Structure of Copolymers of Syndiotactic Polypropylene with Ethylene

A structural characterization of copolymers of syndiotactic polypropylene containing ethylene comonomeric units (PPET) in a wide range of concentrations (0.4-60 mol %) is presented. The copolymer samples are crystalline up to an ethylene content of 18-20 mol %. Ethylene units are partially included in the crystals of both as-prepared and melt-crystallized samples. As-prepared PPET samples basically crystallize in conformationally disordered modifications of form II of syndiotactic polypropylene (sPP) containing kink bands. The amount of kink bands defects, represented mainly by T6G2 sequences, increases with the ethylene content. These modifications are metastable and transform by crystallization from the melt into the most stable form I or form II, depending on the ethylene concentration. PPET samples having low ethylene content (up to 6-7 mol %) crystallize from the melt into the antichiral form I, even though disordered modifications of form I are always obtained. For higher ethylene contents, mixtures of crystals of forms I and II are obtained by crystallization from the melt. The fraction of form II increases with increasing the crystallization temperature and the ethylene content. The form II obtained by melt crystallizations does not present kink-band disorder, all chains being in the more stable (T2G2)n helical conformation. This is the first example of crystallization of the isochiral form II of sPP from the melt at atmospheric pressure.