Make or break: Mg(ii)- and Zn(ii)-catalen complexes for PLA production and recycling of commodity polyesters

Abstract

Recently we reported a series of highly active Al(III)-complexes bearing a catalen ligand support for lactide polymerisation, observing unprecedented activity in the melt. Herein we report diversification of the metal to furnish a series of well-defined dimeric Zn(II)- and Mg(II)-complexes, which were fully characterised by X-ray crystallography and NMR spectroscopy. The production of biocompatible atactic PLA from rac-LA in solution and under industrially preferred solvent-free conditions was demonstrated, typically observing good activity and M_n control with a broad range of dispersities (Đ = 1.08–2.04). Mg(II)-Complexes were shown to facilitate the relatively mild methanolysis of PLA, achieving up to 64% conversion to Me-LA within 8 h at 80 °C in THF. Further kinetic analysis found [Mg(1,3)]₂ to have k_app values of 0.628 ± 0.0536 {4 wt% cat. loading} and 0.265 ± 0.0193 h⁻¹ {8 wt% cat. loading} respectively for the rate of consumption of PLA. Preliminary work extended polymer scope to PET from various sources, demonstrating catalyst versatility.