A co-polymer comprising a repeat unit of formula (I): wherein Ar1, Ar2 and Ar3 each independently represent an aryl or heteroaryl group that may be unsubstituted or substituted with one or more substituents, the copolymer further comprising a conjugation-breaking repeat unit that does not provide any conjugation path between repeat units adjacent to the conjugation-breaking repeat unit. The copolymer may be used as a host material of a phosphorescent material in a light-emitting layer of an organic light-emitting device.