A series of defect-free n-type copolymers poly(naphthalene diimide-alt-indacenodithiophene) P(NDI-IDT) comprising alternating naphthalene diimide (NDI) and indacenodithiophene (IDT) units is prepared using atom-economic direct arylation polycondensation (DAP). Copolymers with varying molecular weights up to Mn,SEC = 113 kg mol-1 are obtained in high yield and used to investigate the optical, thermal and electrical properties as a function of chain length. Two weak endotherms are seen in differential scanning calorimetry experiments at 68 °C and 180-220 °C, depending on molecular weight, which are ascribed to side chain (Tm,1) and main chain (Tm,2) melting, respectively. Thin-film morphologies are weakly crystalline for annealing temperatures below Tm,2, with organic field-effect (OFET) mobilities being on the order of 10-3 cm2 V-1 s-1. Under these conditions very low Urbach energies (EU) between 28 and 30 meV are found. However, thermal annealing above Tm,2 results in amorphous morphologies with hypsochromically shifted optical spectra, strongly increased EU and complete loss of mobility. In comparison to the well-investigated bithiophene analogue PNDIT2, P(NDI-IDT) thin films are significantly less crystalline, and solutions lack the typical features of aggregation. This is ascribed to the additional bulky hexylphenyl side chains of P(NDI-IDT) that hamper main chain ordering and aggregate formation and thus, interchain charge hopping. With intrachain transport being limited as well, field-effect electron mobilities are limited to ∼10-3 cm2 V-1 s-1 and mostly independent of chain length. These results demonstrate the importance of both intra- and interchain transport to electron mobility for NDI main chain copolymers.