Terephthalic acid bis-(thiophen-3-yl-methyl) thioester ( TTMT) was synthesized via the reaction of thiophen-3-yl methanethiol with terephthaloyl dichloride. This 3-functionalized thiophene monomer was polymerized in the presence of pyrrole (Py) upon constant potential application. Spectroelectrochemistry experiments reflected a pi to pi* transition with a band gap energy of 2.4 eV for the copolymer. A dual type electrochromic device (ECD) of P(TTMT-co-Py) and poly(3,4-ethylenedioxythiophene) (PEDOT) was constructed. The device switches between greenish yellow and blue upon application of -2.4 V and -0.8 V, respectively. Optical contrast was calculated as 17.5%, whereas switching time was found as 1.6 s at maximum contrast point.