Silicon alkyls, pyrolysis

Polymeric silicon alkyl compounds Polymeric compounds are often observed in the Wurtz synthesis of dihalodialkyl(aiyl)silanes with metals. The intermediate product is probably a dialkyl(aryl)silylene (see Chapter 7). A polymeric dimethylsilane was found by Burkhard68). A yellow polymeric (SiCH3) was isolated after pyrolysis of SiH4 with ethylene156). 

Silicone paints are formed by controlled hydrolysis and condensation of alkyl alkox-ysilanes, and may be encountered either alone or in formulations with other synthetic resins. The typical structural unit in the polymer chain is dimethyl siloxane, and pyrolysis of such resins takes place with random chain scission and the extended formation of stable cyclic fragments. In Figure 12.14 the pyrogram of a silicone resin is shown, with cyclic siloxane oligomers eluting at the shorter retention times, followed by the linear siloxane fragments. 

It is also possible to produce covalently bonded alkyl MLs on Si(l 11) surfaces using a variety of chemical reactions with passivated H-terminated Si(l 11), but the preparation methods are more complex than the immersion strategy in part due to the higher reactivity of silicon. This is a major achievement because it allows direct coupling between organic and bio-organic materials and silicon-based semiconductors. Both pyrolysis of diacyl peroxides (Linford Chidsey, 1993) and Lewis acid-catalyzed hydrosilylation of alkenes and direct reaction of alkylmagnesium bromide (Boukherroub et al, 1999) on freshly prepared Si(lll)-H produce surfaces with similar characterishcs. These surfaces are chemically stable and can be stored for several weeks without measurable deterioration. Thienyl MLs covalently bonded to Si(l 11) surfaces have also been obtained, in which a Si(l 11)-H surface becomes brominated, Si(lll)-Br, and is further reacted with lithiated thiophenes (He etal, 1998). 

Since an insertion reaction into alkyl or aryl silicon bonds seems to be difficult, a radical mechanism usually takes place in the pyrolysis of such compounds. On the other hand, at Si—H, Si—Hal, Si—OR and Si—N bonds, a silylene mechanism occurs because the insertion reaction has only a small activation energy. 

Boron-containing organosilicon polymers have also been synthesized via a sodium coupling reaction of silicon and boron halides or alkyl boron halides in hydrocarbon solvents [37]. On pyrolysis, mixed ceramics comprising SiC, SiBjt, and Si-(B, C) can be obtained. 

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