NOVELTY - Preparing particles including a material capable of catalyzing the reduction of oxygen or the oxidation of hydrogen, and functionalized by polymers including at least one repeating unit having at least one proton-conducting group, and covalently bonded to a carbon material, comprises (a) contacting particles including the material, with the polymer, and at least one portion corresponding to an organic radical of a compound that is an initiator for atom transfer radical polymerization to obtain the particles grafted with the polymers.    USE - The process is useful for preparing particles (claimed), which are useful to catalyze the reduction of oxygen or oxidation of hydrogen. The particles are useful for the formation of catalytic layers of proton exchange membrane fuel cells.    ADVANTAGE - The particles have improved catalytic activity (preferably for the oxidation of hydrogen or reduction of oxygen) and improved protonic conductivity.    DETAILED DESCRIPTION - Preparing particles including a material capable of catalyzing the reduction of oxygen or the oxidation of hydrogen, and functionalized by polymers including at least one repeating unit having at least one proton-conducting group, and covalently bonded to a carbon material, comprises (a) contacting particles including the material, with the polymer, and at least one portion corresponding to an organic radical of a compound that is an initiator for atom transfer radical polymerization, where the radical includes at least one group capable of being grafted onto the surface of the particles, to obtain the particles grafted with the polymers. An INDEPENDENT CLAIM is included for a polymer comprising a median-organic group having a disulfide (-S-S-) group forming a bridge between two identical parts, where part has a phenyl group having an amide group of formula (-NH-CO-R21), and two-polymer chains comprising at least one repeating unit carrying at least one proton-conducting group, where the chains are connected to the central organic group via R21.    R21 = divalent hydrocarbyl.