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 polymerization activity of the following three catalyst systems towards ethylene has been investigated and compared: 1) methyltitanocene chloride-M+lC1, 2) q3-allyltitanocene-M%AlCl and 3) titanium dichloride tetrahydrofuranate-Me&Cl.The f i s t two catalysts formed homogeneous phases and produced linear polymer; the last catalyst functioned in a heterogeneous medium and formed crosslinked polymer. The titanium (Iv) catalyst was about 30 times more active than the t i t a n i u m 0 system and the titanium(II) catalyst on MgC1, could be made 8 times more active than the t i t a n i u m 0 system. A novel mechanism is proposed to explain the behavior and activity of the titanium@) system.

By means of a multinuclear NMR study of the complexes formed between AlC13 and either Cp2TiC12 or Cp2Ti(CH2SiMe3)C1 in methylene chloride solution, isomeric forms of the resulting 1 : 1 complexes have been detected. The influence of temperature, concentration, ratio of the titanocene chloride to aluminum chloride and nature of the solvent upon the 'H. 13C and nAl NMR spectra has been investigated. The spectral changes caused by the foregoing factors give compelling evidence for a equilibrium in such -Ti(R)Cl AlC13 complexes (R = C1, CH2SiMe3) between contact ion pairs, Cp2TiR Cl AlC13, and solvent-separated ion pairs, Cp2TiR+ AlC14-. Upon experimental variations in temperature, concentration, solvent and ratio of R,AlC13-, to the titanium catalyst, the polymerization activity of the catalyst system towards ethylene was markedly altered. Such changes in activity support the conclusion that the most active sites for polymerization in such systems are the solvent-separated ion pairs.

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