Sol-gel condensation reactions

Sol-gel chemistry and processing was a facile and efficient methodology for the preparation of branched, star-shaped homopolymers. The hydrolysis and condensation reactions are efficiently catalyzed with either simple acids or bases, and it was possible to control the nature of the silicate-like hub by judicious selection of the catalyst. The sol/gel condensation reactions to form the branched pol)miers occurred readily in the solid state above the glass transition temperature. The sol-gel process generates a silicate-like hub that contains... 

Since sol-gel polycondensation and organic polymerization usually take place under different reaction conditions, the sequence of the reactions (organic polymerization prior to sol-gel condensation or vice versa) can be chosen by reaction conditions. 

Since sol-gel condensation and ring opening reaction take place under... 

Depending on the size of the ligand, the inner cavity of these structures had a diameter of 2-4 nm, along with 96 inward-facing hydroxyl groups. Addition of tetramethoxysilane resulted in sol-gel condensation exclusively inside the host, thus producing silica nanoparticles of defined size and shape—the size and shape of the host s inner cavity. Despite the amorphous nature of silica nanoparticles, the polydispersities observed approached unity. No side reactions, such as condensation outside the spheres or... 

Sol-gel condensation polymerizations have also been carried out using either photogenerated acids or bases as catalysts. For example, Hanson and Jensen employed a (diphenylmethyl)trimethylammonium salt as a photolatent source of trimethylamine to conduct the sol-gel condensation of tetraethoxysilane (Scheme 7). Postirradiation heating at 65 °C was necessary to drive the reaction to completion. Crivello and Mao and Croutxe-Baghorn have reported similar sol-gel condensation polymerizations of alkoxysi-lanes conducted using diaryliodonium salts as photoacid generators. 

In this case, there is no distinct sol formation process but rather simultaneous hydrolysis and condensation reactions proceed to form a gel. 

The first step in sol-gel processing is the catalytic hydrolysis of TEOS and the second step is the polycondensation of SiOH moieties framing into silica (Scheme 3.1). In the first step of the reaction, water is present as a reactant while it is the by-product in the second step. It is likely that the molar ratio of TEOS/H2O would influence the sol-gel chemistry and hence the end properties of the resultant hybrids. The most interesting part of the sol-gel chemistry is that the catalytic hydrolysis of TEOS is an ion-controlled reaction, while polymerization of silica is not. Usually, the ionic reactions are much faster than the condensation reactions. The stoichiometric equation showing the silica formation from TEOS is presented in Scheme 3.3. 

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]. 

The fabrication of colloidal silica and optical glasses by the sol-gel process has attracted a great deal of attention (8). The process relies on the hydrolytic polycondensation reactions of alkoxysilanes, usually (EtO)4Si, in which the reactive silanols (EtO)4 Si(OH)n (n = 1-4) are formed. These then undergo acid- or base-catalyzed condensation with both water and alcohol formation, as shown in Scheme 2. 

Many of the structural features of sol-gel-derived inorganic polymers are rationalized on the basis of the stability of the M-O-M condensation products in their synthesis environments. Structures which emerge in solution reflect a successive series of hydrolysis, condensation and, depending on the acid or base concentration, restructuring reactions. M-O-M bonds which are unstable with respect to hydrolysis or alcoholysis are generally absent. During... 

The structures of sol-gel-derived inorganic polymers evolve continually as products of successive hydrolysis, condensation and restructuring (reverse of Equations 1-3) reactions. Therefore, to understand structural evolution in detail, we must understand the physical and chemical mechanisms which control the sequence and pattern of these reactions during gelation, drying, and consolidation. Although it is known that gel structure is affected by many factors including catalytic conditions, solvent composition and water to alkoxide ratio (13-141, we will show that many of the observed trends can be explained on the basis of the stability of the M-O-M condensation product in its synthesis environment. 

The basic sol-gel reaction can be viewed as a two-step network-forming polymerization process. Initially a metal alkoxide (usually TEOS, Si(OCIl2CH )4) is hydrolyzed generating ethanol and several metal hydroxide species depending on the reaction conditions. These metal hydroxides then undergo a step-wise polycondensation forming a three-dimensional network in the process. The implication here is that the two reactions, hydrolysis and condensation, occur in succession this is not necessarily true (8.9). Depending on the type of catalyst and the experimental conditions used, these reactions typically occur simultaneously and in fact may show some reversibility. 

The sol-gel route for silica fabrication is based on the strong tendency of silicic acid in solution to take part in condensation reactions [8,18,58]. It can be presented in the general form as ... 

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