Gelation and precipitation of copolymers from solution

There are several reports in the literature to the effect that when isotactic polystyrene crystallizes from moderately dilute solution, lamellas (precipitate) form at high crystallization temperatures and gels at low temperatures. We report here the occurrence of a very similar phenomenon during the crystallization of ethylene‐butene‐1 copolymers. The observations for the latter system can be given a straightforward explanation based on the demonstrated compositional fractionation that occurs on crystallization. Low co‐unit content copolymers are associated with lamellas. Gels invariably contain much higher co‐unit content. A generalization of these findings is invoked to explain the previous polystyrene results. A new resonance detected by 13C NMR experiments in this polymer can be ascribed to head–head sequences.