Many acrylic and vinylic monomers are often polymerized by a free radical chain addition mechanism. This process consists of a three main steps: initiation, propagation and termination. Free radicals are formed by the fragmentation of the initiator, once formed these radicals propagate by reacting with monomers to form long macroradical chains. In batch reactors, this reaction mechanism is often complicated by the fact that the physical properties of the reacting system change dramatically with conversion during the reaction; in concentrated systems a strong acceleration in the polymerization rate occurs along with the increase in the medium viscosity when the reactions become controlled by diffusion phenomena (gel and glass effect). Because the chain length of the polymer produced during polymerization is directly related to the reaction kinetics, as a consequence also the average molecular weights (Mn and Mw) and the Molecular Weight Distribution (MWD) are strongly affected by the diffusion phenomena.