Crystallization of Propene–Pentene Isotactic Copolymers as an Indicator of the General View of the Crystallization Behavior of Isotactic Polypropylene

The crystallization from the melt in isothermal conditions of metallocene random propene–pentene isotactic copolymers (iPPC5) has been studied. All samples with pentene concentration between 0.5 and 10 mol % crystallize at any crystallization temperature in mixtures of α and γ forms of isotactic polypropylene (iPP) and the amount of γ form increases with increasing crystallization temperature up to a maximum (fγ(max)), which depends on pentene concentration. Pentene defects produce a shortening of the regular propene sequences that in turn induces crystallization of the γ form. At concentrations higher than 6–7 mol %, pentene units are incorporated to a high extent in the crystals of α and trigonal forms, which are stabilized over the γ form, and fγ(max) decreases. The maximum fraction of γ form is, therefore, related to the average length of regular propene sequences and the degree of incorporation of defects in the crystals of α and γ forms. The values of fγ(max) that develop in iPPC5 copolymers have been compared with those that develop in copolymers of iPP with ethylene (iPPC2), butene (iPPC4), and hexene (iPPC6) and in stereoirregular iPPs reported in the literature. Stereoirregular iPPs and iPPC2 copolymers give the same relationship between fγ(max) and the average length of regular propene sequences (LiPP), whereas iPPC4, iPPC5, and iPPC6 copolymers show different behaviors. In particular, iPPC5 copolymers exhibit a behavior intermediate between those of iPPC4 and iPPC6 copolymers. The relationship between fγ(max) and LiPP of iPPC5 copolymers fits perfectly between the relationships found for iPPC4 and iPPC6 copolymers, in agreement with the different types and sizes of comonomers and the different efficiencies of their interruption and inclusion effects. These data give evidence of the general view of the crystallization behavior of iPP, based on the definition of a double role exerted by defects, the interruption effect that shortens the regular propene sequences and favors crystallization of γ form, and the effect of incorporation of defects into the crystalline unit cells of α and γ forms, which favors crystallization of the form that better accommodates the defect into crystals. The relative efficiency of these two effects depends on the type and size of the defect. The different relationships between fγ(max) and LiPP are a result of the equilibrium between interruption and inclusion effects achieved by each defect and confirm that the crystallization of γ form is a perfect indicator of the length of the regular propene sequences and may provide very detailed information on the molecular structure of iPP.