The stability of cyclo-octasulfur, c-S8, results from the existence of two low energy rotational states of S-S bonds at about ± 98°. The S-S-S bond angle is near 107°. When heated, the orthorhombic form, α-S, first melts and then undergoes a polymerization, which is believed to be initiated by the thermal ring opening of c-S8 (the bond energy is about 265 kJ/mol) to form a diradical. Propagation occurs by an attack of a radical on an S8 ring, leading to a ring opening polymerization mechanism. Chains might be terminated with live radicals or by non-specific quenching of the radicals; there will undoubtedly be cyclics as well. Different forms of polymeric sulfur are recognized in the literature – we will lump these together as p-S. A commercial useful form of p-S is compatible with rubbery polymers, making it possible to form a homogeneous blend prior to vulcanization. However, the reversion of p-S to the thermodynamically stable α-S limits the shelf life of p-S. In order to understand the reversion, we have performed high level quantum calculations to evaluate the relative energetics of (1) the reverse of the radical mechanism, whereby polymeric chains spontaneously break to form two radicals which then induce depolymerization by back-biting or similar mechanisms, and (2) an internal loop formation which pinches S8 rings out from long chains via a hypervalent intermediate.

Depolymerization of polymeric sulfur

2019 Fall National Meeting

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