Sulfonated polysiloxane

The use of POSSs as a support for the extractant for selective extraction of heavy metals was studied.471 Sulfonated polysiloxanes were surveyed as solid acids and as supports for precious metal catalysts. 

Polymerization Solvent. Sulfolane can be used alone or in combination with a cosolvent as a polymerization solvent for polyureas, polysulfones, polysiloxanes, polyether polyols, polybenzimidazoles, polyphenylene ethers, poly(l,4-benzamide) (poly(imino-l,4-phenylenecarbonyl)), silylated poly(amides), poly(arylene ether ketones), polythioamides, and poly(vinylnaphthalene/fumaronitrile) initiated by laser (134—144). Advantages of using sulfolane as a polymerization solvent include increased polymerization rate, ease of polymer purification, better solubilizing characteristics, and improved thermal stability. The increased polymerization rate has been attributed not only to an increase in the reaction temperature because of the higher boiling point of sulfolane, but also to a decrease in the activation energy of polymerization as a result of the contribution from the sulfonic group of the solvent. 

In this chapter, we briefly review our work on the synthesis and conducting properties of complexes of lithium perchlorate and polysiloxane comb polymers with oligooxyethylene side chains. Included in the discussion are blends with high-molecular-weight poly(ethylene oxide) (PEO) and some preliminary work on cation-conducting polymers with bound sulfonate groups. 

Wieland S, Panster P (1997) Replacing liquid acids in fine chemical synthesis by sulfonated polysiloxanes as sohd acids and as supports for precious metal catalysts. Stud Surf Sci Catal 108 67... 

Replacing Liquid Acids in Fine Chemical Synthesis by Sulfonated Polysiloxanes as Solid Acids and as Supports for Precious Metal Catalysts... 

The use of solid acid catalysts, consisting of polysiloxanes bearing alkylsulfonic acid groups, is described, in organic synthesis and in technical applications. Although sulfonated polysiloxanes have been reported in the literature and have been found to show excellent activities in comparison to conventional polystyrene based cation exchange resins, no large scale technical use has become known thus far. 

Sulfonated polysiloxanes have been described and reported in the literature for their excellent activities in comparison with polystyrene based cationic exchange resins. ... 

Polycondensation (sol-gel process) of suitable alkyl sulfonic acid functionalized organosilane monomers leads to solids with a siliceous matrix and a high concentration of anchored ligand groups, thus leading to the development of specially designed polysiloxane based solid acid catalysts Deloxan ASP (like in formula 1,2a, 2b, and 3). 

The alkyl sulfonic acid bearing polysiloxane is also used as a support for powder type precious metal catalysts 2 (particle size < 200 pm) that can be used in hydrogenolysis reactions. The conversion of 1 -phenylethanol to ethylbenzene proceeds quantitatively (>99%) with the same rate as achieved with the combination of H2SO4 and a Pd/C catalyst. Therefore,... 

Comparison data were presented, that demonstrate the use of the polysiloxane material as an advantageous substitute for organic cation exchange resins, sulfuric acid, p-toluene sulfonic acid and acidic zeolites. It is demonstrated, that materials like 1 and 2 are cost-efficient and reliable catalysts in esterification, alkylation, and condensation, whereas use of the bifunctional catalyst 3 gives excellent conversions in hydrogenolysis reactions in general. 

Sulfonic acid ion-exchange resins that can catalyze esterification, etherification, addition of alcohols and water to olefins, were covered in Chap. 5. Degussa has made a polysiloxane analogue with alkyl sulfonic acid groups on it.13 It is stable to 230°C, compared with 120oC for the polystyrene-based ones. 

A wide variety of solid-acid catalysts is available [17] acidic clays, zeolites, silica-occluded heteropoly acids, sulfonated polysiloxanes, Nafion (a sulfonated perfluoroalkyl resin) and Nafion-silica composites, and a variety of hybrid sulfonated mesoporous systems (see Chapter 3). 

The different approaches used to synthesize sulfonated polysiloxanes can be distinguished according to synthesis/modification of the solid siliceous network, to the organosilane precursor used, and to the generation of the active sulfonic acid group, as given in Figure 1. 

Aryl- and alkylsulfonated polysiloxanes are available. Aryl sulfonated polysiloxanes are generally synthesized by sulfonation of solid aryl polysiloxanes whereas alkyl sulfonated polysiloxanes are synthesized by starting either with an aqueous solution of the sulfonic acid-functionalized silane or with a highly crosslinked mercapto or polysulfan functionalized polysiloxane. Figure 3 gives an overview of the predominantly synthesized sulfonated siloxane units. 

Method of building-up the siliceous matrix organosilane precursors used generation of sulfonic acid group by postreacting the solid polysiloxanes... 

Figure 1. Approaches to the synthesis of sulfonated polysiloxanes. Any combination of the first and second rows of the table is possible. The second row gives the organo-silane precursors required and whether post-reaction of the solid formed is necessary.

Figure 3. Functional subunits of sulfonated polysiloxanes as attached to the solid phase.

Figure 4. Schematic representation of a cross-linked polysiloxane structure formed by copolycondensation of tetraalkoxysilane, propyltrialkoxysilane and 3-(tris-hydroxy-silyl)propyl sulfonic acid. Note the embedding of the functionalized. siloxanes into the matrix in contrast with simple attachment to the surface of a preformed solid.

Determination of sulfur in the polysiloxane by elemental analysis is used to calculate the capacity of acidic groups in the polysiloxanes. Simple acid-base titration is often misleading if strong bases, e. g. NaOH, are used, because they react not only with the sulfonic acid but also with silanols and even with siloxane bonds, as shown in Eqs (3) and (4). 

Sulfonated polysiloxanes are extremely stable in gas-phase reactions. No degradation of catalytic activity has been found from study of catalysts thermally pretreated under nitrogen up to 300 °C and also after hydrothermal pretreatment,... 

Sulfonated polysiloxanes have been tested in a variety of reactions. A typical reaction used to compare the acid strength of these catalysts with that of other solid acids is dehydration of isopropanol to yield propene [3]. The splitting of ethers, especially MTBE, at temperatures up to 200 °C has also been successfully demonstrated [14]. 

Sulfonic acid-functionalized polysiloxanes are also used in eatalytic aminations,... 

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