Preparation of polysilanes

On the other hand, Shono, Kashimura and coworkers found that alternating the polarity of Mg electrodes in LiC104/THF is quite effective for the formation of Si—Si bonds (Table 19)87,88. This method was successfully applied to the preparation of polysilanes. Sonication resulted in a marked increase in the yields of polysilanes as shown in Table 2087,88. Under higher concentration of dichlorosilane, poly silane of higher molecular weight was obtained. 

The usual procedure in preparations of polysilanes and polygermanes is to circulate the simple hydride through an ozonizer-type discharge until practically all of it has decomposed and the gas being circulated is principally hydrogen. The product hydrides are trapped out in a suitable cold trap in the gas circuit. If the desired product vapors were allowed to circulate continually through the discharge, the product would... 

E. A. Hoess, E. L. O Brien, Preparation of Polysilanes and Polysilarylenea, Tech. Rept. No. 3289, Picatinny Arsenal, Dover, NJ, 1966. 

The preparation of polysilanes is described in detail in Chap. 2 and was first reported by Kipling in 1924 [28]. The concept most commonly used now was developed by Yajima et al. [29], who used dimethylchlorosilane as a starting substance. 

In the past few years, similar reactions using numerous dichlorosilane derivatives were investigated to yield preceramic polymers. Some routes use several different types of chlorosilanes as starting materials (e.g., Refs. 33-35) Analogous to the preparation of polysilanes, polycarbosilanes can also be synthesized using a similar route. Alkyl chloroalkyl chlorosilanes are used as starting materials for this purpose [2]. 

The preparation of polysilanes by the reaction of intermediately formed silylmetallics, derived from cyclosilanes, will be discussed in the next section. 

In addition, the cleavage of octaphenylcyclotetrasilane and decaphenyl-cyclopentasilane with lithium in the presence of chlorosilanes has provided access to a variety of polysilane derivatives. Other reactions describing the use of cyclosilanes for the preparation of polysilanes have been thoroughly reviewed 14),... 

Recently, the condensation-polymerization reaction of a mixture of methylchlorodisilanes has been reported 66). This reaction is catalyzed by silver cyanide, and treatment of the reaction mixtures with methylmag-nesium bromide gave hexamethyldisilane, octamethyltrisilane, decametbyl-tetrasilane, and dodecamethylpentasilane. The condensation-polymerization of pentamethyldisilanyl cyanide and heptamethyltrisilanyl cyanide has also been reported (66). The products of these latter reactions are trimethylsilyl cyanide and higher methylcyanopolysilanes. This scheme allows the preparation of polysilanes in the absence of metals and promises to be an extremely useful synthetic method. 

Metal-assisted dehalogenation reaction, analogous to the well-known Wurtz reaction has been very effectively employed for the preparation of polysilanes which contain catenated organosilicon units. Thus, the reaction of a variety of diorganodichlorosilanes with finely divided sodium in a... 

Catalytic dehydrogenation is an alternative method for the preparation of polysilanes. This reaction involves the dehydrogenation of RSiHa to afford polysilanes of the type [RSiHjn (see Eq. 1.10). 

Before we consider the synthesis of polysilanes in the next section, let us consider if addition polymerization involving disilene monomers can be used for the preparation of polysilanes. Robert West from the University of Wisconsin prepared the first stable disilene in 1981. His synthetic route consisted of photolyzing an acyclic trisilane. Loss of McsSi-SiMcs leads to a silylene intermediate that dimerizes to the stable disilene (Fig. 7.4) [12-13]. 

Although many disilenes have now been prepared and characterized they are not very good monomers for the preparation of polysilanes. The very method of preparing these kinetically stabilized disilenes defeats their use as olefin-like monomers for addition polymerization. However, as will be shown in the next section trapped disilenes or masked disilenes that... 

In spite of its many drawbacks (as will be evident during the course of this discussion), the alkali metal-assisted Wurtz-type coupling reaction remains by far the most widely used procedure for the preparation of polysilanes... 

Additionally, silane monomers containing Si-Cl or Si-H bonds are moisture and air sensitive or even pyrophoric while oligomeric precursors containing Si-Si-bonds can be slightly sensitive. The preparation of polysilanes is challenging and difficult to transfer to an industrial scale and application. 

Scheme 23.1.5 Overview of the Proposed Mechanism for the Preparation of Polysilanes via Borane Catalysis A — linear chain-growth, B — ligand scrambling, C — branching, D — termination.

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