Organometallic compounds of Group III. 52. Active sites for ethylene polymerization with titanium(IV) catalysts in homogeneous media: multinuclear NMR study of ion-pair equilibria and their relation to catalyst activity

Abstract

By means of a multinuclear NMR study of the complexes formed between AlC13 and either CpZTiClz or CpzTi(CHzSiMe3)Cl in chloroalkane solution, isomeric forms of the resulting 1:l complexes have been detected. The influence of temperature, concentration, ratio of the titanocene chloride to aluminum chloride, and the nature of the solvent upon the 'H, 13C, and 27Al NMR spectra has been investigated. The spectral changes caused by the foregoing factors give compelling evidence for an equilibrium in such CpzTi(R)Cl*AlC& complexes (R = C1, CH2-SiMe3) between comtact ion pairs (CIP), Cp2TiR..*Cl.-AlCl3, and solvent-separated ion pairs (SSIP), CpzTiR+JIAlCL-. More polar media and higher dilution have been shown to favor the solvent-separated ion pair isomer over the contact ion pair isomer. In a-basic solvents, such as arenes, evidence is offered that a 1:l:l complex of CpZTi(R)Cl, AlC13, and arene is formed reversibly. The isolation of such a complex with mesitylene and the failure to detect the formation of a complex with hexamethylbenzene indicate the operation of significant steric hindrance to complexation. These observations lead to the proposal that arenes form CT or v1 complexes with the titanocenium cation and thus generate a third type of ion pair, the solvated cation-anion pair (SCAP) CpzTi(+arene)R+AlCL-. Since these NMR studies of the soluble Ziegler polymerization catalyst system CpzTiRCl-AlC13 have permitted the determination of three types of ion pairs in solution, contact (CIP), solvent-separated (SSIP), and solvated cationanion (SCAP), an attempt was made to correlate the polymerization activity toward ethylene of an analogous catalyst system, Cp2TiMeC1-MenA1C1%,, with the dominant type of ion pair present under different experimental conditions. The following variations in the conditions of polymerization were made: polarity and donor character of the solvent, concentration of the catalyst, temperature, ratio of the titanium to the aluminum component, and the nature of the Lewis acid, MenA1C1sn (n = 0-3). From these studies it has been concluded that the solventseparated ion pair is the most active catalyst and that the arene-solvated SCAP (when the polymerization is conducted in arene solution), CpzTi(arene)Me+AlC&-, is the least active catalyst. IntroductionThe polymerization of olefins, diolefins, and acetylenes by the cocatalytic action of transition-metal salts andmaingroup organometallics clearly represents one of the most significant and influential discoveries of modern chemistry.2 Although such Ziegler-Natta catalysts have been recognized for almost 40 years,3 the molecular origin of this synergistic effect has come to light only recently. Particularly through studies of soluble ethylene polymerization catalysts, such as combinations of bis(v5-cyclopentadienyl)titanium(IV) dichloride and alkylaluminum

The discovery of metallocene catalysts and their present state of the art

“…Shilov and Dyachkovsky, 58 Eisch, 59 Jordan, 60 and Bochmann 61 showed that the activity of metallocene catalysts depends on the formation of cationic species. Today, most research groups agree with this statement.…”

The Extraordinary Cocatalytic Action of Polymethylaluminoxane (MAO) in the Polymerization of Terminal Olefins by Metallocenes: Chemical Change in the Group 4 Metallocene Dimethyl Derivatives Induced by MAO

“…Multinuclear NMR and solvent-polarity evidence support the conclusion that solvent-separated ion pairs (10) are the most active initiator of polymerization in the Cp 2 TiCl 2 /MeAlCl 2 catalyst system. [13,14] (11), in this system has been corroborated by its chemical capture with trimethyl(phenylethynyl)silane (12) as adduct 13, whose 3-D structure has been verified by single-crystal X-ray determination [Equation (5)]. [9] Eur.…”

Novel Organoborane Lewis Acids via Selective Boron-Tin Exchange Processes Steric Constraints to Electrophilic Initiation by the Boron Halide

“…Deuteriated solvents (Cambridge Scientific) for NMR measurements of oxygen-and moisture-sensitive substance were vacuum-transferred to the Schlenk glass vessels and stored over 4-Å molecular sieves under argon. Ϫ Nuclear magnetic resonance (NMR) spectra were recorded with Bruker AM 360 ( 4 Sn), external. Ϫ Gas chromatographic analysis (GC) was performed with a Hewlett-Packard Chromatograph, model 5880A, with helium as the carrier gas.…”

“…After dilution with dry diethyl ether, the lithium reagent was treated at 0°C with an ether solution of 10 mmol of dimethyltin dichloride. The reaction was stirred at reflux for 2 h and then poured into a solution of aqueous NH 4 Cl. Usual work-up gave an orange-yellow oil with the desired stannaindane as the major component by NMR criteria.…”

“…The individual interaction of the 1,2-bis(trimethylstannyl)ethyne and the isomeric bis(trimethylstannyl)benzenes with Et 2 BBr produced the corresponding bis(diethylboryl)-derivatives. By contrast, with Et 2 BCl the α,o-bis(trimethylstannyl)toluene gave only odiethylboryl-α-trimethylstannyltoluene and with BCl 3 the α,αЈ-bis(trimethylstannyl)-o-xylene formed only α,αЈ-bisAs part of an ongoing investigation of Ziegler-Natta olefin-polymerization catalysis the present study has explored the synthesis of novel Group-13 Lewis acids for their potential suitability as cocatalysts with cyclopentadienyl transition metal derivatives in such homogeneous polymerizations.[ 4] In this contribution the preparation of organoborane Lewis acids bearing substituents of enhanced electron-withdrawing power (1) and diminished steric demand (2) and possibility multidentate boron centers (3) has been the principal focus of research. The electron-withdrawing group E in 1 could favorably be a halide (F > Cl > Br > I) and an organyl group other than alkyl (sp 3 ) having greater p-character, such as aryl (sp 2 ) or 1-alkynyl (sp).…”