Synthesis, structure and properties of copolymers of syndiotactic polypropylene with 1-hexene and 1-octene

Syndiotactic propene-hexene (sPPC6) and propene-octene (sPPC8) copolymers have been synthesized in a wide range of compositions with a syndiospecific metallocene catalyst. The samples are crystallized up to a comonomer content of nearly 20 mol% with crystallinity, melting and glass transition temperatures that decrease with increasing concentrations of comonomers. The copolymers show elastic properties that depend on the crystallization behavior. The as-prepared samples with a comonomer content lower than 2 mol% crystallize in mixtures of the helical form I and form II of syndiotactic polypropylene (sPP), the latter containing conformational kink band defects and the former containing a b/4 shift disorder. Both sPPC6 and sPPC8 copolymers crystallize from the melt in disordered modifications of form I characterized by disorder in the alternation of right- and left-handed two-fold helical chains along the axes of the orthorhombic unit cell of form I of sPP. At high crystallization temperatures, more ordered modifications of form I develop for samples with a low comonomer concentration and form II surprisingly crystallizes in samples with a high comonomer content. The crystallization of form II rather than the ordered form I in melt-crystallized samples is a valuable result because it indicates that it is possible to tune the polymorphic transformations and the elastic properties of these copolymers through tuning of the branches concentrations.