Polymerization of Nonpolar Alkene Monomers

Titanocene and zirconocene dichlorides (Cp2MtCl2 with Mt = Ti, Zr) were the Hrst metallocenes studied [Breslow and Newburg, 1957 Natta et al., 1957a]. The metallocene initiators, like the traditional Ziegler-Natta initiators, require activation by a Lewis acid coinitiator, sometimes called an activator. AIRCI2 and AIR3 were used initially, but the result was initiator systems with low activity for ethylene polymerization and no activity in a-olehn polymerization. The use of methylaluminoxane (MAO), [A1(CH3)0] , resulted in greatly improved activity for ethylene polymerization [Sinn and Kaminsky, 1980]. The properties of MAO are discussed in Sec. 8-5g. MAO has two functions alkylation of a transition metal-chloride bond followed by abstraction of the second chloride to yield a metallocenium 

The positive charge on the transition metal in XXVII is a consequence of the tetravalent oxidation state of the transition metal in Cp2TiCl2. The active sites in traditional Ziegler-Natta polymerizations may be neutral because the transition metal is trivalent in those initiators (Secs. 8-4e, 8-4h-l). The group 3 metallocene initiators have neutral metal centers because those metals are trivalent. 

Propagation proceeds in a manner similar to that described for the traditional Ziegler-Natta initiators. The transition metal has two active sites—the polymer chain is held at one site (the one occupied by a methyl group in XXVII) and monomer at the other site (shown as the vacancy ). The reactivity of the active sites is high because the counteranion, which is either (ClMAO) or (OR MAO) or a mixture of the two, is a weakly coordinating anion. Reactivity is decreased when the counterion is strongly coordinating. 

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TRADITIONAL ZIEGLER-NATTA POLYMERIZATION OF NONPOLAR ALKENE MONOMERS... 

ZIEGLER-NATTA POLYMERIZATION OF NONPOLAR ALKENE MONOMERS 651 TABLE 8-3 Slereoerrors versus Type rf Stereocontrol... 

METALLOCENE POLYMERIZATION OF NONPOLAR ALKENE MONOMERS 679 Propagation proceeds by successive additions of monomer to the propagating species ... 

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