Ti. Koranyi et al., Surface science approach to the preparation and characterization of model Ziegler-Natta heterogeneous polymerization catalysts, TOP CATAL, 7(1-4), 1999, pp. 179-185
Ziegler-Natta heterogeneous catalytic systems are extensively used to polym
erize ethylene and propylene. Some industrial catalysts consist of TiCl4 ch
emisorbed on activated MgCl2 and subsequently reduced and alkylated by reac
tion 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 contr
olled 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 electr
on irradiation-induced and metallic magnesium-induced chemical vapor deposi
tion. Angle-resolved XPS studies indicate that these films consist of a few
layers of TiCl2 with one monolayer of TiCl4 chemisorbed on its surface. Th
e exposure of these titanium chloride films to the co-catalyst AlEt3 produc
es an active model Ziegler-Natta catalyst. XPS analysis reveals the presenc
e of TiCl2Et on the catalyst surface: this is believed to be the active sit
e. Prolonged reaction with the co-catalyst reduces the titanium sites to Ti
ClEtn (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 Lew
is bases.