The mechanism and kinetics of the pressure-induced polymerization of acetylene were studied by Raman spectroscopy. The polymerization reaction occurred in the orthorhombic phase at room temperature and pressures above 3.5 GPa. Dominant formation of trans-polyacetylene suggested that the monomer underwent trans opening of the triple bond and polymerized along the diagonal of the bc plane of the unit cell. The reaction was described as an idealized one-step and one-dimensional growth process by an Avrami equation with an exponent 1.34.
Cyanoacetylene underwent polymerization reaction in a solid phase at pressures above 1.5 GPa. The Raman study of the reaction product showed that the polymer had a conjugated linear backbone with CN pendant groups. The Raman spectra for this substituted polyacetylene demonstrated a resonance behavior similar to that reported for trans-polyacetylene. The optical gap associated with the π–π* transition in the conjugated system was smaller than that of trans-polyacetylene, probably due to the resonance interactions between the CN triple bonds and the conjugated double bonds.
Raman spectra were measured for liquid (0–0.7 GPa), crystalline cubic (0.7–0.9 GPa), and orthorhombic (0.9–3.5 GPa) phases in C2H2 at room temperature. For the orthorhombic phase, the high-pressure behavior of the librational and internal vibrations was obtained in a wide pressure range. The frequencies of all the librational modes increased monotonically with pressure, while those of the internal modes showed a variety of frequency shifts depending on vibrational mode. These high-pressure data will be of great use for the construction of theoretical models for the intermolecular interactions in crystalline C2H2.
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