A monomer having general Formula (I) which may be substituted or unsubstituted: where E and E are the same or different and are reactive groups capable of undergoing chain extension; X is O, S, NR.sub.5, R.sub.5C--CR.sub.6 or R.sub.5.dbd.CR.sub.6; Y is O, S, NR.sub.7, R.sub.7C--CR.sub.8 or R.sub.7C.dbd.CR.sub.8; R.sub.5, R.sub.6 R.sub.7 and R.sub.8 are the same or different and each is independently H or a substituent group; and each Ar is the same or different and is independently a substituted or unsubstituted aryl or heteroaryl group. ##STR00001##