Hexamethyldisilazane-modified silica aerogel nanoparticles were used for in situ copolymerization of styrene and methyl methacrylate via activators generated by electron transfer atom transfer radical polymerization (AGET ATRP) to synthesize well-defined random poly(styrene-co-methyl methacrylate) nanocomposites. Unique characteristics of the synthesized hydrophobic silica aerogel nanoparticles were evaluated by FTIR, thermogravimetric analysis (TGA), nitrogen adsorption/desorption isotherm, SEM, and TEM. Conversion and molecular weight determinations were carried out using gas and size exclusion chromatography, respectively. Adding of hydrophobic silica aerogel nanoparticles by 3 wt% results in a decrease of conversion from 92 to 76%. In addition, molecular weight of the copolymer chains decreases from 39,147 to 33,092 gmol(-1); however, the polydispersity index increases from 1.51 to 2.03. Copolymers composition was evaluated using H-1 NMR spectroscopy. Increasing thermal stability of the nanocomposites is demonstrated by TGA. Differential scanning calorimetry shows a decrease in glass transition temperature from 69.1 to 59.3 degrees C by addition of 3 wt% hydrophobic silica aerogel nanoparticles.