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An investigation of the catalytic behavior of the dimethylated zirconocenes Me2SiCp*NtBuZrMe2 [Cp* = C5(CH3)4; 1Me], Me2SiCp2ZrMe2 (2Me), Cp2ZrMe2 (3Me), Ind2ZrMe2 (4Me), Me2SiInd2ZrMe2 (5Me), Et(2‐MeInd)2ZrMe2 (6Me), and Me2Si(2‐MeInd)2ZrMe2 (7Me) with the combined activator triisobutylaluminum (TIBA)/CPh3B(C6F5)4 (Al/Zr = 250; B/Zr = 1) in ethylene polymerizations at increased monomer pressures (5–11 bar, 30 °C) was carried out. Sterically opened zirconocenes in ternary catalysts gave rise to active species effective in the formation of low molecular weight polyethylenes (PEs). These active species tended to increase the PE molecular weight [1Me (2100) < 2Me (20,000) < 5Me (89,000) < 3Me (94,500)] under similar conditions. PE obtained with 4Me showed a bimodal gel permeation chromatography curve with a 64% peak area [weight‐average molecular weight (Mw) = 43,000] and a 36% peak area (Mw = 255,000). The increase in sterical demands from the zirconocenes was also demonstrated by the reduction of the chain transfer to monomer, the reinsertion of vinyl‐ended PE chains, and their ability for isomerization. These reactions were most pronounced for the zirconocenes 1Me and 2Me. The active species responsible for the formation of low molecular weight PEs deactivated quickly. The zirconocenes 6Me, 7Me, and (2‐PhInd)2ZrMe2 (8Me) bearing substituent at the 2‐position of the indenyl ring was activated with TIBA alone, yielding active species effective in ethylene and propylene polymerizations. PEs formed with 6Me–8Me complexes activated with TIBA had high molecular weights. An increase in the Al/Zr ratio in the catalytic system 8Me/TIBA from 50 to 300 led to an enhancement of the molecular weight of polypropylene (PP) samples from oligomeric products to an viscosity‐average molecular weight of 220,000. The increase in the molecular weights of PPs with an increase in the propylene concentration was also observed. An analysis of the catalytic performance of the 8Me/TIBA system showed first‐order dependency of the initial polymerization rates on the TIBA concentration and close to second‐order dependency on propylene. The second‐order dependency on the monomer concentration is explained in terms of the monomer participation in the initiation step of the polymerization reaction. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1915–1930, 2001

The article describes that sterically hindered isobutylaluminum aryloxides with bulky t Bu substituents at 2,6-positions of aryl fragment, i.e. (2,6-di-Bu (1-TTBP)) and (2,6-di-Me(2-BHT)) can serve as cocatalysts for metallocene complexes. Isobutylaluminum aryloxides have been applied for activation of rac-Et(2-MeInd) 2 ZrMe 2 in homopolymerization of ethylene, propylene, copolymerization of ethylene and propylene, and terpolymerization of ethylene, propylene, and 5-ethylidene-2-norbornene at Al/Zr 5 300 mol/mol. The type of R substituent at 4-position has a significant effect on catalyst activity. The catalytic system with 1-TTBP showed the highest activity in all homo-and copolymerization processes. Diisobutylaluminum aryloxides provide much higher activity to the systems in all polymerization processes and stronger ability for propylene incorporation in copolymer than diaryloxides. The activities of the systems with isobutylaluminum aryloxides are similar or exceed that of the system with MAO as activator as have shown for propylene polymerization.