Effect of the phenyl ring substituent on stereoselectivity in the ring-opening polymerization of the rac-lactide initiated by salen aluminum complexes

A number of unreported salen aluminum complexes bearing Schiff base ligands starting from (R,R)-1,2-diammoniumcyclohexane mono-(+)-tartrate salt were synthesized. These complexes were characterized by ¹H, ¹³C NMR, and elemental analysis. These complexes were employed as initiators for the ring-opening polymerization (ROP) of L-lactide and rac-lactide. Complex 3 (R = Br) showed the highest activity for the ROP of L-lactide among these complexes, and complex 2 (R = ⁱPr) possessed the highest stereoselectivity for the ROP of rac-lactide among these aluminum isopropoxides. The kinetics studies of the polymerization employed complex 2 as initiator indicated that the polymerization rate was first-ordered in lactide and initiator.