Alkoxysilanes

Anotlier important modification metliod is tire passivation of tire external crystallite surface, which may improve perfonnance in shape selective catalysis (see C2.12.7). Treatment of zeolites witli alkoxysilanes, SiCl or silane, and subsequent hydrolysis or poisoning witli bulky bases, organophosphoms compounds and arylsilanes have been used for tliis purjDose [39]. In some cases, tire improved perfonnance was, however, not related to tire masking of unselective active sites on tire outer surface but ratlier to a narrowing of tire pore diameters due to silica deposits. 

Alkoxysilanes Alkyd coatings Alkyd paints Alkyd resins... 

Poly(ethylene oxide). The synthesis and subsequent hydrolysis and condensation of alkoxysilane-terniinated macromonomers have been studied (39,40). Using Si-nmr and size-exclusion chromatography (sec) the evolution of the siUcate stmctures on the alkoxysilane-terniinated poly(ethylene oxide) (PEO) macromonomers of controlled functionahty was observed. Also, the effect of vitrification upon the network cross-link density of the developing inorganic—organic hybrid using percolation and mean-field theory was considered. 

The successfiil synthesis of a transparent soHd polymer electrolyte (SPE) based on PEO and alkoxysilanes has been reported (41). The material possessed good mechanical properties and high electrical conductivity (around 1.8 x 10 S/cm at 25°C) dependent on the organic—inorganic ratio and PEO chain length. 

The physical properties of commercial alkoxysilanes are provided in Table 1. Two classes of silane esters have very distinct properties and are generally considered apart from alkoxysilanes. Sdatranes are compounds derived from trialkanolamines and have siHcon—nitrogen coordination. These are generally hydrolytically stable and have unique physiological properties (3). A second special class of monomeric esters are cycHc diesters of polyethyleneoxide glycols designated sila-crowns, which have appHcation as catalysts (4). Neither silatranes nor sila-crowns are considered herein. 

The most significant difference between the alkoxysilanes and siUcones is the susceptibiUty of the Si—OR bond to hydrolysis (see Silicon compounds, silicones). The simple alkoxysilanes are often operationally viewed as Hquid sources of siUcon dioxide (see Silica). The hydrolysis reaction, which yields polymers of siUcic acid that can be dehydrated to siUcon dioxide, is of considerable commercial importance. The stoichiometry for hydrolysis for tetraethoxysilane is... 

The synthesis of triethoxysilane (28) and trimethoxysilane (29) has also been achieved by direct process. In 1980 there were no direct processes for the production of alkoxysilanes. In 1995 Silbond in Weston, Michigan, and Carboline in St. Louis, Missouri, operated processes for the production of tetraethoxysilane in the United States, and OSi/Witco announced start-up of a process to produce triethoxysilane and tetraethoxysilane in TermoH, Italy. 

Apart from the direct action of an alcohol on a chlorosilane or siUcon, the only other commercial method used to prepare alkoxysilanes is transesterification. 

Other preparative methods for alkoxysilanes, in approximate order of declining utOity, are given by the following equations (34—40) ... 

The analogous reaction between anhydrides and alkoxysilanes also produces acyloxysilanes. The direct reaction of acids with chlorosilanes does not cleanly lead to full substitution. Commercial production of methyltriacetoxysilane direcdy from methyltrichlorosilane and acetic acid has been made possible by the addition of small amounts of acetic anhydride or EDTA, or acceptance of dimethyltetraacetoxydisiloxane in the final room temperature vulcanising (RTV) appHcation (41—43). A reaction that leads to the formation of acyloxysilanes is the interaction of acid chlorides with silylamides. 

Glass Frosting. Deposition of siUcon dioxide is used to impart a translucent coating on glass (qv) (50). The surfaces are either exposed to tetraethoxysilane or tetramethoxysilane under high moisture conditions, or the alkoxysilanes are ignited and the resulting powder is appHed to the surface. 

Relative hydrolysis and condensation rate studies of multifunctional silanes, Si(OR), under acidic and basic catalysis showed that the first (OR) group hydroly2es much more readily than subsequent groups (195). Sdanol—sdanol condensation is much slower than sdanol—alkoxysilane condensation, even if the alkoxysilane is monofunctional, thus suggesting that chain extension is insignificant ia the presence of a cross-linker (196—199). 

MQ resins are commercially manufactured by one of two processes the ethyl sihcate or the sodium sihcate process. In the ethyl sihcate process, these resins were first prepared by cohydrolysis of tetraethoxysilane and trimethylchlorosilane in the presence of an aromatic solvent (eq. 34). This process is versatile and reproducible it can be used to prepare soluble MQ resins with M/Q ratios ranging between 0.6 and 4. The products of these reactions typically contain high levels of residual alkoxysilane groups. 

Other organic—inorganic hybrids include poly(ethyloxazoline)—siUca, poly(vinyl alcohol)—siUca, poly(arylene ether) ketone—siUca, polyimide—siUca, polyozoline—sihca, poly(ethylene oxide)—siUca, and polymers—modified alkoxysilane. 

The hybrid nanocomposite polymer-silica materials on the basis of oligomer alkoxysilane (polyethoxysilane (PES) obtained from tetraethoxysilane) with polycaproamide, polyacrylonitrile, chitosan and zirconyl oxychloride were investigated. 

Similar reactions can also be written for the alkoxysilanes but in commercial practice the chlorosilanes are favoured. These materials may be prepared by many routes, of which four appear to be of commercial value, the Grignard process, the direct process, the olefin addition method and the sodium condensation method. 

Alkyl silyl ethers are cleaved by a variety of reagents Whether the silicon-oxygen or the carbon-oxygen bond is cleaved depends on the nature of the reagent used Treatment of alkoxysilanes with electrophilic reagents like antimony tri-fluonde, 40% hydrofluonc acid, or a boron tnfluonde-ether complex results in the cleavage of the silicon-oxygen bond to form mono-, di-, and tnfluorosiloxanes or silanes [19, 20, 21) (equations 18-20)... 

The general mechanism of how alkoxysilanes form bonds with a fiber surface that contains hydroxyl groups is depicted in Figure 9. 

Alkoxysilanes undergo hydrolysis, condensation (catalysts for alkoxysilane hydrolysis are usually catalysts for condensation), and a bond formation stage under base as well as under acid catalyzed mechanisms. In addition to this reaction of silanols with hydroxyls of the fiber surface, the formation of polysiloxane structures also can take place. 

Figure 9 Scheme of the alkoxysilanes bond formation with hydroxyi groups contained by the fiber surface. 

Grafting from silica particles, silicon wafers, and related surfaces usually involves attaching a chlorosilanc or alkoxysilane derivative. Thus alkoxyamincs (e.g, 361,744,749 3627 0) and a wide variety of ATRP initiators (e.g. 363751) have been attached directly to surfaces and used to initiate grafting from" processes. 

The brush phases are made from the mono-chlorsilanes or the mono-alkoxysilanes which attach a single organic moiety to each reacted hydroxyl group. For example using octyldimethylchlorsilane, dimethyloctyl chains would be attached to the surface like bristles of a brush, hence the term brush phase. In all bonding processes, some hydroxyl groups remain unreacted which would allow the polar character of the silica play a part in retention and also render the surface liable to dissolution in water and dilute acids. For this reason the material is capped by treatment with either trimethylchlorsilane or hexamethyldisilazane to eliminate any unreacted or stearically... 

The Tg-based octa-anion TglO lg can be seen as a model for the well-known D4R found in inorganic structures such as zeolite A. Synthesis of TglO Jg can be achieved more readily than many other POSS species and it can be obtained in quantitative yield from the reaction of a tetra-alkoxysilane with H2O (10 equiv./Si) in the presence of Me4NOH (1 equiv./Si) in methanol at room temperature for 1 day (Figure Alcoholysis of T8[OSiMe2H]g causes terminal Si-O bond... 

As well as phenyl derivatives, other products have been prepared by hydrolysis of alkoxysilanes such as cyclohexenyl or naphthyl derivatives as well as heterocyclic compounds based on thienyl rings (Table 31, entries 28-33). Few practical applications have been reported for this type of compound, except for the styryl compound T81C6H4-A-CFI = CH2]s, and the fluorinated TslCeFsJs which have been used in polymer blending (Table 31, entries 13 and 28). ... 

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