Alkoxysilanes structure

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. 

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... 

Effect of Alkoxysilane on Structure and Properties of Rubber-Sihca... 

EFFECT OF ALKOXYSILANE ON STRUCTURE AND PROPERTIES OF RUBBER-SILICA HYBRID NANOCOMPOSITES... 

The combination of different analytical techniques offers the possibility for a complete characterization of building materials impregnated with liquid alkyl-alkoxysilanes RSi(OC2H5)3. The results derived from IR, NMR and SIMS spectra can be summarized in the schematic layer structure shown in Figure 5. 

The exact mass determination of emitted secondary ions yields informations about the molecular structure of the thick film. In the low mass area the fragmentation of the quasimolecular cation m/e = 163, i.e. (EtO)3Si+ derived from the alkoxysilane monomer gives rise to the positive ions m/e = 135, 119, 107, 91, 79, 63. The resulting fragmentation pattern can be described as follows ... 

In a typical sol-gel synthesis, the metal or the compounds of main group elements undergo hydrolysis and condensation reactions giving gel materials with extended three-dimensional structures. As shown in Equation 5.32 for silicon, addition of an acid or base catalyst to a solution of an alkoxysilane reagent such as tetrame-thoxysilane (TMOS), water and methanol leads to the hydrolysis of the Si-OMe bonds to form Si-OH functional groups. 

Poly(ethylene oxide). The synthesis and subsequent hydrolysis and condensation of alkoxysilane-terminated macromonomers have been studied (39,40). Using 29Si-nmr and size-exdusion chromatography (sec) the evolution of the silicate structures on the alkoxysilane-terminated polyethylene oxide) (PEO) macromonomers of controlled functionality was observed. Also, the effect of vitrification upon the network cross-link density of the developing inorganic—organic hybrid using percolation and mean-fidd theory was considered. 

Individual examples of monomeric silanols such as triethylsilanol [10] and phenylsilanetriol [II] have been prepared under regimens different from those used for surface treatments and they exhibit extended stability. To date, no monomeric silanetriols have been isolated from aqueous hydrolysates of alkoxysilanes. The kinetics of silanetriol condensation have been studied [12]. The conditions which promote the hydrolysis of alkoxysilanes also promote condensation of silanols the persistence of monomeric silanetriols for more than a few hours in typical solutions is unlikely. However, the persistence of silanols in reaction mixtures containing condensed structures have been observed empirically [ 13] and by JVSi-NMR [ 14,15],... 

Several studies using these tools have examined the effects of the silane ester structure on the hydrolysis rates for reactions catalyzed by hydronium ion, hydroxide anion, and general bases [35, 36, 38, 41-43]. In the following sections, we will discuss how the structure of the alkoxysilane impacted on the hydrolysis rate for each type of catalysis and, using these data, speculate about the possible reaction mechanisms. 

Spontaneous reaction rates—hydrolysis. Although the spontaneous hydrolysis of alkoxysilanes has been reported, there are insufficient data to develop an understanding of how structure impacts on rate or to give insights into the reaction mechanisms [35-37, 40]. The spontaneous hydrolysis usually contributes only a small amount to the observed rate of hydrolysis under most conditions. In many instances, the contribution of the spontaneous hydrolysis is so small that it is not detectable [25, 39, 42, 43]. The impact of the silane structure on the spontaneous hydrolysis of alkoxysilanes will therefore not be discussed. 

The hydrolysis of alkoxysilane esters has been extensively studied. The influences of the silane ester structure and acids, bases, solvent, and temperature on the reaction rates and kinetics have been investigated. Linear free energy relationships, such as Taft equations and Brensted plots, were used to gain... 

The relative rates of the consecutive hydrolysis reactions and the rate of condensation to form siloxane bonds depend on the structure of the silane molecule. Data such as those presented here provide the knowledge necessary to permit one to tailor the composition of species of alkoxysilane compounds in solution to obtain the desired rate and degree of hydrolysis of alkoxy groups, and the extent of siloxane bond formation. This is accomplished by the use of several easily observed bands in the mid-infrared spectrum. 

A relatively new area that involves silicon-containing materials is the synthesis of ultrastructure materials, that is materials in which structure can be controlled at the level of around 100 A. An example of such a synthesis is the sol-gel hydrolysis of alkoxysilanes (organosilicates) to give silica, Si02.1-14 The reaction is complicated, involving polymerization and branching, but a typical overall reaction may be written... 

---