Polysilane polymers

In this chapter, an introduction and overview of polysilane chemistry will be presented, concentrating on the linear high polymers (polysilanes) and their technological applications. Polysilane polymers were reviewed in 1986,(1) and a more general review of polysilane chemistry appeared in 1982.(2)... 

Research in polysilane polymers grew slowly at first after this reawakening. But within the past few years, both the unusual scientific interest and the technological possibilities of the polysilanes have been recognized, and activity in this field has increased sharply. Commercial manufacture of both poly dimethylsilylene) and "polysilastyrene" is now being carried out in Japan, so that these two polymers are readily available in quantity. 

Molecular weights of polysilane polymers depend upon the exact method of synthesis, as well as the purity of the dichlorosilane starting materials. Bimodal molecular weight distributions are commonly reported, as shown in Figure 1, but under some conditions... 

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

Even more complicated changes take place in solid films of polysilane polymers as the temperature is changed. These are discussed in the chapter by Miller et al. in this volume.(31)... 

Soluble polysilane polymers can also be used as precursors to silicon carbide. The first such application, using (PhMeSi)n-(Me2Si)m copolymers ("Polysilastyrene"), was to strengthen silicon nitride ceramics. The Si3N4 ceramic body was soaked in polysilane and refired, leading to the formation of silicon carbide whiskers in the pore spaces and a consequent increase in strength. (U)... 

Thin films of this polymer generate third harmonic radiation upon irradiation at 1064 nm, believed to be due to a three-photon resonance. Thus polysilane polymers may eventually find use in laser technology. 

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. 

The present calculations on po1y di-/j-hexylsilylene) are consistent with the hypothesis that the origin of thermochromlc behavior in selected polysilane polymers resides in a change in population of conformational states along the silicon backbone with temperature. 

Further complications with silanes arise from lack of convenient syntheses and difficulties in separation. Nevertheless, compounds from n = I to n = 8 have been isolated, including both straight-chain and branched-chain compounds We should nol judge silicon s tendency to catenate by looking at these hydrides, however, since a much dilfereni result is obtained when substituents other than hydrogen are present.7 Flutters other than inherent Si—Si bond strength must be involved because it is possible to isolate a large number of polysilane polymers 8... 

In polysilane polymers, the polymer backbone is made up entirely of silicon atoms. Therefore these materials differ from other important inorganic polymers, the siloxanes and phosphazenes, in which the polymer chain is heteroatomic. Structurally, they are more closely related to homoatomic organic polymers such as the polyolefins. However, because the units in the main chain are all silicon atoms, the polysilanes exhibit quite unusual properties. The cumulated silicon-silicon bonds in the polymer chain allow extensive electron delocalization to take place, and this delocalization of the sigma electrons in the Si-Si bonds gives the polysilanes unique optical and electronic properties. Many of the potential technical uses, as well as the remarkable properties, of polysilanes result from this unusual mobility of the sigma electrons. 

Linear polysilane polymers, properly called poly(silylene)s, can be obtained as homopolymers or copolymers. Continuation of the polysilane chain consumes two of the four valences of each silicon atom the other two are taken up by pendent groups, which may be the same (5.1) or different (5.2). Copolymers (5.3), which contain two or more kinds of silicon atoms, can be made up from units like those in 5.1 or 5.2. A typical example is the copolymer of Me2Si and PhMeSi units,... 

Yet we know now that polysilane polymers are stable to heat up to almost 300 °C, are inert to oxygen at ordinary temperatures, and are only mildly susceptible to hydrolysis. The principal weakness of polysilanes, as materials, is not any of these properties it is that they become degraded when exposed to ultraviolet light. 

The first clear description of a polysilane polymer appears in a classic paper by C. A. Burkhard of the General Electric Co. Laboratories published in 1949.3... 

Figure 5.1 Number of papers published on polysilane polymers and oligomers per year, plotted against time, showing the dramatic growth in the literature since 1984.

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... 

It now seems evident that the solid state conformations of polysilanes are much more complicated and perplexing than had been thought. Determination of the actual conformations and solid state structures of polysilane polymers is likely to occupy chemists for many years. 

Many kinds of polymers containing silicon atoms in the main chain have been synthesized. A few examples are shown in Figure 1. By a wide margin, the most important are the polysiloxanes, commonly called silicones, which are now the basis for a worldwide industry.Also described in this article are the polysilane polymers, which are of interest because of their special properties reflecting extensive delocalization of the a-electrons along the Si-Si backbone see a-Bond). ... 

As the name implies, polysilane polymers consist of chains made up exclusively of sihcon atoms. (In the ht-erature these polymers are named either as polysUanes or poly(silylenes) thus (Et2Si) can be called polydiethylsilane or poly(diethylsilylene).) Unlike the heteroatomic polysiloxanes, with alternating silicon and oxygen atoms in the polymer backbone, the poly silanes are homoatomic and therefore structurally closer to aUcene polymers. However, because the atoms in the main chain are all silicon, the polysilanes show quite special properties. ... 

Burkhard, who reported the synthesis of polydimethylsilane, [Me2Si] , from sodium metal and dimethylchlorosilane in toluene (equation 31). This polymer is a highly crystalline substance which decomposes without melting and is insoluble in all common solvents. Its intractable nature may have contributed to the relative neglect of polysilane polymers over the next three decades. 

This method, which is the standard for polysiloxanes, is much less useful for polysilanes. One reason is that polysilane polymers are thermodynamically unstable compared to cyclosilanes. Thus, any method of polymerization leading to equilibration will produce cyclosilanes exclusively the linear polymers can only be kinetic products. 

Less is known about the configurations of polysilane polymers. Dialkylpolysilanes such as (MeSi-n-Bu) made by sodium condensation appear to be atactic, from Si NMR studies, but arylalkylpolysilanes made by the same procedure appear to be partially tactic. Anionic ringopening polymerization of (PhSiMe)4 (Section 7.3) leads to a (PhSiMe) polymer with greater, but still incomplete, stereoregularity. 

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. 

Baney and co-workers (iO) used the known Si-Si and Si-Cl bond redistribution of methylchlorodisilanes to prepare poly(methylchlorosilane)s. Pyrolysis of these branched polysilane polymers gives nearly stoichiometric amounts of silicon carbide (equation 6). 

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. 

To quote Robert West. ".Although the myth that silicon i.s nut capable of e.viensive catenation still persists, very large cyclic polysilanes have been synthesized as well as polysilane polymers with molecular weights in the hundreds of thousands." Pure Appl. Cliem. 19K2. 1041-11150. 

---