Hydrolysis/condensation reaction

In the case of products prepared from hydrolysis/condensation reactions, the time of reaction varies from a few hours to more than 3 months. Generally, the crude products precipitate from the reaction mixture and pure compound can be obtained by simple filtration followed by washing or recrystallization. However, the yields are often low and rarely exceed 50%. 

Either particulate sol or polymeric sol has been used for thin film coatings. The polymeric sol was fabricated by partial hydrolysis of corresponding metal alkoxide. If the rate of hydrolysis or condensation is very fast, then some kinds of organic acids, beta-dicarbonyls, and alkanolamines have been used as chelating agent in sol-gel processes to control the extent and direction of the hydrolysis-condensation reaction by forming a strong complex with alkoxide. [2]. 

Other cyclic monomers have been prepared and polymerized through fast ROP. The main focus has been first on bisphenol A carbonate oligocyclic monomers (Brunelle et al., 1994). The oligocyclic monomers were prepared using an amine-catalyzed reaction of bisphenol A bischloroformate, via an interfacial hydrolysis/condensation reaction that also produces linear oligomers and polymers, depending on the structure and concentration of the tertiary amine (Aquino et al., 1994, Table 2.28). 

The approach of Qide et al. to produce thin films of TiQ2 via a sol-gel strategy uses a mixture of titanium isopropoxide, ethanol, water, and triethanolamine. The chemistry is similar to what is used in the silicate gel where a hydrolysis/condensation reaction is exploited (Wright and Sommerdijk, 2001) ... 

A comment needs to be made on the yields of reactions. Indeed, hydrolysis-condensation reactions can yield well-defined organotin oxo-clusters, but they can also yield ill-defined organotin oxo-polymers. 

Work by Elferink et al. [59b] has shown that further optimisation of the microstructure can be obtained by lowering the temperature of the hydrolysis-condensation reaction. This results in better control of porosity and pore size distribution. 

For the coating reactions with APS (Table I), a procedure similar to that described by Philipse and Vrij (4) was applied. Philipse and Vrij used the coupling agent 3-methacryloxypropyltrimethoxysilane to coat colloidal silica. Because an alcosol was used, the actual concentrations of water as listed in Table I have to be corrected for the (not exactly known) amount consumed in the hydrolysis-condensation reactions. After addition of the APS to 400 mL of alcosol, the solution was stirred slowly for an hour. After 2 h of refluxing, 250 mL was distilled slowly for an hour. The remaining unreacted APS was removed in four centrifugation redispersion steps. The ultracentrifuge used was from Beckmann (L5-50B), and the sediment was redispersed each time in absolute ethanol. 

Mesostructure Diversity Mesostructured polymorphs of sUica prepared under alkaline conditions have attracted the most scientific interest so far. However, ordered mesoporous silicas can also be prepared under acidic and neutral conditions. Several reasons make silica-based materials the most widely studied systems, namely, a large variety of possible mesostmctures with various pore network connectivities, narrow pore size distribution, a precise control over hydrolysis-condensation reactions due to lower reactivity of silicates, enhanced thermal stabUity and a vast variety of... 

The preparation of nanocomposite membranes by intra-membrane growth within a proton exchange membrane was first described by Mauritz et al. [45-47]. The then novelty of this approach and the breadth and depth of these studies warrant the following discussion of the results, which in many ways laid the foundation for future work in this area. This group made use of the hydrophilic ionic cluster regions of Nafion for confined, sulfonic acid group catalysed, hydrolysis/condensation reactions of impregnated alkoxides. Nafion membranes were first swollen in ethanol/water, then tetraethoxy-silane (or aluminium, titanium and zirconium alkoxides) permeated from one side of the membrane. In addition to the concentration profile of in-... 

TiO -PP nanocomposite was prepared in-situ with the assistance of corotating twin screw extmder [68], Composite was prepared by the injection of 30 wt% of titanium n-butoxide precursor to achieve 9.3 wt% of Ti02, after hydrolysis-condensation reaction. The titanium n-butoxide-PP mixture was treated in hot water at 80°C for 72 h. During this time period, the following hydrolysis and condensation reactions occur in precursor, which leads to the formation of in-situ TiO -PP nanocomposites. 

In a recent report on synthesis of silica [190], a reverse emulsion was prepared with isooctane as the continuous oil phase and an aqueous solution of HCl or HNO3 (catalyst) as the droplet phase. Tetraethyl orthosilicate was added to the emulsion it diffused from the oil phase to the droplets, underwent hydrolysis/ condensation reactions and formed silica gel particles. The reaction by-product ethanol decreased the stability of the emulsion. To avoid this, block copolymer surfactants Atlox and Hypermer were used (A-B-A type, where A = poly-12-hydroxy stearic acid and B = polyethylene oxide). 

A brief summary of the information available till around 1990 [211] shows the use of three reverse microemulsion systems for the synthesis of (amorphous) silica particles AOT /isooctane /water, AOT /benzyl alcohol/ decane /water and NP-5 /cyclohexane/ water. Note that the dispersed water phase had a dissolved base (NH4OH) or acid (HCl) as catalyst in it. Tetraethyl orthosilicate (TEOS) was added to the reverse microemulsions, leading to hydrolysis-condensation reaction and formation of silica particles. The size of the particles depended on the experimental conditions, but could go down to about 15 nm. 

Esquena etal. [246] used two model W/O microemulsions for the synthesis of silica through hydrolysis-condensation reactions of TEOS with NH3 as catalyst ... 

The acid catalyzed hydrolysis/condensation reactions yield chain-like polyzirconoxanes, when performed with appropriate acac/Zr and HsO/Zr ratios. The stabilizer (Y, Ce, Ca or Mg) is introduced either as a salt (chloride, acetate or nitrate), a complex such as Ce (3) 2, 4-pentane dionate, or an alkoxide (yttrium triisopropoxide, Y (0 Pr)3). Furthermore, various additives [92-93] are often used to control the dehydration and the porosity of the zirconia gel, to improve its ability to form, or to act as a plasticizing additive (e.g., poly(ethyleneglycol)). 

Roughly speaking, hydrolysis/condensation reactions are nucleophilic substitutions (olations, oxolations). Generally, the rates of hydrolysis and condensation are different, depending upon the pH of the medium. When the medium is acid, a polymer-like gel is obtained, whereas when the pH is basic, a colloidal gel is prepared. The precursor molecules can be either organic or inorganic ones. 

The discovery of the direct process of the synthesis of organochlorosi-lanes took these compounds from the realm of laboratory curiosity to commercially important materials. The chemistry of the polymerization methods for the assembly of polysiloxanes is discussed in the subsequent sections. Polysiloxanes are synthesized by two prineipal methods (a) ringopening polymerization of cyclosiloxanes and (b) condensation polymerization involving a hydrolysis/condensation reaction of diorganodichlorosi-lanes or condensation reaction between two difunctional diorganosilanes. 

FIG U RE 7.12 The results of NMR analysis for the hydrolysis/condensation reaction of MPS in methanol solution. (From J. S. Lee, S. Gomez-Salazar, and L. L. Tavlarides. Reactive and Functional Polymers 49 159-172, 2001. With permission.)... 

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