Beuermann, S.; Buback, M.; Hesse, P.; Lacik, I.
Free-radical propagation rate coefficient of non-ionized methacrylic acid in aqueous solution from low monomer concentrations to bulk polymerization
Macromolecules 2006, 39, 184-193.
 

The propagation rate coefficient, k_p, for free-radical polymerization of nonionized methacrylic acid (MAA) in aqueous solution has been studied via pulsed laser polymerization (PLP) in conjunction with aqueous-phase size-exclusion chromatography (SEC). The PLP-SEC experiments were carried out between 20 and 80 °C within the entire concentration range from dilute solution containing 1 wt % MAA up to bulk MAA polymerization. The k_p values which are determined under the assumption that the relevant monomer concentration at the radical site is identical to the known overall MAA concentration decrease by about 1 order of magnitude between 1 and 100 wt % MAA. This significant lowering is almost entirely due to a reduction in the Arrhenius preexponential factor, A(k_p), whereas the activation energy, E_A(k_p), stays essentially constant. The decrease in A(k_p) is assigned to intermolecular interactions between the transition state (TS) structure for MAA propagation and an MAA environment being significantly stronger than the ones between this TS structure and an H₂O environment. In an MAA-rich environment, the barrier to rotational motion of the relevant degrees of motion of the TS thus experiences enhanced friction, which is associated with a lowering of the preexponential factor and thus of k_p.