Aqueous emulsion polymerization is by far the most important method for synthesizing polymer nanoparticles. In this work we report a new type of surfactant-free emulsion polymerization process for one-step formation of monodisperse poly(methyl methacrylate)@SiO2 core-shell nanoparticles. The strategy utilizes a silica precursor polymer, hyperbranched polyethoxysiloxane (PEOS), whose chemical structure can be adapted by hydrolysis at water/oil interfaces for their stabilization. The core-shell particles are obtained simply by adding a monomer/PEOS solution into water and subsequent heating to initiate polymerization. A high-shear emulsification step that is undesirable for industrial scale-up is not needed. In contrast to most emulsion polymerization processes where water soluble initiators are employed, an oil soluble initiator is required in this process for achieving narrow particle size distribution and precise control of silica content in the particles. As the polymerization proceeds, the growing chains are precipitated from the aqueous phase forming polymer particles, which are in turn stabilized by partially hydrolysed amphiphilic PEOS molecules. After the polymerization is completed, the conversion of PEOS continues depositing silica onto the silica-coated polymer particles. This method allows an easy control of silica shell thickness by varying the PEOS concentration and the particle size can be adjusted by changing the pH of the aqueous phase.