Rh+(2,5-norbornadiene)[(eta(6)-C6H5B-(C6H5)(3)] a zwitterionic Rh(I) complex, shows excellent activity for the polymerization of monosubstituted acetylenes under mild conditions. The reaction of phenylacetylene and its p- and m-substituted derivatives proceeds rapidly to give yellow soluble polymers with a cis configuration of the main polymer chain and with a high number-average molecular weight (>10(5)) in high yields. The o-substituted derivatives give stereoregular red crystalline polymers which are scarcely soluble in organic solvents. The Rh complex has a moderate activity for the polymerization of tert-butylacetylene, cyclohexylacetylene, and 3-ethyl-1-pentyne to afford stereoregular cis polymers in quantitative yields.