The synthesis of aluminium nanoparticles in a polypropylene (PP) matrix by a sol-gel process in the melt was investigated. The study was performed using two aluminium precursors, aluminium isopropoxide and aluminium acetylacetonate, to assess the effect of the stereochemical hindrance of the precursor organic groups on reactivity. The hybrid nanocomposites were prepared in a batch mixer under constant processing conditions and characterized by several techniques. Chemical and morphological characterization proved that aluminium formed covalent bonds with polypropylene modified with maleic anhydride and the presence of aluminium nanoparticles. Moreover, TEM micrographs of both nanocomposites synthesised revealed a lower amount of nanoparticles when aluminium acetylacetonate was used. The values of the activation energy determined explained the higher reactivity of aluminium isopropoxide. The reaction mechanisms were proposed for both precursors. (C) 2012 Elsevier Ltd. All rights reserved.