THE ROLE OF ION-PAIR EQUILIBRIA ON THE ACTIVITY AND STEREOREGULARITY OF SOLUBLE METALLOCENE ZIEGLER-NATTA CATALYSTS

By means of temperature-dependent multinuclear NMR studies, electrical conductivity measurements and polymerization activity assessments, the nature of the resulting complexes formed between metallocene alkyls or metallocene dihalides and Group 13 Lewis acids was investigated comprehensively. The influences of solvent polarity, concentration, temperature and the strength and proportion of the Lewis acid upon the structure of the metallocene-lewis acid complexes furnish cogent evidence for the existence of an equilibrium between contact ion pairs (CIP) and solvent-separated ion-pairs (SSIP). Three systems were considered in this study: 1) Cp(2)TiCl(2) + AlCl3; 2) bis(($) under bar n-butylcyclopentadienyl)zirconium dimethyl + (C6F5)(3)B; and 3) bis(($) under bar n-butylcyclopentadienyl)-zirconium dichloride + MAO. More polar media and higher dilution have been shown to favor the solvent-separated ion-pair isomer over the contact ion-pair isomer. In ($) under bar pi-basic solvents, such as arenes, evidence indicates that a 1:1:1 complex of the metallocene, the Lewis acid and the arene is formed reversibly (solvated cation-anion pair, SCAP). Polymerization activities toward ethylene or propylene, as measured in these studies or reported in the literature, support the conclusion that the solvent-separated ion pairs are the most active catalyst sites but are less stereoselective in syndiotactic polymerization. In a related study, the Me(3)Al-content of MAO could be reduced to zero by toluene-evaporations at 25 degrees C. The Me(3)Al-free MAO, upon admixture with (CD3)(3)Al, rapidly displayed an H-1 NMR signal revealing the regeneration of ''free'' Me(3)Al. This observation is consistent with the rapid methyl group exchange between the terminal Me(2)Al groups of MAO chains, Me(2)Al-(-OAlMe-)-(n)Me, and the added (CD3)(3)Al and supports the suggestion that these terminal Me(2)Al groups are the most reactive structural unit of MAO either toward external Me(3)Al or toward metallocene dialkyls or dihalides, In this view such terminal Me(2)Al groups are responsible for the heightened cocatalytic activity of MAO in Ziegler-Natta catalysts. This view conforms with the heuristic Steiger-Kaminsky hypothesis.