In a kinetically-controlled step-growth polymerization, the molecular weight distribution (MWD) of the polymers produced can be controlled via its reaction path. In this paper, the effect of intermediate monomer feed on the MWD in a batch reactor or a continuous plug flow reactor (PFR) is investigated, and a kinetic model is proposed for MWD development. The present model considers the interchange reactions during polymerization. The variance of the MWD can be increased by the intermediate monomer feed method while the interchange reactions counteract this effect. The present model has given satisfactory predictions for the polycondensation of adipic acid and 1,6-hexanediol.