“Nonstrained” γ-Butyrolactone-Based Copolyesters: Copolymerization Characteristics and Composition-Dependent (Thermal, Eutectic, Cocrystallization, and Degradation) Properties

Despite several anticipated advantages of the bioderived γ-butyrolactone (γ-BL) as an effective comonomer to modulate materials properties of its copolyesters, the currently unmet challenge hinders access to such copolyesters with high γ-BL incorporations due to unfavorable thermodynamics toward the ring-opening polymerization of the highly stable, typically referred to as “nonstrained”, γ-BL. Here we report the effective copolymerization of γ-BL with two common cyclic esters with very different monomer thermodynamic polymerizability, ε-caprolactone (ε-CL) and δ-valerolactone (δ-VL), leading to a series of relatively high molecular weight (Mn up to 135 kg/mol) random copolyesters with unprecedented levels of γ-BL incorporations (up to 84.0 mol %) and thus providing access to γ-BL-based copolyesters in the entire composition range needed for comprehensive investigations into the composition-dependent physical properties and degradation behavior of the resulting copolyesters. This copolymerization was enabled by the judiciously chosen metal and organic catalysts that exhibit different kinetic behavior or monomer selectivity, designed to more effectively compete the “nonstrained” γ-BL against the relatively high-strained lactones toward ring-opening. The successful synthesis of the copolyesters with high γ-BL incorporations of >50 mol % led to the discovery of the eutectic phase of the γ-BL/ε-CL copolymer with a eutectic temperature Teu of 11.0 °C and a eutectic composition Xeu of 66.0% γ-BL; thus, at this composition, the copolymer becomes a viscous liquid at room temperature, although the two constituent homopolymers are semicrystalline solids. Other important composition-dependent properties of γ-BL-based copolyesters, including thermal transitions, cocrystallization, as well as thermal and hydrolytic degradation behaviors, have also been examined.