Hybrid nanoparticles with a silica core and grafted poly(methyl methacrylate) (PMMA) or poly(n-butyl methacrylate) (PBMA) chains were prepared via activators generated by electron transfer for atom transfer radical polymerization (AGET ATRP) at room temperature under high pressure. Due to enhanced propagation rate constant and reduced termination rate constant for polymerizations conducted under high pressure, the rate of polymerization was increased, while preserving good control over polymerization when compared to ATRP under ambient pressure. Molecular weights of greater than 1 million were obtained. The PMMA and PBMA brushes exhibited "semi-diluted" or "diluted" brush architecture with the highest grafting densities approximate to 0.3 chain.nm(-2).