Silicon alkoxides, polycondensation

The hydrolytic polycondensation of silicon alkoxides of general formula Si(OR)4 or R/ Si(OR)4 , where the non-reactive organofunc-tional group R acts as a network modifier, is carried out in the presence of dopant molecules resulting in the formation of highly porous, reactive organosilicates whose applications span many traditional domains of chemistry. 

Silica-based materials obtained by the sol-gel process are perhaps the most promising class of functional materials capable to meet such a grand objective. In the sol-gel process liquid precursors such as silicon alkoxides are mixed and transformed into silica via hydrolytic polycondensation at room temperature. Called soft chemitry or chimie douce, this approach to the synthesis of glasses at room temperature and pressure and in biocompatible conditions (water, neutral pH) has been pioneered by Livage and Rouxel in the 1970s, and further developed by Sanchez, Avnir, Brinker and Ozin. 

Generally, two common methods, the Stober method and the reverse microemulsion method are used for synthesis of silica nanoparticles. As derivatives of a sol-gel process, both methods involve hydrolysis of a silicon alkoxide precursor to form a hydroxysilicate followed by polycondensation of the hydroxysilicate to form a silica nanoparticle [44]. 

The amorphous silica matrixes are porous network structures that allow other species to penetrate [44]. Thus, the doped dye molecules have the ability to react with targets. However, the reaction kinetics is significantly different than the molecules in a bulk solution. In the synthesis of DDSNs, commonly used silicon alkoxides including TEOS and TMOS have tetrahedron structures, which allow compact polycondensation. As a result, the developed silica nanomatrix can be very dense. The small pore sizes provide limited and narrow pathways for other species to diffuse into the silica matrix. 

Silica gels may also be produced by the hydrolysis and polycondensation of silicon alkoxides, eg, tetraethylorthosilicate, Si(OC2H5)4. This is often... 

FIGURE 2. Glassy silica needles (2 mm x 30 gm) produced in copious amount by a marine sponge. Each needle contains an occluded axial filament comprised of silicateins, enzyme-like proteins that catalyze and spatially direct the polycondensation of silicon alkoxides and silicic acid at neutral pjj41,42 Reprinted from Reference 3, copyright (1999), with permission from Elsevier Science... 

Making an aerogel involves base-catalyzed hydrolysis and polycondensation reactions of silicon alkoxides in an alcohol followed by 24 hours of aging in the alcohol solvent to produce an alcogel. Then the alcohol is removed from the pores of the alcogel... 

This problem can be overcome by controlling the conditions for hydrolysis and polycondensation, for example by (1) pre-hydrolysis of the silicon alkoxide, (2) avoidance of an excess of water, (3) use of a catalyst that accelerates the slowest reaction, i.e., that of the silicon alkoxide or (4) chelating the aluminum alkoxide to slow down the fastest reaction. 

As recently reviewed by Hench and West (2), an entirely different technique developed since 1846, in which silicon alkoxides such as Si(OCH3)4 and Si(OC2H5)4 [with the colloquial acronyms TMOS and TEOS] were hydrolyzed and subsequently underwent polycondensation under controlled conditions (losing alcohol vapor) to form quite clearly transparent, moderately viscous to almost vitreous materials. Such a glass has close relations to the limpid silica glass obtained today by melting quartz... 

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]. 

Weakly basic membranes can be obtained if one of the R s is hydrogen (19) atom transfer radical polymerization (ATRP) using BPPO the product of (20) can further conduct hydrolysis and polycondensation processes due to silicone alkoxide. 

Alkoxide Method. In this paper alkoxide method means that silicon alkoxides such as Si(OCH3)4 and Si(OC2H5)4 are employed as source materials. These silicon alkoxides dissolved in water and alcohol solutions are subjected to hydrolysis and polycondensation, in order to make bulk gels. Drying is carried out under atmospheric pressure. When the wet gel is dried under supercritical pressures and temperatures, the method is called aerogel method here, even if the silicon alkoxides are employed as source materials. 

In Class-II materials components are chemically linked by strong covalent or iono-covalent bonds. The molecules used as starting building blocks possess two distinct functionalities alkoxy groups (R-O-M bonds) and metal-carbon (M-C) links. The alkoxy groups can be formed into an oxo-polymer network by hydrolysis-polycondensation reactions in a sol-gel. Hybrids can be obtained from organically modified silicon alkoxides such as polyfunctional or polymer functionalised alkoxysilanes. The network-forming functionalities can be covalently connected in a sol-gel in several ways ... 

Layered silicates belonging to the smectite clays family can be delaminated or exfoliated in the formation of the well-known polymer-clay nanocomposites initially developed by Fukushima and Inagaki at the Toyota Central Laboratories [54]. In a similar way, Letaief and Ruiz-Hitzky [55] using silicon alkoxides able to promote sol-gel processes have successfully achieved the hydrolysis and polycondensation of Si(OR)4 compounds within the interlayer space of smectite clays giving rise to a silica network with the concomitant delamination of the silicate layers [8,55-57]... 

The Stober process [26] uses ammonium hydroxide as the catalyst to increase the pH above the isoelectric point of soluble silica (silicic acid) [113]. The pH causes repulsion between newly formed silica particles and terminates polycondensation [113]. Therefore, after primary particles form due to hydrolysis (Figure 43.4), some condensation occurs to form spherical secondary particles, but bonds do not form between the particles, so secondary particles remain as particles (Figure 43.2b). The final size of the spherical silica powder can be controlled by pH, type of silicon alkoxide, and reaction temperature. If the particles are below a certain size ( 200nm), they enter cells [25,114]. Once inside the cells, they do not inherently cause the cell to change behavior, and they can be used to carry therapeutic agents, for example, small drug molecules. 

The formation of a sol-gel porous material is through a hydrolysis-polycondensation reaction. An example is given in equation 1 with the methoxide of silicon (tetramethyl-orthosilicate, TMOS), but many other alkoxides, aryl oxides and acyl oxides can be used, as well as Si—N and Si—Cl compounds. 

The sol-gel process for the production of silica is an industrially widely apphed procedure. The process is usually conducted in homogeneous solution, and it is possible to manufacture nano- or micro-sized particles, clear macroscopic silica objects (monoliths), such as fibres or lenses [17]. Sol-gel processing is commonly conducted at quiescent temperature, and no increased pressure is necessary, which makes this method suitable for LLC templating. The starting material for the sol-gel preparation of silica is an orthoester (alkoxide) of the general structure Si(OR)4, which is hydrolysed in order to formally yield silicic acid. The latter undergoes polycondensation into a three-dimensional network of silicon... 

Since a discussion of the properties and structures of metal alkoxides is essential for an understanding of the chemical processes during hydrolytic polycondensation, a short introduction to the chemistry of metal alkoxides is given first. This overview is restricted to the alkoxides of the network forming elements relevant to sol-gel processing (i.e., alkali and alkaline earth alkoxides, organometallic alkoxides, or alkoxides with metal-metal multiple bonds are not treated) excluding silicon. Three comprehensive books on metal alkoxides have appeared. ... 

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