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The application of pulsed-laser polymerization size-exclusion chromatography (PLPSEC) has made it possible to unambiguously determine propagation rate coefficients (kp) in radical polymerization in a single experiment. Accurate Arrhenius parameters are now available for a wide range of monomers, where previously great uncertainty prevailed even for very common commodity monomers. While propagation rate coefficients are largely insensitive to solvent effects and to monomer concentration, water is a significant exception for many monomers. The currently accepted model attributes changes in kp to a decrease in the Arrhenius pre-exponential factor when water is replaced with monomer in the environment of the transition state, reducing degrees of rotational freedom, but this is problematic in terms of statistical thermodynamics.
We hypothesise that partitioning arises for these monomers due to the robust ice-like structure of liquid water and that, prior to the total disruption of the water structure by dissolved monomer, aqueous solutions are effectively heterogeneous on the propagation timescale with polymer and monomer excluded from regions where water displays medium range order, and restricted to regions of disordered water exhibiting a much higher effective concentration for polymerisation. We are utilizing molecular dynamics simulations to study the extent of water structure disruption and the extent of clustering of monomer molecules in water/monomer mixtures in order to determine if the trends obtained are consistent with experimental observations.
1. S. Beuermann, M. Buback, P. Hesse, I. Lacik, Macromolecules 2006, 39, 184. |
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