Polystannane



Tin is the only known metallic element that has been reported to form organo-metallic polymers comprising a backbone of metal atoms that are bonded to each other by covalent bonds. Such polymers, referred to as polystannanes, are expected to exhibit electronic activity, because of electron delocalization along the tin backbone.



Introduction


Remarkably, the first preparation of oligo- or polystannanes was published by Löwig already in 1852 [1], only 2 years after Franklands report [2] on the isolation of organotin compounds (considered to be the first publication on such compounds). Löwig prepared oligo- or polystannanes by an exothermic reaction of iodoethane with a Sn/K or a Sn/Na alloy, in the presence of quartz sand which was used to control the reaction rate. After work-up of the reaction mixture, fractions of organotin compounds with elemental compositions close to those of oligo(diethylstannane)s or poly(diethylstannane) were obtained. Cahours [3, 4] obtained similar products and attributed the formation of the so-called stannic ethyl to a reaction of the Wurtz type. Interestingly, already in 1858, stannic ethyl was formulated as a polymeric compound denoted with the composition n(SnC4H5) [5]. In 1917 Grüttner [6], who reinvestigated results on hexaethyl-distannanes(H5C2)3Sn-Sn(C2H5)3 reported by Ladenburg in 1870 [7] 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 [8], and indeed a bathochromic shift 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 [9].

The Wurtz reaction [10] is still used for the preparation of poly(dialkylstannane)s. Treatment of dialkyltin dichlorides with sodium lead to polystannanes of high molar mass, however, in low yields and with formation of (cyclic) oligomers during the reactions [11-16]. Other synthetic efforts to prepare high molar mass polystannanes by electrochemical reactions [17, 18] or by catalytic dehydropolymerization of dialkylstannanes (R2SnH2) were also made [16, 19-22]. Unfortunately, frequently, the polymers prepared by those methods were not isolated and typically contained significant fractions of cyclic oligomers.

Synthesis and Characterization of Pure Linear Polystannanes




A facile dehydropolymerization of dialkytin dihydrides (R2SnH2) with Wilkinson’s catalyst, was found in 2005 [23], which provide polystannanes without detectable amounts of "cyclic"-byproducts. This convenient, rapid and high-yield synthesis was employed to produce a variety of polystannanes comprising different side groups. The polymers synthesized were of a yellow color, featured consistencies that ranged from soft to honey-like, number average molar masses of 10 to 70 kg/mol and a polydispersity 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, probably by a stannylen (SnR2) insertion.

The poly(dialkylstannane)s were found to be thermotropic and displayed first-order phase transitions 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 phase, as determined by X-ray diffraction.

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 [24]. By partial oxidation of the material with SbF5 conductivities of 0.3 S cm-1 could be monitored [16].

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). Interestingly, 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.