Binary mixtures of Cp2ZrMe2, Cp2ZrCl2, dimethylaluminum chloride, trimethylaluminum, and methylaluminoxane (MAO), as well as Cp2ZrMe2 with boron-based activators, have been studied by in situ IR spectroscopy (Cp = cyclopentadienyl, Me = methyl). The position of a strong band near 800 cm(-1), corresponding to the out-of-plane vibration of the Cp hydrogen atoms, is sensitive to the bonding environment around Zr and can be used to monitor reactions and the formation of new products in these mixtures. Harmonic frequencies determined by density functional theory correlate well with experimental values and have been used to assist in the interpretation of the data. The frequency of the Cp out-of-plane vibration, ranging from 797 to 832 cm(-1) in our experiments, is found to increase with increasing electron density on the Cp ring and decreasing Zr-Cp distance. In the mixture of MAO and Cp2ZrMe2, a stable complex is rapidly formed at low Al/Zr ratios. A mechanism that may explain the need for a large MAO excess is proposed for the activation of metallocenes with MAO. The proposed mechanism involves the formation of dimers or oligomers of MAO cages that tend to dissipate the charge of the anion. This destabilization of the Cp2ZrMe2-MAO complex facilitates the formation of the catalytically active cation.

No abstract

No abstract

Propylene was polymerized at varying trimethylaluminium (TMA) concentration with a homogeneous binary metallocene catalyst system activated by methylaluminoxane (MAO) in an attempt to better understand interactions between active catalyst sites and to clarify the role of the TMA as a chain shuttling agent. TMA‐free polymerization conditions were obtained by chemical treatment of MAO solution with 2,6‐di‐tert‐butyl‐4‐methylphenol (BHT). A binary catalyst system consisting of catalyst precursors diphenylmethyl(cyclopentadienyl)(9‐fluorenyl)zirconium dichloride (1) producing high Mw syndiotactic polypropylene and rac‐dimethylsilylbis(4‐tert‐butyl‐2‐methyl‐cyclopentadienyl)zirconium dichloride (2) producing low Mw isotactic polypropylene was investigated. At the studied polymerization conditions, chain shuttling between the active catalysts caused by TMA was confirmed. The chain shuttling reactions caused changes in catalyst activity, molecular weights, melting behavior, and polymer microstructure. We propose that TMA is capable to transfer a growing polymer chain from catalyst 2 to catalyst 1, and a stereoblock copolymer is formed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1364–1376, 2007