The Double Role of Comonomers on the Crystallization Behavior of Isotactic Polypropylene:  Propylene−Hexene Copolymers

A study of the crystallization behavior of isotactic propylene−hexene copolymers prepared with metallocene catalysts is reported. These copolymers crystallize from the melt as mixtures of α and γ forms of isotactic polypropylene (iPP). The amount of γ form first increases very rapidly with increasing hexene concentration, up to 2−3 mol %, and then rapidly decreases for hexene concentrations higher than 4−5 mol %, and samples with hexene contents higher than 10 mol % (and lower than 15−16 mol %) crystallize from the melt exclusively in the α form. This is due to the fact that at high concentrations hexene units enter to high extents into crystals of α form, driven by density increase, favoring crystallization of the α form instead of that of the γ form. These data have confirmed that the crystallization of γ form of iPP is not only related to the value of the average length of the regular propylene sequences but also related to the different degrees of inclusion of stereodefects and constitutional defects in the crystals of α and γ forms.