Metal-polymer complex, catalytic

Reactions of Cjq with metal carbene complexes also yield the [6,6] methano-fullerenes [392]. These adducts are probably not formed via a carbene addition, but via a formal [2-1-2] cycloaddition under formation of a metalla cyclobutane intermediate. The Fischer carbene complex [mefhyl(methoxymethylene)]pentacarbonyl chromium can be utilized to prepare l,2-mefhyl(methoxymethano)-fullerene in 20% yield [392]. A tungsten carbene complex was primarily used to initiate the formation of a polyacetylene polymer, but it was discovered that addition of to the complex-polymer-mixture improves the polymerization and dramatically increases the catalytic activity of the carbene complex [393]. can be integrated into the polymer via carbene addition. 

This time-constant rate is proportional to the a-TiCU amount which proves that, at least formally, the over-all polymerization process is really a catal3rtic one, with regard to the a-TiCh. The catalytic behavior of -TiCU is, in any case, connected with the existence on its surface of metal-lorganic complexes which act in the polymerization only if a-TiCU is present. This makes stereospecific polymerization processes (of coordinated anionic nature) very different from the better known polymerization processes, initiated with free radicals. In the latter process, the initiator is not a true catalyst, since it decomposes during the reaction, forming radicals which are bound to the dead polymer on the contrary, in the case of stereospecific polymerization, each molecule of polymer, at the end of its growing period, can be removed from the active center on the solid surface of the catalyst which maintains its initial activity. 

The first example of catalysis by a polymer-metal complex was presented by Lautsch et al. u3 Metalloporphyrin was linked to a poly(phenylalanine) chain by a peptide bond. The catalytic properties of this polymer-Fe(III)porphyrin complex were compared with Fe(III)porphyrin in the oxidative reaction of phenylenedi-amine. The catalytic activity of the polymer complex was twice as large as that for the corresponding analog. 

Polymer-metal complexes often exhibit high efficiency in the catalytic decomposition of hydrogen peroxide. The following reasons for this activity have been advanced, (i) Some polymer-metal complexes contain incomplete complexes due to steric hindrance, and this contributes to their catalytic activity121, 122. (ii) In other polymer complexes, the coordinate bond between polymer ligand and metal ion is relatively weak and the substrate coordinates with high frequency124. ... 

Metal complexes immobilized on crosslinked polymer matrices are hard to characterize by the customary physicochemical techniques, which is why no quantitative studies have been made on the catalytic activity of polymer complexes. 

The hydrolysis of sodium pyrophosphate was effected by using some metal complexes of poly(methacrylacetone) as catalysts138. The catalytic activity of the polymer complexes declined in the following order Zr(IV)0 > U(VI)02 >... 

The importance of the interaction with photons in the natural world can hardly be overstated. It forms the basis for photosynthesis converting carbon dioxide and water into more complex plant-associated structures. This is effectively accomplished employing chlorophyll as the catalytic site (this topic will be dealt with more fully later in the chapter). Chlorophyll contains a metal atom within a polymeric matrix, so it illustrates the importance of such metal-polymer combinations. T oday, with the rebirth of green materials and green chemistry use of clean fuel—namely, sunlight—is increasing in both interest and understanding. 

The observation that the metal carbene complex, (CO)5W = C(Ph)2 [22], catalyzed the polymerization of cyclic olefins to ring opened polymers containing the diphenylmethylene unit of the catalyst provided additional evidence that carbenes were involved in the catalytic cycle. The formation of the initiating metal carbenes in the classic systems that consist of transition metal halides and alkylating agents was proposed to involve metal alkylation followed by oc-hydrogen loss, Eq. (6). Methane and propene were detected in the early stages of these reactions [23]. 

More recent developments in the mechanistic aspects of the alkene metathesis reaction include the observation that the alkene coordinates to the metal carbene complex prior to the formation of the metallacyclobutane complex. Thns a 2 - - 2 addition reaction of the alkene to the carbene is very unlikely, and a vacant coordination site appears to be necessary for catalytic activity. It has also been shown that the metal carbene complex can exist in different rotameric forms (equation 11) and that the two rotamers can have different reactivities toward alkenes. " The latter observation may explain why similar ROMP catalysts can produce polymers that have very different stereochemistries. Finally, the synthesis of a well-defined Ru carbene complex (equation 12) that is a good initiator for ROMP reactions suggests that carbenes are probably the active species in catalysts derived from the later transition elements. ... 

In recent years, there has been much interest in the study of polymer complexes, particularly in relation to their catalyst properties, thermostability and biomedical effects. Mainly these studies have been directed towards the use of insoluble polymers. An account of the soluble polymeric metal complexes and their catalytic activity has been published recently A brief outline of the studies in relation to the n M-chelation of some linear hydrophilic polymers and their functional derivatives with chelating groups and some polymeric molecular complexes is given in this section. 

In addition to the advantages associated with immobilizing homogeneous transition metal complexes on polymers for catalytic reactions, there are certain disadvantages and unresolved problems. One problem characteristic of all the chemistry associated with surface modified materials is the difficulty in determining what the actual chemical structures present on the heterogeneous... 

Polymer supported Pd(II)-glycine complex was found to be stable upto 150°C on the basis of DTA-TG analyses. A change in morphology of the catalyst is indicative of anchoring the ligand as well as the metal ions on the surface of polymer. The catalytic activity of the catalyst was tested for oxidation of toluene under various conditions. The recycling efficiency... 

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