Cooperative polymerization of racemic lactide by catalyst enantiomers

Poly(lactic acid) (PLA) is a biodegradable polymer produced from lactide via ring opening polymerization (ROP). Recently, we introduced an aluminium catalyst based on a chiral salan ligand featuring a bipyrrolidine bridge. This catalyst exhibited a unique stereocontrol in ROP of racemic lactide: the enantiomerically-pure catalyst led to atactic PLA, whereas the racemic catalyst led to a highly heterotactic PLA - a microstructure consisting of alternating insertion of the two lactide enantiomers. This microstructure stems from cooperation between the two catalyst enantiomers that exchange polymeryl chains after each monomer insertion step. In this work, we explored these findings. New bipyrrolidine based salan ligands featuring different substituents on the phenol rings were designed and their corresponding aluminium complexes were prepared. We found that our original findings could be reproduced, namely, that the enantiomerically-pure catalysts led to atactic PLA, while the racemic catalysts led to heerotactic PLA, and that the degree of the heterotacticity could be tuned by the bulk of the phenolate substituents. The implications of these findings in related polymerizations will be presented.