New pi-conjugated polymers (2) with the thioketene dimer unit were prepared by cycloaddition polymerization of bis(thioketene) derived from bis(triphenylphosphonium chloride) (1). The structures of the polymers were compared with that of 2,4-dibenzylidene-1,3-dithietane (3), which was prepared by dimerization of phenylthioketene derived from benzyltriphenylphosphonium chloride, by use of IR and H-1 NMR. The lower energy absorption edges of 2 were located at longer wavelengths than that of the model compound 3, due to developments of pi-conjugations in 2. The cyclic voltammogram of 3 showed irreversible two-step oxidation peaks at 0.25 and 0.61 V vs Ag/Ag+, indicating that the electron-donating ability of 3 is stronger than that of tetrathiafulvalene (TTF). The electron-donating properties of the polymers 2 were also confirmed by cyclic voltammetry and were found to depend on aromatic structures in 2. The polymer 2 formed charge-transfer (CT) complexes with TCNQ (1:1 ratio of the repeating unit in 2 to TCNQ) in DMSO. The degrees of CT in the complex were investigated by UV and IR measurements. In the solid state, the complex of 2b contained partially charge-transferred TCNQ (degree of CT = 0.59), resulting in higher conductivity than that of 2a.