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

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 ?₁ and ?₂, 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 Å.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.