Dichlorosilanes, reductive coupling

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... 

GPC traces of complete products of the reductive coupling of dichlorosHanes with Na In refluxing toluene. Hexylmethyl-dichlorosilane, phenylmethyldichlorosilane. 

GPC traces of products of the reductive coupling of hexylmethyl-dichlorosilane with Na in refluxing toluene. A, with rapid stirring B, with slow stirring. 

Matyjaszewski Krzysztof, Dorota Gresta, Hrkach Jeffrey S, Hwan Kyu Kim (1995) Sono-chemical synthesis of polysilylenes by reductive coupling of disubstituted dichlorosilanes with alkali metals. Macromolecules 28 59-72... 

Polysilylenes are usually prepared by the Wurtz-type reductive coupling of dichlorosilanes with alkali metals. 

Sonochemical homopolymerization of dichlorosilanes in the presence of sodium is successful at ambient temperatures in nonpolar aromatic solvents (toluene or xylenes) only for monomers with a-aryl substituents. Dialky 1-dichlorosilanes do not react with dispersed sodium under these conditions, but they can be copolymerized with phenylmethyldichlorosilane. Copolymers with a 30-45% content of dialkylsilanes were formed from equimolar mixtures of the corresponding comonomers. Copolymerization might indicate anionic intermediates. A chloroterminated chain end in the polymerization of phenylmethyldichlorosilane can participate in a two-electron-transfer process with sodium (or rather two subsequent steps separated by a low-energy barrier). The resulting silyl anion can react with both dichlorosilanes. The presence of a phenyl group in either a or P position in chloroterminated polysilane allows reductive coupling, in contrast to peralkyl species, which do not allow the reaction. Therefore, dialkyl monomers can copolymerize, but they cannot homopolymerize under sonochemical conditions. 

Polymerization by reductive coupling must start at the slow reaction between sodium and monomer and is followed by much faster reactions involving polymeric species. This sequence is synonymous to having a much faster propagation compared with initiation, as usually happens in the chain-growth process. Otherwise, no high-molecular-weight polymer could be formed in the presence of excess sodium or dichlorosilane. 

The electrochemical reduction of chlorosilanes has received significant research interest because this reaction provides an efficient route to disilanes and polysilanes having Si-Si bonds [Eq. (46) [162, 166-176]. The electrode material and the supporting electrolyte seem to be important factors for the effective homo coupling. The sacrificial metal anodes have been found to be quite effective for the reductive coupling of chlorosilanes, and extensive work has been devoted to this field. In fact, various sacrificial anodes such as Hg [167,171], Mg [172], Cu [173,174], Ag [171], and A1 [174-176] were found to be effective for the reductive coupling of chlorosilanes to form disilanes. The application of this method to dichlorosilanes gives rise to the formation of polysilanes [Eq. (47)]. 

The polymers are prepared from disubstituted dichlorosilanes by reacting them with alkali metal dispersions in a reductive coupling process. The polpierizations appear to have the characteristics of chain-growth rather than step-growth reactions. ... 

Attempts have been made to prepare polysilanes containing the 8-dimethylaminonaphth-l-yl ligand.863 The coupling reaction of 8-dimethylaminonaphth-l-yl lithium with MeSiClj has given the dichlorosilane 909, whose mild reduction with Mg has surprisingly yielded the disilane 910 rather than the expected polysilane (Scheme 128). The formation of the disilane may be rationalized by the insertion of a transient silylene 904 into an Si-N bond of... 

Several studies of the mechanism of the sodium coupling reaction have been carried out, and a mechanistic model has emerged which accounts for most of the experimental observations. The reaction chain appears to be anionic in nature. The first step is believed to be the slow reaction of the dichlorosilane with sodium to give an anion (equation 34). The chain-extending process is the reaction of polymer silyl anions with dichlorosilane molecules (equation 35), followed by rapid reduction of the chlorine-terminated polymer to the anion (equation 36). Reduction must take place at the surface of a sodium particle. As polymerization occurs, some molecules may break free from the sodium surface once in solution away from the metal, chain extension would cease. This portion may give rise to the low-molecular weight polymer. Other molecules probably become entangled at the sodium surface and so cannot break free. They therefore continue to react with fresh dichlorosilane, and eventually... 

Polysilanes are synthesized by following five methods (1) reductive condensation of dichlorosilanes with alkali or alkali earth metals (Wurtz-type coupling), (2)... 

The remarkable electronic behavior of these materids has stimulated considerable effort to develop selective syntheses of polysilanes with well-defined structure. Typical synthetic procedures include the Wurtz coupling of dichlorosilanes,(5-5) anionic polymerization of masked disilenes, (9-72) anionic ring-opening polymerization of strained cyclosilanes,(i5-i7) electrochemical reduction of chlorosilanes,(75-27) and transition metal catalyzed dehydrogenative coupling of phenylsilanes (eq. l).(22-55)... 

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