The different crystalline modifications of TiCl3, a catalyst component for the polymerization of α‐olefins. I: α‐, β‐, γ‐TiCl3. II: δ‐TiCl3

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Abstract
Crystalline TiCl3 is the typical solid component of the catalytic system for the stereo‐specific polymerization of α‐olefins; hence a great interest in the knowledge of its crystal structure arises. Four different crystalline modifications of TiCl3 (α, β, γ and δ) are known to date; three (β, γ, and δ) were first described in our laboratory. A preliminary structural distinction among them can be made in terms of fiber‐shaped (β) and layer (α, γ, and δ) modifications. The crystal structure of the β‐modification can, in fact, be considered as built up by linear macromolecules, while identical bidimensional (planar) macro molecules, packing with different sequences of shifts, between themselves, can originate an α‐, or γ‐, or δ‐TiCl3 crystal. This close structural analogy explains the great similarity in physical and chemical properties of the last three modifications. In the first part of this paper, the most important structural features of α, β, and γ‐modifications are described; in the second, the structure of the most recently discovered δ‐TiCl3 is more deeply analyzed. The presence of diffuse halos instead of sharp reflections in its Debye spectra clearly indicates the presence of a certain structural disorder; mathematical calculations (not fully reported here), based on the alternative assumption of two different statistical models of shift successions between structural layers, have led us to find the most probable succession type. As the possible shifts between adjacent structural layers cannot be different in type, in δ‐TiCl3, from those which take place in the ordered α‐ or γ‐modifications, we can say that, in δ‐TiCl3, the intervention of disorder in shift successions leads to something intermediate (in a statistical sense) between the two stated structures.