Cationic polymerization

Cationic polymerization is a chain-growth process in which positively charged active chain ends add monomers, commonly alkenes or cyclic ethers.

Carbocation: A molecular ion in which a carbon atom bears a positive charge. Many cationic chains grow from carbocationic centers that add monomer molecules.
Alkene: A hydrocarbon containing at least one carbon–carbon double bond. Electron-rich alkenes are common monomers because a cation can add across their double bond.
Polyisobutylene: A polymer of isobutylene valued for its low gas permeability and chemical stability. Cationic polymerization is the principal route to polyisobutylene and its controlled molecular weights.
Anionic polymerization: A chain-growth polymerization in which negatively charged chain ends add monomers. It uses nucleophilic anionic centers rather than the electrophilic cations that define this process.
Initiation (polymer chemistry): The step that creates an active center capable of starting a polymer chain. Initiation generates the positively charged species that begins cationic chain growth.
Vinyl ether: An alkene with an oxygen atom bonded directly to one of its double-bond carbons. Its oxygen substituent stabilizes the cation formed during propagation.
Butyl rubber: An elastomer made mainly from isobutylene with a small amount of isoprene. Its isobutylene-rich backbone is produced by low-temperature cationic copolymerization.
Free-radical polymerization: A chain-growth polymerization in which radical chain ends add monomers. Radical centers offer a different route to chain growth and tolerate different monomer substituents.
Propagation (polymer chemistry): The repeated addition of monomer units to an active polymer chain end. Each propagation step extends the chain while preserving its cationic active center.
Isobutylene: A branched alkene, also called 2-methylpropene, with formula C₄H₈. Its substituted double bond supports cationic polymerization to polyisobutylene.

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