The catalysis by an immobilized preparation of Candida antartica lipase B (Novozyme-435)of transesterification or trans acylation between poly(epsilon -caprolactone), PCL, and poly(omega -pentadecalactone), PPDL, was studied. These reactions between macromolecules were performed in toluene or without solvent (bulk) at 70-75 degreesC. In bulk, for PCL (M-n = 9.2 x 10(3)) and PPDL (M-n = 4.3 x 10(3)), PDL*CL/CL*PDL diad sequences were observed by C-13 NMR within 30 min. By increasing the reaction time from 30 to 60 min, the average-sequence length of CL (LICL) and PDL (mu (PDL)) repeat units along chains decreased from 18 to 2 and 23 to 2, respectively. Transacylation between PCL (M-n = 44.0 x 10(3), PDI 1.65) and PPDL (M-n = 40.0 x 10(3), PDI 1.71) was also studied. To reduce diffusion constraints, the reaction was performed in toluene. Multiblock copolymers (M-n = 18.2 x 10(3) g/mol, PDI 1.92) were formed after 1 h. By increasing the reaction time to 30 h, random Poly(CL-co-PDL) (M-n = 31.2 x 10(3) g/mol, PDI 1.87) was formed. Transacylation reactions between polyesters are believed to involve intrachain cleavage by the lipase to form an enzyme-activated-chain segment, followed by reaction of this activated segment with the terminal hydroxyl unit of another chain. This hypothesis is supported by the finding that acetylation of chain end hydroxyl units causes a large decrease in the rate of transacylation between PCL and PPDL chains.