Alkoxysilanes, polymerization

Alkali-developable silicon-containing positive photoresists, development, 175 Alkoxysilanes, polymerization,... 

Silica-based microparticles obtained by the sol-gel process also have limitations that must be known and, when possible, faced. One problem arises with the tendency to gel of the colloidal silica particles formed as a side product in the tetra-alkoxysilane polymerization reaction. In this case, the addition of a sequestering agent or the reduction in the amount of colloidal silicate particles in the suspension leads to aqueous suspensions of silica shell miaocapsules having improved storage stability. ... 

The successful entrapment of enzymes within alkoxysilane-polymerized sol-gel matrices has been perhaps the most nontrivial molecular doping of these materials. The expected pressure development upon polymerizations, the release of alcohols, and the low probability that the active site will remain open to the pore size all made it quite improbable that this entrapment will succeed. Following the report of Braun et al. in 1990 [39] that in fact the opposite is true, and not only that such entrapments keep the enzymes alive, but their stability is also... 

Copolymers of a cationic monomer and a vinyl alkoxysilane may be prepared by conventional vinyl polymerization techniques. These techniques include solution polymerization in water and emulsion polymerization with either free radical initiators or redox initiators. 

In agreement with the NMR results, the bulky alkyl-alkoxysilanes RSi(OC2H5>3 (R = i-C4H9 and n-CgH ) are not able to form polymeric or spherical siloxanes under the conditions used for this study. The observed TOF-SIMS spectra are similar to the El spectra of the pure liquids and, therefore, underline the presence of physically adsorbed species [5,9]. 

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... 

Why are strong bases such as NaOH not used to catalyze the hydrolysis of alkoxysilanes Why does polymerization occur in the pH range... 

A relatively new area that involves silicon-containing materials is the synthesis of ultrastructure materials, that is materials in which structure can be controlled at the level of around 100 A. An example of such a synthesis is the sol-gel hydrolysis of alkoxysilanes (organosilicates) to give silica, Si02.1-14 The reaction is complicated, involving polymerization and branching, but a typical overall reaction may be written... 

In the rest of the chapter we will consider separately stepwise and chainwise polymerizations. A small separate section will be devoted to the hydrolytic condensation of alkoxysilanes. This system exhibits such a large departure from the usual assumptions involved in the description of network formation that it merits particular consideration. 

A second, equally powerful means to prepare such materials relies on traditional inorganic polymerization tools, most notably sol-gel polymerization.24 25 A number of excellent reviews have appeared on this subject as well.5,12,17 In sol-gel processing, the functional monomer [i.e., an organoalkoxysilane such as 3-aminopropyltrimethox-ysilane (APTMS)] is combined with the cross-linking agent [i.e., a tetrafunctional alkoxysilane such as tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS)], a catalyst (such as hydrochloric acid or ammonia), and the template molecule. The resultant sol can be left to gel to form a monolith, which can then be dried, sieved, and extensively washed to remove the template. Alternatively, the sol can be spin coated, dip coated, or electrodeposited on a surface to yield a thin film, which can be subsequently washed with a solvent to remove the template and yield the imprinted cavities. 

The surface silanols can still catalyze the polymerization of triethoxysilane molecules even if there is one or more adsorbed layers of water on the surface. Presumably the triethoxysilane molecule H-bonds to the surface via water molecules after which it requires only an electron shift to make a silanol proton available to the alkoxysilane molecule. This sequence is illustrated by the following structures ... 

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

In most cases, di- and trifunctional reagents will polymerize in the presence of water forming linear (for difunctional) and branched alkoxysilanes (for trifunctional modihers). Trace amounts of water are always present on the surface of polar silica. 

---