Applying Photochemistry to Hydroxylated Polymer Recycling, Tacticity Modification, and Single-Crystal Diamond Surface Functionalization

Abstract

Photoredox catalysis provides a powerful means to access radical-based reactivity using photocatalysts that selectively absorb light and disburse it through single-electron transfer events. The resulting radical intermediates engage in myriad reactions, such as additions to π-bonds, cross couplings, atom abstractions, and bond cleavages. The merits of this reactivity have been thoroughly embraced in organic synthesis and the pharmaceutical industry, but less so in areas of research further outside organic chemistry. This thesis broadly describes the application of photoredox catalysis to problems in materials science. The first chapter details the use of photocatalytic proton-coupled electron transfer (PCET) to promote the depolymerization of thermoplastics bearing hydroxyl groups along their backbone, specifically a commercial phenoxy resin and derivatives of polyolefins. During PCET, an excited-state photocatalyst and a weak Brønsted base jointly remove an electron and a proton from the hydroxyl O–H bond, forming oxygen-centered alkoxy radicals that prompt the cleavage of an adjacent backbone C–C bond. This mechanism is effective for the conversion of these hydroxylated polymers to well-defined and isolable products in a mild, catalytic manifold. The second chapter describes the extension of this depolymerization method to the chemical recycling of thermoset thiol epoxy resins, whose crosslinked structure makes them notoriously challenging to recycle via mechanical means. This process proved to be scalable and tolerant of various additives that are employed in commercial epoxies, such as colorants and a flame retardant. This section also addresses our efforts to degrade aliphatic epoxy resins. The third chapter summarizes the ongoing development of a photocatalytic hydrogen atom transfer (HAT) reaction to modify the tacticity of polymers. Capitalizing on the ubiquity of C–H bonds in synthetic polymers, this reaction utilizes an excited-state oxidant, quinuclidine, and thiobenzoic acid to break and reform C–H bonds, allowing racemization of backbone stereocenters. The fourth chapter discusses the application of photocatalytic HAT to the functionalization of C–H bonds on single-crystal diamond surfaces to enable broader use of nitrogen-vacancy (NV) based quantum sensing. HAT provides mild access to carbon-centered radicals on the diamond surface that can form bonds to fluorine, nitrogen, sulfur, and chlorine.