Polyadditions of 1,4-benzenedithiol (BDT) to bis(alkoxyallene)s, such as 1,4-bis(allenyl-oxy)xylene (3) and 1,4-bis(allenyloxy)benzene (4), were carried out in benzene at 25 degrees C by irradiation with a high pressure mercury lamp. Thiol groups were added to the terminal double bonds of the allenyloxy groups selectively to afford polymers containing reactive carbon-carbon double bonds in the main chain, similar to the radical polyadditions using azobis(isobutyronitrile) (AIBN). The molecular weight of the polymer obtained from BDT and 3 was 10 times higher than that of the polymer produced in the radical polyaddition with AIBN; whereas the molecular weight of the polymer from BDT and 4 was similar to that in the radical polyaddition, probably because of poor solubility of 4 and the polymer toward benzene. The geometrical structure of carbon-carbon double bonds in the polymer isomerized from an E to Z structure with reaction time by virtue of both the addition elimination of thiyl radical to the double bonds and the UV irradiation.