(255cg) Exploiting Latent, Long-Lived Reactive Species in Covalently Cross-Linked Networks

Although free radicals are typically extremely reactive species with negligible recombination activation energies, resulting in diffusion-controlled termination rates, several examples of stable radical species exist. For example, trityl (i.e., triphenylmethyl) radicals in solution exist in equilibrium with their quinoid dimer form owing to the radical stability afforded by resonance and steric stabilization, while the unpaired electron of nitroxides, resonance-stabilized and sterically-shielded nitroxyl radical species, is persistent and commonly employed as mediator for controlled radical 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 photochromic, piezochromic, and thermochromic nature for more than five decades and have found industrial utility as radical polymerization initiators. First synthesized in 1960, they have since been investigated for use in photographic films as leuco dye photooxidants, color proofing systems, and as radical polymerization photoinitiators. The photochromism exhibited by HABIs is attributable to their homolytic cleavage upon irradiation to yield two strongly colored lophyl (i.e., triphenylimidazolyl) radicals that are relatively stable in oxygen-saturated surroundings and show slow recombination rates; thus, analogous to the influence of temperature on the trityl radical/quinoid dimer equilibrium, light can be employed to shift the lophyl radical/HABI dimer equilibrium. Here, we describe the incorporation of HABI-based functional groups into the backbone of cross-linked polymer networks, characterize lophyl radical generation and recombination kinetics, and examine the light-mediated cleavage of a high fraction of HABI-bearing network strands, temporarily decreasing the cross-link density and enabling rapid, photo-mediated polymer healing.