Vanadium complexes aluminum halide

The chemistry of Lewis acids is quite varied, and equilibria such as those shown in Eqs. (28) and (29) should often be supplemented with additional possibilities. Some Lewis acids form dimers that have very different reactivities than those of the monomeric acids. For example, the dimer of titanium chloride is much more reactive than monomeric TiCL (cf., Chapter 2). Alkyl aluminum halides also dimerize in solution, whereas boron and tin halides are monomeric. Tin tetrachloride can complex up to two chloride ligands to form SnCL2-. Therefore, SnCl5 can also act as a Lewis acid, although it is weaker than SnCl4 [148]. Transition metal halides based on tungsten, vanadium, iron, and titanium may coordinate alkenes, and therefore initiate polymerization by either a coordinative or cationic mechanism. Other Lewis acids add to alkenes this may be slow as in haloboration and iodine addition, or faster as with antimony penta-chloride. 

Alternating Copolymers. The reactivity of polar monomers, e.g. acrylonitrile, methyl methacrylate or maleic anhydride can be enhanced by com-plexing them with a metal halide e.g., zinc or vanadium chloride) or an organo-aluminum halide (ethyl aluminum sesquichloride). These complexed monomers participate in a one-electron transfer reaction with either an uncom-plexed monomer or another electron-donor monomer, e.g. olefin, diene, or styrene, forming alternating copolymers with free-radical initiators. 

Equation (19-20) requires a linear relationship between the polymerization rate Vp and the product [ C oi ] /mon independent of the chemical nature of the metal alkyl. Such behavior has also been observed for the 4-methyl pentene-l/VCh/aluminum trialkyl system (Figure 19-3). Thus, the active centers are actually formed by the transition metal halides. An alkylated vanadium species, but definitely not a bimetallic complex, acts as active center. 

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