The mechanism of cationic polymerization is a kind of repetitive alkylation reaction.
Electron donating groups are needed as the R groups because these can stabilize the propagating species by resonance. Examples:
Propagation is usually very fast. Therefore, cationic vinyl polymerizations must often be run at low temperatures. Unfortunately, cooling large reactors is difficult and expensive. Also, the reaction can be inhibited by water if present in more than trace amounts, so careful drying of ingredients is necessary (another expense).
Cationic vinyl polymerization is plagued by numerous side reactions, most of which lead to chain transfer. It is difficult to achieve high MW because each initiator can give rise to many separate chains because of chain transfer. These side reactions can be minimized but not eliminated by running the reaction at low temperature. Here are a few examples:
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