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.
Sterically hindered aluminum methyl compounds derived from reaction of hindered phenols with AlMe 3 (i.e., MeAl(BHT) 2 and MeAl(BHT*) 2 ; BHT ) 2,6-di-tert-butyl-4-methylphenoxide; BHT* ) 2,4,6-tritert-butylphenoxide) are useful scavenging agents in olefin polymerization using metallocene catalysts. They do not, or only slowly, react with activators such as B(C 6 F 5 ) 3 or [Ph 3 C][B(C 6 F 5 ) 4 ] at 25 °C, nor do they coordinate to or react with metallocenium ion-pairs derived from metallocene dialkyls and these activators. A mixture of AlMe 3 and a large excess of MeAl(BHT) 2 proves advantageous for catalysts that are susceptible to reaction with BHT-H, the hydrolysis product of MeAl(BHT) 2 . Ethylene polymerization experiments establish that the activity of [Cp 2 ZrMe][MeB(C 6 F 5 ) 3 ] is only slightly inhibited by AlMe 3 in the presence of a significant excess of MeAl(BHT) 2 . Spectroscopic studies have revealed that AlMe 3 is in equilibrium with MeAl(BHT) 2 , forming Me 2 Al(BHT). At low temperature using 13 C NMR spectroscopy, a 1:1 mixture of AlMe 3 and MeAl(BHT) 2 is shown to consist of Al 2 Me 6 , MeAl-(BHT) 2 , and primarily Me 2 Al(µ-BHT) 2 AlMe 2 . A higher temperature, both intra-and intermolecular exchange of both Al-Me and Al-BHT groups, coupled with the temperature dependence of the various equilibria involved, lead to 1 H and 13 C NMR spectra that are consistent with monomeric Me 2 Al(BHT). 1 H and 19 F NMR spectroscopic studies of mixtures of the ion-pairs [Me 2 C(Cp)IndMMe][MeB(C 6 F 5 ) 3 ] (M ) Zr, Hf) or [Me 2 SiCp 2 ZrMe][MeB(C 6 F 5 ) 3 ] with various quantities of AlMe 3 in the presence of MeAl(BHT) 2 were conducted. The AlMe 3 -mediated degradation of ion-pairs that are susceptible to B(C 6 F 5 ) 3 dissociation is largely absent in the presence of excess MeAl(BHT) 2 , although reversible formation of [Me 2 SiCp 2 Zr(µ-Me) 2 AlMe 2 ][MeB(C 6 F 5 ) 3 ] and related adducts is observed at low ratios of MeAl(BHT) 2 to AlMe 3 .
The study of counterion effects in metallocenecatalyzed olefin polymerization has led to significant advances in our understanding of this process. 1 The counterion, though weakly coordinating, is known to affect catalyst stability and activity 2 as well as polymer properties such as molecular weight 1,2 and tacticity 3 in a direct manner. It is not unreasonable to state that the interaction of the counteranion with the metal center 4 is decisive in affecting catalyst performance. The search for more active catalysts has focused on activators in which two group 13 atoms are linked via an anionic bridge. 5 This type of activator can give rise to very active catalysts, in comparison to those formed using mononuclear activators, and it is interesting to speculate as to the possible role of such species in polymerizations involving the latter. 5a Recent studies on 1-hexene polymerization using [en(Ind) 2 ZrMe][MeB(C 6 F 5 ) 3 ] indicate that excess B(C 6 F 5 ) 3 has no effect on the kinetics of initiation and propagation. 2h However, enhancement of ethylene or propene polymerization activity was noted when using 2 vs 1 equiv of Al(C 6 F 5 ) 3 , and metallocene or constrainedgeometry catalysts. 6a This was attributed to "doubleactivation" of the catalyst precursor, on the basis of the X-ray structure of [Me 2 Si(Ind) 2 Zr][MeAl(C 6 F 5 ) 3 ] 2 . An alternative explanation based on transient formation of [Me 2 Si(Ind) 2 ZrMe][(C 6 F 5 ) 3 Al(µ-Me)Al(C 6 F 5 ) 3 ] 6a featuring a more weakly coordinating dinuclear anion 6b was not excluded. It should be noted that a related dicationic complex, [( t Bu 3 PdN) 2 Ti][MeB(C 6 F 5 ) 3 ] 2 , is inactive in ethylene polymerization. 6c More recently, the use of excess [Ph 3 C][B(C 6 F 5 ) 4 ] significantly increases the rate of a number of metallocene-catalyzed propene polymerizations; 7 only a portion of this activation has been attributed to an increase in active site concentration. 2 During the course of some mechanistic work involving the activation of Me 2 X(Cp)IndZrMe 2 (1, X ) C, Si) using (1) (a) Bochmann, M.; Lancaster, S. J.; Hannant, M. D.; Rodriguez, A.; Schormann, M.; Walker, D. A.; Woodman, T. . (2) For recent kinetic and related mechanistic studies: (a) Landis, C. R.; Sillars, D. R.; Batterton, J. M. J. Am. Chem. Soc. 2004, 126, 8890-8891. (b) Rodriguez-Delgado, A.; Hannant, M. D.; Lancaster, S. Lancaster, S. J.; Walker, D. A.; Beck, S.; Thornton-Pett, M.; Bochmann, M. J. Am. Chem. Soc. 2001, 123, 223-237. (3) (a) Chen, M.; Roberts, J. A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 4605-4625. (b) Busico, V.; Cipullo, R.; Cutillo, F.; Vacatello, M.; Van Axel Castelli, V. Macromolecules 2003, 36, 4258-4261. (c) Mohammed, M.; Nele, M.; Al-Humydi, A.; Xin, S.; Stapleton, R.; Collins, S.
Ethylene polymerization was studied using a variety of iminophosphonamide (PN2) complexes of zirconium. Bis(PN2) dichloride complexes [Ph2P(NR′)2]2ZrX2 (X = Cl; 1a: R′ = p-tolyl; 1b: R′ = Bn; 1c: R′ = C6F5) or dimethyl complexes (X = Me; 2a: R′ = p-tolyl; 2b: R′ = Bn) and cyclopentadienyl(PN2)zirconium dichloride complexes [η5-C5R′′5][R2P(NR′)2]ZrCl2 (3a: R′ = p-tolyl, R = Ph, R′′ = H; 3b: R′ = SiMe3, R = Et, R′′ = H; 3c: R′ = C6F5, R = Ph, R′′ = H; 3e: R′ = 3,5-(CF3)2Ph, R = Ph, R′′ = H; 3f: R′ = 3,5-(CF3)2Ph, R = Ph, R′′ = Me) or dimethyl analogs [η5-C5H5][R2P(NR′)2]ZrMe2 (4a: R′ = p-tolyl, R = Ph; 4b: R′ = SiMe3, R = Et) were evaluated under a range of conditions using methylaluminoxane (PMAO) activator. Complexes 1 and 2 behave as precursors to single-site polymerization catalysts under the conditions studied, while complexes 3 or dialkyls 4 show more complex behavior and formation of poly(ethylene) with a bimodal molecular weight distribution. In contrast, activation of dialkyl complexes 4 with [Ph3C][B(C6F5)4] and polymerization in the presence of small amounts of PMAO or TIBAL as scavenger, led to single-site behavior. PMAO reacts with the neutral dialkyls via ligand abstraction to produce a number of P-containing species that may explain the multi-site behavior observed when using this activator. Dialkyls 4 react cleanly with [Ph3C][B(C6F5)4] in haloarene or even dichloromethane solution to furnish the corresponding cationic alkyls 5, which were characterized by multinuclear NMR spectroscopy. Fluxional dinuclear species are formed in the presence of excess dialkyl and these are susceptible to CH activation to form µ-Me,µ-CH2 complexes one of which could be isolated in pure form. The cationic alkyls initiate the polymerization of 1-hexene at room temperature in chlorobenzene solution, but extensive chain transfer occurs and the systems are not living.Key words: single site, early metal olefin polymerization catalysis.
. In this article we argued that the equilibrium depicted in eq 1 on p 194 was irreversible, on the basis of the value of K ≈ 10 4 M -1 estimated from the fit of the data to the equation depicted in Figure 1. This conclusion is true only if the active species on the right-hand side remains in this state following every insertion. If the active species reverts to the dormant state following each insertion, then the equilibrium depicted in eq 1 is coupled to an irreversible reaction and the expression used to fit the data in Figure 1 is not valid.Under the latter conditions, and if the steady-state approximation holds for the active species, it can be shown that where [Zr] with K ) k 1 /k -1 and k p being the propagation rate constant for monomer coordination and insertion involving the activated species. It can be appreciated from this expression that without independent knowledge of K or n it is not possible to unambiguously estimate either quantity from the data. Also, this expression reduces to the equation depicted in Figure 1 in the limit n f 0.On p 195 we report that K ) 6 M -1 at -60°C for the equilibrium depicted in eq 2 involving pairs 3 and intermediate 4 and from the Supporting Information (Figure S-2) one can estimate that K ≈ 0.02 M -1 at 30°C from the T dependence of K over the range -60 to -30°C (i.e., there is very little of intermediate 4 present above -20°C).If the value of K for the equilibrium depicted in eq 1 has this same magnitude, one can easily show that, in the above expression, the term (1 + r)a , 1 + n for all values of n with r e 1000 at [Zr] 0 ) 5 × 10 -6 M. The expression shown above would then simplify to thus indicating that little or no enhancement of R p should be observed if double activation analogous to that shown in eq 2 is involved during propagation. Thus, either double activation of the propagating intermediate is somehow much more favorable than suggested by the results using model ion pairs or an alternate explanation is needed to account for the kinetic behavior at steady state.
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