Characterization of the Ziegler-Natta olefin polymerization system: surface science studies on model catalysts

Modem surface science techniques have been applied to the heterogeneous Ziegler-Natta catalysis system, polymerizing alpha -olefins to produce polyolefins, and revealed the correlation between the catalyst surface structure and polymer properties. Two types of thin films - TiCl(x)/MgCl(2) and TiCl(y)/Au - were fabricated on an inert gold substrate, using chemical vapor deposition methods, to mimic the high-yield catalysts of MgCl(2)-supported TiCl(4) and TiCl(3)-based catalysts, respectively. Once activated with triethylaluminum (AlEt(3)) vapor, both catalysts were active for polymerization of ethylene and propylene in the absence of excess AlEt(3). The model catalyst films were as active as the high-surface-area industrial catalysts. Both catalysts were terminated with chlorine at the surface but had different surface structures. The TiCl(x)/MgCl(2) film had a distribution of two structures: the dominant sites had the (001) basal plane of these halide crystallites and the minority sites had a non-basal plane structure. The surface of the TiCl(y)/Au film assumed only the non-basal plane structure. These structural differences resulted in different tacticity of the polypropylene produced with these catalysts. The TiCl(x)/MgCl(2) catalyst produced both atactic and isotactic polypropylene, whereas the TiCl(y) catalyst without the MgCl(2) support produced exclusively isotactic polypropylene. The titanium oxidation states did not appear to be an important factor in determining the tacticity of the polypropylene. Copyright (C) 2001 John Wiley & Sons, Ltd.