Asymmetric Kinetic Resolution Polymerization of Racemic Lactide Mediated by Axial‐Chiral Thiourea/Phosphazene Binary Organocatalyst

Abstract

AbstractAsymmetric kinetic resolution polymerization (AKRP) provides an ideal way to obtain highly isotactic polylactide (PLA) with superior thermal‐mechanical properties from racemic lactide (rac‐LA). However, the development of a new catalytic system with concurrent high activity and selectivity at ambient temperature remains a great callenge. Here, a series of simple and effective binary organocatalytic pairs containing axial‐chiral thioureas and commercially available phosphazene bases were designed. These chiral binary organocatalytic pairs allow for both high polymerization activity and moderate enantioselectivity for AKRP of rac‐LA at room temperature, yielding semi‐crystalline and metal‐free stereoblock PLA with a melting temperature as high as 186 °C. The highest kinetic resolution coefficient (krel) of 8.5 at 47 % conversion was obtained, and D‐LA was preferentially polymerized via kinetic resolution with a maximum selectivity factor (kD/kL) of 18.1, indicating that an enantiomorphic site control mechanism (ESC) was involved.