Roberto Zannetti

The activation of MgCl2-supported ziegler-natta catalysts: A structural investigation

Abstract A mathematical procedure for the investigation of the lattice disorder by stacking faults in polycrystalline solids has been used in studying samples of MgCl2 activated by ball milling for various times in the presence of TiCl4. These samples have been investigated by X-ray diffraction and by measurements of surface area, crystallite dimensions, Ti content after washing and catalytic activity in the polymerization of ethylene. A good relationship has been confirmed between activation time and activity; furthermore, the structural variations introduced by ball milling are mainly associated with rotational disorder of the Cl-Mg-Cl triple layers. Nevertheless, the analysis by X-ray diffraction is not unequivocal about the relationships between structure and activity: other significant parameters, such as the surface area and the porosity, are probably very important.

Small-angle X-ray scattering from high-density polyethylene: lamellar thickness distributions

Abstract Small-angle X-ray diffraction patterns were recorded for a number of high-density polyethylene samples and were successively analysed by a fit with the calculated patterns corresponding to certain theoretical models. The parameters determined by the above fits were as follows: long-period, crystallinity, mean dimensions of the crystalline lamellae, amorphous thickness, crystallinity and lamellar dimension distributions. Regarding the latter, two mathematical functions were used for the fits, i.e. a symmetrical and an asymmetrical type, and for all of the samples examined the function was determined which gives the best results. Finally, a correlation is suggested between the polymer molecular weight and lamellar dimension distributions.

A re-examination of the crystal structure and molecular packing of α-nylon 6

Abstract During studies of polymorphism of polyamides with an even number of carbon atoms, it was thought worthwhile to re-examine the crystal structure and molecular packing of the α-form of nylon 6 by more advanced instrumentation and computing facilities. X-ray diffraction measurements on mono-oriented fibres confirm that nylon 6 crystallizes in the monoclinic system, with crystallographic parameters: a = 9.71 A , b = 8.19 A , c = 17.40 A (fibre axis), γ = 115°, 8 monomeric units in the unit cell and space group P22. The crystal packing can be represented by parallel sheets of hydrogen-bonded antiparallel chains; the sheets are all oriented in the same sense and alternately displaced 3.73 A along the c axis. The hydrogen bond distance is 2.98 A. Although the structure was substantially similar to that previously found, some differences were observed; they are discussed on the basis of theoretical calculations.

The activation of MgCl2-supported ziegler-natta catalysts—II: Correlation between activity and structural disorder

Abstract The properties have been examined for Ziegler-Natta catalysts comprising TiCl4 (Ti contents 2.0, 3.4 and 4.2%) supported on MgCl2 activated by dry ball-milling up to 250 hr. The samples have been investigated by measurements of catalytic activity in polymerization, of surface area and of some structural parameters related to the crystal disorder introduced by ball-milling. It is shown that the TiCl4 content affects the activation; a relationship has been found between the crystal disorder of the MgCl2 support and the catalytic activity.

Small- and wide-angle X-ray scattering analysis of Ziegler–Natta catalysts: structural disorder, surface area and activity

Abstract An analysis was carried out of some Ziegler–Natta catalysts precursors by small- and wide-angle X-ray scattering in order to get information on the crystallites dimensions, structural disorder and surface areas. The results were correlated with catalytic activities in polymerization of propylene. It was shown that the structural variations, which mainly influence the catalytic activity, are associated with the rotational disorder of the Cl–Mg–Cl triple layers and that the crystallites dimensions and surface areas play a determinant role in the same activity. Furthermore it was shown that the surface areas of the examined samples, determined by BET, are less meaningful than the data obtained by small-angle X-ray scattering, probably for the presence of intra-particle closed pores and/or microporosity.

A crystallographic study of disorder in titanium trichloride, a component of Ziegler-Natta catalysts for stereospecific polymerization

Abstract The profiles of X-ray diffraction patterns have been examined for several powder samples of δ-TiCl3, obtained by mechanical activation of γ-TiCl3 and presenting different catalytic activities in the Ziegler-Natta stereospecific polymerization of propylene to isotactic polypropylene. Particular attention has been paid to the δ-TiCl3 samples showing the best catalytic properties. On the basis of a previous investigation by Allegra, a mathematical treatment has been developed and a disorder function has been elaborated taking into account, in calculating the profiles of the X-ray diffraction intensities, not only the disorder effects but also the sizes of the δ-TiCl3 crystallites. The selected experimental pattern (δ-TiCl3, highly activated) has been well reproduced in this way and fitted by overlapping the calculated spectra of two disordered forms, labelled ϵ1 and ϵ2, which differ in the relative amounts cubic and hexagonal sequences of the Cl-Ti-Cl layers constituting the violet forms of TiCl3. The best fit was achieved by introducing into the calculations crystallite sizes of about 70 A. The results of the structural investigation are also discussed in terms of activity of the δ-TiCl3 based catalyst for the stereospecific polymerization of propylene.

Crystal structure of the C form of stearic acid

The crystal structure of the C form of stearic acid has been determined from X-ray powder diffraction patterns, The crystals are monoclinic, space group P21/a, with unit-cell dimensions: a= 9·36, b= 4·95, c= 50·7 A. β= 128 °15′ and Z= 4. The crystal structure has been determined by trial-and-error methods to R 0·086. The structural parameters are compared with those of similar compounds, in particular the same acid in the polymorphous B form.

Synthesis and crystal structure of the MgCl2(CH3COOC2H5)2·(CH3COOC2H5) adduct

The reaction of α-MgCl2 with boiling ethyl acetate affords MgCI2(CH3COOC2H5)2·(CH3COOC2H5), which is obtained as crystals suitable for X-ray analysis only from the mother liquor. M=315.5, orthorhombic, space group P21221 (No. 18), a=25.077(3), b=8.616(1), c=7.345(1) A, V=1587.0(3) A3, Z=4, Dx=1.32 g cm−3,λ A(Mo Kα)=0.71069 A, μ=4.17 cm−1, F(000)=664, T=298 K, observed reflections: 1667, R=0.059 and Rw=0.069. The structure is composed of polymeric chains of MgCl2(CH3COOC2H5)2 and the ethyl acetate molecules occupy a mutually trans position.

Preparation, characterization and properties of some lanthanide oxides

Abstract The formation of oxides of the types Ln2O3 and Ln1Ln2O3 is reported. They have been obtained by thermal decomposition of homo- and heterodinuclear complexes of the type Ln2(L)(NO3)4·xH2O or Ln1Ln2(L)(NO3)4·xH2O where H2L is the macrocycle obtained by condensation of 2,6-diformyl-4- chlorophenol and 1,8-diamino-3,6-dioxaoctane. The composition and properties of the oxides, particularly of DyEuO3 and DyNdO3, have been studied by means of X-ray powder diffraction, electron microscopy and X-ray fluorescence techniques together with magnetic investigations. The products obtained have been compared with those prepared by thermal decomposition of the appropriate Ln(NO3)3·xH2O or, where available, with the commercial oxides. The polymorphism of the oxides has been also investigated.

Relationship between crystallization regimes and melting phenomena in isotactic polypropylene

Abstract A study has been made of the relationship between crystallization regimes and multiple peaks in the melting endotherms of isotactic polypropylene (iPP). Some samples of this polymer can show multiple melting peaks in differential scanning calorimetry (DSC) scans: this phenomenon has been studied and associated with a regime III of crystallization, giving rise to a phase which appears disordered from a crystallographic point of view. During melting, this phase recrystallizes, giving a more ordered phase which melts at a higher temperature. On the contrary, crystallization in regime II produces a crystalline phase which melts with a single endotherm. These phenomena have been studied by DSC, by optical microscopy in polarized light and by X-ray diffraction; the results are discussed in terms of the proposed interpretation.