Alkoxysilanes basicity

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). 

Sometimes, the removal can be turned to good advantage synthetically. Thus, phenoxysilanes can be expected to be cleaved more readily than alkoxysilanes because - OPh is more basic than - OR, hence a better leaving group. 

Alkoxysilane hydrolysis—effects of substituents. A series of hydrolysis studies [4-9] have elucidated general trends. Under basic conditions, the hydrolysis of alkoxy groups usually takes place in a stepwise manner. Carbon-bonded substituents can have profound effects on the rate of hydrolysis. With the... 

Most commercial alkoxysilanes (except aminoalkylsilanes) have limited water solubility until the alkoxysilane groups are converted to hydrophilic silanol groups by hydrolysis. Acids or bases may be used to catalyze hydrolysis. The slowest rate is at approximately neutral pH (pH 7). Each change of pH by 1 pH unit in either the acid or the basic direction produces a ten-fold acceleration in hydrolysis rate, assuming an excess of available water. Thus, at pH 4, the hydrolysis is about 1000 times faster than at pH 7. The anion of any acid (e.g. acetate) may also further accelerate the hydrolysis. 

Hydrolysis of alkoxysilanes is catalyzed by acidic and basic catalysts. Base accelerates the reaction much more efficiently than acids do. But since it proceeds by a nucleophilic attack of the hydroxide ion on silicon, the reaction rate is strongly... 

The methods used to increase the water resistance of a glass microsphere foam are basically those applied to glass-reinforced plastics, filled thermoplasts, and elastomers, viz. hydrophobic adhesion compounds are added to binder and microsphere dressing 147). The compounds added are alkyl alkoxysilane derivatives, amino or epoxy alkoxysilanes for epoxy and phenolic resins, vinyl or methacryloxy alkoxysilanes for polyester resins. The dressing agents used are aminoethoxysilanes (y-aminopropyl-... 

Oxidative addition of the silane to the metal is fast and reversible 30 therefore unless the pentacoordinated silane drastically slows down the oxidative addition process, pentacoordination will not alter the rate of the reaction at this stage of the cycle. The increased reactivity of le may be explained by the attack of the alcohol on the pentacoordinated silane that would form after oxidative addition (Figure 9A). The rate of the alcohol addition is increased by the higher reactivity of the pentacoordinated silicon center. This may explain the slower reactivity for those alkoxysilanes that cannot form this intramolecular coordination complex due to the absence of a nearby Lewis basic atom. We had observed during the comparison of aliphatic alcohol to benzyl alcohol that the nucleophilicity of the alcohols has an effect on the rate of the reaction. This is evidence that the alcohol and the silane are involved in the rate-determining step with 10 % Pd/C catalytic system. 

Our idea that intramolecular pentacoordination may accelerate the reaction was tested by using an alcohol with a neighboring ether group to act as a Lewis base (Figure 9B). Ethers are known to be more basic then the carbonyl of an ester.31 The mono-alkoxysilane from... 

L. S. Whatley, and K. J. Lake Hydrogen Bonding Studies. V. The Relative Basicities of Ethers, Alkoxysilanes and Siloxanes and the Nature of the Silicon-Oxygen Bond. J. Amer. chem. Soc. 83, 761—764 (1961). 

In our studies, the nano-sized sihca was synthesized using cheap sodium orthosilicate and sodiirm metasihcate rather than expensive alkoxysilanes. An inverse microemulsion containing NP-5/NP-9 as surfactant and cylcohexane or petroleum ether as the oil was used to carry out the hydrolysis and condensation of sodium orthosilicate or metasilicate using an acidic medium [152, 153]. The spherical silica particles of size 10-20 nm were obtained in a system using cyclohexane as the oil and with sodium orthosihcate of 0.01-0.1 M [152]. The particle size increased as the concentration of sodium orthosihcate and the pH were increased. Sihca particles prepared in basic conditions were more uniform in size than those prepared in an acidic medium. But calcined sihca powders with larger specific areas (350-400 mVg) were obtained for those prepared in an acidic medium. 

Within the scope of systematic investigations on basic reactions in industrial silicone syntheses in the system - alkoxysilane, silanol, siloxane, alcohol, water, catalyst, solvent - after previous investigations on the equilibrium of the silanol alkoxysilane heterocondensation and the reverse siloxane cleavage with alcohol (Eq. 1) [1]... 

This behavior can be explained in terms of different oxide surface acidities. Whereas silica (pH 4) exhibits acidic properties, alumina surfaces shows a much more basic character (pH 8) [5] and, therefore, allows further hydrolysis of the alkoxysilanes. The influence of the surface pH on the characteristics of anchored molecular structures has also been reported by Deo and Wachs [6] for the preparation of supported vanadia catalysts via incipient wetness. 

Instead of the chlorosilanes, other reactive silanes can be used as well. Alkoxysilanes are less reactive than chlorosilanes, but will form bonded phases in the presence of acidic or basic catalysts. Silazanes, on the other hand, are as reactive or more reactive than chlorosilanes. A well-known example is the reaction of the silica surface with hexamethyldisilazane ... 

Adsorption of alkoxysilanes onto cellulose fibers also holds promise for modification of the cellulose surface. The basic formula of the silane-coupling agents used has an organofunctional group on one side of the chain and an alkoxy group on the other. Abdelmouleh studied the adsorption of several prehydrolyzed alkox-... 

Both the heat-cured and RTV silicones are formed by condensation polymerization and are sometimes referred to as alkoxy- or acetoxy-cured systems. The basic hydroxysilane starting materials are produced by the hydrolysis of mixtures of mono-, di-, and trichlorosilanes as depicted in Figure 3.7. " Condensation with elimination of water produces hydroxyl-terminated alkylpolysiloxanes. These resins can be further polymerized in the presence of alkoxysilanes such as propylorthosilicates to yield the RTV silicones (Figure 3.8). Catalysts such as organotitanates or dibutyltindilaurate may accelerate the curing process. 

A mixture of N-p-tolyldiphenylacetamide and hexamethylcyclotrisilazane heated 4 hrs. at 240 diphenylacetonitrile. Y 73%. - The dioice of silazanes, chloro-silanes, or alkoxysilanes allows the reaction to be run under mildly basic, acidic, or neutral conditions. If the former reagents are used, the reaction may be followed by NH3- or HCl-liberation respectively. The products can be easily isolated by distillation if volatile or by filtration if solid. F. e., also from unsubst. amides and aldoximes, and f. silicon compds. as reagents, s. W. E. Dennis, J. Org. Chem. 35, 3253 (1970) nitriles from unsubst. amides with hexamethylphosphor-amide s. R. S. Monson and D. N. Priest, Can. J. Chem. 49, 2897 (1971). 

Water repellence of fabrics made of p-aramide, which were coated with a basic sol (pre-hydrolyzed alkoxysilane, crosslinking agent, ethanol) modified with an additional hydrophobizing agent... 

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