MO AND MM STUDY OF HOMOGENEOUS PROPYLENE POLYMERIZATION WITH A ZIRCONOCENE CATALYST

摘要

The stereoregulation in a-olefins polymerization is one of the most important capabilities of Ziegler-Natta catalysts. Homogeneous ansa-metallocene complexes of group 4 metals have been shown to produce isotactic, syndiotactic and crystalline-amorphous stereoblock polypropylene. Molecular mechanics and ab initio methods have been used to model the catalytic sites [1, 9 - 13]. The results of these calculations have been used not only to confirm experimental results but also to predict the tacticity obtained with certain catalyst precursors. The design of stereochemical control based on these calculations can help the experimentalists to save time and efforts. We have used a semi-empirical Molecular Orbital Model (PM3) to study the stereoselectivity of the insertion reaction of propylene using the zirconocene Et(Ind)_2ZrCl_2 (1), as catalyst precursor. The energy of the re and si olefin π-complex intermediates and their corresponding transition states are compared. The energy difference between a 1,2 and 2,1 insertion is evaluated. A Molecular Mechanics Model, using the SYBYL force field [14], is also used to compare the energy differences of the intermediates due only to distortions from ideal bond distances, bond angles and dihedral angles together with contributions of non-bonded interactions.