Polysilanes are organosilicon compounds with the formula (R2Si)n. They are relatives of traditional organic polymers but their backbones are composed of silicon atoms. They exhibit distinctive optical and electrical properties. They are mainly used as precursors to silicon carbide. [1] The simplest polysilane would be (SiH2)n, which is mainly of theoretical, not practical interest. [2]
The first polysilane, poly(dimethylsilylene), [(CH3)2Si]x, was reported in 1949 by Charles A. Burkhard (1916 - 1991) of General Electric. It was prepared by heating sodium metal with dimethyldichlorosilane:
The modified Wurtz coupling of dichlorosilanes remains a viable and general route to high molecular weight, linear polysilane derivatives. [4] This reaction is conducted at elevated temperature in an inert solvent using a dispersion of the alkali metal. The polymerization stops with the addition of an alcohol. The major limitation with the Wurtz-type polymerization is that the substituents must tolerate the vigorous reaction conditions. The reaction works well for methyl, benzyl, and phenyl substituents. [5] With the rigorous conditions, the yield of the product ranges from a few percent to approximately 50%. Similarly, potassium-graphite (KC8) can be used at much lower temperatures than those required for traditional Wurtz coupling. [6] This reaction typically produces a trimodal distribution of products: a low molecular weight fraction and two higher molecular weight fractions. The low molecular weight fraction consists of five and six-membered rings, i. e. [SiR2]5 and [SiR2]6. Formation of these rings competes with the growth of the polymer. [6] Another method for the synthesis of polysilanes is dehydrogenative coupling of silanes.
The product obtained by Burkhard was difficult to work because it was insoluble in organic solvents. Interest in the polysilanes resumed in the early 1980s when it was reported that [(CH3)2Si]x can be converted to silicon carbide by thermolysis.
Polysilanes range from highly crystalline (and generally insoluble) to amorphous materials, which are more soluble in organic solvents. Decreasing the symmetry and lengthening the organic substituents lowers the crystallinity. Many polysilanes are rubbery elastomers. When doped with oxidizing agents (SbF5, iodine, FeCl3, ferrocinium), the polymers become semiconductors. Most are stable to nearly 300 °C and, in contrast to the polysilicon hydrides, are inert to oxygen at normal temperatures. They are not easily hydrolyzed. Polysilanes exhibit photoconductivity, although degrade when exposed to ultraviolet light. [5] The hydrogen atoms of the higher-dimensional polysilicon hydrides may also be substituted with organic side-groups to give random network organosilicon polymers but these retain the polysilyne base name, for example, as in polymethylsilyne. 29Si NMR spectroscopy provides insights into the microstructure of a polymer. If resonances are broad, oligomerization is likely; if they are sharp, some sort of pattern in the silicon backbone can be inferred.
Yajima and coworkers discovered that the pyrolysis of [Me2Si]n leads to the formation of SiC fibers. This transformation has kindled research on polysilanes and their derivatives. [6] As preceramic polymers, polycarbosilanes can be used to produce dense silicon carbide and silicon oxycarbide through pyrolysis in inert atmospheres. Photopolymerisation of modified polysilanes in stereolithography followed by ceramization is an emerging route towards the additive manufacturing of ceramics. [7]
Polysilanes exhibit σ-delocalization. This characteristic stems from the low ionization energy for electrons in Si-Si sigma bonds relative to that of C-C sigma bonds, for instance. Accordingly, they absorb strongly in the UV-region (300-400 nm) due to intense σ-σ* electronic transitions.6 Polysilanes degrade in the presence of UV light since σ-σ* electronic transitions can be thought of as bonds breaking, often precluding some applications. Dialkyl polysilanes tend to have a band gap of about 4.5 eV. Introduction of an aryl substituent to each silicon lowers the band gap to about 3.5 eV, making for a borderline semiconductor. [5] [6]
Polysilynes are a related class of organosilicon compounds with the formula (RSi)n (R = alkyl). They are more highly cross linked than polysilanes and have been less studied.
In inorganic chemistry, silenes, or disilalkenes, are silicon compounds that contain Si=Si double bonds. The parent molecule is disilene, Si2H4.
In organosilicon chemistry, a siloxane is an organic compound containing a functional group of two silicon atoms bound to an oxygen atom: Si−O−Si. The parent siloxanes include the oligomeric and polymeric hydrides with the formulae H[OSiH2]nOH and [OSiH2]n. Siloxanes also include branched compounds, the defining feature of which is that each pair of silicon centres is separated by one oxygen atom. The siloxane functional group forms the backbone of silicones [−R2Si−O−SiR2−]n, the premier example of which is polydimethylsiloxane (PDMS). The functional group R3SiO− is called siloxy. Siloxanes are manmade and have many commercial and industrial applications because of the compounds’ hydrophobicity, low thermal conductivity, and high flexibility.
In inorganic chemistry, chlorosilanes are a group of reactive, chlorine-containing chemical compounds, related to silane and used in many chemical processes. Each such chemical has at least one silicon-chlorine bond. Trichlorosilane is produced on the largest scale. The parent chlorosilane is silicon tetrachloride.
Organosilicon chemistry is the study of organometallic compounds containing carbon–silicon bonds, to which they are called organosilicon compounds. Most organosilicon compounds are similar to the ordinary organic compounds, being colourless, flammable, hydrophobic, and stable to air. Silicon carbide is an inorganic compound.
Silanes are saturated chemical compounds with the empirical formula SixHy. They are hydrosilanes, a class of compounds that includes compounds with Si−H and other Si−X bonds. All contain tetrahedral silicon and terminal hydrides. They only have Si−H and Si−Si single bonds. The bond lengths are 146.0 pm for a Si−H bond and 233 pm for a Si−Si bond. The structures of the silanes are analogues of the alkanes, starting with silane, SiH4, the analogue of methane, continuing with disilane Si2H6, the analogue of ethane, etc. They are mainly of theoretical or academic interest.
Organogermanium chemistry is the science of chemical species containing one or more C–Ge bonds. Germanium shares group 14 in the periodic table with carbon, silicon, tin and lead. Historically, organogermanes are considered as nucleophiles and the reactivity of them is between that of organosilicon and organotin compounds. Some organogermanes have enhanced reactivity compared with their organosilicon and organoboron analogues in some cross-coupling reactions.
Silicon compounds are compounds containing the element silicon (Si). As a carbon group element, silicon often forms compounds in the +4 oxidation state, though many unusual compounds have been discovered that differ from expectations based on its valence electrons, including the silicides and some silanes. Metal silicides, silicon halides, and similar inorganic compounds can be prepared by directly reacting elemental silicon or silicon dioxide with stable metals or with halogens. Silanes, compounds of silicon and hydrogen, are often used as strong reducing agents, and can be prepared from aluminum–silicon alloys and hydrochloric acid.
Bis(trimethylsilyl)amine (also known as hexamethyldisilazane and HMDS) is an organosilicon compound with the molecular formula [(CH3)3Si]2NH. The molecule is a derivative of ammonia with trimethylsilyl groups in place of two hydrogen atoms. An electron diffraction study shows that silicon-nitrogen bond length (173.5 pm) and Si-N-Si bond angle (125.5°) to be similar to disilazane (in which methyl groups are replaced by hydrogen atoms) suggesting that steric factors are not a factor in regulating angles in this case. This colorless liquid is a reagent and a precursor to bases that are popular in organic synthesis and organometallic chemistry. Additionally, HMDS is also increasingly used as molecular precursor in chemical vapor deposition techniques to deposit silicon carbonitride thin films or coatings.
In polymer chemistry, an inorganic polymer is a polymer with a skeletal structure that does not include carbon atoms in the backbone. Polymers containing inorganic and organic components are sometimes called hybrid polymers, and most so-called inorganic polymers are hybrid polymers. One of the best known examples is polydimethylsiloxane, otherwise known commonly as silicone rubber. Inorganic polymers offer some properties not found in organic materials including low-temperature flexibility, electrical conductivity, and nonflammability. The term inorganic polymer refers generally to one-dimensional polymers, rather than to heavily crosslinked materials such as silicate minerals. Inorganic polymers with tunable or responsive properties are sometimes called smart inorganic polymers. A special class of inorganic polymers are geopolymers, which may be anthropogenic or naturally occurring.
Dimethyldichlorosilane is a tetrahedral organosilicon compound with the formula Si(CH3)2Cl2. At room temperature it is a colorless liquid that readily reacts with water to form both linear and cyclic Si-O chains. Dimethyldichlorosilane is made on an industrial scale as the principal precursor to dimethylsilicone and polysilane compounds.
Methyltrichlorosilane, also known as trichloromethylsilane, is a monomer and organosilicon compound with the formula CH3SiCl3. It is a colorless liquid with a sharp odor similar to that of hydrochloric acid. As methyltrichlorosilane is a reactive compound, it is mainly used a precursor for forming various cross-linked siloxane polymers.
In organosilicon chemistry, polysilazanes are polymers in which silicon and nitrogen atoms alternate to form the basic backbone. Since each silicon atom is bound to two separate nitrogen atoms and each nitrogen atom to two silicon atoms, both chains and rings of the formula [R2Si−NR]n occur. R can be hydrogen atoms or organic substituents. If all substituents R are hydrogen atoms, the polymer is designated as perhydropolysilazane, polyperhydridosilazane, or inorganic polysilazane ([H2Si−NH]n). If hydrocarbon substituents are bound to the silicon atoms, the polymers are designated as Organopolysilazanes. Molecularly, polysilazanes [R2Si−NH]n are isoelectronic with and close relatives to polysiloxanes [R2Si−O]n (silicones).
Polysilicon hydrides are polymers containing only silicon and hydrogen. They have the formula where 0.2 ≤ n ≤ 2.5 and x is the number of monomer units. The polysilicon hydrides are generally colorless or pale-yellow/ocher powders that are easily hydrolyzed and ignite readily in air. The surfaces of silicon prepared by MOCVD using silane (SiH4) consist of a polysilicon hydride.
Polysilicon halides are silicon-backbone polymeric solids. At room temperature, the polysilicon fluorides are colorless to yellow solids while the chlorides, bromides, and iodides are, respectively, yellow, amber, and red-orange. Polysilicon dihalides (perhalo-polysilenes) have the general formula (SiX2)n while the polysilicon monohalides (perhalo-polysilynes) have the formula (SiX)n, where X is F, Cl, Br, or I and n is the number of monomer units in the polymer.
In organosilicon chemistry, polysilynes are chemical compounds with the formula [RSi]n, where R can be hydrogen, or organyl. Although their name suggests a relationship to alkynes, polysilynes are a class of silicon-based random network polymers primarily composed of tetrahedral silicon atoms, each connected to one hydrogen or carbon and three Si atoms. These compounds are prepared by Wurtz coupling of alkyltrichlorosilanes :
The term preceramic polymer refers to one of various polymeric compounds, which through pyrolysis under appropriate conditions are converted to ceramic compounds, having high thermal and chemical stability. Ceramics resulting from the pyrolysis of preceramic polymers are known as polymer derived ceramics, or PDCs. Polymer derived ceramics are most often silicon based and include silicon carbide, silicon oxycarbide, silicon nitride and silicon oxynitride. Such PDCs are most commonly amorphous, lacking long-range crystalline order.
Polymer derived ceramics (PDCs) are ceramic materials formed by the pyrolysis of preceramic polymers, usually under inert atmosphere.
In organosilicon chemistry, silanes are a diverse class of charge-neutral organic compounds with the general formula SiR4. The R substituents can any combination of organic or inorganic groups. Most silanes contain Si-C bonds, and are discussed under organosilicon compounds. Some contain Si-H bonds and are discussed under hydrosilanes.
(Trimethylsilyl)methyllithium is classified both as an organolithium compound and an organosilicon compound. It has the empirical formula LiCH2Si(CH3)3, often abbreviated LiCH2tms. It crystallizes as the hexagonal prismatic hexamer [LiCH2tms]6, akin to some polymorphs of methyllithium. Many adducts have been characterized including the diethyl ether complexed cubane [Li4(μ3-CH2tms)4(Et2O)2] and [Li2(μ-CH2tms)2(tmeda)2].
Dodecamethylcyclohexasilane is the organosilicon compound with the formula Si6(CH3)12. It is one of the more readily prepared and easily handled polysilanes. Dodecamethylcyclohexasilane is produced by reduction of dimethyldichlorosilane with sodium-potassium alloy: