Nanoparticle formulations of drugs, printing inks, sun screens, and other hydrophobic organic compounds have distinct advantages in bioavailability, color intensity, and aesthetics, for example. Common requirements of these applications are control of particle size and surface functionality. We present work on Flash Nano Precipitation to produce stable nanoparticles at high concentrations, using amphiphilic diblock copolymers to direct self-assembly. Uniform rapid mixing is crucial for particle formation. We characterize the mixing times scales in an impinging jet and four-stream vortex mixer using “Bourne” competitive reactions. Uniform particles with tunable sizes from 50-500 nm can be prepared. A new approach wherein block copolymers are formed reactively during the particle growth process is presented. This reactive block copolymer formation enables the process to perform at even higher solids loading, and introduces another time scale to the particle formation process that must be controlled.