Of interest upon initiating this work was whether lipase-catalysts would exhibit control during chain growth in product characteristics such as end-group and polydispersity. In one study (Mahapatro et al., 2004a), we focused on condensation polymerizations between adipic acid and 1,8-octanediol. Polymerizations were conducted in the absence of solvent and without use of groups that activate monomer carboxylic acids. Under a wide range of reaction conditions and without fractionation, products formed had Mw/Mn values ≤ 1.5 (Fig 1). Such low polydispersity values demonstrate that CALB catalyzes chain growth with chain-length selectivity. This ability of CALB to form products of substantially lower polydispersity than would have been attained by conventional chemical catalysts is an important attribute of CALB-catalyzed condensation reactions. Growth of polymer chains was found to be nearly invariable for reaction temperatures from 65 to 90 °C (Fig 1). Two contributing factors are diffusion constraints that limit further chain growth and that CALB reduces the activation energy of the rate-determining step so that increases in temperature have less effect on rate. Another study reported the successful use of CALB catalysis for polymerizations of linear aliphatic hydroxyacids (Mahapatro et al., 2004b). Polymerizations were performed in bulk, at 90 °C, using 10% w/w relative to monomer of Novozym 435 (macroporous beads that contain 10% w/w of lipase B from Candida antarctica). There was little difference in the time profile of DPavg values for w-hydroxyacids with chain lengths of C-16, C-12, and C-10. These polymerizations gave products with DPavg about 105 and Mw/Mn of 1.5 by 24 h. However, shorter chain substrates (e.g. 6-hydroxyhexanoic acid) polymerized relatively slower. The apparent insensitivity to w-hydroxyacid chain lengths between C-10 and C-16 was surprising given the general chain-length selectivity of lipases. Similar to Novozyme-435-catalyzed diacid/diol polycondensations, Mw/Mn values tended toward 1.5 as polymerizations progressed. This result is in contrast to Mw/Mn values of about 2.0 for Novozyme-435-catalyzed lactone polymerizations. Fundamental aspects of the polymerization mechanism that lead to these very different polydispersity values are not yet understood. Plausible explanations are that polydispersity values of lactone polymerizations are broadened by slow chain initiation and a greater tendency (than in condensation polymerizations) to undergo transesterification reactions. Nevertheless, the narrow dispersity of polymers from Novozyme-435 condensation polymerizations can provide beneficial solid-state properties such as enhanced crystallization rates.