Metallocene catalysts, polymer-supported

Takahashi, T., Nakano, H., Uchino, H., Tayano, T., and Sugano, T. 2002. Study of clay mineral "support-activator" in metallocene catalyst. Polymer Preprints 43 1259-1260. 

Machado, F., Lima, E. L., Pinto, J. C., McKenna, T. F. (2011). An experimental study on the early stages of gas-phase olefin polymerizations using supported Ziegler—Natta and metallocene catalysts. Polymer Engineering and Science, 51(2), 302—310. 

Besides silica, numerous other materials, including alumina, zeolites, clays, and organic polymers, have been used as supports for metallocene-based catalysts. In some cases, a covalent bond between metallocene and polymer support has been achieved through prior attachment of part of... 

Han, J. J., Lee, H. W., Yoon, W. J., Choi, K. Y. Rate and molecular weight distri-bntion modeling of syndiospecific styrene polymerization over silica-supported metallocene catalyst. Polymer, 48, 6519-6531 (2007). 

Chu, K.-J., Soares, J. B. P. Effect of experimental conditions on ethylene polymerization with in-situ supported metallocene catalysts./. Polym. Sci, Polym. Chem., (2000) 38, pp. 1803-1810... 

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.)...

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. 

Natta postulated that for the stereospecific polymerization of propylene with Ziegler-Natta catalysts, chiral active sites are necessary he was not able to verify this hypothesis. However, the metallocene catalysts now provide evidence that chiral centers are the key to isotacticity. On the basis of the Cossee-Arlman mechanism, Pino et al. (164,165) proposed a model to explain the origin of stereoselectivity The metallocene forces the polymer chain into a particular arrangement, which in turn determines the stereochemistry of the approaching monomer. This model is supported by experimental observations of metallocene-catalyzed oligomerization. 

The most common industrial method of producing a supported metallocene catalyst is to treat the support with MAO first and then adsorb the metallocene on it. Many procedures for such preparations have been reported. The properties of the support provide some control of the morphology of the polymer produced the polymer particles are often replicates of the catalyst particles (220). 

This difference shows that when additional MAO is used as scavenger, a supported metallocene catalyst in a slurry process gives a mixture of polymers, some produced on the supported catalyst and some in the homogeneous phase. 

It is estimated that by the year 2000 nearly every second new polyolefin plant will run with metallocene systems. A lot of polymer-producing companies try to use mainly supported metallocene catalysts in their running plants, the so-called drop in technology others build plants specially equipped for these new types of catalysts. 

Due to the use of advanced, highly active and selective solid-state catalysts (sections 7.3), the processes described above produce polymers from which neither stereoirregular polymer components nor catalyst residues need be removed. This has resulted in substantial reductions in the costs of investments, energy and maintenance, compared to slurry processes with first-generation catalysts. Ongoing developments are aimed at increased process flexibility and at process adaptation to the use of supported metallocene catalysts (Section 7.4). 

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. 

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