Polysilanes

Polysilanes are polymers in which there is catenation of silicon, that is, where silicon atoms are bonded to each other in a continuous manner. Synthesis of polysilanes involves the Wurtz coupling of diorganodichlorosilanes with sodium metal (Eq. 2-239) [Baldus and Jansen, 1997 Corriu, 2000 Manners, 1996 Miller and Michl, 1989 West, 1986 West and Maxka, 

1988 Yajima et al., 1978]. The reaction is typically carried out in a hydrocarbon solvent such as toluene, xylene, or octane at temperatures above 100°C. Polymerization can be achieved at ambient temperature in the presence of ultrasound, which produces high temperature and 

The use of a monoalkyltrichlorosilane in the Wurtz-type polymerization is reported to yield (RSi) referred to as polyalkylsilyne [Bianconi et al., 1989]. The elemental composition and NMR spectra of the polymer suggest a three-dimensional crosslinked structure. 

Polysilanes (or polysilylenes) consist of a silicon-catenated backbone with two substituents on each silicon atom (Structure 1). The groups R and R attached to the silicon chain can be of a large variety. Polysilanes with alkyl and/or aryl substituents have been the most thoroughly investigated [1-3], whereas polysilanes having at least a heteroatom substitution such as H, Cl, OR, NR2 have received much less attention [4]. The number of silicon atoms is usually from several hundreds to several thousands. 

Organosilanes in Radical Chemistry C. Chatgilialoglu 2004 John Wiley Sons, Ltd ISBN 0-471-49870-X 

Polysilanes can be regarded as one-dimensional analogues to elemental silicon, on which nearly all of modern electronics is based. They have enormous potential for technological uses [1-3]. Nonlinear optical and semiconductive properties, such as high hole mobility, photoconductivity, and electrical conductivity, have been investigated in some detail. However, their most important commercial use, at present, is as precursors to silicon carbide ceramics, an application which takes no advantage of their electronic properties. 

Among the various synthetic procedures for polysilanes is the Harrod-type dehydrogenative coupling of RSiH3 in the presence of Group 4 metallocenes (Reaction 8.1) [5,6]. One of the characteristics of the product obtained by this procedure is the presence of Si—H moieties, hence the name poly(hydrosilane)s. Since the bond dissociation enthalpy of Si—H is relatively weak when silyl groups are attached at the silicon atom (see Chapter 2), poly(hydrosilane)s are expected to exhibit rich radical-based chemistry. In the following sections, we have collected and discussed the available data in this area. 

Polysilanes (alternative denotations polysilylenes, poly-catena-silicons) of the general structure shown in Chart 7.11 exhibit an absorption band in a relatively long-wavelength region, i.e. between 300 and 400 nm, reflecting the cr-conjuga-tion of electrons in the silicon chain. 

The lifetime of the excited state giving rise to main-chain cleavage is shorter than 100 ps [44]. 

Chart 7.11 Chemical structure of a base unit of polysilane. 

Polysilanes have also been synthesized by dehydrogenative polymerization of silanes (i.e. S1H2R2 -[-Si(R2H-) using zirconocene and h ocene catalysts. So far, these 

The photodegradation and photo-oxidative degradation of different poly-silicones include two main groups polysilanes (section 4.15.1) and poly-siloxanes (section 4.15.2). 

Polysilanes (4.93) are polymers which are rapidly photodegraded by UV irradiation, both in the solid state [172, 173, 898, 1392, 1393, 1486, 2099, 

Polysilanes have an intense absorption in the UV to near-UV region owing to the (T-delocalization along the linear Si—Si polymer backbone (Fig. 4.15). 

The photodegradation of polysilanes occurs by the random chain scission of the Si—Si bond with formation of very reactive silyl (4.94) radicals [1050]  

The quantum yield for the chain scission of poly(methylphenylsilane) in solution is 0 = 0.97, whereas in a thin film, 0 = 0.17 [2135]. The lower quantum yield of chain scission in a film, in comparison to that in solution, can be explained by the cage effect (in the solid), which hinders free motion and favours recombination of reactive sites so formed. 

Polymers with silicon-silicon single bonds in the backbone have been known for some time. It was only within the last lQ-15 years, however, that high molecular weight materials were developed. Behind the current interest in these materials is a realization that they have various potential applications in ceramic fibers, in microlithography,in photoconductionand as nonlinear optical materials. 

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.  

The above illustrated reaction with sodium dispersions requires a greater than 80 °C temperature to proceed satisfactorily. When mixtures of different dialkylsubstituted dichlorosilanes are reacted in this manner, copolymers form.  

Recently, an ambient-temperature sonochemical reductive coupling process was developed. The reaction is carried out in the presence of an ultrasound and results in relatively high (Mn = 50,000-100,000) molecular weight materials with narrow molecular weight distributions. In addition, it was reported that polymers can also be formed by anionic ring-opening polymerization of 

Many different polyarylsiloxanes were reported in the literature. Only a few of them are in general industrial use at present, though many exhibit interesting physical properties and might be used in the future. Preparation of one such material [164] starts with a reaction of aniline with dichlorosilane in the presence of an HCl scavenger  

The product, dianilinosilane, is reacted with diphenols, like hydroquinone  

Polymers prepared by the above procedure have molecular weights up to 80,000 [164]. It is also possible to start with diphenoxysiloxane and catalyze the reaction with sodium or lithium metals. Reactions of cyclic silazanes with arylene disalanols yield polymers with molecular weights as high as 900,000 [164]  

7 Step-Growth Polymerizaticni and Step-Growth Polymers 

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The polysilanes oi polysHylenes (25), an unusual class of inoiganic polymers, saw a very intense period of investigation during the 1980s. 

Ring-Opening Polymerization. As with most other inorganic polymers, ring-opening polymerization of cyclotetrasilanes has been used to make polysilanes (109,110). This method, however, has so far only been used for polymethylphenylsilane (eq. 12). Molecular weights (up to 100,000) are higher than from transition-metal catalyzed polymerization of primary silanes. 

Polymerization ofiVIasked Disilenes. A novel approach, namely, the anionic polymerization of masked disilenes, has been used to synthesize a number of poly(dialkylsilanes) as well as the first dialkylamino substituted polysilanes (eq. 13) (111,112). The route is capable of providing monodisperse polymers with relatively high molecular weight M = lO" — 10 ), and holds promise of being a good method for the synthesis of alternating and block copolymers. 

Electrochemical Synthesis. Electrochemical methods have also been investigated for the synthesis of polysilanes, but these have so far yielded low molecular weight materials (113,114). 

The polysdanes are normally electrical insulators, but on doping with AsF or SbF they exhibit electrical conductivity up to the levels of good semiconductors (qv) (98,124). Conductivities up to 0.5 (H-cm) have been measured. However, the doped polymers are sensitive to air and moisture thereby making them unattractive for practical use. In addition to semiconducting behavior, polysilanes exhibit photoconductivity and appear suitable for electrophotography (qv) (125—127). Polysdanes have also been found to exhibit nonlinear optical properties (94,128). 

Magnesium does not form stable Grignard reagents with siUcon haUdes, although some siUcon halohydrides do react, forming polysilanes (15). 

Dehydrogenative Coupling of Hydride Functional Silanes. The autocouphng of dihydridosilanes was first observed usiag Wilkinson s catalyst (128). A considerable effort has been undertaken to enhance catalyst turnover and iacrease the molecular weight of polysilane products (129) because the materials have commercial potential ia ceramic, photoresist, and conductive polymer technology. 

Another fertile area of current interest is the synthesis of stable homocyclic polysilane derivatives.Typical examples are cyclo-(SiMe2)7, > (cyclo-Si5Me9)-(SiMe2) -(cyclo-SisMeg), n = 2-5, and several new permethy-lated polycylic silanes such as the colourless crystalline compounds bicyclo[3.2.1]-Si8Mei4 (mp 245°), bicyclo[3.3.1]-Si9Mei6 (mp >330°) and... 

Silicon centered radicals can be generated by transfer to silanes and by photolysis of polysilanes. Rate constants for addition to monomer are several orders of magnitude higher than similar carbon centered radicals.453,43 The radicals have nucleophilic character. 

Investigations of silicon-metal systems are of fundamental interest, since stable coordination compounds with low valent silicon are still rare [64], and furthermore, silicon transition-metal complexes have a high potential for technical applications. For instance, coordination compounds of Ti, Zr, and Hf are effective catalysts for the polymerization of silanes to oligomeric chain-silanes. The mechanism of this polymerization reaction has not yet been fully elucidated, but silylene complexes as intermediates have been the subject of discussion. Polysilanes find wide use in important applications, e.g., as preceramics [65-67] or as photoresists [68-83],... 

The coordinated silylenes in both the iron and the chromium compounds can be photolytically activated Photolysis of the complexes in the presence of triphenylphosphine gives the trans-silylene-phosphine complex, which in a second step is transformed into the trnns-bisphosphine compound by excess phosphine. If the silylenes are not trapped, polysilanes are isolated in almost quantitative... 

These and similar complexes of Ti and Zrare effective catalysts for the formation of polysilanes from primary silanes [117-120]. 

The activation of silylene complexes is induced both photochemically or by addition of a base, e.g. pyridine. A similar base-induced cleavage is known from the chemistry of carbene complexes however, in this case the carbenes so formed dimerize to give alkenes. Finally, a silylene cleavage can also be achieved thermally. Melting of the compounds 4-7 in high vacuum yields the dimeric complexes 48-51 with loss of HMPA. The dimers, on the other hand, can be transformed into polysilanes and iron carbonyl clusters above 120 °C. In all cases, the resulting polymers have been identified by spectroscopic methods. 

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],... 

Recently, the compounds CpCp Hf(Cl)Si(SiMe3)3 and CpCp Zr(Cl)Si(SiMe3)3 have been used as homogenous catalysts for the formation of polysilanes. 

Polysilanes (or polysilylenes) consist of a silicon-catenated backbone with two substituents on each silicon atom. The two groups attached to the silicon chain... 

Analogous to terminal alkenes, the reaction of 123 with valeraldehyde and cyclohexanone under radical-based conditions allowed for the preparation of the corresponding functional polysilanes 126 (Reaction 90). The efficiency of Si-H bond replacement was 80-85%... 

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