Polycondensation alkoxysilanes

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. 

An alternative strategy to obtain silica immobilised catalysts, pioneered by Panster [23], is via the polycondensation or co-condensation of ligand functionalised alkoxysilanes. This co-condensation, later also referred to as the sol-gel process [24], appeared to be a very mild technique to immobilise catalysts and is also used for enzyme immobilisation. Several novel functional polymeric materials have been reported that enable transition metal complexation. 3-Chloropropyltrialkoxysilanes were converted into functionalised propyltrialkoxysilanes such as diphenylphosphine propyltrialkoxysilane. These compounds can be used to prepare surface modified inorganic materials. Two different routes towards these functional polymers can be envisioned (Figure 3.4). One can first prepare the metal complex and then proceed with the co-condensation reaction (route I), or one can prepare the metal complex after the... 

The preferentially employed approach for the fabrication of inorganic (silica) monolithic materials is acid-catalyzed sol-gel process, which comprises hydrolysis of alkoxysilanes as well as silanol condensation under release of alcohol or water [84-86], whereas the most commonly used alkoxy-silane precursors are TMOS and tetraethoxysilane (TEOS). Beside these classical silanes, mixtures of polyethoxysiloxane, methyltriethoxysilane, aminopropyltriehtoxysilane, A-octyltriethoxysilane with TMOS and TEOS have been employed for monolith fabrication in various ratios [87]. Comparable to free radical polymerization of vinyl compounds (see Section 1.2.1.5), polycondensation reactions of silanes are exothermic, and the growing polymer species becomes insoluble and precipitates... 

Fig. 11 Preparation of alkoxysilanes functionalized with protected sugars for using in polycondensation reactions...

Precrosslinked poly(organosiloxane) particles are synthesized (Scheme 2) by emulsion polycondensation/polymerization of (functionalized) alkoxysilanes and (optionally) cyclic siloxanes [6, 7]. 

The hydrolysis of (EtO)4Si (and the snbseqnent polycondensation of Si OH containing molecnles) has been the snbject of considerable investigation because of its importance in the fabrication of glasses and colloidal silica via the sol gel process (see Sol-Gel Synthesis of Solids). Hydrolysis of the Si-0 bond in alkoxysilanes is also very widely used to attach silicon compounds to surfaces and in coupling reactions. 

At present it seems that immobilization of silica-based particles within a packed capillary by hydrothermal treatment or sol-gel adhesion represent a simpler approach to the preparation of silica-based monoliths for capillary electrochromatography [302,332-334]. Particle fixation is achieved through adhesion by silica precipitated in the interparticle space released from the particles by hydrothermal treatment, or formed by hydrolysis and polycondensation of a solution of alkoxysilanes (sol-gel process). Since only relatively low temperatures are used in both processes, chemically bonded phases can be immobilized as easily as silica. The selectivity and separation efficiency of immobilized particle beds is generally similar to that of slurry packed columns prepared from the same stationary phases. 

Chojnowski, J. Rubinzstajn, S. Fortuniak, W. Kurjata, J., Synthesis of Highly Branched Alkoxysilane-Dimethylsiloxane Copol)miers by Nonhydrolytic Dehydrocarbon Polycondensation Catalyzed by Tris(pentafluorophenyl)borane. 

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 ... 

Alkyl alkoxysilanes undergo quick hydrolysis and polycondensation in acidic medium. This has been exploited in combination with self-assembly of long-chain alkyl trialkoxysilanes driven by hydrophobic interactions that opened prospects for surfectant- and template-free synthesis of ordered mesoporous silicas. This approach has been developed strongly by Shimojima and Kuroda [24]. 

Inorganic materials, particularly those based on silica, are also very useful for enzyme entrapment. Silica is biocompatible and has excellent mechanical properties. Some other important parameters such as surface area and pore size can be modulated [36,37]. The sol-gel process involves hydrolyzing silane precursors (tetra-alkoxysilanes and alkoxysilane) followed by their polycondensation to form ceramic materials [38]. Conventional sol-gel matrices have tight pores of approximately 1.5 nm diameter [28,39]. These prevent diffusion of the enzyme out of the matrix and can also stabilize enzyme structures, thereby improving thermal and chemical stabilities and increasing storage abilities [40]. Fim et al. immobilized alcalase from Bacillus licheniformis... 

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