Grafting non-polar polymer chains on the surface of polar nanoparticles (e.g., silica) is an effective way to enhance particle/matrix interactions and thus promote their homogeneous dispersion within a non-polar matrix, which leads to improved mechanical properties. Current grafting methods have not yet produced nanoparticles with well-defined polymers on the surface and high grafting density. Here, we employed anionic polymerization high vacuum techniques to synthesize ω-triethoxysilyl polystyrene (PS-TEOS) and PS-b-PI macromonomers followed by hydrolysis/condensation of macromonomers, leading to the in situ formation of grafted (grafting through) silica nanoparticles with either PS or polystyrene-b-polyisoprene (PS-b-PI) chains. The molecular characteristics of the precursors PS-TEOS and PS-b-PI-TEOS were determined by proton nuclear magnetic resonance, size exclusion chromatography, and matrix-assisted laser desorption/ionization time-of-flight. The formation of PS (PS@SiO 2 NPs) and PS-b-PI (PS-b-PI@SiO 2 NPs) nanoparticles was verified by Fourier transform infrared spectroscopy, 29 Si solid-state NMR, transmission electron microscopy, thermogravimetric analysis, and dynamic light scattering. Blends of PS@SiO 2 with commercially available PS and PS-b-PI@SiO 2 with anionically synthesized thermoplastic elastomer (PS-b-PI-b-PS) were obtained either in melt by extrusion or in solution by evaporation. The role of polymer@SiO 2 on the mechanical properties and morphological features of the matrices was examined by tensile testing and scanning electron microscopy. The proposed general method controls the molecular weight, chemical composition, particle size, and grafting density of nanoparticles and effectively improves the mechanical characteristics of the two families of PS-based nanocomposites.