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 °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 H 2 O 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 .