TWO-SITE MODEL FOR HETEROGENEOUS ZIEGLER-NATTA POLYMERIZATIONS.

A model based on a two-sites theory for heterogeneous Ziegler-Natta polymerizations was developed and used to predict degree of polymerization and polydispersity, using concepts from monomer mass transfer and multiplicity of sites. It is shown that this relatively simple model is capable of predicting broad MWD irrespective of the values of the Thiele modulus, something that the theory based only on diffusion does not, provided that the ratios between the activity of the two sites or the ratios between their initial concentration are large enough. It presents some of the characteristics found experimentally, that is, constant polydispersity and DPn as a function of time. For the case evaluated it was shown that the broad MWD is not due to restrictions in monomer availability but a consequence of the presence of different types of sites.