Disilane fraction

The disilane fraction refers to a fraction boiling over the range of about 150°-l 60° C which is obtained by fractionation of the higher boiling fraction of methylchlorosilanes produced by the so-called direct synthesis (141, 214). It is composed mainly of 1,1,2-trichlorotrimethyldisilane and 1,1,2,2-tetrachlorodimethyldisilane, somewhat contaminated by siloxanes (6, 22, 27, 114,125,126, 134). 

If silver cyanide is used as the catalyst, chloropentamethyldisilane as well as the disilane fraction undergoes disproportionation, although in this case good conversions require employment of relatively large quantities of silver cyanide (187). 

Organometals containing metal-metal bonds have become more important in recent years. Of especial interest are those compounds with both representative and transition metals. These have been known for over 30 years 130), but have become well known only comparatively recently 69) they provide a link (both literally and figuratively) between the two areas of organometallic chemistry. The growing commercial interest in the silicones has spurred research in organosilicon chemistry, especially polysilane chemistry. Work in this area has been reviewed by Kumada 159). The disilane fraction from the direct synthesis of methylchlorosilanes contains a mixture of compounds of type (CHa) -Clg. Sig, which are readily converted to hexamethyldisilane. This may then be converted to the chloride in a two-step synthesis 160) ... 

The chloride then serves as starting point for other derivatives. The ready availability of the disilane fraction in large quantities has made disilanes and higher polysilanes easy to acquire in bulk, and they have been studied more than other compounds with metal-metal bonds. 

Minor products from the direct reaction, all normally produced in amounts less than 1%, include SiCU, HSiCH, Me4Si, Me(Et)SiCl2, disilanes, especially Me2Si(Cl)Si(Me)Cl2 and (MeSiCl2)2, disiloxanes, CH4 and other hydrocarbons, and H2. The reaction effluent contains all of these, plus excess MeCl, and finely divided silicon particles containing up to 20% copper as well as other metals. These solids are separated, and usually treated to recover the copper. Unreacted MeCl is separated, purified, and recycled. The monosilane fraction is distilled out and then separated into its components by careful fractional distillation. The disilane fraction can also be distilled out, and may be upgraded into useful chloromonosilanes by cleavage reactions, as shown in equation (8). 

Several catalysts were tested, and Baney et al. used 1 mass% BU4PCI to disproportionate a disilane fraction, and also pure SiCl2Me-SiCl2Me as well as... 

The disilane fraction or pure SiCl2Me-SiCl2Me is heated to reflux and condenses in the catalyst bed. A stream of argon removes the volatile monosilanes formed, while the oligosilane mixture flows back into the flask. An improvement of the rheologic properties of the polysilane has heen achieved by co-polymerization of SiCl2Me-SiCl2Me with styrene or by... 

As initial products containing the Si Si unit we used 1,2-dichlorotetramethyldisilane, 1,1,2,2-tetra-chlorodimethyldisilane, hexachlorodisilane and the previously unknown tetramethyldisilane-1,2-diol, which we just synthesized by cautious hydrolysis of 1,2-dichlorotetramethyldisilane. Both chloromethyldisilanes are available starting from the so-called disilane fraction of the direct chloromethylsilane synthesis (Huls-Silicone) by methods already described in the literature [7, 8] and partly improved by us. 

Using hexamethylphosphoramide as the solvent, only the second reaction occurs. Disilane also reacts with potassium in 1,2-dimethoxyethane to form KS1H3, although S1H4 and nonvolatile polysHanes are also produced (28,31). Pure crystalline KSiH prepared from SiH and potassium in 1,2-dimethoxyethane has been obtained by slow evaporation of the solvent. WhenHquid ammonia is used as the solvent, only a small fraction of SiH is converted into metal salt most of the SiH undergoes ammonolysis (32). 

The increase in the deposition rate rj (Fig. 63d) corresponds to the increase in the ion flux (Fig. 63c) the fraction of arriving ions per deposited atom, / ,, is constant at about 0.25. Such observations have also been reported by Heintze and Zedlitz [236], who furthermore suggested that the deposition rate may well be controlled by tbe ion flux. The kinetic ion energy per deposited atom, max, is also constant and amounts to about 5 eV. As was shown in Section 1.6.2.3, the material quality as reflected in the refractive index 2 eV (Fig. 63e) and the microstructure parameter R (Fig. 63f) is good 2 cv is around 4.25, and R is low (<0.1). The depletion of the silane stays constant at a value of 4.0 0.4 seem in this frequency range. The partial pressures of silane, hydrogen, disilane (1.3 x 10 - mbar), and trisilane (2 x 10 mbar) in the plasma are also independent of frequency. Similar... 

The Miiller-Rochow-Synthesis [16,17] (direct synthesis of methylchlorosilanes) provides as byproduct a high boiling fraction consisting essentially of 1,1,2-trimethyltrichlorodisilane and 1,2-dimethyltetrachlorodisilane [18]. Starting with these disilanes Wacker-Chemie has developed different ways to produce silicon carbide [19, 21] and silicon carbonitride [22] fibers. 

Brus et al. prepared isolated silicon particles by high temperature pyrolysis of disilane with a subsequent passivation of the surface by oxidation [33]. The particles of various size are then processed by high-pressure, liquid-phase, size exclusion chromatography to separate sizes and obtain various fractions of monosize particles. Such particles represent an almost ideal model of silicon quantum dots. 

During the fractionation of the DPR, no SiH-containing disilanes were isolated. This is most probably because metal chlorides such as AICI3 will result in Si-Si bond cleavage. Gupper noted that ClH2SiSiH2Cl and ClH2SiSiHCl2 were easily separated and purified by fractional distillation. 

The disilanes with x = 2 and 3, which is the main portion of the high-boiling fraction, are converted to monosilanes by subsequent cleavage with hydrogen chloride. 

The higher boiling residues obtained in the Direct Synthesis of methylchlorosilanes (from elementary silicon and methyl chloride) include a variety of compounds which contain silicon-silicon bonds. A particularly useful fraction (b.p. 150°-160°C) contains methylchlorodisilanes of the type, (CH3) Cl6-nSi2. The use of this fraction for the preparation of disilanes has been described (2, 62-65) however, few applications leading to the formation of higher polysilanes have been reported. 

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