A new layer structure of VCL₃ showing stacking‐fault type disorder

Abstract

It is well known that the most stereospecific catalysts for the polymerization of a-olefins to isotactic polymers are the Ziegler-Natta heterogeneous catalysts. The most important component of these catalysts is a crystalline metal halide of the MtX, type characterized by a layer lattice (violet TiCl,, VCl,, etc.). In this communication, attention has been paid to the structural features of the active forms of some of these catalysts or even their supports.Ball-milling is quite a common procedure used to increase the activity of most solid catalysts. Generally, when a layer type structure catalyst undergoes ball-milling, the powder X-ray diffraction pattern of the ground catalyst exhibits considerable structural modification with respect to that of its starting material. This modification is observed with metal halides that can exist in at least 2 crystalline forms: a hexagonal close-packing and a cubic close-packing of the halogen atoms.Among the 3 crystalline forms, a-, /Iand y-TiCl,, in the best known Ziegler-Natta catalyst'), two of them, aand y-TiC1, are highly active in the polymerization of a-olefins. The violet forms aand y-TiC1, have a layer lattice structure and are triply layered with a metal layer between 2 chlorine layers. Only two thirds of the octahedral sites present are occupied by titanium atoms. The a-form consists of hexagonal closepacking of chlorine atoms, whereas a cubic close-packing of chlorine atoms is present in the y-form.When aor y-TiC1, is activated by grinding, a new violet modification is obtained.This new form, known as 6-TiC13, shows higher catalytic activity than its precursors. &-Tic!, has a triply layered structure and consists of both hexagonal close-packed and cubic close-packed chlorine atoms stacked randomly along the c crystallographic axis. This type of stacking may be called a polytypic disorder.The most active second generation Ziegler-Natta catalysts are those supported on MgCl,. The two crystalline forms of MgCl, (a and /I) are prepared chemically. The a-form, discovered long ago by Ferrari'), has a layer lattice with a cubic close-packing of chlorine atoms. Quite recently, a new form, B-MgCl,, having a hexagonal close-0173-2803/85/$01 .OO