Molecular mechanics and stereospecificity in Ziegler-Natta 1,2 and cis-1,4 polymerizations of conjugated dienes

In the framework of Porri's mechanism for the 1,2 and cis-1,4 diene polymerizations, a geometrical and nonbonded energy analysis on possible catalytic intermediates, which include a cyclopentadienyl group coordinated to the transition metal, is presented. Besides pre-insertion intermediates, post-insertion intermediates presenting a `back-biting' coordination of the growing chain are considered. The post-insertion minimum energy intermediates are structurally very close to the endo-endo pre-insertion intermediates (presenting endo coordinations for both the allyl group of the growing chain and diene monomer), independently of the kind of insertion (1,2 or cis-1,4) and of the diene (butadiene, 4-methyl-1,3-pentadiene or (Z)-pentadiene). In our hypothesis the energy differences between the considered post-insertion intermediates are close to those present in the transition states, and this assures the isospecificity and the syndiospecificity of the models for the cis-1,4 polymerization (of 4-monosubstituted monomers) and for the 1,2 polymerization, respectively. This is in qualitative agreement with the stereoregularities of the polymers obtained with a catalytic system based on cyclopentadienyltitanium trichloride. Support for a 1,2 insertion mechanism for butadiene involving an endo-endo pre-insertion intermediate is given by comparison between the calculated minimum energy post-insertion intermediate and the crystalline structure of a catalytic cobalt complex.