tionally substituted with 1 to 3 methyl and/or ethyl groups with the proviso that one of R4 and R5 is a hydrogen atom and the other of them is not a hydrogen to a solvent which is unreactive with the compound (I) or (II) and in which the produced polymer is hardly or not soluble in a concentration of 0.3 to 4.5% by weight as converted to the polymer weight, and then polymerizing the compound (I) o