Zn(Ii)- And Mg(Ii)-Complexes Of A Tridentate {Onn} Ligand: Application To Poly(Lactic Acid) 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 are reported. The production of biocompatible atactic poly(lactic acid) (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 degrees C in THF. Further kinetic analysis found Mg(1)(2) and Zn(1)(2) to exhibit k(app) 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 poly(ethylene terephthalate) (PET) and poly(epsilon-caprolactone) (PCL) degradations, 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 degrees 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.