AbstractIsothermal crystallization of isotactic poly(1‐butene) fractions by calorimetry and microscopy was studied for a range of molecular weights from 96 000 to 964000. An Avrami exponent of 2 was found indicating a two‐dimensional growth rate of the crystal, following a predetermined nucleation mode, proving that such an exponent is independent of both temperature and molecular weight. The temperature and molecular weight dependence of the growth rate of spherulites and overall crystallization rates was analyzed by means of the theoretical expression given by Hoffman and Lauritzen. The product of the lateral and basal surface free energies of the crystallites, σ·σe, increases with molecular weight and approaches its limiting values when a certain molecular weight is at