Exploring ligand substituent effects on stereoselective polymerization of racemic lactide using aluminium salen-type complexes

Abstract

A number of unreported aluminium complexes bearing chiral cyclohexylsalen ligands were prepared and utilized for the asymmetric kinetic resolution polymerization (AKRP) of rac-lactide (LA). In this contribution, these chiral complexes displayed an interesting stereoselectivity-switch behavior toward rac-LA polymerization. (R,R)-Salen[6-Cl] displayed the highest activity among these complexes for the ROP of rac-LA, and (R,R)-salen[4,6-Br,^tBu] demonstrated excellent stereoselectivity for the ROP of rac-LA with enriched isotactic PLA (P_m = 0.88, CEC; P_m = 0.93, ESC). Kinetic study employing the complex (R,R)-salen[4,6-Br,^tBu] as an initiator revealed first-order dependence on monomer concentration with a rate constant ratio k_L/k_D = 30. Asymmetric kinetic resolution polymerization provides evidence that the enantiomorphic site control mechanism plays a major role in the ROP of rac-LA mediated by aluminium catalysts with a bulkier electron donor located near the metal center, whereas the ROP of rac-LA mediated by Al(salen) catalysts bearing electron-withdrawing groups mainly occurs via the chain end control mechanism.