AbstractA series of well defined polyethylene-b-polystyrene diblock copolymers (ExSyz, where x and y represent the composition in weight % and z the molecular weight in Kg/mol) has been synthesized in a wide composition range by sequential anionic polymerization. The molecular weight of the PE block was kept constant. A fractionated crystallization behavior was observed for the PE block within E26S74105 (PE cylinders) and E11S89244 (PE spheres). When the PE blocks form a continuous or percolated phase (PE, E79S2141 and E53S4751), a classic self-nucleation behavior (where the usual three self-nucleation domains are obtained) was observed. When the PE block is located within isolated microphases (having dimensions on the nanometer scale) and a fractionated crystallization was detected (E26S74105 and E11S89244), the fraction of crystals formed at higher temperatures exhibits a classic self-nucleation behavior, while those crystals that crystallized at the largest supercooling (lower exotherms) can only be self-nucleated at lower temperatures where annealing of unmolten material has already started. An unusual fractionated crystallization behavior for isolated, spherical PE microphases (E11S89244) is reported.