Owing to the inherent reactivity of organic radicals, the identification of photo- cleavable radical polymerization terminators that do not initiate polymerization has proven challenging. Moreover, to obtain wavelength-selective photoinitiation and photoinhibition in a formulated resin, initiating species need to be generated by irradiation at one wavelength while inhibiting species are generated at a second, independent wavelength, necessitating complementary photoinitiatior and photoinhibitor absorbance spectra. Hexaarylbiimidazoles (HABIs) to generate poorly reactive lophyl radicals upon UV light irradiation which recombine slowly and do not directly initiate the radical-mediated chain growth polymerization of (meth)acrylates; nevertheless, in the presence of hydrogen-donating species such as thiols, HABIs are well-known as photoinitiators of radical polymerizations. Here, we discuss the potential of HABIs as efficient radical polymerization photoinhibitors when used in conjunction with complementary photoinitiators, such as the camphorquinine/tertiary amine system. By monitoring the polymerization of formulated acrylate, methacrylate, and maleimide/vinyl ether resin formulations with Fourier transform infrared (FT-IR), UV-vis, and electron paramagnetic resonance (EPR) spectroscopic techniques, we establish the temporal and spatial control over the cure of these resins through wavelength-selective photoinitiation and photoinhibition via the photo-generation of polymerization-inhibiting lophyl radicals.

Hexaarylbiimidazoles as efficient photoinhibitors of radical-mediated chain growth photopolymerizations

256th National Meeting (2018)

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