A copolymer comprising a repeat unit of Formula (I) and at least one further repeat unit: Formula (I) wherein: wherein: Ar1 and Ar2 are each independently selected from aryl and heteroaryl, each of which is independently unsubstituted or substituted with one or more substituents; each R is independently a substituent; each n is independently 0, 1 or 2; each m is independently 0, 1, 2 or 3, and at least one of the repeat units of the polymer is a partially conjugating repeat unit.