Studies of the kinetics and mechanism of Candida antarctica Lipase B (CALB) catalyzed epsilon-caprolactone (epsilon-CL) polymerizations in toluene were performed. The kinetic plot of ln ([M](0)/[M](t)) vs time was carried out to 96% epsilon-CL conversion and M-n 11970. The plot is linear (r(2) = 0.998), indicating that termination did not occur and the propagation rate is first order with respect to monomer concentration. Changes in the water (e.g., initiator) concentration did not change the polymerization rate but did change the number of chains [R-OH]. Thus, the polymerization is zero order with respect to [R-OH] and initiator concentration. A plot of ln k(app) VS In [enzyme] gave 0.7 as the reaction order of the enzyme concentration. The apparent activation energy for Novozyme-435 catalyzed epsilon-CL polymerization in toluene is 2.88 kcal mol(-1). This is well below 10.3 kcal mol(-1), the activation energy for aluminum alkoxide catalyzed epsilon-CL polymerization in toluene. Upward deviation from linearity for M-n vs fractional epsilon-CL conversion and decreases in the number of chains was accentuated by low enzyme water contents and high monomer conversion. These results are consistent with a competition between ring-opening chain-end propagation and chain growth by steplike polycondensations. CALB was irreversibly inhibited by modification with paraoxon at the lipase active site (Ser(105)). The modified enzyme was no longer active for the polymerization. This supports that the polymerizations studied herein occurred by catalysis at the active serine residue (Ser(105)) and not by other chemical or nonspecific protein-mediated processes.