Highly Active N,O Zinc Guanidine Catalysts for the Ring-Opening Polymerization of Lactide

New zinc guanidine complexes with N,O donor functionalities have been prepared, characterized by X-Ray crystallography and examined for their catalytic activity in the solvent-free ring-opening polymerization (ROP) of technical-grade rac-lactide at 150 °C. All complexes showed a high activity. The fastest complex [ZnCl2(DMEGasme)] (C1) produced colorless poly(lactide) (PLA) after 90 min with a conversion of 52 % and high molar masses (Mw = 69100, PD = 1.4). The complexes have been tested with different monomer-to-initiator ratios to determine the rate constant kp. Furthermore, a polymerization with the most active complex C1 has been monitored by in situ Raman spectroscopy. Overall, conversion up to 90 % can be obtained. To clarify the mechanism end-group analysis has been performed. All four complexes combine robustness against impurities in the lactide with high polymerization rates, and they represent the fastest robust lactide ROP catalysts to date, opening new avenues to a sustainable ROP catalyst family for industrial use.