An efficient kinetic model based on the pseudo-homopolymerization (PHP) and reduced stiffness by quasi-steady state approximation (RQSSA) methodology is presented to compute the molecular weight distribution (MWD) in the activator regenerated by electron transfer via atom transfer radical copolymerization. The copolymerization of butyl methacrylate and butyl acrylate is taken as basis of the study; validation with the experimental data results in good agreement, and a deviation between the prediction of the Mayo–Lewis equation and the model estimation is presented. The MWD is dependent on the fed monomer composition. A semibatch process with an extra addition of reduced agent resulted in an increased polymerization rate; however, this affects the process control, and a broader MWD is obtained. The computational time expended to compute the MWD is on the order of seconds or minutes, which can be reduced by using a new expression to calculate the bounds of the MWD.