Organoalkoxysilanes

Organoalkoxysilanes are utilized in different application sectors, because under application conditions no acidic cleavage products are formed as is the case with organo-halosilanes. In addition intentional hydrolysis, e.g. in silicone chemistry, is generally easier to control than in the case of organohalosilanes. 

Organoalkoxysilanes can be produced by the stoichiometric reaction of organohalosilanes with alcohols according to the following equation  

Organoalkoxysilanes are formed by reacting organohalosilanes with alcohols with the sitnultaneous removal of hydrogen chloride 

A prerequisite for a high degree of reaction is removal of the hydrogen chloride formed either by addition of a base such as tertiary amines or by appropriate control of the reaction. 

Industrially this is attained by the utilization of a reaction distillation column, in which a silane-alcohol mixture is fed into the upper one third. 

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Consider a trialkoxysilane as co-precursor for an initial 1 1 mixture of alkoxysilane and organoalkoxysilane, and recall that in the case of ORMOSIL condensation takes place only through elimination of water27 and not by alcohol formation. The mechanism involves hydrolysis and condensation reactions ... 

These polysiloxanes were also found to be synthesized by the condensation reaction between organosilanes and organoalkoxysilanes with release of alkane as an inert by-product. For this polycondensation, B(C6F5)3 was used as an effective catalyst [88]. This process involves cleavage of C-0... 

A second, equally powerful means to prepare such materials relies on traditional inorganic polymerization tools, most notably sol-gel polymerization.24 25 A number of excellent reviews have appeared on this subject as well.5,12,17 In sol-gel processing, the functional monomer [i.e., an organoalkoxysilane such as 3-aminopropyltrimethox-ysilane (APTMS)] is combined with the cross-linking agent [i.e., a tetrafunctional alkoxysilane such as tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS)], a catalyst (such as hydrochloric acid or ammonia), and the template molecule. The resultant sol can be left to gel to form a monolith, which can then be dried, sieved, and extensively washed to remove the template. Alternatively, the sol can be spin coated, dip coated, or electrodeposited on a surface to yield a thin film, which can be subsequently washed with a solvent to remove the template and yield the imprinted cavities. 

A large number of organoalkoxysilanes can be purchased or synthesized to help impart selectivity to the matrix. These functional monomers can be selected to complement specific functional groups on the template molecule to provide the materials with chemical specificity in addition to shape selectivity. 

The ability to change the functional group, R, via the judicious choice of the organoalkoxysilane (many of which are commercially available) is particularly... 

Figure 20.2 Selected examples of organoalkoxysilanes used to create imprinted silica.

The noncovalent self-assembly method relies on less specific types of interactions, including electrostatic, hydrogen bonding, metal ion coordination, Van der Waals forces, and/or hydrophobic attractions.8 In this approach, the template and the polymerizable monomer (i.e., an organoalkoxysilane) are judiciously chosen so that they will have complementary interactions. Removal of the template is easier than in the covalent assembly approach and often involves simply washing the materials in a suitable solvent.8... 

In an interesting illustration of the first strategy, Sakaguchi and coworkers covalently attached hemoglobin to an aminopropyl silica particle and then polymerized organoalkoxysilanes on the surface of the hemoglobin-modified silica particle.85 The template was removed via treatment with oxalic acid.85 In more recent work, Zhang and coworkers utilized a similar approach. In their case, the sphere was made from the functionalized biopolymer, chitosan.86 The model template protein, bovine serum albumin, was covalently attached to the chitosan microsphere and then coated with a composite sol prepared from TEOS and an aminosilane.86... 

Oxidation of Si—C bonds. The carbon-silicon bond of organoalkoxysilanes is cleaved by 30% H202 or m-chloroperbenzoic acid in the presence of a base or under neutral conditions in the presence of KHF2 (equation I). Only one alkoxy group on silicon... 

Silica can be drastically altered by reaction with organochlorosilanes or organoalkoxysilanes giving Si—O—Si—R linkages with the surface. The attachment of hydrocarbon chains to silica produces a non-polar surface suitable for reversed-phase chromatography where mixtures of water and organic solvents are used as eluents. The most popular material is octadecyl-silica (ODS-silica) which contains C g chains, but materials with Cj, C, Cg, and C22 chains are also available. 

Organic species can also be chemically bonded to the oxide network via Si-C bonds. Many organoalkoxysilane precursors, R4 Si(OR), are commercially available. They contain nonhydrolyzable Si-C bonds, so that organic moieties are not removed upon hydrolysis. Depending on the nature and amount of organic and inorganic components, it becomes possible to tailor the microstructure and even obtain multifunctional materials. 

Figure 5 Molecular structure of selected organoalkoxysilanes trifunctional A-3-trimethoxy-sily 1-propyl-pyrrole (6) difunctional polydimethylsiloxane (7) a multifunctional bridged polysilsesquiox-ane (8)...

Organoalkoxysilanes are widely used for coatings production [1], as electrolytes [2], adhesives [3], crosslinking agents [4, 5], water-repellent components for glass, building materials, and textiles [6-8]. 

Organoalkoxysilane HPC was studied by means of Si NMR spectrometry [13, 14] and gas chromatography [15]. Tetraethoxysilane (TEOS), methyltriethoxysilane (MTES), phenyltriethoxy-... 

OTES is currently the most promising organoalkoxysilane for use as a water repellent [18 - 20]. For the determination of optimum conditions for the use of OTES, its HPC processes have to be investigated under various conditions. However, there is a lack of literary data concerning the processes of OTES condensation and hydrolysis. Therefore, this pq)er deals with the investigation of OTES HPC for electrophilic catalysis. 

Although there are a variety of routes presently available for siloxane production, only two are of commercial importance. These include hydrolytic reactions of organohalosilanes or organoalkoxysilanes and redistribution type polymerizations of... 

The difficulties encountered in LLC can be overcome by the use of chemically bonded stationary phases or bonded-phases. Most bonded phases consist of organochlorosilanes or organoalkoxysilanes reacted with micro-particulate silica gel to form a stable siloxane bond. The conditions can be controlled to yield monomeric phases or polymeric phases. The former provides better efficiency because of rapid mass transfer of solute, whereas the polymeric phases provides higher sample capacity. BPC can be used in solvent gradient mode since the stationary phase is bonded and will not strip. Both normal-phase BPC (polar stationary, non-polar mobile) and reversed-phase BPC (non-polar stationary, polar mobile) can be performed. The latter is ideal for substances which are insoluble or sparingly soluble in water, but soluble in alcohols. Since many compounds exhibit this behaviour, reversed phase BPC accounts for about 60% of published applications. The main disadvantage of silica bonded phases is that the pH must be kept between 2 to 7.5. However, bonded phases with polymer bases (polystyrene-divinylbenzene) can be used in the pH range of 0 to 14. 

Synthesis and Characterization of Colloidal Model Particles Made from Organoalkoxysilanes... 

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