Molecular weight distribution predictions for heterogeneous Ziegler-Natta polymerization using a two-site model

In the past there have been important improvements in several areas of Ziegler-Natta polymerizations, such as activity and selectivity enhancement. However, there still remains disagreement on the origins of the broad molecular weight distribution (MWD) characteristic of these polymers. Researchers have taken sides on either of the two groups explaining this phenomenon. These are (a) multiplicity of sites, and (b) diffusional limitations. Both theories have some points in their favour. To the knowledge of the authors, no work has been done combining the two. It is the purpose of this work to try to fill the gap. A model is developed and used to predict degree of polymerization and polydispersity, using concepts from monomer mass transfer and multiplicity of sites. For the cases evaluated it is shown that the broad MWD is not due to restrictions in monomer availability but is a consequence of the presence of different types of sites.