Regiospecificity and Chemoselectivity of the

In the coordination polymerisation of butadiene, the last inserted monomeric unit, which is the butenyl group attached to the metal atom as an -allylic ligand, can assume one of two forms, anti and syn  

CH2] and 1,3-pentadiene (CH2=CH—CH=CH-CH3), two different positional isomers can be formed instead. Isoprene can give two isomeric -allylic groups  

3-pentadiene, in principle the following f/3-allylic groups can exist (again,  

On the basis of 1H NMR investigations it was shown that, in polymerisation with most stereospecific catalysts, isoprene gives, on insertion, predominantly  

The chemoselectivity of the polymerisation, i.e. the formation of 1,4 as against 1,2 monomeric units in resulting polymers of conjugated dienes, depends on whether enchainment of the incoming monomer occurs at the Ci or at the C3 atom of the last inserted monomeric unit. Enchainment via the Mt-Ci bond (in both the anti and syn forms) gives rise to the formation of a 1,4 monomeric unit (cis or trans respectively), whereas enchainment via the Mt-C3 bond (in both the anti and syn forms) gives rise to a 1,2 unit. Both cases of the reaction in the butadiene polymerisation system are illustrated below  

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