Recent advances in propylene polymerization with MgCl₂ supported catalysts

Abstract

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.

Effect of diether as internal donor on MgCl2‐supported Ziegler–Natta catalyst for propylene polymerization

“…Propene (polymerization grade, from Yanshan Petrochemical Co.), TiCl 4 (from Beijing Yili Fine Chemical Co. Ltd) MgCl 2 (from Yingkou Science Chemical Co. Ltd.), and triethylaluminium (TEA, from Aybemarle Co., 95% purity) were used without further purification. Hexane and heptane were dried over molecular sieve (4 Å), then re-fluxed for 24 h over sodium, and distilled before use under nitrogen (extra pure grade, 99.99% purity).…”

“…2 The most widely used electron donors are phthalate and alkoxysilane compounds as internal and external donors, respectively. 3 With the discovery of 2,2-disubstituents-1,3-dimethoxypropane(1,3-diether), 4 a novel and simplified generation of MgCl 2 -supported catalysts had been developed. These diethers (used as internal donors) are known for having the property to produce highly active and stereospecific catalysts without any external donor.…”

Effect of hydrogen on the ethylene polymerization process over Ziegler-Natta catalysts supported on MgCl2(THF)2. II. Kinetic studies

“…12 The reaction between TiOH bonds and the carbonyl group of the ester has been suggested as a possible cause. 20 Summing up the discussion presented above, various hypotheses can be considered to explain such a negative effect from hydrogen on the activity of catalyst in the ethylene polymerization process:…”

XPS study of Al‐Alkyl effect on Ziegler–Natta catalytic system

“…All types of chemical reactions such as reduction, alkylation, and complexation of Ti species leading to formation of active sites in Ziegler Natta catalysts are related to Al-alkyl. [4][5][6] For example, Interaction of the surface Ti(IV) species with trialkylaluminum includes the following processes: reduction of Ti 4+ species to Ti 3+ and Ti 2+ species, alkylation of Ti species with formation of Ti-C bonds, and complexing between the reduced Ti species and AlCl x R 3-x compounds. Therefore, Al-alkyl is of importance both in laboratory research and industrial production.…”