(576e) Reprocessable Covalent Polymer Networks with Robust Properties and Providing for Recycling, Upcycling, Sustainability, and Circularity

Traditional covalent polymer networks or cross-linked polymers have been unable to be recycled for high-value applications because the permanent cross-links prevent melt flow needed for typical reprocessing. Rubber tires exemplify this challenge. In the United States where polymer recycling is done with some regularity, roughly one-half of spent rubber tires are burned for energy and 30% are transformed into a fine rubber crumb that is used for a variety of low-value applications, with much of the remainder being placed in tire dumps or lost to the environment. This is unfortunate as a significant minority of the more than 800 billion pounds of polymers and plastics produced annually on a world-wide basis are networks. Here, I will highlight some of the research that my research group has done in developing polymer networks that can be reprocessed (i.e., recycled) multiple times with full recovery of cross-link density and associated properties. In one class of studies, we have developed multiple ways to make reprocessable polyurethane (PU) networks or analogous systems that could substitute for PU networks. This is a very important class of polymer networks as PUs rank fifth or sixth among all polymer species in annual world-wide production and most commercial PUs are networks. We have demonstrated that PU networks can be made fully reprocessable if the polymerization reaction is run off-stoichiometry by 20%, with hydroxyl groups in excess relative to isocyanates, and the cross-linking unit is tetrafunctional rather than trifunctional. Nevertheless, this approach is less than ideal because of the highly toxic nature of isocyanates and their precursor, phosgene, and the fact that current PU networks are typically made at or very near stoichiometric balance. We have developed three analogs to PU networks that exhibit better reprocessability. In one case, we have created the first reprocessable polythiourethane (PTU) networks from reaction of isocyanates with thiols, achieving fully reprocessable PTU networks when the polymerization reaction is run of stoichiometry by 10% (excess thiol). The reactions and reprocessing steps are rapid, which is of major importance for commercial application, and provide for robust properties. However, the PTU networks still involve the use of isocyanates as reactants, which is considered less than desirable from a human-health standpoint. To address the concerns associated with the use of isocyanates as reactants, we have also developed fully reprocessable networks of non-isocyanate polyurethanes or NIPUs, the most common example of which is polyhydroxyurethane (PHU) made by reaction of cyclic carbonates with amines. With PHU networks, full reprocessabilty can be achieved with networks produced via polymerization reactions are run at stoichiometric balance. We have achieved a wide range of properties associated with PHU networks, found good ways to make PHU network nanocomposites highly reprocessable, and have developed bio-based and biowaste-based PHU networks for added sustainability. However, because of hydroxyl groups adjacent to the urethane linkages in these polymers, PHU networks can be hygroscopic, leading to property reduction after exposure to and sorption of water. Furthermore, the reactions associated with PHU production are typically slow in comparison with PU production. We have addressed these last two issue by very recently developing the first fully reprocessable non-isocyanate polythiourethane (NIPTU) networks. These NIPTUs networks are reprocessable with full recovery of cross-link density and associated properties and have the advantages of high reactivity and high reprocessability, low water sorption in comparison with PHUs, and the ability to be made from biowaste. Time permitting, we will also describe a dynamic covalent cross-linking method that we have developed that could be applied to the polymer precursors associated with the cross-linked rubber in rubber tires.