Lactonic sophorolipid (LSL) is a naturally occurring macrocyclic monomer that undergoes ring opening metathesis polymerization (ROMP) via an entropy driven mechanism (ED-ROMP) Typically, gel permeation chromatographic analysis of poly(LSL) showed products consist of about 70% polymer with M-n up to about 180K (M-w/M-n 1.6-1.8) coexisting with 10% of oligomer and 20% monomer. Detailed kinetic studies for LSL ROMP were performed using two classic metathesis catalysts (i.e., G2 and G3). G2 exhibited apparent first order propagation, although its slow initiation caused subsequent events of secondary metathesis that decreased molecular weight An induction period observed for G2 at 33 and 45 degrees C largely disappears at 60 degrees C with an increase in the apparent rate constant (K-p(app)) of 11 times. G3 gave fast initiation even at 33 degrees C while plots of ln{[M](0)/[M](t)} versus reaction time for G3 show that k(p) continuously decreased, implying a decline in G3 catalytic activity Plots of ln{[M](0)/[M](t)} versus reaction time for G2 are linear, suggesting apparent first order kinetic behavior. From analysis of an Arrhenius plot for G2-catalyzed LSL polymerization in THF, the activation energy (E-a) of propagation is 18 +/- 3 kcal/mol. By keeping [LSL] constant at 0.54 M, G2-catalyzed LSL ED-ROMP (60 degrees C, THF) gave a plot of M-n versus [monomer]/[initiator] ratio close to that of the theoretical curve based on a living polymerization model Hence, despite pronounced secondary metathesis in ED-ROMP, polymerization kinetics with G2 closely resembled living behavior. The length of the induction period for G2-catalyzed polymerizations is inversely proportional to the solvent dielectric constant (epsilon(DCM) > epsilon(THF) > epsilon(CHCl3)). Finally, this work provides an important example of how complex structures derived from nature can be transformed into unique macromolecules.