Thermodynamics, Closed-Loop Recycling, and Marine Degradation of Poly(alkylene oxalate)s Synthesized through Ring-Opening Polymerization

An ideal eco-friendly polymer should be biobased/CO2-derived, closed-loop recyclable, environmentally degradable, and cost-effective. Overall, poly(alkylene oxalate)s (PAOx) demonstrate significant promise as eco-friendly polymers. In this article, ring-opening polymerization of the cyclic ester monomers p-dioxane-2,3-dione (PDD), dimeric p-dioxepane-2,3-dione (d-PDP), and dimeric 6,6-dimethyl-p-dioxepane-2,3-dione (d-PDP2Me) was employed to synthesize high-molecular-weight and well-defined structural PPDD, PPDP, and PPDP2Me in a controlled manner. Compared to polycondensed PAOx, the ring-opening polymerized PAOx exhibit enhanced thermal and mechanical properties, in addition to excellent degradation properties in both simulated seawater and pure water. Notably, through depolymerization–distillation processes, PPDD, PPDP, and PPDP2Me were recycled to monomers with yields exceeding 98.0% and GC purities exceeding 99.0%, demonstrating excellent closed-loop recyclability. Kinetics and thermodynamics research revealed that PDD possesses a moderate ceiling temperature (369 °C in bulk), while d-PDP and d-PDP2Me exhibit high ceiling temperatures (633 and 505 °C, respectively) and clarified the different thermodynamic behaviors among PDD, d-PDP, and d-PDP2Me during polymerization and depolymerization procedures.