Abstract: A polymer comprises repeat units of formula (I): wherein: R1 independently in each occurrence is a substituent, and two substituents R1 may be linked to form a ring; R2 in each occurrence independently is H or a substituent, and two substituents R2 may be linked to form a ring; R3 independently in each occurrence is a substituent; Ar1 independently in each occurrence is an aryl or heteroaryl group that may be unsubstituted or substituted with one or more substituents; x is 0, 1 or 2; y is 0, 1 or 2; and z independently in each occurrence is 0, 1 or 2. Such repeat units have a relatively deep LUMO level that may reduce the barrier to transport of electrons from the electron-transporting layer to the light-emitting layer of an organic light emitting diode having an active layer comprising such a polymer.