Three reactants, polyhedral oligosilsesquioxanes (POSS) (c-C 6 H 11 ) 7 (Si 7 O 9 )(OH) 3 (1), butylethylmagnesium (2) and titanium tetrachloride TiCl 4 (3), react in three different ways according to the order of addition to form three model catalysts, A2, B2 and C2. A2, which is characterized as a bimetallic siloxane cage model catalyst [(c-C 6 H 11 ) 7 (Si 7 O 12 )MgTiCl 3 ] n ( n 14= 141,2), contains a singly bonded (Ti[bond]O[bond]Si) with a doubly bonded Mg([bond]O[bond]Si) 2 . B2 contains a doubly bonded Ti([bond]O[bond]Si) 2 with a singly bonded Mg([bond]O[bond]Si). C2 contains a triply bonded Ti([bond]O[bond]Si) 3 . As catalyst precursors the model catalysts demonstrate different activity and catalytic behavior toward ethylene polymerization in the presence of triethylaluminum. Three types of M x –O y –Si z bonding structures are proposed as bonding models for silica-supported Ziegler–Natta catalysts. Copyright © 1999 John Wiley & Sons, Ltd.
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