Dynamic Urethane and Urethane-like Chemistries in Reprocessable Polymer Networks: Structure, Property, and Sustainability Xi Chen Conventional cross-linked polymers are widely used owing to their outstanding stability and performance. However, permanent cross-links in these polymer networks prevent them from being recycled or reprocessed at the end of life, leading to major sustainability and economic losses. The recycling issue associated with polyurethane (PU) thermoset wastes exemplifies

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... problems and remains an unmet scientific and technological challenge. This dissertation study describes the development of reprocessable polymer networks based on dynamic urethane and urethane-like chemistries to achieve excellent recovery of properties associated with cross-link density after reprocessing. Polyhydroxyurethanes (PHUs) serve as promising sustainable alternatives for traditional PU networks, which have reduced reliance on isocyanate precursors. Hydroxyurethane linkages derived from five-membered cyclic carbonates and primary amines exhibit a dual-mechanism dynamic nature under stimulus, which involves both dissociative and associative pathways. With proper choice of molecular structure and filler-surface functionality, PHU networks and silicananoparticle-reinforced network composites exhibit full recovery of properties associated with cross-link density after reprocessing. The hydroxyurethane cross-link is also incorporated in other different types of polymers to achieve reprocessability, including polybutadiene, poly(tetramethylene oxide), and polydimethylsiloxane networks. Depending on the backbone and chain extender structures, the resulting networks exhibit various morphological behaviors, crosslink densities and property recovery efficiencies. Additionally, the sustainability of non-isocyanate 4 polyurethane networks is further advanced by developing fully reprocessable PHU networks using renewable bio-based precursors. In addition to the hydroxyurethane chemistry, thiourethane dynamic chemistry provides another simple and efficient pathway to produce recyclable substitutes for traditional PU networks, which also involves both dissociative and associative mechanisms during network rearrangement. Upon optimization involving slightly off-stoichiometric-balance reactions, polythiourethane networks exhibit full property recovery after multiple reprocessing steps. Moreover, thiol monomers can potentially be recovered by solvolysis of polythiourethane networks, which provides a second route for sustainable recycling. Finally, with judicious molecular design, off-stoichiometric-balance reactions, and proper choice of catalyst, the intrinsic dynamic nature of urethane linkages is also demonstrated to yield full reprocessability in conventional PU networks, which is highly desirable for the commercial PU industry. Due to the dynamic nature of urethane bonds, PU networks can be degraded in ethylene glycol under relatively mild conditions, which indicates the potential to recover alcohol monomers from waste PU network materials. 5 ACKNOWLEDGEMENT First and foremost, I would like to thank my advisor, John Torkelson, for guiding and mentoring me during my graduate study. This five years at Northwestern University is pleasant and memorable, and I attribute all my academic accomplishments to John. His dedication to excellence and positive attitude has made me a rigorous and motivated researcher. I am truly grateful for his time and help on building up my critical thinking, problem solving and communication skills. I would also like to thank my thesis committee members for providing useful suggestions for my research projects: Prof.