AbstractThe evolution of the morphology of the PVC grains during the mass polymerization of vinylchloride monomer (VCM) has been studied. After nucleation of primary particles, a rapid flocculation step occurs that leads to grains. Continued polymerization inside the grains results in a progressive reduction in the pore volumes. Continuing the reaction to completion, if appropriate by adding a further amount of monomer, causes densifying of the grains leading, in some cases, to vitreous grains, which therefore are related to the cleanliness of the reactors used. The changes in the size of the constituent compartment affect both the process of preparation and the processing of the final product: degassing rate of monomer, and plasticizing rate. Thus, control of the degrees of conversion, associated with perfect cleaning of the reactor, which is easy in the case of the new vertical reactors, constitutes the key factors in the reproducibility of the process and the quality of the PVC produced.