Depolymerization of Polymeric Sulfur

Cyclo-octa-sulfur, c-S8, in the orthorhombic crystal, α-S, has two low energy rotational states of S-S bonds at about ± 99°; the SSS bond angle is ca. 108°. On heating, α-S first melts and then undergoes a polymerization. Many different forms of polymeric sulfur are recognized in the literature – we will lump these together as p-S. A commercially useful form of p-S is compatible with rubbery polymers, but the reversion of p-S to the stable α-S occurs readily. To understand the reversion, we have performed high-level quantum calculations to evaluate the relative energetics of (1) the reverse of the radical polymerization mechanism, and (2) internal loop formation that pinches c-S8 rings out from long chains via a hypervalent intermediate. We show that both mechanisms are viable routes to depolymerization.