Photopolymerization of vapor-deposited 2,5-distyrylpyrazine films

2,5-Distyrylpyrazine (DSP) was vapor-deposited onto a surface of potassium chloride obtained by cleavage. The DSP films were composed of crystallites of two modifications, a photoactive α-form and a photostable γ-form, depending on the evaporation conditions. The former appeared as tabular and as laminate crystallites and the latter as needle-like ones. The electron diffraction pattern from the tabular and the laminate crystallites showed single crystal patterns of the α crystal form projected along the b -axis and the a -axis, respectively. By photo-irradiation the color of the film changed from yellow to white. The photo-irradiated film was identified by means of infrared spectroscopy as poly-DSP containing cyclobutane rings. The electron diffraction pattern obtained from the polymer film showed a similar net pattern as that of the monomer crystal, although the lattice spacings shrinked slightly in the direction of the a - and c -axes and elongated in the direction of the b -axis. The polymer film was of extremely high crystallinity and the 200 and 020 lattice fringes with spacings of 0.91 and 0.54 nm, respectively, were observed in the polymer film. The photopolymerization mechanism is discussed and it is concluded that the photopolymerization of DSP films occurs within the monomer matrix through a homogenous reaction and that it proceeds through a direct rearrangement of molecules from the monomer to the polymer crystal.