Despite several anticipated advantages of the bioderived gamma-butyrolactone (gamma-BL) as an effective comonomer to modulate materials properties of its copolyesters, the currently unmet challenge hinders access to such copolyesters with high gamma-BL incorporations due to unfavorable thermodynamics toward the ring-opening polymerization of the highly stable, typically referred to as "nonstrained", gamma-BL. Here we report the effective copolymerization of gamma-BL with two common cyclic esters with very different monomer thermodynamic polymerizability, epsilon-caprolactone (epsilon-CL) and delta-valerolactone (delta-VL), leading to a series of relatively high molecular weight (M-n up to 135 kg/mol) random copolyesters with unprecedented levels of gamma-BL incorporations (up to 84.0 mol %) and thus providing access to gamma-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" gamma-BL against the relatively high-strained lactones toward ring-opening. The successful synthesis of the copolyesters with high gamma-BL incorporations of >50 mol % led to the discovery of the eutectic phase of the gamma-BL/epsilon-CL copolymer with a eutectic temperature T-eu of 11.0 degrees C and a eutectic composition X-eu of 66.0% gamma-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 gamma-BL-based copolyesters, including thermal transitions, cocrystallization, as well as thermal and hydrolytic degradation behaviors, have also been examined.