pi-Conjugated polymers (2) having electron-donating dithiafulvene units in the main chain were prepared by the addition reaction of aldothioketenes derived from sc-conjugated diynes (1,4-diethynylbenzene, 4,4'-diethynylbiphenyl, 2,7-diethynylfluorene, and 1,3-diethynylbenzene) with their alkynethiol tautomers. The structures of the polymers were supported by IR and H-1 NMR spectra and by elemental analyses. The obtained polymers were soluble in DMSO and DMF and partially soluble in acetone and methanol. The absorption peak due to the pi-pi* transition of 2a (prepared from 1,4-diethynylbenzene) was largely bathochromic shifted in comparison with that of 2-benzylidene-4-phenyl-1,3-dithiole (3) as a model compound. This red shift is ascribed to an effective expansion of the pi-conjugation system in 2a. In the case of 2d (prepared from 1,3-diethynylbenzene), the absorption maximum was same as that of 3. This result was caused by the fact that the pi-conjugation is not expanded through 1,3-disubstituted benzene. The cyclic voltammograms show that an anodic shift and a broadening of the oxidation peak for the dithiafulvene unit for 2a was observed compared with that of 3. Before doping, 2a exhibits an electrical conductivity of 3 x 10(-7) S cm-l as measured on a cast film from 1,3-dimethyl-2-imidazolidinone. Oxidation with iodine raised the electrical conductivity to 1 x 10(-3) S cm(-1).