plural, R1 may be the same or different, and a structural unit (II) represented by the general formula (II) ##STR5## 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 substituent having at least one member selected from ##STR6## and a metal, or a hydrogen atom; and when R3 is a hydrogen atom, n is an