title: Crystal structure of the trigonal form of isotactic polypropylene as an example of density-driven polymer structure.
abstract: Propene-hexene copolymers crystallize in a new polymorphic form of isotactic polypropylene when the concentration of hexene is higher than nearly 10-15 mol %. The hexene units are included in the crystals, inducing an increase of density that allows crystallization of 3-fold helical chains in a trigonal unit cell according to the space group R3c or Rc, where the helical symmetry of the chains is maintained in the crystal lattice. The structure of this new form is similar to those of isotactic polybutene and polystyrene and does not crystallize in polypropylene homopolymer because it would have too low density. The crystal structure of isotactic polypropylene is therefore no longer an exception to the principles of polymer crystallography, but the new structure represents the fulfillment of these principles and indicates that the packing of polymer molecules is mainly driven by density.
authors.affiliation: Dipartimento di Chimica, Università di Napoli Federico II, Complesso Monte Sant'Angelo, Via Cintia, 80126 Naples, Italy. claudio.derosa@unina.it
authors.forename: Claudio
authors.initials: C
authors.lastname: De Rosa
authors.forename: Finizia
authors.initials: F
authors.lastname: Auriemma
authors.forename: Paolo
authors.initials: P
authors.lastname: Corradini
authors.forename: Oreste
authors.initials: O
authors.lastname: Tarallo
authors.forename: Stefania
authors.initials: S
authors.lastname: Dello Iacono
authors.forename: Eleonora
authors.initials: E
authors.lastname: Ciaccia
authors.forename: Luigi
authors.initials: L
authors.lastname: Resconi