Dynamic combinatorial library synthesis. The rapid and reversible nature of the trisulfide metathesis in DMF motivated us to explore this chemistry in the preparation of a dynamic combinatorial library. Such libraries have been explored extensively in drug discovery 48 and supramolecular chemistry. Disulfide exchange has been used previously in these dynamic covalent systems, but it often suffers from low reaction rates and sensitivity to pH. In contrast, S-S metathesis of trisulfides in DMF is extremely fast and can generate a dynamic covalent library in minutes at room temperature. No exogenous reagent or initiator is required: DMF was simply added to a mixture of trisulfides, and all expected trisulfide permutations were formed within 5 minutes (Figure and S217-S223). This dynamic covalent system showcases the high rate and reversibility of the novel trisulfide metathesis, as well as its fidelity in generating only trisulfide products under these conditions. Platform for chemically recyclable polymers. The rapid and reversible nature of the featured trisulfide metathesis also prompted us to apply this reaction to the synthesis of poly(trisulfide) polymers. It was envisioned that polymerization of a monomer containing two trisulfide could be favored by formation of a volatile co-product (such as dimethyl trisulfide) along with the target polymer. We were also excited by the prospect of depolymerizing the polymer by the same S-S metathesis reaction. Such chemically recyclable polymer systems are becoming increasingly important in the effort to make plastics and other macromolecular materials more sustainable and aligned with the principles of a circular economy. First, a bis(trisulfide) monomer (Figure ) was prepared on a multi-gram scale in 3 steps from commercially available 1,12-dibromododecane using a series of high-yielding substitution reactions (S224-S233). This liquid substance was stable when stored neat and did not react in polar solvents such as toluene, even when heated to 100 ºC (S234-S236). In contrast, polymerization could be induced by simply dissolving the monomer in NMP. To drive the reversible reaction to polymer product, the mixture was heated to 60 ºC and a vacuum (1 mbar) was applied to remove the dimethyl trisulfide co-product by distillation (Figure polymer, with melting transitions detected at 41.5 and 46.9 ºC (Figure ). This semi-crystalline nature reflects the regular, unbranched structure of the polymer, which can be considered an analogue of high-density polyethylene (HDPE). The infrared spectrum (Figure ) is also very similar to that of HDPE. Importantly, Raman spectroscopy revealed high fidelity propagation of the trisulfide structure throughout the polymer, with a characteristic signal for the trisulfide at 487 cm -1 (Figure ). Comparison of the Raman spectra of authentic disulfide or tetrasulfide reference samples revealed that no disulfides and no tetrasulfides were detectable in the polymer (S240). Energydispersive X-ray spectroscopy (EDX) indicated a uniform distribution of sulfur across the surface of the polymer (S244). The poly(trisulfide) could also be readily processed by injection molding to provide free-standing objects (S251).