Naphthalenediimide (NDI)-based polymers co-polymerized with thienyl units are an interesting class of polymer semiconductors because of their good electron mobilities and unique film microstructure. Despite these properties, understanding how the extension of the thienyl co-monomer affects charge transport properties remains unclear. With this goal in mind, we have synthesized a series of NDI derivatives of the parent poly{[N,N-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5-(2,2-bithiophene) (P(NDI2OD-T2)), which exhibited excellent electron mobility. The strategy comprises both the extension of the donor o-conjugation length and the heteroatomic fusion of the thiophene rings. These newly synthesized compounds are characterized experimentally and theoretically vis-a-vis with P(NDI2OD-T2) as the reference. UV-vis data and cyclic-voltammetry are adopted to assess the effect of the donor modification on the frontier energy levels and on the bandgap. Intra-molecular polaronic effects are accounted for by computing the internal reorganization energy with density functional theory (DFT) calculations. Finally electrons and holes transport is experimentally investigated in field-effect transistors (FETs), by measuring current-voltage characteristics at variable temperatures. Overall we have identified a regime where inter-molecular effects, such as the wavefunction overlap and the degree of energetic disorder, induced by the different donor group prevail over polaronic effects and are the leading factors in determining electrons mobility.