Novel alternating conjugated copolymers (PI-M consisting of an electron-deficient benzothiadiazole and a variety of electron-rich thiophene-arene-thiophene units were synthesized by palladium-catalyzed polycondensations (Stille and Suzuki reactions), aiming at processable materials with a reduced optical band gap. The structures of P1-P6 were confirmed by H-1 NMR and C-13 NMR, and their molecular weights were determined by size exclusion chromatography. In the Suzuki polycondensation, the role of the catalyst [Pd(PPh3)(4) and Pd(OAc)(2)] on the resulting molecular weight was investigated. Pd(OAc), enhances the molecular weight of the polymers for both thiophene and phenylene bis-boronic esters as compared with Pd(PPh3)(4). The optical properties of the polymers were examined in solution and the solid state. The polymers with n-octyl substituents (P1, P4, P5, and P6) on the thiophene rings possessed less-planar structures as a result of torsional steric hindrance, and their absorption spectra appeared blueshifted as compared with their unsubstituted analogues (P2 and P3). The electrochemical properties of the polymers were studied using cyclic voltammetry. Although the alkyl substitution affects the oxidation potential, only marginal differences in the reduction potentials were observed.