Fast-Tracking the l -Lactide Polymerization Activity of Group 4 Metal Complexes of Amine Tris(phenolate) Ligands

Zirconium complexes of amine tris(phenolate) ligands wherein the substituents ortho to the phenolate oxygens are aryl groups are introduced, and their application in the catalysis of ring-opening polymerization of l-lactide at 180 °C is described. Spectroscopic and crystallographic characterization revealed that for ortho-phenyl substituents, bridging dinuclear complexes were obtained, whereas for ortho-mesityl-substituents, fluxional mononuclear complexes were obtained. Melt polymerizations employing catalyst ratios reaching as low as 5 ppm for unpurified l-lactide and 1 ppm for purified l-lactide revealed ultra-high activities for the zirconium complex featuring the mesityl-phenolate substituents, yielding high-molecular-weight stereoerror-free poly(l-lactic acid) that exhibited enhanced stability toward thermal degradation. Slightly lower activities were found for the corresponding hafnium complex and for the zirconium complex featuring the phenyl-phenolate substituents. These catalysts could thus serve as drop-in replacement for the industrially employed stannous octanoate.