Metallocene–methylaluminoxane catalyst for olefin polymerization. II. Bis‐η5‐(neomenthyl cyclopentadienyl)zirconium dichloride

Abstract

AbstractBis(neomenthyl cyclopentadienyl)zirconium dichloride/methyl aluminoxane (η5‐(NMCp)2ZrCl2/MAO) catalyst has been investigated for ethylene polymerization. About 51% of the Zr forms active sites more or less instantaneously according to quenching with tritiated methanol. There is an initial drop of rate of polymerization, Rp, of about 30% which remains constant thereafter. The catalytic activity increases monotonically with temperature; it is proportional to [MAO]1.75 at a constant [Zr] = 1.5 μM and proportional to [Zr]−1.2 at a constant [MAO] = 64.5 mM. At very large [MAO]/[Zr], the catalyst has extremely high activity; κp = 5 × 103 (Ms)−1 at 50°C. There is also facile chain transfer to aluminum, κ = 0.14 s−1 at 50°C. Both κp and κ are about 30 times greater than the corresponding rate constants for MgCl2 supported TiCl3 catalysts. The TiCl3/MgCl2 and (NMCp)2/MAO catalysts have nearly the same activation energy for propagation (ca. 7 kcal/mol−1). The higher activity of the latter is due to its larger preexponential factor in κp. The dependence of catalytic activity on the [MAO]/[Zr] ratio may be explained by rapid association‐dissociation equilibria of MAO involving acid‐base and/or electron deficient bridge complexation.