Review 1739

Summary: Monomodal microwaves have overcome the

safety uncertainties associated with the precedent domestic
microwave ovens. After fast acceptance in inorganic and
organic syntheses, polymer chemists have also recently
discovered this new kind of microwave reactor. An almost
exponential increase of the number of publications in this
field reflects the steadily growing interest in the use of
microwave irradiation for polymerizations. This review intro-
duces the microwave systems and their applications in poly-
mer syntheses, covering step-growth and ring-opening, as
well as radical polymerization processes, in order to sum-
marize the hitherto realized polymerizations. Special atten-
tion is paid to the differences between microwave-assisted
and conventional heating as well as the ‘‘microwave effects’’.

Results of search on number of publications on microwave-

assisted polymerizations, sorted by year.

Microwave-Assisted Polymer Synthesis:

State-of-the-Art and Future Perspectives
Frank Wiesbrock, Richard Hoogenboom, Ulrich S. Schubert*
Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology and Dutch Polymer Institute
(DPI), Den Dolech 2, 5600 MB Eindhoven, The Netherlands
Fax: (þ31) 40 247 4786; E-mail: u.s.schubert@tue.nl

Received: July 15, 2004; Revised: August 16, 2004; Accepted: August 18, 2004; DOI: 10.1002/marc.200400313
Keywords: microwave irradiation; monomodal microwave reactor; radical polymerization; ring-opening polymerization; step-
growth polymerization

1. Introduction waves between infrared and radio waves. Most commercial

microwave systems, however, utilize an irradiation with a
Microwave irradiation is a well-known method for heating frequency of 2 450 MHz (wavelength l ¼ 0.122 m) in order
and drying materials and is utilized in many private to avoid interferences with telecommunication devices. The
households and industrial applications for this purpose. It corresponding electric fields oscillate 4.9
109 times per
offers a number of advantages over conventional heating, second and consequently subject dipolar species and ionic
such as noncontact heating (circumventing the decomposi- particles (as well as holes and electrons in semiconductors
tion of molecules close to the walls of the reaction vessel), or metals) to perpetual reorientation cycles. This strong
instantaneous and rapid heating (resulting in a uniform agitation leads to a fast noncontact heating that is
heating of the reaction liquor), and highly specific heating (approximately) uniform throughout the radiation chamber.
(with the material selectivity emerging from the wavelength A large number of reactions, both organic and inorganic,
of microwave irradiation that intrinsically excites dipolar undergo an immense increase in reaction speed under
oscillation and induces ionic conduction).[1,2] microwave irradiation compared with conventional heat-
Microwave ovens operate with electromagnetic non- ing. Apart from this main advantage, significant improve-
ionizing radiation with frequencies between 300 GHz and ments in yield and selectivity have been observed as a
300 MHz. The corresponding wavelengths span a range consequence of the fast and direct heating of the reactants
from 1 mm to 1 m, exhibiting the medial position of micro- themselves. Furthermore, high-pressure synthesis is easily

Macromol. Rapid Commun. 2004, 25, 1739–1764 DOI: 10.1002/marc.200400313 ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1740 F. Wiesbrock, R. Hoogenboom, U. S. Schubert

accessible for reactions performed in closed vessels, faci- received broad interest in all branches of chemistry ever
litating the use of low boiling solvents and thereby paving since then.
the way to environmentally benign reaction conditions. Monomodal microwave systems (Figure 1) heat only one
It should be mentioned, however, that these ameliora- reaction vessel at a time. Consequently, the volume of the
tions might be mainly ascribed to the one main character- reaction (heating) chamber is diminished, and the resulting
istic of reactions performed under microwave irradiation: a homogenous irradiation field uniformly heats the reaction
combination of the large increase in reaction speed (caused liquor. Most of these microwave reactors operate with
by the high temperatures utilized) and the selective standardized reaction vessels that are closed by septa,
excitation that circumvents the formation of by-products allowing for accurate control of the pressure (by the bowing
or the exhaustion of catalysts and reduces the time available of the septum) as well as the temperature inside the reaction
for the desired product to decompose.[2] In the literature, on vial (detected by an IR sensor that is directed towards the
the other hand, a controversial discussion has arisen as to walls of the vessel). Depending on this online monitoring,
whether the increase in reaction speed not only emerges the irradiation power may be adjusted (‘‘dynamic field
from thermal effects, but also from (nonthermal) ‘‘micro- tuning’’, DFT) to maintain the desired reaction conditions
wave effects’’.[2–5] These days, a strong tendency to dismiss (like a fixed temperature and a threshold for maximum
microwave effects is discernible, benefiting from the pressure), or be switched off, when the chosen parameters
observations made during reactions performed in mono- are exceeded. This way, a powerful tool is provided for
modal microwave systems as reaction devices. This minimizing the risk of hazardous explosions that are
new class of reactors[6] was introduced to a large group of associated with reactions performed in domestic micro-
scientists at the beginning of the millennium and has wave ovens;[3] furthermore, exothermic reactions may be

Frank Wiesbrock was born in Gütersloh (Germany) in 1976. He studied chemistry at the Technische
Universität München (Germany) where he received his degree in 2001. During his studies, he
additionally attended a practical course dealing with switchable adhesives at the EADS (Ottobrunn,
Germany) for six months. In the course of his PhD thesis under the supervision of Prof. H. Schmidbaur
(chair of inorganic chemistry at the Technische Universität München), he investigated the complexation
behavior of b-amino carboxylates towards biologically important metal ions. He was awarded his
doctorate in 2003. In January 2004, he joined the research group of Prof. U. S. Schubert at the
Eindhoven University of Technology (The Netherlands) as a postdoctoral research associate on
microwave-assisted polymerization reactions. His main research interests are metal complexation,
bioinorganic chemistry and polymer synthesis.

Richard Hoogenboom was born in 1978 in Rotterdam (The Netherlands). In 2001 he obtained his M.Sc.
degree in chemical engineering at the Eindhoven University of Technology. His undergraduate research
concerning the quadruple hydrogen bonding of the 2-ureido-4[1H]-pyrimidinone unit in water was
carried out in the group of Bert Meijer (Eindhoven, The Netherlands). After a three months internship
within the group of Andrew Holmes (Cambridge, United Kingdom), he started in November 2001 his
PhD work under the supervision of Ulrich Schubert (Eindhoven, The Netherlands) focusing on
supramolecular initiators for controlled polymerization techniques and automated parallel synthesis of
well-defined polymers.

Ulrich S. Schubert was born in Tübingen in 1969. He studied chemistry at the Universities of Frankfurt
and Bayreuth (both Germany) and the Virginia Commonwealth University, Richmond (USA). His PhD
work was performed under the supervision of Professor Eisenbach (Bayreuth, Germany) and Professor
Newkome (Florida, USA). In 1995 he obtained his doctorate with Professor Eisenbach. After
postdoctoral training with Professor Lehn at the Université Strasbourg (France) he moved to the
Technische Universität München (Germany) to obtain his habilitation in 1999 (with Professor Nuyken).
From 1999 to spring 2000 he held a temporal position as a professor at the Center for NanoScience at
the Universität München (Germany). Since the summer of 2000 he has been a Full-Professor at the
Eindhoven University of Technology (Chair for Macromolecular Chemistry and Nanoscience). His
awards include the Bayerischen Habilitations-Förderpreis, the Habilitandenpreis of the GDCh
(Makromolekulare Chemie), the Heisenberg-Stipendium of the DFG, the Dozenten-Stipendium of the
Fonds der Chemischen Industrie and a VICI award of NWO. The major focus of his research is organic
heterocyclic chemistry, supramolecular materials, combinatorial material research, nanoscience and
tailor-made macromolecules.

Macromol. Rapid Commun. 2004, 25, 1739–1764 www.mrc-journal.de ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
 Microwave-Assisted Polymer Synthesis: State-of-the-Art and Future Perspectives 1741

Figure 1. Microwave reactors for chemical syntheses. A: Emrys Liberator (Biotage,

Sweden, www.biotage.com); B: CEM Discover BenchMate (CEM, USA, www.cem.com)
Copyright CEM Corporation; C: Milestone Ethos TouchControl (Milestone, Italy,
www.milestonesci.com); D: Lambda MicroCure2100 BatchSystem (Lambda, USA,
www.microcure.com).

subjected to analogous supervision, preventing them from 2. Step-Growth Polymerization

running out of control.
These days, step-growth polymerizations are the most
The online measurement of temperature and pressure
extensively investigated polymerization reactions under
not only shifts the reliability of microwave reactors from
microwave irradiation. This is not accidentally because of
hazardous to safe, it also allows for the determination of the
the nearness of polymer and organic chemistry most discer-
reaction conditions inside the vials. This way, it has been
nible with this reaction type. A plethora of data has been
elucidated for the majority of reactions that temperature
collected, in particular for polyamides and polyimides.
effects or the selectivity in heating are solely responsible for
Polyethers and polyesters, as well as phase transfer and C–C
the advantages experienced by using microwave irradia-
coupling reactions, will be presented later in this section.
tion. Furthermore, a high degree of reproducibility and a
potential for optimization results from high-pressure
syntheses. The ease of access to superheated reaction 2.1. Polyamides
conditions facilitates the way to short reaction times, low-
Amide linkages are most abundant in nature because of
boiling solvents and, additionally, to halogen-free solvents
their involvement in peptides, proteins, and enzymes. In
(‘‘green chemistry’’), the latter as a consequence of the
order to simulate the evolution of life from the assembly of
improved solubility at higher temperatures. In this respect,
nondirected amino acid sequences to extrinsically directed
preliminary findings for polymerizations carried out in
ones, Yanagawa et al. subjected amino acid amides to
domestic microwave ovens might be taken into critical
repeated hydration–dehydration cycles in a domestic
account as far as the measurement of the reaction conditions
or reproducibility and up-scaling issues are concerned.
With their intriguing properties, monomodal micro-
wave reactors have started a new era in several branches
of organic chemistry, such as library synthesis[7–10] or
solid-phase synthesis,[11–14] overcoming the long reaction
times previously associated with these fields. A similar
breakthrough seems to be at the outset for polymer science,
as indicated by the almost exponential increase of
publications on microwave-assisted polymerizations
(Figure 2).[15]
With this background, the present review aims at sum-
marizing the hitherto realized polymerizations under micro-
wave assistance. In contrast to previous reviews,[1,16–18]
the preparation of dental materials, polymer modification
reactions, and curing processes are excluded. Instead, the
focus of this review is directed towards microwave-assisted
polymer synthesis, including step-growth, free and con- Figure 2. Number of publications on microwave-assisted poly-
trolled radical, as well as ring-opening polymerizations. merizations, sorted by year (updated on June 23, 2004).[15]

Macromol. Rapid Commun. 2004, 25, 1739–1764 www.mrc-journal.de ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1742 F. Wiesbrock, R. Hoogenboom, U. S. Schubert

microwave oven.[19] The stock mixture was composed of not determined. The type of solvent did not influence the
equimolar amounts of glycinamide, alaninamide, valin- inherent viscosity (therefore, neither the molecular weight)
amide, and aspartic acid a-amide as model substances for of the polymers. The amount of solvent, however, played a
molecules formed at an early stage of the chemical evolu- (minor) role, as the use of a smaller amount of solvent
tion. Continued solvation in aqueous solution and subse- resulted in a higher final temperature in a shorter reaction
quent evaporation of the water in a microwave oven resulted time, affording polymers with higher inherent viscosities.
in the formation of polypeptides with molecular weights of Similar observations have been made for the polymer-
up to 4 000 Dalton. The molecular weights were found to ization of equimolar amounts of diamines and dicarboxylic
be independent of the number of hydration–dehydration acids, the ‘‘nylon salts’’. Imai et al. investigated the poly-
cycles; a saturating yield of 10% was obtained after ten merizations of aliphatic diamines and dicarboxylic acids
cycles. Comparison experiments with conventional heating for the preparation of polymers of the composition
showed that the microwave did not affect the maximum [–NH(CH2)xNHCO(CH2)yCO–]n (with the combinations
molecular weight attainable, but it improved the yield of the x/y ¼ 6/4, 6/6, 6/8, 6/10, 8/4, 12/4, 12/6, 12/8, 12/10)[20–22]
polypeptides by a factor of 100. The authors ascribe this as well as the polymerizations of aromatic diamines (like
success to the shortness of the heating periods, keeping 4,40 -methylenedianiline, 4,40 -oxydianiline, 1,3- and 1,4-
undesired side reactions, such as the hydrolysis of the phenylenediamine) and dicarboxylic acids (like isophthalic
terminal active amide groups, to a minimum. acid and terephthalic acid) to yield polymers of the structure
Artificial polyamides have been derived from amino [–NH–Ar–NHCO–Ar0 –CO–]n after a Yamazaki phospho-
acids as well as nylon salts (for the synthesis of polyamides rylation reaction (Scheme 1b and c).[23] For the polymeriza-
from caprolactam, see Section 3). Utilizing a domestic tions involving aromatic monomers, a domestic microwave
microwave oven, Imai et al. polymerized amino acids of reactor was additionally supplied with a Teflon insulated
the composition H2N(CH2)xCOOH (x ¼ 5, 6, 10, 11, 12) thermocouple (Figure 3). Like the polymerizations of amino
(Scheme 1a).[20,21] As a result of the incapability of the acids, these reactions were also carried out in high-boiling
monomers to absorb microwave irradiation, the polymer- solvents with high dielectric moments. The products were
ization reactions were carried out in solution with the obtained after short reaction times (less than one minute) in
solvent additionally acting as absorber. Solvents with high high yields (85–96%), and the polyamides exhibited
boiling points and high dielectric constants, like m-cresol, medium to high inherent viscosities (up to 0.86 dL  g1).
o-chlorophenol, ethylene glycol, sulfolane, and N-cyclo- For one series of aliphatic diamines and dicarboxylic acids
hexyl-2-pyrrolidone, proved to be most successful. As (x ¼ 6, 8, 12; y ¼ 4), the difference between continuous and
expected, diphenyl ether, with a high boiling point but a low periodic microwave irradiation was investigated.[22] In final
dielectric constant, failed to support the polymerization. consequence, the polymerizations carried out under peri-
The syntheses of the polyamides were carried out in open odic microwave irradiation allowed for easier temperature
reaction vials that allowed for the evaporation of the control and gave polymers with a higher inherent viscosity
solvents; within five minutes, polymers with large inherent (compared with continuous heating). These findings are
viscosities of around 0.5 dL  g1 were obtained. Average taken into account in the monomodal microwave systems
molecular weights and their distributions, however, were available today in dynamic field tuning (see Section 1).

Scheme 1. Polyamides derived from microwave-assisted polymerizations: (a) nylon 6

from e-amino caproic acid as an example of polymers of the composition [–NH(CH2)xCO–]n
(x ¼ 5); (b) nylon 6,6 from adipic acid and hexamethylene diamine as an example of
polymers of the structure [–NH(CH2)xNHCO(CH2)yCO–]n (x ¼ 6, y ¼ 4); (c) Nomex from
isophthalic acid and 1,3-methylene diamine as an example of aromatic polyamides.

Macromol. Rapid Commun. 2004, 25, 1739–1764 www.mrc-journal.de ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
 Microwave-Assisted Polymer Synthesis: State-of-the-Art and Future Perspectives 1743

high inherent viscosities were also obtained in (medium to)

good yields after short reaction times (below one minute).

2.2. Polyimides
A large number of the presented polyimides contain the
pyromellitoyl unit in their polymer chains. Imai et al.
investigated the step-growth polymerization of aliphatic
diamines H2N–(CH2)x–NH2 (x ¼ 6–12) with both pyro-
mellitic acid and its diethyl ester (Scheme 2a).[21,26,27]
Analogously to the synthesis of polyamides (Section 2.1.),
the reactions were performed in solution in a domestic
Figure 3. Experimental setup for the measurement of tempera- microwave oven. Among the investigated solvents, all of
ture in a domestic microwave oven (according to reference [23]).
them with high boiling points and high dielectric constants,
1,3-dimethyl-2-imidazolidone (DMI) proved to be best
suited as primary microwave absorber and solvent for the
Mallon and Ray, moreover, showed that the increase in monomers and polymers. Polyimides with inherent vis-
reaction rates (for nylon 6,6) was determined by the enhan- cosities of 0.7 dL  g1 (for x ¼ 12) were obtained within
ced diffusion rates under microwave irradiation (Figure 4; two minutes from pyromellitic acid as starting material.
cf. Section 2.2).[24] Especially for pyromellitic acid diethyl ester as monomer,
The high-pressure synthesis of polyamides was investi- the microwave-assisted polymerization proved its virtues
gated by Pourjavadi et al. for the Yamazaki phosphorylation by producing a series of polyimides with inherent visco-
reaction of aliphatic dicarboxylic acids (x ¼ 2, 4, 6, 8) and sities of up to 1.6 dL  g1. For the reaction of dodecameth-
aromatic diamines (like 1,4-phenylendiamine and 2,5-bis- ylene diamine with pyromellitic acid, a direct comparison
(4-aminophenyl)-3,4-diphenylthiophene).[25] The educts between microwave irradiation and conventional heating
were dissolved in N-methylpyrrolidone (NMP) and trans- was made by control experiments. The solution polymer-
ferred to the (domestic) microwave oven in a polyethylene ization in the microwave was shown to proceed much
screw-capped cylinder. With this reaction, polymers with faster than the corresponding solid-state synthesis under

Figure 4. Setup for the determination of the diffusion rates in the course of the
polymerizations of nylon 6,6 and poly(ethylene terephthalate) (according to reference [24]).

Macromol. Rapid Commun. 2004, 25, 1739–1764 www.mrc-journal.de ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1744 F. Wiesbrock, R. Hoogenboom, U. S. Schubert

Scheme 2. Types of polyimides synthesized in microwave reactors: (a) polyimide

synthesis from the reaction of hexamethylene diamine with pyromellitic acid diethylester;
(b) preparation of rigid polyimides with third-order nonlinear optical properties from
pyromellitic acid and benzoguanamine in a two-step procedure (by polyamic acid); (c)
polyimides derived from the reaction of polyurea precursors and pyromellitic acid
dianhydride.

conventional heating during the initial five minutes. After polyimides with rigid structures from pyromellitic acid
these five minutes and with concomitant inherent viscos- dianhydride and aromatic diamines (such as benzoguan-
ities of around 1 dL  g1, however, the inherent viscosities amine or 3,30 -diaminobenzophenone) or polyurethane-
of the polymers that were prepared in the microwave oven prepolymers prepared from benzoguanamine and toluene-
only slightly increased with prolonged irradiation times, 2,4-diisocyanate (Scheme 2b and c).[28–32] The preparation
while those from the polymers that were synthesized of the polymers followed a two-step procedure, the first step
with conventional heating still increased to values of up to of which was aimed at the microwave-assisted synthesis of
2 dL  g1. No explanation was provided for these a polyamic acid precursor under reflux conditions (in
observations; instead, Imai et al. emphasized the micro- methanol or tetrahydrofuran). This precursor polymer
wave’s superiority to conventional heating during the first was recovered from the reaction liquor, mixed with pyro-
five minutes. mellitic acid dianhydride, and was subjected to a solid-
In order to provide a new series of polymers with third- phase polymerization in a domestic microwave oven. The
order nonlinear optical properties, Lu et al. synthesized imidization reaction strongly profited from microwave

Macromol. Rapid Commun. 2004, 25, 1739–1764 www.mrc-journal.de ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
 Microwave-Assisted Polymer Synthesis: State-of-the-Art and Future Perspectives 1745

irradiation and the reaction time was decreased from 5 h to that was assessed as further proof for the postulated
8 min.[30] Furthermore, increased inherent viscosities and, mechanism. A direct comparison of microwave irradiation
hence, improved third-order nonlinear optical properties of and conventional heating for the synthesis of poly(ethylene
the polymers were observed and referred to the reduced terephthalate) was carried out by Gilmer et al. at 140 8C.[35]
reaction times. They reported that microwave irradiation did not enhance
The use of polyamic acid prepared under microwave the reaction speed in the case of identical temperatures and
irradiation for grafting procedures has been also described reaction times.
by Lu et al.[33] For that reaction, a domestic microwave oven Currently, with a major debate concerning the existence
was supplied with an external voltage controller to allow for or nonexistence of microwave effects, all three reports
the modulation of the power. In this way, the temperature should be taken into critical account. A steadily growing
inside the reaction vial could be manually maintained at a number of experiments gives rise to the last of the presented
favored value (Figure 5). theories and predicts the microwave to be a fast and efficient
Kinetic measurements were performed for the imidiza- heating device with no further influence on most of the
tion reaction under microwave irradiation. For the polyamic reactions performed. For the other two theories, however, it
acid derived from 3,30 ,4,40 -benzophenone-tetracarboxylic should be mentioned that the observed ameliorations may
acid and diaminodiphenylsulfone, Ward and co-workers originate from the selective excitation of dipoles (which is
showed that the increase in reaction rates and the concomi- characteristically most pronounced for water molecules).
tant decrease of the apparent activation energy was a
consequence of the microwave irradiation’s selectivity to
2.3. Poly(amide imide)s
excite dipoles.[34] According to that theory, a temperature
enhancement of 50 K in the vicinity of the excited Mallakpour et al. synthesized optically active pyromellitoyl
dipole moment was predicted. Mallon and Ray also ascrib- polymers as model substances for column materials in
ed the increase in reaction speed [for nylon 6,6 and enantioselective chromatography.[36,37] The polymeriza-
poly(ethylene terephthalate)] to the selectivity of the tion reaction was proceeded by the preparation of optically
microwave irradiation, based on findings from a specially active pyromellitoyl derivatives (under conventional heat-
designed microwave reactor setup (Figure 4).[24] However, ing), namely N,N0 -(pyromellitoyl)-bis-L-phenylalanine di-
they proposed that enhanced diffusion rates (of small acid chloride and the corresponding L-leucine compound.
molecules involved in the ring-closing imidization reac- Starting from these monomers, two series of optically
tion) were responsible for the observable increase of active poly(amide imide)s were investigated: The bis-L-
reaction rates and the decrease of the apparent activation phenylalanine compound was reacted with six differently
energy. This increase in the reaction speed was found to be 5,5-disubstituted hydantoin compounds (Scheme 3a) and a
most pronounced for water molecules that are known to bis-L-leucine derivative with six aromatic diamines, respec-
optimally absorb microwave irradiation, a phenomenon tively. The polymerizations were carried out in a domestic
microwave oven in o-cresol solution because of the low
absorbance of the monomers. Polymers with inherent
viscosities of up to 0.5 dL  g1 were obtained within 10 min.
Longer exposure times led to partial degradation of the
polymers.
Another library of poly(amide imide)s was prepared by
Mallakpour et al. from the reactions of optically active
N,N0 -(4,40 -carbonyldiphthaloyl)-bis-L-phenylalanine di-
acid chloride and the corresponding L-alanine and L-leucine
compounds, with eight aromatic diamines, all of them with
a rigid scaffold (Scheme 3b).[38–40] The two monomers
were dissolved in o-cresol and exposed to microwave
irradiation for 7 to 10 min in a domestic microwave oven to
yield polymers with large inherent viscosities in the range
of 0.22 to 0.85 dL  g1. A comparison of microwave-
assisted polymerization in a Teflon vessel with that in a
porcelain dish showed that (for identical reaction times) the
polymers recovered from the porcelain dish had larger
inherent viscosities.[38] Furthermore, for two series of
Figure 5. Modification of a domestic microwave oven in order to compounds, the microwave-assisted polycondensation
maintain the favored reaction temperatures by adjusting the was compared with a solution polymerization supported
supplied power (according to reference [33]). by trimethylsilyl chloride (for the activation of the

Macromol. Rapid Commun. 2004, 25, 1739–1764 www.mrc-journal.de ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1746 F. Wiesbrock, R. Hoogenboom, U. S. Schubert

Scheme 3. Poly(amide imide)s prepared in microwave reactors, Part I: (a) optically active
poly(amide imide)s from pyromellitic diacid anhydride, L-amino acids, and 5,5-disubstituted
hydantoin derivatives; (b) polymers containing a carbonyldiphthaloyl unit; (c) poly(amide
imide)s with tetrahydro-2-thioxopyrimidines in the main chain.

diamines).[38,39] These reference experiments were per- step-growth polymerizations of N,N0 -(pyromellitoyl)-bis-
formed under a nitrogen blanket, followed by a subsequent L-isoleucine diacid chloride and six aromatic diamines (cf.
increase of the temperature to ambient conditions. From the beginning of this Section).[41]
properties of the polymers, it could be concluded that the Findings from the above-described series were suc-
microwave irradiation was superior to the catalyzed solu- cessfully transferred to the synthesis of polymers from
tion polymerization. In a more recent publication, these N,N0 -(4,40 -carbonyldiphthaloyl)-bis-L-alanine diacid chlor-
findings were repeatedly observed for the corresponding ide with bulky derivatives of tetrahydropyrimidone and

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 Microwave-Assisted Polymer Synthesis: State-of-the-Art and Future Perspectives 1747

tetrahydro-2-thioxopyrimidine (six compounds) (Scheme of compounds was synthesized by Mallakpour et al. from
3c).[42] The monomers were dissolved in o-cresol and N,N0 -(4,40 -hexafluoroisopropylidene)-bis-phthaloyl-L-leu-
irradiated for 10 min in a domestic microwave oven to yield, cine diacid chloride and a group of ten aromatic diamines
(quasi) quantitatively, polymers with inherent viscosities of (Scheme 4a); the polymers exhibited viscosities of 0.50 to
0.25 to 0.45 dL  g1. 1.93 dL  g1.[45]
A similar group of optically active poly(amide imide)s The synthesis of optically active poly(amide imide)s
was prepared by Faghihi et al. from N,N0 -(4,40 -carbonyl- containing Epiclon as structural unit additionally empha-
diphthaloyl)-bis-L-alanine diacid chloride and six repres- sized the universality of the concept for the microwave-
entative hydantoin and thiohydantoin derivatives.[43] assisted step-growth polymerization introduced by
Analogously, the polymerizations were carried out in solu- Mallakpour et al.[45,46] The Epiclon-containing diacid
tion in o-cresol in a domestic microwave oven to produce chlorides were obtained with conventional heating from
polymers within irradiation times of 10 min. Viscosity reactions involving L-leucine and L-phenylalanine
measurements, however, were not performed. A similar set (Scheme 4b). The polymerizations themselves with seven

Scheme 4. Poly(amide imide)s prepared in microwave reactors, Part II: (a) polymers from
N,N0 -(4,40 -hexafluoroisopropylidene)-bisphthaloyl-L-leucine diacid chloride and aromatic
diamines; (b) Epiclon-containing polymers; (c) polymers from 4-(40 -N-1,8-naphthalimido-
phenyl)-1,2,4-triazolidine-3,5-dione and isophorone diisocyanate.

Macromol. Rapid Commun. 2004, 25, 1739–1764 www.mrc-journal.de ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1748 F. Wiesbrock, R. Hoogenboom, U. S. Schubert

different aromatic diamines were carried out in N- conventional and within 20 min with microwave heating
methylpyrrolidone solution in a domestic microwave oven. (domestic microwave oven, 650 W), respectively, exhibit-
After exposure times of 5 min, poly(amide imide)s with ing an acceleration factor of 70 for the transfer to the micro-
inherent viscosities of 0.12 to 0.22 dL  g1 were obtained. wave reactor. Prolonged exposure times to microwave
For both series, control experiments were carried out to irradiation were found to induce the undesirable formation
elucidate the microwave’s superiority over formerly of cyclic oligomers as shown by matrix-assisted laser-
exercised polymerization techniques: The low-temperature desorption ionization time-of-flight mass spectrometry
solution polycondensation (in N-methylpyrrolidone with (MALDI-TOF MS) (Figure 6).
trimethylsilyl chloride as promotor), as well as the high- The polymerization of a derivative of lactic acid, namely
temperature analogon, rendered polymers with comparable L-2-hydroxy-3-phenylpropanoic acid, was studied by Liu
properties. However, significantly longer reaction times in et al. (Scheme 5a).[53] The polymerizations were carried out
the range of hours were employed for these alternatives to with variable time and power in a domestic microwave oven.
microwave-assisted polymerizations. With maximum power (510 W), polymers with number-
Similar variations in the polymerization techniques average molecular weights of 1 800/3 900/5 400 and PDI
for the synthesis of poly(amide imide)s from N,N0 -(4,40 - values of 1.8/1.0/1.4 (given by the authors) were obtained
sulfonediphthaloyl)-bis-L-phenylalanine diacid chloride within 0.5/1.5/2.5 h, respectively. The formation or non-
and its L-leucine congener with seven aromatic diamines formation of cyclic by-products was not discussed.
showed analogous tendencies.[47,48] Like poly(lactic acid), polyanhydrides also belong to the
The approach to polymers with another type of amide class of biodegradable polymers and represent potential
linkages in the main chain, the polyureas, has also been materials for drug delivery applications. The preparation of
described by Mallakpour et al.[49–51] From 4-(40 -acetami- polyanhydrides is a two-step process under conventional
dophenyl)-1,2,4-triazolidine-3,5-dione and the 40 -N-1,8- heating, comprising the synthesis of a polyanhydride pre-
naphthalimidophenyl derivative, as well as 4-(40 -tert-butyl- polymer, its isolation, purification, and subsequent poly-
phenyl)-1,2,4-triazolidine-3,5-dione, the three of them merization. Mallapragada and co-workers showed that
prepared under conventional heating, the targeted polymers the polymerization can be carried out in a single step,
were obtained by the reaction with three different diiso- admittedly with intermediate removal of unreacted acetic
cyanates (hexamethylene diisocyanate, isophorone diiso- anhydride (Scheme 5b).[54] The dicarboxylic acid (like
cyanate, and toluene-2,4-diisocyanate) (Scheme 4c). The sebacic acid) and excessive acetic anhydride were placed in
polymerizations were carried out in a domestic micro- a high-pressure vial that was capped and irradiated for 2 min
wave oven in solution in dimethylacetamide[49,51] and in a microwave oven (General Electrics, 1 100 W). Imme-
N-methylpyrrolidone[50], with pyridine,[49,51] triethyl- diately after the reaction, the vial was decapped and the
amine,[50,51] or dibutyltin dilaurate[51] as catalysts. Poly- unreacted acetic anhydride was evaporated from the hot
ureas with inherent viscosities of 0.06 to 0.20 dL  g1 were reaction solution by an inert gas flow. After eventual addi-
obtained after 8 min irradiation time. Control experiments tion of a catalyst (SiO2, Al2O3, or glass beads), the vial was
were performed comprising steplike heating procedures as recapped and heated for 4 to 25 min in order to produce the
well as reflux conditions, both of them performed under polyanhydride (Scheme 5b). As a result of the elimination
conventional heating. Even with a broad variation of the of a complete step of the reaction procedure, reaction times
catalysts utilized, it was found that microwave irradiation were shortened from four days to a few minutes. Glass
was superior to the other polymerization techniques, beads proved to be the most successful support for that kind
shortening reaction times from up to 24 h down to 8 min. of microwave-assisted polymerizations: Polymers with
number-average molecular weights of 11 400 Dalton were
obtained within 25 min.
2.4. Polyethers and Polyesters
The only example of a polyether synthesized in a micro-
Currently, few data on the microwave-assisted preparation wave so far is a poly(phenylene vinylene)-ether (PPVether)
of polyethers and polyesters by step-growth polymeriza- obtained from the polymerization of 1-chloro-4-methox-
tions have been collected (the use of phase-transfer ylbenzene in solution in alkaline dimethyl sulfoxide
catalysis in the microwave-assisted synthesis of polyethers (Scheme 5c).[55] Alimi et al. heated the reaction mixture
will be discussed in Section 2.7). The homopolymerization to 200 8C in a domestic microwave oven (that was modified
of D,L-lactic acid (2-hydroxypropanoic acid) was suc- to enable online measurement of the temperature), stopped
cessfully applied to microwave reactors by Zsuga and co- the exposure as soon as the targeted temperature was
workers.[52] This reaction can be performed as bulk reached, and continued stirring at room temperature for six
polymerization with conventional as well as with micro- hours. The polymer was obtained in 43% yield; two dif-
wave heating because of the high dipole moment of the ferent oligomer fractions soluble in chloroform and dich-
monomer. Oligomers with molecular weights in the range loromethane were additionally recovered with yields of
of 600 to 1 000 Dalton were obtained after 24 h with 52 and 5%.

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 Microwave-Assisted Polymer Synthesis: State-of-the-Art and Future Perspectives 1749

Figure 6. Excerpts from the MALDI-TOF MS spectra of the microwave-assisted bulk

polymerizations of D,L-lactic acid after 10 min (bottom), 20 min (middle), and 30 min (top),
respectively. Ln represents the linear polymers composed of n monomers, Cn the cyclic
analogue. Apparently, the formation of cyclic by-products becomes more likely with
prolonged reaction times. Reprinted with permission from reference [52], Copyright John
Wiley and Sons.

2.5. Poly(ether imide)s and Poly(ester imide)s decreased from 115 to 60 8C when the reaction was
performed in a (domestic) microwave reactor instead of an
Yu and co-workers observed a significant increase in reaction oil bath. The authors ascribed these adavantages to the good
rates for the imidization reaction between poly(tetramethyl- absorbance of the microwave irradiation by the water
ene oxide)glycol di-p-aminobenzoate and benzene-tetracar- molecules released during the imidization reaction.
boxylic acid dianhydride.[56] The reaction times were Analogously to the synthesis of optically active poly-
reduced from 12 to 3 h and the reaction temperatures were (amide imide)s, Mallakpour et al. also prepared poly(ester

Scheme 5. Microwave preparation of polyethers and polyesters: (a) homopolymerization

of L-2-hydroxy-3-phenylpropanoic acid; (b) polyanhydrides from dicarboxylic acid and
acetic anhydride; (c) PPV ether from the homopolymerization of 1-chloromethyl-4-
methoxylbenzene in alkaline solution in DMSO.

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1750 F. Wiesbrock, R. Hoogenboom, U. S. Schubert

imide)s with chiral carbon atoms in the polymer chain that ide, however, were found not to be formed under microwave
additionally contained pyromellitoyl units (cf. Section irradiation, but from low-temperature polycondensation in
2.3.).[57,58] The compounds N,N0 -(pyromellitoyl)-bis-L- chloroform with triethylamine as catalyst.[62]
phenylalanine diacid chloride and N,N0 -(pyromellitoyl)-
bis-L-leucine diacid chloride were synthesized with
2.6. Polymerizations Involving C–C
conventional heating. The step-growth polymerizations
Coupling Reactions
involving these diacid chlorides and a series of aromatic
diols with rigid scaffolds (like phenolphthalein, bisphenol Contrary to their well-established use in organic chemistry
A, 4,4-hydroquinone, and others) were performed in a and the resulting numerous applications in microwave-
domestic microwave oven. In this way, a basic library of improved organic synthesis,[7–14] microwave-assisted poly-
14 optically active poly(ester imides) was synthesized (cf. merizations by C–C coupling reactions have gained
Scheme 3). As a result of the low dipole moments and the attention only very recently. Apart from the C–C coupling
corresponding low absorbance of the educts, the polymer- reactions described hereinafter, so far only one metathesis
ization reactions were carried out in o-cresol, with o-cresol polymerization of phenylacetylene under microwave irra-
acting as solvent and as primary microwave absorber at the diation has been reported. Utilizing in situ generated
same time; 1,4-diazabicyclo[2.2.2]octane was added as [(arene)M(CO)3] complexes (M ¼ Cr, Mo, W) as catalysts,
catalyst. Within reaction times of 10 min, polymers with the reaction time was reduced from 24 h (under reflux
inherent viscosities in the range of 0.10 to 0.27 dL  g1 conditions of a solution in 1,2-dichloroethane) to 1 h (in a
were readily obtained. Comparison experiments under domestic microwave oven).[63] The temperature for the
solution conditions using triethylammonium chloride as a microwave-assisted polymerizations was not measured. It
phase-transfer catalyst only produced oligomers as was may be concluded, however, that it was higher than conven-
concluded from the improved solubility in methanol. tional reflux conditions as specially designed long-necked
A similar synthetic study resulted in a 14-membered round-bottom flasks were utilized in order to perform the
library with optically active carbonyldiphthaloyl anhydride reactions under high pressure.
derivatives, namely N,N0 -(4,40 -carbonyldiphthaloyl)-bis-L- Carter investigated the nickel(O)-mediated coupling
phenylalanine diacid chloride as well as the corresponding polymerization of 2,7-dibromo-9,9-dihexylfluorene in
L-alanine compound, and a set of aromatic diols with rigid solution in toluene.[64] Previous findings for the reaction
scaffolds (cf. Scheme 3).[59,60] It was found that the poly- performed under conventional heating had shown that the
merizations involving the L-phenylalanine derivative[59] polymerization reaction has to be preceded by an activation
could not be performed under microwave irradiation, but step for the preparation of the catalyst. The polymerization
only at low temperatures under phase-transfer catalysis. reaction itself may last up to 24 h and characteristically
With the L-alanine compound,[60] on the other hand, a shows a low reproducibility and a hindered access to high-
microwave-assisted synthesis was successfully carried molecular-weight polymers because of the decreasing
out in o-cresol without the addition of catalysts. Polymers solubility of the polymers with an increasing degree of
with inherent viscosities of 0.35 to 0.58 dL  g1 were polymerization. Under microwave irradiation, the reaction
obtained within 12 min in a domestic microwave was carried out in capped vials in a microwave reactor
oven. The difference in reactivity for the two carbonyl- specially designed for chemical synthesis (SmithCreator,
diphthaloyl anhydride derivatives is not explained by PersonalChemistry), resulting in high-temperature and
Mallakpour et al. high-pressure synthesis. The polymers were obtained in
A similar study focused on the reaction of 4,40 - almost quantitative yield after 10 min at 250 8C. As a result
(hexafluoroisopropylidene)-N,N0 -bis-(phthaloyl-L-leucine) of this tremendous decrease in reaction times, side reactions
diacid chloride with aromatic diols, all of them with rigid were kept to a minimum, and the polymerization conse-
fixation of the hydroxy function (cf. Scheme 4).[61] For a quently could be run as a one-step routine with no prior
systematic investigation, three different solvents (with a activation of the catalyst in a separate procedure. In addition,
bifunctional role as solvent and primary microwave absor- polymers with high molecular weights of 100 000 Dalton (or
ber) were tested; o-cresol proved to be best suited for those even higher) were formed. The molecular weights could be
purposes compared with m-cresol and 1,2-dichloroben- controlled by the addition of a monofunctional end-capping
zene. Reactions in a domestic microwave oven were unit (4-bromobiphenyl) to yield polymers with molecular
completed within five minutes and yielded polymers with weights from 5 000 to 40 000 Dalton and PDI values from
inherent viscosities of 0.50 to 1.12 dL  g1. Comparison 1.65 to 2.22, respectively (Scheme 6a).
experiments with conventional heating, carried out in 1,2- The preparation of poly(pyrazine-2,5-diyl) from 2,5-
dichlorobenzene, only yielded polymers with inherent dibromopyrazine in a CEM Discover apparatus has been
viscosities of 0.09 to 0.18 dL  g1, even after 24 h reaction described by Yamamoto et al.[65] Using the same nickel(O)-
time. The corresponding polymers derived from N,N0 -[4,40 - based catalyst system, a similar shortening of reaction times
carbonyl-bis-(phthaloylimido)]-bis-L-leucine diacid chlor- down to 10 min was observed.

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 Microwave-Assisted Polymer Synthesis: State-of-the-Art and Future Perspectives 1751

Scheme 6. Microwave-assisted C–C coupling reactions: (a) nickel(O)-mediated

polymerization of 2,7-dibromo-9,9-dihexylfluorene with 4-bromobiphenyl as end-capping
unit; (b) quinquethiophenes from microwave-assisted bulk polymerization, catalyzed
by palladium halides. (COD: cycloocta-1,5-diene; DPPF: 1,10 -bis(diphenylphosphino)-
ferrocene).

The Suzuki coupling reaction has been transferred to temperatures, for both microwave irradiation and conven-
microwave conditions by Barbarella and co-workers for the tional heating, will be published in the near future.
preparation of thiophene oligomers.[66] The synthesis of
quinquethiophenes, for example, was achieved from bulk
2.7. Phase-Transfer Catalysis
conditions of 2-thiophene boronic acid and dibromo precur-
sors with three thiophene units, catalyzed and promoted by Only a few examples of microwave-assisted phase-transfer
[PdCl2(dppf)], KF, and KOH in a Synthewave 402 appara- catalyzed polymerizations have been collected so far, all of
tus (Prolabo) (Scheme 6b). With a maximum temperature of them aiming at the preparation of polyethers. A first set of
70 8C, yields of 74% were obtained after reaction times of investigations, performed by Hurduc et al., focused on
10 min. The synthesis of polymers instead of oligomers, polymers from the step-growth polymerization of 3,3-
however, was not carried out. bis(chloromethyl)oxetane and several bisphenols.[69] The
Leadbeater et al. described the successful preparation of targeted compounds were prepared both with conventional
biaryl compounds by Suzuki coupling reactions in a micro- heating and microwave assistance (domestic microwave
wave reactor.[67] First applications of Suzuki and Stille oven) from a water/nitrobenzene two-phase system with
cross-coupling reactions for the preparation of semicon- tetrabutylammonium bromide as phase-transfer catalyst.
ducting polymers were carried out in a monomodal Compared with conventional heating for 5 h, the same
microwave reactor (CEM Discovery) by Scherf and co- polymer yields could be obtained under microwave irradi-
workers.[68] Reaction times for the preparation of five ation in the temperature range of 95 to 100 8C (depending
representative polymers with number-average molecular on the bisphenol’s chemical structure) within 90 min. The
weights in the range of 11 300 to 15 400 Dalton (and decrease in reaction times is not commented on in detail;
comparably narrow molecular-weight distributions with the acceleration observed, however, may depend on the
PDI values around 1.8) were found to decrease from 3 days enhanced reaction temperatures. The analysis of the ther-
in the case of conventional heating down to a few minutes mal behavior of the polymer revealed higher glass
under microwave irradiation. The corresponding reaction temperatures and melting points for the polymers synthe-
temperatures, however, were not measured automatically in sized in the microwave oven, indicative of higher molecular
a straightforward manner. Instead, the applied power was weights. In a more recent publication,[70] a similar set of
varied and found to require optimization for each specific microwave-assisted polymerizations of 3,3-bis(chloro-
reaction. The microwave reactor proved its virtues partic- methyl)oxetane and several bisphenols has been performed
ularly in the case of the Stille cross-coupling polymeriza- in a monomodal microwave system (Synthewave 402,
tion between the electron rich (and consequently less Prolabo) and compared with results from conventional
active) 1,5-dioctyloxy-2,6-dibromonaphthalene and 5,50 - heating. The difference in reaction times, 4 to 6 h in the case
bis(trimethylstannyl)-2,20 -bithiophene by yielding poly- of conventional heating as opposed to 20 min under
mers with higher molecular weights (13 700 compared with microwave irradiation (for the preparation of oligomers
5 100 Dalton, the latter under conventional heating). In with oligomers’ degrees of 6 to 8 and comparable yields in
order to determine the reasons for this pronounced the range from 64 to 97%), was found to be very pronounc-
microwave effect, results from experiments at identical ed. However, explanations for the increase in reaction rates

Macromol. Rapid Commun. 2004, 25, 1739–1764 www.mrc-journal.de ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1752 F. Wiesbrock, R. Hoogenboom, U. S. Schubert

1,8-dimesyloctane (Scheme 7).[72,73] A solution of equi-

molar amounts of the corresponding educts in toluene was
subjected to phase-transfer catalyzed polymerization in the
presence of tetrabutylammonium bromide and aqueous
potassium hydroxide under microwave irradiation (Synthe-
wave 402, Prolabo) as well as conventional heating. For
the isosorbide-containing polymers, it was found that the
reaction rates increased under microwave irradiation,
providing polymers in yields of around 70% within
30 min. Under conventional heating, reaction times of
24 h were necessary for similar conversions. Furthermore,
the polymers synthesized in the microwave reactor showed
higher molecular-weight averages according to findings
from gel permeation chromatography (GPC) and MALDI-
TOF MS analysis. In the case of the isoidide derivative,
comparable yields were observed, but also for that con-
gener, a strong tendency to higher polymerization degrees
under microwave assistance was observable, as a heavy
weight fraction (insoluble in methanol) was formed in
relatively higher yield (39–67% compared with 5–12%).
Moreover, the two different modes of activation showed
specific mechanisms of chain termination: The polyethers
Figure 7. Modification of a domestic microwave reactor for the prepared under conventional heating had hydroxylated
phase-transfer catalyzed synthesis of poly(ether imide)s. Rep- ends, and those synthesized in the microwave reactor
rinted with permission from reference [71], Copyright John Wiley exhibited ethylenic group ends (Scheme 7). In a more recent
and Sons. publication, these characteristics were repeatedly recog-
nized for the phase-transfer catalyzed step-growth poly-
merizations of 1,8-dibromo- and 1,8-dimesyloctane with
(and the nonimproved oligomers’ degrees) are not provided. isosorbide derivatives (two isosorbides linked by alkyl or
Zhang and co-workers, on the other hand, ascribed the glycol chains, possessing two equivalent alcohol functions
increase in reaction speed for the phase-transfer catalyzed in exo positions).[74]
step-growth polymerization of bisphenol A and bis(chlor-
ophthalimide) in a domestic microwave oven to the selec-
tive excitation of the phenol anion and the resulting
3. Ring-Opening Polymerizations
enhanced reactivity (Figure 7).[71]
Loupy and co-workers observed shifts in selectivity (as a Aliphatic polyesters are an important class of biodegrad-
consequence of microwave irradiation) for the polyether- able polymers and are utilized in biomedical and phar-
ification of isosorbide (1,4:3,6-dianhydro-D-sorbitol) or maceutical applications.[75] The physical and chemical
isoidide (1,4:3,6-dianhydro-L-iditol) with 1,8-dibromo and properties, as well as the degradability, can be easily tuned

Scheme 7. Step-growth polymerization of isoidide with 1,8-dibromooctane and the

different mechanisms of chain termination, depending on the activation source.

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 Microwave-Assisted Polymer Synthesis: State-of-the-Art and Future Perspectives 1753

by the synthesis of copolymers[76] or the involvement of ing polymerization of e-caprolactone with stannous octoate
more advanced structures like comb-shaped, hyper- [Sn(Oct)2] as catalyst and maleic acid as initiator.[82] Up to
branched, or graft (co)polymers.[77] In this section, the maximum irradiation times of 135 min (360 W), the molec-
microwave-assisted polymerization of cyclic lactones (and ular weights were found to increase. If the reaction time was
similar monomers) is described in detail. In addition, the elongated further, the molecular weights decreased as may
polymerization of other cyclic monomers (e.g., e-capro- be depicted from Figure 9 (left). This phenomenon was
lactams and 2-oxazolines) will be discussed. ascribed to the occurrence of transesterification reactions.
Irradiation of a mixture of e-caprolactone, maleic acid
(20:1), and stannous octoate [Sn(Oct)2] for 25 min with
3.1. Aliphatic Polyesters
different microwave powers, on the other hand, induced a
The microwave-assisted ring-opening polymerization of e- nearly linear increase of the (weight) average molecular
caprolactone, exploiting a home-built monomodal micro- weights of the polymers with microwave power (Figure 9,
wave reactor (Figure 8), was first reported by Albert et al.[78] right). In addition, the microwave-assisted polymerization
The microwave was equipped with a temperature control was performed in the presence of Ibuprofen (Ibuprofen is a
and an online viscometry determination. It was demon- nonsteroidal anti-inflammatory drug)[86] in order to synthe-
strated that e-caprolactone could be successfully polymer- size a poly(e-caprolactone)-based system for the controlled
ized with titanium tetrabutylate as catalyst. A comparison release of Ibuprofen. Moreover, the microwave-assisted
of the microwave-assisted polymerization with thermal polymerization was performed in the presence of Sn(Oct)2
polymerization did not show a microwave effect since the without additional initiator, whereby a higher polyme-
observed changes were minor and within the experimental rization rate was observed (compared with thermal
error. Ever since, many reports have appeared in the liter- heating).[81,83,84] Zhuo and co-workers demonstrated the
ature dealing with the microwave-assisted ring-opening possibility of using various acids (maleic acid, adipic acid,
polymerization of e-caprolactone. The contributions are succinic acid, benzoic acid, and chlorinated acetic acids)
divided according to the catalysts applied and discussed in as initiators for the microwave-assisted synthesis of poly-
the following section. (e-caprolactone).[87–90] The main advantage of solely using
The most commonly used catalyst for the ring-opening an acid as initiator is the exclusion of metal ions (as
polymerization is stannous octoate [Sn(Oct)2]. Consequen- catalysts) from the reaction mixture. Those acid-initiated
tly, most microwave-assisted polymerizations were also polymerizations also showed an increased reaction speed
performed utilizing this catalyst.[79–85] Scola and compared with thermal polymerizations. The maleic acid-
co-workers have shown the possibility of producing poly- initiated polymerizations were also performed in the pres-
(e-caprolactone) under microwave irradiation (multimode ence of different amounts of Ibuprofen, yielding a drug
with temperature control) at temperatures ranging from 150 release system for which the release rate could be tuned by
to 200 8C with water or butanediol as initiator.[79,80] The the amount of incorporated Ibuprofen.[87,88] In addition, it
polymerizations were accelerated from 12 h under thermal was shown that the molecular weights of the synthesized
heating (110 8C) to 2 h in the microwave (150, 180, and poly(e-caprolactone)s depend on the strength of the utilized
200 8C), whereby the polymers obtained under microwave acid initiator: The molecular weights decreased with an
irradiation showed thermal and tensile properties similar to increasing acid strength. Those new insights were subse-
those of the conventionally produced materials. Zhuo and quently applied for the degradation of poly(e-caprolactone)
co-workers investigated the microwave-assisted ring-open- under microwave irradiation in the presence of a strong

Figure 8. Schematic representation of the home-built monomodal microwave including

in situ viscometry determination as employed by Albert et al. (according to reference [78]).

Macromol. Rapid Commun. 2004, 25, 1739–1764 www.mrc-journal.de ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1754 F. Wiesbrock, R. Hoogenboom, U. S. Schubert

Figure 9. Left: Increase of molecular weight (squares) and conversion (circles) with time
for the microwave polymerization of e-caprolactone. Right: Increase of molecular weight
against applied microwave power. Reprinted with permission from reference [88], Copyright
VSP International Science Publishers.

acid.[89,90] Increased polymerization rates were also obtain- To summarize, numerous studies have been performed
ed by Koroskenyi and McCarthy who reported kinetic on the microwave-assisted polymerization of e-caprolac-
investigations on the microwave-assisted polymerization of tone. For a large number of reactions, enhanced polymer-
e-caprolactone.[85,91] Furthermore, the optimal conditions ization rates were observed (mainly with multimodal
were applied for the successful grafting of poly(e-caprolac- microwaves).
tone) onto potato starch (for a review on the properties Similar to the polymerization of e-caprolactone, the poly-
of potato starch, for example, see reference [92]). Zinc- merization of L-lactide,[85] D,L-lactide,[95] and trimethylene
catalyzed polymerizations of e-caprolactone were reported carbonate[96] with stannous octoate were reported to
by both Zhuo and co-workers[81] and by Madras and co- proceed faster under (multimodal) microwave irradiation
workers.[93] The latter used a multimodal microwave oven if compared with thermal polymerizations.
without controlling devices for temperature or pressure. To
prevent overheating, the microwave irradiation was sup-
3.2. Other Ring-Opening Polymerizations
plied in cycles of 40 or 50 s as it is commonly done with
multimodal (domestic) microwaves ovens. Figure 10 shows Besides the ring-opening polymerization of e-caprolactone,
the heating profile discernible for the polymerization of some other cyclic monomers were also successfully poly-
e-caprolactone with heating cycles of 50 s. The nonlinear merized utilizing microwave synthesizers. Scola and
regression of the temperature could be used as a kinetic co-workers reported the o-caproic acid-initiated polymer-
parameter. In addition, the activation energy for pulsed ization of e-caprolactam resulting in the formation of nylon
microwave-assisted polymerizations (5.3 kcal  mol1) was 6.[79,80] The comparison of the microwave-synthesized
calculated to be lower than that for polymerizations
performed with continuous thermal heating (13.4 kcal 
mol1). The activation energy for the microwave polymer-
izations (with cycled microwave irradiation) was calculated
from the molecular weight variation during the polymer-
ization. Therefore, the calculated activation energy for the
microwave polymerization might be questioned since it
excludes any depolymerization and transesterification
reactions that might occur with applied temperatures up
to 200 8C. Barbier-Baudry et al. studied the ring-opening
polymerization of e-caprolactone using lanthanide halides
as catalysts.[94] The polymerizations performed in a mono-
modal microwave system yielded polymers with higher
molecular weights and lower PDI values compared with
polymers synthesized thermally at the same temperature. In
addition, it was shown that the PDI of the resulting poly-
mers could be decreased from 3.18 to 1.58 by increasing the
Figure 10. Temperature profile obtained for the pulsed micro-
microwave power from 200 to 300 W. This effect is assign- wave (50 s cycles) irradiated polymerization of e-caprolactone.
ed to faster heating that suppressed secondary transfer Reprinted with permission from reference [93], Copyright John
reactions. Wiley and Sons.

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 Microwave-Assisted Polymer Synthesis: State-of-the-Art and Future Perspectives 1755

polymers with commercial nylon 6 showed that the melting 4. Radical Polymerizations
points of the polymers synthesized in a microwave oven
Free radical polymerization is a widely used technique in
were lower because of different crystalline domains. Accor-
industry for the production of numerous bulk materials, for
ding to the authors, tensile testing exhibited smaller yield
example, polystyrene and poly(methyl methacrylate). The
strain and strain to break and, in addition, an eight times
main drawback of the free radical polymerization technique
larger tensile modulus for the microwave-synthesized nylon
is the poor control of the molecular weights and poly-
6. The polydispersity indices, however, were not determin-
dispersity indices. Therefore, controlled free radical techni-
ed and, thus, no definite explanation could be given for
ques have been introduced during the last decades. In this
those phenomena. The same group also described the
section, investigations on the different radical polymeriza-
microwave-assisted copolymerizations of e-caprolactone
tions (‘‘classical’’, emulsion, and controlled free radical
and e-caprolactame.[97] Compared with thermal polymer-
polymerizations) under microwave-irradiation will be
ization, higher yields and higher amide contents were ob-
discussed.
tained using microwave irradiation, although similar
molecular weights were found. As a result of the higher
amide content, the glass-transition temperature of the
4.1. Free Radical Polymerization
microwave-synthesized polymers was also higher.
Takeuchi and co-workers described the microwave- Microwave-assisted free radical polymerizations were
assisted copolymerization of ethylene isophtalate cyclic initially investigated by Gourdenne et al. in 1979.[100] The
dimer with bis(hydroxyethyl)terephthalate and titanium cross-linking of an unsaturated polyester with styrene was
potassium oxilate.[98] The polymerization could be per- performed under microwave irradiation. In a later contribu-
formed successfully in bulk within 60 min. tion, it was shown that hydroxyethyl methacrylate (HEMA)
Very recently, Schubert and co-workers investigated the could be polymerized under microwave irradiation without
microwave-assisted living cationic ring-opening polymer- the addition of a radical initiator.[101] It was demonstrated
ization of 2-ethyl-2-oxazoline.[99] Performing the poly- that the temperature of the reaction mixture for both, a
merization at temperatures of up to 200 8C in acetonitrile, microwave-irradiated polymerization and a thermal poly-
the polymerization was accelerated by a factor of 400, merization, gave similar profiles, although the microwave-
maintaining the living character of the polymerization. assisted polymerization was significantly faster. Madras
Figure 11 shows the conversion (represented by ln{[M0]/ and Karmore reported the accelerated polymerization of
[Mt]}) plotted against time for the microwave-assisted methyl methacrylate (MMA) under microwave irradia-
polymerization of 2-ethyl-2-oxazoline at several temper- tion.[102] According to the authors, an equilibrium between
atures in the range of 80 to 180 8C, demonstrating the first polymerization and depolymerization was reached within
order kinetics of the monomer consumption. Comparison 10 min, resulting in equal polymer distributions for diffe-
with thermal polymerizations elucidated that the accelera- rent initiator and monomer concentrations as shown in
tion resulted only from thermal effects and not from Figure 12. Boey and co-workers described an increase in
(nonthermal) microwave effects, as the polymerization polymerization rates for the microwave-assisted polymer-
with thermal heating (well beyond the boiling point of izations of methyl methacrylate (MMA), styrene (S), and
acetonitrile) in a high-pressure NMR tube revealed methyl acrylate (MA).[103,104] The corresponding polymer-
analogous polymerization rates. izations were performed at three different microwave
powers (200, 300, and 500 W). The conversion profiles
were shown to be similar for the different microwave
powers in terms of radiation energy. Recently, Ritter and co-
workers reported the synthesis of (meth)acrylamides and
the subsequent polymerization of those monomers in a
monomodal microwave synthesizer.[105] The (meth)acry-
lamides could be synthesized from (meth)acrylic acid and
the corresponding amines in a single step. The polymeriza-
tion of these new monomers (using azodiisobutyronitrile
(AIBN) as initiator) under microwave irradiation has also
proved to be successful. In a final step, it was demonstrated
that the microwave-assisted polymerization of acrylic acid
(initiated by AIBN) in the presence of hexylamine or
benzylamine resulted in the in situ formation of a copoly-
Figure 11. Kinetic results for the microwave-assisted cationic mer of acrylic acid and acrylamide (Scheme 8a). Sitaram
ring-opening polymerization of 2-ethyl-2-oxazoline (according to and Stoffer described the microwave-assisted polymeriza-
reference [99]). tion of styrene with different radical initiators:[106,107]

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1756 F. Wiesbrock, R. Hoogenboom, U. S. Schubert

wave-irradiated polymerizations reached the final tempera-

ture faster than the conventionally heated samples. In
contrast to those results, Boey and co-workers reported an
accelerated polymerization of styrene under microwave
irradiation.[104,108] Furthermore, a sharp and large auto-
acceleration (Trommsdorf effect) was observed under
microwave irradiation, whereas only a gradual auto-
acceleration effect was present during conventional heat-
ing. A clear explanation of this difference between thermal
and microwave polymerization was not revealed, but the
authors suggest that it might result from an increase in the
polymerization rate. This sharp auto-acceleration was also
observed for the microwave polymerization of methyl
methacrylate but not for methyl acrylate. This absence of a
sharp transition for MA could be explained by chain-
Figure 12. Variation of the molecular weight with time for the transfer reactions that occur during the polymerization of
polymerization of methyl methacrylate under microwave irradia- MA. Bovin and co-workers investigated the polymerization
tion with different monomer and initiator concentrations repres- of 4-nitrophenyl acrylate in a monomodal microwave
ented by different symbols. Reprinted with permission from reactor (Scheme 8b).[109] The synthesized poly(nitrophenyl
reference [102], Copyright Society of Chemical Industry.
acrylate)s (PNPAs) are interesting precursors for the
synthesis of polyacrylamides that can be obtained by the
Solutions of 5 mol percent initiator in styrene were irradi- reaction of PNPA with amino-functionalized compounds.
ated for two minutes in a domestic microwave oven at Faster polymerizations were observed using microwave
800 W. Significant polymerization within two minutes only irradiation instead of conventional heating. In addition, the
occurred with AIBN, tert-butyl peroxybenzoate, and tert- polymers from the microwave-supported synthesis had
amyl peroxybenzoate as initiators. In addition, the micro- significantly lower PDI values, which implies that the
wave-assisted and conventional polymerizations were termination of the polymer chains mainly proceeded by
comparable regarding the conversions and the molecular disproportionation instead of recombination, which is
weights of the polymers obtained. However, the micro- dominant in the case of thermal heating. Finally, the high

Scheme 8. Free radical polymerization under microwave irradiation: (a) simultaneous

polymerization and amidization of acrylamide; (b) polymerization of 4-nitrophenyl acrylate;
(c) fullerene-initiated polymerization of N-vinylcarbazole.

Macromol. Rapid Commun. 2004, 25, 1739–1764 www.mrc-journal.de ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
 Microwave-Assisted Polymer Synthesis: State-of-the-Art and Future Perspectives 1757

reproducibility of the microwave-assisted polymerizations

and the possibility of up-scaling without a change of the
reaction parameters were described.[109] Cai and co-work-
ers investigated the fullerene-initiated charge-transfer bulk
polymerization of N-vinylcarbazole (N-VC) under micro-
wave irradiation (Scheme 8c).[110] Fullerenes form
Cþ.
60  N-VC
.
ion-radical pairs that initiate the polymer-
ization of N-VC. Microwave-assisted polymerizations were
found to proceed much faster than conventional thermal
polymerizations. However, the authors admittedly mention-
ed that the main reason probably was the higher temperature
during the course of the microwave polymerizations.
Mattos and co-workers described the possibility of perform-
ing the free radical bulk polymerizations of vinyl acetate,
styrene, methyl methacrylate, and acrylonitrile utilizing a
domestic microwave oven.[111] The polymerizations pro-
ceeded at least 60 times faster (compared with conventional
heating) utilizing azodiisobutyronitrile (AIBN) as initiator.
Moreover, the polymerization of the highly absorbing
acrylonitrile could also be performed in the absence of
AIBN. Elsabee et al. reported the microwave-assisted
homopolymerization of N-p-bromophenylmaleimide Scheme 9. Solid-phase microwave-assisted copolymerization
(BrPMI) with AIBN.[112] The homopolymerization of this of (a) maleic anhydride with dibenzyl maleate, (b) maleic anhy-
solid monomer could not be performed under conventional dride and allylthiourea, (c) dibutyltin maleate and allyl thiourea,
heating at 135 8C with AIBN as initiator, but occurred and (d) dibutyltin maleate and stearic acid vinyl ester.
readily within 10 min using microwave heating.
Random bulk copolymerizations of hydroxyethyl metha- a series of solid-state random copolymerizations using
crylate HEMA and methyl methacrylate MMA were various maleate derivatives (Scheme 9).[115–118] For the
investigated by Rodriguez and coworkers.[113,114] The different combinations of monomers, the reactivity ratios
microwave-assisted polymerizations were finished within were determined. The copolymerization of maleic anhy-
45 min compared with 125 min using conventional heating dride with dibenzyl maleate (Scheme 9a) was performed by
as shown in Figure 13. In addition, the polymers synthesized a 32 s microwave irradiation (power-regulated to obtain
under microwave irradiation showed higher molecular 45 8C) of a ground mixture of the monomers.[115] Subse-
weights and lower PDI values than the thermally synthesized quently, the resulting copolymers were used for the pre-
polymers (microwave irradiation: 1.36–2.08, conventional paration of superabsorbent oil resins, obtained by the
heating: 4.1), although the microstructure and the physical copolymerization and cross-linking of a variety of acry-
properties of the polymers did not change. Lu et al. described lates. The copolymerization of ground maleic anhydride
and allylthiourea (Scheme 9b) yielded water-soluble
polymers with metal ion complexing abilities.[116] Both
the water solubility and the metal complexing capacities
could be influenced by changes in pH. The third inves-
tigated system, the copolymerization of dibutyltin maleate
and allyl thiourea (Scheme 9c), gave heat-stabilizing
organotin polymers.[117] The mechanism of the copolymer-
ization was investigated by the addition of a radical
scavenger directly after microwave irradiation. An increase
in microwave power led to an increase in radical concen-
tration. In addition, it was demonstrated that the polymer-
ization of this monomer combination did not occur in the
absence of initiators with conventional heating, whereas
the same procedure yielded polymers under microwave
Figure 13. Comparison of the monomer conversion as a function irradiation. Furthermore, a drastic increase in reaction
of time for the bulk copolymerization of MMA and HEMA under
both microwave irradiation (A) and conventional heating (B). speed from several hours down to a few minutes was
Reprinted with permission from reference [114], Copyright VSP observed for the solid-phase copolymerization of itaconic
International Science Publishers. acid and acrylamide, sodium acrylate and N,N-methylene

Macromol. Rapid Commun. 2004, 25, 1739–1764 www.mrc-journal.de ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1758 F. Wiesbrock, R. Hoogenboom, U. S. Schubert

4.2. Controlled Radical Polymerizations

With a few exceptions only, it is mainly the group of X. Zhu

who has transferred controlled radical polymerizations to
microwave irradiation. Wisnoski et al., for example, have
prepared novel resins in a monomodal microwave reactor
(SmithSynthesizer, Biotage).[121] A TEMPO-methyl resin
(TEMPO: 2,2,6,6-tetramethylpiperidine-N-oxyl) was react-
ed with various functionalized styrenes or 4-vinyl pyridine
at 185 8C for 10 min. The resin beads obtained were large
(>500 mm) and exhibited spherical shapes (Figure 15).
Hence, the authors strongly assumed a controlled radical
mechanism (involving the TEMPO radical) in the course of
the polymerization. The high-loading resins (>5.5 mmol 
g1) were prepared in the range of 10 min, which is in fact
Figure 14. Radical concentration as a function of microwave 150 times faster than with conventional heating. Further-
irradiation energy for the microwave polymerization of
allylthiouerea (AT) with and without additional carriers. Reprinted more, conventional heating (at an identical temperature of
with permission from reference [117], Copyright John Wiley and 185 8C) failed to yield resins of similar size, loading, or
Sons. uniform shape. Further investigations aimed at the deter-
mination of the reasons for this pronounced microwave
biacrylamide, acrylamide and maleic acid anhydride, and effect are in progress.
also for the copolymerization of dibutyltin maleate and With these findings in mind, it is interesting to note that
stearic acid vinyl ester (Scheme 9d).[118] To further improve Zelentzova et al. observed a re-initiation of the AIBN-
the microwave absorption, the effect of carriers was in- initiated (AIBN: 2,20 -azoisobutyronitrile) and TEMPO-
vestigated.[117–120] It was demonstrated that aluminum mediated radical polymerization of methyl methacrylate
oxide enhanced the reaction speed of the copolymerization under microwave irradiation at 70 8C.[122] The radical
of dibutyltin maleate and allyl thiourea more than silicon species were detected by means of electron spin resonance
oxide (Figure 14).[117,118] A more detailed study revealed (ESR) spectroscopy. The authors ascribe this observation to
that aluminum oxide (an alkaline carrier) was the best car- the decomposition of the intermediately formed nonradical
rier for acidic or neutral monomer combinations, and TEMPO adduct with the 2-cyanopropan-2-yl radical
silicon oxide (an acidic carrier) was better for alkaline (derived from AIBN).
systems.[119] This effect was ascribed to the higher Zhu and co-workers described the homogeneous atom
dielectric constant resulting from acid–base interactions. transfer radical polymerizations (ATRP) as well as the reverse
Another study performed by Lu and co-workers revealed atom transfer radical (solution) polymerizations (RATRP) of
that free radical polymerizations (2-ethylhexyl acrylate in methyl methacrylate with the following initiator/catalyst/
solution and solid-phase polymerization of acrylamide) in solvent systems: EBB/CuCl-PMDETA/DMF, PhCH2Cl/
the presence of silicon oxide or aluminum oxide proceeded CuCl-bpy/CCl4, pTsCl/CuSCN-PMDETA/CCl4, AIBN/
by a radical mechanism, whereas in the presence of CuBr2-bpy/ACN, AIBN/CuCl-bpy/ACN, AIBN/FeCl3-
magnesium oxide the same polymerizations occurred by PPh3/DMF, AIBN/CuBr2-TMEDA/DMF, and pTsCl/CuBr-
both radical and anionic mechanisms leading to grafting bpy/CCl4 (EBB: ethyl-2-bromobutyrate; PMDETA:
onto the magnesium oxide surface.[120] N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine; DMF:

Figure 15. Photographs of the TEMPO-methyl resin before (left) and after loading with
p-bromostyrene (middle) and a mix of m- and p-chloromethylstyrene (right). Reprinted with permission
from reference [121], Copyright Elsevier Science Ltd.

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 Microwave-Assisted Polymer Synthesis: State-of-the-Art and Future Perspectives 1759

N,N-dimethylformamide; bpy: 2,20 -bipyridine; AIBN: 2,20 - it was shown that for reaction temperatures higher than
azoisobutyronitrile; ACN: acetonitrile; TMEDA: tetra- 110 8C, the molecular weight distributions became sig-
methyethylenediamine; pTsCl: p-tosyl chloride).[123–130] nificantly broader (PDI > 1.4), exhibiting severe deviations
The temperatures of the reaction mixtures were adjusted, from an ideally controlled radical polymerization and
if appropriate, by the boiling points of the solvents in an thereby limiting the use of microwave reactors for the
open reflux system, or by placing the reactors into a ther- reaction conditions employed.
mostat bath. Especially for low concentrations of the
corresponding initiators and catalysts, the (modified)
4.3. Emulsion Polymerizations
domestic microwave reactor proved to be superior to con-
ventional heating in terms of reaction speed, molecular Zhu et al. performed the emulsion polymerization of
weights, and the narrowness of the molecular weight styrene with potassium persulfate as initiator and sodium
distributions. Influences of the microwave irradiation on the dodecyl sulfonate as emulsifier under conventional heating
glass temperatures or stereoregularities of the poly(methyl as well as under microwave irradiation.[134] To ensure a
methacrylate)s, however, were negligible.[130] The first- direct comparison of the polymerizations performed under
order kinetics for the monomer consumption were illu- the two different activation modes, the reactions in the
strated by the linear dependency of the ln{[M0]/[Mt]} modified domestic microwave oven with adjustable mean
values on reaction time. An explanation for the fivefold output power were performed at a constant temperature
increase in reaction rates, however, was not provided in (around 70 8C) that was held by a thermostat with tetra-
these references. Corresponding findings were also obtain- chloroethylene, which hardly absorbs microwave irradia-
ed for the atom transfer radical polymerization (ATRP) of tion (Figure 16). Compared with conventional heating, the
octyl acrylate with the initiator/catalyst/solvent system polymerization rate was effectively enhanced because of
EBB/CuCl,bpy/CCl4.[131] In a more recent publication, the increase of the decomposition rate of potassium
Zhu and co-workers performed the atom transfer radical persulfate by a factor of 2.4. Consequently, the amount of
polymerization (ATRP) of methyl methacrylate in hexane, particles formed within identical reaction times was higher
with the initiator a,a0 -dichloroxylene and the catalysts CuCl in the case of microwave activation. Although polymers
and PMDETA.[132] During the course of this polymeriza- with higher average molecular weights were formed under
tion, and by additional investigations that aimed at deter- microwave irradiation, almost identical glass-transition
mining the nature of the increase in reaction rates under temperatures and similar atacticity/isotacticity ratios of
microwave irradiation, it was observed that the dissociation around 75:25 (determined by 13C NMR spectroscopy) for
of CuCl and consequently the concentration of copper ions the polymers obtained from conventional as well as micro-
in solution were enhanced when the polymerization was wave heating exhibited that their physical properties and
performed in a microwave reactor. The authors attributed microstructures were almost independent of the type of
the increase in reaction speed observable for all the ATRP activation source. The corresponding findings were obtain-
reactions under microwave assistance to this finding. On ed for the emulsion polymerization of methyl methacrylate
this occasion, it should be pointed out that the use of
thermostat baths does not necessarily reveal the reaction
temperatures in the reaction vial, especially if solid ionic
particles are involved in the reaction. These particles optim-
ally absorb microwave irradiation because of its elec-
tromagnetic character (cf. Section 1). Consequently, the
noncontact heating in the microwave reactors might heat
the reaction mixture well beyond the temperature of the
thermostat bath. Hence, a direct comparison with findings
from reactions performed under conventional heating
would be no longer valid.
For the atom transfer radical polymerizations (ATRP)
of methyl methacrylate with the initiator/catalyst/solvent
systems EBIB/CuCl-NHPMI/p-xylene and EBIB/CuCl-
NHPMI/DMF, however, Zhang and Schubert observed no
acceleration with microwave irradiation [EBIB: ethyl-2-
bromo-isobutyrate, NHPMI: N-Hexyl-2-pyridylmethani-
mine].[133] To ensure a valid comparison of the experiments, Figure 16. Experimental setup for the emulsion polymerization
of styrene in a domestic microwave oven additionally equipped
the polymerizations were conducted under conventional with adjustable mean output power and a thermostatic bath.
heating and in a monomodal microwave reactor (Emrys Reprinted with permission from reference [134], Copyright John
Liberator, Biotage) at identical temperatures. Furthermore, Wiley and Sons.

Macromol. Rapid Commun. 2004, 25, 1739–1764 www.mrc-journal.de ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1760 F. Wiesbrock, R. Hoogenboom, U. S. Schubert

to the good absorption of the irradiation by the water

molecules and the resulting higher reaction temperatures.
Wu and co-workers prepared latex particles from styrene
and the co-monomers methyl methacrylate, butyl metha-
crylate, ethyl acrylate, and maleic anhydride by an emul-
sion polymerization in aqueous acetone with potassium
persulfate as initiator in a domestic microwave oven.[140]
The hydrodynamic radii were determined utilizing a modi-
fied commercial laser light scattering spectrometer and
found to have a narrow distribution. In fact, the hydro-
dynamic radii decreased with an increasing acetone content
in the reaction mixture because of the corresponding
decrease of interfacial tension. The concentration of the
comonomer also influenced the hydrodynamic radii. It was
mentioned that the preparation of such particles with
conventional heating requires longer reaction times and
Figure 17. Modification of a domestic microwave reactor in
order to conduct batch, semicontinuous, and continuous processes, yields particles with a broad distribution; the corresponding
utilized for the emulsion polymerization of styrene. Reprinted values or references, however, are not given.
with permission from reference [139], Copyright Elsevier Science Polymers that contain rare earth metal ions have been
Ltd. studied extensively because of their applications for lumi-
nescence or laser materials. For example, Zhang and co-
workers transferred the emulsion polymerization of methyl
under identical conditions (cf. reference [134] and methacrylate (in the presence as well as the absence of
Figure 17).[135] europium cations) to a modified domestic microwave
Similar experiments were studied by Palacios and oven.[141] Assuming identical temperatures in the control
Valverde in order to investigate the characteristics of the experiments under conventional heating, the reaction was
two different energy sources.[136] In control experiments found to proceed faster with microwave assistance, eluci-
performed at 50 8C, they found a decrease in reaction times dated by the dependency of the particle size on time. After
from 6 h to 8.3 min under microwave irradiation. For reaction times of 1 h, radii of 165 and 215 nm, respectively,
initiator concentrations lower than 1.5
102 mol  L1, were measured. After prolonged reaction times of 5 h,
the polymerization rates were effected by the corresponding however, the radii were almost identical at around 215 nm,
concentration, and the ratio of the corresponding ln values independent of the heating source. SEM images of those
{ln(rp)/ln[I]} was found to be constant at 0.28. From particles (recovered from reaction mixtures without euro-
literature data of the Smith–Ewart theory,[137,138] this ratio pium ions) consequently were similar to each other
was known to be at a constant value of 0.4; the authors (Figure 18a and b). The authors ascribe the higher reaction
ascribe this decrease from 0.4 to 0.28 to the facilitated rates in the case of microwave activation to the good
activation of the initiator by the microwave irradiation absorbance of the irradiation by the water molecules. In this
which renders this step of the mechanism less important. context, it is worth mentioning that Mülhaupt and co-
For this and above cited two experiments, it should be workers observed no difference in terms of molecular
stressed that the use of thermostatic baths does not reveal weights, polydispersities, stereoregularities, particle sizes,
the temperatures in the reaction mixture. Therefore, a direct and particle-size distributions between conventional and
comparison between microwave and conventional heating microwave heating for the dispersion polymerization of
is not (necessarily) valid, and the origin of the accelerated methyl methacrylate in heptane.[142] The corresponding
reaction rates, selective excitation of the ionic species experiments were performed in a specially equipped
versus higher reaction temperatures, cannot be deducted microwave reactor (cf. reference [78] and Figure 8). Zhang
reliably. and co-workers (cf. herein and above) also recorded SEM
Correa et al. performed the emulsion polymerization in a images of the PMMA particles obtained from europium
specially modified domestic microwave reactor that allow- containing reaction mixtures. They found a distribution of
ed for batch, semicontinuous, and continuous processes the europium ions mainly on the surface (Figure 18c), a
(Figure 17).[139] In order to overcome the risk of hazardous phenomenon that was explained by the fact that the octonate
explosions associated with prolonged irradiation times, a anion, with a hydrophobic alkyl chain, was used as counte-
rectangular pulse function was utilized to heat the reaction rion for the europium ion and prevented the hydrophilic
emulsion. A decrease in reaction times by a factor of 70 was trivalent cation to enter the interior of the particles.
observed for the emulsion polymerization performed under He et al. investigated the persulfate-initiated soapless
microwave irradiation. The authors ascribe this adavantage emulsion polymerization of butyl methacrylate in ethanol

Macromol. Rapid Commun. 2004, 25, 1739–1764 www.mrc-journal.de ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
 Microwave-Assisted Polymer Synthesis: State-of-the-Art and Future Perspectives 1761

Figure 18. SEM images of PMMA particles obtained from (a) conventional heating, (b)
microwave heating, and (c) microwave heating with europium ions present in the reaction
mixture. Reprinted with permission from reference [141], Copyright John Wiley and Sons.

under microwave irradiation.[143] Control experiments with irradiation (compared with conventional heating). The
conventional heating showed a tenfold increase in reaction enhanced reaction rates have been found to originate from
rates for the polymerization under microwave assistance, thermal effects (higher temperatures) for a large number of
originating from the accelerated decomposition of the these reactions. Concomitant with shorter reaction times,
persulfate anion. The influences of the monomer/initiator side reactions are reduced to a minimum, and consequently
amounts and the addition of ethanol on the monomer the purity and the polymer properties improve.
conversion, the particle sizes, and their distributions were For some reactions, however, it seems that these advan-
examined and found to be in full agreement with trends tages do not result from higher reaction temperatures, but
observable under conventional heating. from the selective excitation of one of the educts involved.
This might be especially true for ions and (zerovalent)
metals that are known to intrinsically absorb microwave
irradiation. Pronounced effects of that type are significantly
5. Conclusion and Outlook
related to the fields of metal catalysis (cf. C–C coupling
The use of microwave irradiation as a heating source for reactions) and reactions involving ionic species (cf. ring-
polymerization reactions is a rapidly growing branch in opening polymerizations). Shifts in selectivity, originating
polymer science as may be determined from the exponen- from the specific excitation by the microwave irradiation,
tial increase in the number of publications originating from have been observed as well (cf. the phase-transfer catalyzed
that field (Figure 2). With the advent of monomodal step-growth polymerization of sorbide and 1,8-dibromooc-
microwave reactors (Figure 1) at the beginning of the tane, for example).
millennium, the safety uncertainties, like fires or explosions In this respect, some of the findings and the correspond-
that are likely to accompany organic reactions under ing explanations might be taken into account: With
microwave irradiation, have been overcome. Controlling comparably few data on (monomodal) microwave-assisted
the temperature and pressure inside the reaction vials not polymerizations available these days, general conclusions,
only shifts the reliability of this type of reactors from valid for a whole series of (similar) educts or a specific
hazardous to safe, but also allows for a more detailed insight reaction type, cannot be drawn. Further problems might
into the run of the polymerization. also arise from the lack of knowledge of the actual reaction
So far, most of the polymerizations performed under conditions, such as temperature or pressure in the case of
microwave irradiation may be entitled as ‘‘well-estab- domestic microwave ovens (resulting from cold and hot
lished’’ polymerization reactions, including the step- spots), especially in the cases where thermostat baths are
growth preparation of polyamides, polyimides, polyethers, utilized, disallowing a direct comparison with conventional
and polyesters, the ring-opening polymerizations of (almost heating experiments.
exclusively) e-caprolactams and e-caprolactones, and the With the polymerizations reviewed in this article, how-
free radical polymerizations of some well-investigated ever, (monomodal) microwave reactors have been intro-
monomers like styrene or methyl methacrylate. More duced as powerful tools for the synthesis of polymers:
advanced techniques among the reactions investigated com- Reaction times are greatly shortened, paving the way to
prise the phase-transfer catalyzed polymerizations and C–C higher yields, improved selectivities, and environmentally
coupling reactions, as well as controlled radical and benign methods (shorter reaction times, low-boiling
emulsion polymerizations. solvents). Consequently, more data is very likely to be
For the vast majority of reactions, a significant increase in accumulated in the near future, probably focusing on more
reaction speed becomes discernible under microwave and more advanced techniques. A direct comparison with

Macromol. Rapid Commun. 2004, 25, 1739–1764 www.mrc-journal.de ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1762 F. Wiesbrock, R. Hoogenboom, U. S. Schubert

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