ional vinyl monomer, to give an emulsion of a phosphorus-containing hydrophilic copolymer, and coagulating the copolymer emulsion with a coagulant selected from the group consisting of a tertiary amine, a quaternary ammonium salt and an amine compound represented by the formula (2): ##STR6## wherein R3 is a hydrocarbon group, R4 and R5 independently represent divalent hydrocarbon radicals, r and s