The influence of average molar mass and shear on the crystallization of isotactic polypropylene (iPP) has been studied. The Weissenberg number, a dimensionless parameter consisting of the product of the shear rate and a characteristic relaxation time (gamma(0)lambda(r)), provides a criterion to locate the morphological transition from spherulitic to extended growth. The Weissenberg criterion applies to polymers of high and of low molecular weight. This is attributed to the importance of the initial nucleation process. Comparison of materials of different molar masses and various shear histories is allowed by the introduction of the normalized crystallization time tau, which is the ratio of the experimental time and a characteristic crystallization time lambda(Vvmax) that needs to be defined from the several possible choices. Rheological and rheooptical experiments furnish the material characteristic relaxation and crystallization time scales, respectively. The time lambda(Vvmax), defined with light scattering as the moment at maximum density fluctuation invariant Qeta, is chosen here as a characteristic crystallization time scale since it is especially sensitive to the early stages of crystal growth that are responsible for the liquid-to-solid transition. DSC and WAXD have been used to further analyze the effect of shear on the populations of crystalline fractions present in the cooled iPP samples.