Zirconocene catalysts for olefin polymerization: A comparative DFT study of systems with Al- and B-containing activators

The energies of formation and the heterolytic dissociation energies for the ion pairs Cp′2ZrMe+A− (active sites in olefin polymerization) in the presence of Al- and B-containing activators including low-molecular-weight ones X(C6F5)3 (X = Al, B) and a number of models for Al-sites in polymethylaluminoxane (MAO) were obtained from DFT calculations. The reaction mechanisms were thoroughly studied and the energy characteristics of the reactions of the ion pairs Cp′2ZrMe+A− with ethylene molecule (Cp′ = η5-C5H5, η5-C5Me5, A− = MeB(C6F5)3−, MeAl(C6F5)3−, and [(C6F5)3Al–Me–Al(C6F5)3]− or three models for anions in MAO-containing systems [Me–AOTMA]−, [Me–2AOTMA]−, [Me–3AOTMA]−, AOTMA = Me2AlO(Al2Me5)) were calculated. Heterolytic dissociation energy (energy needed for complete separation of counterions) is found to be a crucial parameter which determines the energy characteristics of the polymerization reaction. We propose that the involvement of the second and third Lewis acid Al-sites in the formation of the ion pair could explain why high Al:Zr ratios (>1 for Al(C6F5)3 and ≫1 for MAO) are necessary for a high catalytic activity of zirconocenes, whereas for B(C6F5)3 an equimolar B:Zr ratio is quite sufficient.