Polymerization silicon alkoxides

Poly(vinyl-acetate), the polymer chosen for this study, is functionally incapable of covalently bonding with the polymerizing silicon alkoxide, TEOS. Therefore, the Si02/PVAc composites may be considered class I hybrid materials. Reasons for interest in mixing PVAc with silica include its excellent optical transparency, a refractive index close to that of silica, and its solubility in ethanol-water mixtures. The solubility of PVAc in ethanol-water mixtures allows the polymer to be added directly to a typical alkoxide sol-gel formulation of alkoxide, alcohol, water, and catalyst, followed by tiie polymerization of the inorganic network around the organic polymer domains. 

The polymerization of silicon alkoxides, Si(OR)4, to produce inorganic glasses proceeds through these three reactions and is commonly referred to as the sol-gel process. Reactions of trialkoxysilane-terminated molecules R OR 2)3 are similar and give hybrid organic-inorganic networks,... 

Silica nanoparticles are commonly prepared by polymerization of appropriate precursors such as silicates, silicon alkoxides, or chlorides (Fig. 11.2).2 Besides the industrial methods, which rely mainly on condensation of sodium silicate in water induced by sodium removal through ion exchange, three different synthetic methods are currently used in research labs to prepare silica nanoparticles loaded with organic molecules. In the first method, proposed by Kolbe in 1956s and developed by Stober and coworkers in the late 1960s,6 the particles are formed via hydrolysis and... 

Small molecule imprinting in sol-gel matrices has received considerable interest in recent years, undoubtedly due to the flexibility offered by the sol-gel process.5 Two different approaches have been utilized covalent assembly and noncovalent self-assembly.9 In the covalent assembly approach, the polymerizable functional group (i.e., the silicon alkoxide group) is covalently attached to the imprint molecule. The functionalized imprint molecule is then mixed with appropriate monomers (i.e., TMOS) to form the imprinted materials. After polymerization, the covalent bonds are cleaved to release the template and leave the molecular recognition pocket. Figure 20.4 shows a diagram of this process. 

Another route for the production of materials involves the reaction of hydrolysis-condensation of metal alkoxides with water. We study here the important case of amorphous silica synthesis. In this case [38,39,44], silicic acid is first produced by the hydrolysis of a silicon alkoxide, formally a silicic acid ether. The silicic acids consequently formed can either undergo self-condensation, or condensation with the alkoxide. The global reaction continues as a condensation polymerization to form high molecular weight polysilicates. These polysilicates then connect together to form a network, whose pores are filled with solvent molecules, that is, a gel is formed [45],... 

Silicon alkoxides are rapidly hydrolyzed by water, eventually to hydrous silica, but polymeric hydroxo alkoxo intermediates occur. Organo alkoxides such as the silyl ester (MeO)3SiMe and its derivatives are widely known.72... 

Figure 4 Polymer-silica hybrid materials hydrogen bonding between silanol groups of hydrolyzed silicon alkoxides and carboxylate groups of preformed poly(A, A-dimethylacrylamide) (a) or in situ polymerized A, A-dimethylacrylamide/methylene bisacrylamide precursors (b) favors the formation of homogeneous organic-inorganic networks...

In the acidic route (with pH < 2), both kinetic and thermodynamic controlling factors need to be considered. First, the acid catalysis speeds up the hydrolysis of silicon alkoxides. Second, the silica species in solution are positively charged as =SiOH2 (denoted as I+). Finally, the siloxane bond condensation rate is kinetically promoted near the micelle surface. The surfactant (S+)-silica interaction in S+X 11 is mediated by the counterion X-. The micelle-counterion interaction is in thermodynamic equilibrium. Thus the factors involved in determining the total rate of nanostructure formation are the counterion adsorption equilibrium of X on the micellar surface, surface-enhanced concentration of I+, and proton-catalysed silica condensation near the micellar surface. From consideration of the surfactant, the surfactants first form micelles as a combination of the S+X assemblies, which then form a liquid crystal with molecular silicate species, and finally the mesoporous material is formed through inorganic polymerization and condensation of the silicate species. In the S+X I+ model, the surfactant-to-counteranion... 

The general theory of nucleation and polymerization in aqueous systems, in which silica shows some solubility, is discussed in detail in Iler s book (3). However, very little was known at the time the book was published (1979) about the polymerization of silica when Si(OH)4 is formed in nonaqueous systems. Progress made up to 1990 in the understanding of the hydrolysis and condensation of silicon alkoxides that leads to silica gels or to silica sols of large particle diameter are lucidly discussed by Brinker and Scherer (8). Brinker s chapter in this book (Chapter 18) includes a clear explanation of the difference between hydrolysis and condensation of aqueous silicates and silicon alkoxides. 

In the evolution of solids from solution, a wide spectrum of structures can be formed. In Fig. 4, a simple schematic representation of the structural boundary condition for gel formation is presented. At one extreme of the conditions, linear or nearly linear polymeric networks are formed. For these systems, the functionality of polymerization /, is nearly 2. This means there is little branching or cross-linking. The degree of cross-linking is nearly 0. In silica, gels of this type can be readily formed by catalysis with HCl or HNO3 under conditions of low water content (less than 4 mol water to 1 mol silicon alkoxide). The ideal fractal dimension for such a linear chain structure is 1. The phe-... 

For a nonmolecular sihcon source, silica is obtained as a gel formed from a non-homogeneous solution and subsequently treated hydrothermally. Silica can be prepared, for instance, by acidification of a basic aqueous solution, and when the reaction conditions are properly adjusted, a porous silica gel is obtained. Most frequently, two types of chemical reactions are involved silicate neutralization producing silicic acids, followed by condensation polymerization of the silicic acid species. In the case of molecular silicon sources, solvent and catalyst are usually first combined to form a homogeneous solution to which a silicon alkoxide is then added. In both... 

Sanchez J. 1994. Kinetics and Models of Silicon Alkoxide Polymerization. [AVM] Cairncross R.A. 1994. Solidification Phenomena During Drying of Sol-to-Gel Coatings. [LEE, LES]... 

In the case of silicon alkoxides, hydrolysis occurs by the nucleophilic attack of the oxygen contained in water on the silicon atom and is most rapid and complete when acid (e.g. HCl, CH3COOH, HF,. ..) or basic (e.g. NH3, KOH, amines,. ..) catalysts are employed. Under acidic conditions, it is likely that an alkoxide group is protonated in a rapid first step. Electron density is withdrawn from silicon, making it more electrophilic and thus more susceptible to be attacked by water. Under basic conditions it is likely that water dissociates to produce nucleophilic hydroxyl anions in a rapid first step. The hydroxyl anion then attacks the silicon atom. Polymerization... 

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