Electronic transitions in catalyst systems: Olefin polymerization by titanium trichloride-aluminum alkyl

Abstract

The electronic energy levels were calculated for the molecular orbitals of the octahedral complex, RTiCl 4-olefin, which is postulated to be an intermediate in polymerization by TiCl 3-aluminum alkyl catalyst systems. In this complex, an electronic transition from the highest filled orbital to the half-filled orbital above corresponds to the initial step in the olefin insertion reaction which has been proposed by many as the propagation step in the polymerization. The energy of this transition was calculated to be near the known values of 10–14 kcal/mole for the activation energy of the polymerization reaction. Consequently, this suggests that the electronic transition may be the controlling step in the reaction. The calculated electronic energy levels for the octahedral complex, CH 3CH 2TiCl 4, and the tetrahedral complex, CH 3TiCl 3, were used to judge the relative stability of these complexes. In both complexes, the lowest electronic transition corresponds to an initial step in the breaking of the TiC bond. For CH 3TiCl 3, the energy of this transition is 3.2 eV, which is much too high for thermal excitation and probably accounts for its known stability. The energy of the corresponding transition in CH 3CH 2TiCl 4 is 1.07 eV, which indicates that this complex is much less stable than CH 3TiCl 3. However, the transition is high enough to account for the stability of the complex under polymerization conditions in the absence of olefins. The electronic energy levels were calculated by use of an IBM computer program designed by Professor R. Hoffman, Cornell University, and based upon the linear combination of atomic orbital-molecular orbital method with a modified form of the Wolfsberg-Helmholz approximation.