Photoinitiator properties of 2-substituted amido and acryloxyanthraquinones

Abstract

The photophysical and photoinduced polymerization activities of 2-acrylamido, 2(2-ethylhexyl) amido and 2-acryloxyanthraquinones have been determined in solution media. The photoinduced polymerization activities of the two amido derivatives in methyl methacrylate (MMA) are greater than that of the acryloxy derivative. This is consistent with the two former derivatives exhibiting longest wavelength absorption maxima some 50 nm red shifted from that of the 2-acryloxy derivative. The shorter wavelength absorption band of the latter exhibits a small blue shift with increasing solvent polarity indicative of an nπ ∗ configuration. All three compounds appear to show little evidence of any charge-transfer interaction with the solvent environment. Fluorescence and phosphorescence analysis of the three compounds confirm the existence of low lying nπ ∗ excited states and rapid intersystem crossing to the next lowest lying triplet nπ ∗ excited states. Photoinduced polymerization of MMA monomer gave polymers with a higher molecular weight for the two acrylated derivatives. Second derivative u.v. analysis showed the anthraquinone chromophore for each of the molecules to be incorporated in the polymer (PMMA) to the same extent indicating the absence of any copolymerization with the acrylated groups of the anthraquinones. Incorporation of the anthraquinone through the semiquinone radicals may be an important mechansim in all cases. In the case of the 2-(2-ethylhexylamido) anthraquinone, alkylamido radicals formed by inter or intra-molecular hydrogen atom abstraction may be formed which can behave as initiators of the polymerization. The role of the triplet state in photoinduced polymerization was confirmed by the observation of a strong inhibiting effect in air. Furthermore, the use of both a radical trap (nitroxyl free radical) and an electron trap (tetracyanoethylene) also strongly inhibited the polymerization of MMA indicating the role of both radical and electron transfer processes in the photopolymerization reaction.