Mechanistic insight into the stereochemical control of lactide polymerization by salan-aluminum catalysts

Alkyl aluminum complexes of chiral salan ligands assembled around the 2,2′-bipyrrolidine core form as single diastereomers that have identical configurations of the N donors. Active catalysts for the polymerization of lactide were formed upon the addition of benzyl alcohol. Polymeryl exchange between enantiomorphous aluminum species had a dramatic effect on the tacticity of the poly(lactic acid) (PLA) in the polymerization of racemic lactide (rac-LA): The enantiomerically pure catalyst of the nonsubstituted salan ligand led to isotactic PLA, and the racemic catalyst exhibited lower stereocontrol. The enantiomerically pure catalyst of the chloro-substituted salan ligand led to PLA with a slight tendency toward heterotacticity, whereas the racemic catalyst led to PLA of almost perfect heterotacticity following an insertion/auto-inhibition/exchange mechanism. The importance of being (diastereomerically) earnest: Chiral 2,2′-bipyrrolidine-based salan ligands give Al complexes as single diastereomers, which are active catalysts for stereoselective polymerization of racemic lactide. The heterotacticity of the obtained poly(lactic acid) increased dramatically upon increasing the enantiomeric impurity of the catalyst as the result of an insertion/auto-inhibition/exchange mechanism.