Alternating copolymerization of l-lactide and ε-caprolactone via enantiomorphic site and chain-end synergistic control

Abstract

The alternating copolymerization of chiral and achiral cyclic esters, such as commercial L-lactide (L-LA) and ε-caprolactone (CL), remains a significant challenge in polymer synthesis. Here, we introduce an enantiomorphic site and chain-end synergistic control strategy to achieve the first highly alternating copolymerization of L-LA and CL. The polymerization proceeds via a dynamic mechanism: first, the chiral L-LA-terminated chain end and the enantiomorphic site of the catalyst synergistically preclude the homopropagation of highly reactive L-LA; subsequently, the rate-determining insertion of the less reactive achiral CL after the L-LA-terminated chain end, generating an achiral CL-terminated chain end, relieves steric hindrance, which allows the subsequent insertion of highly reactive L-LA. This dynamic control yields highly alternating poly(L-LA-alt-CL) (P_alt up to 0.91). The highly alternating copolymerization of L-ethylglycolide (L-EG) and CL (P_alt = 0.96) further validates the universality of this synergistic control for constructing alternating cyclic ester copolymers.