Ambient cationic ring-opening polymerization of Dibenzo[c,e]oxepine-5(7H)-thione (DOT): Thermal and nucleophile-initiated depolymerization

• Ambient cationic ring-opening polymerization of a thionolactone. • No need for inert atmosphere or dry solvents. • Novel polythioester is fully degradable through different methods. • Cationic initiator influences thermal stability. • Thermal depolymerization is controlled and gives thiolactone building block. 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 remaining challenge consists in tuning the degradation behaviour of polymers to ensure stability during use and efficient recyclability after use. 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 and 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.