ma400291c_si_001.pdf (535.45 kB)
Ring-Opening Metathesis Polymerization of a Naturally Derived Macrocyclic Glycolipid
journal contribution
posted on 2013-05-14, 00:00 authored by Yifeng Peng, John Decatur, Michael A. R. Meier, Richard A. GrossLactonic
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.