Regardless of numerous anticipated advantages on the bioderived γ-butyrolactone (γ-BL) as an efficient comonomer to modulate supplies properties of its copolyesters, the presently unmet challenge hinders access to such copolyesters with high γ-BL incorporations as a consequence of unfavorable thermodynamics toward the ring-opening polymerization in the hugely steady, commonly referred to as “nonstrained”, γ-BL. Here we report the effective copolymerization of γ-BL with two typical cyclic esters with very diverse monomer thermodynamic polymerizability, ε-caprolactone (ε-CL) and δ-valerolactone (δ-VL), leading to a series of comparatively high molecular weight (Mn as much as 135 kg/mol) random copolyesters with unprecedented levels of γ-BL incorporations (as much as 84.0 mol %) and hence giving access to γ-BL-based copolyesters inside the whole composition variety required for extensive investigations in to the composition-dependent physical properties and degradation behavior in the resulting copolyesters. This copolymerization was enabled by the judiciously selected metal and organic catalysts that exhibit unique kinetic behavior or monomer selectivity, made to a lot more correctly compete the “nonstrained” γ-BL against the relatively high-strained lactones toward ring-opening. The successful synthesis on the copolyesters with high γ-BL incorporations of >50 mol % led to the discovery of the eutectic phase from the γ-BL/ε-CL copolymer using a eutectic temperature Teu of 11.0 °C along with a eutectic composition Xeu of 66.0% γ-BL; hence, at this composition, the copolymer becomes a viscous liquid at space temperature, though the two constituent homopolymers are semicrystalline solids. Other significant composition-dependent properties of γ-BL-based copolyesters, such as thermal transitions, cocrystallization, at the same time as thermal and hydrolytic degradation behaviors, have also been examined.