Dithiafulvene oligomers (3) were prepared by cycloaddition polymerization of aldothioketenes with their alkynethiol tautomers derived from 1,4-diethynylbenzene (2) with the addition of 1-ethynyl-4-methylbenzene (1) as a monofunctionalized compound. Different feed ratios of 2/1 were used to control the molecular weights of 3. The structures of 3 were confirmed by IR and H-1 NMR spectroscopies in comparison with those of 2-(4-tolylidene)-4-tolyl-1,3-dithiol (4) as a model compound, which was obtained by the treatment of lithium 2-tolylethynethiolate with water in Et2O. The number-average degree of polymerization (D]P) and the number-average molecular weight were measured by gel permeation chromatographic and H-1 NMR analysis. DP increased with an increasing feed ratio of 2/1. The ultraviolet-visible spectra of 3 in diluted acetonitrile showed that the absorption maxima of 3 increased with an increasing DP of 3. These redshifts are ascribed to an effective expansion of the pi-conjugation system in 3. The oligomers exhibited a maximum conjugation length of seven repeating units. The redox properties of 3 were examined by cyclic voltammetry. The oxidation half-peak potentials (E-p/2) of 3 were slightly cathodically shifted with increasing DP. (C) 2003 Wiley Periodicals, Inc.