A Theoretical Study of Homogeneous Ziegler−Natta Catalysis

Abstract

In this paper we have investigated, using a DFT (B3LYP) computational approach, the insertion process of ethylene in the titanium−carbon bond, which represents a fundamental step in the Ziegler−Natta polymerization reaction. The DFT results have been validated by a comparison with the results obtained at the MP2 and CASPT2 levels. The two following models have been considered: (i) the cationic species Cl2TiCH3+ reacting with an ethylene molecule which emulates the positive part of a solvent-separated ion pair (CH3)2AlCl2- || Cl2TiCH3+); (ii) the bimetallic species H2Al(μ-Cl)2TiCl2CH3 also reacting with ethylene and which mimics the possible bimetallic complexes or tight ion pairs that can originate from the catalyst−cocatalyst interaction. In the former case the process is highly exothermic (−45.5 kcal mol-1) and is characterized by an insertion energy barrier of about 5 kcal mol-1. In the latter case the energetically most favored channel is a two-step reaction path that requires the overcoming of a first barrier of about 5.6 kcal mol-1 to form an intermediate and of a second barrier of 5.8 kcal mol-1 to reach the insertion transition state. We suggest that in the real conditions used to carry out the reactions both reaction channels (bimetallic complex and separated ion pair) are simultaneously available and that their relative importance and the resulting reaction rate are determined by the solvent polarity: the more polar the solvent, the more important the reaction path involving the cationic species. Self-consistent isodensity polarized continuum model (SCI-PCM) computations have shown that the insertion barriers decrease with the increasing polarity of the solvent.