Polysilane polymers reactions

Scheme 1. Possible Reactions tor the Photodegradation of Polysilane Polymers,...

Polymers with triflate groups react with alcohols to form alkoxysubstituted polysilanes. This reaction occurs readily in the presence of bases. The best results were obtained using triethylamine and hindered pyridine. In Fig. 3c the NMR spectrum of the reaction mixture containing the excess of triethylamine is shown, the methyl groups from the polymer chains absorb in the range typical for alkoxysilanes. Reaction in the presence of unsubstituted pyridine leads to the formation of insoluble polymer probably by attack at the p-C atom in the silylated pyridine. 

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. 

For poly silanes made by Wurtz reaction, the presence of functional groups in polysilane polymers is limited by the vigorous conditions of the sodium condensation reaction. Any substituents that react with molten sodium metal must therefore be introduced after the polymer is synthesized. Halogen atoms can be introduced into alkenylpolysilanes by HBr or HC1 addition reactions, as shown for the 3-cyclohexenyl-ethyl polymers in equation (21).37 Chlorine atoms have also been introduced into arylpolysilanes by chloromethylation, in which -CH2C1 groups are substituted onto phenyl rings in the polymer.38... 

Soluble polysilane polymers of high molecular weight were first prepared several years ago (1-3). The method commonly used is the reductive coupling of dialkyldichlorosilanes with sodium in refluxing toluene. Relatively few studies of the mechanism of the reaction or discussions of the probable reaction intermediates have been reported. 

The condensation of secondary silanes requires more rigorous conditions and no catalyst has yet been reported to couple tertiary silanes efficiently [140]. The great interest in polysilane polymers and their potential application come from their unusual electronic, optical, and chemical properties [142], particularly as preceramic materials [143]. The reaction constitutes the only alternative to Wurtz coupling for the formation of silicon-silicon bonds. 

SCHEME 4. Possible reactions for the photodegradation of polysilane polymers... 

In 1971, a short communication was published [54] by Kumada and co-workers reporting the formation of di- and polysilanes from dihydrosilanes by the action of a platinum complex. Also the Wilkinson catalyst (Ph3P)3RhCl promotes hydrosilation. If no alkenes are present, formation of chain silanes occurs. A thorough analysis of the product distribution shows a high preference for polymers (without a catalyst, disproportionation reactions of the silanes prevail). Cross experiments indicate the formation of a silylene complex as intermediate and in solution, free silylenes could also be trapped by Et3SiH [55, 56],... 

The history and development of polysilane chemistry is described. The polysilanes (polysilylenes) are linear polymers based on chains of silicon atoms, which show unique properties resulting from easy delocalization of sigma electrons in the silicon-silicon bonds. Polysilanes may be useful as precursors to silicon carbide ceramics, as photoresists in microelectronics, as photoinitiators for radical reactions, and as photoconductors. 

GPC traces of poly(phenylmethylsilylenes) prepared in the ultrasonication bath are shown in Fig. 1. In contrast to thermal condensation, monomodal high molecular weight polymer is formed. Oligomeric cycles (mostly cyclic pentamer), formed usually in high yield (cf. Table 1), can be very easily separated from the reaction mixture by precipitation with isopropanol. The molecular weight of polysilanes decreases and polydispersity increases with temperature. 

Because the coupling reaction is usually carried out in excess sodium ([Na]o/[Si-Cl]o=1.2), degradation of the polymer is possible. We had prepared polysilanes and tested this possibility by their reaction with... 

The discovery that soluble high molecular weight polysilanes may be prepared by the reductive coupling of dichlorodialkylsilanes by alkali metals (1,2) has led to considerable work on the properties of this interesting class of polymers (3,4,5). The preparation of the polymers leaves much to be desired as frequently the high polymer is only a minor product. Mechanistic studies of the reaction with a view to improving the relevant yields have been few (6). The major ones by Zeigler (7,8,9) showed that a silylene diradical was not involved in the reaction, and stressed the importance of polymer solvent interactions. 

Polysilanes containing phenyl groups are thermally stable up to 350°C while methylmethoxypoly-silanes without phenyl groups decompose at 250 - 300°C. When methylmethoxyphenylpolysilanes are heated to 350°C methylmethoxysilanes distil off and a polymer is formed which is nearly free of methoxy groups. In contrast to the thermal reaction of polydimethylsilanes no polycarbosilane structures can be observed even if polyborodiphenylsiloxane is added as catalyst (Figure 3.). 

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