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

Abstract The energies of formation and the heterolytic dissociation energies for the ion pairs Cp′ 2 ZrMe + A − (active sites in olefin polymerization) in the presence of Al- and B-containing activators including low-molecular-weight ones X(C 6 F 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′ 2 ZrMe + A − with ethylene molecule (Cp′ = η 5 -C 5 H 5 , η 5 -C 5 Me 5 , A −  = MeB(C 6 F 5 ) 3 − , MeAl(C 6 F 5 ) 3 − , and [(C 6 F 5 ) 3 Al–Me–Al(C 6 F 5 ) 3 ] − or three models for anions in MAO-containing systems [Me–AOTMA] − , [Me–2AOTMA] − , [Me–3AOTMA] − , AOTMA = Me 2 AlO(Al 2 Me 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 6 F 5 ) 3 and ≫1 for MAO) are necessary for a high catalytic activity of zirconocenes, whereas for B(C 6 F 5 ) 3 an equimolar B:Zr ratio is quite sufficient.