Assignment for Chemical Kinetics
Kinetics of Cationic Polymerization
Submitted to: Dr. Erum Jabeen
Ahmad Saeed
BX-500148
Cationic polymerization:
A type of chain growth polymerization in which a cationic initiator transfers
charge to a monomer which then becomes reactive. This reactive monomer
goes on to react similarly with other monomers to form a polymer. The types
of monomers important for cationic polymerization are limited to alkenes
with electron-donating substituents and heterocycles. Similar to anionic
polymerization reactions, cationic polymerization reactions are very sensitive
to the type of solvent used. Specifically, the ability of a solvent to form free
ions will show the reactivity of the propagating cationic chain.
Difference between Anionic and Cationic polymerization
Some differences between cationic and anionic polymerization:
The most important and the basic difference between a cationic and anionic
polymerization is the generation of active centers.
Anionic active center Cationic active center
• Rates for cationic polymerization are faster (1 or more orders of magnitude
faster than anionic or free radical)
• Cationic polymerization reactions are very reactive, difficult to control and
stabilize:
→ more transfer occurs
→ more side reactions
→ more difficult to form “living” systems
→ hard to make polymers with low PDI or block copolymers
• Living cationic polymerization reactions are only possible for a specific
subset of monomers.
• Most industrial cationic processes are not living.
Living Process:
In polymer chemistry, living polymerization is a form of chain growth
polymerization in which the ability of a growing polymer
chain to terminate has been removed. Living polymerization is desirable
because it offers precision and control in macromolecular synthesis. This is
important since many of the advantageous properties of polymers result from
their structure and molecular weight. Since molecular
weight and dispersity are less controlled in non-living polymerizations, this
method is more desirable for designing materials of different kinds.
Kinetics of Cationic Polymerizations:
The ionic polymerizations possess free radical mechanism for their
propagation/establishment.
Free radical mechanisms comprise of three basic steps: (1) Initiation (2) Propagation (3)
Termination.
But for the Initiation of cationic polymerization reaction, active center with a cationic charge
is required which is usually provided by Lewis acids.
Lewis acids / Friedel-Crafts catalysts:
Lewis acids are the most common compounds used for initiation of cationic
polymerization. The more popular Lewis acids are SnCl4, AlCl3 and BF3 etc.
Although these Lewis acids alone can also induce polymerization itself, the
reaction occurs much faster with a suitable cation source. The cation / co-
catalyst can be water, alcohols, or even a carbocation donor such as an ester or
an anhydride etc.
If we take;
AlCl3 = Lewis acid
Cation source or Co-catalyst = HCl
Monomer = M
M+ = Propagating monomer chain
K = Rate constant for catalyst co-catalyst reaction.
K(i) = Rate constant for initiation step.
K(p) = Rate constant for propagation step.
K(t) = Rate constant for termination step.
Reaction of catalyst and co-catalyst
K
HCl + AlCl3 ⇌ H+(AlCl4)-
Rate constant (K) = ¿¿
Rearranging: H+(AlCl4)- = K [HCl] [AlCl3] -----{1}
Initiation:
K (i)
H+(AlCl4)- + M → HM+(AlCl4)-
Rate ( Ri ) = Ki [H+(AlCl4)- ][M] ----------{2}
Putting equation 1 in equation 2, we get;
Ri = Ki K [HCl] [AlCl3] [M] ----------- {3}
Propagation:
K (p)
HM+(AlCl4)- + M → HM+M(AlCl4)-
Rate ( Rp ) = Kp [ M+] [M] -----------{4}
Where M+ means the propagating chain.
Termination:
K (t)
HM+M(x-1) (AlCl4)- + M → Mx + HM+(AlCl4)-
Rate (Rt) = Kt [ M+] [M] ------------{5}
At Steady State
Rate of initiation = Rate of Termination
By using equations {3} and {5},
Ri =Rt
Ki K [HCl] [AlCl3] [M] = Kt [ M+] [M]
By rearranging and solving,
Ki K
[ M+] = Kt [HCl] [AlCl3] ---------{6}
Inserting equation {6} in equation {4},
K i Kp K
Rp = Kt [HCl] [AlCl3] [M]
Here, we can predict kinetically that the rate of propagation of
cationic polymerization depends on the concentration of catalyst as
well as the Monomer.
Degree of Polymerization
Rp
Xn = Rt
Putting values from equations {4} and {5}
Kp
= K p¿¿ = Kt ------------------{7}
According to equation 7 we are able to conclude that the
degree of polymerization is independent of the concentration
of monomers as well as catalysts.
It only depends on the rate constants for the propagation
and termination steps.
Uses of Cationic Polymerization Kinetics:
The largest commercial application of cationic polymerization is in
the production of polyisobutylene products which
include polybutene and butyl rubber. These polymers are having a
variety of applications from adhesives and sealants to protective
gloves and pharmaceutical stoppers etc. The reaction conditions for
the synthesis of every type of isobutylene product vary depending on
the desired molecular weight and what type(s) of monomer(s) is
used.
1. Butyl Rubber Polymerization:
Butyl rubber polymerization is carried out as a continuous process
with AlCl3 as the initiator. Its low gas permeability and good
resistance to chemicals make it advantageous for a variety of
applications such as protective gloves, electrical cable insulation, and
even basketballs. Large scale production of butyl rubber started
during World War II, and roughly 1 billion pounds per year are
produced in the U.S. as of today.
2. Polymerization of butene:
Polybutene is another copolymer, containing roughly 80%
isobutylene and 20% other butenes (usually 1-butene). The
production of these low molecular weight polymers is done within a
large range of temperatures (−45 to 80 °C) with AlCl3 etc. Depending
on the molecular weight of these polymers, they can be used as
adhesives, sealants, plasticizers, additives for transmission fluids, and
a variety of many other applications.
3. Polymerization of Terpenes:
Other polymers formed by cationic polymerization are
homopolymers and copolymers of poly-terpenes, such as plant
derived products.
Polymer of Terpene
4. Polymerization of Alkyl Vinyl Ethers:
The homo-polymerization of alkyl vinyl ethers is accomplished only
by cationic polymerization.
Alkyl Vinyl Ethers
