Ring-opening polymerization of lactide using chiral salen aluminum complexes as initiators: high productivity and stereoselectivity

A family of aluminum complexes bearing chiral salen ligands derived from (R,R)-1,2-diammonium-cyclohexane mono-(+)-tartrate salt and modified by salicylaldehyde were prepared. The complexes were characterized by H-1, C-13 NMR and elemental analysis. These complexes were used as initiators for the ring-opening polymerization (ROP) of L-lactide and rac-lactide. Complex 2 (R-1 = R-2 = Cl) showed the highest activity among these complexes for the ROP of L-lactide, and complex 3 (R-1 = Bu-t, R-2 = Cl) showed the highest stereoselectivity for the ROP of rac-lactide with enriched isotactic polylactide with a P-m of 0.91. The kinetic data of the polymerization employing complex 3 as initiator indicated that the polymeric rate was first-order in both lactide and in initiator.