Polysiloxane matrix

A simple alternative to the Lindlar reduction process has been recognized.95 The palladium(II) acetate-catalyzed polymerization of triethoxysilane in water produces finely divided palladium metal dispersed on a polysiloxane matrix with concomitant hydrogen evolution. Addition of (EtO)3SiH to the mixture of an alkyne and all other necessary constituents allows facile and selective reduction of the alkyne without an external hydrogen source ... 

The introduction of organofunctional groups, i.e. the use of alkoxysilanes of the type (R O Si—X—A, where A is a functional organic group and X is a chemically inert spacer permanently linking Si and A, results in a more extensive chemical modification of the materials76. The properties of the organic functions A supplement the properties of the polysiloxane matrix formed by hydrolysis and condensation of the Si(OR )3 and Si(OR )4 units. 

A new procedure also carries out hydrogenation without added hydrogen. Triethoxysi-lane and 5 mol% of palladium acetate in a mixture of THF and water yields finely divided palladium dispersed on a polysiloxane matrix with concomitant hydrogen evolution44. Alkenes, present in this reaction mixture, are transformed to the corresponding saturated hydrocarbons in 100% yield at room temperature (equation 1). 

In this chapter two different kinds of phase materials are presented the first is a chiral diamide selector bonded to a polysiloxane matrix the other selector system is a calix[4]arene with chiral residues which is attached to a polysiloxane backbone (Sect. 2.4). These systems were used in direct optical methods based on a change in the refractive index or the optical thickness of a transparent polymeric layer. 

Lipodex E [octakis(3-0-butanoyl-2,6-di-0- -pentyl)-y-cyclodextrin] dissolved in a polysiloxane matrix (SE-54) was used as a chiral receptor. The binding strength of the analyte molecules depends on the interaction mech-... 

For special applications it is of interest to incorporate also non-functional, e.g. propyl-, or functional siloxane units with cocatal)dic properties, e.g. mercapto-functionalized siloxanes, into the polysiloxane matrix. One example in which this is very important is the industrial production of bisphenol-A (eq. 2). 

A co-catalytically active mercapto group was incorporated within the polysiloxane matrix. The desired 4,4 -bisphenol-A is produced with a selectivity of 95-96% whereas the total selectivity for the 4,4 - and 2,4 -bisphenol-A is higher than 99%. These results are achieved with extremely short residence times as compared to the state-of-the-art technology. Whereas the use of polystyrene resins gives selectivities of about 4 to 5% less at WHSV = 0.2 - 0.5 h, the above reported excellent selectivities obtained with the Deloxan catalysts are reached at space velocities from 0.5 up to 2.0 h". ... 

As already discussed, there are two parts of the cyclodextrin molecule that can be derivatized, but only one part significantly effects its chiral selectivity. Referring to the cyclodextrin molecule in figure 5.15, the OH group on the 6-position is usually alkylated and as the group is free to rotate, it blocks the base aperture to the cone and thus the 6-position offers no chiral selectivity. In addition, the alkylation of the 6-position can increase the solubility of the cyclodextrin in the polysiloxane matrix. 

Based on the above results, we have proposed a classical Chalk-Harrod-like mechanism for the Pd nanocluster catalyzed reaction (Scheme 3.10). " The first step involves the reduction of Pd(OAc)2 to zero-valent palladium nanoparticles followed by stabilization with a polysiloxane network. These particles followed the conventional pathway of oxidative-addition and reductive-elimination cycles to generate sUyl esters. After the transformation was over, these palladium colloids precipitated out of the solution as a black solid supported on a polysiloxane network. The presence of the soluble polysiloxane matrix, which was not fully condensed in the form of silica, allows particle redispersion— thus allowing particles to be reused as catalyst... 

Production of amorphous silica during thermal degradation of polysiloxane matrix imder oxidizing atmosphere, which results in... 

Figure 21.6 Preparation of highly active Pd nanoparticles embedded in a polysiloxane matrix.

Figure 21.7 On-column hydrogenation over the reactor. SEM and Si/Pd EDX measure-highly active Pd nanoparticles. Capillaries of ments show the coating of the fused silica only 2 cm coated with Pd nanoparticles sta- micro capillaries (i.d. 250, 0.25 pm film thick-bilized in a polysiloxane matrix are used as ness).

To extend ocRGC to the catalytic asymmetric Gosteli-Claisen rearrangement, the following experimental setup is described. Catalytically active separation columns were used as the micro-capillary reactor and are subsequently coupled with enantioselective separation columns to perform analysis of the reaction products. The inner surface of fused-silica capillaries (i.d. 250 pm) was coated with [Cu (R,R)-Ph-box ](SbFg)2 (R R)-5 dissolved in an inert polysiloxane matrix... 

TEOS and TMOS can form up to four bonds to the polysiloxane matrix. In this case, as crosslinking proceeds the possibility of any relaxation processes vanishes (28) since the probability that two of the Si-O-(GEL) bonds will open at the same time is extremely low. To eliminate cracking we need to allow relaxation to proceed. This implies that the maximum number of bonds between an Si atom and the gel must be 3. Thus, upon a temporary rehydrolysis of any Si-O-(GEL) bond, its number of bonds with the gel is reduced to 2, which allows mobility of that Si atom, as illustrated in Figure 3. It is noteworthy that by applying this precondition, we completely shift the problem from a sol-gel a ng-drving process towards a polymer-crosslinking one. 

The addition of the filler into a polysiloxane matrix will also influence the porosity of the resulting material. Differences in permeabihty between porous and amorphous sihcone membranes have been determined [73]. The authors concluded that the trcuisport of gcises through porous rubber membranes is primarily due to capihary flow-through pores, whereas the transport of gcises through amorphous rubber films is attributed to activated diffusion, hi facf structural analysis of a PMDS-sihca mixture by small-angle neutron... 

Papers [98, 99] studied the dimethylsiloxane system by the NMR method. It is shown that the value is 8 1 regardless of the oligomer chain length (4 samples with the molecular weight from 300 to 1000) and the swelling degree (from 5 to 20%) in the cross-linked polysiloxane matrix. The value of the orientation order parameter decreases abruptly imder the low-molecular dilution. 

---