The atom transfer radical polymerization (ATRP) of styrene onto poly-(vinylidene 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 2,2Ј-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°C. The degree of grafting increased linearly with time, thereby indicating first-order kinetics, regardless of the polymerization temperature. Furthermore, 120°C 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°C. 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-(styreneblock-tert-butyl acrylate)] membranes were also synthesized by ATRP.

A versatile method for the preparation of proton exchange membranes (PEMs) by the combination of radiation chemistry of polymers with nitroxide-mediated living free radical graft polymerization with subsequent sulfonation is presented. Thus, poly(vinylidene fluoride) (PVDF) membranes were first irradiated with electron beam and then the free radicals formed were immediately quenched with 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO). In the second step, the produced TEMPOcapped macroinitiator sites were utilized in nitroxide-mediated living free radical graft polymerization of styrene or for the controlled graft copolymerization of styrene and N-phenylmaleimide onto the PVDF membrane. In the final step, the membranes were either directly sulfonated, or in the former case, alternatively the alkoxyamine moieties at the polymer chain ends were substituted by a maleimide derivative prior to sulfonation. The introduction of the N-phenyl maleimide moieties into the grafted chains significantly increased the thermo-oxidative stability of the PEMs as determined by thermogravimetric analysis (TGA). In this work, also a comparison between these membranes prepared by the controlled nitroxide-mediated graft polymerization and those membranes prepared by the conventional preirradiation grafting method in terms of grafting yields, thermo-oxidative stability, water uptake, and ion exchange capacity, proton conductivity, etc. is presented. Noteworthy is the fact that the membranes using the controlled grafting technique are grafted through the membrane already at a degree of grafting of 14%, whereas the penetration limit for the membranes prepared by conventional radiation-induced grafting is approximately 30% as determined by SEM-EDX analysis. Furthermore, preliminary H 2/O2 fuel cell tests showed promise for the development of this type of PEMs prepared by means of a nitroxidemediated living free radical grafting process. Thus, already the PVDF membrane that had been grafted with styrene by means of controlled radical polymerization could after sulfonation be used in a fuel cell for 930 h at 70 °C without any drop in current density. In contrast, according to our previous studies, PVDF membranes prepared by conventional preirradiation grafting of styrene fail within 150 to 200 h under similar conditions.

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