Immobilized Metallocene Catalysts

The cyclopentadienyl group is another interesting ligand for immobilization. Its titanium complexes can be transformed by reduction with butyl lithium into highly active alkene hydrogenation catalysts having a TOF of about 7000 h 1 at 60 °C [85]. Similar metallocene catalysts have also been extensively studied on polymer supports, as shown in the following section. 

A unique immobilization strategy was developed by Alt, who synthesized metallocenes with alkene functionalities and employed these functionalized metallocene catalysts for polymerization. During the polymerization process, the metallocene catalysts are consumed as a co-monomer, leading to the generation of polymer-supported metallocene catalysts. 

Metallocenes immobilized on solid support materials have been successfully introduced in industry as polymerization catalysts for the production of new application-oriented polymer materials. Industrial polymerizations, which are carried out either as a slurry process in liquid propylene or as a gas-phase process (Section 7.2.3), require that catalysts are in the form of solid grains or pellets soluble metallocene catalysts thus have to be supported on a solid carrier. 

In spite of the initial enthusiasm and the large amount of work that has been carried out in the meantime, any important practical application of an immobilized metal complex catalyst is not known so far. Promising and really satisfying results could only be obtained to a small extent in selective hydrogenation reactions in the synthesis of chemical specialities and pharmaceutical products and in polymerization reactions with immobilized metallocene catalysts [21]. The situation for the latter type of reactions, however, is different. 

One of the most recent trends is to use nanobuilding blocks, or crystal tectonics. 106,187,188 The idea is to use a pre-formed inorganic fragment with the desired porous or catalytic properties which is then simply immobilized into a matrix, as, for example, in POSS or titanium alkoxide fragments.189 Alternatively, the fragment may be coordinated to the surfactant template.189,190 Incorporation of metallocene catalyst-terminated silanol dendrimers into silica was achieved through sol gel processing methods.191... 

Figure 9.19 Synthesis of a heterogeneous metallocene catalyst with immobilized MAO. (Adapted from Koppl et al., 2001.)...

Chapter 6), grafting-from polymerization and polymer brushes (Chapter 6), living controlled radical polymerization (Chapter 6), metallocene-based Ziegler-Natta catalysts (Chapter 9), immobilized metallocene catalysts (Chapter 9), and oscillating metallocene catalysts (Chapter 9). 

Supported metallocene catalysts are an important field of research for the polymer industry and shall be mentioned here only briefly. Single-site olefin polymerization catalysts have been extensively investigated and are now achieving wide acceptance in the polyolefin industry. In order to achieve commercial significance, these soluble metallocene catalysts have to be immobilized on a carrier. The challenges of supporting these catalysts have been addressed in many creative ways and this topic has been taken up in an excellent review by Carnahan et al. [87]. This paper mainly describes anchoring techniques to silica, but these concepts may also be applied for other purposes. 

The right choice of supporting material as well as the choice of suitable properties (pore size, specific surface, chemical surface composition) are important factors influencing the immobilization of the metallocene catalyst and the fragmentation of the support during polymerization. Commercially applied porous silica gels are prepared by neutralization of aqueous alkali metal silicate with acid. The pore structure and pore size distribution can be controlled by the type of chemical reaction and experimental conditions. ... 

As an alternative to the suspension process. Witco GmbH developed (1995) a technique which immobilizes the active compounds on a spray-dried silica support by utilizing a fluidized bed reactor. They claimed to produce supported metallocene catalysts with a controllable distribution of active centers achieved by using the three different supporting methods. Controlling the addition of tri-methylaluminum and water in the different reactor... 

Because of these circumstances, the high excess of MAO is demanded to provide sufficient MAO cages. It is interesting to note that the catalytically active share of MAO can be separated from the rest when a homogeneously activated metallocene catalyst is heterogenized by self-immobilization. In these cases, Zr/Al = 80 ratios were found. 

Figure 12-9.. A scheme of the self-immobilization of a metallocene catalyst.

Kinetic data on olefin polymerization by polymer-immobilized zirconocene are scarce. It is generally accepted that homogeneous metallocene catalysts contain uniform active sites however, if they are immobilized on a polymer support, the MWD polymer production becomes broader compared with a homogeneous catalyst [103]. Kinetic analysis of gas-phase ethylene polymerization catalyzed by (CH3)2[Ind]2ZrCl2 bound at a hydroxylated copolymer of styrene with divinylbenzene and previously activated with MAO (0.17 wt.% Zr) has been carried out [104]. The influence of temperature (333 to 353 K), ethylene partial pressure (2 to 6 atm) and MAO level (molar ratio of MAO to zirconium from 2600 to 10,700) were studied. The activity of the catalyst in the gas-phase process changed from 5 to 32 kg PE (g of Zr atm h)It is possible that there are two types of active site. They are stable to temperature and deactivated by the same mechanism. A first-order reaction takes place. The propagation rate constants of two active sites show a similar dependence on temperature. 

Supported metallocene catalysts were combined with KIO montmorillonite by Weiss et al. During preparation of the clay, bentonite was treated with mineral acid, causing some of the octahedral alumina sheet to dissolve. This created dendritic silica, whose hydroxyl termination served to immobilize an alkylaluminum cocatalyst (either AlMes or Al Bus). The supported cocatalysts activated the simple metallocene dichlorides II and XI,... 

Based on this model, microreactors might be fabricated by immobilizing different types of single-site metallocene catalysts — or even catalyst cascades — on suitable supports for in situ production of novel polyolefin blends and other environmentally friendly polyolefin materials. 

MAO may be used as a homogeneous solution in a solvent such as toluene, as a solid chemical dissolvable in polymerization media, as a supported compound, termed SMAO, immobilized on relatively inert solid media such as Si02, AI2O3, or MgCb, or finally, as a mixture of solution MAO with SMAO. Generally, the heterogenization of a metallocene catalyst for polymerization occurs via one of three different methods ... 

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