Propylene polymerizations with a binary metallocene system—Chain shuttling caused by trimethylaluminium between active catalyst centers

Abstract

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 TMA 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 Mw syndiotactic polypropylene and rac‐dimethylsilylbis(4‐tert‐butyl‐2‐methyl‐cyclopentadienyl)zirconium dichloride (2) producing low Mw isotactic polypropylene was investigated. At the studied polymerization conditions, chain shuttling between the active catalysts caused by TMA 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. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1364–1376, 2007

Bimodal polypropylene through binary metallocene catalytic systems: comparison between hybrid and mixed heterogeneous catalysts

“…By applying this arithmetic calculation to the nine physical mixtures carried out, it can be concluded that the variation between calculated and experimental catalytic activity is 14.5 % on average. Nevertheless, this correlation does not hold for hybrid catalysts, because when hybrid catalysts are employed, the participation of each active center is random [12][13][14][15][16], so catalytic activity may change depending on the homogeneity in the hybrid catalyst when both metallocenes are anchored on the silica support. Thus, in this case the activities are considerably Kg PP/mol Zr·h·bar, which is closer to 2100 achieved by the hybrid catalyst.…”

“…Thus, in this case the activities are considerably Kg PP/mol Zr·h·bar, which is closer to 2100 achieved by the hybrid catalyst. In comparison with individual catalysts, hybrid catalysts present a decrease of activity due to the interaction between both active catalytic centers [12][13][14][15][16].…”

“…However, this method involves additional processes and increases costs considerably [9], and 3) Combination of a binary or multicomponent catalytic system in a single reactor with the subsequent cost saving. This last method can comprise a combination of a heterogeneous Ziegler-Natta catalyst and a homogeneous metallocene catalyst providing a means to control the MWD and polydispersity of the polyolefin [10] but a disadvantage of this particular binary system comes from triethyl-aluminium (TEA) used to activate Ziegler-Natta since it has a serious detrimental effect on metallocene active sites [11] Therefore, an economical and attractive route is to use two or more metallocenes simultaneously yielding a polypropylene mixture consisting on the polymer fractions produced by each metallocene [12][13][14][15][16]. In this sense, it is important to decide which metalocenes will be present in the catalytic system, since they should have different kinetic responses, such as different propagation and termination rate constants, in order to produce a blend of polymeric chains with different predominant molecular weights.…”

Isobutylaluminum aryloxides as metallocene activators in Homo‐ and copolymerization of olefins

“…Sterically hindered phenols 2,6-di-t Bu,4-Me-C 6 H 2 OH (BHT-H), 2,4,6-tri-t Bu-C 6 H 2 OH (BHT*-H), and methylaluminum diaryloxides ((BHT) 2 AlMe and (BHT*) 2 AlMe) are sometimes used as the component of catalyst system in olefin polymerization. Phenols are applied for binding of free trimethylaluminum (TMA) [7][8][9][10][11][12][13][14][15] containing in commercial MAO up to 30 mol % since TMA is capable of blocking the active sites. [16][17][18] Sterically hindered phenols are inert toward catalytic intermediates even at high excess to metallocene and cause no lowering of the catalytic activity.…”

“…At the same time, it was demonstrated that adding of phenol to reaction medium enables to control the molecular weight and microstructure of derived (co)polymers. 11,12,14,15 Methylaluminum diaryoxides with tert-butyl substituents in ortho-positions of aryloxy groups simultaneously act as moisture impurities scavanger. 9,10 Several publications about application of phenols in order to obtain highly effective activators of metallocene complexes should be noted.…”