The S-S metathesis of disulfides is widely used in synthetic and biological chemistry, but generally requires the addition of a chemical reagent or other energy input. B) The S-S metathesis of trisulfides can be induced thermally at elevated temperatures and proceeds through a radical mechanism. This process is very slow, taking hours to reach equilibrium. Disulfides and tetrasulfides also form in this reaction. C) In this study, extremely rapid and reagent-free S-S metathesis of trisulfides was found to occur in polar aprotic solvents such as DMF. The discovery and understanding of this unusual reaction enabled diverse applications in dynamic covalent library synthesis, selective modification of complex natural products, and S-S metathesis polymerization and depolymerization for recyclable plastics.