In this work, a new controllable and continuous free radical polymerization process was developed and characterized in a coaxial capillary microreactor. In this process, the monomer solution was first dispersed into monodispersed droplets followed by thermal-initiated polymerization in the following capillary immersed in an oil bath. Poly(butyl acrylate) prepared in this microreactor possessed a much higher average molecular weight (M-n) and far lower polydispersity index (PDI) than that produced in a typical stirred vessel. The microreactor method possesses two unique advantages which allow for the optimization of the free radical polymerization process. First, the use of highly monodispersed droplets as polymerization units ensures that the polymerization process occurs uniformly in each individual droplet. Second, the small droplet size, on the order of several hundred micrometers, greatly enhances heat transfer efficiency with no heat accumulation within the droplets during polymerization. A simplified numerical simulation was used to show the superiority of the microreactor in effectively removing polymerization heat due to the miniaturization of the droplets to submillimeter scale. Simulation results also demonstrated that, in contrast to polymerization processes occurring in macroreactors, the polymerization conducted in the microreactor proceeded in a nearly isothermal condition. Experimental results in the microreactor showed that the molecular weight distribution was mainly determined by the size of the droplet, while the molecular weight of the polymer could be adjusted by changing the reaction temperature and 2,2-azobis(isobutyronitrile) concentration. This type of microreactor can potentially be applied to research involving the mechanisms of highly exothermic free radical polymerization processes and can also be used as an efficient tool for their controllable preparation.