Two and three-state statistical models, which are mathematical devices used to partition observed poly(propylene) sequence distributions, are usually evaluated on how well they reproduce observed overall pentad/heptad distributions from C-13 NMR. In this study, sequence distribution curves vs crystallization rates are also employed to judge the behavior of a particular model. A supercritical fluid fractionation of a poly(propylene) prepared with a MgCl2-supported Ziegler-Natta catalyst provided a series of samples that had progressively decreasing racemic contents as the molar masses increased. A modification was made to an existing three-state statistical model that gave sequence distribution curves that satisfied the fractionation behavior as well as linear spherulitic growth rates and polymorphic behaviors observed for the series of Ziegler-Natta poly(propylene) fractions. The results indicate that MgCl2-supported Ziegler-Natta isotactic poly(propylene)s contain significant amounts of a high molar mass, extremely isotactic poly(propylene) molecules with average isotactic sequence lengths exceeding a thousand or more. There is a second predicted major type of polypropylene molecule, which has some isotactic blocks of opposite handedness, an average isotactic sequence length of around 200 and a sequence length distribution curve that partially overlaps with that of the highly isotactic component. Finally, a third and minor component is a stereoblock type of poly(propylene) molecule, which has an average isotactic sequence length of no more than 5-25 repeat units. This microstructural distribution, predicted by the newly modified three-state statistical model, is also consistent with that reported in the literature for a similar MgCl2-supported Ziegler-Natta poly(propylene) where the intermolecular microstructural distribution was derived from a TREF study.