Theoretical study on a reaction pathway of Ziegler - Natta - type catalysis a )

Ab initio SCF-LCAO-MO all-electron calculations have been performed for the catalyzed reaction, related to the Ziegler-Natta polymerization process TiCl.·Al(CH3)3+C2H. -+TiCl •. Al(CH3h·C3H7. The energy of the Ti-Al--ethylene complex during the reaction pathway is associated to the following processes: First, the ethylene coordinates to the catalyst complex and subsequently it reacts with the methyl moiety producing an activation energy barrier mainly due to the breaking of the ethylene double bond. The computed barrier height (15 kcal/mole) is in reasonable agreement with experimental activation energies. The identification of the energy barrier is based on the analysis of the electron population, bond energies, and molecular orbitals of the system as they evolve during the reaction. A comparison with the qualitative picture resulting from the classical model of Cossee and with semiempirical calculations is presented. From these calculations a model is derived which leads to a tentative suggestion for the enhancement of the catalytic activity. We consider this suggestion (regardless of its practical value), as well as the model, as examples of the type of theoretical results that can be furnished by quantum mechanics.