An ion-exchanger with polyanionic molecular brushes was synthesized by a "grafting from" route based on "surface-controlled reversible addition-fragmentation chain transfer polymerization" (RAFT). The RAFT agent, PhC(S)SMgBr was covalently attached to rnonodisperse-porous poly(dihydroxypropyl methacrylate-co-ethylene dimethacrylate), poly(DHPM-co-EDM) particles 5.8 mu m in size. The monomer, 3-sulfo-propyl methacrylate (SPM), was grafted from the surface of poly(DHPM-co-EDM) particles with an immobilized chain transfer agent by the proposed RAFT protocol. The degree of polymerization of SPM (i.e. the molecular length of the polyanionic ligand) on the particles was controlled by varying the molar ratio of monomer/RAFT agent. The particles carrying polyanionic molecular brushes with different lengths were tested as packing material in the separation of proteins by ion exchange chromatography. The columns packed with the particles carrying relatively longer polyanionic ligands exhibited higher separation efficiency in the separation of four proteins. Plate heights between 130-200 mu m were obtained. The ion-exchanger having poly(SPM) ligand with lower degree of polymerization provided better peak-resolutions on applying a salt gradient with higher slope. The molecular length and the ion-exchanger group content of polyionic ligand were adjusted by controlling the degree of polymerization and the grafting density, respectively. This property allowed control of the separation performance of the ion-exchanger packing.