Relationship to Ziegler-Natta polymerization

For isotactic polymer formation steric control is not lost even when some ethene units are introduced by copolymerization. This shows that the addition of the propene then occurs by primary insertion , with [Mt]—CH2P as chain carrier and the stereoregularity controlled by the chirality of the metal centre. On the other hand for syndiotactic polymer formation steric control is lost when ethene units are introduced by copolymerization. It may be concluded that the addition of the propene then occurs by secondary insertion , whereby the chain carrier is [Mt]—CHMeP and the stereoregularity is controlled by the chiral centre at the 

Firm evidence for an equilibrium between a tungsten(IV) methyl complex and a tungsten(VI) methylene hydride complex was first provided by Cooper (1974a,b, 1979). 

If equilibrium (10) exists in a catalyst system for ZNP there is the possibility that ZNP actually proceeds via [2- -2] addition of the olefin to the metal carbene complex by the mechanism shown in sequence (11). 

Racemic titanocene alkenyl chloride, of which one enantiomer is shown in eqn. (12), gives a 50 50 mixture of cis and trans isomers, as determined from the NMR spectrum of the cis/trans mixture of methylcyclopentanes obtained by hydrolysis of the product. However, the complex Ta(=CHCMe3)(H)(I)2(PMe3)3 adds ethene in the presence of PMe3 to yield Ta[=CH(CH2) CMe3](H)(I)2(PMe3)3. It is known that, for complexes of this type, a-hydrogen transfer is rapid compared with the rate of addition of ethene, so that this polymerization may well proceed by the mechanism shown in sequence (11) (Turner, H.W. 1983). 

Two catalysts have been found which under limited conditions give polymers 

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As in Ziegler-Natta polymerization, steric control of the propagation step may involve either the interaction of the monomer with a chiral metal centre (enantiomorphic sites model), or the interaction of the monomer with the chiral centres in the repeating unit(s) adjacent to the metal centre (chain-end model). (The relationship to Ziegler-Natta polymerization will be considered further in Section VIII.C.3.)... 

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