Polymerization of phenylacetylene using rhodium catalysts can be carried out with high efficiency in compressed (liquid or supercritical) CO2. Using the complex [(nbd)Rh(acac)] (nbd = 12,5-norbornadiene, acac = acetyl acetonate) as a catalyst precursor, the polymerization rate in CO2 is considerably higher than in conventional solvents such as THF or hexane. The resulting polymers consist of THF-soluble (mainly cis-transoidal) and THF-insoluble (mainly cis-cisoidal) species. The polymer characteristics such as stereoregularity and molecular weight are compared to those obtained by polymerization in conventional solvents. Addition of {4-F(CF2)(6)(CH2)(2)C6H4}(3)P gives rise to a ligand-modified rhodium catalyst with high solubility in compressed CO2 and leads to an increase in selectivity toward the cis-transoidal polymer.