The mechanism of the action of aluminum-containing activators in ziegler-type systems

The structure of adducts I and II formed by Cp 2 * Zr(R)Me (R = Me, Et; Cp* = η 5 -C 5 H 5 , η 5 -C 5 Me 5 ) with two Al(C 6 F 5 ) 3 molecules was studied. The formation of adducts I and II was shown to be favorable energetically.The ion pair [Cp* 2 ZrR] + [(C 6 F 5 ) 3 Al-Me-Al(C 6 F 5 ) 3 ] - (adduct II) was considered as a catalytically active particle. These species were characterized by a high degree of charge separation. Thanks to this property, they can act as catalysts for olefin polymerization. The catalytic activity was found to be determined by the energy difference between I and II, that is, the concentration of II in its equilibrium mixture with I. The introduction of donor substituents into the Cp ring and polymer chain growth (the transition from R = Me to Et) should stabilize ion pair II compared with the corresponding double adduct I. The conclusions drawn were extended to polymethylalumoxane-containing systems.