general formula (I): ##STR2## wherein R1 is H or CH3 ; each of R2 and R3 is an alkyl group having 1 to 2 carbon atoms; R4 is H or an alkyl group of 1 to 2 carbon atoms; A' is an oxygen atom; B' is an alkylene group of 2 to 4 carbon atoms; and X-- is an anionic counterion; and (b) a second monomer comprising (meth)acrylamide in an aqueous solution of a polyvalent anionic salt, wherein the polymeriz