Tunable Copolymerization of Oxetane and Anhydrides via a Nonsymmetric Three-Center Four-Electron Iodine Activation...

The copolymerization of low-strain cyclic ethers, such as oxetane and anhydrides, still presents opportunities for improving compositional control in polymer synthesis. In this study, we report a halogen-bond catalysis platform based on a nonsymmetric three-center four-electron ([O···I···O]⁺) iodine activation mode that enables highly selective and tunable copolymerization of oxetane and anhydride. This active catalytic structure mediates oxetane insertion through a dynamic equilibrium between nonsymmetric and symmetric iodine-carboxylate complexes ([O__oxetane···I···O__carboxylate]⁺ versus [O__carboxylate···I···O__carboxylate]⁺), thereby regulating ester and ether linkage ratio. Mechanistic study reveals that nonsymmetric iodine activation enforces near-perfect alternating copolymer (>99 mol% selectivity in ester linkage) between oxetane and cyclic anhydrides. Notably, the catalytic system exhibits broad monomer scope, enabling the copolymerization of oxetane with a variety of cyclic anhydrides to yield structurally diverse polyesters. Furthermore, by tuning the carboxylate/I₂ ratio, systematic control over the ester-to-ether content in the resulting polyesters can be achieved. This work establishes halogen-mediated nonsymmetric activation as a versatile and programmable platform for constructing composition-defined polyesters from low-strain cyclic ethers.