Simultaneous crystallization of isotactic and syndiotactic sequences of polypropylene

Polypropylene samples containing 45% isotactic and 20% syndiotactic pentads were characterized by calorimetry and solid-state NMR. Iso- and syndio-sequences are able to crystallize in the same sample to form a complex mixture of several allomorphs: monoclinic alpha (2) phase together with disordered alpha phases for isotactic and extended chain together with helix forms for syndiotactic stereosequences. The fraction of each phase was established quantitatively, due to the limited overlapping of the signals at room and high temperature. The partition of stereosequences between the amorphous and crystalline phases was determined at room and high temperature. The distribution of the stereosequences in the amorphous phase was established at high temperature at the pentad level; thus, it was possible to determine the stereosequences already crystallized at that temperature. Hydrogen spin-lattice relaxation times in the rotating frame, measured under spin diffusion conditions, demonstrate the segregation of iso- and syndiotactic crystallites and the intimacy of the phases of the same tacticity.