Counterion Effects on Propylene Polymerization Using Two-State ansa-Metallocene Complexes

Abstract

Propylene polymerization using unsymmetrical, ansa-metallocene complexes Me(2)Y(Ind)CpMMe(2) (Y = Si, C, M = Zr, Y = C, M = Hf) and the co-initiators methyl aluminoxane (PMAO), B(C(6)F(5))(3), and [Ph(3)C][B(C(6)F(5))(4)] was studied at a variety of propylene concentrations. Modeling of the polymer microstructure reveals that the catalysts derived from Me(2)Si(Ind)CpZrMe(2) and each of these co-initiators function under conditions where chain inversion is much faster than propagation (Curtin-Hammett conditions). Surprisingly, the microstructure of the PP formed was essentially unaffected by the nature of the counterion, suggesting similar values for the fundamental parameters inherent to two-state catalysts. The tacticity of PP was sensitive to changes in [C(3)H(6)] in the case of catalysts derived from Me(2)C(Ind)CpHfMe(2) and PMAO, or [Ph(3)C][B(C(6)F(5))(4)], but the average tacticity of the polymer produced at a given [C(3)H(6)] decreased in the order [Ph(3)C][B(C(6)F(5))(4)] > PMAO. With B(C(6)F(5))(3), the polymer formed was more stereoregular, and its microstructure was invariant to changes in monomer concentration. The PP pentad distributions in this case could be modeled by assuming that all three catalyst/cocatalyst combinations function with different values for the relative rates of insertion to inversion (Delta) but otherwise feature essentially invariant, intrinsic stereoselectivity for monomer insertion (alpha, beta), while the relative reactivity/stability (g/K) of the isomeric ion-pairs present seems to be only modestly affected, if at all. Similar conclusions can also be made about the published propylene polymerization behavior of the C(s)-symmetric Me(2)C(Flu)CpZrMe(2) complex with different counterions. For every counterion investigated, the principle difference appears to be the operating regime (Delta) rather than intrinsic differences in insertion stereoselectivity (alpha). Surprisingly, the ordering of the various counterions with respect to Delta does not agree with commonly accepted ideas about their coordinating ability. In particular, catalysts when activated with B(C(6)F(5))(3) appear to function at low values of Delta as compared to those featuring B(C(6)F(5))(4) (less coordinating) and FAl[(o-C(6)F(5))C(6)F(4)](3) (more coordinating) or PMAO (more coordinating) counterions where the ordering in Delta is MeB(C(6)F(5))(3) < B(C(6)F(5))(4) < FAl[(o-C(6)F(5))C(6)F(4)](3) approximately PMAO. Possible reasons for this behavior are discussed.

(Butadiene)metallocene/B(C6F5)3 Pathway to Catalyst Systems for Stereoselective Methyl Methacrylate Polymerization: Evidence for an Anion Dependent Metallocene Catalyzed Polymerization Process

“…different mechanisms, 48 their observed anion dependent stereochemical performance may add to the cumulating evidence for the involvement of different types of ion pairs in metallocene catalyzed polymerization reactions,43,45 similar to what is well established in the chemistry of reactive organic carbenium ion systems.49 …”

“…23, 27 Since the anions cannot kinetically control the actual polymerization process in the CurtinHammett situation via their reversible equilibration with the active species, we must assume that the actual active species (e.g., 23/24) are in fact also ion pairs and that the anions involved here exert a pronounced influence on the respective transition states of the product formation, similar to what has been discussed for anion dependent R-olefin polymerization at metallocene catalysts. [43][44][45][46][47] Unless the polymerization reactions of the catalyst systems 9 and 18, respectively, proceed by …”

Kinetic and mechanistic aspects of metallocene polymerisation catalysts

“…The rrrr pentad intensities for different anions are determined by the ratio of the rate of insertion to the rate of inversion. Remarkably, there is essentially no anion effect with catalyst 22 where inversion (chain swinging) is always much faster than propagation [84]. It is clear therefore that close proximity of the anion is required to suppress chain swinging and to ensure stereoregular chain propagation.…”

Propylene Polymerization with 1,2‘-Bridged Bis(indenyl)zirconium Dichlorides

“…For bis(indenyl)zirconocenes, the influence of the bridge on the stereoselectivity generally follows the trend H 2 C < Me 2 C < C 2 H 4 < Me 2 Si. 2 For the C 1 symmetric metallocenes 2 and 3, it is possible that the higher stereoselectivity of 2 is a consequence of a higher probability of insertion at the stereoselective site due to a higher degree of site epimerization (also called "backskip") 16,21,26 for the ethylene-bridged metallocene relative to the silicon-bridged metallocene, but further studies are necessary to support this hypothesis. The molecular weights of the polypropylenes derived from the 2-phenyl-substituted systems increase in the order CH 2 < Me 2 Si < CH 2 CH 2 (4 < 3 < 2), whereas the productivity follows the trend 2 < 4 < 3.…”