Derivation of the isotactic block length distribution is made for polypropylenes having Bernoullian (chain-end control) or hemiisotactic microstructures. For each of these tacticities, the isotactic block length distribution depends on a single stereochemical parameter and M-n, the number-average molecular weight. For polypropylene with Bernoullian statistics, the number of isotactic blocks of length n is given by N-n = (1 - sigma)(2)(sigma)((n-1)) (M-n/42.08). For hemiisotactic polypropylene, the number of isotactic blocks of length one is given by N-1 = [(0.5)(1 - alpha)(2) + (0.5)(1 -alpha)](M-n/42.08) and the number of isotactic blocks of length n (where n is an odd integer greater than one) is given by N-n = (0.5)(1 - alpha)(2)(alpha)((n-1)/2)-(M-n/42.08). The distributions are compared and contrasted for hypothetical polypropylenes and real polypropylenes for which sigma, alpha (C-13 NMR) and M-n (GPC) have been determined. For polypropylene with M-n = 100 000 an elastomeric morphology is calculated to exist for sigma = 0.752-0.827 (Bernoullian) and for alpha = 0.575-0.693 (hemiisotactic). The lower limit defines the presence of at least two crystallizable isotactic segments (n greater than or equal to 21) per chain; the upper limit defines the point at which the calculated percent crystallinity exceeds 10%, and the properties are dominated by the crystalline phase at the expense of the amorphous phase. The elastomeric properties observed in hemiisotactic polypropylenes with alpha approximate to 0.6 are readily explained by the statistical presence of crystallizable isotactic blocks in the presence of an otherwise amorphous medium. In contrast, the calculations suggest that most elastomeric Bernoullian polymers (i.e., atactic with sigma near 0.5) rely on chain entanglements rather than crystallization phenomena for their elastomeric properties since the calculated distributions do not render a sufficient number and arrangement of crystallizable isotactic blocks.