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

By means of a multinuclear NMR study of the complexes formed between AlCl3 and either Cp2TiCl2 or Cp2Ti(CH2SiMe3)Cl in chloroalkane 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 the nature of the solvent upon the H-1, C-13, and Al-27 NMR spectra has been investigated. The spectral changes caused by the foregoing factors give compelling evidence for an equilibrium in such Cp2Ti(R)Cl.AlCl3 complexes (R = Cl, CH2SiMe3) between comtact ion pairs (CIP), Cp2TiR...Cl...AlCl3, and solvent-separated ion pairs (SSIP), Cp2TiR+ parallel-to AlCl4-. More polar media and higher dilution have been shown to favor the solvent-separated ion pair isomer over the contact ion pair isomer. In pi-basic solvents, such as arenes, evidence is offered that a 1:1:1 complex of Cp2Ti(R)Cl, AlCl3, 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 sigma or eta1 complexes with the titanocenium cation and thus generate a third type of ion pair, the solvated cation-anion pair (SCAP) Cp2Ti(eta1-arene)R+AlCl4-. Since these NMR studies of the soluble Ziegler polymerization catalyst system Cp2TiRCl-AlCl3 have permitted the determination of three types of ion pairs in solution, contact (CIP), solvent-separated (SSIP), and solvated cation-anion (SCAP), an attempt was made to correlate the polymerization activity toward ethylene of an analogous catalyst system, Cp2TiMeCl-MenAlCl3-n, 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, MenAlCl3-n (n = 0-3). From these studies it has been concluded that the solvent-separated ion pair is the most active catalyst and that the arene-solvated SCAP (when the polymerization is conducted in arene solution), Cp2Ti(arene)Me+AlCl4-, is the least active catalyst.