In this work, the use of a nanoreactor is demonstrated to rapidly prepare monodisperse polymer nanoparticles in water-based dispersion consisting of 4-arm star polymer via the R-group RAFT approach. It is shown that by heating a nanoparticle assembler above its lower critical solution temperature (LCST), stabilized nanoparticles are formed that act as a template for the 4-arm star RAFT-mediated polymerization of styrene. Monodisperse nanoparticles of size (between 40 and 90 nm) containing monodisperse polymer of desired molecular weight (30-95 k) are obtained with little star-star coupling due to compartmentalization. The nanoreactor technique allows independent control over the size and molecular weight with a 4-arm star topology.