The grafting through method was employed to study the effect of nanoclay confinement on the atom transfer radical polymerization (ATRP) of styrene. An ammonium salt containing a double bond on its structure was used as a clay modifier. Employing ATRP to polymerize styrene in the presence of modified montmorillonite resulted in a finely well-defined polystyrene nanocomposite. The gas chromatography (GC) results showed the linear increase of ln(M-0/M) versus time, which indicated the controlled behavior of the polymerization. Another confirmation of the living nature of the polymerization was the linear increase of molecular weight against monomer conversion concluded from the gel permeation chromatography (GPC) data. Nanoclay exerted acceleration on the polymerization of free polystyrene chains. The poly-dispersity indexes of polymer chains increased by the addition of nanoclay. In the case of clay-attached polystyrene chains, number and weight-average molecular weights were lower than that of freely dispersed polystyrene chains. The polydispersity index of the clay-attached chains was higher in respect to the freely dispersed polystyrene chains. The living nature of polymer chains was more elucidated by Fourier transform infrared spectroscopy (FTIR). Exfoliation of the clay layers in the polymer matrix of polystyrene nanocomposite containing the lowest amount of nanoclay has proven by Transmission Electron Microscopy (TEM). (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 409-417, 2012