A pi-conjugated poly(alpha-dithienylen-dithiafulvene) (2) was obtained by the oxidation polymerization of 2,6-bis(2-thienyl)-1,4-dithiafulvene (1) as a dithiafulvene monomer derived from 4-(2-thienyl)-1,2,3-thiadiazole. When a solution of 1 in CHCl3 was added to a stirred solution of FeCl3 in CHCl3, only the low-molecular-weight product 2 was obtained. The mixture was stirred for 15 h with an N-2 flow. The polymerization at higher temperatures resulted in polymers with large insoluble fractions. A higher molecular weight polymer was obtained by the oxidation polymerization of a charge-transfer complex of 1 with 7,7,8,8-tetracyanoquinodimethane (compound 3). In contrast to 2, polymer 4 was readily soluble in dimethyl sulfoxide, dimethylformamide, and acetone and partially soluble in tetrahydrofuran and methanol and had a larger molecular weight (peak top molecular weight = 37,000). The conductivity of polymer 4 was 3 orders of magnitude larger than that of polymer 2. (c) 2005 Wiley Periodicals, Inc.