A polymer for use in an optical device comprising a first, optionally substituted, repeat unit of formula ##STR00001## and a second, optionally substituted, repeat unit of formula ##STR00002## wherein each Ar and Ar' is the same or different and comprises an optionally substituted aryl or heteroaryl group and optionally a third, optionally substituted, repeat unit in a molar ratio of no greater than 5%, the third repeat unit having a formula --Ar--N(Ar)--Ar-- and having a single nitrogen atom in its backbone.