Reaction and diffusion of free radicals within a latex particle are investigated for the entry rate in emulsion polymerization when the entry requires that the oligomeric free radical reach a certain size. Transient expressions obtained for the entry rate show that the events within the particle do play a role in the transients but that they have no role in the steady-state entry rate. The transient behavior is shown to reveal useful information on the critical size of the oligomeric free radical. Initiator and monomer concentrations control the approach to the stationary state. A concise expression is obtained for a condition that has to be satisfied for the usual stationary state assumption.