We report on the controlled synthesis of naphthalenediimide-bithiophene copolymers with varying ratios of 2,6- and 2,7-linkage (parent polymer: poly{[N,N'-bis (2-octyldodecyl)-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} P(NDI2OD-T-2)). Monomers with different ratios of 2,6- and 2,7-regioisomers could be isolated, leading to n-type polymers with tunable regioregularity. With increasing content of 2,7-linkage in the polymer backbone the aggregation tendency can be drastically reduced. Both the reduction behavior and the electron transport properties are very similar for all systems. For the regioregular system we find mobility values of 3 x 10(-1) cm(2)/(V s); the regioirregular systems become comparable with values as high as 7 x 10(-2) cm(2)/(V s) when temperature annealing is employed. This means that regioregularity seems to have an overall lower effect in comparison to classical systems such as poly(3-hexylthiophene). Tuning regioregularity in such systems therefore provides a powerful tool to optimize processing without deteriorating device performance. We even find that device performance of bulk-heterojunction solar cells with a polythiophene derivative as donor strongly improves upon use of the regioirregular systems.