We studied ring-collapsed radical alternating copolymerization (RCRAC) of hexaphenylcyclohexastibine (cyclo-(PhSb)(6)) and several acetylenic compounds in the presence of a catalytic amount of 2,2'-azobis(isobutyronitrile) (AIBN) in tetrahydrofuran at 70 degrees C. Although the monosubstituted acetylenic monomers, phenylacetylene (1a), 4-cyanophenylacetylene (1b), and 4-methoxyphenylacetylene (1c), gave low molecular weight poly(vinylene stibine)s, RCRAC of dimethyl acetylenedicarboxylate (1d) and cyclo-(PhSb)(6) provided a corresponding poly( vinylene stibine) (2d) of which the number-average molecular weight was 4300 by gel permeation chromatographic analysis. The structure of 2d was supported by H-1 and C-13 NMR spectra and elemental analysis. The structure and the molecular weight of 2d were insensitive to solvents and the feed ratio of the monomers. When hexaphenylcyclohexaarsine (cyclo-(PhAs)(6)) was used in RCRAC with 1d, no polymerization proceeded. These results were derived from different reactivity of pnictogen radicals. When the results of RCRAC of cyclo-(PhAs)(6) and that of 2 are compared, the arsenic radicals had more reactivity to the ethynyl groups and the antimony radicals had more reactivity toward the vinyl radicals.