Polymer analogous thionation of the n-type conjugated polymer PNDIT2 is investigated using Lawesson's reagent (LR). Detailed high-temperature NMR. spectroscopic investigations show that due to the copolymer structure, two out of the four available carbonyl groups present in the naphthalene diimide (NDI) comonomer are sterically less hindered and react preferentially. This leads to regioselective thionation in the trans-configuration even for a large excess of LR For high degrees of O/S conversion, signals of minor intensity show up in addition pointing to undesired side reactions. These signals could not be eliminated despite further optimized reaction conditions including different aromatic solvents and reaction temperatures. Compared to PNDIT2, the resulting 2S-trans-PNDIT2 features strong aggregation, lower solubility, an 80 nm bathochromic shift of the charge-transfer band, a by 0.22 eV lower LUMO energy level, a lower thermal stability, and higher melting temperatures (T-m). As the combination of the lower thermal stability and higher melting points renders the characterization of thermal transitions challenging, fast scanning calorimetry (flash-DSC) is successfully used to determine T-m. With increasing O/S conversion, Tm first increases but then decreases, which is ascribed to a combined effect of stronger main chain interactions and increasing chemical defects. Microstructural order and field-effect electron mobilities decrease with increasing O/S conversion compared to PNDIT2.