Influence of trimethylaluminum on kinetics of rac-Et(Ind)2ZrCl2/aluminoxane catalyzed ethylene polymerization

• Ethylene was polymerized by rac-Et(Ind)2ZrCl2/aluminoxane (MAO, dMAO or MMAO).
• Concentration of active centers and its changes with time were determined.
• Three kinds of active centers were differentiated by MWD deconvolution.
• Free TMA was found to take part in the formation of contact ion pairs.

Ethylene polymerization with rac-Et(Ind)2ZrCl2/aluminoxane catalysts were conducted under different conditions, including varied reaction time, cocatalyst and cocatalyst concentration. Methylaluminoxane (MAO), dried MAO (dMAO) and modified MAO (MMAO) were used as cocatalyst, respectively. Changes of polymerization rate, number of active centers ([C∗]/[Zr]) and polymer molecular weight distribution (MWD) with time were traced. A method of counting active centers using 2-thiophenecarbonyl chloride as quenching agent was applied to measure [C∗]. MWD curves of polymer were deconvoluted into three Flory components, which were assigned to polymer formed by loosely associated ion pairs, contact ion pairs and ion pairs with medium cation-anion distance. The loosely associated ion pairs produce polymer of low molecular weight, and the contact ion pairs produce high molecular weight polymer. Free trimethylaluminum (TMA) in the system was found to be an important component for the formation of contact ion pairs. In ethylene polymerization activated by different cocatalysts, gradual increase of [C∗]/[Zr] with time until 30 min was observed, meaning that complete activation of all the metallocene molecules takes rather long time. Comparisons of [C∗]/[Zr] ratios at 30 min in systems activated by different cocatalysts lead to the conclusion that TMA takes part in formation of dormant active centers. The role of TMA as an efficient chain transfer agent has also been proved.

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