Propagation Rate Coefficient of Free-Radical Polymerization of Partially and Fully Ionized Methacrylic Acid in Aqueous Solution

Abstract

Pulsed-laser polymerization (PLP) in conjunction with aqueous-phase size-exclusion-chromatography (SEC) was employed to determine the propagation rate coefficient, kp, 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 kp. At 5 wt % MAA, kp is lowered by about 1 order of magnitude in passing from nonionized to fully ionized MAA, whereas the associated decrease in kp at 40 wt % MAA is only by 20%. The changes of MAA kp 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(kp). The observed effects are primarily of entropic origin, but slight changes in activation energy, EA(kp), also seem to play a role. An expression is given which allows for estimates of MAA kp as a function of degree of ionization, monomer concentration, and temperature.