Sol-gel polymerization

Fig. 2. The sol—gel polymerization of resorciaol with formaldehyde (a) and (b) of melamine with formaldehyde. Reproduced from Refs. 13 and 14,...

Klemperer, W. G. Ramamurthi, S. D. 1988. Molecular pathways in silica sol-gel polymerization. In Better Ceramics Through Chemistry III, edited by Brinker,... 

Figure 9.4 Reaction conditions exert a strong influence on the course of a sol-gel polymerization reaction. Basic pH, higher temperatures, and greater dilutions favor the formation of rings and ring clusters, as shown in the pathway on the left. Acidic pH, lower temperatures, and higher concentrations favor the formation of chains and dendritic structures.

The confinement of a relatively large number of dye molecules in the small volume of a nanoparticle may trigger collective phenomena otherwise not observable in bulk solution. This has been demonstrated by Prasad and coworkers in the case of an ORMOSIL pH sensor.69 The PEBBLEs contain a naphthalenylvinylpyridine derivative (NVP) as pH-sensitive fluorescent dye which has been functionalized with a triethoxysilane anchor by reaction with an excess of (3-isocyanatopropyl)triethoxysi-lane (ICTES). The sol-gel polymerization in aqueous micellar solution of the NVP-ICTES derivative with VTES gives spherically shaped 33 nm silica nanoparticles in which the dye is covalently linked to the silica matrix and uniformly distributed in the nanoparticle volume. The NVP dye responds ratiometrically to protons, with a... 

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. 

Greater flexibility in terms of the processing conditions. Because sol-gel polymerization involves mild reaction conditions, biological molecules, water soluble molecules, and thermally sensitive molecules can be utilized as templates. Such molecules are often difficult to use as templates when traditional free radical polymerization in nonaqueous solvents is employed. 

In this context the supramolecular crystalline [15-22] or hybrid materials [23-35] can be prepared and constitutionally self-sorted by using an irreversible kinetic process like crystallization or sol-gel polymerization. The self-selection is based on constitutional internal interactions of library components, resulting in the dynamic amplification of self-optimized architectures, during the phase change process. With all this in mind, the second part will be devoted to sol-gel resolution of dynamic molecular supramolecular libraries, emphasizing recent developments, especially as pursued in our laboratory. 

The dynamic self-assembly processes of such supramolecular systems undergoing continous reversible exchange between different self-organized entities in solution may in principle be connected to kinetically controled sol-gel process in order to extract and select an amplified supramolecular device under a specific set of experimental conditions. Such dynamic marriage between supramolecular self-assembly and in sol-gel polymerization processes which synergistically might communicate leads to constitutionnal hybrid materials. ... 

Mauritz, K.A., Organic-inorganic hybrid materials perfluorinated ionomers as sol-gel polymerization templates for inorganic alkoxides. Mater. Sci. Eng. C, 6, 121, 1998. 

It is prepared under conditions of controlled hydrolysis, by a sol-gel polymerization of an alkoxysilicate (e.g. tetraethoxysilane) in the form of an emulsion, under the effect of base-catalysed hydrolysis. Initially, tiny particles are formed (0.2 pm) which grow in a regular manner, by various methods, to form spheres that attain a few micrometres in diameter. 

Scheme 3A-E. Mechanism of inorganic coating of organogelators. Schematic representation for the creation of paper-like roU silica by sol-gel polymerization of TEOS in the organogel state of 4 A mixtiue of gelator and TEOS B gelation C sol-gel polymerization of TEOS and adsorption onto the cationic gelator D before calcination E paper-like roll multilayer structure of the sihca formed after calcination. Reprinted with permission from [49]. Copyright 2000 American Chemical Society...

Cn-doped monolithic organic aerogels were prepared by sol-gel polymerization of a resorcinol derivative containing an ion-exchange moiety with binding sites for metal ions with the aim of determining their structure [23,24] and electrochemical [26] and transport properties [25]. Thus, the potassium salt of... 

---