The aim of this work was to apply the concept of nano-reactors to the reactive extrusion process of the free radical grafting of maleic anhydride onto polypropylene (PP), an important process in the polymer industry. Pre-confinement of dicumyl peroxide in the lamellar structures of organically modified montmorillonite allowed slowing down the release of primary radicals and consequently to significantly improve the selectivity of the grafting of maleic anhydride onto PP and decrease the selectivity of PP chain scission. To the authors' knowledge, this is the first time that a non-chemical method is found to solving the inherent problem facing this complicated process: increasing the grafting degree is always concomitant to increasing PP chain scission. (c) 2006 Elsevier Ltd. All rights reserved.