Candida antarctica lipase B-catalyzed transesterification: New synthetic routes to copolyesters

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 withou t solvent (bulk) at 70-75 degreesC. In bulk, for PCL (M-n = 9.2 x 10(3)) an d 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, t he average-sequence length of CL (LICL) and PDL (mu (PDL)) repeat units alo ng 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), PD I 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, rand om Poly(CL-co-PDL) (M-n = 31.2 x 10(3) g/mol, PDI 1.87) was formed. Transac ylation reactions between polyesters are believed to involve intrachain cle avage by the lipase to form an enzyme-activated-chain segment, followed by reaction of this activated segment with the terminal hydroxyl unit of anoth er chain. This hypothesis is supported by the finding that acetylation of c hain end hydroxyl units causes a large decrease in the rate of transacylati on between PCL and PPDL chains.