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Ionic polymerization is a chain-growth polymerization in which the propagating chain end bears a formal charge and is paired with a counterion. Its two principal classes are anionic polymerization, propagated by carbanionic or heteroatom-anionic centers, and cationic polymerization, propagated by carbocations, oxonium ions, or related electrophilic centers. Monomer addition preserves the ionic chain carrier at the newly extended terminus. Counterion identity, solvent polarity, temperature, aggregation, and ion pairing govern propagation rate, regioselectivity, stereochemistry, transfer, and termination. The behavior of benzylic ions in azaborine-containing styrenes illustrates how ion stability controls ionic-polymerization reactivity ^[1]. The term excludes radical polymerization and coordination–insertion polymerization because their propagating centers are respectively odd-electron species and metal-bound insertion sites. Living cationic polymerization suppresses transfer and termination sufficiently to preserve electrophilic chain ends ^[2]. Living anionic polymerization applies the corresponding control principle to nucleophilic chain ends ^[3].