Possible effects on the polyethene chain structure of trimethylaluminum coordination to zirconocene catalysts

Abstract

Ethene was polymerized with the catalytic systems L2ZrCl2/MAO/TMA (where L = Cp, Me5Cp, or Me4Cp; Cp = η5‐cyclopentadienyl; MAO = methylaluminoxane; and TMA = trimethylaluminum) at 60 °C, 2 bar, and AlTMA/Zr ratios of 0–2700. The polymerization activity was reduced with the addition of TMA for L = Cp but was almost unaffected for the methyl‐substituted catalysts. Increasing the TMA concentration resulted in a lower molecular weight of the polymer, with the largest effect for L = Me5Cp. A gel permeation chromatography analysis of the polymers revealed a high molecular weight shoulder and a nearly bimodal distribution for L = Me5Cp at high TMA concentrations. A possible explanation of such a shoulder in terms of long‐chain branching was ruled out by dynamic viscosity measurements. The origin of this effect more likely stemmed from competition between chain transfer to aluminum and β‐hydrogen transfer reactions at two different sites, one TMA‐sensitive and one TMA‐insensitive. Polymerizations at various pressures and temperatures substantiated this assumption. A clue to the underlying mechanism came from investigations of chain transfer to TMA studied with density functional calculations. Complexation of Me3Al to Zr was much stronger for L = Cp than for L = Me5Cp. However, the overall chain‐transfer barrier was much higher for L = Cp. These results agreed both with the reduced activity for L = Cp and with the strongly reduced molecular weight for L = Me5Cp observed with the addition of TMA. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3566–3577, 2001

Effect of immobilization of titanocene catalyst in aralkyl imidazolium chloroaluminate media on performance of biphasic ethylene polymerization and polyethylene properties

“…The signal at 3.53 ppm results from the presence of hydroxyl groups (-OH) in the polymer chain. This indicates that the alkylaluminium compound also participates in the termination reaction of the polymer chain [57,58]. Furthermore, intensity of this signal increases with the increase of the alkylaluminium activator concentration, which proves occurrence of this termination mechanism.…”

Zirconocene Propylene Polymerisation: Controlling Termination Reactions

“…Assuming insignificant chain transfer to MAO, the degree of polymerisation for metallocene catalysts (M n ) can be described by the following equation: [11,24] …”

“…Even though some TMA is practically always present in metallocene polymerisations activated by MAO, the role of TMA in the activation process and the effect of TMA concentration on catalyst activity often get little attention and it is not well understood. Lately Liu et al [11,12] reported that the effect of TMA on polymerisation behaviour is dependent on the structure of the metallocene precursor in ethylene polymerisations.…”

Kinetics and mechanism of metallocene‐catalyzed olefin polymerization: Comparison of ethylene, propylene homopolymerizations, and their copolymerization

“…Mechanistic studies through detailed polymer chain structure characterization have made important achievements . Studies on the polymerization kinetics have been conducted by more research groups for understanding the mechainsm . It is evident that kinetic studies based on direct measurement of the active center concentration in the polymerization system can lead to easier construction of a comprehensive mechanistic model.…”