droplets were nucleated. The direct nucleation becomes narrower as the number of passes through the
of the monomer droplets can be enhanced if the droplet size is high-pressure homogenizer increases. The effect is more pro­
reduced and the surface area of the droplets is large as com­ nounced in the first passes, and it has been found that the
pared with that of the micelles. Hence, droplet nucleation number of passes can be considerably reduced if the coarse
should prevail over the other nucleation mechanisms. What is emulsion is first sonified.97
known as ‘miniemulsion polymerization’ is basically an oil­ Hexadecane and cetyl alcohol are the most widely used
in-water emulsion, in which the size of the monomer droplets costabilizers, but since these compounds do not polymerize,
has been considerably reduced (50–500 nm) by combining a
suitable emulsifier and an efficient emulsification technique
and by stabilizing the resulting ‘nanoemulsion’ (the so-called Poiseuille
flow
‘miniemulsion’) against diffusional degradation by using a
costabilizer (a low-molar-mass hydrophobic compound). Coalescence
Under this condition, the surfactant is adsorbed on the large
surface area of the monomer droplets and hence (ideally in a
Pressure
well-formulated miniemulsion) the surfactant that should
remain available to form micelles is negligible and micelles
are not present. Therefore, if a water-soluble initiator is added
to the system and oligoradicals are formed in the aqueous
phase, they preferentially enter into monomer droplets that
become polymer particles; namely, the main nucleation
mechanism is droplet nucleation. The droplet nucleation is a
unique feature of the miniemulsion polymerization83 that
allows the production of polymers that cannot be produced by
Turbulence
any other polymerization in dispersed media technique.84–93 If
all the monomer droplets present in the original miniemulsion
Coarse
capture radicals, then all the droplets become polymer particles. emulsion
This has been taken as an inherent feature of the miniemulsion
polymerization, but it hardly takes place in practice.94 Namely,
in addition to droplet nucleation other nucleation mechanisms
such as homogeneous and micellar nucleation as well as droplet
coagulation and degradation might also take place. Elongational flow
A miniemulsion formulation includes water, monomer(s), Figure 8 Monomer miniemulsion formation in a high-pressure
a costabilizer, and the surfactant and initiator systems. homogenizer.
 Vinyl Polymerization in Heterogeneous Systems 475

they contribute to the formation of volatile organic com­ 1

pounds (VOCs) and hence reactive costabilizers (that will φ h = 0.08
covalently attach to the polymer) are preferred. Hydrophobic 0,9
chain transfer agents, monomers, initiators, or polymers have φ h = 0.06
been used. The role of the costabilizer is to avoid or minimize 0,8 φ h = 0.04
the degradation of the droplets by diffusion of the monomer
from small to large droplets (Ostwald ripening). The stabiliza­
0,7
tion of the monomer droplets against diffusion is governed by φ h = 0.02

r1e /r10
the thermodynamics of the system, namely, the chemical
0,6
potential of the monomer (given by the partial molar Gibbs
free energy of monomer) in the monomer droplets in the φ h = 0.01
0,5
presence of small amounts of costabilizer. This can be
expressed by eqn [6].98,99
0,4
ΔGm 2Vm σ φ h = 0.005
¼ lnðφm Þ þ ð1−mmh Þφh þ χ mh φm
2
þ ½6
RT rRT 0,3
In eqn [6], φm and φh are the volume fraction of monomer and
costabilizer in the monomer droplets, respectively, mmh the 0,2
2 4 6 8 10
ratio of the molar volume of the monomer (Vm ) and the
r20 /r10
costabilizer (Vh ), χmh the interaction parameter, σ the dro­
plet–water interfacial tension, and r the droplet radius. Under Figure 10 Effect of the volume fraction of costabilizer on the stability of
maximum swelling equilibrium conditions ΔGm ¼ 0 and the monomer droplets: r10 = 25 nm, mmh = 0.5, χ = 0.4, Vm = 10−4 m3 mol−1,
eqn [6] can be solved for different costabilizers (by varying σ = 5  10−3 N m−1.
mmh) and for different sizes of droplets (by varying r).
Figure 9 presents these results. A low value of mmh corresponds
The effect of the amount of costabilizer on the stability of a
to a costabilizer with a molar mass much larger than that of the
miniemulsion can be predicted by solving eqn [6] for the
monomer (for instance, a polymer) and higher values corre­
swelling equilibrium of monomer in the presence of costabili­
spond to costabilizers with molar masses closer to that of the
zer when droplets of different sizes coexist, which is typically
monomer (i.e., hexadecane). It can be seen that low­
the case after a homogenization step. For illustrative purposes,
molar-mass costabilizers lead to superswelling in contrast
Asua94 solved the equation for the case with two types of
with the modest swelling achieved by a polymer used as a
droplet sizes. Figure 10 displays the effect of the costabilizer
costabilizer. Figure 9 also shows that the larger the droplet,
concentration in the monomer droplets on the stability of the
the higher the swelling.
monomer droplets. r1e and r2e are the pseudo-equilibrium
An important issue when formulating a monomer mine­
droplet radii and r10 and r20 the droplet radii before diffusional
mulsion is the amount of costabilizer required to keep the
degradation of the small droplet to the large ones.
miniemulsion stable, at least during the polymerization time.
Figure 10 shows that the droplet stability significantly
increases as the amount of costabilizer increases. However,
volume fractions above 0.04 only lead to a slight increase in
1000 the droplet stability.
It is worth noting that the results presented here are equili­
brium values and that monomer diffusion is a kinetic process
mmh = 0.5 that might take some time. As the driving force for the mono­
Monomer/costabilizer (vol/vol)

mer diffusion is the difference in chemical potential in the

droplets of different sizes, thermodynamic not only affects the
100 mmh = 0.33 final state, but also the rate at which Ostwald ripening occurs.

mmh = 0.1 3.14.2.1.3(ii) Homopolymerization and copolymerization

Miniemulsion homopolymerizations of vinyl chloride, VAc,
10
MMA, BA, styrene, Veova10, dodecyl methacrylate, and stearyl
methacrylate have been reported.94,100,101 In miniemulsion
homopolymerization, once the polymer particles are formed,
mmh = 0 the process evolves in a similar manner as in interval III of a
conventional emulsion polymerization, that is, in absence of
monomer droplet phase. The differences in the polymerization
1 rates observed when comparing conventional emulsion and
0 50 100 150 200 250
miniemulsion polymerizations can be attributed to the differ­
rdroplet (nm)
ent number of polymer particles formed in each process, which
Figure 9 Effect of the droplet size and type of costabilizer on maximum can be substantially different depending on the initiator sys­
swelling: χ = 0.5, Vm = 10−4 m3 mol−1, σ = 5  10−3 N m−1, T = 533 K. tems employed.
476 Vinyl Polymerization in Heterogeneous Systems

The situation is more complex when a copolymerization is with the conversion as in the miniemulsion process. This dif­
considered because as two or more monomers are involved and ference in the evolution of the volume fraction of polymer in
the partitioning of the monomer between the phases might be the polymer particles might have a significant impact on the
different, this may lead to variations in the copolymer compo­ microstructure. For instance, for the polymerization of acrylate
sition. In emulsion copolymerization, the evolution of the monomers that form long-chain branches (by intermolecular
copolymer composition depends, in addition to the reactivity chain transfer to polymer) and eventually gel polymer (inso­
ratios, on the partition of the monomer between the aqueous luble polymer network), if termination by combination is the
and polymer particle phases (see Section 3.14.2.1.2(iv)). predominant chain termination event,104,105 then the amount
Furthermore, if the monomer is hydrophobic enough, trans­ of gel polymer that can be formed by means of a batch mini-
port limitations through the aqueous phase might control the emulsion polymerization process is smaller than that produced
concentration of the monomer in the polymer particles. On the by emulsion polymerization. This was recently reported for the
other hand, in miniemulsion polymerization, the transport of polymerization of 2-EHA and its copolymerization with MMA
monomer is reduced to such levels that the incorporation of (2-EHA/MMA = 90:10)106 and also for the copolymerization of
hydrophobic monomers is favored as compared with conven­ styrene and butadiene.107,108
tional emulsion polymerization, and the copolymer
compositions achieved in batch miniemulsion copolymeriza­ 3.14.2.1.3(iii) Synthesis of hybrid particles
tion are closer to those expected from the Mayo–Lewis As explained above, droplet nucleation is a unique feature of
equation (eqn [3]) under bulk conditions. This trend was the miniemulsion polymerization process, and this nucleation
experimentally observed by several authors who investigated mechanism has prompted the discovery of new applications
the copolymer composition produced in batch emulsion and that were not possible by other conventional dispersed phase
miniemulsion copolymerization using monomers with differ­ polymerization techniques. This is in particular the case for the
ent water solubilities and reactivity ratios.102,103 incorporation of highly hydrophobic materials or materials
The microstructure (MMD, gel content, branching, and that are unable to diffuse through the aqueous phase (poly­
crosslink densities) of the polymer might be affected by the mers and inorganic particles) to produce waterborne polymer–
different segregation levels in emulsion and miniemulsion polymer and polymer–inorganic nanocomposite dispersions.
The efforts to synthesize waterborne hybrid polymer–
copolymerization. Figure 11 presents the expected evolution
polymer nanoparticle dispersions are due to the expected
of the volume fraction of polymer in the polymer particles as a
synergetic behavior of the positive properties of each polymer
function of conversion for batch emulsion and miniemulsion
phase. Hybrid latexes made of alkyd resins,72,92,109,110 polyur­
polymerization processes. Since miniemulsion polymerization
ethanes,111–113 polydimethylsiloxane (PDMS),50 polyester,114
can be roughly approximated to a completely segregated sys­
and other polymers115 have been reported. The polymer resin
tem (no transport of matter between the droplets), each
can be used as the sole costabilizer, but long-chain acrylates
nanodroplet can be considered as a batch reactor in which a
(such as stearyl acrylate) are also used to increase the stability of
bulk polymerization takes place. Therefore, the initial volume
the monomer miniemulsions. The main polymer phase in the
fraction of polymer in the polymer particles (at the time of
hybrid system is produced in situ by polymerization of suitable
nucleation of the droplet) should be close to zero, and it will
monomers. Typically, acrylic and acrylic–styrene copolymer
increase with conversion. By contrast, in emulsion polymeriza­ formulations are used to take advantage of the weather and
tion the volume fraction of polymer will be that corresponding water resistance of the acrylic polymers.
to the saturation of the particles by monomer (as soon as the For the synthesis of hybrid polymer–polymer latexes by
particles are formed), and it will remain at the saturation level miniemulsion polymerization, the homogenization step is car­
until the monomer droplet phase is depleted (the conversion at ried out as explained above, but it should be taken into account
which this occurs depends on the monomer–polymer system). that the viscosity of the organic phase strongly affects the size of
Beyond this point, the volume fraction of polymer will increase the droplets that can be achieved. Thus, the higher the polymer
content in the organic phase, the larger the droplet size will be
for the same energy applied because of the higher viscosity of
the organic phase.97
Volume
fraction In most of the cases, limiting conversion was found during
polymer the polymerization of the hybrid miniemulsions in batch reac­
tor because the growing radical transferred to the polymer resin
Emulsion that then became a radical sink.89,116,117 However, this could
be easily overcome working in semibatch mode or by
post-polymerizing with suitable redox initiator systems.109
The morphology of the hybrid particles depends on the com­
patibility of both polymer phases. The compatibility is
Miniemulsion enhanced if the acrylic polymer is covalently linked to the
polymer resin, that is, if most of the acrylic polymer is grafted
onto the resin (acrylic degree of grafting (ADG)) and a large
Conversion fraction of the resin is also incorporated to the acrylic polymer
Figure 11 Evolution of the volume fraction of polymer in the particles as (resin degree of grafting (RDG)). It has been found that the
a function of conversion for emulsion and miniemulsion polymerization RDG controls the morphology of the particles. For alkyd–
processes. acrylic hybrids, RDG values greater than 35% are necessary to
 Vinyl Polymerization in Heterogeneous Systems 477

produce homogeneous particles, provided that the acrylic poly- (a)

mer is almost completely grafted (ADGs above 90%).109
Hybrid polymer–inorganic nanocomposite latexes have also 0.9
been synthesized by miniemulsion polymerization.

Δnanofiller-monomer
Encapsulation of inorganic material in polymer particles has
attracted the interest of scientists working in the field of cos­
metics, coatings, adhesives, and in pharmaceutical or biomedical 0.6
applications. TiO2, CaCO3, silica, clay, carbon nanotubes, quan­
tum dots, and magnetite are some examples of nanoparticles
that have been incorporated to polymer particles by using mini-
emulsion polymerization.87 In most of the applications, the full 0.3
encapsulation of the inorganic nanoparticles by the polymer is
required (biomedical applications) but in other cases (coatings
and adhesives) there is no sufficient proof demonstrating that 0.3 0.6 0.9
the encapsulated morphology will provide better properties than Δnanofiller-water
other possible morphologies.
(b)
The encapsulation of inorganic nanoparticles by means of
miniemulsion polymerization requires the following: (1) the 0.9
nanoparticles to be hydrophobic enough to be dispersed homo­

Δnanofiller-monomer
geneously in the monomer and costabilizer phase; (2) the
formation of monomer nanodroplets with the inorganic mate­
rial in it; and (3) the polymerization of all (or at least a large 0.6
fraction) of the nanodroplets avoiding other possible nucleation
mechanisms. The success on the encapsulation of the inorganic
nanoparticles by miniemulsion polymerization depends on the
interplay of several parameters. Thus, the compatibility of 0.3
the modified nanoparticles and the monomers (nanoparticle–
monomer interfacial tension, σnanofiller–monomer) and the
interaction of the nanoparticle with the aqueous phase 0.3 0.6 0.9
(nanoparticle–aqueous phase interfacial tension, Δnanofiller-water
σnanofiller–water) are the key parameters to determine the achiev­
able morphologies. Figure 12 presents equilibrium Figure 12 Equilibrium morphology maps for hybrid monomer–
nanoparticle miniemulsion nanodroplets calculated by means of a Monte
morphologies of hybrid monomer–nanoparticle nanodroplets
Carlo simulation algorithm. (a) Disklike nanoparticles. Reproduced with
calculated by means of a Monte Carlo simulation algorithm for permission from Micusik, M.; Bonnefond, A.; Reyes, Y.; et al. Macromol.
different interaction of the nanoparticles with the monomer and React. Eng. 2010, 4, 432.118 Copyright Wiley-VCH Verlag GmbH & Co.
water (Δnanofiller–monomer and Δnanofiller–monomer in Figure 12 are KGaA. (b) Spherical nanoparticles. Water phase is depicted in light gray,
interaction parameters that are proportional to the interfacial monomer phase in darker gray, and nanofiller in black.
tensions). Figure 12(a) shows the equilibrium morphologies
achieved with nanoparticles with disklike structure (to mimic
clay platelet particles)118 and Figure 12(b) shows the case of nucleation and the lack of monomer transport during the
nanoparticles with spherical shape (i.e., silica, quantum dots). polymerization have made miniemulsion polymerization
The simulation indicates that to encapsulate the inorganic very attractive for the development of a new portfolio of pro­
nanoparticle in the monomer droplets, the monomer and nano­ ducts. In addition to the synthesis of hybrid latexes, the
particle should be very compatible (low monomer–nanoparticle applications for which the miniemulsion polymerization is
interfacial tension) and, on the other hand, the nanoparticle unique with respect to conventional emulsion polymerization
should be very hydrophobic in the case of disklike nanoparticles are CRP in dispersed systems, catalytic polymerization, ionic
(Δnanofiller–monomer = 0.9) and hydrophobic (Δnanofiller–water = polymerizations, polymerization of very hydrophobic mono­
0.6–0.9) in the case of spheres. In the other cases (less compa­ mers, and step-growth polymerization in aqueous dispersed
tible nanoparticle–monomer system and less hydrophobic systems. In addition, it offers advantages in other applications
nanoparticles), the thermodynamically stable morphologies such as the synthesis of high solids content latexes (the broad
are armored structures with the nanoparticles preferentially PSDs obtained might help in reducing the viscosities of the
located at the monomer–water interface; when the nanoparticle latexes and hence increase the solids content) or for process
is very hydrophilic and incompatible with the monomer (large intensification using CSTR or tubular loop reactor (the typical
monomer–nanoparticle interfacial tension), the nanoparticle is oscillations due to the intermittent micellar nucleation are
preferentially located in the aqueous phase. avoided using miniemulsion polymerization).119
However, the implementation of miniemulsion polymeriza­
3.14.2.1.3(iv) Examples of industrial applications tion in industry is challenging because industry will only adopt
The importance of the miniemulsion polymerization techni­ this method provided that new and improved materials can be
que relies on the potential to synthesize waterborne polymeric produced and that the technology required to run the process is
dispersions that cannot be produced by means of conventional available at a reasonable cost. The former is true because hybrid
emulsion polymerization. The unique feature of the droplet alkyd–acrylic coatings and polyurethane–acrylic adhesives with
478 Vinyl Polymerization in Heterogeneous Systems

enhanced performance that cannot be synthesized by other 50

polymerization techniques have been recently discovered. The
incorporation of inorganic nanoparticles into polymer particles

Interparticle spacing (nm)

has also been shown, but not all the systems studied at the 40
300 nm
laboratory scale are ready for commercialization because of the
low solids content, reduced range of stability of the dispersions,
30 50 nm 75 nm
and the need for complex modifications of the inorganic materi­
als that make the industrialization very challenging.
20
3.14.2.1.4 Microemulsion polymerization 20 nm
Microemulsions are thermodynamically stable oil-in-water emul­
10
sions, in which the droplet size (10–100 nm) is smaller than that
in conventional emulsions (macroemulsions: 1–100 μm) and
miniemulsions (50–500 nm). To prepare a stable microemul­ 0
sion, large amounts of anionic or cationic surfactants (more 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
than 10–20% in the formulation or at similar levels as the Volume fraction
monomer) and short-chain alcohols (e.g., n-pentanol) or other
Figure 13 Interparticle spacing vs. volume fraction of particles as a
cosurfactants are employed.120,121 Upon polymerization of the function of particle diameter. The horizontal line represents the safe
microemulsion with a suitable initiator (either water soluble or interparticle distance.
oil soluble), polymer latexes with particle sizes in the 10–50 nm
range and very high molar masses (>106 g mol−1) can be
synthesized. represents an arbitrary safe (i.e., minimum) distance between
The mechanisms of particle nucleation and particle growth particles to account for situations that would lead to an exces­
and hence the kinetics of the microemulsion polymerization sive increase of viscosity of the dispersion and/or high risk of
are not well understood yet. Although there are common fea­ coagulation. Figure 13 shows that, for a given volume fraction
tures between microemulsion and emulsion polymerization, of the dispersion, the larger the particle diameter, the higher the
the two processes present significant differences in the final distance between particles and the higher the solids content
products. In contrast to emulsion polymerization, the number that can be reached before the double or adsorption layers of
of models describing the kinetics and mechanisms of micro­ neighboring particles interact (i.e., crossing of the IPS and of
emulsion polymerization is scarce.122–127 For a slightly the straight line). Therefore, the crosspoints roughly represent
water-soluble monomer such as styrene, the following features the maximum achievable solids content for monodisperse
are generally assumed: (1) radicals generated in the aqueous latexes with the given particle sizes.
phase diffuse predominantly to monomer-swollen micelles; Xu and Gan130 have recently reviewed the attempts made
(2) radical termination in the aqueous phase is negligible; to produce latexes with high solids content and small parti­
(3) radical entry into preformed particles is negligible; (4) ter­ cle sizes using microemulsion polymerization. The most
mination of radical growth in the particles is predominantly by promising technique is what can be regarded as semibatch
chain transfer to the monomer followed by desorption of the microemulsion polymerization. Basically, the approach con­
radicals; and (5) a 0-1 system is assumed for the radical dis­ sists in preparing a thermodynamically stable
tribution in the particles. microemulsion (using large amounts of surfactant and with
When microemulsion polymerization was first developed in the help of a cosurfactant at low solids content, namely,
the 1980s, it was seen as a promising technique to produce latexes typical microemulsion conditions) that is polymerized in
with very small particle sizes and high molar masses. However, batch up to full conversion. Then, additional monomer is
the large amounts of surfactants, namely, the low polymer to added to the microemulsion to increase the solids content,
surfactant ratios, and the low solids content achievable by this and hence, reducing the amount of surfactant with respect to
method, have prevented the industrial implementation of this polymer of the original microemulsion, and very likely
technique. In spite of this limitation, there is a strong interest to increasing the particle size from that of the original micro-
obtain nanosized (< 50 nm) latexes with high polymer contents
emulsion. Although it might be viewed as a seeded
(high solids content) and lower surfactant concentrations; ideally
semibatch emulsion polymerization, there is a fundamental
without (or with low amounts of) emulsifier. A rough idea of the
difference with respect to this common process; when the
maximum solids content achievable for narrow unimodal latexes
monomer feeding starts, in addition to the seed polymer
can be obtained by considering the interparticle space (IPS) given
particles, there are plenty of micelles in this system (in
by eqn [7]:128
other words, the emulsifier concentration is well above the

1=3 ! CMC). The empty micelles compete with the polymer parti­
φn
IPS ¼ 2r −1 for φ ≤ φn ≤ 0:64 ½7 cles for the monomer and radicals, and hence both
φ
formation of new particles and growth of the preformed
In eqn [7], r is the particle radius, φn the maximum packing ones in the batch microemulsion take place during the feed­
factor of the monodisperse particles, and φ the volume fraction ing period. Different monomers and different addition
of the particles. Figure 13 plots the interparticle distance (eqn strategies have been reported and the most interesting results
[7]) as a function of the volume fraction of particles (solids are presented in Table 1. The results are in agreement with
content) for unimodal latexes.129 The straight line in the plot the limitation presented in Figure 13, namely, unimodal
 Vinyl Polymerization in Heterogeneous Systems 479

Table 1 Summary of works that employed semibatch microemulsion polymerization to produce latexes with high solids content, low particle size, and
reduced amount of surfactant

Particle Polymer/surfactant ratio; Solids content

Initial charge: microemulsion? size (nm) type of surfactant Monomer addition (wt.%) Reference

131
Yes <100 15:1; CTAB (cationic) Winsor I-like 15
132
Yes, M/S = 1:1– 5:1 + 1­ 10–20 7–10:1 (for Sty, BMA, and Continuous addition (2–3 h) 10–30
pentanol BA) and 25:1 (for MA); (30 for MA)
SDS (anionic)
Yes, M/S = 3:1 (VAc) 30 → 70 3:1 → 30:1; Aerosol OT Stepwise; shots every hour 3 → 30 133

(anionic) (six)
134
Not given <40 20:1; SDS (anionic) Continuous addition (3–5 h) 30–40
135
Yes, M/S = 1:2 (only CTAB) 50–80 15:1; CTAB/PEO-R-MA40 Continuous addition (4 h) of 10
Mixture cationic/nonionic monomer and surfmer
surfmer
136
Yes, M/S = 1:2 (BA) <60 20:1; mixture of SDS/AOT Continuous addition until <30
(anionic) latex viscosity is gel-like
Yes, M/S = 1:2.2 (S) 26 → 45 3:1; DTAB (cationic) Stepwise (5 ml every 20 min) 40 137
138,139
Yes, M/S = 1:5 (for S, BMA) 15–60 15:1; CTAB (cationic) or SDS Hollow fiber feeding <20
and M/S = 1:1.25 (for MMA) (anionic)
1-pentanol
140
No, micellar solution 25–60 10:; DOWFAX 2A1 (plus Continuous addition 30–45
acrylamide as (includes monomer,
cosurfactant) DOWFAX, acrylamide)
141
No, micellar solution 16–40 18:1; SDS (anionic) Continuous addition 13–36
(water-soluble initiator, (MMA + 1-pentanol in 1 h)
ammonium persulfate)
142
No, micellar solution 15–30 160–?/1; SDS (anionic) Continuous addition (MMA <20
(oil-soluble initiator, AIBN) for 1.5 h)

M/S, monomer/surfactant; PEO-R-MA40, ω-methoxy poly(ethylene oxide)40 undecyl α-methacrylate.

latexes with particles equal or below 20 nm cannot be last entry142 are interesting because they demonstrated
produced in volume fractions above 25–28%, unless gel-like that using small amounts of SDS with an oil-soluble initiator
or extremely viscous dispersions are allowed, which is not (N,N′-azobisisobutyronitrile (AIBN)) led to very small poly
the case for the majority of commercial applications. Using (n-butyl acrylate) latex particles (between 20 and 30 nm)
true microemulsion conditions, the best results (highest with concentrations of SDS as low as 0.6 wt.% at solids
solids content and lowest particle size with the lowest sur­ content below 20 wt.%.
factant concentration) were obtained by Ming et al.132 and In addition to the synthesis of latexes with high solids
Ramirez et al.136 The former were able to produce poly content and small particle size, in the past decade, microemul­
(methyl acrylate) latexes with solids content of 30 wt.%, sion polymerization has been used to synthesize a wide range
number-average particle size of 14.5 nm with broad PSD of materials. For instance, several works have incorporated
(Dv/Dn = 1.5), and c. 3 wt.% SDS based on monomer and inorganic materials such as carbon nanotubes,143 ZnO
small amount of 1-pentanol. With less polar monomers nanoparticles (UV absorption),144 montmorillonite clay,145
(S, BMA, BA, and MMA), they could not reproduce these and quantum dots (luminescence probes)146 to produce
results: for similar particle sizes, solids contents were below nanocomposites. Furthermore, nanogels,147 conductive poly-
20 wt.% and larger surfactant concentrations were required. pyrrole and polyaniline latexes,148,149 polyurethanes using
The latter synthesized poly(n-butyl acrylate) latexes immiscible monomers,150 and polymers in water-in-scCO2
with solids content of 30 wt.% and particles size in the microemulsions151 have been also prepared by microemul­
40–55 nm range, with a mixture of SDS and Aerosol OT sion polymerization.
anionic surfactants at total concentrations of 5 wt.% with
respect to monomer. The last three entries of Table 1 are 3.14.2.1.5 Aqueous dispersion and precipitation
not formally microemulsion polymerization because they polymerizations
do not form a microemulsion (namely, a thermodynami­ 3.14.2.1.5(i) Aqueous dispersion polymerization
cally stable oil-in-water emulsion), although the authors Dispersion polymerization8,12,152–156 is a way of forming
used this name to describe the processes. As shown in the polymer particles from an initially homogeneous monomer
first column, these works started from a micellar solution solution. While the monomer is soluble in the selected solvent,
(rather than from a microemulsion as in the other works in the formed polymer is not and precipitates. In the presence of a
the table) produced with large amounts of surfactant and the stabilizer, this leads to particles. In the vast majority of the
monomer (in addition to a cosurfactant, either acrylamide situations, the continuous phase is an organic solvent or a
or 1-pentanol in the first two cases, respectively) was slowly mixture of water and alcohol, and the typical particle diameter
fed into the micellar solution. The results reported in the is in the 200 nm–20 μm range (see Section 3.14.3.1.2). In pure
480 Vinyl Polymerization in Heterogeneous Systems

water, there are very few examples and most of them are related free radical polymerization (SFRP))173 and atom transfer radical
to the synthesis of polymer particles able to respond to an polymerization (ATRP),171,172 which operate by a reversible
external stimulus, such as pH or temperature (for instance, termination mechanism, and reversible addition-fragmentation
polymers exhibiting a lower critical solution temperature chain transfer (RAFT),174–176 which operates by a reversible
(LCST), such as poly(N-isopropylacrylamide) (PNIPAM)). transfer process. All these techniques are well described in the
Those may be useful in the domain of biomedical applications corresponding chapters of this comprehensive. The application
and drug delivery.157–159 of the three major CRP types, as well as more recently developed
CRP methods, to aqueous dispersions will be described in this
3.14.2.1.5(ii) Aqueous precipitation polymerization section. It will be shown that the behavior of CRP in aqueous
Precipitation polymerization is a general case of dispersion dispersed systems can vary considerably from behavior in bulk
polymerization in which no stabilizer is added. Although the or in solution. Challenges arising from partitioning of the med­
monomer is soluble in the aqueous phase, the polymer precipi­ iating species into the aqueous phase, radical exit from
tates and forms a second phase. In the typical case of acrylonitrile, particles, and reactions of the mediating species with other
the monomer is not a good solvent for the polymer and the components may cause loss of control and/or poor colloidal
polymerization takes place essentially at the polymer–water stability.
interface. The developments of CRP in aqueous dispersed systems have
been reported in several review articles,88,177–187 which stressed
that, although challenging, the target was considered as particu­
3.14.2.2 Controlled Radical Polymerization larly important due the intrinsic qualities of these processes.
3.14.2.2.1 Overview of CRP in aqueous dispersed systems There are indeed significant fundamental and practical incentives
Since free radical polymerization proceeds with fast to conduct CRP in aqueous dispersions. For economic reasons,
self-termination of the propagating radicals and is often domi­ aqueous dispersions are often the best alternative for large-scale
nated by chain transfer to the monomer in emulsion production, providing excellent heat transfer, ease of mixing,
polymerization, it does not allow the synthesis of well-defined process flexibility such as semibatch addition of reagents during
polymer chains. In particular, the synthesis of polymers with polymerization, and ease of handling/transporting the final
controlled molar mass, narrow MMD along with well-defined latex. Recently, it has been shown that important polymer
end group and chain structure is almost impossible. Indeed, property advantages, such as improved livingness, may also be
emulsion polymerization leads to polymers with very high realized when running CRP in an aqueous dispersion. Moreover,
molar masses and sometimes microgels in the case of acrylates, CRP was considered to offer additional advantages than simply
VAc, and butadiene. For some applications, it is highly useful to controlling the polymer at the molecular level. It can be, for
reduce the chain length using chain transfer agents. Classical instance, the design of particle composition and morphology,
molecules such as thiols are often used in the industrial produc­ the way to nanostructured organic particles and to hybrid
tion of latexes. Of lower industrial importance, but with great nanocomposites, and so on, for a variety of new potential
promises are the catalytic chain transfer (CCT) agents, essentially applications.
cobalt-based molecules, which show exceptionally high chain
transfer constants and lead to low-molar-mass poly(methacrylic 3.14.2.2.2 Nitroxide-mediated radical polymerization
esters) with a terminal double bond.160–170 In NMP, also known as SFRP, a stable free radical (nitroxide)
A better control of the polymer at the molecular level was reversibly terminates propagating macroradicals to yield dor­
made possible with the advent of the CRP techniques.84,171–176 mant polymer chains with an alkoxyamine end group
CRP is a particular way of performing radical polymerization in (Figure 14). Because the equilibrium favors the dormant species,
which the propagating radicals are subjected to reversible deac­ the propagating radical concentration is most often lower than
tivations during the polymerization. They are in equilibrium that in conventional radical polymerization. Under typical poly­
with dormant chains, in much larger concentration than the merization conditions, a dormant chain is activated every
radicals themselves. This situation leads to the formation of
102–103 s on average, and the formed macroradical adds
polymer chains, which exhibit controlled and predictable
1–5 monomer units prior to deactivation. Deactivation is
molar mass (the Mn increases linearly with monomer con­ very fast, at almost diffusion-limited rates, such that the deacti­
version), narrow MMD (i.e., low polydispersity index vation of a propagating radical occurs
10−4–10−3 s after it is
(PDI) = Mw/Mn, with Mw the weight-average molar mass), and activated. Irreversible termination is ideally minimized although
reactivable end group. Therefore, CRPs allow a high degree of it cannot be completely eliminated. Accumulation of nitroxide is
control of polymer microstructure so that the synthesis of struc­ a direct effect of termination (the so-called persistent radical
tures such as di- and triblock copolymers, star polymers, and effect (PRE)),188 and shifts the equilibrium toward the dormant
comblike graft copolymers can be made. Unlike ionic polymer­ state, thereby suppressing the polymerization rate. Elevated tem­
izations, which sometimes require very low temperatures, peratures (
90–135 °C) are necessary to achieve reasonable
rigorous purification of reagents, and careful exclusion polymerization rates.
of moisture and oxygen, CRP can be conducted under typical Although several nitroxides have been reported for bulk/solu­
conventional free radical polymerization conditions, and do not tion NMP, 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) and
even require purification of reagents. Several methods have been N-tert-butyl-N-(1-diethyl phosphono-2,2-dimethylpropyl) nitr­
proposed to reach that goal and all of them can be classified in oxide (Figure 15), better known by its trade name ‘SG1’ (trade
two main categories according to the mechanism of the deacti­ name of the Arkema Group), have been most commonly used
vation step. The three major types of CRPs are in aqueous dispersed systems. TEMPO requires higher polymer­
nitroxide-mediated radical polymerization (NMP) (or stable ization temperatures than SG1 (
120–135 °C for TEMPO vs.
 Vinyl Polymerization in Heterogeneous Systems 481

N CH
O
ka +
N
O
kd
n
n
Alkoxyamine Polystyrene TEMPO
(dormant polymer radical) radical
Figure 14 Nitroxide-mediated radical polymerization, illustrating reversible activation and deactivation of polystyrene by TEMPO. The activation–
deactivation equilibrium constant is defined as K = ka/kd.

O – +
O Na
S
O N O O
CH P OC2H5 SDBS
N
OC2H5
O
O (CH2)15CH3
Figure 15 Structure of the nitroxides TEMPO (left) and SG1 (right).
DOWFAX 8390
SO3Na SO3Na

Figure 16 Structure of the surfactants SDBS and DOWFAX 8390, com­

90–120 °C for SG1). Other differences between these two nitr­ monly used in high temperature NMP emulsions and miniemulsions.
oxides include their partitioning behavior between aqueous and
organic phases, sensitivity to the pH of the aqueous phase, and
the stability of the nitroxide at reaction conditions. TEMPO is a 3.14.2.2.2(i) TEMPO-mediated polymerizations
more stable radical than SG1, which means it is more prone to Initial efforts to conduct TEMPO-mediated NMP in aqueous
accumulation over time. This phenomenon requires another dispersions logically used an emulsion polymerization approach.
source of radicals, such as those arising from thermal polymer­ While living polymer was obtained, poor colloidal stability and
ization (in the case of styrene polymerization) or from added coagulum formation plagued these early attempts.88,177 Initially,
initiator, to consume the excess TEMPO. Consequently, polymer­ the reason(s) for the colloidal stability problems was not under­
ization of non-styrenic monomers (e.g., acrylates) is difficult with stood but the underlying causes have recently been elucidated
TEMPO, while it is readily achieved with SG1. in studies by Pohn et al.198 and Maehata et al.199 Pohn et al.198
Among the various processes of polymerization in aqueous simulated the mass transfer of monomer between droplets and
dispersed systems, nitroxide-mediated suspension polymeriza­ seed particles during TEMPO-mediated styrene emulsion poly­
tions, where the particle sizes are comparatively large (>1 µm), merization. Simulation results showed that the presence of
have been reported in a few articles only.189–193 In general, they small particles, which form during the early stages of emulsion
behave similarly to bulk polymerization, with the challenges polymerization, will (counterintuitively) lead to polymerization
described above not being as prominent as when particle sizes occurring preferentially in the large droplets, suggesting that
are below 1 µm, like in miniemulsion and emulsion polymer­ TEMPO-mediated ab initio emulsion polymerizations will not
ization, which have been far more studied. be feasible. By selectively inhibiting polymerization in the mono­
There are limited choices for surfactant at the higher tempera­ mer droplets using the highly hydrophobic 4-stearoyl-TEMPO,
tures required for NMP. Sulfates undergo hydrolysis at higher Maehata et al.199 experimentally demonstrated that droplet
temperatures and are generally not suitable, while sulfonates polymerization was indeed responsible for the formation of
have excellent hydrolytic stability and do function effectively at large particles that can lead to coagulum formation. A modified
elevated temperatures (100–140 °C). The two most commonly TEMPO-mediated emulsion polymerization process was then
used in NMP miniemulsions are sodium dodecyl benzene developed in which droplet polymerization was suppressed,
sulfonate (SDBS) and DOWFAX 8390 (Dow Chemical Inc.) and was shown to be capable of giving coagulum-free latexes.
(Figure 16). SDBS is usually available only in a technical grade Another modified SFRP emulsion polymerization process using
while DOWFAX 8390 is a mixture of mono- and dihexadecyl a nanoprecipitation approach was able to yield a stable latex
disulfonated diphenyloxide sodium salts. Polymerization rates with mean particle diameters
400–500 nm.200
may be significantly influenced by [SDBS].194,195 SDBS is able to Miniemulsion polymerization has proven to be robust pro­
generate radicals in the presence of ascorbic acid,196 suggesting cess for many types of CRP, including TEMPO-mediated
additional radical generation is the most likely explanation NMP. Mathematical modeling of TEMPO-mediated
for the observed effects of [SDBS]. Pan et al.197 examined the miniemulsion201–207 has provided insight into many of the
effects of [DOWFAX 8390] on TEMPO-mediated styrene mini- underlying phenomena and how they affect kinetics and poly­
emulsions but did not observe notable effects on the rate of mer properties. A study of interfacial mass transfer of nitroxides
polymerization, Mn, or PDI. predicted that phase equilibrium for [TEMPO] should be
482 Vinyl Polymerization in Heterogeneous Systems

maintained.203 However at larger droplet diameters correspond­ result of the PRE.188 Block copolymers (polystyrene-b-poly
ing to suspension and emulsion polymerizations, the predicted (n-butyl acrylate)) made in TEMPO-mediated miniemulsions
equilibration times are significantly greater, giving rise to the have been reported.220,221 Addition of ascorbic acid enabled
possibility of diffusion-controlled reactions. high conversion (>99%) in both blocks.221 TEMPO-mediated
For TEMPO-mediated styrene miniemulsions, polymeriza­ homopolymerization of BA in bulk222 and miniemulsion222,223
tion rates are nearly independent of the water solubility of the has also been reported where again semibatch ascorbic acid
nitroxide even if partition coefficients differ significantly addition (or its oil-soluble derivative, ascorbic acid 6-palmitate)
(e.g., TEMPO and 4-hydroxy-TEMPO).201 The reason for this was used.
surprising result is that thermal autoinitiation of styrene dom­
inates the phase partitioning behavior. In the absence of thermal 3.14.2.2.2(ii) SG1-mediated polymerization in miniemulsion
initiation with monomers such as BA, the rate of polymerization SG1 (Figure 15) is a versatile nitroxide suitable for the poly­
is significantly faster in systems with more water-soluble TEMPO merization of styrene, acrylates, and even methacrylates (when
derivatives (4-hydroxy-TEMPO) compared to systems with copolymerized with
10 mol.% styrene224,225 or acryloni­
TEMPO. Zetterlund and Okubo208 corroborated these findings. trile226 to impart livingness to the system). SG1 is an acyclic
TEMPO-scavenging additives can increase polymerization β-phosphonylated nitroxide with a more favorable equilibrium
rate, although the role of various additives may be more com­ constant than TEMPO. For highly reactive monomers such as
plex than just simple reaction with nitroxide. Camphorsulfonic BA, additional SG1 is required to maintain a controlled
acid was used in TEMPO-mediated styrene miniemulsion poly­ polymerization.
merizations.209 In another way, semibatch addition of ascorbic Farcet et al.227 found that the initial ratio of [SG1]/
acid in TEMPO-mediated styrene miniemulsion polymeriza­ [MONAMS] (an SG1-based oil-soluble alkoxyamine) played
tions is a powerful technique for enhancing the polymerization an important role in determining polymerization rate in the
rate and achieving high conversions at low reaction times.210,211 miniemulsion polymerization of BA. [SG1]/[MONAMS] ratios
High conversions (>98%) were obtained in 2–3 h (which is of
0.035–5 provided good control, with temperature influen­
faster than industrial styrene polymerizations) while PDI cing the optimum ratio. Insufficient quantities of SG1 gave
remained low (< 1.3). Livingness was actually higher when the higher rates but at the expense of broad MMDs. Excessive quan­
rate was increased, as the reduction in reaction time led to less tities of SG1 suppressed the rate. Conversions greater than 70%
dead chain formation through disproportionation (i.e., H trans­ were reported for Mn
25–30 kg mol−1 and PDI
1.4–1.6. The
fer from the propagating radical to the nitroxide). observed particle diameters were larger than typically obtained
Although TEMPO is typically used in the range of
120– in miniemulsion (400–650 nm) but colloidal stability, a more
135 °C, it is possible to use temperatures
100 °C through important concern, was good.
judicious addition of a nitroxide scavenger such as ascorbic The Charleux laboratory also used a water-soluble SG1-based
acid.212 Although the PDIs were larger than values achieved at alkoxyamine, known as MAMA or by its current trade name
higher temperatures, the polymer livingness remained high. An BlocBuilder MA® (Arkema Group)228 (Figure 17) in
advantage of polymerizing at lower temperatures with TEMPO SG1-mediated miniemulsion polymerization. BlocBuilder MA®
is that the disproportionation rate is much less.201 is unique in that it has a carboxylic acid moiety that imparts water
Most published studies have focused on making narrow solubility when ionized. At pH >
5.5, it exists in the ionized
MMD with CRP, and far less attention has been given to under­ (sodium salt) form while at lower pH it remains in acid form.
standing the behavior of CRP in the presence of crosslinking BlocBuilder MA® also has a higher dissociation rate constant than
agents. In conventional bulk polymerization systems, in addi­ the oil-soluble MONAMS and does not require additional SG1 to
tion to the desired intermolecular crosslinking, intramolecular give a controlled polymerization, even for BA. The ionized alkox­
crosslinking is also important. Intramolecular crosslinking yamine is soluble in the aqueous phase, but becomes sufficiently
results in microgel formation and heterogeneous network hydrophobic to enter droplets or particles after adding a few
formation. In CRP, the lower chain lengths (compared to con­ monomer units, much like water-soluble initiators in conven­
ventional polymerization) result in lower apparent reactivity tional (mini)emulsion polymerization. High initiation
of the pendent unsaturated sites, which in turn yields more efficiencies were observed for BA. However, for styrene, initiation
uniform networks without microgel formation that also have efficiencies were low, which was attributed to low oligoradical
greater swelling capability.213 Differences have also been entry rates from the aqueous phase due to slow aqueous phase
observed, however, between crosslinking behavior in bulk styrene polymerization. Addition of small amounts of methyl
and miniemulsion CRP systems. Okubo and Zetterlund have acrylate (MA) significantly improved the efficiency by increasing
examined crosslinked styrene–divinylbenzene systems,214–219 the propagation rate of oligomeric radicals in the aqueous phase,
noting distinct differences between bulk and miniemulsion yielding a far more effective miniemulsion process. This study
behavior with miniemulsions exhibiting faster kinetics than provided the framework for subsequent development of
bulk. Crosslink densities and pendent vinyl group conversion SG1-mediated emulsion polymerization.
also differed between the bulk and miniemulsion; miniemul­
sions showed much lower crosslink densities and slower 3.14.2.2.2(iii) SG1-mediated polymerization in emulsion
conversion of the pendent vinyl groups. TEMPO is generally Unlike TEMPO that is not readily amenable to a true emulsion
not well suited to the polymerization of acrylates or methacry­ polymerization process, a seeded emulsion polymerization
lates, although copolymerizations with styrene are better process using the water-soluble SG1-based alkoxyamine
controlled. Often TEMPO-mediated acrylate homopolymeriza­ BlocBuilder MA® has been developed.229–231 BlocBuilder MA®
tions progress with reasonable control to low conversions allows effective aqueous phase initiation, which is key for an
(< 5–10%) and then cease due to TEMPO accumulation as a emulsion polymerization process. Low-molar-mass seed latex
 Vinyl Polymerization in Heterogeneous Systems 483

(a) (b)

O N O O N O
Na+ −
O P O HO P O
O O O O

COOH COOH

(c)
O CH2 CH2 O
O N O O N O
O P 3
O O P O
O O

Figure 17 The SG1-based alkoxyamine BlocBuilder MA® in ionized (a) and nonionized (b) forms, (c) and the difunctional DIAMA.

is first prepared and then swollen with monomer and polymer­ particle size effects did exist, influencing both polymerization
ized to yield final latex particles by chain extension. The use of a rate and livingness. Smaller particles showed lower rates of
seed eliminates monomer droplet formation early in the polymerization than larger particles as well as higher livingness.
polymerization and, therefore, prevents droplet polymerization The lower rate in smaller particles likely results from the follow­
and consequent colloidal instability. ing: (1) geminate recombination of thermally generated radicals,
The difunctional derivative DIAMA has also been used in leading to reduced thermal initiation rates; and (2) enhanced
emulsion polymerization (Figure 17),230,231 enabling a signifi­ deactivation of propagating radicals with nitroxide (the so-called
cant reduction in particle size and narrower distribution confined space effect). Enhanced deactivation is also predicted to
compared to latexes prepared using monofunctional BlocBuilder reduce termination rates and, therefore, to increase livingness in
MA®. Polystyrene-b-poly(n-butyl acrylate)-b-polystyrene triblock smaller particles. Delaittre and Charleux236 examined compart­
copolymers were made using the difunctional alkoxyamine, and mentalization with SG1-mediated emulsion polymerizations,
allowed nanostructured particles to be formed upon internal and did not observe effects in the formation of poly(acrylic
phase separation.232 A refined semibatch process shortened over­ acid)-b-polystyrene amphiphilic block copolymers. Because
all process time compared to earlier efforts.231 SG1 has higher water solubility than TEMPO but more impor­
Water-soluble macroalkoxyamines were used in the tantly because it exhibits a lower rate constant of recombination
SG1-mediated surfactant-free, ab initio, batch emulsion polymer­ with the propagating radicals, it can rapidly diffuse between
ization of styrene, BA, 4-vinyl pyridine,67,68,233 and MMA (with phases to equilibrate its concentration, and therefore compart­
<10% styrene comonomer).225 They led to amphiphilic block mentalization is unlikely to be important.
copolymer nanoparticles by simultaneous chain growth and
self-assembling in situ. PAA macroalkoxyamines terminated by 3.14.2.2.3 Atom transfer radical polymerization
SG1 were very efficient in particle stabilization, but the initiating In ATRP (also known as transition metal-mediated polymeri­
efficiency was rather low.67,68 Poly(methacrylic acid-co-styrene) zation), a halogen atom is transferred from a catalyst–ligand
macroalkoxyamines (again, small amounts of styrene com­ complex to a propagating macroradical (Figure 18). Dormant
moner are required to preserve control and livingness with chains are activated by a transition metal complex in its lower
methacrylate monomers)225,234 were used as the first block oxidation state (e.g., CuBr/ligand) whereas propagating macro-
in the in situ synthesis of amphiphilic block copolymers of radicals are deactivated by a catalyst complex in a higher
poly([methacrylic acid-co-styrene]-b-[methyl methacrylate-co­ oxidation state (e.g., CuBr2/ligand). The ligand plays a critical
styrene]), with very high crossover efficiency. This method has role in determining the reactivity of the catalyst complex as well
the advantage of using a single molecule (i.e., the macroalkox­ as affecting its solubility in the reaction medium. In dispersed
yamine) as the initiator, the control agent, and the stabilizer of aqueous polymerizations, the ligand should be highly hydro­
the formed particles. phobic to prevent partitioning of catalyst into the aqueous
phase. Of particular concern is loss of the deactivator; Cu(II)

3.14.2.2.2(iv) Compartmentalization in NMP (mini)emulsions

In conventional emulsion polymerization, compartmentaliza­ kact
tion of the propagating radicals gives higher reaction rates and Pn-Br + CuIBr/L Pn + CuIIBr2/L
higher molar masses compared to bulk/solution processes by kdeact
reducing the termination rate. Initially, it was thought that in kp
reversible termination systems (NMP, ATRP), compartmentali­
zation effects would not exist, although theoretical modeling +M
studies suggested that they may exist at sufficiently small par­ Figure 18 Atom transfer radical polymerization (ATRP) showing acti­
ticle size.205–207 Maehata et al.235 demonstrated experimentally vation of Br-terminated polymer chain by Cu(I) complex, and deactivation
in TEMPO-mediated styrene miniemulsion polymerizations that of propagating macroradical by Cu(II) complex.
484 Vinyl Polymerization in Heterogeneous Systems

species are usually more water soluble than the Cu(I) species high Brij 98 concentrations (
75 wt.% Brij 98 vs. the monomer),
that activate dormant chains. Reduction in the concentration of a refined process used only 12 wt.% versus the monomer.
deactivator results in loss of control and significant chain ter­
mination. Deleterious reactions of the catalyst are a potential
3.14.2.2.3(iv) ATRP in miniemulsion
concern in ATRP in aqueous dispersions, and can include
Conventional (forward) ATRP with bipyridine ligands, a system
hydrogen abstraction (e.g., from monomers or solvents), reac­
that works well in bulk and solution, is poorly suited for aqueous
tion with monomers containing acid groups, and reactions
systems, primarily because of the sensitivity of the Cu(I) species
with anionic surfactants. Unlike NMP, ATRP does not require
to air. Reverse ATRP uses Cu(II), which is far more tolerant of
high temperatures and is much more versatile than NMP in the
exposure to air that occurs when the miniemulsion is created by
range of monomers it can polymerize. Important process inno­
high shear (e.g., using microfluidization or sonication).238,239
vations have occurred in dispersed aqueous phase in recent
High activity, hydrophobic ligands such as CuBr2-tris[2-di
years. Of special interest is activator generated by electron
(2-ethylhexyl acrylate)aminoethyl]amine (EHA6TREN) or
transfer (AGET) ATRP that has proven to be a versatile and
bis 2-pyridylmethyl)octadecylamine (BPMODA) (Figure 19)
robust system that uses less air-sensitive Cu(II) complexes.237
show much better performance for reverse ATRP in miniemul­
A limited range of suitable surfactants exists for dispersed
sion. Catalyst–ligand complexes used in ATRP aqueous
phase ATRP. Anionic surfactants such as sulfates and sulfonates
dispersions must be fully soluble in monomer, unlike bulk or
poison the catalyst. Most studies have used nonionic surfac­
solution ATRP where heterogeneous catalysts can function effec­
tants (Brij 98, PEO20 oleyl ether)237–239 or Tween 80 (PEO
tively. Although reverse ATRP displays much better overall
sorbitan monooleate),240–244 although the cationic surfactant
performance than forward ATRP in miniemulsion, there are
cetyltrimethylammonium bromide (CTAB) has been shown to
issues with unpredictable induction periods and mediocre con­
give superior colloidal stability (especially at higher tempera­
trol of molar mass, both arising from variability in the initiation
tures) and yields smaller particles.245
efficiency. Many of the challenges with reverse ATRP were
resolved by development of the simultaneous normal and
3.14.2.2.3(i) ATRP in suspension
reverse initiation (SNRI) process.
There are limited reports of ATRP in suspension polymeriza­
SNRI, like reverse ATRP, uses the less oxygen-sensitive Cu(II)
tion,246–248 including their use for encapsulating polar organic
catalyst but addresses the problem of unpredictable initiation
solvents.249–251 As with NMP, suspension polymerizations with
efficiency that is experienced with reverse ATRP by employing
larger particle sizes tend to behave similar to bulk polymeriza­
alkyl halide as the primary initiator. A small amount of free
tions, with issues such as catalyst partitioning not as problematic
radical initiator is also added (a
5:1 ratio of alkyl halide to
as when particle diameters are in submicrometer range.
initiator is employed). The activating Cu(I) species is generated
in situ by reduction of the Cu(II) to Cu(I) as the free radical
3.14.2.2.3(ii) ATRP in emulsion initiator decomposes. In a series of papers,255–257 Matyjaszewski
As with NMP, early efforts to conduct ATRP in aqueous disper­ demonstrated the suitability of the SNRI process for polymeriza­
sions used emulsion polymerization, and employed alkyl halide tion of BMA, BA, and styrene.
initiators with Cu(I)/ligand catalyst complexes.238–240 Similar to
NMP, severe colloidal stability problems were often encountered
in emulsion ATRP. Significant difficulties were also observed C18H37

using reverse ATRP in emulsion,242 although seeded ATRP emul­

N
sion polymerization was more successful than ab initio
polymerizations.243,244 A seed latex of poly(i-butyl methacrylate) N N

was made by miniemulsion polymerization using CuBr/dNbpy

BPMODA
(4,4′-dinonyl-2,2′-dipyridine) with ethyl-2-bromoisobutyrate
OR O
initiator, and then swollen with styrene and polymerized.
Although the polymerizations were controlled, colloidal stability O
C C
OR
was problematic. Chan-Seng and Georges252 obtained better col­
loidal stability using a seeded nanoprecipitation technique first N
developed for emulsion NMP in the ATRP of styrene.
OR O

3.14.2.2.3(iii) ATRP in microemulsion C N C

While emulsion ATRP is not a robust process, a new approach O N N OR

using an initial microemulsion polymerization253,254 appeared

to be considerably more promising. Min and Matyjaszewski253
extended miniemulsion ATRP to microemulsion using AGET C C
RO O RO O
ATRP. Particle sizes of
40 nm were obtained for polystyrene
and poly(methyl methacrylate) latexes. A two-step emulsion
polymerization process was subsequently developed, thereby
R:
eliminating the need for a miniemulsion process.254 The first
stage of the two-step emulsion polymerization process was an
AGET microemulsion polymerization, yielding
30–40 nm
EHA6TREN

particles that were then swollen with monomer and polymerized. Figure 19 ATRP ligands BPMODA and EHA6TREN, commonly used in
Final particle sizes were
90 nm. While initial experiments used ATRP miniemulsions.
 Vinyl Polymerization in Heterogeneous Systems 485

CuIIBr2/L

Normalized SEC response

Reducing
agent

kact
Pn-Br + CuIBr/L Pn + CuIIBr2/L
kdeact
kp

+M
Figure 20 Activator generated by electron transfer (AGET) ATRP.
Activating species CuIBr/L is generated in situ by reduction of CuIIBr/L.
19 21 23 25 27 29
Retention time (min)
In the SNRI process, contamination by homopolymer arising
from the free radical initiator prevents synthesis of high-purity Figure 21 Size-exclusion chromatography traces for the miniemulsion
reverse ATRP of n-butyl methacrylate. T = 60 °C. Conversion increases
block or star polymer structures. AGET ATRP (Figure 20) utilizes
from right to left: conversion = 11%, Mn = 222 500 g mol−1, PDI = 1.56;
a reducing agent to convert Cu(II) to Cu(I), instead of relying on
conversion = 23%, Mn = 345 000 g mol−1, PDI = 1.47; conversion = 74%,
radicals from initiator decomposition.237,258 The reducing agent Mn = 859 000 g mol−1, PDI = 1.24; conversion = 83%,
also scavenges oxygen, increasing the tolerance of the system Mn = 989 900 g mol−1, PDI = 1.24. Reprinted from Simms, R. W.;
to air. Water-soluble ascorbic acid as a reducing agent is well Cunningham, M. F. Macromolecules 2007, 40, 860,245 with permission
suited to miniemulsions. Best results were obtained when the from the American Chemical Society.
ratio of ascorbic acid/Cu(II) varies from
0.1 to 0.5/1 (ascorbic
acid reduces two equivalents of Cu(II)). In the miniemulsion
polymerization of BA initiated by ethyl-2-bromoisobutyrate, Thomson and Cunningham263 simulated the highly active cata­
well-controlled polymerization was obtained with final PDI lyst–ligand system (CuBr/EHA6TREN) and BMA, with particular

1.2. Linear block and three-arm star copolymers (poly(methyl focus on the PDI and livingness of the growing chains. They
acrylate)-b-polystyrene) were also prepared. found there is a defined range of particle sizes where the rate of
Most publications dealing with ATRP, especially with aqueous polymerization can be enhanced above that of bulk polymeriza­
systems, have yielded Mn <
80 kg mol−1, with the typical Mn tion while maintaining excellent control, with an expected PDI
values being much lower. Simms and Cunningham259 recently and degree of termination below that of bulk polymerization.
showed that ATRP is suitable for preparing much higher molar Furthermore, while the polymerization rate is controlled by
masses in miniemulsion. Using a redox initiation system the equilibrium ratio of Cu(I)/Cu(II) for bulk ATRP, in compart­
(ascorbic acid/hydrogen peroxide) with a CuBr2/EHA6TREN mentalized system, the rate is controlled by enhanced
(Figure 19) catalyst, they were able to produce poly deactivation and also the relative concentration of Cu(I) and
(n-butyl methacrylate) with Mn
106 g mol−1 and PDI
1.25. Cu(II), which are dependent upon the size of the particles.
Conversions >80% were achieved in
8 h, with mean particle Experimental evidence showed compartmentalization in
diameters
100 nm. The evolution of the MMDs showed excel­ ATRP miniemulsions reduced polymerization rate (confined
lent livingness, even at Mn > 800 000 g mol−1 (Figure 21). space effect), and more importantly, improved control over the
polymerization264 when the number of chains was small (high
3.14.2.2.3(v) Compartmentalization in ATRP target Mn) for the system CuBr/EHA6TREN-n-butyl methacrylate.
As discussed with NMP, it has been generally believed that While in a conventional emulsion polymerization, segregation
compartmentalization effects do not exist in aqueous ATRP dis­ effects cause an increase in the rate, in ATRP, the confined space
persions and some studies have shown similar kinetics and effect dominates the kinetics and results in a decrease in rate.
MMDs for bulk and miniemulsion experiments.255,260 As also
reported for NMP, however, compartmentalization can influence 3.14.2.2.4 Reversible addition–fragmentation chain transfer
the rate of polymerization, the degree of control of the PDI, and The reversible deactivation process in a RAFT mechanism is
the livingness of the polymer formed in aqueous dispersed phase governed by a chain transfer reaction between an active macro­
ATRPs. Kagawa et al.261 have conducted simulations to explore molecule and a dormant one.174–176,265,266 The latter is most
questions about possible compartmentalization and partition­ generally end-functionalized by a thiocarbonylthio group (from
ing effects in dispersed ATRP for CuBr/dNbpy-mediated systems. a dithioester, a dithiocarbonate, a dithiocarbamate, or a trithio­
They predicted compartmentalization effects may be evident for carbonate (TTC), Figure 22), and the exchange reaction relies on
particle diameters < 70 nm, resulting in lower polymerization an addition–fragmentation process. The poly(methacrylic ester)s
rate but higher livingness. Zetterlund et al.262 simulated compart­ with a terminal double bond derived from CCT polymerization
mentalization for styrene polymerization using CuX/dNbpy can also be used, although their reactivity is lower than the
(X = Br or Cl) catalyst. They found compartmentalization always thiocarbonylthio counterparts.267 Technically, the polymeriza­
improved livingness because of reduced termination (segrega­ tion system requires the use of a classical radical initiator
tion effect). Control was improved as a result of the confined decomposing at low to moderate temperature (generally below
space effect but only for particles that were sufficiently small. The 100 °C) in conjunction with a RAFT agent, which is consumed
magnitude of compartmentalization effects increased with in the early stage of the polymerization to create the dormant
increasing target molar mass (i.e., fewer chains per particle). chains. The latter are also active as macromolecular RAFT agents
486 Vinyl Polymerization in Heterogeneous Systems

R S C S
Z

Z = activating group
R = leaving / initiating group Dithioesters
Z = -Ph
-CH3
R = -CH2-Ph
-CH2-Ph
-CH(CH3)-Ph
-C(CH3)2-Ph
-C(CH3)2-CN Dithiocarbonates (xanthates)
-C(CH3)2-COOEt Z = -O-Ph
-C(CH3)3 -O-Et

Dithiocarbamates
Z = -NEt2
pyrrole
pyrrolidone

Trithiocarbonates
Z = -S-R
Figure 22 Main families of RAFT agents.

throughout the polymerization. Consequently, a linear increase highly hydrophobic (for instance, a hydrophobic macromolecu­
of the Mn with monomer conversion is expected (the lar RAFT agent),270 and hence, unable to diffuse through the
number-average degree of polymerization (DPn) is calculated aqueous phase. For more hydrophilic control agents, exit of
by the ratio of the initial monomer concentration over the primary leaving radicals from the particles and termination in
chain transfer agent concentration, multiplied by monomer con­ the aqueous phase can be an issue, and may lead to rate retarda­
version), along with narrow MMD and the formation of block tion and poor control over molar masses.271 In addition,
copolymers by subsequent polymerization of a different mono­ colloidal stability problems were encountered in some particular
mer. Multistep reactions and/or the use of a multifunctional examples, mainly with ionic surfactants, and were assigned to a
chain transfer agent allow complex architectures to be elaborated. superswelling effect explained by the presence of a large concen­
The polymerization kinetics follow the classical steady-state tration of short chains within the monomer droplets.272 In spite
assumption, meaning that compartmentalization effect is of these difficulties, the miniemulsion process was very success­
expected to be the same as in classical emulsion polymerization. ful for RAFT in aqueous dispersed systems and allowed block
In some cases, especially with the dithiobenzoate-based RAFT copolymers to be synthesized with good control over molar
agents, a significant rate retardation effect is often observed.268 mass, MMD, and chain architecture. Special morphologies
The development of the RAFT method in aqueous dispersed such as capsules with a liquid core and a well-defined polystyr­
systems is quite recent88,177–179,181–185,187 and started essen­ ene shell have been prepared using this method.273
tially in miniemulsion polymerization processes. It is only very
recently that successful attempts in emulsion polymerization 3.14.2.2.4(ii) RAFT in emulsion polymerization
systems have been disclosed. In comparison to miniemulsion The development of RAFT in true emulsion polymerization
and emulsion polymerizations, the suspension process has processes was more challenging than in miniemulsion. A gen­
hardly been applied.269 eral difficulty of RAFT in aqueous dispersed systems, and
particularly emulsion polymerization, is related to the need
for a radical initiator in conjunction with the RAFT agent.
3.14.2.2.4(i) RAFT in miniemulsion polymerization
Consequently, it is not always easy to control the locus where
In a first approach, RAFT was mainly studied in miniemulsion,
reversible transfer will take place, and this may have important
because the technique allows the complex nucleation and
and sometimes deleterious consequences on the control over
mass transport processes of an emulsion polymerization to be
molar mass and MMD. Again, the most important parameters
avoided by preforming monomer droplets that act as nanor­
to consider are both the water solubility and the reactivity of
eactors throughout the polymerization. The reactivity of the
the chain transfer agent.
chain transfer agent (i.e., the value of the chain transfer con­
stant) is of primary importance, but its effect on molar mass 3.14.2.2.4(ii)(a) Low-molar-mass RAFT agents With mod­
and MMD should be the same in miniemulsion as in homo­ erately hydrophobic chain transfer agents (i.e., mainly soluble
geneous systems. The other parameter of highest influence is in the monomer phase but sufficiently water soluble to diffuse
the water solubility of the chain transfer agent and its partition from the monomer droplets to the particle, through the aqu­
coefficient between water and the monomer phase. The most eous phase) exhibiting low chain transfer constant like
hydrophobic transfer agents have been easily used in miniemul­ dithiocarbonate (also called xanthates), the nucleation step
sion polymerization, with good control over both the kinetics was not modified with respect to a classical radical emulsion
and the molar masses. Actually, the miniemulsion polymeriza­ polymerization due to the initial formation of long hydropho­
tion process is particularly convenient when the RAFT agent is bic chains. Therefore, the colloidal properties of the latexes
 Vinyl Polymerization in Heterogeneous Systems 487

O the loci of the next controlled polymerization step performed

under starved-feed conditions. Following the same strategy,
S S Božović-Vukić et al.281 synthesized poly(4-vinyl pyridine) chains
(a) HO C12H25
in a mixture of toluene and ethanol under RAFT control, and the
S TTCA obtained macromolecular RAFT agent was further used in emul­
O
sion polymerization under acidic conditions. In both cases, the
(b) S S characteristics of a controlled polymerization were observed
HO C4H9 together with good colloidal properties of the formed
surfactant-free latexes. The latter were composed of well-defined
S
amphiphilic block copolymers, with a stabilizing hairy layer
O
(core-shell particles) coming from the initially synthesized hydro­
O S S philic segments. The method has been well studied for various
(c) H3C O C12H25
monomers and various hydrophilic blocks and allowed more­
n
S PEO-TTC over fundamental studies to be performed on the nucleation step
and on the rate of entry of radicals in particles.282–284
Figure 23 Amphiphilic RAFT agents used in emulsion polymerization. To avoid droplet nucleation, another method aimed at lock­
ing the RAFT groups within preformed particles before the
were good in the presence of a classical surfactant, although addition of the hydrophobic monomer in one shot. For this,
dibenzyltrithiocarbonate was used as a hydrophobic RAFT
control over molar mass and MMD was far from excellent due
agent for the copolymerization of styrene and AA in bulk.285
to the poor reactivity of the chosen control agents. The poly­
merization was found to be retarded when the amount of At incomplete monomer conversion, alkaline water was added
xanthate was increased. This result was assigned to an enhanced slowly to the solution to induce a phase inversion process and
hence form living amphiphilic copolymer aggregates swollen
exit of the primary radicals formed upon transfer in the parti­
cles, and to an unexpectedly slow rate of entry of the with the remaining monomer. Upon a shot addition of styrene,
oligoradicals from the aqueous phase.274 With highly reactive the polymerization was resumed with chain extension of the
TTC-functionalized copolymers along with particle growth. The
RAFT agents, the emulsion polymerization was more difficult
to achieve owing to the fast formation of a large concentration method led to stable latex particles composed of well-defined
of short oligomers. The latter hampered the nucleation step copolymer chains, but again, it needed several steps to reach
the goal.
and led to colloidal instability in the presence of monomer
droplets and sometimes very poor control over the chain
3.14.2.2.4(ii)(c) Amphiphilic or water-soluble macro­
growth along with severe rate retardation. The conclusion was
molecular RAFT agent in ab initio, batch conditions In the
that droplet nucleation had to be avoided, which motivated
very first works, water-soluble macromolecular RAFT agents
further works using either starved-feed conditions or multistep
were tested in ab initio batch emulsion polymerization, with the
methods (see the following sections). A surface-active RAFT
idea that the hydrophilic block would chain extend through a
agent, the sodium salt of 2-(dodecylthiocarbonothioylthio)­
transfer reaction and form in situ an amphiphilic block copolymer
2-methylpropanoic acid) (TTCA; Figure 23(a)), was further
able to participate in particle stabilization. The technique would
tested275 as TTCA is a widely employed TTC, known for con­
be very helpful to replace the low-molar-mass surfactants, which
trolling efficiently the polymerization of a large variety of
exhibit high mobility and hence deleterious effect in coating
monosubstituted ethylenic monomers276 in homogeneous
applications. With dithiobenzoate as the RAFT group and poly
conditions. In the batch emulsion polymerization, however,
(methacrylic acid)286 or protonated poly(diethylaminoethyl
the success was limited to specific conditions. In the homopoly­
methacrylate)287 or protonated poly(dimethylaminoethyl metha­
merizations of styrene or BMA, the kinetics were either extremely
crylate) combined with a PEO segment,288 good colloidal
slow (styrene) or the molar masses were not controlled (BMA).
stability was achieved but not the control over molar mass and
In contrast, for the batch emulsion copolymerization of BMA
MMD of the polymer chains forming the particles. Similar results
with either BA or styrene, a good control was achieved over
were obtained with dithiocarbonate-functionalized dextran
both the macromolecular structure of the chains and the colloi­
used in the emulsion polymerization of VAc,289 or with
dal stability of the particles, in the presence or in the absence
TTC-functionalized polyacrylamide in the batch emulsion
of additional surfactant.
polymerization of styrene.290 In most cases, the emulsion poly­
merization kinetics exhibited an induction period, during which
3.14.2.2.4(ii)(b) Amphiphilic macromolecular RAFT agents formation of the amphiphilic block copolymer was assumed to
in starved-feed conditions or in a two-step method In order take place. The latter self-assembled into micelles, in which the
to avoid droplet nucleation, a first way was to avoid the presence emulsion polymerization could progress. In such a situation,
of droplets by working under starved-feed conditions. An amphi­ control over all the characteristics of the polymer formed in this
pathic asymmetrical trithiocarbonate RAFT agent was used latter stage is not necessarily a major target, but the method offers
(Figure 23(b)) for the aqueous polymerization of AA in alkaline a new tool toward surfactant-free emulsion polymerization,
conditions to form in situ a water-soluble oligomeric RAFT which is of high industrial relevance.
agent.277–280 Then, a hydrophobic monomer was slowly added, In contrast, when PEO-based amphiphilic TTC RAFT agents
and the system led to the formation of amphiphilic block copo­ (PEO-TTC; Figure 23(c)) were used in ab initio, surfactant-free,
lymer micelles upon chain extension of the hydrophilic segments batch emulsion polymerization, the reactions were fast and led
in the absence of monomer droplets. These micelles became then to good colloidal stability together with excellent control over
488 Vinyl Polymerization in Heterogeneous Systems

molar mass and MMD. These results were fully achieved for the macromolecular chain transfer agent. In these systems, due to
emulsion polymerization of BA291 and its copolymerization their high water insolubility, the chain transfer agents were used
with MMA over a broad composition range.292 For styrene, as hydrophobes for the miniemulsion formulation. In contrast,
the polymerization was rather slow and the control was less ITP failed in controlling the polymer molar mass in emulsion
efficient. PEO-TTC was shown to react very fast at the early stage polymerization due to the incapacity of the chain transfer agents
of the polymerization, leading to well-defined amphiphilic of diffusing from the monomer droplets toward the particles.
block copolymer chains and hence self-stabilized, block copo­ RITP304 was further developed with the idea of using a classical
lymer particles. In consequence, no PEO chains remained free radical initiator in conjunction with molecular iodine to generate
in the final latex, all of them being covalently bound to the iodinated chain transfer agents in situ.305 Besides its application
polymer forming the particles. So far, this macromolecular to the miniemulsion polymerization of styrene,306 it was very
RAFT agent structure can be considered as the most effective for convenient for the use in ab initio emulsion polymerization due
surfactant-free, ab initio, batch emulsion polymerization. Quite to the water solubility of iodine and of sodium iodide later used
recently, a macromolecular RAFT agent composed of AA and as a precursor.298,307 Using RITP in aqueous emulsion, advantage
PEO acrylate units based on a TTC functional group was shown was further taken of the higher reactivity and solubility in water
to lead to nonspherical morphologies in the batch emulsion of AA compared to BA to synthesize, in one step, amphiphilic
polymerization of styrene. In particular, very long nanofibers poly(acrylic acid-co-n-butyl acrylate) gradient copolymers able to
were formed and were composed of poly(acrylic acid-co- self-assemble into particles.308
PEO acrylate)-b-polystyrene amphiphilic block copolymers
self-assembled by polymerization-induced micellization.293 3.14.2.2.5(ii) TeRP in aqueous dispersed systems
The TeRP309 proceeds by the two activation–deactivation pro­
3.14.2.2.4(iii) RAFT in aqueous dispersion polymerization cesses, namely, thermal dissociation of the C-TeCH3 terminal
The use of hydrophilic macromolecular RAFT agents as both bond and degenerative transfer of the terminal –TeCH3 group.
stabilizer and control agent in emulsion polymerization was However, when an external source of free radicals is used at low
easily transposed to aqueous dispersion polymerization in temperature, it only proceeds by degenerative transfer. Okubo
which the monomer is completely soluble in the water phase et al.310 used a water-soluble poly(methacrylic acid) with a –Te­
while the corresponding polymer is not. An et al.294 polymer­ CH3 terminal group to synthesize poly(n-butyl acrylate) latex
ized N-isopropylacrylamide using a water-soluble poly(N, particles by chain extension of the hydrophilic segment in
N-dimethylacrylamide) RAFT agent. The polymerization emulsion polymerization. The system resulted in very small
temperature was above the LCST of the newly formed polymer particles with controlled polymer chains exhibiting an amphi­
block. Therefore, chain extension led to its precipitation and philic structure.
self-assembling, resulting in the creation of self-stabilized
particles. Hydrogel nanoparticles able to swell with a decrease 3.14.2.2.5(iii) CoMRP in aqueous dispersed systems
of the temperature were similarly obtained with the additional Similar to TeRP, CoMRP follows the dual mechanism of rever­
use of a crosslinker during the dispersion polymerization. sible termination and degenerative chain transfer.311 It was
Similar temperature-sensitive nanogels of poly(N, applied in suspension312,313 and in miniemulsion for the poly­
N-diethylacrylamide) were prepared using a series of double merization of VAc and allowed well-defined polymers to be
hydrophilic macromolecular RAFT agents with a PEO first prepared at low temperature (0–30 °C) with quite a fast rate.
block and a poly(N,N-dimethylacrylamide) second block The miniemulsion process yielded latexes with small particles
formed by chain extension of PEO-TTC.295 (diameter of approximately 100 nm) and good stability.314
CoMRP is one of the best methods (beside RAFT using a
3.14.2.2.5 Other CRP methods xanthate as a chain transfer agent) to produce poly(vinyl acetate)
Besides the most common CRP methods used to control the with controlled molar mass, and its successful implementation
radical polymerization of vinylic monomers, other techniques to an aqueous dispersed system is an important step.
have been developed over the time, and have been tested in
miniemulsion or emulsion polymerization as well. These
3.14.2.3 Other Vinyl Polymerization Methods
methods are described in the corresponding chapters of this
comprehensive. They are iodine transfer polymerization (ITP; 3.14.2.3.1 ROMP in aqueous dispersed systems
and the reverse method, RITP), organotellurium-mediated CRP ROMP in homogenous processes (e.g., bulk or solution) is used
(TeRP), and cobalt-mediated radical polymerization (CoMRP). to manufacture a range of industrially important polymers,
including polynorbornene and its derivatives, polycyclooctenes
3.14.2.2.5(i) ITP and RITP in aqueous dispersed systems and polydicyclopentadiene. In addition to its established com­
The method is based on the reversible exchange of a terminal mercial importance, ROMP (and especially functionalized
iodine atom between a propagating radical and a dormant norbornenes) has recently become the subject of intense
chain.296–298 ITP using C6F13I as a chain transfer agent was research interest for applications such as preparing chiral poly­
shown to be very effective in the miniemulsion polymerization mers, brush-like peptide-bearing polymers for tissue engineering
of styrene299 and in the formation of polystyrene-b-poly(n-butyl and drug delivery,315 membrane transporters in multicompo­
acrylate) diblock copolymers.300 More recently, the method nent sensors,316 antibacterial and hemolytic polymers,317
allowed original triblock copolymer architectures to be achieved antimicrobial polymers,318 and microcellular foams.319
such as poly(vinyl acetate)-b-polydimethylsiloxane-b-poly Reviews on ROMP are available in the literature.320–324
(vinyl acetate)301,302 and polystyrene-b-polydimethylsiloxane­ ROMP is a living polymerization, and as such it enables
b-polystyrene303 from an α,ω-polydimethylsiloxane excellent control of the polymer microstructure, and is well
 Vinyl Polymerization in Heterogeneous Systems 489

Cy CI CI Cy
Cl N P Ru P N Cl
Cy Cy
Ph
n

Figure 24 Principle of ring-opening metathesis polymerization of norbornene.

suited for designing advanced polymers (for applications such 3.14.2.3.1(ii) Emulsion and miniemulsion polymerization
as those referenced above) with tailored polymer chain struc­ Emulsion-type polymerizations for ROMP of norbornene and
ture (e.g., block, graft copolymers) or functionalized with its derivatives were first reported using hydrates of Ru, Ir, and
desired reactive groups for further chemical reaction. The Os four decades ago.335–337 However, polymerization rates
basic ROMP mechanism is shown in Figure 24. were very low. ROMP using water-soluble ruthenium carbene
Despite the commercial importance and research interest complexes as catalysts was used to polymerize functionalized
in ROMP, little has been published on developing an emulsion 7-oxanorbornenes not only in water and methanol, but also in
ROMP process. There is considerable incentive for conducting aqueous emulsions.338,339 Cationic water-soluble aliphatic
ROMP in aqueous dispersed systems, including many of the phosphines were used in the synthesis of the ruthenium
same reaction engineering benefits that have made emulsion carbene complexes. The polymer polydispersity was low
and suspension polymerization commercially important pro­ (1.1–1.3), although few details of the dispersed phase poly­
cesses for free radical polymerization. Aqueous dispersions merization were reported.
would eliminate (or greatly reduce) the use of organic solvents, Claverie et al.340 reported the ROMP in emulsion of nor­
and provide reaction engineering advantages such as facile bornene as well as that of the less strained monomers
mixing, heat transfer, residual monomer removal, and product 1,5-cyclooctadiene and cyclooctene. First-generation (benzyli­
transport. The limited papers that have been published have dene-bis(tricyclohexylphosphine)dichlororuthenium) and
established the feasibility, in principle, of doing ROMP in second-generation (benzylidene[1,3-bis(2,4,6-trimethylphe­
aqueous dispersions, but most have been of limited scope nyl)-2-imidazolidinylidene]dichloro(tricyclohexylphosphine)
and/or were plagued by difficulties such as colloidally unstable ruthenium) Grubbs catalysts used with norbornene yielded
dispersions, poor control over particle size and/or molar mass,
stable latexes with minimal coagulum using emulsion poly­
and low reaction rates.
merization with an anionic surfactant. Fast reaction rates
ROMP has been reported in emulsion, miniemulsion,
(>80% conversion in less than 30 min) were typical, and
dispersion, and suspension. Caution should be used in inter­
molar masses were high, ranging from
2  105 to
preting reported process types in the literature; however, these
2  106 g mol−1. Mean particle diameters ranged from about
various processes are not as well defined for nonradical
50 to 150 nm. These polymerizations resemble free radical
polymerizations as they are for radical polymerizations. In
emulsion polymerizations, with aqueous phase initiation and
particular, the locus of chain initiation is sometimes ambigu­
the presence of monomer droplets that act as monomer reser­
ous and the mechanisms of particle nucleation may not be well
voirs during polymerization. With 1,5-cyclooctadiene
understood.
and cyclooctene, the Grubbs type 1 and 2 catalysts gave
very low yields.340 The more active and faster initiating
3.14.2.3.1(i) Dispersion polymerization
third-generation Grubbs catalysts (dichloro-di(3­
Early ROMP catalysts were intolerant to polar functionality in
monomers, resulting in severe catalyst poisoning. Pioneering bromopyridino)-N,N′-dimesitylenoimidazolino-RuCHPh) are
efforts by Novak and co-workers325,326 and Feast and needed for these monomers; however, they are quite hydro­
Harrison327 demonstrated that ruthenium-based catalysts phobic and cannot, therefore, be easily used in emulsion
could not only polymerize functional monomers but could polymerization, which requires facile transport of the catalyst
do so in an aqueous environment. Novak and Grubbs325 through the aqueous phase from monomer droplets to parti­
were able to polymerize functionalized 7-oxanorbornenes cles. The use of miniemulsion polymerization provided a
under air in a completely aqueous environment using the solution to this problem, since the hydrophobic catalyst
complex RuII(H2O)6(p-toluenesulfonate)2. Ruthenium, iri­ could be readily dissolved in the monomer phase (which con­
dium, and osmium chlorides were used to polymerize tained a small amount of toluene) and stable latexes without
7-oxanorbornenes in water.327 These developments in coagulum formation were reported.
water-tolerant catalysts led to work in dispersion polymeriza­ Gnanou’s group has extensively explored the use of ROMP
tion first and then other forms of dispersed phase in emulsion and miniemulsion in addition to dispersion and
polymerizations (suspension, emulsion, miniemulsion). suspension.332,341 The use of ROMP in tandem with ATRP in
Booth and co-workers328–330 generated a stable latex miniemulsion has proven to be an effective route for making
(< 100 nm) in the dispersion polymerization of exo,exo-2,3­ biphasic or ‘Janus particles’, consisting of polynorbornene and
bis(methoxymethyl)-7-oxanorbornene. Gnanou and co­ poly(methyl methacrylate) domains.342,343 They were able to
workers331–334 further developed the ROMP dispersion use the same ruthenium-based catalyst for both, achieving
polymerization process, and in particular showed the impor­ simultaneous ROMP of norbornene and ATRP of MMA, using
tance of stabilizer design. miniemulsion polymerization. Stable latexes with minimal
490 Vinyl Polymerization in Heterogeneous Systems

coagulum were formed, with mean particle diameters of polyethylene, linear low-density polyethylene, isotactic propy­

200 nm. The polymer polydispersities were typically
1.8, lene, high impact polypropylene, and ethylene-propylene
not as narrow as can be achieved in ideal ROMP or ATRP but rubbers). Polyolefins compete with other polymer families in
still reasonable. most of the markets but their presence in markets where a thin
film that adheres on a substrate is required is very limited.
3.14.2.3.1(iii) Suspension polymerization These markets are dominated by waterborne polymer disper­
Suspension polymerization was applied to prepare polynor­ sions synthesized by emulsion polymerization of styrene,
bornene crosslinked beads suitable for use as supports in butadiene, (meth)acrylates, and vinyl ether monomers that
organic synthesis.344 The monomers used included norbor­ are more expensive than olefins, which are directly obtained
nene, norborn-2-ene-5-methanol, and crosslinking agents in the steam cracking process. Therefore, there is a strong inter­
including bis(norborn-2-ene-5-methoxy)alkanes, di(norborn­ est in including olefins as part of the waterborne dispersion
2-ene-5-methyl)ether, and 1,3-di(norborn-2-ene-5-methoxy) formulations. However, this is not an easy task because water­
benzene. The initial resins, which were unsaturated, were sub­ borne dispersions are mostly polymerized by free radical
sequently modified using hydrogenation, hydrofluorination, polymerization, which has very limited capability to polymer­
chlorination, or bromination to yield saturated resins with ize olefins and controlling the microstructure of the polymer.
varying properties. They were reported to be superior to more Ethylene is polymerized by free radical polymerization but
traditional styrene–divinylbenzene resins due to reduced inter­ α-olefins are not. Polymerization of ethylene in aqueous
ference in electrophilic aromatic substitution reactions (e.g., phase is possible, but the homopolymer has little application
Friedel–Crafts acylation and nitration). because it is not film forming. The copolymerization with
Quémener et al.332 reported the suspension polymerization acrylates is challenging because of the very different reactivity
of both 1,5-cyclooctadiene to yield polybutadiene beads using ratios (racrylate = 13.94; rethylene = 0.01) and the low solubility of
the organic soluble catalyst (PCy3)2Cl2RuCHPh. High mono­ ethylene in water that favors the incorporation of the acrylate.
mer conversions were obtained but colloidal stability was poor Furthermore, high temperatures and pressure are necessary,
using the electrosteric stabilizer poly(diallyl methylammonium which involve high capital investment.
chloride), prompting a change to various copolymer stabilizers, Ziegler-Natta, Phillips, and metallocene catalysts are exten­
of which a graft copolymer (polybutadiene-g-PEO) and a sively used to produce polyolefins by catalytic polymerization.
macromonomer stabilizer (polystyrene-b-PEO) yielded much These catalysts allow a good control of polymer microstructure
improved stability. The mean particle diameters were
20 µm, and large productivities, but they are based on early transition
quite small for suspension polymerization. metals (Ti, Zr, Cr, and V), which are oxophilic, and hence
sensitive to water. Therefore, they cannot be used in aqueous
3.14.2.3.2 Ionic polymerizations systems although some relative success has been recently
Cationic polymerization of vinyl monomers is extremely sensi­ reported in the polymerization of styrene with metallocene
tive to water traces as the polymerization proceeds by highly catalysts.366 Late transition metals (Ru, Co, Rh, Ni, and Pd)
electrophilic propagating carbocations. The polymerization in are much less oxophilic, and hence they may be used in water
aqueous dispersed systems is, therefore, an extremely difficult systems.367 In the past 30 years, a great deal of work has been
task, not to say an impossible one.345 Nevertheless, attempts done to develop late transition metal catalyst to polymerize
have been made using water-tolerant Lewis acids such as ethylene and copolymerize it with acrylates in both solvent and
B(C6F5)3 and ytterbium triflate for the suspension polymeriza­ aqueous phases.368–370 The neutral nickel complexes of [P,O]
tion of p-methoxystyrene,346–352 its miniemulsion chelating agents developed by Keim and co-workers371,372 were
polymerization,353–355 its dispersion polymerization,356 or the first ones but only yielded low-molar-mass oligomers.
even its emulsion polymerization357 with presumably an inter­ Better yields of the catalyst and higher molar masses were
facial mechanism. The controlled character of the obtained by similar Ni(II)-P,O-based catalysts with a modified
polymerization was studied. To reach high-molar-mass poly­ ligand.373–375 However, they were still not able to copolymer­
mers (i.e., several thousands of g mol−1) instead of oligomers, ize ethylene with other polar monomers. The cationic versions
the polymerization had to be transported inside the monomer of Ni and Pd metal complexes of neutral multidentate ligands
droplets, using oil-soluble superacid initiators, namely, ‘Lewis with nitrogen donor bulky groups were more effective and
acid–surfactant combined catalysts’ (LASC).357 particularly the Pd(II) catalyst allow the copolymerization
The anionic polymerization in aqueous dispersed systems with acrylates at weight ratios as high as 25%.376 However,
concerns mainly the alkyl cyanoacrylate monomers, which can the activity of the catalyst strongly decreased by increasing the
polymerize spontaneously at a very fast rate in the presence of concentration of the acrylate monomer.368 Other groups377–379
water. (Nano)particles358,359 and nanocapsules360–362 were did also report catalysts (based on neutral Ni and Pd complexes
synthesized by emulsion, miniemulsion, or inverse miniemul­ with [P,O] ligands) that were able to copolymerize ethylene
sion polymerization processes.363 They mainly find applications and acrylates in ethanol and toluene, but molar masses were
in the biomedical domains364,365 and received for that reason a relatively small (<10 000 g mol−1) and the incorporation of the
huge interest, which makes it impossible to be exhaustive in this acrylate reduced the productivity of the catalyst, which was also
chapter. modest in most of the cases.
More recently, monometallic palladium catalysts380–382
3.14.2.3.3 Catalytic polymerization containing a sulfonated phosphine ligand have been developed
The world production of polymers is about 260 million tons that are able to homopolymerize ethylene and copolymerize
per year and half of the production is made of polyolefins ethylene with acrylates and other polar monomers in both
(including low-density polyethylene, high-density solution and aqueous phase.383,384 In solution,385 the activity
 Vinyl Polymerization in Heterogeneous Systems 491

of the catalyst for homopolymerization of ethylene was very applications as flocculants, thickeners, and drag-reducing agents
high and high-molar-mass polyethylene was obtained. The in a variety of industries, including water and sewage treatment,
copolymerization with acrylates was possible, but catalyst activ­ pulp and paper mills, coatings, paints, and oil recovery. Usually,
ity strongly decreased with the degree of incorporation. In high-molar-mass polymers are required. Inverse emulsion poly­
aqueous phase,386 both homopolymerization of ethylene and merizations provide the same reaction engineering advantages
copolymerization of ethylene with acrylates was effective, but as most dispersed phase polymerizations, including improved
for the emulsion polymerization of ethylene, the activity was 20 heat transfer, mixing, and low viscosity. The hydrophilic dis­
times smaller and the activity of the catalyst in the emulsion persed phase often contains water, in some cases, because the
copolymerization of ethylene and MA decreased by a factor of monomers are solids that cannot be readily dispersed in the
2 for an incorporation of 2.7 mol.% of acrylate monomer. In continuous organic phase (e.g., acrylamide, sodium styrene sul­
both cases, the solids content of the latexes was modest (< 8%). fonate). Addition of water to the final latex induces phase
Miniemulsion polymerization has also been used to produce inversion, and the polymer can usually be dissolved without
polyethylene dispersions using two families of neutral Ni(II) too much difficulty, even at high molar masses. Initiators can
catalyst originally developed for nonaqueous systems: nickel(II) be soluble in either phase. Inverse emulsion and miniemulsion
phosphinoenolato complexes and nickel(II) salicylaldiminato polymerizations are kinetically stable, while inverse microemul­
complexes.387,388 The water-insoluble catalysts were dissolved sions are transparent and thermodynamically stable. Inverse
in an organic solvent and then miniemulsified. The catalysts microemulsions have smaller particle size (< 40–50 nm) than
were highly active yielding polyethylene latexes with 30% solids
inverse emulsions or miniemulsions, and contain much higher
and high molar masses. Both molar mass and polymerization
surfactant levels. The particles in inverse microemulsion latexes
rate decreased when the catalysts were used in copolymerization
can be so small that each particle contains only
1–10 polymer
with α-olefins to produce a branched polymer. Latexes with very
chains. Micelles may persist throughout the polymerization
small particles (10 nm) have been prepared using this family of
that gives rise to an increasing particle number. Inverse mini-
catalysts in microemulsion polymerization.389 Water-soluble
emulsions are less common, but have recently become
versions of nickel(II) phosphinoenolates390 and salicylaldimi­
important in the CRP of hydrophilic monomers.
nato391 complexes did also yield polyethylene latexes with
Typical oil phases include aromatic and aliphatic compounds
small particles (20 and 4 nm, respectively).
such as cyclohexane, heptane, toluene, xylene, isooctane, and
Miniemulsion copolymerizations of α-olefins and ethylene
paraffin oil. The stabilizer (or emulsifier) plays a critical role in
have been carried out using the α-olefin to prepare the mini-
the polymerization and must be chosen carefully, with regard
emulsion (i.e., without using a solvent). Particles with diameters
of 50–100 nm and low-molar-mass amorphous polymers were given to monomer(s) selection, polymerization temperature,
obtained.392 mixing conditions, and the fraction of monomer in the aqueous
Sauca393 recently reported on the use of monometallic pal­ phase. In these low dielectric constant hydrocarbon continuous
ladium catalyst containing a sulfonated phosphate ligand in the phases, electrostatic stabilization is not feasible and steric
homopolymerization of ethylene and copolymerization of ethy­ stabilization is thus required for inverse emulsions. Unlike
lene with acrylates and acrylate macromonomers in solution oil-in-water emulsions, inverse emulsions require oil-soluble
(toluene) and miniemulsion polymerization. Sauca found that stabilizers. These are usually fatty acid esters of sorbitan (e.g.,
in the copolymerization of ethylene with acrylates performed in sorbitan monooleate), PEO derivatives, or triblock polymer
aqueous systems, the type of surfactant (anionic vs. nonionic) based on a PEO midblock with 12-hydroxystearic acid-based
had a strong influence over the polymerization rate, latex stabi­ esters for the outside blocks.394
lity, and acrylate incorporation. This was attributed to the Stabilizers are used above the CMC so that micellar nuclea­
capability of the surfactant to disperse the entering acrylate. tion is the predominant mechanism of new particle formation.
When poly(acrylate) macromonomers were used in the copoly­ Free radicals initiate chain growth in the organic phase, where
merization of ethylene, it was found that macromonomers the chains propagate until they reach a critical length and enter
synthesized by high-temperature polymerization of acrylate either a monomer-swollen micelle or an existing particle.
were not incorporated into the polymer backbone whereas Monomer droplets provide reservoirs that supply the growing
macromonomers produced by CRP, in particular those synthe­ particles. Nucleation continues until all micelles are consumed,
sized by ATRP, showed the highest degree of incorporation. after which the particles grow until all monomer has been
consumed. If water-soluble initiators are used, polymerization
in droplets may be significant, depending on droplet size.
3.14.3 Vinyl Polymerization in Nonaqueous Inverse microemulsion polymerization, which has very high
Dispersed Systems particle numbers compared to conventional emulsion poly­
merization, tends to give very high molar masses as chains are
3.14.3.1 Conventional Radical Polymerization not terminated by entry of radicals from the organic phase. The
3.14.3.1.1 Inverse emulsion, miniemulsion, high stabilizer concentration gives rise to a high number of
and microemulsion polymerizations micelles, and, therefore, most newly initiated chains in the
Inverse emulsions are water-in-oil emulsions that provide a continuous phase enter micelles rather than particles. In this
convenient and commonly used approach for preparing colloi­ case, molar mass is determined by chain transfer to monomer
dal dispersions of water soluble or hydrophilic polymers, or possibly to the organic phase solvent. While there are simi­
including crosslinked polymers.394,395 Water-soluble polymers larities between the mechanisms of conventional and inverse
represent an important class of commercial materials, finding emulsion polymerization, the kinetics of inverse emulsion
492 Vinyl Polymerization in Heterogeneous Systems

polymerization are generally more complex and not as well subsequent work involved the use of more polar solvents,
understood. particularly alcohol–water mixtures. Also popular are mixed
solvent–nonsolvents including alcohol–toluene mixtures,
3.14.3.1.2 Dispersion polymerization alcohol–dimethyl sulfoxide mixture, dimethylformamide–
3.14.3.1.2(i) Dispersion polymerization in organic solvents methanol, and dimethylformamide–toluene. Typical poly­
The preparation of polymeric particles in the 1–10 µm range meric stabilizers include poly(vinyl pyrrolidone) (PVP),
remains a challenge. Modified emulsion polymerization pro­ poly(vinyl alcohol), poly(hydroxy alkyl celluloses), and PAA.
cesses involving the use of multiple, sequential stages can be
used to make particles up to
1–2 µm, while microsuspension
polymerization becomes increasingly difficult for particles 3.14.3.1.2(ii) Dispersion polymerization in ionic liquids
under 10 µm. Dispersion polymerization, however, provides a Ionic liquids have increasingly attracted attention in recent years
one-step process for preparing 1–10 µm particles, with mono­ as alternative solvents for a variety of applications, including
disperse PSD if desired. Monodisperse particles in this size catalysis, chemical synthesis, separation processes, dissolution
range have applications in chromatography, instrument cali­ of cellulosics, and synthetic polymers, and as a medium for
bration standards, xerographic toners, and health sciences. polymerization. The synthesis of polymer particles in ionic
A brief description of dispersion is given below. More detailed liquids has been reported using techniques such as condensation
reviews are available.152–156 polymerization, chemical oxidative polymerization, and cationic
In a dispersion polymerization, the reaction mixture (mono­ ring-opening polymerization. Only a few publications have
mer, initiator, optionally chain transfer or crosslinking agent, appeared, however, utilizing free radical polymerization to
stabilizer (surfactant), and solvent phase) is initially homoge­ synthesize polymer particles.396–398 Polymerization in ionic
neous. The solvent phase is chosen so that monomer(s) is fully liquids has been recently reviewed.399,400 Attractive properties
soluble but the polymer is insoluble. In the early stages of a of ionic liquids include low flammability, low volatility, high
dispersion polymerization, free radical initiator decomposes to boiling points, and good thermal stability in addition to their
initiate chains. The chains are initially miscible but when they conductivity. Furthermore, their solubilization properties can be
reach a given length (which depends on the solvent/monomer readily tailored by varying the cation and/or the anion. A large
composition), they precipitate. Individual chains are too small number of ionic liquids have been reported for solubilizing
to remain stable, and therefore aggregation of the individual polymers.401
chains into discrete particles occurs until the aggregates reach a Polystyrene particles, polystyrene/poly(methyl methacry­
particle size that can be stabilized by the available surfactant. late) composite particles, and polystyrene/PAA core-shell
Stabilization is steric in nature. (Precipitation polymerization is particles were synthesized using dispersion polymerization in
a similar, related process except no surfactant is added.) The new the ionic liquids diethyl(2-methoxyethyl)methylammonium bis
polymer particles will swell with monomer, although a portion (trifluoromethanesulfonyl)imide ([DEME][TFSI]) or 1-butyl­
of the monomer remains in the continuous phase. The relative 3-methylimidazolium tetrafluoroborate ([Bmim][BF4]).396,397
partitioning of the monomer between the particle phase and the Styrene, initiator (AIBN), and surfactant (PVP) are all soluble
continuous phase depends on the thermodynamic properties of in the ionic liquid so that the initial reaction medium is homo­
monomer, polymer, and solvent. The other components in the geneous. The polymer however is insoluble, so that particles are
formulation (initiator, chain transfer agents, crosslinking agents) formed as the polymer precipitates in the early stages of poly­
will also partition between the phases. The ‘critical point’ is merization. As with dispersion polymerization in conventional
reached when all of the particles have been sufficiently stabilized solvents, surfactant choice is critical. The stabilizer must be
by surfactant, after which no new particles are nucleated. soluble in the reaction medium but possesses sufficient affinity
After the critical point is reached, particles continue to grow for the particle surface that it adsorbs onto the surface once
by different mechanisms. (1) Particles can capture growing polymer chains begin precipitating. Inadequate affinity for the
oligomeric radicals in the continuous phase. (2) Individual polymer particles will result in the stabilizer residing primarily in
chains that precipitate can aggregate with existing particles. the continuous phase and not on the particle surface, during
(3) Monomer within the particles polymerizes. Chain initia­ polymerization. Dispersions of
10% solids with narrowly dis­
tion in the continuous phase continues throughout the tributed particles were made at 70 °C. Mean particle diameters
polymerization, but chain initiation also occurs within the were
300–400 nm. Thermal polymerization of the styrene in
particles resulting in two loci of polymerization. If the particle ([DEME][TFSI]) was also performed at 130 °C in the absence of
nucleation stage occurs fairly quickly, all particles are nucleated free radical initiator. Stable dispersions were formed without
at about the same time and narrow or monodisperse PSDs requiring an autoclave reactor.
are more likely. However, solvent and surfactant choice are PAA particles were made using the ionic liquid N,
also critical concerns if monodisperse particles are desired. N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(tri­
Variations in solvent composition, solvent/monomer ratio fluoromethanesulfonyl)amide ([DEME] [TFSA]).398 PAA,
(which affects the overall solvency of the system), monomer being a hydrophilic polymer, required both a different ionic
polarity, and surfactant type and concentration are all impor­ liquid and a different surfactant than the hydrophobic poly­
tant variables in determining the final PSD. An added styrene. PVP, which was an effective stabilizer for polystyrene
complexity of dispersion polymerization is that the solvency particles, proved to be ineffective with PAA. However, poly
of the continuous phase changes continually during polymer­ (vinyl alcohol) was able to effectively stabilize PAA particles.
ization as monomer is consumed. Although crosslinking agents were not used in the formulation,
Although the original early work in dispersion polymeriza­ crosslinked polymer was obtained, likely as the result of acid
tion was conducted in hydrocarbons such as cyclohexane, most anhydride formation although this does not occur in solvents
 Vinyl Polymerization in Heterogeneous Systems 493

such as hexanes. The ([DEME][TFSA]) could also be reused, an 3.14.3.2 Controlled Radical Polymerization
important consideration given the high cost of ionic liquids.
The development of CRP in nonaqueous dispersed systems was
envisioned with the aim of controlling simultaneously the poly­
mer chain characteristics along with the colloidal properties of
3.14.3.1.2(iii) Dispersion polymerization in scCO2
the so-formed polymer particles. However, in comparison with
CO2 transforms into a supercritical state at relatively mild con­
CRP in aqueous dispersed systems, the nonaqueous systems
ditions (Tc = 31.1 °C; Pc = 7.39 MPa), making it suitable for a
were much less studied, although in the past years the number
variety of chemical process applications, including selective
of articles is in constant progression. Two main types of systems
separations, chemical syntheses, polymer processing, and poly­
were considered: the classical organic solvents and scCO2.
merizations. It has received extensive attention as a potential
alternative solvent for traditional chemical processes that use
large volumes of organic compounds. Increasingly stringent 3.14.3.2.1 Dispersion polymerization in organic solvent
environmental regulations on emissions of VOC are driving NMP represents the first CRP technique to be tested in nonaqu­
research into alternative ‘greener’ solvents. scCO2 has attracted eous dispersion polymerization.408–411 The monomer was
widespread interest for polymerization. The ability to tune the styrene in all published examples. Although it was possible to
solubility properties of scCO2 by varying temperature and control the polymer chain characteristics in terms of molar mass
pressure provides considerable flexibility in designing polymer­ and MMD, the difficulty was to achieve the formation of stable
ization processes. scCO2 is inexpensive, inert, nontoxic, and particles at the high temperature needed for NMP (typically
nonflammable with low viscosity and high diffusion rates above 110 °C). This was related to both the nucleation step
that facilitate heat and mass transfer. In some cases, homoge­ and the stability over the course of the polymerization. In gen­
neous polymerizations can be conducted in scCO2; however, eral, when stable particles were achieved, the control over the
for many free radical polymerizations, the vinyl monomers are particle size was poor and the PSD was very broad. The solvent
soluble in the scCO2 while their polymers have only limited type was directly responsible for the quality of the nucleation
solubility (with the exception of silicon and fluoropolymers step: at high temperature and for the initially formed short
that have excellent solubility in scCO2). Dispersion or precipita­ chains, the solvent may not be appropriate to induce their fast
tion polymerizations are, therefore, commonly used processes. and efficient precipitation. The first stage of the reaction may
Although the solubility of many polymers in scCO2 is low, the then mainly take place in solution rather than in dispersion. It
was moreover supposed that part of the soluble polymer pre­
solubility of scCO2 in polymers is often appreciable, resulting in
cipitated when the temperature of the reaction medium was
significant swelling (
5–15%) of the polymer by CO2 and
decreased rather than during the polymerization itself. The
accompanied by the effects of plasticization including lower Tg.
enhancement of the polymerization rate was further shown to
DeSimone et al.402 reported the first dispersion polymeriza­
have a positive effect on the PSD.411 Although controlled in term
tion in scCO2 by free radical polymerization. Using MMA
of average molar masses, the polymers exhibited higher PDIs
initiated by AIBN at 65 °C and 204 bar, dramatic differences
than in bulk, which may possibly be the result of an unfavorable
were observed in final conversion and molar mass depending
partitioning of the nitroxide in the different phases of the
on whether stabilizer was used. In the absence of stabilizer
system.
(precipitation polymerization), conversions and rates were
The ATRP of 4-vinylpyridine initiated by a PEO-based
low, ranging from
10% to 40%. However, addition of the
macroinitiator was conducted in an ethanol–water mixture in
fluorosurfactant perfluorooctanoate (PFOA), which is soluble
the absence or in the presence of a divinylic comonomer to
in scCO2, raised conversion to over 90% and allowed molar
crosslink the particle core.412 The system resulted in block
masses to be increased. Furthermore, the polymer existed as copolymer micelles stabilized by the PEO-soluble blocks. To
fairly uniform spheres of
1–3 µm in diameter and was recover­ improve the PSD in dispersion ATRP, a two-stage method was
able as a free-flowing powder. Although PFOA does not have a applied, starting from a standard free radical polymerization,
block or graft copolymer structure characteristic of most stabili­ which allowed the nucleation step to proceed in fast condi­
zers, it does function effectively, likely because the backbone has tions.413 The controlled character of the second polymerization
affinity for the particle surface while the fluoroalkyl groups have step was demonstrated.
greater affinity for the scCO2 phase. PFOA has also been used in Similar to NMP and ATRP, the first articles on RAFT in
polymerizations of styrene and VAc.403,404 dispersion polymerization showed that the quality of control
Motivated by the importance of the stabilizer structure on the along with the PSD was sometimes rather poor.414,415 This
polymer properties, kinetics, and PSD, numerous studies have difficulty was overcome by a delayed introduction of the RAFT
examined different types of polymers as stabilizers in scCO2 agent, after a first step of classical free radical polymeriza­
dispersion polymerization.405–407 In addition to fluorinated tion.416–420 At this stage, the nucleation step was over, which
homo- and copolymers such as PFOA and its methacrylate guaranteed a narrow PSD throughout the polymerization
analogue, random copolymers, block copolymers, comblike course. Another strategy also applied in ATRP by the same
graft copolymers, and reactive macromonomers of varying group412 was to employ a macromolecular RAFT agent able to
molar mass and copolymer composition have been used. play multiple roles in the dispersion polymerization, that is,
Fluorinated polymers and silicone polymers (PDMS) are the control agent and stabilizer (providing it is soluble in the con­
best choices for the portion of the surfactant with affinity for tinuous phase).421 This method led to block copolymers formed
the scCO2 phase, while the lipophilic moieties used for the in situ and able to self-assemble simultaneously to the growth
anchoring block are often chosen to be same as the monomer step. In the presence of crosslinker as performed in the quoted
being polymerized. study, the system allowed small, core-crosslinked micelles with
494 Vinyl Polymerization in Heterogeneous Systems

narrow size distribution to be obtained in an easy manner. In polymerization methods than aqueous systems as they can
some situations however, depending most probably on the tolerate unstable catalysts and active centers that can be inacti­
experimental conditions and type of RAFT group employed, vated by water traces. Therefore, ionic polymerizations can be
the system was not efficient enough to ensure a good control used in such dispersed systems although the number of articles
over the polymer structure, even though the PSD was nar­ really devoted to the synthesis of stable polymer particles is
row.422,423 The RAFT functional group along with the very scarce as far as vinyl monomers are regarded.
concentration of the macromolecular RAFT agent was later In addition to the living dispersion polymerization of vinyl
shown to have a strong influence on the outcome of the disper­ monomers initiated with n-butyllithium442,443 or an enolate444
sion polymerization in terms of both control over the diblock in the presence of a steric stabilizer, the anionic polymerization
copolymer structure and particle size.424,425 Once the difficulties in dispersed systems is also adapted to the synthesis of amphi­
related to the finding of the most appropriate experimental philic block copolymers. Indeed, the synthesis of diblock
conditions were overcome, the RAFT method was shown to be copolymers in a selective solvent that is good for the first block
quite powerful in dispersion polymerization leading to very and bad for the second block is a particular situation of disper­
original block copolymer micelle morphologies such as vesicles sion polymerization. In such a situation, self-assembly of the
and nanotubes.426–429 copolymers during the second polymerization step leads to the
in situ formation of diblock copolymer micelles. This was actually
3.14.3.2.2 Dispersion polymerization in scCO2 a method developed for the anionic polymerization of styrene in
The controlled/living heterogeneous radical polymerization in an aliphatic solvent, using polybutadienyl carbanion as a macro­
scCO2 has been reviewed quite recently.430 The works are initiator.445 It was similarly used for the polymerization of
related to precipitation polymerization, that is, in the absence
divinylbenzene leading to crosslinked particles.446 A very recent
of stabilizer but also dispersion polymerization, in which a
work published by Wang et al.447 follows this same principle and
CO2-philic stabilizer has to be introduced. The latter is usually
provides several examples of polybutadiene-b-polystyrene block
a polymer, either PDMS or a fluorinated polyacrylate.
copolymer nanoparticles of various shapes and morphologies
The SG1-mediated polymerization of styrene was studied in
produced by anionic polymerization in hexane. Other examples
the presence of a reactive PDMS with attached diazoic initiating
of dispersion polymerization can be found in scCO2 dispersing
groups, in conjunction with free nitroxide. The PDMS being
medium, concerning either anionic448 or cationic polymeriza­
soluble in scCO2 should play in addition the role of stabilizer.
tion systems.449–451 Similarly, a few articles describe the cationic
However control over the polymerization was rather poor and
polymerization of styrene,451–454 the anionic polymerization
the system suffered from stability issues.431 Similar results were
of MMA,455 and its group transfer polymerization456 in ionic
obtained when a low-molar-mass initiator was used in the
liquids, a topic which has been well covered by a recent review
presence of a nonreactive diblock copolymer stabilizer
(namely, PDMS-b-poly(methyl methacrylate)), with however article.457
better colloidal properties.432 A PDMS-polystyrene-SG1 macro­
alkoxyamine was further used in similar conditions for the
polymerization of styrene, with the advantage of leading to
diblock copolymers in situ. The system was then much better 3.14.4 Conclusion
controlled in terms of both molar mass and colloidal stabi­
lity.433 The TEMPO-mediated polymerization of styrene was As shown in this chapter, polymerization of vinyl monomers in
also studied with either silicon-based or perfluorinated macro­ heterogeneous systems covers all types of polymerization che­
molecular stabilizers, also synthesized by NMP.434 mistries, along with a broad variety of processes. Some of them
Similarly, the ATRP of MMA was studied following various are widely used in industrial productions. The field remains
strategies: with fluorinated ligand and fluorinated stabilizer,435 very active and new systems (to a large extent due to new
or with a PDMS-Br macroinitiator and a classical ligand,436 or catalysts and controlling agents being developed) are continu­
with a fluorinated macroligand playing roles of both ligand ally being proposed to improve the polymer structure, particle
and stabilizer.437,438 Control over polymerization was rather morphology, surface chemistry, etc. In addition, completely
good in all cases and particle stability was acceptable. new approaches are regularly reported, demonstrating the
The dispersion polymerization of MMA in scCO2 was dynamic and continually evolving character of the research
applied using the RAFT method.439,440 Although very long conducted in this field, where the limits can still be expanded
induction periods were observed together with rate retardation, further.
the systems were particularly well controlled from the macro­
molecular viewpoint, with very narrow MMDs. The use of poly
(vinyl alkylate) hydrocarbon surfactants with a RAFT group at
the chain end was particularly successful for the synthesis of
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 Vinyl Polymerization in Heterogeneous Systems 499

Biographical Sketches
Bernadette Charleux received a PhD degree from the University Claude Bernard in Lyon, France, under the supervision of
Dr. Christian Pichot at the joint research unit CNRS-BioMérieux. She was working on the elaboration of functionalized
latex particles for medical diagnostic applications. She then spent 6 months at the CNRS center of Thiais, France, working in
the domain of conducting polymers. During that period, she got a permanent CNRS researcher position to work in the field
of cationic polymerization in the group of Prof. Jean-Pierre Vairon, at the Laboratoire de Chimie des Polymères at the
University Pierre et Marie Curie, Paris, France. She received the habilitation in 2000 and was appointed full professor in
2001. Since September 2009, she has been a professor at the University Claude Bernard Lyon 1, a senior member of the
Institut Universitaire de France, and director of the research unit Chemistry Catalysis Polymer and Processes. In recent years,
she has focused her research on controlled/living radical polymerization in dispersed systems and on the synthesis of
amphiphilic block copolymers and their self-assembling.

Michael Cunningham completed his doctoral studies in polymerization kinetics at the University of Waterloo under the
supervision of Professor K. F. O’Driscoll and Professor H. K. Mahabadi. He spent 6 years in the Corporate Research Group in
Xerox before accepting an academic position at Queen’s University and initiating a research program on polymer colloids.
In recent years, his research has concentrated on controlled/living radical polymerizations, particularly in dispersed
systems. He has received a Premier’s Research Excellence Award and the Syncrude Canada Award (Canadian Society for
Chemical Engineering) for outstanding contributions in chemical engineering before the age of 40. He holds an Ontario
Research Chair in green chemistry and engineering.

Jose Ramon Leiza graduated from the University of the Basque Country (Donostia, Spain) in 1987, and obtained the PhD
degree in 1991 from the same university, working in the monitoring and control of emulsion polymerization reactors
under the supervision of Prof. J. M. Asua. In 1992, he was appointed as assistant professor in the Department of Applied
Chemistry at the University of the Basque Country. In 1994, he held a 1-year research associate position at the Emulsion
Polymers Institute of Lehigh University (Bethlehem, PA, USA) under the supervision of Prof. M. S. El-Aasser. In 1995, he
was promoted to associate professor. In 2004, he spent a sabbatical year at Queen’s University (Kingston, ON, Canada). In
2010, he was promoted to full professor at the University of the Basque Country. Since 2000, he has been a member of the
Institute of Polymer Materials, Polymat. In 1993, he received the Rhône-Poulenc Award in Clean Technologies organized
by the Association of Engineers of Madrid (Spain). His current research interest is focused on the following topics:
waterborne polymer–inorganic hybrid nanocomposites, polymer reaction engineering aspects of polymerization in dis­
persed media (kinetics, modeling, high solids content formulations), molecularly imprinted polymers in dispersed media,
and polymerization of water-soluble monomers.