A photoinduced cationic polycondensation towards the synthesis of a thieno[3,4-b]thiophene (TT) based homopolymer PTT-L was described. The reaction mechanism was investigated by employing a series of control experiments. It was found that the polymerization initiated with a light-induced radical formation, followed by a cationic propagation, all in solid state, at room temperature, and without any catalyst or solvent. In addition, a reference polymer (PTT-Ni) was synthesized via Kumada catalyst transfer polycondensation (KCTP) to assist in structural characterization of polymer PTT-L. It was shown that both polymers exhibit analogous optical, thermal, electrochemical and electrical properties. This polymerization process can be utilized as a mean to form patterned PTT-L films.