The kinetics and mechanism of seeded dispersion polymerization of methyl methacrylate (MMA) was studied by applying both micron and submicron PMMA seeds. Using a 1.7 mu m PMMA seed (N-p = 1 X 10(12)/L) and a monomer polymer ratio (M/P) of 28/1, secondary nucleation was found to occur and the number of new particles exceeded that produced in a parallel ab initio dispersion polymerization. This was explained by the paradoxical initiator concentration effect seen in dispersion polymerizations where the number of particles decreases with increasing initiator concentration. In contrast, using 194 nm (N-p = 26 x 10(12)/L; M/P = 833/1) and 317 nm (N-p = 5.6 x 10(12)/L; M/P = 714/1) submicron seeds, it was found that the final particle number was similar to (or less in a few cases) the initial seed number over a relatively wide range of initiator concentrations. With increasing initiator concentration, the initial reaction rate increased but the maximum reaction rate decreased slightly. This was explained by increased radical termination particularly in unstable nuclei, leading to a reduced radical entry rate. The reaction rate was found to be moderately dependent on the number of seed particles, but was independent of the seed surface area. (c) 2008 Wiley Periodicals, Inc.