Kinetic elucidation of a novel photoproduct via irradiation of isopropylthioxanthone with maleic anhydride

Isopropylthioxanthone is a popular cosynergist for Type II and three-component photoinitiators used to photopolymerize ultraviolet (UV) curable coatings and can be used in the presence of maleic anhydride derivatives to efficiently cure coatings used in numerous applications. Isopropylthioxanthone (ITX) was shown via ultraviolet-visible (UV-vis) spectroscopy to yield a single photoproduct with maleic anhydride (MA) when irradiated (lambda > = 350 nm). Following irradiation of ITX and MA at set time intervals, high-performance liquid chromatography (HPLC) was used to observe the formation of the photoproduct by monitoring the UV-vis absorbance spectra of the photoproduct and the reactants. Based on the consumption of ITX shown in the HPLC plots, the complete rate law was calculated based on the initial rates. Finally, a possible mechanism is proposed for the formation of the photoproduct which may provide evidence for the reduced initiation efficiency of photoinitiators incorporating ITX and a MA derivative when compared to a similar photoinitiator substituting an N-substituted maleimide for the MA derivative.