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

Abstract

Ethylene polymerizations with catalytic systems Me2SiCp*NtBuZrX2 (1) [Cp* = C5(CH3)4; X = Cl (1Cl), Me (1Me)], triisobutylaluminum (TIBA), perfluorophenylborate CatB(C6F5)4 [Cat = CPh3 (3), Me2NHPh (4)], or Me2SiCp2ZrX2 [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 systems 1Cl(2Cl)/TIBA/3 led to the formation of very high linear molecular weight polyethylene (PE) of Mη ∼2,000,000 with low activity. The replacement of both chlorine ligands in the precatalyst for the methyl ones led to the formation of active species producing low molecular weight PE with high activity. Chain transfer to ethylene was shown to be the main reaction controlling PE chain propagation: kp/ktr ∼20–30 for 1Me/TIBA/3 and kp/ktr ∼350–500 for 2Me/TIBA/3. It was suggested that TIBA was present in the active center first in the form of a neutral heterobimetallic Zr–Al bridged complex followed by the formation of a partially polarized Zr–Al(Cl)R2 (R = iBu) or an unreactive Zr–AlR3 cationic complex by abstraction of the alkyl ligand under the action of borate. It was concluded that AlR3 from the latter cationic complex may be easily reversibly replaced under the specific coordination of ethylene or accumulated α‐olefin, giving rise to highly labile and sterically accessible cationic species. 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)/TIBA/4 were performed to confirm the suggestion. Catalytic systems derived from dichloride complexes in the presence of a σ‐donor substrate also produced low molecular weight PEs with molecular weight characteristics similar to those of products obtained with the dimethylated precatalysts. The specific feature of active species derived from 2Me complexes to isomerize coordinated α‐olefin into trans‐vinylene polymer chains was also revealed. The catalytic behavior of the ternary catalytic system based on 2Me relative to 2Me or 2Cl precatalysts activated with polymethylaluminoxane at different Al/Zr ratios was compared. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1901–1914, 2001

Synthesis of ultra‐low‐molecular‐weight polyethylene wax using a bulky Ti(IV) aryloxide–alkyl aluminum catalytic system

“…In order to ascertain the level of branching in the 'as synthesized' polymer reported in Table 1, the IR spectrum was compared with a standard PE wax sample using the following equation. 48 The branching degree of PEs was determined by taking the value of absorbance of the 1378 cm…”

Effect of Different Aluminum Alkyls on the Metallocene/Methylaluminoxane Catalyzed Polymerization of Higher α‐Olefins and Styrene

“…[54][55][56] Several authors proposed that true active sites are coordinatively unsaturated cationic species [L 2 ZrMe þ ] formed after dissociation of the species (A) and (B), [30][31][32][33][54][55][56] while others claimed that olefin is able to directly replace AlR 3 from the coordination sphere of the species (B). [20,21] In this case, a deactivating effect of the linear aluminum alkyls (TEA, THA) by the higher a-olefins polymerization can be attributed to the formation of the relatively stable heteronuclear complexes with cationic species (similar to (B)) that strongly depressed the catalyst activity. On the contrary, TIBA or mixed Me n i-Bu 3-n Al species, which are formed by means of intermolecular ligands exchange between TMA and TIBA, [14,57,58] can successfully replace TMA from its complex with cationic zirconocene species (B) to form looser ion pairs.…”

“…On the contrary, TIBA or mixed Me n i-Bu 3-n Al species, which are formed by means of intermolecular ligands exchange between TMA and TIBA, [14,57,58] can successfully replace TMA from its complex with cationic zirconocene species (B) to form looser ion pairs. [20,21,57] The last formed active species are labile [20,57] and can dissociate to form active species [L 2 ZrMe þ ] [57] or can be easily reactivated via displacement of TIBA (Me n i-Bu 3-n Al) in the excess of such p donor as a monomer. [20] It should be also noted, that TIBA can react with MAO to form a mixed aluminoxane containing isobutyl groups.…”

“…[20,21,57] The last formed active species are labile [20,57] and can dissociate to form active species [L 2 ZrMe þ ] [57] or can be easily reactivated via displacement of TIBA (Me n i-Bu 3-n Al) in the excess of such p donor as a monomer. [20] It should be also noted, that TIBA can react with MAO to form a mixed aluminoxane containing isobutyl groups. [11,57,59] The increase in the size of alkyl groups in aluminoxane makes the active cationic ion pair (L 2 ZrR þ ÁÁÁMAO À ) looser, this can lead to an increase of the catalyst activity, [11] which correlates well with our experimental results.…”

“…[15,16] It should be also noted that the actual role of ''free'' TMA contaminants in MAO is still under debate. [17] Basically, aluminum alkyls have different effects on homogeneous metallocene mediated olefins polymerization: (i) they can alkylate catalyst precursor L 2 MX 2 , where X ¼ Cl, NMe 2 ; M ¼ Zr, Ti, before its activation by borate reagents [8,16,[18][19][20][21] and even operate as a cocatalyst; [14,[20][21][22] (ii) scavenge traces of impurities which are always present in the monomer or in solvents; [23,24] (iii) act as a chain transfer agent; [12,19,25,26] (iv) form a bimetallic heteronuclear complex with catalyst precursor (L 2 M(mR) 2 AlR 2 ); [8,17,[20][21][22] (v) trap of free TMA admixtures in MAO; [14] (vi) act as a reducing agent for the titanium compound. [27,28] However, as mentioned above, the exact function of aluminum alkyls in metallocene mediated polymerization is still not clear and additional investigations are required.…”