Compounds of general formula (I) ##STR1## characterised in that R.sup.1 and R.sup.2 are independently selected from the group consisting of COOR, CN, aryl, substituted aryl, COOH, halogen, C(O)NHR.sup.4, C(O)NR.sup.5 R.sup.6 ; X and Y are independently selected from the group consisting of H, COOH, COOR, CN, R.sup.3 CO--, C(O)NHR.sup.4, C(O)NR.sup.5 R.sup.6, P(O)(OR.sup.7).sub.2 and SO.sub.2 R.sup.8 ; with the proviso that X and Y are not both H; or X and Y together with the carbon atom to which they are attached form a carbocyclic or heterocyclic ring system which can contain oxygen, sulfur or nitrogen atoms; and R, R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.7 and R.sup.8 are various groups; and polymers and copolymers derived from such compounds.