Transition-metal organic compounds, alkyl groups

Concomitant with continued olefin insertion into the metal-carbon bond of the titanium-aluminum complex, alkyl exchange and hydrogen-transfer reactions are observed. Whereas the normal reduction mechanism for transition-metal-organic complexes is initiated by release of olefins with formation of hydride followed by hydride transfer (184, 185) to an alkyl group, in the case of some titanium and zirconium compounds a reverse reaction takes place. By the release of ethane, a dimetalloalkane is formed. In a second step, ethylene from the dimetalloalkane is evolved, and two reduced metal atoms remain (119). 

The sol-gel process is based on the hydrolysis and condensation of molecular precursors [11]. The most versatile precursors are the metal organic compounds, metal alkoxides, M(OR) . The transition metal is represented by M, n is the valence of the metal, and R is an alkyl group such as methyl, ethyl, propyl, etc. 

Many transition metals and their compounds with organic ligands initiate the polymerization of alkenes and/or dienes. Some of them do not need any special treatment to this end while others require the presence of some organic or mineral compound or a special physical modification. In contrast to ZN catalysts, they are active without an organometal of Groups I—III. They are commonly known as metal alkyl free (MAF) catalysts. Many of their features are, of course, in common with ZN catalysts. MAF catalysts initiate stereoselectively controlled polymerization. Even less is known of their operating mechanism than that of ZN catalysts. It is assumed that propagation also occurs on the transition metal-carbon bond. 

As the first transition series is ascended the metal—carbon bond becomes weaker and the alkyls of iron, cobalt and nickel are stable at ambient temperatures only when coordinated with strong it donors such as 1,1 bipyridyl [12], Organic groupings which form delocalized bonds with the metal are more stable and many tt complexes or 7r-allylic compounds of transition metals are known. The metal—cyclopentadienyl bond is too stable for these compounds to initiate polymerization, but some TT-allyl compounds will polymerize monomers such as butadiene where a comparable structure will be retained in the propagating chain. 

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