KEY WORDSEnzymatic Polymerization I Ring-Opening Polymerization 1 Michaelis-Menten Kinetics I Lactone I Macrolide I Recently, polyester syntheses through enzymatic catalysis with various monomer combinations have been extensively investigated. 1 By utilizing specific enzymatic catalysis, enantioselective 2 · 3 and regioselective 4 polymerizations have been developed. To obtain polymers of higher molecular weight in high yields, activated diesters having 2,2,2-trifluoroethyl, 2,2,2-trichloroethyl, or vinyl leaving groups have been often used as monomers for the enzymatic polycondensation. 5 · 6Enzymatic syntheses of aliphatic polyesters have also been achieved by ring-opening polymerization and copolymerization of lactones. 7 -16 Until now, small-( 4-membered) and medium-size lactones (6-and 7-membered) as well as macrolides (12-, 13-, and 16-membered lactones) were polymerized by lipase catalysts. In particular, these macrolides having much lower anionic polymerizability than e-caprolactone (e-CL) exhibited unusually high reactivity through lipase catalysis. 11 · 12 · 15 The lipase-catalyzed polymerization of the macrolides proceeded much faster than that of e-CL, which is the reversed tendency of chemical reactions such as an alkaline hydrolysis 17 and anionic ring-opening polymerizations.18 To quantitatively evaluate the lipase-catalyzed polymerizability of these lactones, we made MichaelisMenten kinetics of the polymerization. 19 This study is the first example to shed light on the mechanistic aspect via kinetics on the enzymatic ring-opening polymerization of lactones. EXPERIMENTAL Materialse-CL, 12-dodecanolide (DDL), and iso-propyl ether were stored over freshly activated type 4 molecular sieves. Pseudomonas fluorescens lipase (lipase PF) was donated by Amano Pharmaceutical Co. and used without further purification. Kinetic StudyTo a dried 50 mL flask was added e-CL or DDL, 1-octanol (0.039 g, 0.30 mmol), lipase PF (200 mg), and iso-propyl ether (10 mL). The flask was kept at 60°C under gentle stirring. An aliquot (10--20 JlL) of the reaction mixture was removed every 1 h and subjected to gas chromatographic (GC) analysis for determination t To whom correspondence should be addressed.of the monomer conversion. MeasurementsGC analysis was carried out using a Shimadzu GC-14B apparatus equipped with an FID detector and a TC-5 column (GL Sciences). 1 H NMR spectra were recorded on a 400MHz Bruker DPX-400 spectrometer. Water content was measured using a Mitsubishi Kasei V A-06 Karl Fischer titrator. RESULTS AND DISCUSSIONThe enzymatic polymerization of lactones was carried out using lipase PF in iso-propyl ether at 60°C. Monomers used in this study were e-caprolactone (7-membered, e-CL) and 12-dodecanolide (13-membered, DDL). Lipase PF has high catalytic activity for these monomers. 7 · 12 After 48 h, the chloroform-soluble part of the reaction mixture from DDL monomer was analyzed by 1 H NMR spectroscopy. The product was found to be a mixture of the unreacted monomer and the corresponding oligomer. 10 On the b...