Model Catalytic Sites for Olefin Polymerization and Diastereoselectivity in the Cyclopolymerization of 1,5-Hexadiene

The diastereoselectivity of the cyclization step in the cyclopolymerization of 1,5-hexadiene with homogeneous catalysts based on zirconocene/methylalumoxane systems is investigated through the analysis of the nonbonded interactions on model catalytic sites analogous to those proposed for α-olefin polymerizations. A model which does not include any cyclopentadienyl ring presents a small trans specificity: the minimum energy intermediate brings the formation of trans rings and resembles a cyclohexane chair in which the growing chain occupies a pseudoequatorial position (trans-chair conformation). The addition of two cyclopentadienyl rings to the Zr atom, due to interactions with the growing chain which occupies a pseudoaxial position, disfavors the two most stable cis conformations (a cis-chair and a cis-boat). As a consequence, the model with two cyclopentadienyl rings shows a higher trans specificity than the “bare” model. In the presence of two pentamethylcyclopentadienyl rings, the steric interactions of the aromatic ligands tend to disfavor the trans-chair with respect to a cis-twist conformation and the model becomes cis-specific. A good qualitative agreement between the modeling and polymerization results is observed.