Making and un-making poly(trisulfides) with light: precise regulation of radical concentrations via pulsed LED irradiation

Organic polysulfide polymers are highly useful materials with emerging high-value applications as cathode components for Li-S batteries, optics for infrared imaging, sorbents for heavy metal remediation, and novel anti-microbial agents. Despite the increasing use of these sulfur-rich polymers, there are limited methods to synthesize them with control of structure and molecular weight. In this study, we disclose novel photopolymerization methods to make poly(trisulfides) with well-defined sulfur-rank and stereochemistry, narrow dispersity, and controlled molecular weights. The reaction features photochemical cleavage of an S–S bond in a 1,2,3-trithiolan (a cyclic trisulfide) to generate a diradical that can undergo ring-opening polymerization. To generate polymers with molecular weights exceeding 20,000 g/mol, spatial or temporal control of irradiation is required because the same wavelengths of light that initiate the reaction also cleave S–S bonds in the target polymer. Spatial control of photoinitiation was achieved using a photochemical flow reactor that physically separates the polymer products from the light source after synthesis. Temporal control of photoinitiation also allowed these polymers to be made in batch reactors. This control was imparted with pulsed light-emitting diode (LED) irradiation with precisely controlled on and off times. This innovative strategy allowed initiation during short (100 ms) bursts of light, and propagation during longer dark periods (e.g. 900 ms). The pulsed light strategy allowed preservation of the polymer chains during the reaction. In contrast, continuous irradiation led to cleavage of S–S bonds in the polymer and rapid depolymerization. Previously published attempts to make a poly(trisulfide) by photochemical ring-opening polymerization were not viable because continuous irradiation always resulted in depolymerization; our spatially and temporally controlled irradiation methods solve this long-standing problem and provide ready access to decagram quantities of several poly(trisulfides). The reversibility of the polymerization also prompted us to develop new applications of these polymers as recyclable coatings and adhesives, and photoresists for lithography. More generally, we argue that spatial and temporal control of irradiation during photochemical reactions should be considered as a design principle in photochemical synthesis. Pulsed laser techniques have long been used for the study of kinetics and mechanisms of reactions, but the deliberate control of irradiation on and off times has seldom been applied to preparative chemical synthesis. Our photochemical polymerizations using pulsed light reported here illustrate the value of controlling irradiation in space or time to make useful products not accessible with continuous irradiation.