Condensation polymerization silicon alkoxides

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 our laboratory, we have recently conducted gelation studies of silica nanopardcles in microgravity during the STS-95 space shuttle mission (28). Stable silica nanoparticle dispersions may be form either by polymerization of silicic acids in an aqueous system or through hydrolysis and condensation of silicon alkoxides (the sol-gel or Stober route). Comparison of small-angle x-ray scattering (SAXS) measurements of Ludox, a commercial aqueous silicate with acid- and base-catalyzed alkoxides shows that only aqueous silicate sols are uniform, whereas alkoxides generate fractal particles. As Brinker and Scherer point out (29), these results illustrate that sols derived from aqueous silicates are... 

Currently, microelectronics relies on bulk sihca as an important dielectric material that is often used as an insulating template for further reactivity. On the nanoscale, sUica can be synthesized by polymerizing silicic acid in an aqueous system, or through hydrolysis and condensation of silicon alkoxides in the Stober synthesis [51]. The mechanism of these two methods is unique. The first method is dominated by monomers and tetra-functionalized species, such that the resultant sihcate sols are uniform, which means that they are fully hydrolyzed and grow by monomer addition. In contrast, for the second method, di- and tri-functionalized species are dominant for alkoxides. Regardless of the synthesis used, these particles induce a fractal interior with minimal morphological control due to their common template, ammonium hydroxide [51]. 

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... 

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],... 

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... 

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... 

However, in the polymerization of silicon alkoxides discussed in the next section. Drinker and Scherer (1) suggest that the condensation process more likely proceeds via an associative = SiOHR(OH2) intermediate. 

Euclidean objects (dense spherical particles) are most likely to form in systems (e.g., aqueous silicates) in which the particle is slightly soluble in the solvent. In this case, monomers can dissolve and reprecipitate until the equilibrium structure (having a minimum surface area) is obtained. In nonaqueous systems (e.g., silicon alkoxide-alcohol-water solutions), the solubility of the solid phase is so limited that condensation reactions are virtually irreversible. Bonds form at random and cannot convert to the equilibrium configuration, thereby leading to fractal polymeric clusters. 

Sol-gel chemistry is frequently employed in designing random mesoporous structures of silicates (Brinker and Scherer 1990 Corma 1997). Liquid silicon alkoxide precursors (Si(OR) ) are hydrolyzed and condensed to form siloxane bridges, a process that is often described as inorganic polymerization and is represented below ... 

Silica sol is prepared by hydrolysis of a metal alkoxide and subsequent or simultaneous polycondensation (polymerization), which can be either acid or base catalyzed. The acid catalyzed hydrolysis proceeds through an electrophilic attack of the H+ ions. This means that the reactivity decreases as the number of OR groups decreases with the progression of hydrolysis [40-42]. The probability of formation of fully hydrolyzed silicon, Si(OH)4, is thus very small. Since the condensation reaction starts before the silicon alkoxide is completely hydrolyzed, silicon alkoxide molecules will polymerize with non-hydrolyzed alkoxyl groups, in which the degree of crosslinking is low. The gyration radius of these small molecules is typically in the order of 1.5 - 1.7 nm [43]. 

Initially, two different mechanisms had been proposed for silicatein reaction that might describe the condensation reaction of orthosihcic acid or silicon alkoxide (TEOS) substrates (Cha et al., 1999 Fairhead et al., 2008). These models explain the hydrolytic activity of silicatein toward its synthetic substrate, TEOS, or the initial formation of disiUcic acid from orthosilicic acid as the natural substrate, but none of them is able to desoibe the biocatalytic formation of the polymeric silica from its monomeric precursor(s). 

Actually, as discussed in depth in the preceding chapter, hydrolysis and condensation are concurrent. The purpose of writing out these schematic reactions is to point out that the stoichiometric amount of water for hydrolysis is 4 moles for every mole of silicon alkoxide, or 2 moles if condensation goes to completion. However, in the preparation of particles, the ratio of water to TEOS is typically more than 20/1 and the pH is very high, and both of these factors promote condensation. This encourages the formation of compact structures, rather than extended polymeric networks of the kind generally found in alkoxide-derived gels. 

Titanium alkoxides are used for the hardening and cross-linking of epoxy, silicon, urea, melamine, and terephthalate resins in the manufacture of noncorrodable, high temperature lacquers in the sol-gel process as water repellents and adhesive agents (especially with foils) to improve glass surfaces as catalyst in olefin polymerization, and for condensation and esterification. 

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