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 (TiCl_x/MgCl₂). The low solubility of TiCl_x in MgCl₂ leads to the formation of a TiCl_x 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 TiCk 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 TiCl_x (Ti⁴⁺) species react with metallic Mg atoms in underlayers to complete the redox reaction. In the case of the codeposited film, the surface TiCl_x 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.