A superacidic ester group is useful in the production of ionomers for polymer electrolyte fuel cells. A polymer is produced from said vinyl monomer. A process produces a polymer containing a superacidic group. The vinyl monomer contains a haloalkyl ester group of a superacid, wherein the number of carbon atoms of the haloalkyl ester group is no more than 10, and the halogen in the haloalkyl group is chlorine and/or fluorine. A polymer containing said vinyl monomer as a repeating unit can be converted to a polymer containing a superacid group using at least one procedure of 1) heat treating at 50.degree.C. to 350.degree.C., and 2) contacting with a protic compound.