Bimodal Polyethylenes Obtained with a Dual‐Site Metallocene Catalyst System. Effect of Trimethylaluminium Addition

Bimodal polyethylenes were obtained with the dual site Cp*₂ZrCl₂( 1 )/Et(IndH4)₂ZrCl₂( 2 ) metallocene catalyst system with a mixture of methylaluminoxane (MAO) and trimethylaluminium (TMA) as the cocatalyst. Polymer properties can be controlled by the amount of TMA added, monomer pressure, polymerization temperature and the addition of hexene or hydrogen. TMA is suggested to be partly coordinated to the active sites, thereby enhancing termination ( 1 ), increasing comonomer incorporation ( 2 ), but also partially blocking coordination and chain transfer to hydrogen. For the ansa catalyst, hydrogen probably relieves dormant (β‐agostic) sites.