alkoxy group); compounds represented by the general formula, ##STR4## wherein X1, X2, X3 and X4 are independently H, a halogen, or --COOR1 (where R1 is a C1 -C4 hydrocarbon group, an alkali metal or an alkaline earth metal), provided that X2 and X4 should always be substituents other than H; and/or such ethylenic compounds as maleic anhydride and anhydrous maleimide.