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112 Ralf Steudel mer can exist below the floor temperature, and above the floor temperature the polymer concentration rises steeply. The polymer dissolved in liquid S8 must be in equilibrium with all dis- solved species. Steudel et al. have shown in 1984 that the addition of an S7 ring to a polymeric diradical-chain is exothermic and endentropic if pure liquid S7 is considered [124]. Therefore, pure cyclo-S7 is thermodynamically unstable with respect to polymeric chains at all temperatures while in the case of pure cyclo-S6 the ring addition is exothermic and exentropic result- ing in a (very high) “ceiling temperature” for the polymerization above which no polymer can exist [124]. Since the enthalpy and entropy data for the ring addition reactions of S6 and S7 are not accurately known the conse- quences for liquid sulfur can be discussed only qualitatively. At the so-called onset of the polymerization at 157 C liquid sulfur contains about 1% S6 and 5% S7 [93]. The term R·ln c in Eq. (23) has then values of 38 J mol1 K1 for S6 and 25 J mol1 K1 for S7. Under these circumstances the addition of both rings in liquid sulfur to the polymer is exentropic. However, the ceiling temperatures cannot be calculated as long as the necessary polymerization enthalpies and entropies are not known accurately. Whether the polymer in liquid sulfur actually consists of long chains or very large rings does not make any difference since the thermodynamic properties of the ring addition reactions (Eq. 21) are independent of the structure of the polymer as long as it is macromolecular. In fact, it is highly unlikely that the polymer present in liquid sulfur at temperatures below 157 C is exclusively chain-like. This follows not only from the high dissoci- ation enthalpy of S-S bonds of 150 kJ mol1 which is not available at moder- ate temperatures. More convincing is the following argument: the viscosity minimum of liquid sulfur is observed at 157 C at which temperature the melt contains already ca. 3% of polymeric sulfur! At 157 C the viscosity suddenly rises sharply while the polymer content rises only gradually. Obvi- ously, a new type of polymer is formed at this temperature which must be responsible for the dramatic increase in viscosity. This polymer is evidently chain-like and a diradical in agreement with the first observation of free rad- icals near 153 C by ESR spectroscopy [38]. Only chain-like macromolecules can form catenane-like molecules and entangled chains which cannot longer flow and slip easily without breaking covalent bonds and which therefore give rise to the observed viscosity increase. This view is supported by the analytical data of quenched melts which showed the presence of relatively large rings up to at least S35 (see above). There is no reason to exclude even larger rings which, above a certain molecular mass, will be insoluble in car- bon disulfide and therefore qualify as “polymeric sulfur”. On quenching and extraction the long diradical-chains of polymeric sul- fur will pick up some impurity atoms from the solvent or from the air to chemically satisfy the chain ends with, for example, H or OH groups. There- fore, polymeric sulfur prepared in this way is diamagnetic (Sm). If this view is correct the polymer generated at temperatures below 157 C is mainly composed of very large rings, formed by some kind of ring-fusion reaction with no or little involvement of radicals, while the polymer formed at tem-PDF Image | Topics in Current Chemistry
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