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

Summary: Propylene was polymerised at varying trimethylaluminium (TMA) concentration with two metallocenes activated by methylaluminoxane (MAO) in an attempt to better understand the effect of TMA on the activation process, catalyst activity and termination reactions. A chemical treatment of MAO solution with 2,6‐di‐tert‐butyl‐p‐cresol was used to obtain TMA‐free polymerisation conditions. The metallocene precursors under investigation were diphenylmethyl(cyclopentadienyl)(9‐fluorenyl)zirconium dichloride (1) and rac‐dimethylsilylbis(4‐tert‐butyl‐2‐methyl‐cyclopentadienyl)zirconium dichloride (2). Chain transfer to aluminium was the dominating termination route for 1/MAO accompanied with β‐H/β‐CH3 transfer to Zr and β‐H transfer to monomer. It was found that β‐H/β‐CH3 transfer to Zr was favoured over the β‐H transfer to monomer at elevated temperatures, and that polymerisation and β‐H transfer to monomer depended on the same critical reaction. For 2/MAO the detected termination routes were β‐H transfer to Zr and chain transfer to aluminium. A strong activity dependency on TMA concentration was observed. With 1/MAO high TMA concentration decreased and stabilised the activity, whereas TMA free polymerisation conditions at 40 °C increased markedly the activity, indicating that TMA coordinated to the active site of 1/MAO. Surprisingly, with more sterically hindered 2/MAO, high TMA concentration increased the activity and nearly complete activity loss occurred at TMA‐free polymerisation conditions at 40 °C.Metallocene precursors for investigation of propylene polymerisation.magnified imageMetallocene precursors for investigation of propylene polymerisation.

Summary: Ethylene–propylene (EP) copolymerisations were performed with two sterically different metallocenes activated by methylaluminoxane (MAO) in an attempt to better understand the effect of catalyst structure on termination reactions and polymer microstructure. The metallocene precursors under investigation were rac‐dimethylsilylbis(2‐methyl‐4‐phenyl‐1‐indenyl)zirconium dichloride (1) and a more sterically hindered counterpart rac‐dimethylsilylbis(2‐isopropyl‐4‐[3,5‐dimethylphenyl]indenyl) zirconium dichloride (2). For both catalyst systems, the most common termination mechanism was chain transfer to aluminium. In addition, for polymer samples polymerised with 1/MAO, chain growth was terminated by chain transfer to Zr metal in propylene‐rich polymerisations and by chain transfer to ethylene monomer in ethylene‐rich polymerisations. The steric hindrance of 2 was able to suppress the chain transfer to the ethylene monomer, and chain transfer to Zr metal was also found in the ethylene‐rich polymerisations. The greater steric hindrance of 2 also affected the EP copolymer microstructure: regioregularity in the propylene‐rich copolymers was greater and isotacticity less with 2/MAO than with 1/MAO.

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