Hexaarylbiimidazole-based Dynamic Materials and their Utilization

Abstract

Free radicals, atomic or molecular species with an unpaired valence electron, are common reactive species in many chemical reactions. Indeed, radicals often participate in many naturally-occurring biological processes as well as in polymerization reactions as the active propagating centers in both chain- and step-growth polymerizations. One particularly interesting class of radical-generating compounds consists of hexaarylbiimidazoles (HABIs) which have been the focus of significant research activity owing to their photo-, piezo-, and thermo-chromic nature. Upon irradiation, HABIs undergo homolytic cleavage to yield two strongly colored, 2,4,5-triarylimidazolyl radicals that are relatively unreactive with oxygen and show slow recombination rates, attributable to steric hindrance and electron delocalization, and thermally recombine to reproduce the HABI dimer. Because of their unique attributes, HABIs have found industrial utility as proofing papers, photoresists, and radical polymerization initiators. This dissertation details the design and synthesis of novel HABI derivatives to 1) address the deficiencies of conventional HABI photoinitiators, including their poor visible light absorption and low solubility, 2) elucidate the recombination mechanism of lophyl radicals and resolve the apparent disagreement between reported reaction orders, and 3) explore their utilization as a novel class of dynamic covalent bonds to effect the photo-mediated healing of cross-linked polymer networks. A novel, bis(hydroxyhexyl)-functionalized HABI was synthesized and employed as an efficient, visible light-active photoinitiator for thiol–ene resin formulations in the absence of any accompanying photosensitizer. Owing to the high reactivity of thiol functional groups with HABI-derived lophyl radical, in conjunction with the necessarily high thiol concentration in thiol–ene formulations, this HABI photoinitiator effectively initiated thiol–ene polymerizations upon visible light irradiation and exhibited improved visible light absorption and solubility in both organic solvents and resins relative to a commercial HABI photoinitiator. Additionally, the photo-mediated dissociation of HABIs and subsequent dark recombination of lophyl radicals were examined, where analysis of lophyl radical concentration curves revealed that the recombination reactions were well described as 3/2- and second-order reactions for the two respective parent HABIs. Finally, HABI-based functional groups were utilized as a new class of dynamic covalent bonds to demonstrate the photo-mediated healing of cross-linked polymer networks. Novel di- and tetra-functional, polymerizable HABI-based monomers were synthesized and subsequently incorporated into cross-linked polymer networks. The resultant HABI-incorporating cross-linked polymer networks were able to undergo photo-mediated backbone cleavage, temporarily affording reduced cross-link densities and dynamic connectivity rearrangement while under irradiation, then reverting back to stable, static networks upon irradiation cessation. This network stability in the dark, combined with a highly dynamic nature under irradiation, enabled rapid healing rates while precluding the creep that often accompanies the dynamic connectivity of intrinsically-healable polymer networks. Moreover, the cross-link density reduction upon irradiation and long half-life of the photo-generated lophyl radicals tethered to network strands provided sufficient time for effective, successful healing within HABI-incorporating, vitrified, cross-linked polymer networks. The findings described herein open up possibilities in the design and synthesis of novel, intrinsically-healable cross-linked polymer networks, and suggest the suitability of HABIs in advanced, responsive materials applications, including in stress-indicating and self-reinforcing materials.