Developing thioxanthone based visible photoinitiators for radical polymerization

Three new amino-thioxanthone photoinitiators (PIs) with different substituents on nitrogen, including 2-allyl(methyl)amino-9H-thioxanthen-9-one (PI-1), 2-benzyl(methyl)amino-9H-thioxanthen-9-one (PI-2) and 2-butyl(methyl)amino-9H-thioxanthen-9-one (PI-3) were synthesized and characterized. They showed high molar extinction coefficients and very broad absorption range in the visible region. Among them, PI-2 showed the best visible light photoinitiating properties evidenced by the fact that it successfully initiated polymerization of 1,6-hexanedioldiacrylate (HDDA) under xenon light exposure (28 mW cm(-2)) both in the presence and absence of N,N-dimethylaniline (DMA) for 0.5 h with 92% and 52% conversion, respectively. For TMPTA, the conversion of PI-2 and PI-2/N-methyldiethanolamine (MDEA) systems is 32% and 62% under xenon lamp exposure for 10 min. The photoinitiation mechanisms were analysed through EPR, fluorescence spectra and visible light photolysis experiments.