Ring-Opening Metathesis Polymerization of Carbodiimides: Sustainable Catalysis and Extensions to Diazene Metathesis

About This Grant

With the support of the Macromolecular, Supramolecular, and Nanochemistry program in the Division of Chemistry, Dr. Aleksandr V. Zhukhovitskiy of the University of North Carolina at Chapel Hill is developing methods to prepare polymeric materials with a high content of nitrogen atoms embedded within the polymer chains. Polymers form key components of plastics and rubber materials that are, in turn, at the heart of innumerable modern technologies: from vehicle parts to food packaging. Among existing synthetic commodity polymers, most are exclusively composed of carbon, oxygen, and hydrogen. In contrast, Nature’s polymers like proteins have a high content of nitrogen, which is crucial to their remarkable versatility. While some examples of nitrogen-rich synthetic polymers do exist and are even produced industrially (for example, nylons and polyurethanes) the scope of these materials is relatively small, and most lack structural precision, which limits their applications. Previous research led by Dr. Zhukhovitskiy and supported by this program demonstrated that iridium-based catalysts can enable the ring-opening polymerization of cyclic carbodiimides—moieties, wherein a carbon atom is linked to two nitrogen atoms via double bonds—to produce poly(carbodiimide)s that can be readily transformed into an assortment of other valuable polymer classes. This research seeks to improve the precision of this process, as well as to develop catalysts based on alternative, more abundant transition metals like iron. Lastly, the lessons learned from the carbodiimide polymerization will be extended to new nitrogen-containing molecules to ultimately access structurally precise photo-responsive plastics. This research will feature both experimental and computational components at the interface of polymer, organometallic, and physical organic chemistry; as such, this work will train students to be multidisciplinary experts capable of tackling complex modern scientific challenges. The research will transform how industrial and academic scientists access nitrogen-rich polymers. Dr. Zhukhovitskiy and his team will also grow their educational polymer-focused program that targets K-12 students and science teachers in their community. This program will engage students to think about both plastics and catalysis and provide high school STEM teachers with research training that will help them design engaging hands-on projects for their classes, with safety as a top priority. This research will focus on advancing carbodiimide ring-opening metathesis polymerization (CDI ROMP) and enabling diazene ROMP through augmenting our understanding of critical aspects of these transformations. The first objective of this research is to study the process of chain transfer in CDI ROMP, which must be limited to develop living CDI ROMP, and can potentially be exploited to enable catalytic CDI ROMP and precise control of end-group installation. The chemistry of iridium imido and guanidinate complexes, which serve as initiators and catalysts in this context, will be advanced as part of this objective. The second objective is to explore the use of late first-row transition metal imido and guanidinate complexes to initiate and catalyze CDI ROMP. The last objective is to extend the mechanistic insights of CDI ROMP to enable catalytic metathesis of nitrogen-nitrogen double bonds. The latter will, in turn, enable the development of precise synthetic methods toward main-chain poly(diazene)s. Thus, this research simultaneously advances the fundamental chemistry of late transition metal imido and guanidinate complexes, mechanistic design applied to polymer synthesis, and our ability to access precise nitrogen-rich polymer backbones with unprecedented compositions and architectures. With the latter materials in hand, this proposal will also begin to illuminate the effects of different structural aspects of the nitrogen-rich polymer backbones on their thermomechanical properties. 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.