{{Short description, Family of inorganic polymers containing a tin-tin backbone Polystannanes are

organotin compound Organotin compounds or stannanes are chemical compounds based on tin with hydrocarbon substituents. Organotin chemistry is part of the wider field of organometallic chemistry. The first organotin compound was diethyltin diiodide (), discovered by ...

s with the formula (R₂Sn)_n. These polymers have been of intermittent academic interest; they are unusual because heavy elements comprise the backbone. Structurally related but better characterized (and more useful) are the

polysilane 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 use ...

s (R₂Si)_n.

History

Oligo- or polystannanes were first described by Löwig in 1852, only 2 years after

Edward Frankland Sir Edward Frankland, (18 January 18259 August 1899) was an English chemist. He was one of the originators of organometallic chemistry and introduced the concept of combining power or valence. An expert in water quality and analysis, he was a ...

's report on the isolation of the first

organotin Organotin compounds or stannanes are chemical compounds based on tin with hydrocarbon substituents. Organotin chemistry is part of the wider field of organometallic chemistry. The first organotin compound was diethyltin diiodide (), discovered by ...

compounds. Löwig' route involved treating an Sn/K and Sn/Na alloys with

iodoethane Ethyl iodide (also iodoethane) is a colorless flammable chemical compound. It has the chemical formula C2H5I and is prepared by heating ethanol with iodine and phosphorus.''Merck Index of Chemicals and Drugs'', 9th ed., monograph 3753 On contact ...

, in the presence of quartz sand which was used to control the

reaction rate The reaction rate or rate of reaction is the speed at which a chemical reaction takes place, defined as proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per unit ...

. Products with elemental compositions close to those of oligo(diethyl

stannane Stannane or tin hydride is an inorganic compound with the chemical formula . It is a colourless gas and the tin analogue of methane. Stannane can be prepared by the reaction of and . : Stannane decomposes slowly at room temperature to give ...

)s or poly(diethylstannane) were obtained. Cahours obtained similar products and attributed the formation of the so-called "stannic ethyl" to a reaction of the Wurtz type. Already in 1858, "stannic ethyl" was formulated as a polymeric compound denoted with the composition n(SnC₄H₅). In 1917 Grüttner, who reinvestigated results on hexaethyl- distannanes(H₅C₂)₃Sn-Sn(C₂H₅)₃ (reported by Ladenburg in 1870) confirmed the presence of Sn-Sn bonds and predicated for the first time that tin could form chain like compounds. In 1943, it was postulated that “diphenyltin” exists as a type of polymeric material because of its yellow color and indeed a

bathochromic shift Bathochromic shift (from Greek βαθύς ''bathys'', "deep"; and χρῶμα ''chrōma'', "color"; hence less common alternate spelling "bathychromic") is a change of spectral band position in the absorption, reflectance, transmittance, or emissi ...

of the wavelength at maximum absorption with increasing number of Sn atoms was found later in the case of oligo(dibutylstannane)s comprising up to 15 Sn atoms. The

Wurtz reaction In organic chemistry, the Wurtz reaction, named after Charles Adolphe Wurtz, is a coupling reaction whereby two alkyl halides are treated with sodium metal to form a higher alkane. : 2 R−X + 2 Na → R−R + 2 NaX The reaction is of little v ...

is still used for the preparation of poly(dialkylstannane)s. Treatment of dialkyltin dichlorides with sodium lead to polystannanes of high

molar mass In chemistry, the molar mass of a chemical compound is defined as the mass of a sample of that compound divided by the amount of substance which is the number of moles in that sample, measured in moles. The molar mass is a bulk, not molecular, p ...

, however, in low yields and with formation of (cyclic) oligomers. 1-16 Other efforts to prepare high molar mass polystannanes by electrochemical reactions or by catalytic dehydropolymerization of dialkylstannanes (R₂SnH₂) were also made 6 Unfortunately, frequently, the polymers prepared by those methods were not isolated and typically contained significant fractions of cyclic oligomers.

Linear polystannanes

: : Dialkytin dihydrides (R₂SnH₂) were reported in 2005 to undergo dehydropolymerization in the presence of Wilkinson’s catalyst. This method afforded polystannanes without detectable amounts of "cyclic"-byproducts. The polymers were yellow with number average molar masses of 10 to 70 kg/mol and a

polydispersity In chemistry, the dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. A collection of objects is called uniform if the objects have the same size, shape, or mass. A sample of objects that have an incons ...

of 2 – 3. By variation of the catalyst concentration the molar masses of the synthesized polymers could be adjusted. A strong influence of the temperature on the degree of conversion was observed. Determination of the molar mass at different degrees of conversion indicated that polymerization did not proceed according to a statistical condensation mechanism, but, likely, by growth onto the catalyst, e.g. by insertion of SnR₂-like units. The poly(dialkylstannane)s were found to be

thermotropic A liquid crystal phase is thermotropic if its order parameter is determined by temperature. At high temperatures, liquid crystals become an isotropic liquid and at low temperatures, they tend to glassify. In a thermotropic crystal, those phase tr ...

and displayed first-order

phase transition In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states of ...

s from one liquid-crystalline phase into another or directly to the isotropic state, depending on the length of the side groups. More specifically, poly(dibutylstannane) for example showed an endothermic phase transition at ~0 °C from a rectangular to a pure

nematic Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way. The ...

phase, as determined by

X-ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

. As expected, polystannanes were semi-conductive. Temperature-dependent, time-resolved pulse radiolysis microwave conductivity measurements of poly(dibutylstannane) yielded values of charge-carrier mobilities of 0.1 to 0.03 cm2 V⁻¹ s⁻¹, which are similar to those found for pi-bond-conjugated carbon-based polymers. By partial oxidation of the material with SbF₅ conductivities of 0.3 S cm⁻¹ could be monitored.T. Imori, V. Lu, H. Cai, T. D. Tilley, ''J. Am. Chem. Soc.'', 1995, ''117'', 9931. The liquid-crystalline characteristics of the poly(dialkylstannane)s permitted facile orientation of these macromolecules, for instance, by mechanical shearing or tensile drawing of blends with poly(ethylene). Poly(dialkylstannane)s with short side groups invariably arranged parallel to the external orientation direction, while the polymers with longer side groups had a tendency to order themselves perpendicular to that axis.

References

External links

* Fabien Choffat (2007
Polystannane
Doctoral dissertation, Swiss Federal Institute of Technology, Zürich. Polymers Inorganic polymers Conductive polymers Plastics Organotin compounds Tin(II) compounds