Integration of upcycling and closed-loop recycling through alternative cyclization-depolymerization

Currently, to address the problem of end-of-life plastics, scientific researchers are urgently looking for the future generation of polymers and technologies that contribute to upcycling and closed-loop recycling. In this manuscript, an alternative cyclization-depolymerization strategy was developed for integrating upcycling and closed-loop recycling of poly(p-dioxanone) (PPDO), an aliphatic polyester with excellent biocompatibility, biodegradability and mechanical performance. Using a Bronsted acidic ionic liquid as a solvent/catalyst bifunctional agent, the depolymerization and cyclization of linear PPDO (l-PPDO) are realized at relatively low temperatures. Therefore, benefiting from the fully suppressed side reactions, cyclic PPDO (c-PPDO) with a relatively high yield is obtained under such conditions and can be easily separated from other distillable products (mainly the PDO monomer). In a multiple cycle cyclization-depolymerization experiment, c-PPDO was accumulated and its production reached 222 g L-1, which is comparable with that obtained by the most advanced synthesis method of macrocyclic polymers. On the one hand, the upcycling product c-PPDO can convert back to the linear analog by hydrolysis, hence being available for closed-loop recycling again. On the other hand, the other recycling product, i.e., the PDO monomer with very high purity, can repolymerize to l-PPDOs with a high molecular weight and definite terminal groups, therefore achieving highly efficient closed-bi-loop recycling. This alternative cyclization-depolymerization strategy effectively integrated upcycling into closed-loop cycling, making PPDO an ideal candidate for the future generation of green polymers.