Model Ziegler−Natta Polymerization Catalysts Fabricated by Reactions of Mg Metal and TiCl₄:  Film Structure, Composition, and Deposition Kinetics

The film structure, composition, and deposition kinetics of model Ziegler−Natta polymerization catalysts produced by chemical vapor deposition have been studied using temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). Redox reactions of metallic Mg and TiCl4 deposited from the vapor phase on a Au substrate produce the model catalysts composed of titanium chloride and magnesium chloride (TiClx/MgCl2). The low solubility of TiClx in MgCl2 leads to the formation of a TiClx monolayer on top of MgCl2 multilayers characteristic of the supported catalysts. The TiCl4/Mg flux ratio during the deposition controls the oxidation state distribution and surface coverage of TiClx species of the deposited film. The conversion of Mg to MgCl2 is completed in the codeposition process while it is incomplete in the sequential deposition process due to a kinetic barrier for chlorine diffusion. Upon heating the sequentially deposited film, the surface TiClx (Ti4+) species react with metallic Mg atoms in underlayers to complete the redox reaction. In the case of the codeposited film, the surface TiClx species desorb as TiCl4 at temperatures higher than 430 K. After exposure to triethylaluminum (AlEt3) cocatalyst, the model catalyst films are active for propylene polymerization.