A Simple, Selective, and General Catalyst for Ring Closing Depolymerization of Polyesters and Polycarbonates for Chemical Recycling

Abstract

The ability to efficiently and selectively process mixed polymer waste is important to address the growing plastic waste problem. Herein, we report that the combination of ZnCl2 and an additive amount of poly(ethylene glycol) under vacuum can readily and selectively depolymerize polyesters and polycarbonates with high ceiling temperatures (Tc>200 °C) back to their constitute monomers. Mechanistic experiments implicate a random chain scission mechanism and a catalyst structure containing one equivalent of ZnCl2 per ethylene glycol repeat unit in the poly(ethylene glycol). In addition to being general for a wide variety of polyesters and polycarbonates, the catalyst system could selectively depolymerize a polyester in the presence of other commodity plastics, demonstrating how reactive distillation using the ZnCl2/PEG600 catalyst system can be used to separate mixed plastic waste.

Chemical Recycling of Commercial Poly(l-lactic acid) to l-Lactide Using a High-Performance Sn(II)/Alcohol Catalyst System

“…Recycling conditions: neat PLLA, 0.4 mol % Zn­(OAc) 2 , 160 °C. k Data reported in ref . Recycling conditions: neat PLLA, 0.4 mol % Zn­(OAc) 2 , 200–210 °C. l Data reported in ref . Recycling conditions: neat PLLA, 5 mol % ZnCl 2 , 6 mol % PEG, 160 °C. …”

“…The Sn­(Oct) 2 /GEO catalyst system also significantly outperforms all other PLLA depolymerization catalysts reported so far in the literature. − While comparisons of catalysts can be complicated by the different conditions used to test them, the key targets in chemical recycling should be to deliver processes that operate at the lowest temperatures while achieving high rates and l -LA selectivity. At 160 °C, the current catalyst exhibits a 160× higher rate, even at 500× lower loading, than when it was applied in solution (Table , entry 10) .…”

“…While finalizing this article, Byers and coworkers reported PLLA chemical recycling to L-LA, using 10− 20 wt % of a ZnCl 2 catalyst combined with poly(ethylene glycol), PEG. 26 The catalyst system showed excellent selectivity for L-LA (98%) but very low overall rates with a TOF = 1 h −1 . Our objective was to discover highly active catalysts and develop applicable processes for chemical recycling of waste PLLA to L-LA.…”

“…Nevertheless, there are surprisingly few reports of PLLA postconsumer waste recycling to l -LA. − Reactions tend to be hindered by side processes, including the epimerization of l -LA to meso-lactide, and operate at elevated temperatures. − In 2020, Enthaler and co-workers reported a Zn­(OAc) 2 catalyst for PLLA depolymerization to l -lactide (Figure ). The process required relatively high catalyst loading (0.4 mol % or 1:250, Zn­(OAc) 2 :PLLA) and temperatures of 200–210 °C.…”

“…For example Wang, Xu, and co-workers recently reported an efficient Zn-catalyzed “polymer to polymer” recycling method, exploiting PLLA transesterification with alcohols to form oligomers (which were subsequently used to make other polymers). While finalizing this article, Byers and co-workers reported PLLA chemical recycling to l -LA, using 10–20 wt % of a ZnCl 2 catalyst combined with poly­(ethylene glycol), PEG . The catalyst system showed excellent selectivity for l -LA (98%) but very low overall rates with a TOF = 1 h –1 .…”