The concept of the critical chain length for radical entry Z was used for modelling the rate of carboxylation between the particle surface and particle core. An average Z value was computed by a semi-empirical method from the individual Z values for homopolymers. Grafted, buried and water soluble polymerized acid could be computed as well as the molecular weight of the grafted and water soluble species. The model was tested on two different systems involving either styrene/butadiene or styrene/butyl acrylate as the main monomers and acrylic acid as the carboxylated monomer; it gave a good approximation of the actual rate of grafting and molecular weight. In butadiene containing recipes, gel permeation chromatography (g.p.c.) of the aqueous phase showed an unusual peak that might be assigned to the exit of butadiene radical followed by propagation and particle capture.