The product of the propagation rate constant and monomer concentration is a central issue of radical polymerization. Both quantities are strongly interrelated as the determination of a reliable value of the propagation rate constant requires exact knowledge of the monomer concentration. Even for homogeneous polymerization conditions, exact knowledge of the monomer concentration at the reaction site from the molecular perspective is not trivial. For emulsion polymerizations the situation is additionally complicated by mass transfer events and colloid-chemical effects. Molecular modeling appears as an attractive alternative or complement to the deterministic kinetic description of radical polymerizations.