Chemical Recycling of Polyesters to Cyclic Anhydrides and Valuable Chemicals Enabled by Zinc Catalysts

Abstract

Chemical recycling of polyesters to re-form the constituent monomers is essential for a circular plastic economy. The ring-opening copolymerization (ROCOP) of epoxides and cyclic anhydrides represents an important route to polyesters and has received considerable attention over the past decades. In contrast, the chemical recycling of such polyesters has been only sporadically investigated and remains a formidable challenge due to unfavorable kinetics and thermodynamics. Here, we report an advance in the chemical recycling of ROCOP-polyesters to true monomers. A commercial zinc bromide catalyst enables highly efficient depolymerization of ROCOP-polyesters to yield cyclic anhydrides (up to 97% yield) and valuable chemicals (up to 75% yield) at 200 °C under vacuum. Mechanistic investigations, including kinetic studies, control experiments, and DFT calculations, elucidate that depolymerization proceeds primarily via a chain-end backbiting mechanism, assisted by random chain scission. Remarkably, this catalyzed depolymerization strategy is applicable to various ROCOP-polyesters and related block copolymers. Furthermore, we demonstrated the scalability of this process and its high selectivity in recycling ROCOP-polyesters from polyester/PP/PE/PS mixtures.