Compartmentalization effects on the kinetics of the atom transfer radical polymerization (ATRP) of styrene employing polystyrene-X/CuX/4,4'-dinonyl-2,2'-dipyridyl (dNbpy) (X = Br or Cl) in a dispersed system at low conversion at 70 or 75 degrees C have been investigated theoretically using modified Smith-Ewart equations. Compartmentalization always leads to improved livingness (end functionality) because of the segregation effect on propagating radicals reducing the overall termination rate. The control over the molecular weight distribution is, however, only improved if the particles are sufficiently small; that is, a particle size range exists where livingness is improved but control is not. Improved control is obtained as a result of the confined space effect causing an increase in the deactivation rate. The extent of compartmentalization increases with increasing target molecular weight and if Br is replaced with Cl. The present work provides further insight into how one can potentially employ particle size as an experimental parameter to influence ATRP.