Zn(II)- and Mg(II)-Complexes of a Tridentate {ONN} Ligand: Application to Poly(lactic acid) Production and Chemical Upcycling of Polyesters

Abstract

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.

Versatile Chemical Recycling Strategies: Value‐Added Chemicals from Polyester and Polycarbonate Waste

“…Notably, Zn( 2 ) 2 and Zn( 5 )Et were reasonably active at 8 wt%, exhibiting k app values of 0.053 and 0.044 min −1 respectively in THF at 50 °C (Table S3, Entries 2 and 5). Such values are comparable to previously reported Zn II ‐complexes [12g,i] …”

“…In short, Zn( 2‐3 ) 2 and Zn( 4‐5 )Et were taken forward for further kinetic analysis under mild reaction conditions (Figure 3 and Table S3). Reactivity trends could generally be attributed to ligand effects, with the presence of hydrogen bond donors and Lewis acidity of the Zn II centre being particularly important [12e,g,i] . Notably, Zn( 2 ) 2 and Zn( 5 )Et were reasonably active at 8 wt%, exhibiting k app values of 0.053 and 0.044 min −1 respectively in THF at 50 °C (Table S3, Entries 2 and 5).…”

“…We have previously shown ligand flexibility and hydrogen bond donors to dramatically influence the activity of Zn II ‐complexes for polyester degradation [12g,h,i] . Thus, we sought to design new industrially relevant ligands that combine such adventitious attributes.…”

“…Herein, a reaction temperature of 180 °C and an excess of EG (20.6 equiv.) in the presence of Zn( 2 ) 2 (4–8 wt%) were chosen based on previous work (Table S4) [12g] . Zn(OAc) 2 ⋅ 2H 2 O is generally considered the benchmark catalyst, and thus was selected as a commercially available and air‐stable reference [1c] .…”

“…PET aminolysis has also been reported, yielding value‐added terephthalamides for use in the production of additives and high‐performance materials [13d,14] . Catalysts for PET alcoholysis and glycolysis have been widely investigated, although discrete metal‐based examples remain rare [12f,g,15] . Comparatively, BPA‐PC remains vastly underexplored and is limited almost exclusively to the use of organocatalysts [13a,16] .…”