With a mercury-and cyanide-free leach solution developed, the next step was to identify a sorbent that could selectively bind to leached gold and remove it from solution. It was hypothesized that solid polysulfide polymer sorbents could serve a dual role of quenching active oxidants in the leach solution and binding gold selectively. Polysulfide polymers have recently been shown by Hasell and coworkers to have a high affinity for gold ions (39), but their effectiveness in complex leach solutions and methods for gold separation from the sorbent have not been established. Importantly, the introduction of Pyun's inverse vulcanization concept and related copolymerizations of sulfur and sulfur-rich monomers have made this class of polymers widely accessible . In our own prior research, we developed such a copolymer made from unsaturated triglycerides such as canola oil and sulfur using inverse vulcanization (poly-1, Fig. ) , which has subsequently been scaled to tonne quantities for commercial mercury remediation (poly-1, Fig. ) . Because a pathway to scale-up for this polymer has already been established, assessing its effectiveness and selectivity for gold sorption was a logical next-step. In parallel, a new photochemical ring-opening polymerization was invented to provide ready access to a linear polysulfide polymer (poly-2) (Fig. and Fig. ). This polymer was sought as a gold sorbent candidate because of its ability to undergo thermal depolymerization back to the norbornane trisulfide (monomer 2) . This would allow for gold binding and then separation of the polymer from gold by depolymerization and distillation of 2. This concept was recently validated by us using a version of poly-2 prepared by electrochemical polymerization . However, upscaling this electrochemical synthesis is complicated by the requirement to recycle costly electrolytes. It was therefore hypothesized that ring-opening polymerization could be initiated photochemically, which would dramatically simplify the synthesis of poly-2. Notably, the proposed photochemical polymerization had been attempted previously by Emsley and Griffiths for a related cyclic trisulfide, but only low molecular weight species such as trimers were formed . For our gold sorption process, we did not need high molecular weight polymers, but we required a sufficient degree of polymerization to convert 2 (a liquid) into a solid polymer sorbent that could be mixed with the leach solution and recovered by simple filtration. We hypothesized that the photoinitiation process would compete with photochemical cleavage of S-S bonds in the polymer product. To initiate polymerization and minimize cleavage of S-S bonds in the polymer product, a photochemical flow reactor was therefore used to control the initiation event in space and time. In this process, a 1.0 M solution of monomer 2 in THF was passed through a photochemical reactor illuminated with light emitting diodes (365 nm, 22.6 W). The residence time was a mere 1.6 minutes, which was sufficient to initiate photochemical ringopening polymerization and flush the resulting polymeric material to the collection reservoir. The polymer could be selectively precipitated and separated from unreacted monomer, which could be recycled and repolymerized. Poly-2 was isolated in 50% yield (79% based on recovered 2) and displayed a Mw = 5640 g/mol and Đ = 1.8. It was also confirmed in control experiments that poly-2 can be photochemically converted into lower molecular weight species, including regeneration of monomer 2, using the same photochemical reactor (Fig. ). This result highlights the critical importance of residence time, concentration, and continuous flow processing that allows the polymerization to be initiated by light, while minimizing photolysis of S-S bonds in the target polymer. Indeed, by varying the residence time and concentration, molecular weights of poly-2 exceeding 20,000 g/mol were accessible (Table ). 1 H and 13 C NMR spectra were consistent with the target structure (Fig. ), and Raman spectroscopy was consistent with the formation of primarily trisulfide linkages, based on comparison to authentic di-, tri-, and tetrasulfide reference compounds (Fig. ).