The Mechanism of Ethylene Polymerization Reaction Catalyzed by Group IVB Metallocenes. A Rational Analysis Through the Use of Reaction Force

Abstract

A density functional theory study of the ethylene polymerization mechanism catalyzed by metallocene methyl cations based on group IVB (Ti, Zr, and Hf) is presented. The concept called reaction force was applied in order to decompose the activation energy into two parts with the aim of distinguishing the predominance of structural or electronic effects within intervals along the reaction coordinate on each step of the polymerization process. This has implied an alternative rational analysis of elementary chemical reactions associated with the polymerization mechanism under the assumption that the Cossée–Arlman’s mechanism is operating along the whole process. Three reaction models representing elementary chemical reactions of the polymerization process (initiation, propagation, and termination) were used through molecular quantum mechanical calculations. The simplest of metallocene methyl cations (built up by two cyclopentadienyl groups and one methyl group linked to the metal) was used as a catalytic molecular model. Since the main goal was focused on getting information of intrinsic global reactivity of catalytic systems, both solvent and co-catalyst were not modeled in the present work. As a result, energy profiles indicate that ethylene polymerization reactions are better catalyzed by the respective metallocene cation based on Ti, whereas the catalysts based on Zr and Hf present similar characteristics among them, thus supporting experimental results where the molecular weight of polyethylene obtained by means of metallocene cation based on Ti approximately doubles the molecular weights of polyethylene catalyzed by metallocenes based on Zr and Hf. However propagation and termination steps are better catalyzed by metallocene complexes based on Zr and Hf, thus masking the influence of initiation step upon the molecular weight of the obtained polymer and providing more importance to termination step rather than propagation step. This would help to better understand differences presented in the average molecular weight of a same polymer obtained with each of these catalytic systems. As some key steps of the polymerization process would be more favored with one type of metallocene rather than other one, the use of reaction force would help to better understand how to modify energetic barriers and or global changes in energy by perturbating geometrical or electronic structure of catalytic systems. The latter suggests that the ideal polymerization process should be carried out with different catalytic systems depending on the step of polymerization and not with a unique catalyst as has been commonly performed up to now. That might lead to the obtention of a more wide range of new polymeric materials.