Novel epoxide-promoted polymerization of lactides mediated by a zinc guanidine complex: a potential strategy for the tin-free PLA industry

Abstract

A unique example of a bench-stable guanidine-ligated zinc complex has been thoroughly evaluated for its effectiveness towards the ring-opening polymerization (ROP) of lactides (L-LA and rac-LA) with the assistance of epoxides. The present catalytic system exhibits exceptional polymerization activity even under simulated industrial conditions. Specifically, it can reach a turnover frequency (TOF) value of 106 500 h⁻¹ under very low catalyst loading (monomer/catalyst = 50 000/1) at an elevated temperature (180 °C), which surpasses those of traditional zinc analogs, resulting in high molecular weight (32.2 kg mol⁻¹) polylactide (PLA) with an extremely low zinc residue content of 6.7 mg kg⁻¹. Based on kinetic studies and spectroscopic analysis, a plausible coordination–insertion mechanism is proposed, with the potential to generate the Zn–alkoxide (Zn–OR) active species. This research sheds light on the development of an eco-friendly catalytic system with improved efficiency for the lactide polymerization industry.