Radical polyadditions of bifunctional vinyloxiranes [1,4-bis(2-vinylepoxyethyl)benzene (3) and 1,4-bis(2-isopropenylepoxyethyl)benzene (4)] and dithiols are described. 3 and 4 were prepared by the phase transfer reaction of terephthaladehyde and the corresponding sulfonium salts. The radical polyaddition of 4 and aromatic dithiols [1,4-benzenedithiol (5a), 4,4’-isopropylidenebis(thiophenol) (5b), and bis(4-mercaptophenyl) sulfide (5c)] was carried out at 60 degrees C in PhCl in the presence of azobis-(isobutyronitrile) (AIBN) as an initiator to obtain the corresponding polymers with high molecular weights in excellent yields. The same polyaddition also proceeded at 20 degrees C under photoirradiation to give polymers of molecular weights lower than those obtained using AIBN. On the other hand, only a gelled polymer was obtained by the polymerization of 4 and 5a using di-tert-butyl peroxide (DTBP) at a higher temperature (120 degrees C) in PhCl. When 3 was used instead of 4 in the polyaddition with 5a at 60 degrees C, a gelled polymer was also obtained, probably due to cross-linking of the formed polymer by the attack of a propagating thiyl radical on the resulting vinylene group in the main chain. The polyadditions of 4 and aliphatic dithiols gave the corresponding polymers with molecular weights lower than those from 4 and aromatic dithiols. The results of the model reactions of vinyloxiranes [2-phenyl-3-vinyloxirane (1) and 2-isopropenyl-3-phenyloxirane (3)1 and thiols (benzenethiol and benzyl mercaptan) were consistent with those of the radical polyadditions of dithiols to bifunctional vinyloxiranes. The structure of the obtained polymers was confirmed by comparison with IR and NMR spectra of the model compounds obtained from 2 and benzenethiol in the presence of AIBN. These results indicate that the radical polyaddition proceeded mainly through the carbon-carbon bond cleavage of the oxirane ring of 4, accompanied by a small amount of carbon-oxygen bond fission, and led to the formation of the polymers with a main unit having vinyl ether and sulfide groups and a minor unit having hydroxy groups.