Cocatalysts solid

Natta s bimetallic mechanism stipulates that when the catalyst and cocatalyst components are mixed, the chemisorption of the aluminium alkyl (electropositive in nature) occurs on the titanium chloride solid surface which results in the formation of an electron-deficient bridge complex of the structure shown... 

Presently, the importance of Nd allyl compounds as intermediates in Nd carboxylate- and other Nd-based catalyst systems is widely accepted. As various Nd allyl compounds have been synthesized, characterized and successfully tested as polymerization catalysts this view is supported by solid experimental evidence (Sect. 2.1.1.5 and the references therein). Selected Nd allyl compounds exhibit significant polymerization activities without the addition of cocatalysts. In these cases the active species is neutral. But also cationic active Nd species are taken into consideration (Sect. 2.1.1.5) [288,291]. Cationic species also prevail in the presence of non-coordinating anions. 

Figure 1. Conversion of isoprene to solid polymer as a function of Al/Ti molar ratio and alkyl size in tri-n-alkylalumi-num cocatalyst polymerization time—22 hours...

The solid studies have also shown that all trialkylaluminum compounds are more or less equivalent as cocatalysts. Thus with a good solid, such as the 0.9 i-Bu3Al solid, trialkyls—as far apart as the trimethyl- and trioctyl-aluminum—are essentially equivalent. This indicates that the poorer activity of unseparated catalysts prepared from the lower alkyls (trimethyl-, triethyl-aluminum) must be directly related to the resulting reaction products with TiCl4, i.e., the reduced and/or alkylated Ti species (expected to be found in the solid phase) and the alkylaluminum chlorides (unless strongly adsorbed, expected to be found in the liquid phase). 

The active centre of all these catalysts is the Ti-C bond surrounded by suitable ligand fields, mainly from the direction of the transition metal.Even though milling of the solid phase greatly increases the surface and thus also the number of transition metal atoms potencially able to form active centres with suitable cocatalysts, the amount of transition metal inside the crystals remains relatively large. These atoms have no chance of becoming active... 

The work is divided into three parts. The first (Chapters 3 and 4) covers the catalyst (solid components) with particular regard to preparation, structure and role of the components. The second part (Chapter 5) deals with the reactions which take place between catalyst and cocatalyst and give rise to the formation of polymerization centers. The third part regards the polymerization and, apart from a discussion of number and nature of the active centers, reports the hypotheses about the most probable reaction mechanisms as a function of various kinetic parameters. 

The two-component catalytic systems used for olefin polymerization (Ziegler-Natta catalysts) are combinations of a compound of a IV-VIII group transition metal (catalyst) and an organometallic compound of a I-III group non-transition element (cocatalyst) An active center (AC) of polymerization in these systems is a compound (at the surface in the case of solid catalysts) which contains a transition metal-alkyl bond into which monomer insertion occurs during the propagation reaction. In the case of two-component catalysts an AC is formed by alkylation of a transition metal compound with the cocatalyst, for example ... 

Reaction of dialkylmagnesium compounds with selected chlorinated compounds produces finely divided MgCl that can be used as a support for polyethylene catalysts. Other reagents may be used to produce different inorganic magnesium compounds, also suitable as supports. Examples are shown in Figure 4.1. Treatment of these products with transition metal compounds results in a supported "precatalyst." Typically, the transition metal is subsequently reduced by reaction with an aluminum alkyl and the solid catalyst isolated. The solid catalyst and cocatalyst (usually TEAL) may then be introduced to the polymerization reactor. 

As isolated from toluene solution, neat MAO is an amorphous, friable white solid containing 43-44% Al (theory 46.5%). Like most commercially available aluminum alkyls, it is pyrophoric and explosively reactive with water. Freshly prepared MAO solutions form gels within a few days when stored at ambient temperatures (>20 °C). However, lower storage temperatures (0-5 °C) delay gel formation. Consequently, manufacturers store and transport MAO solutions in refrigerated containers. Commercially available MAO contains residual TMAL (15-30%), called "free TMAL" or "active aluminum." The literature is contradictory on the influence of free TMAL on activity of single site catalysts both reductions and increases have been reported (18-20). Perhaps the most important drawback of methylaluminoxane is its cost, which is substantially higher than conventional aluminum alkyls. Despite these untoward aspects, methylaluminoxane remains the most widely used cocatalyst for industrial single site catalysts. 

Tris(pentafluorophenyl)borane, known as "FAB" (structure below), is the most common arylborane used as cocatalyst for single site catalysts. FAB is a strongly Lewis acidic, air-sensitive solid (T 126-131 °C) that is only slightly soluble in hydrocarbon solvents. The structure of FAB is given below. 

The procatalyst [Rh(cod)Cl]2 is an orange, air-stable solid which is commercially available, accessible in one step from RhCls and 1,5-cyclooctadiene [82], The cocatalyst DIOP (Figure 3), the most frequently used optically active phosphine, is also commercially available. A survey of the literature shows that more than half of the numerous studies on the hydrogenation of (Z)-a-acetamido-cinnamic acid have been carried out with in-situ catalysts [79, 81]. 

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