In this work, a branched polymer-grafted cation exchanger was synthesized on a Sepharose FF matrix via atom transfer radical polymerization using 3-sulfopropyl methacrylate potassium salt (SPM) as a functional monomer and 2-(2-bromo-isobutyryloxy)ethyl methacrylate as a branching monomer. The resulting branched poly(SPM)-grafted cation exchangers exhibit typical ionic exchange characteristics for lysozyme adsorption and the maximum adsorption capacity reach 450 mgimL on Sep-BrS-S12B2 whereas gamma-globulin adsorption is more dependent on polymer architecture. By adjusting the ratio of the monomers, very high adsorption capacity for lysozyme and gamma-globulin can be achieved in the more branched Sep-BrL-S4B3 while gamma-globulin has a much higher effective diffusivity in Sep-BrL-S4B3. Experimental evidences show that the performance of polymer -grafted cation exchanger can be improved by regulating polymer architecture. The calorimetric results further indicate that counter-ions are released from polymer and proteins during protein adsorption. With an increase of the adsorption density, adsorbed proteins experience a change of molecular orientation along poly(SPM) chain. In Sep-BrL-S4B3, dynamic binding capacities reached 145 mgimL for lysozyme and 96 mgimL for gamma-globulin, demonstrating that Sep-BrL-S4B3 is a promising type of novel high-capacity cation exchangers. This research gives clue to the design of high-capacity cation exchangers and offers insights into protein adsorption on polymer -grafted cation exchangers. (C) 2017 Elsevier B.V. All rights reserved.