Candida antarctica lipase B (CALB) was successfully used to promote synthesis of aliphatic poly(carbonate-co-ester) copolymers from dialkyl carbonate, diol, and lactone monomers. The polymerization reactions were carried out in two stages: first-stage oligomerization under low vacuum, followed by second-stage polymerization under high vacuum. Therefore, copolymerization of omega-pentadecalactone (PDL), diethyl carbonate (DEC), and 1,4-butanediol (BD) yielded PDL-DEC-BD copolymers with a M-w of whole product (nonfractionated) up to 33 000 and M-w/M-n between 1.2 and 2.3. Desirable reaction temperature for the copolymerization was found to be similar to 80 degrees C. The copolymer compositions, in the range from 10 to 80 mol % PDL unit content versus total (PDL + carbonate) units, were effectively controlled by adjusting the monomer feed ratio. Reprecipitation in chloroform/methanol mixture allowed isolation of the purified copolymers in up to 92% yield. H-1 and C-13 NMR analyses, including statistical analysis on repeat unit sequence distribution, were used to determine the polymer microstructures. The synthesized PDL-DEC-BD copolymers possessed near random structures with all possible combinations of PDL, carbonate, and butylene units via either ester or carbonate linkages in the polymer chains and are more appropriately named as poly(PDL-co-butylene-co-carbon ate).