Stereoselective Ring-Opening Polymerization of rac-Lactides Catalyzed by Aluminum Hemi-Salen Complexes

Abstract

A series of unreported aluminum complexes supported by asymmetrically O,N,N,O-quadridentate hemi-salen ligands were synthesized by binaphthyl-imine derivatives. These complexes were characterized and used as catalysts in rac-lactide or l-lactide polymerization. The X-ray diffraction analysis showed that molecular structures of (S)-3 and (rac)-4 were mononuclear coordination compounds with five-coordinated aluminum atoms in the solid state. Using 2-propanol as cocatalyst, complex (S)-6 revealed the highest activity among these aluminum coordination compounds toward the ring-opening polymerization of l-lactide, and complex (S)-2 displayed the highest stereospecificity for the ring-opening polymerization of rac-lactide, affording partially isotactic polylactide with a Pm of 0.64. The polymerization kinetics using (S)-6 as a catalyst were investigated at great lengths. The kinetics of the polymerization consequences proved that the polymerization was first-order in monomer as well as catalyst. There was a lin...