Ambient Cationic Ring-opening Polymerization of Dibenzo[c,e]oxepine-5(7H)-thione (DOT): Thermal and Nucleophile-initiated Depolymerization

With rising concern over plastic waste accumulation worldwide, the quantitative depolymerization of polymers into small molecule building blocks offers avenues toward a circular polymer economy. But a tuning of the polymer stability versus degradation efficiency remains challenging. Herein, the thionolactone dibenzo[c,e]oxepine-5(7H)-thione (DOT) is shown to undergo cationic ring-opening polymerization (CROP) under ambient conditions without the need for inert atmosphere or dry solvents. Involving S–O isomerization, the polymerization gave polythioesters in near-quantitative conversions with tuneable SEC-measured molar masses from 1.3–50 kg/mol and dispersities between 1.5–2.0. The polythioesters could be degraded with an excess of amine, with substoichiometric amounts of thiolate (which was shown to involve depolymerization from a thiolate ω-end group), or thermally. The latter two conditions produced the thiolactone dibenzo[c,e]thiepine-5(7H)-one (DTO). While anionic ring-opening polymerization (the common route to polythioesters) gives thiol end groups, the CROP presented herein provided end-capped polymers. Interestingly, the choice of initiator (and resulting end cap) was shown to have a drastic influence on the thermal stability. While a boron trifluoride-initiated polymer showed only 6% decomposition when heated to 140 °C without solvent, a comparable methyl triflate-initiated polymer underwent 35% degradation to DTO when heated to the same temperature overnight.