Regioregular polythiophene copolymers 10a-d containing hexyl and 11-hydroxyundecyl side chains have been synthesised, by nickel-catalysed cross-coupling of well-defined Grignard intermediates. The hydroxy groups were protected during the polymerisation as tetrahydropyranyl ethers, and subsequently transformed into azide groups. These azide-functionalised copolymers 11a-d were heated under vacuum, leading to azide decomposition, nitrene formation and crosslinking. The resultant polymer films showed decreased solubility (or insolubility) and a shift in the absorption spectrum to shorter wavelengths dependent on the azide content of the polymer. This colour change is rationalised in terms of the conformational change in the polythiophene backbone, associated with thermochromism, which has been partially fixed by the crosslinking at high temperature. This is supported by the modified thermochromism and photoluminescence behaviour of the crosslinked polymer films.