Research

N-heterocyclic carbenes (NHCs) are versatile and powerful organocatalysts for both small molecule chemistry and polymerizations. The innate sensitivity of NHCs has prompted the use of reversible NHC adducts (i.e. “masked” NHCs) as easily-handled precursors, but common masks (e.g. CO2) have limited tunability for controlled release. Likewise, current on-off “switchable” NHC catalysts rely on modification of the NHC itself, which inherently changes the reactivity of the catalyst and limits the scope of NHC-catalyzed reactions amenable to these methods. In this project, we seek to design fully-reversible NHC adducts with tunable masks that will enable externally-stimulated release, independent of NHC reactivity. Through this method, a larger selection of on-off switchable NHCs will be explored. These fully reversible adducts will not only allow for latent catalysis, but also catalyst recycling on a solid support as well as spatiotemporal control over polymerizations.

Polyurethanes (PUs) are industrially important polymers, but the most common syntheses involve toxic isocyanate precursors. Carbon dioxide is an attractive alternate C1 feedstock for non-isocyanate polyurethanes (NIPUs) due to its low cost, high abundance, and nontoxic nature. We are working to advance the development of CO2-based polyoxazolidinones (POxa), which are an emerging subclass of NIPUs for high temperature applications. Previous POxa with rigid linkers suffer from limited solubility that hinders synthesis and characterization. We have developed a general strategy for achieving soluble POxa through the incorporation of alkyl and alkoxy solubilizing side chains. This approach enables the characterization of key properties (i.e., molar mass and polymer structure) using solution-state methods and provides a platform for establishing structure-property relationships to inform the more rational design of these materials. Beyond POxa, we also have broader interests in other CO2-based NIPUs, such as circularly recyclable materials from waste feedstocks.

The synthesis of polymers with a broad range of properties and chemical space requires techniques that can tolerate the incorporation of diverse monomer classes; however, current methods to achieve materials from multiple mechanisms generally require tedious synthetic manipulations between steps, or an architecture-limiting multi-functional initiator to achieve block copolymers (BCPs). Universal mediators have emerged as a technique to combine radical polymerization with either cationic or anionic polymerizations without the need for any intermediate synthetic steps, but compatibilization of cationic and anionic mechanisms using a universal mediator has previously been unreported. We have focused on thiocarbonyl thio compounds (TCTs) as universal mediators and developed a method to combine cationic and anionic polymerization mechanisms in sequence to synthesize novel poly(vinyl ether)-b-poly(thiirane) block copolymers. This technique also allows for the simple incorporation of radical polymerization to synthesize novel three-mechanism, three-monomer-class triblock terpolymers. Through this system, we seek to expand the available chemical space of BCPs by incorporating different monomer classes, tacticity, and post-polymerization modifications. Further work in this area seeks to expand the scope of this method and characterize the resulting novel BCPs. Thermal characterization of the BCPs suggests microphase separation of the blocks, and thus exploration into the self-assembly of these BCPs and the resulting material properties is of interest.