Zinc Complexes for PLA Formation and Chemical Recycling: Towards a Circular Economy

A series of Zn^II complexes, based on propylenediamine Schiff bases, have been prepared and fully characterized. X-ray crystallography and NMR spectroscopy identified significant differences in the solid and solution state for the Zn^II species. All complexes have been applied to the ring-opening polymerization of l-lactide with emphasis on industrial conditions. High conversion and good molecular weight control were generally achievable for Zn(A-D)₂ , and high-molecular-weight poly(lactic acid) (PLA) was prepared in 1 min at a 10 000:1:33 [lactide]/[Zn]/[BnOH] loading. The more active Zn^II catalysts were also applied to PLA degradation to alkyl lactate under mild conditions. Zn(A-B)₂ demonstrated high activity and selectivity in this process with PLA being consumed within 1 h at 50 °C. Zn(C-D)₂ were shown to be less active, and these observations can be related to the catalysts' structure and the degradation mechanism. Initial results for the degradation of poly(ethylene terephthalate) and mixed feeds are also presented, highlighting the broader applicability of the systems presented.