Polysilanes, linear preparation

In any event, between 1951 and 1975, no papers appeared on polysilane high polymers. However, linear permethylpolysilanes of the type MelSiMezhiMe were prepared and studied, especially by Kumada and his students,(5) and cyclic polysilanes were being investigated in several laboratories.(6,7) Studies of the permethyl-cyclosilanes, (Me2Si)n where n = 4 to 7, showed that these compounds exhibit remarkable delocalization of the ring sigma electrons, and so have electronic properties somewhat like those of aromatic hydrocarbons.(6)... 

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... 

In recent years, dehydrocoupling reactions catalysed by early transition metal complexes have become an increasingly important method for generating catenated species of the p-block elements. In addition to producing cyclic oligomers, this approach is used to prepare linear oligomers and polymers such as polysilanes and polystannanes of the type H(MR2) H (M = Si, Sn) (see Section 10.1.4). ... 

The homologous series of linear polysilanes with the formula (XIII) are prepared by treating appropriate methylchlorosilanes with sodium-... 

The proton NMR spectral data of organopolysilanes have often been published incidental to preparative studies (51, 54, 62, 74, 108, 119, 177, 187, 190). Only recently has a systematic investigation to determine the chemical shifts and coupling constants in linear and cyclic permethylated polysilanes, and to study the effects of substituents on the NMR properties of methyl derivatives of disilane and trisilane been reported (see Table V-VII) (206). 

Kumada et al.359 prepared linear methylpolysilanes up to n = 12 by Wurtz synthesis from different chloromethyldisilanes and found by investigation of the physical constants that the melting points of the silanes up to n = 7 do not rise continuously but follow in a zigzag course since the polysilanes with an odd number of silicon atoms melt at a lower temperature, than those with an even number of silicon atoms, though this effect is less pronounced with higher n. 

Various linear and cyclic fully methylated polysilanes can be made by catalytic retyrangement of Si6(CH3)i4, and the cyclic compounds [Si(CH3)2] ( = 5-7) have been prepared by the action of Li on Si(CH3)2Cl2 in tetrahydrofuran. Structural data for some of these compounds are included in Table 21.1 (P-727). 

Polysilanes are compounds which contain silicon-silicon bonds. Although the disilane Et3SiSiEt3 was reported as early as 18691, and the cyclic perphenyl compounds (Ph2Si) (n = 4-6) were prepared over 70 years ago by Kipping and Sands2, the belief that silicon had limited capability for catenation persisted until quite recently. The isolation within recent years of cyclic dialkylpolysilanes containing up to 40 silicon atoms in a ring, and of linear polymers with more than 40,000 silicon atoms in the chain, have however effectively dispelled this myth. 

Cyclic polysilanes are more stable than chain polymers. Attempts to produce high polymers of polysilanes by ring opening of cyclic compounds are unsuccessful, but conversions from linear polymers or chain oligomers to cyclic compounds are feasible. Most of the limited work in this area is done with permethylpolysilanes. As mentioned in 15.2.4.1.2, the preparation of methylcyclosilanes from Me2SiCl2 and alkali metals proceeds through polymer formation followed by depolymerization, at least in some cases . ... 

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. 

By the way, although a dominant majority of papers concerning the formation of amorphous SiC layers describes appUcations of CVD or PVD techniques, there have also been some attempts to use the polymer route for preparing SiC films. Starting from solutions of various polysilanes or polycarbosilanes, frequently films are formed by spin-coating and pyrolyzed under inert atmosphere [215-218]. Of course, such a procedure does not form a part of this section SiC layers via gas phase reactions . However, in this connection it should be mentioned that polysilanes are also applied to form films via evaporation, not only with the aim to build amorphous and/or crystalline SiC films, but also to use special properties of the polysilane films themselves, i.e. without a subsequent pyrolysis of these films. Such amorphous films are characterized by non-linear optical effects [219, 220] and their properties may be controlled by the uniformity of the orientation of polysilane chains which is susceptible to epitaxial influences [221-223]. 

In recent years, considerable attention has been directed toward a study of the more highly catenated linear and cyclic polysilanes (I, 2, 14, 18). An interesting facet of this recent work involves the preparation of branched polysilanes such as those illustrated below. 

Polysilanes (or polysilylenes) are usually prepared by the Wurtz-type couphng reaction of dichlorodialkylsilanes or, alternatively, via a transition metal-catalyzed dehydrogenation of dialkylsilanes both approaches often exhibit difficulties in terms of controlling the molecular weight and chain-structure, however. Nonetheless, by using masked silylene monomers (l-phenyl-7,8-disilabicyclo[2.2.2]octa-2,5-dienes), a series of novel, well-defined linear poly(silylene)s (M /Mn 1.3) was successfully obtained via an anionic ROP (Scheme 5.11) [147-150]. 

As usual, there are some limitations in attachment of various organic groups to siloxane backbone. In some instances, direct hydrosilylation of an olefin derivative of the intended functional group can lead to serious side reactions it may give poor yield or may simply call for unusual and difficult to prepare intermediates. In any case, hydrosilylation of polymers with Si—H is a good source of preceramic polymeric materials (4,18). A variety of linear polysilanes and polycarbosilanes have been reported, but because they tend to depolymerize on heating and afford little ceramic jdeld, they are not useful as SiC precursors. To increase the... 

Planned syntheses have been devised for the preparation of specific linear polysilanes. This is illustrated by the following example starting from the cyclic hexamer. 

Polysilane copolymers also become conducting when they are doped with oxidizing agents. Treatment of copolymer 3 with arsenic pentafluoride affords a conducting polymer as shown in Table 15.2 [5,7]. Interestingly, when the crosslinked copolymer prepared from irradiation of a linear polysilane 3 with x = 0.5 and y = 1.0 is doped with arsenic pentafluoride, the conductivity of the polymer increases slowly, but eventually reaches a level of 2 x 10 S cm [5]. 

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