The propagation rate coefficient, kp, of methacrylic acid (MAA) in aqueous solution is strongly dependent on monomer concentration. Pulsed laser polymerization (PLP) at 25 °C and ambient pressure in conjunction with polymer analysis via size-exclusion-chromatography (SEC) was used to study whether kp also depends on monomer conversion. As the applicability of the PLP-SEC method is restricted to polymerization up to a few per cent of monomer conversion, situations of higher monomer-to-polymer conversion were achieved by adding to the MAA solution either (i) commercially available high-molecular-weight poly(MAA) or (ii) iso-butyric acid (IBA), which serves as a model component for an associated polymer with chain length unity. Within these experiments, the overall carboxylic acid concentration has been kept constant at 20 wt.-%. Under these conditions, kp of MAA turns out to be independent of the relative amounts of MAA and IBA, at least up to MAA:IBA ratios of 1:3, whereas kp increases by 60 per cent upon replacing half of the MAA content by poly(MAA), which situation corresponds to about 50 per cent monomer conversion in MAA polymerizations with initial MAA contents of 20 wt.-%. This kp value for 50 per cent conversion is close to the one obtained for PLP-SEC experiments at initial MAA concentrations of 10 wt.-%. The presence of poly(MAA) thus does not affect kp, whereas the IBA content has a similar effect on kp as has MAA concentration. The behaviour is understood as a consequence of IBA becoming part of the solvent environment at the radical site within the macroradical coil, whereas addition of poly(MAA) does not affect this intra-coil environment. This finding bears important consequences for the modeling of MAA polymerizations carried out at different initial MAA concentrations and up to different degrees of monomer conversion.