Cationic polymerization

Monomer scope for cationic polymerization is limited to two main types: alkene and heterocyclic monomers. Cationic polymerization of both types of monomers occurs only if the overall reaction is thermally favorable. In the case of alkenes, this is due to isomerization of the monomer double bond; for heterocycles, this is due to release of monomer ring strain and, in some cases, isomerization of repeating units. Monomers for cationic polymerization are nucleophilic and form a stable cation upon polymerization. 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. ==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. Classical protic acids Strong protic acids can be used to form a cationic initiating species. High concentrations of the acid are needed in order to produce sufficient quantities of the cationic species. The counterion (A−) produced must be weakly nucleophilic so as to prevent early termination due to combination with the protonated alkene. The cation source can be water, alcohols, or even a carbocation donor such as an ester or an anhydride. In these systems the Lewis acid is referred to as a coinitiator while the cation source is the initiator. Upon reaction of the initiator with the coinitiator, an intermediate complex is formed which then goes on to react with the monomer unit. The counterion produced by the initiator-coinitiator complex is less nucleophilic than that of the Brønsted acid A− counterion. Halogens, such as chlorine and bromine, can also initiate cationic polymerization upon addition of the more active Lewis acids. 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. In this process, the growing chain is terminated, but the initiator-coinitiator complex is regenerated to initiate more chains. 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. Cationic ring-opening polymerization Cationic ring-opening polymerization follows the same mechanistic steps of initiation, propagation, and termination. However, in this polymerization reaction, the monomer units are cyclic in comparison to the resulting polymer chains which are linear. The linear polymers produced can have low ceiling temperatures, hence end-capping of the polymer chains is often necessary to prevent depolymerization. ==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: :\begin{align} \ce{{I+} + M}\ &\ce{->[{k_{i}}] M+} \\ \ce{{M+} + M}\ &\ce{->[{k_{p}}] M+} \\ \ce{M+}\ & \ce{->[{k_{t}}] M} \\ \ce{{M+} + M}\ &\ce{->[{k_{tr}}] {M} + M+} \end{align} In which I+ is the initiator, M is the monomer, M+ is the propagating center, and \mathit{k_i}, \mathit{k_p}, \mathit{k_t}, and \mathit{k_{tr}} are the rate constants for initiation, propagation, termination, and chain transfer, respectively. 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: :\begin{align} \text{rate(initiation)} &= k_i[\ce{I+}] [\text{M}] \\ \text{rate(propagation)} &= k_p[\ce{M+}] [\text{M}] \\ \text{rate(termination)} &= k_t[\ce{M+}] \\ \text{rate(chain transfer)} &= k_{tr}[\ce{M+}] [\text{M}] \end{align} Assuming steady-state conditions, i.e. the rate of initiation = rate of termination: : [\ce{M+}] = {k_i[\ce{I+}] [\ce{M}] \over k_t} This equation for [M+] can then be used in the equation for the rate of propagation: :\text{rate(propagation)} = {k_p k_i[\ce{M}]^2[\ce{I+}] \over k_t} From this equation, it is seen that propagation rate increases with increasing monomer and initiator concentration. The degree of polymerization, \mathit{X_n}, can be determined from the rates of propagation and termination: :X_n = {\text{rate(propagation)} \over \text{rate(termination)}} = {k_p[\text{M}] \over k_t} If chain transfer rather than termination is dominant, the equation for \mathit{X_n} becomes :X_n = {\text{rate(propagation)} \over \text{rate(chain transfer)}} = {k_p \over k_{tr}} ==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. ==Commercial applications==

The largest commercial application of cationic polymerization is in the production of polyisobutylene (PIB) products which include polybutene and butyl rubber. These polymers have a variety of applications from adhesives and sealants to protective gloves and pharmaceutical stoppers. The reaction conditions for the synthesis of each type of isobutylene product vary depending on the desired molecular weight and what type(s) of monomer(s) is used. The conditions most commonly used to form low molecular weight (5–10 × 104 Da) polyisobutylene are initiation with AlCl3, BF3, or TiCl4 at a temperature range of −40 to 10 °C. 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. Polybutene is another copolymer, containing roughly 80% isobutylene and 20% other butenes (usually 1-butene). The production of these low molecular weight polymers (300–2500 Da) is done within a large range of temperatures (−45 to 80 °C) with AlCl3 or BF3. Depending on the molecular weight of these polymers, they can be used as adhesives, sealants, plasticizers, additives for transmission fluids, and a variety of other applications. These materials are low-cost and are made by a variety of different companies including BP Chemicals, Esso, and BASF. Other polymers formed by cationic polymerization are homopolymers and copolymers of polyterpenes, such as pinenes (plant-derived products), that are used as tackifiers. In the field of heterocycles, 1,3,5-trioxane is copolymerized with small amounts of ethylene oxide to form the highly crystalline polyoxymethylene plastic. Also, the homopolymerization of alkyl vinyl ethers is achieved only by cationic polymerization. ==References==