Stereoselective Ring-Opening Polymerization of <i>rac</i>-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 <i>rac</i>-lactide or l-lactide polymerization. The X-ray diffraction analysis showed that molecular structures of <b>(</b><i><b>S</b></i><b>)-3</b> and <b>(</b><i><b>rac</b></i><b>)-4</b> were mononuclear coordination compounds with five-coordinated aluminum atoms in the solid state. Using 2-propanol as cocatalyst, complex <b>(</b><i><b>S</b></i><b>)-6</b> revealed the highest activity among these aluminum coordination compounds toward the ring-opening polymerization of l-lactide, and complex <b>(</b><i><b>S</b></i><b>)-2</b> displayed the highest stereospecificity for the ring-opening polymerization of <i>rac</i>-lactide, affording partially isotactic polylactide with a <i>P</i>_m of 0.64. The polymerization kinetics using <b>(</b><i><b>S</b></i><b>)-6</b> 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 <i>rac</i>-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)