Effects of diffusion control in a compartmentalized atom transfer radical polymerization (ATRP) system have been investigated by modeling and simulations employing modified Smith−Ewart equations in connection with conversion-dependent rate coefficients for the system styrene/polystyrene-Cl/CuCl/4,4′-di(5-nonyl)-2,2′-bipyridine (dNbpy) at 75 °C. The effects of conversion-dependent rate coefficients for deactivation (kdeact) and termination (kt) were investigated in detail for the particle diameter 30 nm, i.e., a strongly compartmentalized system. The decrease in kdeact at high conversion results in an increase in the number of monomer units added per activation−deactivation cycle, thereby causing slight loss of control relative to when kdeact remains constant. The conversion dependence of kt results in a minor increase in livingness at high conversion relative to when kt remains constant. The individual compartmentalized rates of deactivation and termination have been analyzed in detail based on the partic...