Aspects of coordination polymerization in heterogeneous and homogeneous catalysis. A computational survey

Abstract

Theoretical studies on ethylene polymerization by single site homogeneous catalysts, as well as by Ziegler-Natta heterogeneous catalysts, have been reviewed. Studies on cation-anion interactions in ion-pair systems of the typeprovided indications as to which ancillary ligands on the cation acted as good electron donors, and also revealed the importance of having a weakly coordinating anion to enable the ionpair to separate more easily in solution. Subsequent calculations of barriers to ethylene insertion into the metal-methyl bond in ionpair systems of the type LL 0 MeM-l-B(C 6 F 5 ) 3 [M ¼ Ti, Zr] revealed that the ease of separation of the anion from the cation in solution was an important criterion in determining the activity of the catalysts. The barrier to insertion was found to be the rate determining step during the first insertion of ethylene into the metal-methyl bond, but second insertion studies conducted for two different ion-pair systems showed that the barrier to uptake of the monomer became the rate determining step during the second insertion of the ethylene monomer. Theoretical studies conducted on TiCl 4 /MgCl 2 heterogeneous Ziegler-Natta catalysts provided insights into the nature of the binding of the titanium catalysts (with the Ti in different oxidation states) on to the MgCl 2 support.The results indicated that the support worked best when a few Ti atoms replaced Mg atoms in the crystal lattice of the support. Ethylene polymerization studies, along with investigations on chain termination, led to the conclusion that the TiCl 3 -based edge site on MgCl 2 would be the most promising model for the actual catalytic species. Studies on addition of tetrahydrofuran (THF) to the active sites showed that the presence of the base would lead to higher molecular weights of the polymers, as it acted to increase the barrier to termination (by chain transfer to monomer) more than the barrier to insertion.

Synthesis and Characterization of Dichlorotitanium Alkoxide Complex and Its Activity in the Polymerization of α-Olefins

“…Higher amounts of MAO are often needed in the polymerization of propylene than in the polymerization of ethylene. [111][112][113] We observed a similar trend for the influence of reaction time as in the polymerization of ethylene (compare entries 1-4). Maximum insertion rate was calculated for the shortest reaction time measured, thus strengthening our claim for the decomposition of the active species right from the first steps of the reaction.…”

Catalyst Activation and Chain Transfer Agents in Metallocene Catalysts for Ethylene Polymerization: A Computational Investigation

“…Indeed, for effective olefin polymerization, the ion pair formation step, [catalyst] δ + …..[anion] δÀ , played an important role in metallocene where the lability of zwitterionic bond decides the rate of monomer insertion into the M δ + ….C δÀ bond. [46][47][48] Therefore, a mechanistic study on catalyst activation and olefin polymerization steps using magnesium-and zinc-based cocatalyst and its comparison with aluminium-and boron-based cocatalysts is worth exploring in metallocenes.…”

Gamma-alumina: An Active Support to Obtain Immobilized Electron Poor Zr Complexes

“…From the Zr-butyl intermediate, the next step is the C-C activation, the key step of the mechanism (C-C bond cleavage). b-alkyl transfer has already been proposed for this step in the alkane hydrogenolysis on silica supported Zr hydrides [29][30][31][32]. We have also evaluated this elementary step by calculation on the alumina supported Zr hydride system, and compared it with alternative solutions.…”