While PET is often touted as one of the most recycled plastics, current thermomechanical strategies rich in oxygen and moisture-rich environments at melt conditions lead to an unavoidable loss in molecular weight, necessitating chain extenders to recoup the loss in performance. The limitations of mechanical recycling can be overcome by chemically recycling PET to repolymerizable monomers; however, it mandates high energy demands, corrosive chemical environments, and long reaction times. The search for a sustainable closed-loop strategy has prompted research to redesign the polyester backbone with reactive moieties nicknamed trojan horse units that trigger depolymerization under selective conditions. Through this study, we demonstrated a scalable green synthesis of dimethyl dimethoxy terephthalic acid (DMTPA) co-units for instilling hydrolytic susceptibility into the PET backbone. These counits were synthesized by etherifying dihydroxy terephthalic acid using dimethyl sulfide and dimethyl carbonate, obviating the need for toxic alkylating agents such as methyl iodide and dimethyl sulfate. These reactive units facilitate hydrolysis in the presence of Lewis acids. The dimethoxy pendants give rise to favorable Lewis acid-base pair interactions, potentially alleviating the energy demands to initiate depolymerization. Even a modest 5% incorporation resulted in rapid bulk depolymerization in the presence of residual zinc acetate and other Lewis acids used in esterification and polycondensation. Incorporating reactive counits also diminishes crystallinity and suppresses the melting point, allowing for deeper solvent ingress and rapid hydrolysis to monomers at 180°C. The dosage of counits can be modified to strike a balance between recyclability and engineering performance. These copolymeric formulations provide a strategy to foster sustainability at source by modifying the polyester backbone, thus offering a paradigm shift to a circular economy.

Lewis acid mediated hydrolytic depolymerization of DMTPA-PET-copolymers

Michael Forrester

Appearances