Pulsed-laser polymerization (PLP) in conjunction with aqueous-phase size-exclusion-chromatography (SEC) was employed to determine the propagation rate coefficient, k p, of methacrylic acid (MAA) free-radical polymerization in aqueous solution. This was done over a wide range of degree of MAA ionization, at MAA concentrations between 5 and 40 wt %, and at temperatures from 6 to 80 °C. Depending on monomer concentration, the degree of ionization, α, may largely affect k p. At 5 wt % MAA, k p is lowered by about 1 order of magnitude in passing from nonionized to fully ionized MAA, whereas the associated decrease in k p at 40 wt % MAA is only by 20%. The changes of MAA k p with α are assigned to intermolecular interactions affecting the friction that is experienced by the relevant internal rotations of the transition state structure for propagation. Increasing hindrance of rotational motion is associated with a lowering of the pre-exponential factor, A(k p). The observed effects are primarily of entropic origin, but slight changes in activation energy, E A(k p), also seem to play a role. An expression is given which allows for estimates of MAA k p as a function of degree of ionization, monomer concentration, and temperature.

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Summary: Free‐radical batch polymerization (FRP) of N‐vinyl pyrrolidone (NVP) and N‐vinyl formamide (NVF) monomers in aqueous solution as well as NVP polymerization in organic (n‐butanol) solution has been studied. The differences found in rate of monomer conversion with monomer and solvent choice correlates well with the differences in values of the propagation rate coefficients (kp) and their variation with monomer concentration measured in independent pulsed‐laser polymerization studies, a result demonstrating that a generalized understanding of water‐soluble vinyl monomers can be obtained once their kp differences have been accounted for. A reasonable representation of polymer molecular mass averages and the complete molecular mass distributions for the three systems was obtained by assuming that the rate coefficient for transfer to monomer, polymer, and organic solvent also vary as a function of monomer concentration.