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 TAM 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 M-w syndiotactic polypropylene and rac-dimethylsilylbis(4-tert-butyl-2methyl-cyclopentadienyl)zirconium dichloride (2) producing low Mw isotactic polypropylene was investigated. At the studied polymerization conditions, chain shuttling between the active catalysts caused by TAM 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. (c) 2007 Wiley Periodicals, Inc.