Abstract Enzymatic incorporation of carbohydrate-derived monomer units into hydrophobic polyester backbones represents a promising alternative to obtain new biodegradable oligomers and polymers. Immobilized lipases are efficient biocatalysts for copolymerization of β-butyrolactone and δ-gluconolactone, but only a systematic optimization study was able to highlight the influence of the main reaction parameters on the polymerization degree and on the relative copolymer content of the product. Therefore, experimental design was employed for determination of the optimal ring-opening copolymerization conditions in solventless reaction systems, at temperatures up to 80 °C. The obtained products, cyclic and linear polyesters, have been characterized by FT-IR, MALDI-TOF MS, NMR, and TG analysis, demonstrating the incorporation of gluconolactone unit(s) into the hydrophobic backbone of the polyester and the formation of new bio-based products.