Stereoselective Ring-Opening Polymerization of rac-Lactides Catalyzed by Chiral and Achiral Aluminum Half-Salen Complexes

A new series of aluminum half-salen complexes have been synthesized from either chiral or achiral tridentate Schiff base ligands derived from amino alcohols or amino acids. All aluminum complexes have been shown to be active catalysts for the ring-opening polymerization (ROP) of rac-lactide in toluene at 70 degrees C, producing polylactides with controlled molecular weights and narrow molecular weight distributions. Both chiral and achiral aluminum complexes showed moderate selectivity to the ROP of rac-lactide to produce isotactic polylactide with a P-m value up to 0.76. In addition, epimerization of rac-lactide to meso-lactide was observed during the polymerization process for some of the complexes studied.