Biobased, Non-isocyanate Polythiourethane (NIPTU) Networks: Reprocessability, Enhanced Performance by Inter-chain Disulfide Linkages and End-of-Life Monomer Recovery

Isocyanate-based polyurethanes (PUs) raise concerns about safety, sustainability, and the circular economy. Isocyanates are toxic, and their precursor, phosgene, was a chemical weapon in World War I. Commercial PUs are derived from fossil fuels, a limited resource. Research has yielded biobased PUs, but many biobased PUs are derived from food products needed to feed a hungry world. Most commercial PUs are cross-linked networks, with permanent cross-links preventing melt reprocessing and high-value reuse. Here, we present research on recyclable non-isocyanate polythiourethanes (NIPTUs) derived from non-food biobased sources, e.g., cashew nutshell liquid and glycerol. We address recyclability by designing reprocessable NIPTU networks, with cross-links consisting of dynamic covalent bonds, which yield full recovery of cross-link density and associated properties after multiple melt-recycling steps. We also show that inter-chain disulfide linkages provide NIPTU with enhanced mechanical properties. Finally, using transcarbamoylation dynamic chemistry, we developed a facile solvolysis approach to depolymerize NIPTU to pure monomer with 93% yield. Our work highlights the potential for robust, non-food biobased NIPTU with multiple routes for recyclability.