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

Organic polysulfide polymers are 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 antimicrobial agents. Despite the increasing use of these sulfur-rich polymers, there are limited methods to synthesize them with control of the structure and molecular weight. In this study, we disclose photopolymerization methods to make poly(trisulfides) with a well-defined sulfur-rank, 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 undergoes ring-opening polymerization. Polymers with molecular weights exceeding 20,000 g/mol require spatial or temporal control of irradiation 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 by using a photochemical flow reactor that 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 bursts of light and propagation during longer dark periods. The pulsed light strategy allowed the preservation of the polymer chains during the reaction. In contrast, continuous irradiation led to the cleavage of S–S bonds in the polymer and rapid depolymerization. The reversibility of polymerization also prompted us to develop new applications of these polymers as recyclable coatings and adhesives, and photoresists for lithography.