Fragmentation of support/catalyst particles during propylene bulk polymerization is analyzed by means of a mathematical model including energy and mass balances, with chemical reaction. The rupture phenomenon is specifically considered by the model and analyzed as it proceeds along time. Model predictions concerning the effects of fragmentation on polymerization are discussed. The influence of mass-transfer resistances at the macroparticle and microparticle level, as well as the microparticle nucleus-size effects over the polymerization process, are analyzed. Macroparticle mass-transfer resistance affects both the rate of fragmentation and temperature excursions. Microparticle nucleus-size exerts a strong influence over the whole polymerization process. A small micronucleus-size produces both a delay in the fragmentation process and a greater value of she final catalyst yield. The effects of major critical parameters are evaluated via model simulation, and the results are discussed. The analysis shows that fragmentation depends on the combined effect of the parameters studied. Modeling of the process considering all parameters simultaneously is the proper way of predicting the fragmentation sequence for a given support/catalyst particle. Crystallinity of the produced polymer affects the rate of fragmentation, either increasing or decreasing the rupture rate depending on macroparticle porosity and compactness. Heat transfer conditions in the liquid-phase system make the temperature runaway problem easy to predict and control, in spite of high polymer yields. The design of “tailor-made” support/catalyst macroparticles in accordance with catalytic activity is necessary in order to obtain high yields and controlled process temperatures.