The RAFT-mediated nonaqueous dispersion polymerization of methyl acrylate in isododecane, a nonsolvent for poly(methyl acrylate), was carried out using two soluble poly(2-ethylhexyl acrylate) macromolecular RAFT agents, containing either a dithiobenzoate reactive function or a trithiocarbonate one. The method produced stable colloidal particles, with hydrodynamic diameters below 100 nm. Using poly(2-ethylhexyl acrylate) with a dithiobenzoate end group, strong rate retardation and poor control over the polymer chains were observed. In contrast, when the trithiocarbonate-functionalized poly(2-ethylhexyl acrylate) was used, the formation of monodisperse micellar aggregates of well-defined self-assembled block copolymers was obtained with fast polymerization rates, irrespective of the RAFT agent concentration. Such differences were explained by the dispersed state of the system rather than by the intrinsic reactivity of the soluble macromolecular RAFT agent.