The atom transfer radical polymerization (ATRP) of styrene onto polyvinylidene fluoride)-graft-poly(vinylbenzyl chloride) (PVDF-g-PVBC) membranes was investigated. Novel membranes were designed for fuel-cell applications. The benzyl chloride groups in the PVDF-g-PVBC membranes functioned as initiators, and a Cu-based catalytic system with the general formula Cu(n)X-n/ligand [where X is Cl or Br and the ligand is 2,2'-bipyridyl (bpy)] was employed for the ATRP. In addition, 10 vol % dimethylformamide was added for increased solubility of the catalyst complex in styrene. The system was homogeneous, except for the membrane, when the initiator/copper halide/ligand/monomer molar ratio was 1/1/3/500. As anticipated, the fastest polymerization rate of styrene was observed with the copper bromide/bpy-based catalyst system. The reaction rate was strongly temperature-dependent within the studied temperature interval of 100-130 degreesC. The degree of grafting increased linearly with time, thereby indicating first-order kinetics, regardless of the polymerization temperature. Furthermore, 120 degreesC was the maximum polymerization temperature that could be used in practice because the membrane structure was destroyed at higher temperatures. The degree of styrene grafting reached 400% after 3 h at 120 degreesC. Such a high degree of grafting could not be reached with conventional uncontrolled radiation-induced grafting methods because of termination reactions. On the basis of an Arrhenius plot, the activation energy for the homogeneous ATRP of styrene was 217 kJ/mol. The prepared membranes became proton-conducting after sulfonation of the polystyrene grafts. The highest conductivity measured for the prepared membranes was 70 mS/cm, which is comparable to the values normally measured for commercial Nafion membranes. The scanning electron microscopy/energy-dispersive X-ray results showed that the membranes had to be grafted through the matrix with both PVBC and polystyrene to become proton-conducting after sulfonation. In addition, PVDF-g-[PVBC-g-(styrene-block-tert-butyl acrylate)] membranes were also synthesized by ATRP.