We report the synthesis and characterization of a series of cyclopentadithiophene polymers containing thiophene, selenophene, and tellurophene as comonomers. The effect of the chalcogen atom has been studied by a range of techniques, including thermal, optical, electrochemical, and computational. The results showed that by increasing the size of the chalcogen atom, the optical band gap is reduced mainly due to a downshift in the LUMO energy level. In addition, the larger size also increases the intermolecular heteroatomheteroatom interactions facilitating the formation of polymer aggregates. This led to not only a stronger red-shifted band in the UVvis absorption spectrum as well as raise in the HOMO energy level but also a potential solubility issue for higher molecular weight polymers containing particularly tellurophene units.