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 A lCl3 and either Cp2TiCl2 or Cp2Ti(CH2SiMe3)Cl in chloroalkane solution , isomeric forms of the resulting 1:1 complexes have been detected. Th e influence of temperature, concentration, ratio of the titanocene chl oride 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 chan ges caused by the foregoing factors give compelling evidence for an eq uilibrium in such Cp2Ti(R)Cl.AlCl3 complexes (R = Cl, CH2SiMe3) betwee n comtact ion pairs (CIP), Cp2TiR...Cl...AlCl3, and solvent-separated ion pairs (SSIP), Cp2TiR+ parallel-to AlCl4-. More polar media and hig her dilution have been shown to favor the solvent-separated ion pair i somer 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 h examethylbenzene indicate the operation of significant steric hindranc e to complexation. These observations lead to the proposal that arenes form sigma or eta1 complexes with the titanocenium cation and thus ge nerate a third type of ion pair, the solvated cation-anion pair (SCAP) Cp2Ti(eta1-arene)R+AlCl4-. Since these NMR studies of the soluble Zie gler 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: po larity and donor character of the solvent, concentration of the cataly st, 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 polymeriza tion is conducted in arene solution), Cp2Ti(arene)Me+AlCl4-, is the le ast active catalyst.