Metathesis Chemistry for the Controlled Synthesis of Nitrogen-rich Polymers

Double-bond metathesis is an enabling reaction class that has transformed the synthesis of both small molecules and polymers, leading to innovations across many fields. Alkenes are by far the most studied substrate for this chemistry. In contrast, heteroatom-containing double bonds—hetero-enes—have received considerably less attention. Further development of catalytic hetero-ene metatheses could unlock new synthetic pathways to valuable heteroatom-rich materials. In the context of polymer synthesis, these methods could provide a solution to the persistent challenge of controlling the incorporation of nitrogen into polymer backbones. Resolving this problem would enable the preparation of performance polymers at scale with functionality akin to proteins. Typically, such nitrogen-rich polymers are prepared via step-growth methodologies that inherently limit precision. A chain-growth process founded on hetero-ene metathesis could enable straightforward access to nitrogen-rich polymers of high molecular weights, high chain-end fidelities, and narrow dispersities.In this dissertation, I present my work in the pursuit of such methodologies for the controlled synthesis of polycarbodiimides and polydiazenes. In Chapter 1, an outline of the strengths, challenges, and potential opportunities associated with several known hetero-ene metathesis mechanisms is presented. This outline serves as a framework from which one can approach the metathesis of other hetero-ene substrates with no catalytic precedent. Chapter 2 details the application of this framework in the development of a controlled ring-opening metathesis polymerization of N,N′-dialkyl and N,N′-diaryl carbodiimides initiated and catalyzed by an iridium(III) guanidinate complex. The resulting polycarbodiimides underwent simple post-polymerization modification to produce polyureas, polythioureas, and polyguanidinates with varied architectures. Lastly, Chapter 3 describes further application of mechanistic design toward the unrealized inorganic transformation of catalytic diazene metathesis. A proposed mechanism for this transformation, based on our discoveries in cyclic carbodiimide polymerization, was explored through the synthesis of possible intermediates that contain six-membered iridium metallocycles comprised of five contiguous nitrogen atoms. These complexes were readily prepared through newly discovered reactivity between alkyl triazoline diones and a (pentamethylcyclopentadienyl)iridium(III) tert-butyl imido complex. Taken together, my work advances the understanding of hetero-ene metathesis chemistry and demonstrates its application in polymer science towards the goal of accessing functional polymers with precise structures.