A pi-conjugated poly(dithiafulvene) (2) was obtained by the cycloaddition polymerization of aldothioketene with its alkynethiol tautomer derived from 1,4-bis(1,2,3-thiadiazolyl-4-yl)benzene (1) in a 94% yield. To a mixure of 1 and dimethyl sulfoxide (DMSO)/ethanol (5/1, v/v), KOH was added. After stirring the mixture overnight, piperidine was added to quench the terminal thioketenes. The reaction mixture was then poured into water to obtain the product. The cycloaddition polymerization of aldothioketene derived from 1 with its alkynethiol tautomer was studied under various conditions in several solvent systems. The structure of the polymer was supported by the H-1 NMR and C-13 NMR spectra. The number-average degree of polymerization (DP) of 2 was 8, estimated from the H-1 NMR analysis. Optical properties and electrochemical analysis of 2 were also studied. (C) 2004 Wiley Periodicals, Inc.