Grafting polymers, catalyst immobilization

If the mam goal of the formation of grafted polymer catalysts was to dispose active sites on the surface of the support so as to increase its specificity, gel-immobilized... 

The fact that silsesquioxane molecules like 2 7 contain covalently bonded reactive functionalities make them promising monomers for polymerization reactions or for grafting these monomers to polymer chains. In recent years this has been the basis for the development of novel hybrid materials, which offer a variety of useful properties.3,4 This area of applied silsesquioxane chemistry has been largely developed by Lichtenhan et al.4 With respect to catalysis research, the chemistry of metallasilsesquioxanes also receives considerable current interest.1,2,10,21 As mentioned above, incompletely condensed silsesquioxanes of the type R7Si7Og(OH)3 (2-7, Scheme 4) share astonishing structural similarities with p-tridymite and p-cristobalite and are thus quite realistic models for the silanol sites on silica surfaces.1,2,21 26 Metal complexes derived from 2-7 are therefore commonly regarded as realistic models for industrially important metal catalysts immobilized on silica surfaces.1,2,22 It is... 

By chemical modification of the silica surface it has become possible to design new highly-selective adsorbents and catalysts, active polymer fillers, efficient thickeners of dispersive media. Interest in the modified silicas, in particular, in the activated matrices based on functional organosilicas has quickened in the past few years as a result of the favorable prospects for their application for various kinds of chromatographic separation, preparation of grafted metal complex catalysts, immobilized enzymes and other biologically active compounds [1]. 

Non-cross-linked polymers can be used in this way just as cross-linked polymers can. For example, we have used polyethylene supports with surface grafts to support Pd(0) catalysts [133,134]. In these cases, the polymer-immobilized catalyst is used in exactly the same way as an insoluble polymer-bound catalyst. Such supported catalysts do require that the insoluble polymer be swollen or permeable to substrates or that the catalysts be within a solvent-permeable, thin immobilized graft. While this approach can be useful, it takes no advantage of the polymer s solubility. It is an approach that conceptually is no different than that used with insoluble inorganic supports or with polymers that are by design insoluble by virtue of cross-linking, and is an approach to catalyst immobilization that is not further discussed since this review is focused on polymer-immobilized catalysts that are used under solution-state conditions. 

Choi and co-workers synthesized 6-amino-1-hexanol-immobilized SWCNTs by immersing the acyl chloride-functionalized SWCNTs into a DMF solution of 6-amino-1-hexanol for 12 h, and grafted poly(p-dioxanone) (PDX) from the SWCNT surfaces by in situ ROP in toluene at 100 °C for 3 days with tin(II) 2-ethylhexanoate [Sn(Oct)2] as the catalyst. The 10% weight-loss temperature of grafted polymer is higher than that of free polymer by approximately 20 °C. 

Goto and coworkers attached a surface-immobilizing initiator IHE onto a siliccm wafer (Scheme 5) and prepared polymer brushes by RTCP [66]. The IHE-immobUized silicon wafer was inunersed in a mixture of MMA, 2-cyanopropyl iodide (a free iodide initiator), azobis(isobutyronitrile) (a radical source), and NIS (a catalyst). The system was purged with an inert gas and heated at 70°C for 4 h to induce polymerization. The and M IM values of the free polymer were 15,000 and 1.31, respectively. From the thickness of the graft polymer and the M of the free polymer, the a value was calculated to be 0.28. This result indicates the successful controlled synthesis of a concentrated polymer brush by RTCP. Another example of the graft polymerization is depicted in Fig. 6,... 

Hoveyda and co-workers immobilized an olefin metathesis catalyst on monolithic sol-gel and claimed that the catalytic material is easily recyclable. Barrett and co-workersprepared a recyclable boomerang polymer supported catalyst for olefin methathesis by grafting the preformed catalyst to a polystyrene... 

Many efforts have been undertaken to graft transition metal complexes onto various supports in order to retain the performance of the soluble catalyst precursors and to allow easy separation of the catalysts from the reaction products. Most studies have been concerned with polymers, particularly with functionalized styrene-divinylbenzene resins. This approach to immobilize homogeneous catalysts has been reviewed, with all the strategies to anchor metal complexes on organic or inorganic supports examined (57-59). 

Phase transfer catalysts have been grafted onto the surface of porous capsules to facilitate product purification after reaction, and many types of immobilized cells, mycelia, enzymes, and catalysts have been encapsulated in polymers such as PDMS, PVA, or cellulose. In the specific case of PVA, they are named Lenti-kats, as commercialized by Genialab and used for nitrate and nitrite reduction and in the synthesis of fine chemicals. These beads show minimized diffusion limitations caused by the swelling of the polymeric environment under the reaction conditions. To avoid catalyst leaching, enlargement can be realized by linking them to, e.g., chitosan. 

Poly(ethylene glycol) grafted on crosslinked polystyrene (PEG-PS) resin has often been used as a polymer support for chiral catalysts of reactions performed in aqueous media. Peptides immobilized to PEG-PS resin have been developed and used as a catalyst for direct asymmetric aldol reactions in aqueous media (Scheme 3.19) [42]. When tripeptide-supported PEG-PS 67 was used as chiral catalyst in the reaction between 70 and acetone, the corresponding aldol product 69 was obtained with 73% ee. Kudo further developed the one-pot sequential reaction of acidic deacetalization and enanhoselective aldol reaction by using an Amberhte and PEG-ST-supported peptide catalyst 67 [43]. The enantioenriched aldol product 72 was obtained in 74% isolated yield from acetal 70 in a one-pot reaction (Scheme 3.20). 

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). 

---