Oscillatory adaptive catalysis: Intramolecular chain shuttling regulated by stereo-autocorrection in stereoselective polymerization of lactide

Abstract

In metal-mediated stereoselective polymerization of cyclic esters, racemic catalysts use their enantiomers for enantioselective roles and achieve diverse polymer stereomicrostructures through intermolecular chain exchange. Here, an intramolecular chain shuttling mechanism is achieved by the combination of cis (R,S)-dithiourea and MeOK to overcome limitations on intermolecular polymer exchange and also offer diverse polymer stereomicrostructures. This system exhibits diastereospecificity toward the polymerization of both rac-lactide (rac-LA) and meso-LA, producing highly isotactic PLA (P_m ∼ 0.96) and heterotactic PLA (P_r ∼ 0.92), respectively. Mechanistic studies reveal an “oscillatory adaptive catalysis” (OAC) phenomenon, which is key to achieving dual recognition of the chirality of both the chain end and incoming monomer by using the two switchable chiral centers in catalyst. Such OAC enables dynamic interchange between chiral recognition (that triggers chain propagation) and stereochemical autocorrection (when monomer mismatched) by multi-site cooperativity, which induces chiral-site switching and polymer-chain shuttling intramolecularly within a single catalyst molecule.