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.

A cheap, commercial, and easy-to-process hydrocarbon polymer (syndiotactic polystyrene, s-PS) can be crystallized in two different nanoporous phases (δ and ε) that show large uptake of several guest molecules and hence are suitable for applications in chemical separations and air/water purification as well as in sensorics. The recently discovered ε phase of s-PS is characterized by channel-shaped cavities crossing the unit cells along the c axis rather than by isolated cavities as observed for the already known δ phase. Guest sorption from the δ and ε nanoporous phases eventually leads to two different classes of polymer cocrystals, where planar guest molecules present their planes roughly perpendicular and parallel to the polymer chain axes. The ε phase channels allow cocrystal formation with guest molecules much longer than the chain axis periodicity and, due to possible reactions between guest molecules, open the possibility to achieve new hybrid materials.

A comparative analysis of the polymorphic behavior of samples of isotactic polypropylene (iPP) prepared with heterogeneous Ziegler−Natta catalysts and with a single-center homogeneous metallocene catalyst is presented. Different samples of Ziegler−Natta iPP, prepared with MgCl2-supported catalysts modified by adding different Lewis bases, have been fractionated by extraction with boiling solvents. The irregular fraction, insoluble in diethyl ether and soluble in hexane, crystallizes from the melt almost totally in the γ form. The more stereoregular fractions crystallize instead basically in the α form. This confirms that, even in the case of Ziegler−Natta iPP samples, the γ form may develop by melt-crystallization at atmospheric pressure in fractions containing a high concentration of defects. The relative amount of γ form crystallized from the melt is, however, much lower that that observed in samples of metallocene-made iPP containing comparable amount of defects. Since the γ form crystallizes in chains having short regular isotactic sequences, these data indicate that in Ziegler−Natta iPP samples the regular isotactic sequences are longer than those present in chains of metallocene-made iPP having a similar overall concentration of defects. The different polymorphic behavior of metallocene and Ziegler−Natta iPP samples is related to the different distribution of defects in the polymeric chains, generated by the different kinds of catalytic systems. While in the metallocene-made iPP the distribution of defects along the chains is random, in Ziegler−Natta iPP samples the majority of the defects are segregated in a small fraction of poorly crystallizable macromolecules or in more irregular portions of the chain, so that much longer fully isotactic sequences can be produced, leading to the crystallization of the α form, even for a relatively high overall concentration of defects. These results confirm the idea that the structural analysis of iPP, in particular the crystallization of the γ form, may give information about the microstructure of the polymer chains. The measure of the maximum amount of γ form crystallized from the melt may be used as an indirect method to evaluate the average length of isotactic sequences. This analysis allows concluding that some fractions of Ziegler−Natta iPP are characterized by chains with a stereoblock microstructure, consisting of regular isotactic sequences linked to more irregular sequences. The latter contain the major part of stereodefects mainly consisting in isolated rr triads, r diads, and longer ...rrrr... syndiotatic sequences. The hypothesis of a stereoblock microstructure for some of these less stereoirregular fractions is also consistent with the high degree of crystallinity observed in the samples crystallized from solution or from the melt, despite the high concentration of defects.

Polymeric ''cocrystalline forms,'' that is, structures were a polymeric host and a low-molecular-mass guest are cocrystallized, were early recognized, and in many cases also well characterized by X-ray diffraction studies. However, only in the last two decades cocrystalline forms have received attention in material science, due to the ability (of few of them) to maintain an ordered polymer host structure even after guest removal, thus leading to the formation of ''nanoporous-crystalline forms,'' for which many applications in the fields of molecular separation and sensors have been proposed. Moreover, in the last decade, an accurate control of the orientation of the polymer cocrystalline phases has been achieved, thus leading to a control of the orientation of the guest molecules, not only in the crystalline phase but also in macroscopic films. In addition, on the basis of this orientation control, in the last few years, cocrystalline films where active molecules are present as guests of polymer cocrystalline phases have been proposed for optical, magnetic and electric applications. In the last few years, it has been also discovered that polymer cocrystallization, when induced by nonracemic guest molecules, can produce stable chiral optical films. V C 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: [305][306][307][308][309][310][311][312][313][314][315][316][317][318][319][320][321][322] 2012

A study of the morphology and the mechanical properties of the mesomorphic form of isotactic polypropylene (iPP) that crystallizes in samples of different stereoregularity prepared with metallocene catalysts is reported. Highly isotactic samples slowly crystallized from the melt in the α form show the typical lamellar morphology with organization in spherulites. Bundle-like elongated crystalline entities and needle-like crystals of γ form are instead observed for stereoirregular samples slowly crystallized from the melt in the γ form. All samples crystallize by fast quenching the melt at 0 °C in the mesomorphic form, regardless of stereoregularity. Crystals of the mesomorphic form always exhibit a nodular morphology and absence of lamellar spherulitic superstructures, independent of the stereoregularity. This morphology accounts for the similar good deformability of all the quenched samples, whatever the concentration of stereodefects. For all samples and any stereoregularity, the nodular morphology with absence of spherulites is preserved after annealing and transformation of the mesophase into α form. The formation of a nodular α form accounts for outstanding properties of high ductility and mechanical strength of the annealed samples crystallized in the α form. The most stereoirregular sample with rr concentration of 11% shows elastic behavior either when it is slowly crystallized in the γ form or when it is crystallized in the mesomorphic form and also after annealing and transformation of the mesophase into the nodular α form