Confinement effect of graphene nanoplatelets on the kinetics of styrene atom transfer radical polymerization was studied by a "grafting from" reaction. Graphene oxide was modified by different amounts of (3-aminopropyl) trie-thoxysilane and then alpha-bromoisobutyryl bromide from the hydroxyl groups. Polymerization of styrene in the presence of modified graphene and free initiator, ethyl alpha-bromoisobutyrate, was accomplished at 110 degrees C. Then, effect of various graft densities and different graphene loadings on the heterogeneous graft and free polystyrene chains characteristics and also kinetics of polymerization was studied by gas and gel permeation chromatographies. Efficiency of grafting reactions along with the graft contents was studied by X-ray photoelectron spectroscopy, elemental analysis, and thermogravimetric analysis. Confinement effects of graphene on the relaxation behavior of polystyrene chains and also morphology of the graphenes were studied by differential scanning calorimetry and transmission electron microscopy, respectively. (C) 2014 Society of Plastics Engineers