. . "are each an alkyl group or an aryl group which may be the same or different and are allowed to be substitued, and at least one of the groups represented by R1, R2, R3 and R4 is a group containing double bond, a halogen atom, an epoxy group, an acid anhydride residue, an alkoxy group, an alkoxycarbonyl group, an amino group, an acryloyl group, a methacryloyl group, an acrylamido group, a methacryla" . . . .