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 °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 poly­(ethylene terephthalate)
(PET) and poly­(ε-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 °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.