Among the hard polymers useful for the present invention, vinyl polymers such as polymethyl methacrylate, poly-t-butyl methacrylate, polystyrene, poly-t-butylstyrene, polymethacrylic acid and its salt, polyacrylamide, poly(meth)acrylonitrile, poly-2-chlorostyrene and 3-chlorostyrene are particularly preferred in this embodiment wherein radical polymerization is conducted by using a polymerization