New experiments concerning the mechanism of solid‐state photopolymerization of 2,5‐distyrylpyrazine

Abstract

AbstractThe photoinduced formation of oligomers in single crystals of 2,5‐distyrylpyrazine (DSP) was investigated by X‐ray and electron diffraction techniques and also by gel permeation chromatography (GPC). The DSP crystals were characterized for the presence and nature of dislocations by dark‐field images and etch‐pit studies. Small angle grain boundaries were identified to be the prominent feature, but in general the crystals were highly perfect. A correlation between the sites of the defects and the formation of the photoproduct could not be detected. The oligomers formed in a solid‐solution under appropriate irradiation conditions. Thus, DSP oligomer of number average degree of polymerization P̄n = 3,2 in macroscopic single crystal texture was obtained for the first time using a laser light source at 478 nm. Deterioration of the single crystal texture of DSP usually observed during photopolymerization is explained as being caused by inhomogeneous light distribution which in turn causes an inhomogeneous product distribution with accumulation of large strain fields. These can be made visible by the multiplication of etch pits. Phase separation between oligomer and polymer is observed on further irradiation of the oligomer single crystals. The oligomer phase seems to play the role of an intermediate in the photopolymerization of DSP at wavelengths λ < 400 nm. The molecular weight distribution of the oligomers was studied by GPC. The data are only compatible with the assumption that the rate constant of dimerization is higher than that for the addition of a monomer to an already existing oligomer.