Highly stereoselective bimetallic complexes for lactide and ε-caprolactone polymerization

A series of Schiff-base compounds containing bimetallic ligands and two aluminum centers have been synthesized and characterized by H-1, C-13 NMR and elemental analysis. These compounds could be successfully used as catalysts for the polymerization of rac-lactide and epsilon-caprolactone. The polylactides (PLAs) prepared with these compounds were highly isotactic enriched (P-m = 0.97). The nature and steric hindrance of the ligands coordinated to the central metal ions remarkably influenced the polymer properties. Kinetic studies revealed that the polymerization was first order with respect to each of these compounds and lactide/caprolactone monomers. All the polymerizations were living, with good molecular weight control and relatively narrow molar mass distributions.