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Aside from polymerization techniques, polymer nanoparticles can be generated through the displacement of a solvent with a nonsolvent, i.e., nanoprecipitation. In this study, we utilize a facile process termed Flash NanoPrecipitation (FNP) to generate polystyrene (PS) nanoparticles of several different molecular weights. As compared to PS nanoparticles synthesized by surfactant free emulsion polymerization, nanoparticles prepared by FNP show comparable size distributions when the diameter is less than 150 nm. Furthermore, we illustrate that the sizes of PS nanoparticles prepared by FNP can be fine-tuned by changing the polymer and/or electrolyte concentration. The stabilized nanoparticles contain only the radically polymerized polymer chains, which have sulfate anions at the chain termini and no additional external stabilizers. Calculations of the mechanism of particle formation and stabilization show that the size-dependent electrostatic repulsions between nanoparticles and single collapsed polymer chains control assembly and monodispersity. The ability to independently vary polymer molecular weight and nanoparticle size will enable fundamental studies of the effect of confinement on polymer dynamics in a way not easily achievable by other techniques.