No abstract

Whilst plastics have played an instrumental role in human development, growing environmental concerns have led to increasing public scrutiny and demands for outright bans. This has stimulated considerable research into renewable alternatives, and more recently, the development of alternative waste management strategies. Herein, the aim was to highlight recent developments in the catalytic chemical recycling of two commercial polyesters, namely poly(lactic acid) (PLA) and poly (ethylene terephthalate) (PET). The concept of chemical recy-cling is first introduced, and associated opportunities/challenges are discussed within the context of the governing depolymerisation thermodynamics. Chemical recycling methods for PLA and PET are then discussed, with a particular focus on upcycling and the use of metal-based catalysts. Finally, the attention shifts to the emergence of new materials with the potential to modernise the plastics economy. Emerging opportunities and challenges are discussed within the context of industrial feasibility.

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 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(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.

A series of Mg(ii) and Zn(ii) catalen complexes have been prepared for PLA formation and recycling.