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Solid Sulfur Allotropes 41 romolecular sulfur are known at standard temperature-pressure (STP) con- ditions: Two slightly different fibrous forms and a laminar form. The struc- ture of one of the fibrous allotropes (Sy) is well characterized. The macro- molecular allotropes can be obtained only by special methods of prepara- tion; for example, by quenching of liquid sulfur and stretching of the fibers obtained, by chemical reactions including decomposition of unstable al- lotropes, by UV-Vis irradiation, by drawing filaments from the melt, by sub- limation of sulfur vapor, and under the action of high pressure. The materi- als thus obtained are in general mixtures of the polymer and of rings of dif- ferent sizes. Such preparations are unstable and convert more or less slowly to the stable allotrope a-S8. The lifetime of the polymer content depends critically on the method of preparation and on the temperature of storage. At room temperature lifetimes of up to years have been reported. All of the polymeric forms studied up to now at ambient pressure seem to consist of the same type of helix conformation with lengths of 104 to 106 atoms. Left- handed and right-handed conformers of the helix will normally be present in one and the same allotrope. In the past, the various techniques of preparation led to rather different results and, therefore, the elucidation of the structures of the polymeric al- lotropes was difficult. For example, fibrous sulfur is typically obtained by quenching a sulfur melt from above the polymerization temperature (Tc~432 K) and subsequent extraction of the ring content as well as by stretching the fibers. Since the melt consists, besides the polymer fraction, of rings of different sizes in a temperature dependent equilibrium (see [43, 48, 49, 106]), this composition is commonly believed to be frozen by rapid quenching at temperatures well below 0 C. However, the temperature of the melt, the temperature gradient of quenching (dT/dt), and the final temper- ature of the quenched material have considerable impact on the resultant product. In addition, the time of the washing procedure with respect to the time of stretching induced crystallization determines the polymer content [50, 54, 107, 108]. The configuration of the polymeric molecules and their spatial arrangements in the solid state are affected by the different treat- ments too. It is not unusual in the history of structural determination of sul- fur allotropes that the different findings reported in the literature resulted in a confusing nomenclature. In Table 18 we present a summary of the names used for polymeric sulfur. At that point, we are inclined to emphasize the necessity to label the polymeric allotropes in a consistent manner and—if delivering reports on new allotropes—to give a detailed description of the methods and conditions of preparation. For example, the polymer fraction in the liquid and in the quenched melt was often termed as S regardless of the possibility that the molecular structures might be quite different. Fur- thermore, polymeric sulfur in the solid state comes along in different forms which may have a structural variety ranging from crystalline to amorphous (random coil) states as well as mixtures of both and mixtures of chains with very large rings. Therefore, we recommend to distinguish between the poly- meric molecules in the liquid (S1) and the insoluble, polymeric content of a solid material (S) isolated from the melt, for example. Accordingly, the termPDF Image | Topics in Current Chemistry
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