Regardless of many anticipated positive aspects of the bioderived γ-butyrolactone (γ-BL) as an efficient comonomer to modulate materials properties of its copolyesters, the at the moment unmet challenge hinders access to such copolyesters with high γ-BL incorporations on account of unfavorable thermodynamics toward the ring-opening polymerization from the hugely stable, typically known as “nonstrained”, γ-BL. Here we report the helpful copolymerization of γ-BL with two prevalent cyclic esters with very distinctive monomer thermodynamic polymerizability, ε-caprolactone (ε-CL) and δ-valerolactone (δ-VL), major to a series of reasonably higher molecular weight (Mn as much as 135 kg/mol) random copolyesters with unprecedented levels of γ-BL incorporations (up to 84.0 mol %) and therefore giving access to γ-BL-based copolyesters in the complete composition range necessary for complete 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 diverse kinetic behavior or monomer selectivity, created to extra efficiently compete the “nonstrained” γ-BL against the somewhat high-strained lactones toward ring-opening. The effective synthesis of your copolyesters with high γ-BL incorporations of >50 mol % led for the discovery with the eutectic phase from the γ-BL/ε-CL copolymer using a eutectic temperature Teu of 11.0 °C plus a eutectic composition Xeu of 66.0% γ-BL; hence, at this composition, the copolymer becomes a viscous liquid at room temperature, despite the fact that the two constituent homopolymers are semicrystalline solids. Other essential composition-dependent properties of γ-BL-based copolyesters, such as thermal transitions, cocrystallization, as well as thermal and hydrolytic degradation behaviors, have also been examined.