Lipase-catalyzed intrachain transesterification reactions were studied in dry toluene (70 degreesC) using an immobilized form of Candida antarctica lipase B (Novozyme-435), poly(e-caprolactone), PCL, and hexanol/water as competitive nucleophiles. In addition to NMR and GPC, MALDI-TOF MS was also used to determine product molecular weight and end group structure. Experiments were conducted to determine (i) whether lipases cleave at random or specific sites along a polyester substrate and (ii) how reaction parameters such as the molecular weight of the starting polyester, reaction time, and nucleophile concentration effect the structure of the products formed. For the ratio of CL units to hexanol (22/1 mol:mol) and parent PCL M-n 56 400, calculations based on integration of H-1 NMR. spectra for the precipitated products showed that, after only 10 and 20 min reactions, about 60 and 70% of the PCL chains have terminal hexyl ester groups. Thus, transesterification reactions between PCL and hexanol occur rapidly. By 2 h, Novozyme-435 resolved the higher (A) and lower (B) molecular mass components of the starting polyester. This gave fraction A with M-n and polydisperity (M-w/M-n) of 122 000 and 1.44, respectively. The "mass-selective" transesterification is evident in the GPC traces within 10 min, giving a product with M-n and polydispersity of 1810 and 1.6 (population D). Thus, the lipase cleaves short chain segments with an average length of 16 units by an endo-type mechanism. From 10 min to 24 h, it appears that condensation reactions of population D occur to give fraction C (M-n = 4040 M-w/M-n = 1.8, M-peak = 7240). The results of this work show how enzymatic transesterification reactions can be further developed to provide oligomers with well-defined length and end group structure.