Supported Metallocenes

Although some important aspects concerning the nature of metallocene supported catalysts are not clear at the present stage, the profile of the formation of catalysts (catalyst precursors) with MgCl2, A1203 and Si02 supports and with a methylaluminoxane-pretreated Si02 support can be represented as follows [201, 202] ... 

Metallocene supported catalysts are not limited only to those involving inorganic catalyst carriers. Starch [212] and a-cyclodextrin [213] have also been successfully used as supports for zirconocene-AlMe3 [212] and zircono-cene-fA Me b [213] catalysts capable of producing high molecular weight polyethylene. 

At the end of considerations of metallocene supported catalysts, it should be noted that the catalyst derived from chromocene, biscyclopentadienylchro-mium (Cp2Cr), deposited on silica supports, also exhibits very high activity for the polymerisation of ethylene [214]. The formation of the catalyst has been formulated as follows [215] ... 

Roscoe, S. B., Frechet, X. M. X., Walzer, . F., and Dias, A. X, Polyolefin spheres from metallocenes supported on noninteracting polystyrene. Science 280, 270 (1998). 

TABLE 33 Influence of Reaction Time (or Polymer Yield) on the Elasticity of Polymers Made with Metallocene Supported on Silica-Alumina, Showing that LCB Content Again Drops with Increasing Polymer Yield (Indicated Here by Increasing JC-a and Zero-Shear... 

The dichromate species was associated with LCB production in one path of speculation above (Scheme 26). Alumina is known to support little or no dichromate, which is consistent with the low LCB levels produced. The structure of the alumina matrix probably also contributes to the low LCB levels produced. Indeed, metallocenes supported on acidified alumina also produce much lower LCB levels compared with the same metallocenes supported on silica-alumina. Because the chemistries of these two catalysts are quite different, this similar behavior suggests that the porosity of alumina, which is different from that of silica, may contribute to the LCB behavior [496,544,546]. 

Metal cations Metal cation reduction Metal cations in MeAPO synthesis Metal corrosion prevention Metallocene, supported catalyst 24-0-05 Metallosilicates, microporous Methanol adsorption Methanol amination Methanol conversion Methanol dehydrogenation Methanol formation Methanol in alkylation 15-0-03 25-0-03 Methanol to hydrocarbons Methanol, reagent Methanol, steam reforming Methylamine in MFI synthesis N-Methylation, aniline Methylation, 4-methylbiphenyl Methylation, toluene, model 4-methylbiphenyl, methylation Methylcyclohexane cracking Methylcyclopentane hydroconversion Methylene silanes... 

Figure 3.10 Different metallocene supporting methods (reproduced from Ref. [41]).

Transmission electron microscopy (TEM) is the main technique to detect intercalation and exfoliation for polymer-clay nanocomposites. Polyethylene-clay nanocomposites samples with poor (Figure 3.13a) and good (Figure 3.13b) exfoliation are shown in Figure 3.13 [62]. Uniform exfoliation and distribution of clay nanolayers is obtained by in-situ polymerization only when ethylene is polymerized with a metallocene supported on the organoclay in this case. 

Olabisi, O. Atiqullah, M. Kaminsky, W. Group 4 metallocenes supported and unsupported. J. Macromol. ScL, Rev. Macromol. Chem. Phys. 1997, C37,519-554. 

Ouros ACD, Souzab MOD, Pastore HO. Metallocene supported on inorganic solid supports an unfinished history. J Braz Chem Soc 2014 25(12) 2164—85. 

Fig. 3 Polymerization filling using MAO-tethered fillers as metallocene support...

O. Olabisi, M. AtiquUah, and W. Kaminsky, Group 4 metallocenes Supported and... 

Cocatalysts for metallocene-based olefin polymerization catalyst systems Metallocenes supported on ion exchange resins Olefin polymerization catalysts Process and a catalyst for preventing reactor fouling Supported metallocene catalysts for the production of polyolefins... 

Further simulations have been performed. In contrast to what was observed for bis-cyclopentadienyl metallocenes, mono-cyclopentadienyl systems did reveal a significant barrier to insertion [lOj. However, for all these systems it turned out that insertion only proceeded after the formation of a relatively stable agostic interaction, an observation that clearly supports the Brookhart-Green mechanism. 

Most catalysts for solution processes are either completely soluble or pseudo-homogeneous all their catalyst components are introduced into the reactor as Hquids but produce soHd catalysts when combined. The early Du Pont process employed a three-component catalyst consisting of titanium tetrachloride, vanadium oxytrichloride, and triisobutjlalurninum (80,81), whereas Dow used a mixture of titanium tetrachloride and triisobutylalurninum modified with ammonia (86,87). Because processes are intrinsically suitable for the use of soluble catalysts, they were the first to accommodate highly active metallocene catalysts. Other suitable catalyst systems include heterogeneous catalysts (such as chromium-based catalysts) as well as supported and unsupported Ziegler catalysts (88—90). 

Eluidized-bed reactors are highly versatile and can accommodate many types of polymerization catalysts. Most of the catalysts used for LLDPE production are heterogeneous Ziegler catalysts, in both supported and unsupported forms. The gas-phase process can also accommodate supported metallocene catalysts that produce compositionaHy uniform LLDPE resins (49—51). 

Cyclopentadiene itself has been used as a feedstock for carbon fiber manufacture (76). Cyclopentadiene is also a component of supported metallocene—alumoxane polymerization catalysts in the preparation of syndiotactic polyolefins (77), as a nickel or iron complex in the production of methanol and ethanol from synthesis gas (78), and as Group VIII metal complexes for the production of acetaldehyde from methanol and synthesis gas (79). 

Table 5 Properties of Gas Phase LLDPE (Hexene-1 Comonomer) Using Supported Metallocenes...

Figure 5 High melt tension LLDPEs from supported metallocenes. (From Ref. 2.)...

The goal of precise synthesis of supported mononuclear and polynuclear metal complexes can be traced to the early work of Yermakov [1], Ballard [2], and others. Their work stimulated the hvely field referred to as surface organometalhc chemistry [3-6]. The success and importance of precise synthesis of supported molecular catalysts are illustrated by the apphcation of supported metallocene catalysts for industrial alkene polymerization [7j. 

Figure 34 A polymer-supported metallocene catalyst (51) with a weakly coordinating anion, [B(C6F5)4] , produced from lightly cross-linked, chloromethylated polystyrene beads for olefin polymerization. (Adapted from ref. 75.)...

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