This work aims at shading fundamental insights into mechanisms that dictate the composition and microstructure of polypropylene/poly(ethylene-co-propylene) (PP/EPR) in-reactor alloys produced by the multizone circulating reactor (MZCR) technology, a novel one for polyolefin production. For this purpose, this technology is simulated by a batch process with periodic switching of monomer additions. An increase in the switch frequency leads to a decrease in the fraction of ethylene/propylene random (amorphous) copolymers denoted as EPR and an increase in the fraction and the length of PP segments of ethylene/propylene segmented (partially crystalline) copolymers denoted as EPS. Concomitantly, the size of the dispersed phase domains (EPR) decreases, and its size distribution becomes more uniform as a result of the decrease in the fraction of the EPR and the increase in that of the EPS which acts as a compatibilizer for PP/EPR in-reactor blends. Mechanical properties of the PP/EPR in-reactor blends are also discussed.