a process for preparing polymers having quaternary ammonium groups comprising polymerizing a vinyl monomer component containing a vinyl monomer having a tertiary amino group in the presence of a specific solvent , while quaternizing the tertiary amino group with a quaternizing agent , to produce the vinyl polymers having quaternary ammonium groups in the form of particles dispersed in the solvent , which is possible to easily separate the produced polymers having quaternary ammonium groups from the solvent without requiring a heating step for removing the solvent , and which can produce quaternary ammonium group - containing copolymers having a high content of a hydrophobic vinyl monomer . the quaternary ammonium group - containing copolymers obtained by this process are useful as modifier for plastics , permanent antistatic agent , bactericide or fungicide , chemicals for paper making and water treating agent .