In acrylate polymerizations both SPRs and tertiary MCRs occur. Via pulsed laser polymerization, using a wide range of LPRRs, in conjunction with aqueous-phase size-exclusion chromatography the polymerization of 1.35 mol-L-1 acrylic acid in aqueous solution has been investigated at 6 degrees C. The sigmoidal decrease in the apparent propagation rate coefficient, k(p)(app), towards lower LPRRs is in line with recent predictions. At the highest LPRRs, k(p)(app) approaches the rate coefficient of SPR propagation, k(p)(SPR), whereas the limiting value of k(p)(app) at low LPRRs approaches the effective propagation rate coefficient, k(p)(eff), which allows for an estimate of the fraction of MCRs under polymerization conditions, x(MCR). {graphics}