Catalyst-free molecular recycling of a commercial polyurethane through a novel auto-acidolysis depolymerization technique

Polyurethanes (PUs) are a class of synthetic polymers that rank as the sixth most produced plastic. PU is generally made by reacting a polyol with a diisocyanate and can be thermoset or thermoplastic (TPU), offering diverse properties and applications. However, the versatility of PU poses challenges in waste management. Most end-of-life PUs end up in landfills or are incinerated, releasing hazardous substances. Although TPU can be mechanically recycled, each cycle causes irreversible degradation, leading to downcycling. Molecular recycling, however, is not restricted by the number of reprocessing cycles. This process typically involves depolymerization via a catalyst prior to subsequent polymerization. Current research activities, however, mainly focus on recovering the polyol segment and discarding the dicarbamate segment as waste. Therefore, we aimed for the recovery of the nitrogen-based depolymerization product and demonstrated its potential for molecular recycling through the subsequent synthesis of a poly(ester amide). For this purpose, we selected a commercial TPU based on poly(caprolactone) polyol and hexamethylene diisocyanate. One-pot catalyst-free depolymerization of the aliphatic TPU was achieved in 1 h at 250 °C in water through a combination of hydrolysis and acidolysis reactions. First, hydrolysis of the polyol segments produced 6-hydroxyhexanoic acid (6HHA), which in turn was able to react with the dicarbamate segments via acidolysis. The resulting diamide diol was successfully obtained in high purity and polymerized with adipic acid to yield relatively high-molecular-weight poly(ester amide). This work introduces a new depolymerization technique, namely, auto-acidolysis , which allows depolymerization requiring only water under relatively mild conditions in the absence of hazardous catalysts or organic solvents. Auto-acidolysis could open up possibilities for the recycling of polyester-based PUs and influence polymer design by encouraging the incorporation of depolymerization agents in the polymer chain to streamline molecular recycling at end of life. • Introduces auto-acidolysis : a sustainable and fast recycling technique • Catalyst-free depolymerization of a commercial thermoplastic polyurethane • Molecular recycling of depolymerization product into a poly(ester amide)