A polymer, for use in an organic light emitting device, comprises asymmetrically substituted repeat units of formula (I (a)): wherein R7 represents a substituent bound to the 9-carbon atom of the fluorene ring through a non-aromatic carbon atom, R8, R9 and R11 independently in each occurrence represent H or a substituent with the proviso that at least one R8 is not H; R10 independently in each occurrence is a substituent; and t in each occurrence is independently 0, 1, 2 or 3. R7 is preferably a linear alkyl substituted with one or more groups —(Ar6)w, wherein each Ar6 independently represents an optionally substituted aryl or heteroaryl group, and w is at least 1, for example 1, 2 or 3.