The physical and chemical properties of metal nanoparticles differ significantly front those of free metal atoms as well as from the properties of bulk metals, and therefore they may be viewed as a transition regime between the two physical states. Within this nanosize regime, there is a wide fluctuation of properties, particularly chemical reactivity, as a function of the size, geometry, and electronic state of the metal nanoparticles. In recent years, great advancements have been made in the attempts to control and manipulate the growth of metal particles to prespecified dimensions. One of the main synthetic methods utilized in this endeavor, is the capping of the growing clusters with a variety of molecules, e.g., polymers. In this paper we attempt to model such a process and show the relationship between the concentration of the polymer present in the system and the final metal particle size obtained. The theoretical behavior, which we obtained, is compared with experimental results for the cobalt-polystyrene system.