A THEORETICAL-STUDY OF HOMOGENEOUS ZIEGLER-NATTA CATALYSIS

In this paper we have investigated, using a DFT (B3LYP) computational approach, the insertion process of ethylene in the titanium-carbon bon d, which represents a fundamental step in the Ziegler-Natta polymeriza tion reaction. The DFT results have been validated by a comparison wit h the results obtained at the MP2 and CASPT2 levels. The two following models have been considered: (i) the cationic species Cl2TiCH3+ react ing with an ethylene molecule which emulates the positive part of a so lvent-separated ion pair (CH3)(2)AlCl2- parallel to Cl2TiCH3+); (ii) t he bimetallic species H2Al(mu-Cl)(2)TiCl2CH3 also reacting with ethyle ne and which mimics the possible bimetallic complexes or tight ion pai rs 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 barrie r of 5.8 kcal mol(-1) to reach the insertion transition state. We sugg est that in the real conditions used to carry out the reactions both r eaction channels (bimetallic complex and separated ion pair) are simul taneously available and that their relative importance and the resulti ng reaction rate are determined by the solvent polarity: the more pola r the solvent, the more important the reaction path involving the cati onic species. Self-consistent isodensity polarized continuum model (SC I-PCM) computations have shown that the insertion barriers decrease wi th the increasing polarity of the solvent.