High-Tg Polythioesters From Isomerization Ring-Opening Polymerization of a Renewable Oxanorbornane-Fused Thionolactone.

Polythioesters are gaining increasing interest due to their attractive material properties and improved end-of-life options. However, the lower ring strain of thiolactones compared to lactones limits the accessibility of polythioesters through conventional ring-opening polymerization. Thionolactone isomerization ring-opening polymerization provides an additional thermodynamic driving force, enabling polymer synthesis from more accessible monomers. Nevertheless, reported polythioesters typically exhibit low glass-transition temperatures (Tg), and the impact of sulfur substitution on the polymer(ization) properties remains poorly understood. Here, we report a biobased tricyclic thionolactone (TCTn) that undergoes selective polymerization to yield high-molecular weight P(TCT) (ca. 100 kg mol-1). P(TCT) exhibits one of the highest Tg values reported for polythioesters (171°C), exceeding that of the analogous polyester (130°C). The thermally stable P(TCT) can be efficiently depolymerized to the unstrained thiolactone (TCT) through backbiting depolymerization. Whereas hydrolysis of P(TCT) under heterogeneous aqueous conditions is ten times slower than that of the polyester analogue, homogeneous methanolysis proceeds at comparable rates, indicating that the intrinsic susceptibility of the ester and thioester linkages toward nucleophilic attack is similar. Overall, this work provides insights into the design of sustainable, sulfur-containing polymers, the intricacies of O→S substitution, and expands the space of high-Tg norbornane-fused polymers.