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

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 P-m 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 linear dependence between the rac-lactide conversion and the number-average molecular weight of the macromolecules. The PDI values of these macromolecules were in a narrow range (1.04-1.09).