Molecular mechanics of photopolymerization of 2,5-distyrylpyrazine in solid state

The molecular mechanics of photopolymerization of 2,5-distyrylpyrazine in the solid-state α form is investigated by a combination of Raman and electronic spectroscopic techniques. A stepwise photoreaction is used to stabilize an intermediate oligomer structure. For this purpose, the 476.5-nm argon-ion laser line is used to convert the monomer to the oligomer. The Raman phonon spectra reveal a homogeneous mechanism, with considerable lattice rearrangement, for the oligomerization process, which is also found to produce lattice strain. Further photoreaction of the oligomer is carried out with light of wavelength ≤400 nm from a 200-W mercuryxenon lamp as well as with the 355- and 417-nm laser lines from a Nd: Yag laser system. The Raman phonon spectra, monitored during the conversion of the oligomer to the polymer, show that initially the process starts homogeneously, with considerable lattice rearrangment. Then it turns heterogeneous, with a phase separation accompanied by a gradual ordering of the polymer product lattice. The electronic absorption spectra of the monomer are characterized and related to the stepwise photo process discussed above. Although the emission spectra show a monotonic change during the photoreaction, energy transfer from the oligomer to the monomer and from the polymer to the oligomer obscures and quantitative characterization by emission spectroscopy.