Metallocene catalysts cocatalysts

Polypropylenes produced by metallocene catalysis became available in the late 1990s. One such process adopts a standard gas phase process using a metallocene catalyst such as rac.-dimethylsilyleneto (2-methyl-l-benz(e)indenyl)zirconium dichloride in conjunction with methylaluminoxane (MAO) as cocatalyst. The exact choice of catalyst determines the direction by which the monomer approaches and attaches itself to the growing chain. Thus whereas the isotactic material is normally preferred, it is also possible to select catalysts which yield syndiotactic material. Yet another form is the so-called hemi-isotactic polypropylene in which an isotactic unit alternates with a random configuration. 

In order to enhance the activity of coordination catalysts we typically add a cocatalyst. The cocatalyst works synergistically with the catalyst to allow us to tailor the tacticity and molecular weight of the product while also enhancing the rate of the reaction. An example of a commercially used cocatalyst is methylaluminoxane used in conjunction with metallocene catalysts. 

A final example of homogeneous catalysis is the use of metallocene catalyst systems in chain growth polymerization processes. The metallocene, which consists of a metal ion sandtviched between two unsaturated ring systems, is activated by a cocatalyst. The activated catalyst complexes with the monomer thereby reducing the reaction s energy of activation. This increases the rate of the reaction by up to three orders of magnitude. 

The key to highly active metallocene catalysts is the use of cocatalysts. In an activation step, the cocatalyst creates out of the metallocene a polymerization-active species. At first, methylaluminoxane (MAO) was usually used to activate metallocenes. Nowadays an alternative activation via borane and borate is becoming more and more important [20, 24, 25]. 

A major limitation of such Group IVB metallocene catalysts is that they are air- and moisture-sensitive and not tolerant to heteroatom-containing monomers. In the case of heteroatom-containing monomers the unbonded electron pairs on the heteroatom, such as oxygen, preferentially coordinate to the Lewis acid metal center in place of the carbon-carbon double bond. Some so-called middle and late transition metal organometallics are more tolerant to the presence of such heteroatoms and can be used as effective cocatalysts. These include some palladium, iron, cobalt, and nickel initiators. 

Natta and Breslow reported the first metallocene catalyst in the mid-1950s by using Et2AlCl2 as the cocatalyst [25], The system had very low activity and life-... 

Metallocene catalysts are bicomponents consisting of group four transition metal compounds and cocatalysts. Metallocene compounds have long been known and were used as a compound for Ziegler-Natta catalysts (Table 1) [11],... 

The first kinetic model for propagation in homogeneous systems was proposed by Ewen [47], assuming that the propagation took place as shown in Fig. 9.18. This scheme, shown for Cp2Ti(IV) polymerization of propylene, is representative of the kinetics for dl of the polymerizations with Group IVB metallocenes. In the scheme, species 1 and 4 represent coordinatively unsaturated Ti(IV) complexes that are-formally 16-electron pseudo-tetrahedral species, species 2 represents the interacting catalyst/cocatalyst combination, while intermediate 3 is shown with the monomer coordinated... 

The Phillips catalyst is not alkylated when it goes into the reactor, and metal alkyl cocatalysts are not normally used. Thus, in contrast to Ziegler, Ballard, or metallocene catalysts, the Phillips catalyst has no Cr-alkyl bond into which ethylene may be inserted. Instead, the chromium somehow reacts with ethylene to generate such a bond. This characteristic is not unique, as many catalyst types also display this ability.8 This issue has been the source of much interest and speculation for half a century. On some catalysts, CO reduction is known to cleanly produce Cr(II). Reaction with ethylene could involve a formal oxidation [52,94,141,250-252,269,322-325,339-345] and many pathways involving Cr(IV) have been proposed, sometimes based on organochromium analogs, such as shown in Scheme 8 [94,250-252,315-319,321-325,342,346-349]. 

Both the productivity and stereoselectivity of the oscillating metallocene catalyst (2-PhInd)2ZrCl2 are strongly influenced by the nature of the cocatalyst and reaction conditions, such as temperature, pressure, and monomer concentration. Best performance of the catalyst has been obtained with MAO as the cocatalyst (Wilmes et al., 2002). Both the productivity and [mm/Tzm] are found to increase with decreasing temperature or increasing pressure, with the other variable remaining constant. The effect of monomer concentration [M] on the tacticity of the... 

The kinetics of a propene polymerization which is promoted by Si02-supported metallocene catalysts depends on various factors (1) On the applied reaction engineering (gas-phase, bulk, and slurry polymerization) ° (2) On the degree of catalyst/ cocatalyst distribution on the support and (3) On the chosen reaction conditions and parameters (Figure... 

In the following, we will review the most interesting examples of stereoregular polymerizations and analyze the correlation between ligand substitution and catalyst behavior. Our view of the evolution of metallocene catalysts for polypropene is shown in Chart 4. The strong influence of the polymerization conditions will be discussed in section V. The influence of the cocatalyst type and of the catalyst/ cocatalyst ratio has not been, in our opinion, exhaustively investigated in any case, for MAO-cocatalyzed systems, our experience with chiral zirconocenes is that the aluminum concentration has a major influence on catalyst activity but neither on microstructure nor on i-PP molecular weight when the polymerization is carried out in liquid propene. The available data will also be discussed in section V. 

The polymers obtained with Me2Si(l-Ind)2ZrCl2 activated by either [BusNH] [B(C6Fs)4] or MAO show the same microstructure, indicating that the stereoselectivity of the metallocene catalysts does not depend on the nature of the cocatalyst and that the active species are similar in the two cases. This is a key observation, in accordance with the report of Ewen and our own experience. Nevertheless, the two catalyst systems differ considerably in the mechanism of formation of these active species. Similarly, Miilhaupt et al. found for propene polymerization with rac-Me2Si(2-Me-Benz[e]ind)2ZrX2 (X = Cl, Me)... 

That is, in terms of reaction rates, the molecular weight of polyolefins is given by the ratio between the overall rate of propagation (Rp) and the sum of all rates of chain release (Rr) reactions this means that the molecular weight is dependent on the type of catalyst and the kinetics of the process, that is, the polymerization conditions (polymerization temperature, monomer concentration, catalyst/cocatalyst ratio). Hence, understanding the details of the mechanisms of chain release reactions is the key to molecular weight control in metallocene-catalyzed olefin polymerization. Here, chain release reactions (usually referred to as termination or transfer reactions) are all those steps that cause release of the polymer chain from the active catalyst, with the formation of a new initiating species (see section... 

All these aspects contribute to the character of a metallocene catalyst. Many attempts have been made in the past to design catalysts by molecular modeling. The results were not very satisfying because there are still too many open parameters that must be considered such as the degree of activation of the catalyst precursor, the interaction between catalyst cation and cocatalyst anion, or the role of the solvent. Because of this situation, we preferred the empirical way. The author s group synthesized over 650 met-... 

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