The role of Lewis bases in MgCl2 supported catalysts for olefin polymerization is a subject of continuous interest and discussion in order to obtain more and more active and stereospecific catalysts and to explain their stereoregulating mechanism.
Through molecular calculation and conformational analysis it was possible to identify chelating diethers that have the correct oxygen‐oxygen distance necessary to tightly coordinate with the Mg ions of the support, even in the presence of other strong Lewis acids, and unable to give secondary reactions with TiCl4, AlR3, Ti‐C and Ti‐H bonds. The use of these donors has allowed the synthesis of catalytic systems that are both highly active and stereospecific even in the absence of external donors. Kinetic data of propylene polymerization with these catalyst systems are reported.
The importance of the distance between the donor atoms in bifunctional Lewis bases has been proved also in the case of new classes of internal donors.
Molecular modelling studies have enabled us to formulate models of active sites, located on some corners of MgCl2 crystallites, whose chirality is induced by the presence of a donor molecule in their environment. These models could explain, at least in part, the exceptional increase of isotactic polymer productivity observed for stereospecific catalyst systems, containing only the internal donor, with respect to catalysts lacking the Lewis base and could account for the influence of the donor on the molecular properties of the obtained polymers.
The general problem of the determination of the structure of water molecules surrounding biological molecules has been investigated with a method capable of providing an adequate simulation of liquid water (at room temperature) and ion pairs in aqueous solution (at room temperature). SCF-LCAO-MO computations are presented here for 21 amino acids interacting with one molecule of water located at different positions and orientations around each amino acid. The 1690 computed total energies, E, for the water-amino acid complex have been fitted with an analytical potential of the form E -(-A¡jab/r¡j6 + Bijab/r¡jn + C¡jab/r¡j) + ¿(amino acid) + ¿(water), where i and j designate two atoms, one in the amino acid and the second in the water molecule, a is an index that distinguishes the electronic environment of an atom in the amino acid, b is an index that distinguishes between either a hydrogen or the oxygen atom in the water molecule, A, B, and C are fitting constants, ¿(amino acid) and ¿(water) are the total energies of the amino acid and of the water molecule, respectively, at infinite separation from the other. The standard deviation of the fit (comparing the 1690 SCF-LCAO-MO energies
New models for steric environment of Ti isospecific polymerization sites on MgCI, microcrystals are presented. They directly involve the presence of a donor molecule in order to obtain chiral activable Ti atoms otherwise belonging to isolated adsorbed TiCI, molecules or Ti,C1, dimers which are lacking the required symmetry. The most important steric features of donor molecules have been obtained through structure-activity relationships and molecular comparisons, while their adsorption on MgCI, faces lateral to (001) has been studied through a conformational analysis approach.
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