In carrying out the polymerization in aqueous solution, use is made, as a radical polymerization initiator, a water-soluble polymerization initiator, for example, a persulfate salt such as ammonium persulfate, sodium persulfate or potassium persulfate; hydrogen peroxide; or a water-soluble azo initiator, for example an azoamidine compound such as 2,2???-azobis-2-methylpropionamidine hydrochloride,