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

Poly(L-lactic acid) (PLLA) is a leading commercial polymer produced from biomass, showing useful properties for plastics and fiber applications; after use, it is compostable. One area for improvement is postconsumer waste PLLA chemical recycling to monomer (CRM), i.e., the formation of L-lactide (L-LA) from waste plastic. This process is currently feasible at high reaction temperatures and shows low catalytic activity accompanied, in some cases, by side reactions, including epimerization. Here, a commercial Sn(II) catalyst, applied with nonvolatile commercial alcohol, enables highly efficient CRM of PLLA to yield L-LA in excellent yield and purity (92% yield, >99% L-LA from theoretical max.). The depolymerization is performed using neat polymer films at low temperatures (160 degrees C) under a nitrogen flow or vacuum. The chemical recycling operates with outstanding activity, achieving turnover frequencies which are up to 3000x higher than previously excellent catalysts and applied at loadings up to 6000x lower than previously leading catalysts. The catalyst system achieves a TOF = 3000 h(-1) at 0.01 mol % or 1:10,000 catalyst:PLLA loading. The depolymerization of waste PLLA plastic packaging (coffee cup lids) produces pure L-LA in excellent yield and selectivity. The new catalyst system (Sn + alcohol) can itself be recycled four times in different PLLA batch degradations and maintains its high catalytic productivity, activity, and selectivity.

Automated summary

This study presents an efficient method for chemical recycling of waste Poly(L-lactic acid) (PLLA) using a commercial Sn(II) catalyst and nonvolatile commercial alcohol. The depolymerization of PLLA to yield pure L-lactide (L-LA) can be achieved at low temperatures, and the catalyst system shows outstanding activity and selectivity. Furthermore, the catalyst system can be recycled multiple times without loss of its catalytic productivity.