Alkylpropiolate having n-hexadecyl moiety in the ester group was stereospecifically polymerized with a [Rh(norbornadiene)Cl](2) complex catalyst in alcohol to give rise to cis-transoid polyacetylenes in high yields. The unusually facile cis to trans isomerization for the polymer was found to be induced when the polymer was warmed to even less than ca. 57 degrees C. Under these conditions ESR spectra bearing hyperfine structures due to the radicals generated by the rotational scission in the cis-pi bond were observed. This isomerization induced a large g value shift in the ESR spectra together with an increase of the radical concentration during the isomerization, indicating formation of planar conjugated trans sequences that stabilize mobile unpaired electrons as solitons.