Triisobutylaluminum as cocatalyst for zirconocenes. I. Sterically opened zirconocene/triisobutylaluminum/perfluorophenylborate as highly effective ternary catalytic system for synthesis of low molecular weight polyethylenes

Ethylene polymerizations with catalytic systems Me2SiCp*NttBuZrX(2) (1) [Cp * = C-5(CH3)(4); X = Cl(1Cl), Me (1Me)], triisobutylaluminum (TIBA), perflu orophenylborate CatB(C6F5)(4) [Cat = CPh3 (3), Me2NHPh (4)], or Me2SiCp2ZrX 2 [X = Cl(2Cl), Me (2Me)]/TIBA/3 (4) were performed within a wide range of ethylene pressures of different Al/Zr ratios, and Zr/B = 1. Catalytic syste ms 1Cl(2Cl)/TIBA/3 led to the formation of very high linear molecular weigh t polyethylene (PE) of M-eta similar to2,000,000 with low activity. The rep lacement of both chlorine ligands in the precatalyst for the methyl ones le d to the formation of active species producing low molecular weight PE with high activity. Chain transfer to ethylene was shown to be the main reactio n controlling PE chain propagation: k(p)/k(tr) similar to 20-30 for 1Me/TIB A/3 and k(p)/k(tr) similar to 350-500 for 2Me/TIBA/3. It was suggested that TIBA was present in the active center first in the form of a neutral heter obimetallic Zr-Al bridged complex followed by the formation of a partially polarized Zr-Al(Cl)R-2 (R = Bu-i) or an unreactive Zr-AlR3 cationic complex by abstraction of the alkyl ligand under the action of berate. It was conc luded that AIR, from the latter cationic complex may be easily reversibly r eplaced under the specific coordination of ethylene or accumulated alpha -o lefin, giving rise to highly labile and sterically accessible cationic spec ies. Experiments on ethylene polymerization with the catalytic systems 1Cl( 1Me)/TIBA/3/Ph2NH, 1Cl(1Me)/TIBA/4, 2Cl(2Me)/TIBA/3/ Ph2NH, and 2Cl(2Me)/TI BA/4 were performed to confirm the suggestion. Catalytic systems derived fr om dichloride complexes in the presence of a sigma -donor substrate also pr oduced low molecular weight PEs with molecular weight characteristics simil ar to those of products obtained with the dimethylated precatalysts. The sp ecific feature of active species derived from 2Me complexes to isomerize co ordinated alpha -olefin into trans-vinylene polymer chains was also reveale d. The catalytic behavior of the ternary catalytic system based on 2ME rela tive to 2Me or 2Cl precatalysts activated with polymethylaluminoxane at dif ferent Al/Zr ratios was compared. (C) 2001 John Wiley & Sons, Inc.