Influence of nuclearity and coordination geometry on the catalytic activity of Zn(II) carboxylate complexes in ring-opening polymerization of ε-caprolactone and lactides

Abstract

A series of nine Zn(II) carboxylate complexes stabilized by monodentate pyridyl Schiff base ligands have been synthesized and characterized by IR, NMR, mass spectroscopy and elemental analysis. The molecular structures of three of the nine complexes were established from single-crystal X-ray diffraction studies and found to have different nuclearity (mono-, -di or tri-). As catalysts in the ring-opening polymerization (ROP) of ε-caprolactone (ɛ-CL) and rac-lactide (rac-LA), the carboxylate complexes were active and afforded low molecular weight polymers with moderate to broad polydispersity (Ð) at elevated temperatures. The catalytic activity examined under various polymerization conditions was found to be influenced by the nuclearity and coordination geometry adopted by the carboxylate complexes. Complexes with higher nuclearity and particularly those with mixed carboxylate systems displayed higher catalytic activities with and without alcohol initiators. The dinuclear and trinuclear complexes exhibited superior catalytic activities and for the polymerization of L-LA they gave predominantly isotactic poly-lactides while polymerization of rac-LA gave heterotactic polymers with Pr of 0.68 and 0.69 for complexes 1 and 2, respectively.