Surface science approach to the preparation and characterization of model Ziegler-Natta heterogeneous polymerization catalysts

Ziegler-Natta heterogeneous catalytic systems are extensively used to polymerize ethylene and propylene. Some industrial catalysts consist of TiCl4 chemisorbed on activated MgCl2 and subsequently reduced and alkylated by reaction with an aluminum alkyl (generally AlEt3). Lewis bases are added to the catalytic systems to control the enantio-selectivity for the production of isotactic polypropylene. Our aim is to clarify the chemical composition of the active centers by modern surface science methods. Model catalysts are prepared in the form of ultra-thin films by gas-phase deposition on a gold foil in ultrahigh vacuum. Under these conditions, MgCl2 films grow to controlled thickness via a layer-by-layer mechanism, as revealed by AES and XPS. TiCl4 can be deposited on these films near room temperature by both electron irradiation-induced and metallic magnesium-induced chemical vapor deposition. Angle-resolved XPS studies indicate that these films consist of a few layers of TiCl2 with one monolayer of TiCl4 chemisorbed on its surface. The exposure of these titanium chloride films to the co-catalyst AlEt3 produces an active model Ziegler-Natta catalyst. XPS analysis reveals the presence of TiCl2Et on the catalyst surface: this is believed to be the active site. Prolonged reaction with the co-catalyst reduces the titanium sites to TiClEtn (n= 1 and/or 2). High molecular weight polyethylene and polypropylene are synthesized on these catalysts, as shown by Raman spectroscopy. Highly isotactic polypropylene is produced without need for stereo-regulating Lewis bases.