Seven Simple Dynamic Covalent Chemistries to Transform Thermosets into Thermoplastics: Sustainable Chemical Recycling of Traditionally Non-Recyclable Materials

The production of conventional crosslinked polymer networks and their composites, i.e., thermosets and thermoset composites, was estimated to consume more than 40 billion kg of polymer in 2020. Unfortunately, thermosets cannot be melt-reprocessed into moderate- to high-value products because permanent crosslinks prevent melt flow. We have developed seven simple one-step or two-step chemistries to produce networks and their composites with dynamic covalent crosslinks that are robust at use conditions but allow for melt-state reprocessing at high temperature. Unique to our research group, we have developed three approaches for melt-state reprocessing of addition-type polymer networks and network composites, including those synthesized directly from monomers containing carbon-carbon double bonds and those synthesized from combined polymer and monomer with both containing carbon-carbon double bonds. These approaches allow for full crosslink density recovery after multiple reprocessing steps. We have also demonstrated for the first time with four dynamic chemistries the ability to make PU and PU-like networks, e.g., polyhydroxyurethane and polythiourethanes networks, reprocessable with full recovery of crosslink density. An "Achilles' heel" has been identified regarding the application of dynamic covalent networks, i.e., such networks are subject to creep at elevated or sometimes even room temperature, which is often highly undesirable. We have successfully addressed this issue in several ways. Brief highlights of each of these accomplishments will be presented. Implications for making major gains in the sustainability of networks and network composites will be discussed as will the potential benefits of such chemistry even with regards to sustainability