¹H and ¹³C NMR Study of the Intermediates Formed by (Cp‐R)₂ZrCl₂ Activation with MAO and AlMe₃/[CPh₃][B(C₆F₅)₄]. Correlation of Spectroscopic and Ethene Polymerization Data

Abstract

Using 1H and 13C NMR spectroscopy, cationic intermediates formed by activation of (Cp‐R)2ZrCl2 (R = nBu, tBu and 1,2,3‐Me3) with MAO in toluene were monitored at Al/Zr ratios from 50 to 1 000. The catalysts (Cp‐R)2ZrCl2/AlMe3/CPh3+B(C6F5)4− (nBu, tBu and 1,2,3‐Me3) were also studied for comparison of spectroscopic and polymerization data with MAO based systems. Complexes of type (Cp‐R)2ZrMe+ ← Me−‐AlMAO (IV) with different Me‐MAO− counter anions have been identified in the (Cp‐R)2ZrCl2/MAO systems at low Al/Zr ratios. At Al/Zr ratios of 200–1 000, the complex [(Cp‐R)2Zr(μ‐Me)2AlMe2]+ Me‐MAO− (III) dominates in all MAO‐based reaction systems. Ethene polymerization activity strongly depends on the Al/Zr ratio (Al/Zr = 200–1 000) for the systems (Cp‐nBu)2ZrCl2/MAO and (Cp‐tBu)2ZrCl2/MAO, while it is virtually constant in the same range of Al/Zr ratios for the catalytic system (Cp‐1,2,3‐Me3)2ZrCl2/MAO. The data obtained are interpreted on the assumption that complex III is the actual precursor of active centers of polymerization in MAO based systems.Formation of cationic intermediates by activation with MAO.magnified imageFormation of cationic intermediates by activation with MAO.

Studies of the Nature of the Catalytic Species in the α‐Olefin Polymerisation Processes Generated by the Reaction of Diamido(cyclopentadienyl)titanium Complexes with Aluminium Reagents as Cocatalysts

“…The broadness could be due to the closer position of the methyl group, initially bound to titanium, to the ''diversification'' centres provided by different Lewis acidic sites present in MAO. [62,63] The broadening of Ti-Me resonances of an adduct with MAO has also been reported by Talsi et al [59,64] When the polarity of the solvent is high, as in this case, methyl group abstraction by MAO is favoured and solventseparated ion pairs are preferentially formed.…”

Ethylene Polymerization with a Hafnocene Dichloride Catalyst Using Trioctyl Aluminum and Borate: Polymerization Kinetics and Polymer Characterization

“…The primary function of this class of cocatalysts is to abstract an alkyl group from dialkyl metallocenes and stabilize the resulting cation, which is the actual catalyst site for the polymerization of olefins. Several reactions of metallocenium ion formation were thoroughly investigated, and the presence of cationic species have been confirmed and they were isolated and characterized by NMR …”

“…MAO performs multiple functions during olefin polymerization, such as scavenging catalyst poisons, alkylating transition metal-chloride bonds, and abstracting the second chloride to yield a metallocenium cation with a vacant coordination site that is active for olefin polymerization. [9][10][11] MAO is needed in large excess relative to the metallocene, usually in 100:1 to 10 000:1 ratios to achieve high catalyst activities and stable polymerization kinetic profiles, thereby increasing both the cost and ash content of the polymer. In laboratory scale reactors operated in batch or semibatch mode, MAO is usually added prior to the metallocene to react with catalyst poisons, such as water and oxygen that may be present in the reactor; otherwise, the impurities present in the reactor could deactivate the metallocene catalyst.…”

“…Several reactions of metallocenium ion formation were thoroughly investigated, and the presence of cationic species have been confirmed and they were isolated and characterized by NMR. [9,17,[20][21][22][23][24][25][26][27][28][29][30][31][32] The catalyst Cp*TiMe 3 reacts with B(C 6 F 5 ) 3 forming the methyl-bridged methyl borate species Cp*TiMe 2 (μ-Me)-B(C 6 F 5 ) 3 , which was fully characterized by 1 H and 13 C NMR spectroscopy. [33] The formed Cp*TiMe 2 (μ-Me)B(C 6 F 5 ) 3 species exists in equilibrium with its solvent-separated ion pairs [MeB(C 6 F 5 ) 3 ] − and [Cp*TiMe 2 ] + .…”

“…Triphenylmethyl salts of noncoordinating anions such as trityl tetrakis (pentafluorophenyl) borates, [CPh 3 ] + [(BC 6 F 5 ) 4 ] − , have also been used to activate metallocenes. [9,17] The advantage of this cocatalyst over B(C 6 H 5 ) 3 is that the resulting counter ion is less coordinating than the one formed when B(C 6 F 5 ) 3 is used to activate monocyclopentadienyl precursors. [16] NMR studies on the reaction between Et(Ind) 2 ZrMe 2 and [CPh 3 ] + [(BC 6 F 5 ) 4 ] − by Bochmann and Lancaster [28] showed that a dimeric μ-Me species [Cp 2 ZrMe(μ-Me)MeZrCp 2 ] + B(C 6 F 5 ) 4 − is initially formed, and probably subsequently converted to the monomeric cation species.…”