Characterizing partially crystalline isotactic polypropylene can be carried out by extractions which are based on decreases in solubility with increased crystallinity, which in turn increases with increased isotactic block length. The present investigation employs Monte Carlo techniques to model how crystallization in polypropylene might depend on increase in isotactic block length (with the overall isotacticity either increasing or being held constant), and on its distribution (polydispersity). While isotactic block length was found to have a strong effect on the characteristics of crystallization, it should have less effect on elastomeric moduli calculated from the number density of crystallites (when functioning as volume-less cross-links). In contrast, block-length polydispersity had little or no effect on the crystallization, but a rather unusual effect on mechanical properties. These dependences may be due to the assumption that the elastomeric moduli depended only on the number density of the crystallites, without account for size effects such as large crystallites also functioning as filler particles.