A series of alternating donor-acceptor polymers, consisting of diphenyl-benzodipyrrolidone (BDP) and oligothiophene were synthesized and used in top-gate bottom-contact organic field-effect transistors. The BDP polymers exhibit ambipolar charge transport with high and balanced mobilities up to 0.21 cm2 V-1 s-1 for holes and 0.18 cm2 V-1 s-1 for electrons. Increasing the length of the oligothiophene reduces the electron mobility roughly by an order of magnitude per thiophene unit, but the hole mobility remains high. This behaviour is explained using DFT calculations by a strong localization of the electrons on the BDP units. © 2013 The Royal Society of Chemistry.