In the repeating unit [structural unit (II)] represented by the general formula (II), R2 is a hydrogen atom, halogen atom, cyano group or hydrocarbon residue having 1 to 20 carbon atoms, preferably hydrocarbon radical having 1 to 10 carbon atoms exemplified by saturated hydrocarbon radical, especially alkyl group such as methyl group, ethyl group, propyl group, butyl group, pentyl group and hexyl