Sensation of Cleavage Reactions in Photoinitiator Research. Quantitative Assessment by Magnetic-Resonance Methodology

Photoinitiators are the key components in numerous applications using light-induced curing. Indeed, the photoinitiator is responsible for the light absorption and the production of free radicals which initiate the polymerization process (Scheme 1).However, free radicals can not only be produced by direct irradiation of the photoinitiator (A-B): but also via energy transfer from a sensitizer (S) to the photoinitiator (Scheme 2).The sensitized cleavage of a photoinitiator is an important issue since it allows to use light with various wavelengths independent of the absorption spectrum of the photoinitiator. Such a shift of the absorption wavelength to a desired region could lead to an increase of the cure speed and efficiency. The mechanism of sensitation has been demonstrated on several examples. However a quantitative measure for the photosensitization processes is a prerequisite for the, assessment of photocurable systems. Here we• introduce a novel approach for the quantification of the efficiency of the sensitation process for photoinitiators. The procedure is based on NMR spectroscpy. The sensitiser evaluated is 2-isopropylthioxanth-9-one (ITX) and the photoinitiators are monoacyiphosphineoxide I and the ct-aminoketone 2. We report on the influence of the concentration of ITX on the cleavage of I and the effect of solvent polarity on the sensitation efficiency of 2/ITX.