A method of forming a crosslinked polymer comprising the step of reacting a crosslinkable group in the presence of a polymer, wherein: the crosslinkable group comprises a core unit substituted with at least one crosslinkable unit of formula (I): the crosslinkable group is bound to the polymer or is a crosslinkable compound mixed with the polymer; Ar is aryl or heteroaryl which may be unsubstituted or substituted with one or more substituents independently selected from monovalent substituents and a divalent linking group linking the unit of formula (I) to the core unit; and R is independently in each occurrence H, a monovalent substituent or a divalent linking group linking the unit of formula (I) to the core unit, with the proviso that at least one R is not H.