The underlying nature of the thermal instability of doped poly(3-alkylthiophene)s, a fatal problem for their application, is discussed. The so-called “thermal undoping” is attributed to interactions due to steric hindrance of the long flexible side chains, which twist the conjugated main chain and kick out the dopants. Thus the routes to suppress thermal undoping are to avoid or alleviate the side chain interactions by separating the side chains from each other, or from the main chains, and also leaving space around the main chains to accommodate dopants. Accordingly, poly(3-(4-octylphenyl)thiophene), random copolymers of 3-methylthiophene and 3-octylthiophene, and regular copolymers of thiophene and 3-octylthiophene are prepared. Thermal undoping is significantly suppressed in these polymers. Some of these polymers are soluble and fusible in the neutral state. After doping they become highly conductive, yet remain stable even at elevated temperatures. Thermochromism, solvatochromism and thermal undoping are thus all related to side chain mobility.