The synthesis of a novel organoarsenic polymer is described that contains arsenic atoms in the main chain. Radical copolymerization of a six-membered cyclic diarsine compound, 1,2,4,5-tetramethyltetrahydrodiarsenine (1), with vinyl monomers such as styrene and methyl methacrylate in the presence of 2,2′-azobisisobutyronitrile was carried out. The number-average molecular weights of the copolymer obtained after reprecipitation were estimated to be more than 10,000 by gel permeation chromatography analysis (CHCl3, polystyrene standards). By characterization with 1H NMR and 13C NMR spectra, the copolymer was found to possess no diarsenic linkages in the backbone. The polarity and the conjugative stabilization of the substitution of the vinyl group affected the content of the vinyl monomer unit in the copolymer.