Elucidation of the alternating copolymerization mechanism of epoxides or aziridines with cyclic anhydrides in the presence of halide salts

Organic halide salts in combination with metal or Lewis acids are the most common and essential catalysts in ring-opening copolymerizations (ROCOP). However, the role of organic halide salts was neglected. Here, we have uncovered the complex behavior of organic halide in ROCOP of epoxides or aziridines with cyclic anhydrides. Coordination of the chain-end cations, electron-withdrawing effect, leaving ability of halide atoms, chain-end basicity/nucleophilicity, and terminal steric hindrance cause three types of side reactions: single-site transesterification, substitution, and elimination. Understanding the complex functions of organic halide salts in ROCOP led us to develop highly active and selective aminocyclopropenium chlorides as catalysts/initiators. Adjustable H-bonding interactions of aminocyclopropenium with propagating anions and epoxides create chain-end coordination processes that generate highly reactive carboxylate and highly selective alkoxide chain ends.