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Ring-Opening Metathesis Polymerization of a Naturally Derived Macrocyclic Glycolipid

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journal contribution
posted on 14.05.2013 by Yifeng Peng, John Decatur, Michael A. R. Meier, Richard A. Gross
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 Mn up to about 180K (Mw/Mn 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 °C largely disappears at 60 °C with an increase in the apparent rate constant (kpapp) of 11 times. G3 gave fast initiation even at 33 °C while plots of ln­{[M]0/[M]t} versus reaction time for G3 show that kp 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 (Ea) of propagation is 18 ± 3 kcal/mol. By keeping [LSL] constant at 0.54 M, G2-catalyzed LSL ED-ROMP (60 °C, THF) gave a plot of Mn 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 (εDCM > εTHF > εCHCl3). Finally, this work provides an important example of how complex structures derived from nature can be transformed into unique macromolecules.