A novel addition polymerization of 1,4-benzenediselenol (BDSe) to 1,4-divinylbenzene (DVB) was carried out with various azo radical initiators [dimethyl 2,2’-azobisisobutyrate (DAIB), 1,1’-azobis(1-acetoxy-1-phenylethane) (AAPE), and AIBN] in toluene at 65 and 75-degrees-C under a nitrogen atmosphere. The polymerization proceeded without an induction period, and pale-yellowish powder polymers were obtained in 89% yields for 75 h (DAIB), 89% yields for 24 h (AAPE), and 60% yields for 8 h (AIBN). The molecular weight (M(w)BAR) of the insoluble polymers in toluene was about 4000 (AIBN) to 14,000 (DAIB or AAPE) by GPC. The polymer had an alternating structure of BDSe to DVB units by H-1-NMR, IR analyses, and selenium contents, but the polymer contained the diselenide linkage by Raman spectroscopy. By AIBN initiator, the yield of the polymers did not increase over 60% and higher molecular weight polymer was hardly obtained. According to the model addition reaction of benzeneselenol to styrene by AIBN, it was found that AIBN was consumed by the side reaction between dimethyl-N-(2-cyano-2-propyl) ketenimine derived Apple II from AIBN and benzeneselenol to give the adduct C, MH+ 295 by DCI MS. On the other hand, DAIB and AAPE initiators, which do not form a ketenimine intermediate, gave the polymers of higher molecular weight in a higher yield. The polymer film exhibited high refractive index (n(D)25 = 1.81) and a reversible phase transition between a transparency and an opaque by thermal mode.