Polysilane formation

Reductive dehalogenation cannot be completely controlled, and mostly complicated mixtures were formed which are difficult to separate. Salt elimination opens the possibility of a reaction aimed at polysilane formation. Some examples are shown in Fig. 2. The key compounds are the alkali metal cyclosilanes, which we have isolated via the mercury compounds by the action of sodium/potassium alloy and used for the first time [13]. 

These and similar complexes of Ti and Zrare effective catalysts for the formation of polysilanes from primary silanes [117-120]. 

In 1971, a short communication was published [54] by Kumada and co-workers reporting the formation of di- and polysilanes from dihydrosilanes by the action of a platinum complex. Also the Wilkinson catalyst (Ph3P)3RhCl promotes hydrosilation. If no alkenes are present, formation of chain silanes occurs. A thorough analysis of the product distribution shows a high preference for polymers (without a catalyst, disproportionation reactions of the silanes prevail). Cross experiments indicate the formation of a silylene complex as intermediate and in solution, free silylenes could also be trapped by Et3SiH [55, 56],... 

Recently, the compounds CpCp Hf(Cl)Si(SiMe3)3 and CpCp Zr(Cl)Si(SiMe3)3 have been used as homogenous catalysts for the formation of polysilanes. 

Soluble polysilane polymers can also be used as precursors to silicon carbide. The first such application, using (PhMeSi)n-(Me2Si)m copolymers ("Polysilastyrene"), was to strengthen silicon nitride ceramics. The Si3N4 ceramic body was soaked in polysilane and refired, leading to the formation of silicon carbide whiskers in the pore spaces and a consequent increase in strength. (U)... 

As these experiments indicate, polysilanes can in some cases be converted to silicon carbide directly, without the necessity for formation of polycarbosilane, fractionation, or oxidation. For example, polysilastyrene copolymers can be formed into films or fibers and then crosslinked by irradiation with UV light. The crosslinked polysilane forms silicon carbide when heated to 1100°C in vacuum. (1U This method can be used in a "printing" mode, if a film of polysilane is cast onto a ceramic or metal substrate, then... 

Polymers with triflate groups react with alcohols to form alkoxysubstituted polysilanes. This reaction occurs readily in the presence of bases. The best results were obtained using triethylamine and hindered pyridine. In Fig. 3c the NMR spectrum of the reaction mixture containing the excess of triethylamine is shown, the methyl groups from the polymer chains absorb in the range typical for alkoxysilanes. Reaction in the presence of unsubstituted pyridine leads to the formation of insoluble polymer probably by attack at the p-C atom in the silylated pyridine. 

Polysilanes with alkoxy groups are more light sensitive than conventional polysilanes. They degrade rapidly in the presence of light in agreement with the facile formation of silylene from dialkoxydisilanes. Properties of these polymers are currently being investigated. 

Another interesting feature pertains to the melting points cited for the polymers having m — 6-9. The side chains are now long enough to crystallize themselves, which is apparently the reason that formation of mesophases is suppressed. Such side-chain crystallization has also been involved in polysilane homopolymers... 

The most important process so far has been the reductive elimination of halogens with the formation of Si-Si bonds. Kipping used this reaction and discovered the first perphenylated cyclosilanes, yielding polysilanes as a by-product [8]. Similarly dodecamethylcyclohexasilane was found using dimethyldichlorosilane as a starting material for this reaction by Burkhardt in 1949, but 90% of the yield appeared as poly silane by-products [9]. 

The spun polysilane fibers are cured by oxidation in air at a temperature of 160 - 200°C. The curing process is necessary to permit the conversion of the green fiber to the ceramic fiber without softening during pyrolysis. It is presumed that the oxidation mechanism results in the formation of Si-O-C and Si-O-Si bonds by the reaction of Si-CH3 and Si-Si in the polysilane. 

Fundamental knowledge on the structures and properties of the ladder polysilanes has accumulated in our research for the past 15 years. Some results were unpredictable, including the silicon double helix structure, the domino oxidation, the formation of persistent radical anions, the Diels-Alder reactions at the 1,4-positions of anthracene, etc. These results let us recognize that the construction of novel structures will open the new chemistry. 

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