Zn(II)- and Mg(II)-complexes of a Tridentate {ONN} Ligand: Application to PLA Production and Chemical Upcycling of Polyesters

The synthesis and characterization of two homoleptic Zn(II)- and Mg(II)-complexes based on a simple tridentate {NNO} ligand is reported. The production of biocompatible atactic PLA under industrially relevant melt conditions is demonstrated, noting high activity for Zn(1)2 at room temperature in CH2Cl2 (TOF = 184 h-1). Mg(1)2 and Zn(1)2 were shown to facilitate rapid PLA methanolysis into methyl lactate (Me-LA) under mild conditions, achieving up to 85% Me-LA yield within 30 min at 50 °C in THF. Further kinetic analysis found Mg(1)2 and Zn(1)2 to exhibit kapp values of 0.23 ± 0.0076 and 0.15 ± 0.0029 min-1 respectively {8 wt% cat. loading}, among the highest reported thus far. Zn(1)2 retained excellent activity for both PET and PCL degradation, demonstrating catalyst versatility. Various upcycling strategies (e.g. methanolysis, glycolysis and aminolysis) were employed to achieve a broad substrate scope, which included bis(2-hydroxyethyl)terephthalate (BHET), high value terephthalamides and methyl 6-hydroxyhexanoate. Optimal glycolysis conditions using Zn(1)2 enabled 64% BHET yield within 1 h at 180 °C, a rare example of PET glycolysis mediated by a discrete homogeneous metal-based catalyst. The application of such catalysts for PET aminolysis and PCL methanolysis has been reported for the first time.