A Density Functional Study on the Origin of the Propagation Barrier in the Homogeneous Ethylene Polymerization with Kaminsky-Type Catalysts

Agostic interactions in transition metal compounds

“…␤-Agostic species have been identified as the ground-state structures in several d 0 metal alkyl complexes and are thought in many cases to be the catalyst resting state in olefin polymerizations (63)(64)(65). Species exhibiting ␥-agostic interactions are formed upon insertion of olefins into an ␣-agostic species (Scheme 5) and have also been proposed as possible catalyst resting states (66)(67)(68)(69)(70)(71)(72). These ␥ interactions are thought to inhibit inversion at metal (''chain-swinging'') and thus allow for highly syndiospecific polymerization of propylene, where strict alternation (accomplished through monomer insertion) between sites preferring the si and re faces of propylene is required for high syndiospecificity (57).…”

Ethylene polymerization reactions with Ziegler-Natta catalysts. I. Ethylene polymerization kinetics and kinetic mechanism

“…Theoretical calculations convincingly demonstrate the existence of ␤-agostic interactions in various [(C 5 H 5 ) 2 Zr-R] ϩ complexes, although the reasons for a stronger agostic interaction of the ␤-CH 3 group vs. the ␤-CH 2 group we propose are not clear. [32][33][34][35] Scheme 3 shows the major steps of ethylene polymerization reactions under this assumption. Most of the reactions in this scheme are the same as those in Schemes 1 and 2.…”

Effect of a second ethylene molecule on the insertion of ethylene in zirconocene catalyst systems: A QM semiempirical study

“…The energy barrier obtained for the first stage (alkyl rotation) is greater than for the second one (insertion process), indicating that the rotation around the ZrOC␣ bond is the propagation controlling step; this is in agreement with other studies. 20,22 The energy barrier associated with the alkyl rotation around the ZrOC␣ bond is lowered by about 1 kcal/mol when a companion second ethylene molecule is present. For the other barrier (which is associated with the insertion process), the energy difference is too small.…”

“…13 Other authors have performed studies with PM3(tm) on related transition-metallic compounds. [53][54][55] The C2OC␣ distance was chosen as the reaction coordinate 13,22 (for the nomenclature, see Fig. 1).…”