STEREOCHEMICAL CONTROL IN HOMOGENEOUS ZIEGLER-NATTA CATALYSTS

Racemic ethylenebis(eta5-indenyl)zirconium dichloride (Et[Ind]2ZrCl2) activated with methylaluminoxane (MAO) catalyzed propylene polymerization with varying degree of stereochemical control which decreases greatly with the increase of T(p) (temperature of polymerization. The PP's are characterized by low melting temperature (T(m)), high solubility, and prefers to crystallize in the gamma-modification. The catalytic activity of Et[Ind]2ZrCl2/MAO becomes very small with the lowering of T(p). Very active and highly stereoselective cationic metallocene alkyl, Et[Ind]2Zr+(CH3), was produced by the reaction of Et[Ind]2Zr(CH3)2 with Ph3C+B(C6F5)4-. Comparison of this system with the Et[Ind]2ZrCl2/MAO calalyst showed that in the latter case a quarter of the Et[Ind]2ZrCl2 was converted by MAO to Et[Ind]2Zr+CH3 at room temperature but less than 0.14% of the Zr was so activated at -20-degrees-C. The Et[IndH4]ZrCl2/MAO catalyst was shown to have two kinds of catalytic species one with high propagation rate constant (k(p)) and stereoselectivity and another with low k(p) and poor stereoselectivity. The very narrow molecular weight distribution of the PP produced may be attributed to the fact that the different types of active species have comparable k(p)/k(tr)A, the latter is the rale constant of transfer. Non-symmetric, rac-[anti-ethylidene(1-eta5-indenyl)(1-eta5-tetramethylcyclopentadienyl)-Ti-Cl2 and -(CH3)2 have been synthesized and structures determined. The complexes provide dissimilar steric environment to propagating chains to produce crystalline-amorphous multiblock thermoplastic elastomeric PP. The polymerization process here involves a two-state propagation mechanism.