Open- and Closed-Loop Recycling: Highly Active Zinc Bisguanidine Polymerization Catalyst for the Depolymerization of Polyesters.

Abstract

In this study, the aliphatic N,N-bisguanidine zinc complex [Zn(DMEG2ch)2](OTf)2 ⋅ THF is introduced as a promising candidate for the chemical recycling of (bio) polyesters. This catalyst is highly active in the ring-opening polymerization (ROP) of lactide (LA) and ϵ-caprolactone (CL). The combination of polymerization and depolymerization activity creates new pathways towards a sustainable circular economy. The catalytic activity of [Zn(DMEG2ch)2](OTf)2 ⋅ THF for the chemical recycling of polylactide (PLA) via alcoholysis was investigated by detailed kinetic and thermodynamic studies. It is shown that various high value-added alkyl lactates can be obtained efficiently under mild reaction conditions. Catalyst recycling was successfully tested using ethanol for the degradation of PLA. In addition, LA can be recovered directly from PLA, enabling either open- or closed-loop recycling. Selective PLA degradation from mixtures with polyethylene terephthalate (PET) and polymer blends are presented. For the first time, a cascade recycling reaction of a PLA/polycaprolactone (PCL) blend is tested with a zinc-based bisguanidine catalyst, whereby PLA is degraded selectively at first and subsequent modification of the reaction conditions leads to efficient degradation of the remaining PCL. The highly active, universally applicable benign zinc catalyst allows the implementation of a circular plastics economy and thus the reduction of plastic pollution in the environment.