Pulsed Laser Polymerization

1
PULSED LASER POLYMERIZATION IN ROOM TEMPERATURE
IONIC LIQUIDS S. Harrisson, S. Mackenzie, D. M.
Haddleton Department of Chemistry, University of
Warwick, CV4 7AL, Coventry, UK
Polymerizations of methyl methacrylate (MMA) in
ionic liquids such as bmimPF6 proceed much
faster and to higher molecular weights than in
bulk.1 We investigated the cause of this using
the pulsed laser polymerization (PLP) technique,
which allows the separation of effects due to
propagation (kp) and termination (kt). Our
results showed increasing kp and decreasing kt as
the proportion of IL was increased. Both of these
effects increase the overall rate of
polymerization. Different ionic liquids gave
similar results. We suspect that the increase in
kp is due to increased participation of charge
transfer species in the transition state. These
species would be favored by the high charge
density of the IL medium. Experiments across a
range of temperatures (25-60 C) showed that the
main effect of the IL is to lower the activation
energy of propagation, with no clear influence on
the pre-exponential factor. This is consistent
with a mostly electronic cause. It is not
possible, however, to rule out radical-IL
complexes which could produce similar
effects. The decrease in kt is strongly
correlated with the viscosity of the MMA/IL
solution, suggesting that the termination
reaction is diffusion-controlled, as in bulk
MMA. Hence it appears that the increase in
overall polymerization rate observed in ionic
liquids is due to a mixture of physical
(increased viscosity ? lower kt) and electronic
effects (charge separation ? higher kp).
Effect of bmimPF6 on ktm of MMA at 25 C.
Effect of bmimPF6 on kp of MMA at 25 C.
IUPAC benchmark
Bulk MMA (this work)
20 IL
50 IL
Conclusions Rate of MMA polymerization is
greatly increased in ionic liquids. This is due
to both increase in kp and decrease in
kt. Decrease in kt is due to the increase in
viscosity of the medium. Increase in kp may be
due to charge transfer in the transition state
or formation of radical-IL complexes.
ktm is highly correlated with the viscosity of
the MMA/IL solution.
Point estimates and 95 joint confidence
intervals for Arrhenius parameters of kp of MMA.
IUPAC benchmark value2 for bulk MMA is shown for
comparison.
Pulsed Laser Polymerization The PLP
technique is the IUPAC-recommended method of
determining the rate coefficient of
propagation, kp, in radical polymerizations.2
Recently, the technique has been extended to
allow estimation of
the average termination rate
constant, ?kt?.4 How it works

• Ionic Liquids
• Room temperature ionic liquids (RTILs) are salts
  which are liquid around room
• temperature. The development of these compounds
  dates to 1914, with the preparationof
  ethylammonium nitrate. More recently, there has
  been a revival of interest in RTILs due to their
  potential applications as environ-
• mentally-friendly and catalytically-active
  solvents.3
• bmimPF6
• Some Properties of Ionic Liquids

The main ionic liquid used in this study was
butyl methylimidazolium hexafluoro-phosphate
(bmimPF6). Hexyl and octyl methyl-imidazolium
cations and the tetrafluoroborate anion were also
investigated with similar results. Many different
cations and anions are available, giving ionic
liquids with a wide range of physical and
chemical properties.
kp is given by ? kpM td where ? is the
kinetic chain length (estimated from the low-MW
point of inflection of the MWD).5 ?kt? can be
estimated by ktm, where DPw Rp kp2/ktm M2
(3-?) ktm underestimates the true value of ?kt?
by about 20, but still provides a good
semi- quantitative measure of the effect of
ILs on termination kinetics.4
?
A typical PLP molecular weight distribution.
References
Acknowledgements Thanks to Mr Richard Doyle for
help with the PLP experiments and the EPSRC for
funding (S.H., GR/R16228).
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S. A. F. Seddon, K. R. J. Chem. Soc., Chem.
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D. Haddleton, D. M. ACS Symposium Series Chapter,
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Hong, K. Mays, J. W. Rogers, R. D. ACS
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see a)Wasserscheid, P. Keim, W. Angew. Chem.
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1999, 99, 2071. c)Holbrey, J. D. Seddon, K. R.
Clean Prod. Proc. 1999, 1, 223. 4. Olaj, O. F.
Kornherr, A. Zifferer, G. Macromol. Rapid.
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Gleixner, G. Makromol. Chem. 1985, 186, 2569.