A monomer of formula (III): wherein X is a polymerisable group Ar, Ar1 and Ar2 each independently represent an optionally substituted aryl or heteroaryl group; R1 represents H or a substituent; and Z represents a direct bond or a divalent linking atom or group, wherein Ar1 and Ar2 are linked by a single bond or a divalent linking group selected from CR1R2, SiR1R2, PR1, NR1, O and S wherein R1 and R2 are independently selected from hydrogen; optionally substituted alkyl wherein one or more non-adjacent C atoms may be replaced with O, S, N, C═O and —COO—; alkoxy, aryl, arylalkyl, heteroaryl and heteroarylalkyl. Ar1 and Ar2 are preferably linked by an oxygen atom, and Ar1 and/or Ar2 may be fused to their respective adjacent Ar groups. Ar1 and its adjacent Ar group and/or Ar2 and its adjacent Ar group are optionally fused to form a fluorene unit.