Free-radical propagation rate coefficient of nonionized methacrylic acid in aqueous solution from low monomer concentrations to bulk polymerization

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 degrees 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),