Isomerisation-Polymerisation of Olefins

As already discussed, most Ziegler-Natta and related coordination catalysts polymerise a-olefins by a 1,2-insertion mechanism, yielding highly regio- and stereoregular polymers. When some nickel-based coordination catalysts are used, however, isomerisation of the active species may accompany the propagation, and polymers containing 2, co-coupled monomeric units are formed [183,191], 

Catalysts capable of 2, co-linkage of a-olefins are obtained from the reaction of Ni(0) or Ni(II) compounds, such as Ni(Cod)2 or Ni(All)2 respectively, with aminobis(imino)phosphorane, such as (Me3Si)2NP(=NSiMe3)2, preferably used in equimolar ratio the catalysts are characterised by the appearance of Ni(II) species as has been shown by scheme (18) [182]. 

When linear a-olefins are polymerised, the polymer contains only methyl branches, regularly distributed along the chain with a separation corresponding to the size of the alkyl substituent in the monomer. It may be interesting that the structure of the polymer formed from 1-pentene, poly[2,5-(l- 

Fink et al. [183] developed a scheme explaining the 2,o polymcrisation pathway, which is represented in the case of 1-butene 2,4-propagation by 

It is to be noted that the monomer can only be inserted into a primary nickel-alkyl bond at the end of the growing chain, the insertion is regioselective and only CV-C2 coupling of the growing chain with the next monomer molecule occurs, and the nickel species migrates along the polymer main chain (isomerisation) between two insertions. It should be added that, during the isomerisation, transfer reactions can take place, but not insertions. 

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