CAS: Dithioacetal as a Highly Tunable and Versatile Bond for the Design of Chemically Recyclable and Dynamic Covalent Polymers

With the support of the Macromolecular, Supramolecular and Nanochemistry program in the Division of Chemistry, Ying Yang of the University of Nevada at Reno is using ring-opening and -closing polymerization strategies to prepare recyclable polydithioacetals. Polydithioacetals are long chain macromolecules in which the main polymer backbone contains chemical element sulfur in addition to carbon commonly seen in commodity plastics. This class of polymers is chemically stable and yet has highly modifiable backbones and side groups. As a result, polydithioacetals are promising candidates for recyclable polymers with tunable properties. Research associated with this project will develop an in-depth understanding of the polymerization mechanisms of dithioacetal polymerization, investigate how thermodynamics and kinetics of polymerization are affected by the rich structural variations, and explore using these chemistries to enable dynamic architectural transformations of thermoplastic and thermoset polydithioacetals controlled by stimuli. The results from these fundamental studies have the potential to lead to the development of novel recyclable polymers which can be reprocessed at high temperatures and depolymerized on-demand back into cyclic macrocycles. From the sustainability point of view, the design principle associated with this project provides a very promising and viable solution to the current challenges associated with the widespread usage of non-degradable vinyl plastics. Hence, the developed methodology could be utilized to build readily degradable and sustainable plastics that are amenable to a circular economy life cycle. The research team will be actively involved in disseminating the results via publications and presenting the work at conferences and local events. Furthermore, community outreach will be conducted by establishing close relationships with Hug High School which has a large Hispanic student population. The project will also facilitate the research and education of polymer science at the University of Nevada Reno by integrating polymer chemistry modules into the large general chemistry classes. This research will focus on studying polydithioacetals that are capable of reversible entropy-driven ring-opening polymerization (ED-ROP) and ring-closing depolymerization (RCD) as a versatile platform for the development of recyclable polymers, as well as dynamic covalent systems. The dynamics will be enabled by acid-catalyzed dithioacetal exchange reactions. Building on promising preliminary results, the project aims to develop a detailed understanding of the chemistry and thermodynamics of ED-ROP for dithioacetal macrocycles through three interconnected objectives: (1) understand the polymerization mechanism with different initiator/catalyst systems and obtain controlled “living” cationic polymerization, (2) vary the structures of polydithioacetal backbones and side groups/chains and study their effects on the monomer-polymer equilibrium, and (3) develop reprocessable and depolymerizable polydithioacetal thermosets with controlled degradation by light and mechanical force. These research activities have a strong sustainability component and the potential to generate fundamental chemistry knowledge of relevance to using ED-ROP for the development of inherently practical, recyclable and functional polymers. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.