Cationic Polymerization

Typical catalyst for cationic polymerization include, in order of importance, aprotonic acids (Lewis acids and Friedel Craft halides), protonic (Bronsted) acids, and stable carbenium ion salts. All these are strong electron acceptors. Many of them, particularly the Lewis acids, require a cocatalyst, usually a Lewis base or other proton donor, to initiate polymerization. Those monomers with electron donating 1,1-substituents that can form stable carbenium ions are polymerized by cationic mechanism.

High rate of polymerization at low temperatures is a characteristic of ionic polymerizations. It is often difficult to establish uniform reaction conditions before the reactants are consumed. The polymerization of isobutylene by AICI or BF, takes place within a few seconds at 100 ^oC, producing of molecular weight up to several million. Both rate and molecular weight are much lower at room temperature. These consideration limit the industrial application of cationic polymerization to the above mentioned example.

The most satisfactory of cationic polymerizations involved the carbenium ion as the chain carrier. For example, in the polymerization of isobutylene with boron trifloride catalyst, the first step is the reaction of the catalyst and cocatalyst, for example water, to form a catalyst-cocatalyst complex, that donate proton to an isobutylene molecule to give a carbenium ion, CH₃C. This ion then react with monomer with the reformation of carbenium ion at the end of each step. The "head-to-tail" addition of monomer to ion is the only one possible for energetic reason. Since the reaction in general carried out in a hydrocarbon medium of low dielectric constant, the anion and the growing cationic end form an ion pair.