Immobilized polymerization catalysts

Duchateau, R. (2003) Silsesquioxanes advanced model supports in developing silica-immobilized polymerization catalysts, in Nanostructured Catalysts, Kluwer Academic/Plenum Publishers, New York, pp. 57-83. 

Nanostructured Catalysts, ed. S.L. Scott, C.M. Crudden and C.W. Jones, Kluwer Academic/Plenum Publishers, New York, N.Y., 2003 R 247 R. Duchateau, Silsesquioxanes Advanced Model Supports in Developing Silica-Immobilized Polymerization Catalysts , p. 57... 

The above example outlines a general problem in immobilized molecular catalysts - multiple types of sites are often produced. To this end, we are developing techniques to prepare well-defined immobilized organometallic catalysts on silica supports with isolated catalytic sites (7). Our new strategy is demonstrated by creation of isolated titanium complexes on a mesoporous silica support. These new materials are characterized in detail and their catalytic properties in test reactions (polymerization of ethylene) indicate improved catalytic performance over supported catalysts prepared via conventional means (8). The generality of this catalyst design approach is discussed and additional immobilized metal complex catalysts are considered. 

When combined with the isolation and reactivity studies of the patterned aminosilica (7), the increased activity of the patterned catalysts provide further evidence that the patterning technique developed allows for the synthesis of aminosilicas which behave like isolated, single-site materials (although a true single site nature has not been proven). As the olefin polymerization catalysts supported by the patterned materials show a marked improvement over those materials supported on traditional aminosilicas, these patterned materials should be able to improve supported small molecular catalysis as well. Future improvements in catalysis with immobilized molecular active sites could be realized if this methodology is adopted to prepare new catalysts with isolated, well-defined, single-site active centers. 

A list of examples in this section is not exhaustive rather, they have been chosen to illustrate the different approaches used for immobilization of the catalysts for important classes of organic reactions, namely hydrogenation, oxidation, and coupling reactions. Due to the major industrial importance of olefin polymerization (see Chapter 9.1), and although the objectives of immobilization of polymerization catalysts are rather different from the other examples, some references to this will also be given here. 

Other functionalized supports that are able to serve in the asymmetric dihydroxylation of alkenes were reported by the groups of Sharpless (catalyst 25) [88], Sal-vadori (catalyst 26) [89-91] and Cmdden (catalyst 27) (Scheme 4.13) [92]. Commonly, the oxidations were carried out using K3Fe(CN)g as secondary oxidant in acetone/water or tert-butyl alcohol/water as solvents. For reasons of comparison, the dihydroxylation of trons-stilbene is depicted in Scheme 4.13. The polymeric catalysts could be reused but had to be regenerated after each experiment by treatment with small amounts of osmium tetroxide. A systematic study on the role of the polymeric support and the influence of the alkoxy or aryloxy group in the C-9 position of the immobilized cinchona alkaloids was conducted by Salvadori and coworkers [89-91]. Co-polymerization of a dihydroquinidine phthalazine derivative with hydroxyethylmethacrylate and ethylene glycol dimethacrylate afforded a functionalized polymer (26) with better swelling properties in polar solvents and hence improved performance in the dihydroxylation process [90]. 

Recently, Mecking et al. reported the synthesis of inverse micelles based on a hy-perbranched polyglycerol polymer. Terminal -OH groups were modified with palmi-toyl chloride and gave a polymeric catalyst soluble in organic solvents with hydrophilic core to immobilize water-soluble guest molecules such as PdCl2 or Pd(OAc)2. 

The tethering approach has been applied extensively toward the development of Zr-based polymerization catalysts [32]. A silica-anchored Zr(lV) cyclopentadienyl heterogeneous catalyst has been developed by immobilization of the organometal-Hc precursor pZr(NMe)3 on mesoporous sihca previously adequately silylated by chlorodimethyline-3-ylsilane. The catalyst, which can polymerize ethylene after... 

Palladium containing homogeneous polymeric catalysts PEO-b-P2VP and PS-b-P4VP were synthesized by the immobilization of appropriate palladium salts into vinyl pyridine cores of PEO-b-P2VP and PS-b-P4VP micelles, respectively, followed by the reduction of the palladium. 

McKittrick, M.W. and Jones, C.W. (2004) Toward single-site, immobilized molecular catalysts site-isolated Tr ethylene polymerization catalysts supported on porous silica. J. Am. Chem. Soc., 126, 3052. 

McKittrick, M. W., Yu, K. Q. and Jones, C. W. Effect of metallation protocol on the preparation and performance of silica-immobilized TiCGC-inspired ethylene polymerization catalysts, J. Mol. Catal., A, 2005, 237, 26-35. 

Heterogeneous diene polymerization catalysts based on modified and unmodified silica-supported lanthanide complexes are known as efficient gas-phase polymerization catalysts for a variety of support materials and activation procedures (see Sect. 9). Metal siloxide complexes M(()SiR3 )x are routinely employed as molecular model systems of such silica-immobilized/ grafted metal centers [196-199]. Structurally authenticated alkylated rare-earth metal siloxide derivatives are scarce, which is surprising given that structural data on a considerable number of alkylated lanthanide alkoxide and aryloxide complexes with a variety of substitution patterns is meanwhile available. 

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. 

McKittrick, Michael W., and Christopher W. Jones, Toward Single-Site, Immobilized Molecular Catalysts Site-Isolated Ti Ethylene Polymerization Catalysts Supported on Porous Sihca , JACS Communications (2004) 126,3052... 

Severn JR, Chadwick JC, Duchateau R, Friederichs N (2005) Bound but not gagged -immobilizing single-site a-olefin polymerization catalysts. Chem Rev 105 4073... 

As previously mentioned, one of the primary motivations for the development of site-isolated aminosilicas is to construct a better molecular-level understanding of immobilized catalysts through the use of a more uniformly reactive surface. Within the area of a-olefin polymerizations, another parameter that negatively affects the ability to study well-defined surfaces is the use of methylaluminoxane (MAO) as a catalyst activator. The exact structure of the MAO species has been postulated to exist in a number of different forms, which makes it difficult to elucidate the exact nature of its interactions with the surface [21]. To address this issue, a well-defined sulfonic acid organic/inorganic hybrid material was developed to serve as both a support and a catalyst activator for homogeneous a-olefin polymerization catalysts [22]. 

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