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Three difunctional cleavable photoinitiators based on the a-hydroxyketone structure were tested in photopolymerization reactions with Fourier transform infrared spectroscopy. The excited-state processes were investigated with time-resolved laser absorption spectroscopy and laser-induced photocalorimetry, which allowed the cleavage quantum yields to be determined and estimates of the triplet-state lifetimes and interaction rate constants with a methacrylate monomer to be given. Modeling calculations helped to describe the ground-state properties and showed that the difunctional photoinitiators exhibited different chromophoric groups than the corresponding monofunctional photoinitiator. An examination of the photochemical reactivity versus the practical efficiency observed in the polymerization showed that these difunctional photoinitiators exhibited better efficiency than the parent molecule, mainly because of their increased absorption properties. Their practical efficiency appeared to be better because of better light absorption when standard polychromatic light sources were used.

Different photoinitiators based on Barton thiohydroxamic esters, O-acyl-N-hydroxy-pyridine-2(1H)-thione and O-acyl-N-hydroxy-thiazole-2(3H)-thione derivatives, were tested in photopolymerization reactions through time-resolved FTIR spectroscopy. Good rates of polymerization and final monomer conversion were obtained for some compounds. The excited state processes, investigated by time resolved absorption spectroscopy, lie on a fast singlet state cleavage leading to thiyl and alkyl radicals. Both the pyridine-2(1H)-thiyl and alkyl radicals are able to initiate a polymerization, in contrast with the thiazole-2(3H)-thiyl radical. A triplet state is observed for some derivatives. Computational studies help to describe the excited state properties and show a strong difference in the spin localization in the formed thiyl initiating radicals.

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