DISORDER IN LAYERED TICL3 - SYMMETRY OF THE COORDINATION SITE

The structural layers of TiCl3 consist of two monoatomic layers of chl orine atoms octahedrally coordinated to intercalating Ti atoms. Mechan ical (grinding) and/or chemical treatment of the regular alpha- or gam ma-forms may produce the disordered delta-form, wherein shifts and rot ations between neighboring layers give rise to the largest disorder co mpatible with close-packing of the chlorine layers, accompanied by red uction of crystallite size both along the layers and across them. A Be rnoullian statistical model fully describes the statistical structure of the polycrystalline samples in terms of 4 parameters, two of them r elating with the stacking disorder, the other two with crystallite siz e distribution. The catalytic sites for isospecific alpha-olefin polym erization lie on the side edges of the structural layers. It is shown that Ti atoms on these edges possess two isochiral free valencies, ena bling them to exchange the roles of the (Ti-coordinated olefin) and of the (Ti-growing chain) bonds at each polymerization step with no chan ge of the local chirality, in analogy with the metallocene-based isosp ecific catalysts.