PREPARATION AND SURFACE SCIENCE CHARACTERIZATION OF MODEL ZIEGLER-NATTA CATALYSTS. ROLE OF UNDERCOORDINATED SURFACE MAGNESIUM ATOMS IN THE CHEMISORPTI ON OF TICL4 ON MGCL2 THIN FILMS

Two new synthetic routes for the preparation of model Ziegler−Natta catalysts under UHV conditions are described. The exposure of metallic magnesium to TiCl4 produced titanium chloride films with Ti in the 4+, 3+, 2+, and 0 oxidation states. Stable titanium chloride films could also be obtained by TiCl4 and Mg codeposition on MgCl2 and Au. A TiCl4/TiCl2 film was obtained in this case. The reaction of these systems with AlEt3 produced model catalysts for the polymerization of both ethylene and propylene. XPS is a proper technique for the characterization of the oxidation state of Ti in a variety of titanium chloride surface species. The stability of MgCl2 surfaces with a high concentration of undercoordinated Mg atoms was studied in UHV by Mg gas-phase deposition on a MgCl2 multilayer film. The Mg adatoms were readily coordinated by the Cl ions diffusing from the halide bulk to the surface. Mg-containing MgCl2 faces are thermodynamically unstable, and the fast diffusion of the Cl ions in the MgCl2 matrix a...