Zirconocene Propylene Polymerisation: Controlling Termination Reactions

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.