Novel 2,5-dimethylene-2,5-dihydrothieno[3,2-b] thiophene derivatives such as 2,5-bis[di(ethylthio)methylene]-2,5-dihydrothieno[3,2-b]thiopene (4b) and 2,5-bis [cyano(ethylthio)methylene] -2,5-dihydrothieno [3,2-b]thiophene (4c) were successfully synthesized as isolable crystals. Polymerization behavior of a,5-bis(dicyanomethylene)-2,5-dihydrothieno [3,2-b] thiophene (4a), 4b, and 4c was investigated. 4a, 4b, and 4c are not homopolymerizable with any initiators and also not copolymerizable with vinyl monomers such as styrene (St), methyl methacrylate, and acryronitrile except for an alternating copolymerization of 4a with St. 4a, 4b, and 4c did not copolymerize with 7,8-bis(butoxycarbonyl)-7,8-dicyanoquinodimethane (BCQ) as a highly conjugated comonomer and instead only homopolymer of BCQ was obtained, indicating that they are much less reactive than BCQ. To obtain the relative reactivity among 1c, 2c, and 4c, the rate of addition reaction of 2,2'-azobis(isobutylonitrile) (AIBN) with 4c was compared with those of AIBN with 7,8-bis(ethylthio)-7,8-dicyanoquinodimethane (Ic) and with 2,5-bis [cyano(ethylthio)methylene] -2,5-dihydrothiophene (2c) by NMR spectroscopy and analyzed with the first-order kinetics. The relative reactivity among 1c, 2c, and 4c was found to be as follows: 1c > 4c > 2c. The relationship between structure and reactivity for the quinonoid compounds was discussed.