The segmental dynamics in 1,2-polybutadiene was studied over a broad range of temperature and pressure using dielectric spectroscopy. The glass transition temperature was found to be strongly dependent on pressure (dT(g)/dP = 240 KJGPa). Moreover, while no appreciable change in the shape of the relaxation function with pressure was observed for spectra compared at a fixed value of the relaxation time, the fragility (T-g-normalized temperature dependence) increased with pressure. From the ratio of the isochronal to isobaric expansivities, and consistently from the ratio of the isochoric and isobaric activation energies, temperature was found to exert a stronger influence on the dynamics than does the volume. This is similar to results for other polymers as well as small molecule glass-formers (when the latter lack hydrogen bonds). Finally, a comparison was made of the properties of 1,2-PBD with those of other vinyl polymers having different pendent groups. The different segmental dynamics reflect the manner in which local relaxation is governed by intermolecular cooperativity. Consistent with this interpretation, 1,2-PBD exhibits an activation volume more than 9 times the molar volume of its repeat unit; this is significantly larger than the values for the other vinyl polymers.