Enantioselective polymerization of racemic lactide for stereocomplex poly(lactic acid)

Abstract

Poly(lactic acid) (PLA) is a leading degradable and biocompatible polymer in the plastics industry. Stereocomplex PLA—a mixture of poly(l-lactic acid) and poly(d-lactic acid)—exhibits enhanced mechanical toughness and an elevated melting temperature compared to other PLA stereoisomers. However, the lack of highly enantioselective catalysts has prevented the single-step production of stereocomplex PLA from inexpensive racemic lactide. This work presents the discovery of chiral aluminum catalysts that are highly active for enantioselective lactide polymerization. Using a mixture of chiral catalysts with opposite enantioselectivities allowed for the single-step production of highly isotactic stereocomplex PLA from racemic lactide. The obtained stereocomplex PLA was tougher and more ductile than poly(l-lactic acid), stereoblock PLA, and even conventionally blended stereocomplex PLA. Computational studies revealed that the enantiocontrol exerted by the bimetallic aluminum complexes arises from dispersion interactions between the ligand and lactide within the catalyst cleft.