Heteroselective polymerization of rac‐lactide and syndioselective polymerization of meso‐lactide by chiral aluminum salan catalysts

Poly(lactic acid) (PLA) is a biodegradable polymer produced from lactide via ring opening polymerization (ROP). With two stereogenic centers, the lactide stereoisomers are either the homochiral L-LA and D-LA or meso-LA. Herein, We describe chiral salan-aluminum catalysts that follow the dual-stereocontrol mechanism: a given combination of stereogeneities at the metal site and the proximal center of the last inserted lactidyl (“match”) is active towards lactide having a proximal stereogenic center of the opposite configuration, while the diastereomeric combination of stereogeneities (“mismatch”) is inactive towards any lactide. Consequently, Polymerization of rac-LA by the enantiomerically pure catalysts was sluggish and gave stereoirregular poly(lactic acid) (PLA) because selective insertion to a match diastereomer gives a mismatch diastereomer, while the racemic catalysts showed higher activity and led to highly heterotactic PLA following polymeryl exchange between two mismatched catalyst enantiomers. In contrast, the switching of distal and proximal stereogenic centers following a monomer insertion should turn meso-LA into the ideal substrate for these catalysts, because the selective insertion into a match combination leads to a new match combination. Indeed, these catalysts polymerized meso-lactide much faster than rac-lactide and led to the elusive highly syndiotactic PLA (α=0.96). This polymer featured a T_m of 153 °C matching the highest reported value, and the highest crystallinity (ΔH_m=56 J g⁻¹) ever reported for syndiotactic PLA. With the availability of meso-LA as a byproduct of the industrial production of L-LA and the potentially attractive properties of the highly syndiotactic PLA we found, the search for new catalysts operating by the dual-stereocontrol mechanism is an exciting challenge.