Study on Ring-opening Polymerization of Lactide Catalyzed by Guanidine Zinc Complexes

Poly(lactic acid) (PLA) is one of the most important biodegradable polymers, which has attracted much attention due to its renewable raw materials and excellent biodegradation properties. The quest to develop sustainable and high-performance catalytic technologies as alternatives to the tin catalysts, which are prevalent currently, is a hot topic in this field. Cyclic PLA, contrasting with the extensively produced linear PLA, has garnered significant attention due to its superior crystallinity, thermal stability, and low intrinsic viscosity, and it has been reported that the mixture of cyclic poly(lactic acid) and linear poly(lactic acid) has better hydrolysis resistance and thermal stability. This study reports the design and synthesis of a binuclear guanidine zinc complex, exhibiting exceptional activity (TOF = 7.50x10(3)- 3.45x10(4) h(- 1)) in catalyzing lactide ring-opening polymerization for the efficient production of cyclic poly(lactic acid) under simulated industrial conditions. Gel permeation chromatography (GPC) analysis revealed a narrow, unimodal distribution (1.25- 1.49) of the obtained products, with a molecular weight range of 6.2- 34.3 kg/mol. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) characterization of low molecular weight products confirmed the predominant presence of cyclic PLA, allowing for the proposal of a plausible ring formation mechanism. Moreover, the experiment demonstrated the binuclear guanidine zinc complex's effective catalytic action on a diverse range of lactone monomers, offering a novel avenue for the development of broad-spectrum catalytic systems within the realm of metal catalysis.