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.
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.
XPS and contact angle measurement have been used to study oxygewplasma-treated polypropylene (PP) surfaces aged at variable temperatures. Surface rearrangement leading to low wettability has been observed, without alteration of the surface composition, as determined by XPS. Experimental results have been interpreted in terms of internal rearrangements of a modified layer, < 5 nm thick, formed on top of the PP and immiscible with it.We also modelled the composition of the surface layer and calculated the relative mobility of modified and non-modified polymer chains. On this basis, the experimentally observed behaviour can be interpreted in terms of surface rearrangement driven by a compromise between striving for lower surface tension and maximizing interand intramolecular interactions, mainly hydrogen bonds.The surface composition observed after treatment with plasma, corona, flame or other for enhancing surface tension is then time dependent. For this reason, the procedure used for surface analysis, namely the time allowed for surface equilibration, should be specified in reports.
SUMMARY A correlative approach is illustrated in which some conformational features of Lewis bases used in Ziegler-Natta heterogeneous catalysts for isotactic poly(propy1ene) proved to be molecular descriptors strongly correlating with stereoregulating power and polymerization yield. They are both steric and statistical descriptors and have been obtained through the analysis of the entire rotational hypersurface of the considered compounds. The correlative study selected some of their values as necessary conditions for a good stereoregulating effect. Their calculation for new molecules led to the predictive evaluation of new chemical classes as efficient stereoregulating electron donors and to the choice of the most effective ones.
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