A linear water-soluble quaternary ammonium polymer having a homopolymeric chain of repeating unit of general formula (I): ##STR1## where each of R.sub.1 and R.sub.2 independently represents a member selected from the group consisting of linear or branched alkyl, hydroxyalkyl, alkoxyalkyl groups having from 1 to 6 carbon atoms, X.sup.- represents an anion and n is an integer comprised between 30 and 220 has been prepared. Furthermore, the polymer has a molar mass distribution of less than or equal to 1.5. The polymer is used as a displacer in a displacement mode of chromatography.