292 Enzyme-Catalyzed Synthesis of Hyperbranched Aliphatic Polyesters Sunny Skaria, Mario Smet, a Holger Frey* Institut für Makromolekulare Chemie und Freiburger Materialforschungszentrum (FMF), Albert-Ludwigs-Universität, Stefan-Meier-Str. 21/31, 79104 Freiburg/Brsg., Germany Fax: +49-761-203-4709; E-mail: holfrey@fmf.uni-freiburg.de Keywords: dendrimers; enzymatic polymerization; hyperbranched polymers; polyesters; ring-opening polymerization; Introduction Macromolecular engineering of complex polymer archi- tectures that are based on controlled branching has become an increasingly important issue in polymer science. The interest is driven by the peculiar mechanical, rheological and compatibility properties of these materi- als. Polymers with a branch-on-branch (i. e. tree-like) structure are generally known as cascade-branched or dendritic. [1, 2] In contrast to the perfectly branched dendri- mers prepared in a stepwise manner, the synthesis of such materials is based on one synthetic step only, i.e. poly- condensation of AB m monomers or polymerization of inimer-type AB* structures that can be regarded as “latent AB m monomers”. [3] The resulting hyperbranched polymers possess a randomly branched topology, i. e. branching is not achieved for every monomer unit incor- porated and additional linear units are present. [4] Despite numerous recent developments in the field of hyper- branched polymers, the main challenge lies in the devel- opment of synthetic strategies that permit to prepare hyperbranched materials on the basis of readily available commercial monomers. Hyperbranched polyesters repre- sent a particularly important class of materials that are of interest for specialty coatings and nanostructured net- works as well as for numerous biomedical applications. [5] However, almost all syntheses published for hyper- branched polyesters to date require rigorous conditions, such as temperatures exceeding 150 8C and catalysts like sulfuric acid. [6] The past 15 years have seen enormous progress in the use of enzymes in organic media to catalyze a wide variety of small molecule transformations. [7] Initially, enzyme- catalyzed polymerizations [8] reported between 1991 and 1999 using, e. g., e-caprolactone (e-CL), [9] d-valerolac- tone [10] and c-butyrolactone [11] as monomers, required long reaction times (days) and produced low-molecular-weight polyesters (M — n generally lower than 5 6 10 3 g/mol). A breakthrough was achieved with the use of immobilized Lipase B from Candida antarctica (Novozyme 435; CALB) for the polymerization of e-CL as well as x-penta- decalactone in toluene or other apolar solvents. [12] The state of the art in the field has been summarized in a com- prehensive review recently. [13] The synthesis of hyperbranched aliphatic polyesters has been reported by Hedrick et al. as well as FrØchet et al. who used e-CL-based inimers, polymerized in an Sn-cata- lyzed self-condensing ring-opening polymerization (SCROP). [14, 15] However, these cyclic inimers have to be Communication: Hyperbranched aliphatic copolyesters have been prepared by copolymerization of e-caprolactone with 2,2-bis(hydroxymethyl)butyric acid, catalyzed by immobilized Lipase B from Candida antarctica (Novo- zyme 435) under mild conditions. Via this novel combina- tion of ring-opening AB polymerization and AB 2 polycon- densation, the degree of branching (DB) and, conse- quently, the density of functional end groups can be con- trolled by the comonomer ratio in the feed (0 a DB a 0.33). Macromol. Rapid Commun. 2002, 23, No. 4 i WILEY-VCH Verlag GmbH, 69469 Weinheim 2002 1022-1336/2002/0403–0292$17.50+.50/0 a Permanent address: Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001 Heverlee (Leuven), Belgium. Macromol. Rapid Commun. 2002, 23, 292–296