Cationic polymerization
cationic polymerization: An ionic polymerization in which the kinetic-chain carriers are cations.[1]
In
Monomers
Monomer scope for cationic polymerization is limited to two main types:
Alkenes
Cationic polymerization of olefin monomers occurs with olefins that contain electron-donating substituents. These electron-donating groups make the olefin nucleophilic enough to attack electrophilic initiators or growing polymer chains. At the same time, these electron-donating groups attached to the monomer must be able to stabilize the resulting cationic charge for further polymerization. Some reactive olefin monomers are shown below in order of decreasing reactivity, with heteroatom groups being more reactive than alkyl or aryl groups. Note, however, that the reactivity of the carbenium ion formed is the opposite of the monomer reactivity.[5]
Heterocyclic monomers
Heterocyclic monomers that are cationically polymerized are
Synthesis
Initiation
Initiation is the first step in cationic polymerization. During initiation, a carbenium ion is generated from which the polymer chain is made. The counterion should be non-nucleophilic, otherwise the reaction is terminated instantaneously. There are a variety of initiators available for cationic polymerization, and some of them require a coinitiator to generate the needed cationic species.[7]
Classical protic acids
Strong
Lewis acids/Friedel-Crafts catalysts
Carbenium ion salts
Stable carbenium ions are used to initiate chain growth of only the most reactive alkenes and are known to give well defined structures. These initiators are most often used in kinetic studies due to the ease of measuring the disappearance of the carbenium ion absorbance. Common carbenium ions are
Ionizing radiation
Ionizing radiation can form a radical-cation pair that can then react with a monomer to start cationic polymerization. Control of the radical-cation pairs is difficult and often depends on the monomer and reaction conditions. Formation of radical and anionic species is often observed.[5]
Propagation
Propagation proceeds by addition of monomer to the active species, i.e. the carbenium ion. The monomer is added to the growing chain in a head-to-tail fashion; in the process, the cationic end group is regenerated to allow for the next round of monomer addition.[6]
Effect of temperature
The temperature of the reaction has an effect on the rate of propagation. The overall activation energy for the polymerization () is based upon the activation energies for the initiation (), propagation (), and termination () steps:
Generally, is larger than the sum of and , meaning the overall activation energy is negative. When this is the case, a decrease in temperature leads to an increase in the rate of propagation. The converse is true when the overall activation energy is positive.[6]
Chain length is also affected by temperature. Low reaction temperatures, in the range of 170–190 K, are preferred for producing longer chains.[6] This comes as a result of the activation energy for termination and other side reactions being larger than the activation energy for propagation.[5][6] As the temperature is raised, the energy barrier for the termination reaction is overcome, causing shorter chains to be produced during the polymerization process.[6]
Effect of solvent and counterion
The solvent and the counterion (the gegen ion) have a significant effect on the rate of propagation. The counterion and the carbenium ion can have different associations according to intimate ion pair theory; ranging from a covalent bond, tight ion pair (unseparated), solvent-separated ion pair (partially separated), and free ions (completely dissociated).[2][6]
The association is strongest as a covalent bond and weakest when the pair exists as free ions.[6] In cationic polymerization, the ions tend to be in equilibrium between an ion pair (either tight or solvent-separated) and free ions.[2] The more polar the solvent used in the reaction, the better the solvation and separation of the ions. Since free ions are more reactive than ion pairs, the rate of propagation is faster in more polar solvents.[6][8]
The size of the counterion is also a factor. A smaller counterion, with a higher charge density, will have stronger electrostatic interactions with the carbenium ion than will a larger counterion which has a lower charge density.[2] Further, a smaller counterion is more easily solvated by a polar solvent than a counterion with low charge density. The result is increased propagation rate with increased solvating capability of the solvent.[6]
Termination
Termination generally occurs by unimolecular rearrangement with the counterion. In this process, an anionic fragment of the counterion combines with the propagating chain end. This not only inactivates the growing chain, but it also terminates the kinetic chain by reducing the concentration of the initiator-coinitiator complex.[2][6]
Chain transfer
Chain transfer can take place in two ways. One method of chain transfer is hydrogen abstraction from the active chain end to the counterion.[6][8][9] In this process, the growing chain is terminated, but the initiator-coinitiator complex is regenerated to initiate more chains.[5][6]
The second method involves hydrogen abstraction from the active chain end to the monomer. This terminates the growing chain and also forms a new active carbenium ion-counterion complex which can continue to propagate, thus keeping the kinetic chain intact.[6]
Cationic ring-opening polymerization
Cationic
Kinetics
The rate of propagation and the degree of polymerization can be determined from an analysis of the kinetics of the polymerization. The reaction equations for initiation, propagation, termination, and chain transfer can be written in a general form:
In which I+ is the initiator, M is the monomer, M+ is the propagating center, and , , , and are the rate constants for initiation, propagation, termination, and chain transfer, respectively.[5][6][10] For simplicity, counterions are not shown in the above reaction equations and only chain transfer to monomer is considered. The resulting rate equations are as follows, where brackets denote concentrations:
Assuming steady-state conditions, i.e. the rate of initiation = rate of termination:[6][10]
This equation for [M+] can then be used in the equation for the rate of propagation:[6][10]
From this equation, it is seen that propagation rate increases with increasing monomer and initiator concentration.
The degree of polymerization, , can be determined from the rates of propagation and termination:[6][10]
If chain transfer rather than termination is dominant, the equation for becomes[6][10]
Living polymerization
In 1984, Higashimura and Sawamoto reported the first living cationic polymerization for alkyl vinyl ethers. This type of polymerization has allowed for the control of well-defined polymers. A key characteristic of living cationic polymerization is that termination is essentially eliminated, thus the cationic chain growth continues until all monomer is consumed.[11]
Commercial applications
The largest commercial application of cationic polymerization is in the production of polyisobutylene (PIB) products which include
Butyl rubber, in contrast to PIB, is a copolymer in which the monomers isobutylene (~98%) and isoprene (2%) are polymerized in a process similar to high molecular weight PIBs. Butyl rubber polymerization is carried out as a continuous process with AlCl3 as the initiator. Its low gas permeability and good resistance to chemicals and aging make it useful 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/year are produced in the U.S. today.[2]
Polybutene is another copolymer, containing roughly 80% isobutylene and 20% other butenes (usually
Other polymers formed by cationic polymerization are homopolymers and
References
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- ^ ISBN 978-0-471-27400-1.
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- ^ Robello, Douglas R. (2002). "Chem 421: Introduction to Polymer Chemistry – Cationic Polymerization". Department of Chemistry, University of Rochester. Archived from the original on 20 July 2011. Retrieved 20 March 2011.
- ^ ISBN 978-0-8247-9463-7.
- ^ ISBN 978-0-8493-9813-1.
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