Hydrogenated polysilanes

Tanaka, H. et al. 2007. Solution-processed Si02 films using hydrogenated polysilane based liquid materials. Tech. Digest of SID 07. pp. 188-191. 

Unlike the limited synthetic routes towards hydrogenated polysilanes, a relatively broad spectrum of methods is available for alkyl and aryl substituted polysilanes. The first compound containing a silicon-silicon bond and bearing organic substituents was hexaethyldisilane described by Friedel and Ladenburg in 1869 who reacted Si2le with ZnEt2 [10]. 

The polysilanes are inert to air and atmospheric moisture, and are not attacked by mild reagents such as dilute acids, etc. However in a solvent such as THF, solvolysis of the Si-Si bond by strong bases is fairly rapid. With alcohols or water and bases, hydrogen is produced ... 

A similar dichotomy was observed in the titanium catalyzed polymerization of primary silanes coupled to the hydrogenation of norbornene (20). At low catalyst concentration (ca. 0.004H), essentially complete conversion of norbornene to an equimolar mixture of norbornane and bis-phenylsilyl- (and/or 1,2-diphenyl-disilyl)norbornane was observed. Under these conditions no evidence for reduction of titanium was obtained. At higher catalyst concentrations (> 0.02M) rapid reduction of the dimethyltitanocene to J, and 2 occurs and the catalytic reaction produces mainly polysilane (DPn ca. 10) and norbornane in ca. 80 per cent yields, and silylated norbornanes in about 20 per cent yield. 

The ladder polysilanes can be used as useful probes for obtaining information on the stereochemistry of Si-Si bond cleavage reactions.60 Watanabe and co-workers64 and West and co-workers65 reported that the strained Si-Si bonds of cyclotetrasilanes such as [(z-Pr)2Si]4 and (Et2Si)4 are cleaved by hydrogen chloride, hydrochloric acid, and hydrobromic acid. However, to the best of our knowledge, no report has dealt with the... 

Finally, silicon-based polymers, especially with hydrogen lateral groups, are very interesting, but they are not yet explored sufficiently. There are many unknown properties in these materials, including the details of the photopolymerization process and a-Si formation from polysilane. Additional academic work in this field is expected and necessary to make the solution processing of silicon devices more convenient and reliable. 

It was proposed that an initially generated silyl radical 3, by reaction of i-BuO radical and polysilane 2, attacks another silicon atom in the same backbone to give a cyclic polysilane that contains an acyclic chain and another silyl radical (Scheme 8.1) [12]. The last silyl radical can either cyclize or abstract a hydrogen atom from another macromolecule, thus propagating the chain degradation. The reaction in Scheme 8.1 is an example of intramolecular homolytic substitution (ShO, a class of reactions discussed in Chapter 6. 

Further complications with silanes arise from lack of convenient syntheses and difficulties in separation. Nevertheless, compounds from n = I to n = 8 have been isolated, including both straight-chain and branched-chain compounds We should nol judge silicon s tendency to catenate by looking at these hydrides, however, since a much dilfereni result is obtained when substituents other than hydrogen are present.7 Flutters other than inherent Si—Si bond strength must be involved because it is possible to isolate a large number of polysilane polymers 8... 

Polymer Modification. The introduction of functional groups on polysilanes using the alkali metal coupling of dichlorosilanes is extremely difficult to achieve. Some polymers and copolymers with 2-(3-cyclohexenyl)ethyl substituents on silicon have been made, and these undergo hydrogen halide addition to the carbon—carbon double bond (94,98). 

Recently, several studies have been made of the photolysis of disilanes or polysilanes in the presence of an electron-deficient alkene using a photosensitizer (such as phenanthrene) and acetonitrile as solvent. These conditions result in the addition of silyl groups to one end of the alkene double bond and hydrogen to the other end (equation 18) and evidently involve the reaction of the radical anions of the electron-deficient silene with silyl radicals67 (see also Section VIII.A). 

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