The preparation of nanocomposites, including styrene, tertbutylstyrene, and SiO2 nanoparticles, in toluene solution was attempted by in situ polymerization using a cyclopentadienyltitaniumtrichloride-methylaluminoxane, CpTiCl3-MAO, initiator system. SiO2 nanospheres (ca. 20nm in diameter) were synthesized by the sol-gel method. The nanoparticles' surface was modified with hexadecyltrimethoxysilane (Mod-SiO(2)Nps) in order to improve the interactions with the polymer. The polymerization activity increased as the proportion of p-methyl styrene was increased in the initial feed. With respect to the effect of the incorporation of nanoparticles in the reactions, the catalytic activity increased slightly in the presence of 5wt% of nanospheres compared to neat copolymerization without any nanoparticles. Our studies achieved a convenient route through in situ polymerization, avoiding further treatment of the nanocomposite. The thermal stability of the PS increased with nanoparticle incorporation. The effect of SiO2-Npts on the catalyst's activity and on the thermal properties of the resulting nanocomposites was determined.