starting to decompose at similar temperatures. This observation aligns well with literature in which the thionation of naphthalene diimide-bithiophene copolymers was studied and thionation reduced their thermal stability. This can be attributed to the lower stability of thioamides compared to amides. All samples appear completely amorphous, no crystallization or melting point was observed in DSC thermograms. Glass transition temperatures (Tg) were determined from the inflection point of the second and third heating cycles of the DSC thermograms (Figure ). Increased thionation increases in Tg with a linear correlation, except for S16 (Figure ). This can be attributed to an increased rotational barrier on carbon-nitrogen bonds for the thioamides repeating units. Chain flexibility mainly depends on the rotational flexibility of backbone substituents. Case in point, after (partial) reduction of POx polymers to poly(N-alkyl ethylene imine)s, which yields freely inverting tertiary amines lower Tg values are observed. Furthermore, the increased double-bond character of the thioamides is corroborated by IR spectroscopy. The absorbance of C-N str,amide at 1060 cm -1 82 is decreased noticeably with increased thionation degree with a new band emerging at 950 cm -1 , which we attributed to C-N str thioamide (Figure ).