Crystal structure of isotactic propylene-hexene copolymers: the trigonal form of isotactic polypropylene.

A study of the structure of isotactic propylene-hexene random copolymers, prepared with single-center metallocene catalysts, is reported. For low concentrations of hexene comonomeric units, up to nearly 10 mol%, copolymers crystallize in the alpha form of isotactic polypropylene. Copolymers with hexene contents higher than 10 mol% crystallize in a new polymorphic form that melts at nearly 50 °C. The hexene units are partially included in both crystals of alpha form and of the new form. The crystallization of the new crystalline form allows incorporation of a high amount of hexene units. The melting temperature and the crystallinity decrease rapidly with increasing hexene content. A slower decrease of melting temperature and crystallinity is observed at high hexene concentrations because of the crystallization of the new form. The crystal structure of the new form has been studied by analysis of X-ray powder and fiber diffraction patterns of samples containing hexane concentration higher than 10 mol%. Chains in three-fold helical conformation of propylene-hexene copolymers are packed in a trigonal unit cell according to the space group R3c or R3menoc. The values of a and b axes of the unit cell depend on hexene concentration, and values of a = b = 17.5 Å and c = 6.5 Å have been found for the sample with 26 mol% of hexene. The structure contains high degree of disorder due to the constitutional disorder of the random copolymer chains that produces disorder in the positioning of the lateral groups in the unit cell. Moreover statistical disorder in the up-down positioning of the helical chains and slight disorder in the orientation of chains around the three-fold axes are present. The inclusion of hexene units in the crystals induces a suitable increase of density that allows crystallization of three-fold helical chains in the trigonal form, where the helical symmetry of the chains is maintained in the crystal lattice. The structure is similar to that of form I of isotactic polybutene. This form does not crystallize, and has never been observed so far for the polypropylene homopolymer because, in the absence of bulky side groups, it would have a too low density. This structure represents an example of entropy driven phase formation.