Inorganic silica polymers polymerization

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... 

For select polymers that are soluble, the method is very simple. The polymerization of the silica precursor occurs around the preformed polymer chains or domains. When the organic polymer is formed before the inorganic polymer, the result is a sequential organic/inorganic interpenetrating polymer network. 

Historically, polysiloxane elastomers have been reinforced with micron scale particles such as amorphous inorganic silica to form polysiloxane microcomposites. However, with the continued growth of new fields such as soft nanolithography, flexible polymer electronics and biomedical implant technology, there is an ever increasing demand for polysiloxane materials with better defined, improved and novel physical, chemical and mechanical properties. In line with these trends, researchers have turned towards the development of polysiloxane nanocomposites systems which incorporate a heterogeneous second phase on the nanometer scale. Over the last decade, there has been much interest in polymeric nanocomposite materials and the reader is directed towards the reviews by Alexandre and Dubois (4) or Joshi and Bhupendra (5) on the subject. 

This approach has been used to imprint theophylline [74], which was attached to the surface of silica gel particles and an imprinted polymer assembled around the inorganic particles. When polymerization was complete, the silica was dissolved away with hydrofluoric acid. This yielded imprinted polymer particles with a narrow pore size distribution with diameters between 254 and 257 A compared with conventionally prepared imprinted polymers which have pore diameters in the range 30-1000 A. The imprinted polymer showed a high selectivity for theophylline over the related compounds caffeine and theobromine. 

Scheme 8 Synthesis of a silica nanowire using the polymer brush template approach, (a) ATRP multi-initiator poly[2-(2-bromoisobutyryloxy)ethyl methacrylate] (PBEM) with degree of polymerization of 3,200 (b) cylindrical polymer brush (CPB) with side chains of 20 APTS units (c) core-shell CPB with an additional 57 OEGMA units (d) soluble organo-sUica hybrid nanowires with a crosslinked silsesquioxane network in the core (e) inorganic silica nanowires after pyrolysis. Reprinted with permissitm liom Yuan et al. [40]...

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