Asymmetric kinetic resolution polymerization of racemic lactide: enhanced enantiomorphic site control

Asymmetric kinetic resolution polymerization (AKRP) provides an ideal way to obtain highly isotactic polymers and optically pure monomers. However, highly enantioselective AKRP process is still rarely reported to date. And the AKRP of racemic lactide (rac-LA) remains a challenge. Here, in this paper, the AKRP of rac-LA was achieved by means of an enhanced enantiomorphic site control, delivering isotactic polylactide (PLA) and enantiomeric enriched lactide (LA) with the highest kinetic resolution coefficients (k_rel) to date. Different from common (Salen)Al complex, a series of new chiral (BisSalen)Al complexes were designed and synthesized to form a more confined chiral microenvironment and further improve specific discrimination of rac-LA. The NMR analysis, kinetic studies, the activation parameters and thermal properties of the resulting PLAs provided strong evidence for enhanced enantiomorphic site control. This work enables highly enantioselective AKRP of rac-LA.