Dr. Ralf Widmaier from the BASF AG Polymer Research in Ludwigshafen is awarded the Schering Prize 2001 for his research on “Fragmentable carriers for co-catalysts in metallocen-catalyzed polymerization”.
Fragmentable carriers for co-catalysts in metallocen-catalyzed polymerization
Two different spherical nanodisperse silica gels with particle diamters of 235 and 10-20 nm were modified in suspension with alkoxy- and chlorosilanes carrying amino-, ether- siloxane- and vinylfunctionalities.
The carriers were examined using various techniques: surface and particle size measurements, scanning and transmission electron microscopy, energy-dissipative x-ray absorption (EDXA), 27al-solid state nmr as well as elemental analysis. The modification of the silica resulted in interlinkind and surface modification silica spheres. The degree of interlinking depended on the structure of the silane, the water-content during the modification reaction and the stoichiometric ratios of silanol-groups (on silica), water and silanes.
These modified silicas were modified with various simple aluminiumorganyls like trimethylaluminium (TMA), triethylaluminium (TEA), diethylaluminiumhydride (DEAH) and diisobutylaluminiumhydridy (DIBAH) and oligomeric methylaluminoxane (MAO) to yield heterogeneous cocatalysts for the metallocene-catalyzed polymerization of olefins. The modification also was done in two-step-reactions using simple aluminiumorganyl and MAO or MAO in both steps. Unmodified silica was also used to heterogenize MAO: Two donor-atom-stabilized aluminiumorganyls were also grafted. The aluminium-contents were determined using aluminium-atom absorption spectroscopy (Al-AAS). The values ranged from 0,74 mmol Al/g up to 6,07 mmol Al/g. The amount of aluminium being grafted is determined by the nature of the surface modification of the silica, i.e. the procedure and the functionalities in the silanes being used. Selected examples of the aluminiumorganyl-modified carriers as well as all of the MAO-modified carriers were tested as cocatalyst in the polymerization of ethylene using zirconocenedichloride as catalyst.
The morphologies of all resulting polymers were characterized by scanning electron microscopy. Some polymers were examined using EDXA to determine the elemental compositions. Thus the remainings of the silica and the aluminiumorganyls could be localized. The fragmentation of the cocatalysts Ð monitored by the consumption of ethylene over time Ð could be verified by the electron microscopic examinations.
The polymerization activities of the heterogeneous cocatalysts reached above 100 percent of the activity of the homogeneous MAO/ZrCp2Cl2. The polymerization activities of selected cocatalysts were also examined on their dependency on polymerization duration and Al.Zr-ratio.
For comparison commercially available non-spherical silica gels were also used. At 1000 ¡C dehydroxylated silica was directly functionalized with aluminiumorganyls by reaction with DEAH. The achieved aluminium content ways as low as 0,09 mmol Al/g. Aminosilane-modified silica gel was also reacted with titanium tetrachloride and with butyllithium/titanium tetrachloride (two step-reaction). Thereby titanium amido chloride-modified silica gel with titanium contents of up to 0,23 mmol Ti/g were obtained (determined by Ti-AAS).
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