Steric Control in Polymerization of Conjugated Dienes

The subject has also received considerable attention. Nevertheless, the mechanism is still not fully understood. It is reasonable to assume that the form and structure of the catalyst and the valence of the transition metals must play a role. The conformation of the monomer (s-cis or s-trans) is probably also important. 

It is known that CoCl2/Al(C2H5)Cl can polymerize 1,3-pentadiene but not the cis isomer. This suggests that a two-point coordination is required. Several reaction schemes that provide for an attack at either Ci or C4 position were proposed over the years. One mechanism for polymerization of butadiene suggests that complexes of the catalysts in solvents of low dielectric constant will either act as ion pairs or as independent solvated entities. Also, the growing chain may be bound by either a tt or a a linkage, and it is suspected that a continuous a- n isomerism is possible  

The insertions of the monomers is believed to occur in two steps. the first one, the incoming 

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