High-pressure infrared spectra were measured in a diamond anvil cell at room temperature and pressures up to 13.7 GPa. Transitions from a liquid to a crystalline cubic and to a crystalline orthorhombic phase were observed in the molecular phase at 0.7 and 1.0 GPa, respectively. Polymerization occurred in the orthorhombic phase at 4.2 GPa, accompanied by the appearance of new peaks indicating formation of C=C double and saturated C-C single bonds. The saturated single bonds showed an increase in quantity on further compression beyond 4 GPa, while the double bonds showed no significant change. The spectrum of the reaction product recovered from 14 GPa indicated formation of cross-linked polymers containing trans- and cis-C=C and saturated C-C single bonds.