Recent intensive research into the development of high mobility polymer semiconductors has achieved field effect mobilities higher than 10 cm2/Vs, when fabricated as a polymer thin film transistor. However, most of this work has been aimed at achieving a high mobility/reliability in polymer semiconductors, with very few reports regarding the use of non-chlorinated solvents in the processing of polymer semiconductors. From an industrial point of view, the chlorinated solvent that is currently the most widely used in the processing of polymer semiconductors should be excluded owing to its high environmental cost. Here, we show that controlling the random copolymerization between two different diketopyrrolopyrole-based conducting units by varying the relative quantity of each represents a suitable synthetic strategy to increase the solubility of polymer semiconductors in a non-chlorinated solvent, without compromising high charge carrier mobility. Highly performing and reliable polymer thin film transistors processed from environmentally benign solvents such as tetralin are demonstrated for the first time, resulting in a mobility of greater than 5 cm2/Vs.