PLP−SEC Study into Free-Radical Propagation Rate of Nonionized Acrylic Acid in Aqueous Solution

Pulsed laser polymerization (PLP) in conjunction with size-exclusion chromatography (SEC), both being carried out in the aqueous phase, was used to determine propagation rate coefficients, k<INF>p</INF>, of nonionized acrylic acid (AA) at temperatures between 2 and 25 °C and monomer concentrations, c<INF>AA</INF>, from 1 to 10 wt %. The product k<INF>p</INF>c<INF>AA,local</INF> is the primary experimental quantity deduced via the PLP−SEC technique. Assuming c<INF>AA,local</INF> to be identical to overall monomer concentration, c<INF>AA</INF>, yields apparent k<INF>p</INF> values, which, upon enhancing c<INF>AA</INF>, first increase and, after passing through a maximum at around 3 wt % AA, significantly decrease. A k<INF>p</INF> value as high as 180 000 L·mol^-1·s^-1 was determined for 3 wt % AA at 25 °C. The decrease observed toward higher AA concentration is fully consistent with what has been found in a preceding study into k<INF>p</INF> of nonionized AA at monomer concentrations of 20 and 40 wt %. At constant temperature, variations in apparent k<INF>p</INF> by about a factor of 3 are seen in the range up to 40 wt % AA. Discussion of the measured rate data suggests that it is primarily c<INF>AA,local</INF> that changes as a function of monomer concentration rather than k<INF>p</INF>. As a consequence of strong hydrogen bonds between polymer segments, between polymer segments and AA monomer, and between both these species and water, the AA concentration at the radical site may significantly differ from overall c<INF>AA</INF>. The assignment of the observed changes in apparent k<INF>p</INF> to c<INF>AA,local</INF> is supported by PLP−SEC experiments in which appreciable amounts of propionic acid (PA) have been added to aqueous AA solutions. The addition of PA significantly reduces apparent k<INF>p</INF>. Addition of NaCl to an aqueous solution of AA in its nonionized form, on the other hand, does not affect apparent k<INF>p</INF>. Whether the observed changes in k<INF>p</INF>c<INF>AA,local</INF> are entirely due to c<INF>AA,local</INF> differing from c<INF>AA</INF> or whether also the “true” propagation rate coefficient varies cannot be safely decided on the basis of the presently available data.