Modification of polysilanes

Polysilanes can be elaborated, after their synthesis, by appropriate chemical methods [1-4, 61]. Representative methods of such possibilities are discussed in this section. 

Polysilanes containing Si-H groups can be reacted further, in a variety of ways. Waymouth and coworkers have shovra that firee-radical-assisted addition of C=C or C=0 bonds can be effected (Fig. 7.19) [62]. 

Free-radical-assisted hydrosilylation can be carried out on poly(phenylsilane), H(PhHSi)nH. Various types of reactive substrates such as 1-hexene, cyclohexanone etc., have been utilized and about 70-90% of substitution was observed without much chain degradation (Fig. 7.19). 

Similarly, the conversion of the Si-H unit in poly(phenylsilane) into the more reactive Si-Cl and Si-Br functional group can be easily effected by the reaction with carbon tetrachloride and carbon tetrabromide, respectively (Fig. 7.19) [63]. The chlorination or bromination does not affect the Si-Ph substituent but only involves the Si-H bonds. In these reactions about 80% of the Si-H bonds can be converted into the corresponding Si-X bonds. These halogenated polysilanes are reactive polymers that can react with other types of nucleophiles such as alcohols to afford polysilanes containing alkoxy groups [63]. 

The phenyl substituent in poly(phenylmethylsilane)s can also be activated by its chlorination in a Sn(fV)-catalyzed reaction (see Eq. 7.15) [65]. 

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The second approach to linear polysilanes is based on the modification of polysilanes prepared by the reductive coupling method. The severe conditions of this reaction allow only alkyl or aryl substituents at the silicon atom in the starting dichlorosilane. Therefore only alkyl or aryl substituted polysilanes are known. We have successfully prepared new polysilanes with pendant alkoxy and amino side groups. This approach allows fine tuning of the properties of... 

Functionalization of polysilanes by chemical modification (post-polymerization) was covered in COMC II (1995) (chapter Organopolysilanes, p 101), where the formation of precursor polysilanes with potentially functionalizable side groups such as chloride, type 34 (via HCI/AICI3 chlorodephenylation of PMPS), 6 triflate, type 35 (via triflate replacement of phenyl groups)135,137 or alkyl halide (via chloromethylation of phenyl groups,138,139 type 36, or addition of HC1 or HBr to double bonds140) was discussed. Four other precursor polysilanes, which utilize the reactivity of the Si-Cl or Si-H bond, have been successfully applied in functionalization since COMC (1995) perchloropolysilane, 17 (see Section 3.11.4.2.2.(i) for synthesis),103 poly[methyl(H)silylene-f >-methylphenylsilylene],... 

Went, M. J. Sakurai, H. Sanji, T. Modification and Functionalization of Polysilanes. 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 419-437. 

The chemical modification of fullerenes has received considerable attention in the last decade in order to achieve new applications to material sciences [27]. Fuller-ene-bonded polysilane derivatives might be expected to show high conductivity since Ceo-doped polysilane is found to be a good photoconductor [28]. Therefore, a variety of silylated derivatives have been obtained to date, although the available methods are limited to the photoinduced addition of various silanes to Ceo-... 

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. 

Semi-empirical understanding of PM-transition characteristics and main chain stiffness of rod-like polysilanes thus leads to the new idea that minute structural modification of the achiral and chiral alkyl side groups could critically modify the transition characteristics, as indeed demonstrated in a series of 16-based copolymers, poly ((S)-3,7-dimethyloctyl-3-methylbutylsilane)-co-((S)-3,7-dimethyloctyl-2-methylpropylsilane) (22) and poly ((S)-3,7-di-methyloctyl-3-methylbutylsilane)-co-((P)-3,7-dimethyloctyl-2-methylpropyl-silane) (23) (see Scheme 5). 

The conformational structures of polysilane main chains at the macro-and microscopic levels are controllable by suitable choice of the side chain structures. Similarly, it is also the side chain which controls the optoelectronic properties by effecting the optical band gap. In the case of phenyl-substituted polysilanes, electronic interaction between the delocalized Si chain cr-bonding orbitals and the it-orbitals of the aryl groups causes a dramatic modification of both the band gap and conformational properties [61,83]. These aryl-containing polysilanes may be potential candidates for applications in a molecular-based chiroptical switch and memory in the UV/visible region. On the other hand, the precise control of helical polymers is now a subject of great interest and importance, due to the tech-... 

Several other studies on Nicalon-based ceramic fibers have also been conducted in addition to the investigation of oxidation curing of PCS fibers and effect of oxygen in tensile strength of SiC fibers [51]. Similarly, studies dealing with the chemistry, characterization, modification, use, and applications of polysilanes and polycarbosUane are also available [52]. 

As mentioned above, the cleavage of Si-Si-bonds in polysilanes upon irradiation with UV-light is very efficient and results in the formation of silicon radicals. These radicals react with olefins to initiate radical polymerizations. Especially the polymerization of methylmethacrylate and styrene with a variety of polysilanes as photoinitiators has been studied in detail [104]. The advantage of this kind of initiation is the possibility to prepare polysilane-polyolefin hybrids [105]. All modifications of radical polymerizations, such as the atom transfer radical polymerization (ATRP), are possible with polysilanes as photoinitiators [106]. 

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