) ##STR22## wherein R2 is a hydrogen atom, halogen atom, cyano group or hydrocarbon residue having 1 to 20 carbon atoms; n is an integer from 0 to 20; R3 is a substitutent having at least one member selected from ##STR23## and a metal, or a hydrogen atom; and when R3 is a hydrogen atom, n is an integer from 1 to 20, said styrenic copolymer being characterized in that said structural unit (II) is c