Silicon hexamers

Relatively few examples are known which utilize an oxidative addition reaction of metal hydrides to necessarily low valent silicon compounds. Seyfert s hexame-thylsilirane (31) could be used as a source of dimethylsilylene to perform an... 

Oligomer silicone compounds with the Si—N bond in the chain have recently been used more and more often. In particular, of great practical interest is the simplest representative of this class of compounds, hexame-thyldisilazane, which is used as one of the components for certain medications, in gas chromatography and in the synthesis of organic substances. 

The system with which we have begun our investigations is the styrene-dimethylsiloxane system. The dimethylsiloxane blocks should be considerably less compatible with polystyrene blocks than either polybutadiene or polyisoprene since the solubility parameter of dimethylsiloxane is much farther from that of polystyrene than are the solubility parameters of polybutadienes or of polyisoprenes (17), no matter what their microstructure. Furthermore, even hexamers of polystyrene and of polydimethylsiloxane are immiscible at room temperature and have an upper critical-solution temperature above 35°C (18). In addition, the microphases in this system can be observed without staining and with no ambiguity about the identity of the phases in the transmission electron microscope (TEM) silicon has a much higher atomic number than carbon or oxygen, making the polydimethylsiloxane microphases the dark phases in TEM (19,20). 

A Py-GC study [7] with pyrolysis done at 500° C showed numerous peaks corresponding to the isoprene dimers, trimers. .. up to hexamers eluting in clusters of peaks. The separation was done on a methyl silicone 5% phenyl silicone type column with FID detection. The results from a Py-GC/MS study [8] where natural rubber was pyrolysed at 580° C in a Curie point Py-GC/MS on-line system showed similar results. The TIC trace of the pyrolysate with the separation done on a 60 m Carbowax column, 0.32 mm i.d., 0.25 pm film thickness, with the temperature gradient of the GC oven between 40°... 

It is worth noting that trimethylsilyllithium (LiSiMes) also crystallizes as the hexamer (LiSiMe3)6 based on a Lie trigonal antiprism like that in (cyclohexylLi)6, held together by pg-trimethylsilyl groups in which the silicon atoms are effectively hypercoordinate, forming three normal two-center Si-C bonds and one four-center SiLig bond. 

Silicones have been claimed to improve combing, enhance feel, reduce flyaway, increase shine, reduce drying time, and lock in color [54-56], The most frequently used silicone is dimethicone, which is a polydimethylsiloxane. Other important silicones are dimethiconol, which is a dimethylsiloxane terminated with hydroxyl groups, amodimethicone, which is an amino-substituted silicone, dimethicone copolyol, which is a dimethylsiloxane containing polyoxyethylene and/or propylene side chains, and cyclomethicone, which refers to a class of cyclic dimethyl polysiloxanes ranging from trimer to hexamer ... 

Polycarbosilanes can exhibit thermal stabilities greater than the analogous polysiloxane , contrary to their relative bond energies (Si—C, 78 kcalmol Si—O, 106 kcal mol" ). Thus polydimethylsilane is polymerized by heating the hexamer to 300-500°C, forming a product with thermal stability greater than 500°C and that can be spun into fiber, which in turn can be thermally oxidized, forming j5-silicon carbide, SifCHj) -,. 

Fig. 15 Rupture of a strained Si-O bond in a siloxane hexamer by attack from a water molecule in the surroundings." (i) shows the approach of the water molecule, (ii) shows the point at which the oxygen atom in the water molecule is attracted to the silicon atom inducing rupture, and (iii) shows the rupture products where a proton has been transferred to the ruptured oxygen of the siloxane and the hydroxyl group goes to the silicon. Reprinted with permission from E. M. Lupton, F. Achenbach, J. Weis, C. Brauchle and I. Frank, J. Phys. Chem. B, 2006, 110, 14557-14563. Copyright 2006 The American Chemical Society.

Thus each polymer species is in equilibrium with a certain concentration of monomer in solution. Three-dimensional polymer units larger than the double-three-ring hexamer or similar species can be considered to be the core of a particle. A polycyclic octamer or decamer is not actually a silica particle because there is no anhydrous SiOj core. As additional monomer condenses with SiOH groups on these oligomers they also condense with themselves until the oligomer remains as an anhydrous SiO core surrounded by a layer of silica in which the silicon atoms retain outwardly disposed silanol groups as shown in Figure 3.17. 

Another important research field aims at controlling the molecular and structural ordering of semiconducting PT in view of improving its charge transport properties. A major advance in this direction has been realized in 1987 at CNRS, Thiais, with the synthesis of sexithiophene (6T), the linear hexamer of thiophene, and its use to fabricate an organic transistor whose performances are close to those of silicon-based devices. The spectacular increase of the carrier mobility in polycrystalline... 

Another imique one-pot synthesis of calixfuran analogues is that of the silicon-bridged calixfurans. Treatment of furan with two equimolar amounts of n-BuIi/TMEDA/f-BuOK followed by slow addition of Me2SiCl2 afforded the cycHc tetramer 8 and hexamer 9 in 16% and 10% yields, respectively (Scheme 2) [26,27]. 

Silicone resins for coatings resemble some phenol-formaldehyde polymers in that polymerization takes place in two stages. A trifunctional monomer such as phenyl-trichlorosilane may be hydrolyzed to the relatively stable hexamer (and related structures modified by comonomers for better solubility). 

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