Suitable alkyl (meth)acrylate for use in the step (2) mentioned above include alkyl acrylates such as methyl acrylate, ethyl acrylate, n-propyl acrylate, n-butyl acrylate, and 2-ethylhexyl acrylate; and alkyl methacrylates such as hexyl methacylate and 2-ethylhexyl methacrylate.