Olefin Polymerization with Homogeneous Ziegler−Natta Catalysts:  A DFT Quantum-Mechanical Study of the Reactions of Cp₂MtCH₃Cl Complexes (Mt = Ti, Zr) with Al(CH₃)₃ and MAO

Abstract

The reactions of Cp 2 MtMeCl (Cp = η 5 -C 5 H 5, Mt = Ti, Zr, Me = CH 3 ) with AlMe 3 and with MeAl[OAl 2 H 4 ] 2 O, as a model of the cocatalytic site of methylalumoxane (MAO), were studied by means of DFT quantum-mechanical calculations. The energy required to form the Cp 2 MtMe + cation from dissociation of Cp 2 MtMeCl·AlMe 3 and different kinds of chlorine-bridged and oxygen-bridged Cp 2 MtMeCl· MAO model adducts accounts for the higher cocatalytic activity exhibited by methylalumoxane with respect to AlMe 3 in olefin polymerization. Both the presence of highly acidic aluminum atoms and negative charge dispersion power of XMAO - macroanions (X = Cl, Me) are essential features in determining low dissociation energies. Possible key roles played by monomer and AlMe 3 content of MAO in affecting active species formation are suggested. The computational results support the idea that, in the absence of degradative side reactions, titanium and zirconium MAO-based systems should exhibit comparable catalytic activity.