Synthesis of Polysilanes

The polymer [SiCl2]n is not soluble in any organic solvent and is extremely moisture sensitive. However, it has been structurally characterized by X-ray crystallography and solid-state Si NMR. Thus, the X-ray crystal structure of this polymer shows an all-trans conformation. The Si-NMR spectrum of this compound in the solid state shows a resonance at 8 3.9. The polymer [SiCl2]n has two reactive chlorine atoms on silicon and therefore the possibility of replacing these by other substituents by a nucleo- 

Other polysilanes that contain organic side-groups can be prepared essentially by the following procedures. 

Anionic polymerization of masked disilenes and strained cyclosilanes [18] 

In addition to these general synthetic procedures, other methods of polysilane synthesis include electrochemical reduction of diorganodichlorosilanes [22], condensation reactions between appropriate acyclic difimc-tional reagents such as Li(Ph2Si)5Li and Cl(Me2Si)5Cl [4]. These latter methods are not discussed here. 

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Electrochemical Synthesis. Electrochemical methods have also been investigated for the synthesis of polysilanes, but these have so far yielded low molecular weight materials (113,114). 

The presence of alkali metal during reductive coupling practically prevents the synthesis of polysilanes with groups which can be easily cleaved under basic conditions, such as pendant alkoxy or amino substituents. Polysilanes with pendant functional groups are not known. We have recently discoverd a very efficient method of functionalization of polysilanes. 

Table 2 Synthesis of Polysilanes with Carboxylic Acids...

With the success of CgK in the syntheses of 41 and related co-polymers, the same methodology was applied to the synthesis of polysilanes bearing pendant crown ether substituents, viz. 42-44,150,151 which were synthesized in yields from 6% to 28%, with molecular weights of the order of 104. The polymers were found to be water soluble and thermochromic in accordance with the Schweizer thermochromism theory (see Section 3.11.5.2.4).152... 

Sakurai, H. Yoshida, M. Synthesis of Polysilanes by New Procedures Part 1 Ring-opening Polymerizations and the Polymerization of Masked Disilenes. In Silicon-based Polymers The Science and Technology of their Synthesis and Application-, Jones, R. G., Ando, W., Chojnowski, J., Eds. Kluwer Dordrecht, 2000 pp 375-399. 

More recently Kashimura et al. [96] has investigated the effect of ultrasound on the electroreductive synthesis of polysilanes, polymer germanes and related polymers using magnesium electrodes. They found that the presence of ultrasound greatly facilitated the reactions. 

Polysilanes are polymers in which there is catenation of silicon, that is, where silicon atoms are bonded to each other in a continuous manner. Synthesis of polysilanes involves the Wurtz coupling of diorganodichlorosilanes with sodium metal (Eq. 2-239) [Baldus and Jansen, 1997 Corriu, 2000 Manners, 1996 Miller and Michl, 1989 West, 1986 West and Maxka,... 

Synthesis of Polysilanes. The most commonly utilized method is based on the Wurt/ type alkali metal coupling or dichlorosilanes. Other synthesis methods include dehydrogenative coupling, ring-opening polymerization, polymerization of masked disilenes. electrochemical synthesis, and polymer modification. 

Soluble and well-characterized polygermane homopolymers. (R Ciel,. and their copolymers with polysilanes have been prepared by the alkali metal coupling of diorgano-stbstiluled dihalogermancs, via clectrochemicul methods, and by tnmsition-metal catalyzed routes, as with the synthesis of polysilanes. 

WurtZ-Type Coupling of Dihalosilanes. Several approaches have been developed for the synthesis of polysilanes. However, the most only utilized method is based on the Wurtz-type alkali metal coupling of dichlorosilanes. Both homo- and copolymers can be prepared this way (eq. 

The familiar Wurtz synthesis of polysilanes from dichlorosilanes and alkali metal evidently does not involve free silylenes. See R. G. Jones, R. E. Benfield, R. H. Cragg, A. C. Swain and S. J. Webb, Macromolecules, 26, 4878 (1993). 

A comprehensive review of the progress in the general synthesis of polysilanes is presented in Koe.2 Only a brief selection of the advancements in... 

Ultrasound has been used for efficient and clean synthesis of polysilanes as well as further conversion of polysilanes to SiC by combining a reducing... 

R. G. Jones and S. Holder, Synthesis of polysilanes by the Wurtz reductive coupling reaction , in Reference 4, pp. 353-374. 

Fig. 4 Approaches to the synthesis of polysilane block copolymers by living polymerization techniques (a) via anionic polymerization of masked disilenes (b) via anionic ring-opening polymerization of cyclotetrasilanes...

The standard method for synthesis of polysilanes follows the original preparation of (Me2Si) by Burkhard. Diorganodichlorosilanes are treated with finely dispersed sodium metal in an inert diluent, usually above the melting point of sodium. Homopolymers are obtained from single dichlorosilanes, while cocondensation of mixtures of dichlorosilanes yields copolymers (equation 33). Toluene is the most commonly used solvent, but other aliphatic or aromatic solvents are also effective. After completion of the reaction, the mixture is quenched with alcohol and/or water to destroy any excess sodium and silyl anion, then filtered and... 

Anionic polymerization of masked disilenes has been used for the synthesis of polysilanes. The monomers are bridged disilabicyclooctadienes such as (8). Treatment of these with an organohthium compound converts them to living, anionically terminated polysilanes (equation 37). The... 

Because of the low solubility of most poly silanes in combination with expandable processing, that is, purification including the above mentioned hydrolysis steps, product yields and purity are often unsatisfactory if obtained by reductive coupling of chlorosilanes. A further negative aspect of this approach is the limited functional group tolerance. Accordingly, only the synthesis of polysilanes with inert pendant groups such as alkyl or aryl can be adequately achieved. 

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