[4-[(3,5-Di-tert-butyl-4-hydroxyphenyl)(3,5-di-tert-butyl-4-oxocyclohexa-2,5-dien-1-ylidene)methyl]phenyl]acetylene (6), was synthesized and polymerized to yield a solvent-soluble polyacetylene poly[[4-[(3,5-di-tert-butyl-4-hydroxyphenyl)(3,5-di-tert-butyl-4-oxocyclohexa-2,5-dien-1-ylidene)methyl]phenyl]acetylene] (5a). The polymer gave its polyradical derivative 5b through the chemical oxidation with active PbO2. 5b was stable, whose spin concentration could be increased up to 4.8 x 10(23) spins per molar monomer unit. ESR analysis indicated localization of unpaired electrons in the galvinoxyl residue. ESR analysis also suggested an exchange-narrowing and interpolymer dipole-dipole interaction.