Polymerization Properties of a Heterogeneous Ziegler−Natta Catalyst Modified by a Base: A Theoretical Study

The effects of an external base (tetrahydrofuran, THF) on the ethylene polymerization properties of the TiCl4/MgCl2 heterogeneous Ziegler−Natta catalyst are examined using density functional methods. THF is found to bind to all of the proposed active sites and most strongly to those formed from TiCl3. THF is also found to bind to the aluminum−alkyl compounds that may be present, and an analysis of equilibrium concentration distributions shows that the most common place for a THF to bind is to an aluminum−alkyl monomer or a TiCl3-based site. The polymerization and termination mechanisms of the remaining sites are examined. It is found that the binding of an ethylene molecule to the site is weakened, and the barrier to insertion of that ethylene molecule into the Ti−C bond is increased by the presence of a THF molecule. However, the polymer produced by each site is expected to have a higher molecular weight than in the absence of THF because the barrier to termination is increased even more than that of ins...