Today, there is an intense search for new electron donor polymers for the active layers of organic solar cells. The synthesis of copolymers, in which there is a mix of electron donor monomers with others that are electron acceptors, is a currently employed approach that is used to obtain new polymers with more interesting electronic properties. Usually, the comonomers have a very different structure and the energies of the frontier electronic levels are unpredictable. We demonstrate in this work that the approach of using similar comonomers allows for the provision of copolymers with predictable energies of the frontier electronic levels. We explore the properties of copolymers consisting of monomers of poly(3-hexylthiophene), poly(3-hexyloxythiophene) and some poly(3-hexylthiophene) derivatives. The results indicate that the copolymers have intermediate frontier orbital energies when compared to the parent homopolymers and these energies are easy to predict. This approach is an alternative way of designing a copolymer with favorable tuning of the energies of the frontier electronic levels and is supported by experimental evidence. POLYM. ENG. SCI., 56:479-487, 2016. (c) 2016 Society of Plastics Engineers