A novel addition polymerization of 2-cyano-1,4-bentenedithiol to 1,4-diethynylbenzene was carried out by UV irradiation in toluene at 50 degrees C under nitrogen atmosphere. The polymerization proceeded readily, and a pale-yellowish conjugated polymer containing sulfur atoms and cyano groups (($) over bar M(n) = 20,400-80,800) was obtained in a 60-80% yield for 120-250 min. The polymer was found to be 1 : 1 alternating structure of anti-Markownikoff’s type [GRAPHICS] and was insoluble in conventional organic solvents. Since the polymer having molecular weight of the order of 10(4) had a softening point at 115 degrees C, a thin polymer film was obtained by heat press. TG analysis of the polymer indicated its decomposition point at about 620 degrees C under argon atmosphere. The electrical conductivity of the polymer pellet was 10(-10) S/cm at 300 K without doping and on the order of 10(-5) S/cm on I-2 doping. Furthermore, the electrical conductivity of the undoped polymer pellet reversibly changed from the order of 10(-10) S/cm at 300 K to 10(-7) S/cm at 435 K with temperature variation, accompanying with increasing carrier density and mobility. The polymer pellet (($) over bar M(n) = 80,800) aged at 250 degrees C for 5 min under nitrogen atmosphere exhibited the order of 10(-7) S/cm at 300 K. Thermal treatment of the polymers was thought to cause spreading of conjugated system through molecular rearrangement supported by x-ray diagrams. An absorption edge of diffuse reflectance spectra of the polymer (($) over bar M(n) = 80,800) was 635 nm and shifted to 880 nm by heat treatment of the polymer.