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.

A series of well-defined mono- and dimeric Zn(ii)-complexes were prepared and fully characterised by X-ray crystallography and NMR spectroscopy.

ZnII‐complexes bearing half‐salan ligands were exploited in the mild and selective chemical upcycling of various commercial polyesters and polycarbonates. Remarkably, we report the first example of discrete metal‐mediated poly(bisphenol A carbonate) (BPA‐PC) methanolysis being appreciably active at room temperature. Indeed, Zn(2)2 and Zn(2)Et achieved complete BPA‐PC consumption within 12–18 mins in 2‐Me‐THF, noting high bisphenol A (BPA) yields (SBPA=85–91 %) within 2–4 h. Further kinetic analysis found such catalysts to possess kapp values of 0.28±0.040 and 0.47±0.049 min−1 respectively at 4 wt%, the highest reported to date. A completely circular upcycling approach to plastic waste was demonstrated through the production of several renewable poly(ester‐amide)s (PEAs), based on a terephthalamide monomer derived from bottle‐grade poly(ethylene terephthalate) (PET), which exhibited excellent thermal properties.

Nine new complexes based on thioether appended iminophenolate (ONS) ligands have been prepared and fully characterized in solution by NMR spectroscopy.

A series of novel Al(iii) complexes are prepared that show remarkable activity under the industrially preferred conditions.