Grafting polymeric chains on the surface of nanoparticles is an effective way to enhance particle-matrix interaction and their homogeneous dispersion within the matrix, leading to improved physical and mechanical properties. However, current methods for grafting polymeric chains onto nanoparticles have not produced well-defined polymers with high grafting density. Here, we employed anionic polymerization high vacuum techniques to synthesize in-situ grafted silica nanoparticles with either polystyrene (PS), polystyrene-b-polyisoprene (PS-b-PI) or 3- miktoarm star polymers (3-µ stars polymer) ((PS)2PS, (PS)2PI, and (PS)2PI-b-PS) by hydrolysis/condensation of ω-tetraethyl orthosilicate (TEOS) of PS, PS-b-PI and 3-µ star polymers. The molecular characteristics of the precursors PS-TEOS, PS-b-PI-TEOS, and 3-µ star polymer-TEOS were determined by 1H NMR, SEC, and MALDI-ToF. The synthesis of PS (PS@SiO2 NPs), PS-b-PI (PS-b-PI@SiO2 NPs), and 3-µ star polymer ((PS)2PS)@SiO2, (PS)2PI)@SiO2, and (PS)2PI-b-PS@SiO2 NPs) nanoparticles was verified by FT-IR, 29Si solid-state NMR, TEM, TGA, and DLS. Blends of PS@SiO2 with commercially available PS and PS-b-PI@SiO2 and 3-µ star polymer@SiO2 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@SiO2 on the mechanical and morphological properties was examined by tensile testing and SEM. In the fifth chapter, a high grafting density of polymer on the surface of SiO2 NPs approach that may also be employed in industry was presented. TEOS will be present in every repeating unit in the second block rather than simply the end group as it was in the first three chapters. First, anionic polymerization of PS functionalized with bromide group to utilize as a macroinitioter. Then, the macroinitioter (PS-Br) will initiate the polymerization of VTES (vinyltriethoxysilyl), ATMS (alyltrimethoxysilyl), and SETMS (styrene ethyltrimethoxysilyl) via atom transfer radical polymerization (ATRP) to form PS-b-PVTES, PS-b-PATMS, and PS-b-SETMS. SEC, 1H NMR, FT-IR, 29Si solid-state NMR, TEM, and DLS were used to confirmed copolymer grafting on SiO2 NPs. Lastly, a tensile test was used to study the mechanical properties of the nanocomposite. The proposed 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.