Interpretation of tacticity

As in Ziegler-Natta polymerization, steric control of the propagation step may either involve the interaction of the monomer with a chiral metal centre (enantiomorphic sites model), or it may involve the interaction of the monomer with the chiral centres in the repeating unit(s) adjacent to the metal centre (chain-end model). 

If the propagating metal carbene complex does not have a predetermined vacant position, but is instead trigonal-bipyramidal or tetrahedral, it may still behave as in 

The second assumption may be regarded as proved beyond reasonable doubt. It is known that the additions of most electrophiles to norbomene occur at the exo face (March 1977) also, the stmcture of the metallacyclobutane formed by the 

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