Cyclosilanes polymerization

Polysilanes have been the first class of precursors used to prepare silicon carbide ceramics. In all cases, on pyrolysis, polysilanes are converted into polycarbosilanes by a Kumada rearrangement prior to the formation of an amorphous silicon carbide network. Several synthetic routes including dehydro-polymerization, ring-opening polymerization of strained cyclosilanes, polymerization of masked disilenes, or base catalyzed disproportionation reactions of disilanes have been described to prepare linear or branched polysilanes but despite its drawbacks the Wurtz-coupling route remains the method applied most, especially to prepare linear polysilanes. 

Three permethylated cyclosilanes of the formula [(CH3)2Si]B with n=5-7 are prepared by allowing dimethyldichlorosilane to react with sodium-potassium alloy in tetrahydrofuran. From this reaction, an insoluble polymeric mass and a crystalline solid are produced (21, 177). The latter consists, for the most part, of dodecamethylcyclohexasilane but it also contains decamethylcyclopentasilane and tetradecamethylcycloheptasilane in small quantities. The amounts of both five- and seven-membered cyclosilanes are much increased if the reaction is worked up immediately after the addition of the dichlorosilane is completed with little or no refluxing. The three cyclosilanes can be separated by preparative gas chromatography 21). 

This method, which is the standard for polysiloxanes, is much less useful for polysilanes. One reason is that polysilane polymers are thermodynamically unstable compared to cyclosilanes. Thus, any method of polymerization leading to equilibration will produce cyclosilanes exclusively the linear polymers can only be kinetic products. 

Keyword Polysilanes / Cyclosilanes / Photolysis / Ring-Opening Polymerization... 

Anionic polymerization of masked disilenes and strained cyclosilanes [18]... 

The first examples of well-defined cyclosilanes, also reported by Kipping, were products obtained from the reaction of sodium with dichlorodiphenyl-silane in boiling toluene. Three distinct crystalline compounds were isolated they were designated as Compound A, Compound and Compound C. Other amorphous solids and intractable glues were also obtained, although now they appear to have been polymeric in nature. Compound A was also prepared in comparable yield from chloro-phenoxydiphenylsilane and sodium. 

One other organic substituted cyclosilane has been reported in the Utera-ture. Burkhard prepared dodecamethylcyclohexasilane (9) by the reaction of dichlorodimethylsilane with sodium in hydrocarbon solvents. Under these conditions, the cyclosilane was formed in low yield along with polymeric materials, one of which was characterized as a dimethylsilylene polymer with an average degree of polymerization of 55 units. ... 

The remarkable electronic behavior of these materids has stimulated considerable effort to develop selective syntheses of polysilanes with well-defined structure. Typical synthetic procedures include the Wurtz coupling of dichlorosilanes,(5-5) anionic polymerization of masked disilenes, (9-72) anionic ring-opening polymerization of strained cyclosilanes,(i5-i7) electrochemical reduction of chlorosilanes,(75-27) and transition metal catalyzed dehydrogenative coupling of phenylsilanes (eq. l).(22-55)... 

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