Fully Reprocessable, Non-isocyanate Polyurethane Networks: Dual Thionourethane and Disulfide Cross-links in Non-Isocyanate Polythiourethane Networks Provide Advantages over Polyhydroxyurethane Network Analogues*

Polyurethanes (PUs) rank sixth among all polymer species in annual worldwide production, with the vast majority in the form of permanent networks that have insufficient reprocessability to allow for high-value recycling or reprocessing. In addition, traditional PUs are synthesized from toxic isocyanates, leading to safety and human health hazards. Here, we report on the first study of the reprocessability and properties of non-isocyanate polythiourethane (NIPTU) networks with dual cross-links, thionourethane-based and disulfide-based, the latter obtained by auto-oxidation of pendant thiol groups. To provide a rational comparison, we have also produced structurally analogous polyhydroxyurethane (PHU) networks that contain only one cross-linker species, hydroxyurethane. Such PHU networks are commonly referred to as non-isocyanate polyurethane (NIPU) networks. Both NIPTU networks and PHU networks were made suing predominantly biobased or bio-derivable starting materials. With advantages in reactivity due to the thiol-based nature and in cross-link density and mechanical properties because of the dual cross-link nature, NIPTU networks can be favorable alternatives to PHU networks as replacements for PU networks. Additionally the NIPTU networks exhibit better water resistance with factor of ~3 reductions in water sorption relative to their PHU analogues. Both NIPTU and PHU networks exhibit excellent reprocessabilty with complete recovery of cross-link density after multiple reprocessing steps as well as potential as self-healing polymers. Notably, the NIPTU networks exhibit excellent creep resistance up to 80-100 degrees C as well as a high (> 90%) recovery of pure monomer, one of the best yields among reported studies of chemical recycling of polymers. Thus, the NIPTU networks also exhibit advantages in sustainability and circularity. Our NIPTU networks exemplify how renewable, non-isocyanate PU-like materials can be developed with both high-performance characteristics and the potential to contribute meaningfully to the circular economy.