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

摘要

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(C _6F _5) _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′ = η ~5-C _5H _5, η ~5- C _5Me _5, A - = MeB(C _6F _5) _3 ~-, MeAl(C _6F _5) _3 ~-, and [(C _6F _5) _3Al-Me- Al(C _6F _5) _3] ~- or three models for anions in MAO-containing systems [Me-AOTMA] -, [Me-2AOTMA] -, [Me-3AOTMA] -, AOTMA = Me _2AlO(Al _2Me _5)) 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(C _6F _5) _3 and ?1 for MAO) are necessary for a high catalytic activity of zirconocenes, whereas for B(C _6F _5) _3 an equimolar B:Zr ratio is quite sufficient.