Polyhedral silsesquioxanes

The use of small polyhedral silsesquioxanes as molecular models of silica surfaces has been studied increasingly in recent years, because catalysts, etc., attached to such species are relatively easy to handle and characterize when compared to bulk silica. The preparation and structures of silsesquioxanes used in this type of work are described later in Sections II,C, IV,E, and IV,F. Further recent examples may be found in Ref. 5. 

By far the most important and most thoroughly investigated representatives among the incompletely condensed polyhedral silsesquioxanes are the heptameric trisilanols R7Si70g(0H)3 (Scheme 4). 

The chemistry of polyhedral silsesquioxanes and their homo derivatives is being studied extensively without imitating nature. Natural compounds of these types have not been found. The first reports on compounds of the general structure (XSiOi s) date back to the second half of the nineteenth century. Substances of this type with X = H and CgHj were obtained by hydrolysis of XSiClj by... 

Sprung and Guenther were the first to postulate that the hydrolysis of organyltrifunctional monomers XSiYj involves a consecutive formation of linear, cyclic, polycyclic and, finally, polyhedral siloxanes. They assumed the chain growth to be of a random character. This hypothesis was further developed by Brown and Vogt who studied the hydrolysis of cyclohexyl- and phenyltrichloro-silane They suggested the formation of polyhedral silsesquioxanes and their homo... 

Several chemical synthesis routes to black glass precursors have also been developed, as will be discussed below. In general, the properties of the materials produced by both methods are much the same. For example, Agaskar described the use of several novel polyhedral silsesquioxanes as precursors to microporous organolithic materials . His approach was to synthesize functionalized polyhedral silsesquioxanes (equations 41 and 42)136. 

Hexanuclear manganese complexes of polyhedral silsesquioxanes, supported on Si02, C, or A1203, evolve 02 from H20 when oxidized by Ru(bpy)33+ [166], The Mn ions (Figure 34) form a sandwiched ring structure and are octahe-drally coordinated by four equatorial x2-siloxy groups, one axial i6-chloridc ion, and one O from H20 or ROH. 

Similarly, numerous different nanoparticles, including organomodified clays,3 nanoparticles of silica,4 layered double hydroxides (LDH),5 or polyhedral silsesquioxanes (POSS),6 have been combined with intumescent formulations in polymeric materials to create large synergistic effects (see Chapter 12 for more details) the nanoparticles acting as char reinforcer or char expander that result in differences in terms of FR properties. 

Lichtenhan J D et al. (1993) Silsesquioxane-siloxane copolymers from polyhedral silsesquioxanes. Macromolecules 26 2141-2142... 

Choi [3] prepared photoresist resins useful at 157 and 193 nm containing polyhedral silsesquioxane substituents as illustrated (Vll) below. 

Fig. 23 Various polyhedral silsesquioxane backbone-based liquid crystals...

Zhang and Laine [77] reported on vinyl cubic polyhedral silsesquioxanes (vinylSiOi,5)8 (XI) and ((vinylMe2SiO)SiOi.5)8 (XIV) that were epoxidized using ten equivalents of m-chloroperoxybenzoic acid (m-CPBA) per cube (Scheme 4). 

This approach has also been employed for synthesis of highly enan-tiomerically enriched cychc silanes with sihcon-centered chirahty [141]. Zhang et al. [142] prepared second-generation dendrimers based on polyhedral silsesquioxane cores with up to 72 terminal groups at their surface (Scheme 13). 

Repetitive hydrosilation/allylation of vinyl-functionahzed polyhedral silsesquioxanes (XLIV) produced chlorosilyl- (XLV and XLVII) and allyl-derivatized (XLVI and XLVIII) dendrimers, respectively. Hydroboration/oxi-... 

On the other hand, GC/MS studies on the polycondensation of C2HsSiCl3 [11] suggest a two step mechanism, including rapid hydrolysis of the monomer and formation of linear, cyclic, and polycyclic products. The polyhedral silsesquioxanes are generated during a second and considerably slower step via cleavage and rebuilding from (strained) polysiloxane molecules. 

Accordingly, polycyclic siloxanes can be considered as the direct precursors to polyhedral silsesquioxanes and, therefore, represent possible sources for T and T OH isomers, respectively The following figure schematically shows the formation of polyhedral silsesquioxanes via intramolecular condensation. T (OH)4 siloxanes lead to different T (OH)2 isomers with symmetrical (1) and anti-symmetrical (II) position of the remaining reactive groups (- ) [12]. The formation of spherical T silsesquioxanes, however, can only occur from the type (I) siloxanes. 

An interesting new approach in this area has been reported with the synthesis of novel polymeric zirconocene-silsesquioxanes 134. " The condensation of zirconocene derivatives with polyhedral silsesquioxanes led to... 

Cage rearrangement is an established procedure to prepare polyhedral silsesquioxanes (RSiOi.5)n with n > 8. " Normally, the portion of compounds with n > 12 is very small and difficult to characterize. As shown previously, SEC in combination with MALDI-TOF-MS is a very powerful tool to identify... 

An interesting new approach in this area has been reported with the synthesis of novel polymeric zirconocene-silsesquioxanes (4.15) with R=cyclohexyl [42, 43]. The condensation of zirconocene derivatives with polyhedral silsesquioxanes led to amorphous polymers in high yields ( 90%) that were characterized by NMR spectroscopy and elemental analysis. These high molecular weight (M =14,000) materials exhibited high thermal stability (to ca. 475-515 °C) and, surprisingly, 4.15 a was found to be stable to both air and hydrolysis. 

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