Muqtar Mohammed

Organic Chemistry, Polymer Chemistry



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    ABSTRACT: Propene polymerization using ansa-metallocene dialkyl complexes [Me2C(Cp)IndMMe2] (1a; M=Zr) and (1b; M=Hf) was studied using B(C6F5)3 as a stoichometric activator. The initial rates of propene uptake as well as the steady-state rates of propene consumption are dramatically increased in the presence of excess borane using 1a and to a lesser extent 1b. 1H and 19F NMR experiments at B:Zr ratios of 1:1 and 1.2:1 reveal that the by-products produced following consumption of monomer are also sensitive to B:Zr stoichiometry. Under the former conditions, initiation is inefficient and unreacted ion-pairs [Me2C(Cp)IndZrMe][MeB(C6F5)3] (2a–b) effect C–H activation of the unsaturated oligomers that form to produce π-allyl complexes [Me2C(Cp)IndZr(η3-2-R-C3H4)][MeB(C6F5)3] (4a) [R=–(CH2CHMe)nCH2CH2CH3] and methane; these oligomeric complexes were identified by comparison to π-allyl complexes 4b (R=Me) and 4c (R=CH2CMe3) prepared from ion-pairs 2a–b and either isobutene or 2,4,4-trimethylpentene. Complex 4b was structurally characterized and presents a highly distorted π-methallyl ligand. In the presence of excess borane, initiation is efficient, and the principal by-products formed are Zr-borohydride complexes Me2C(Cp)IndZrMe(μ-H)B(C6F5)3 (5a–b). The major isomer present was identified by comparison to the spectroscopic data observed for its Hf-analog 6 with could be isolated from these reactions and was structurally characterized. Complexes 5 are detected after consumption of monomer is complete whereas complex 6 was formed competitively during consumption of monomer and both are relatively unreactive towards subsequent monomer insertion. Thus, the formation of these μ-borohydride complexes is a reversible deactivation pathway for some ansa-metallocenium ions, and that excess borane may serve to reactivate them towards further insertion.
    No preview · Article · Aug 2005 · Polyhedron
  • Márcio Nele · José Carlos Pinto · Muqtar Mohammed · Scott Collins
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    ABSTRACT: Temperature (T) effects on the microstructure of polypropylene made with metallocene catalysts have been investigated with the theoretical framework originally developed by Coleman and Fox and extended to the stereospecific polymerization of propylene with two-state ansa and fluxional metallocene catalysts. T effects on the polymer microstructure are mainly due to factors other than changes in the intrinsic stereoselectivity of the two states. The model has been applied to the stereosequence distributions of polypropylene prepared with the C1-symmetric Me2Si(Ind)(Flu)ZrCl2 complex, activated with methyl aluminoxane, over a range of T and monomer concentration ([M]) values. The use of these two variables, in combination with the Coleman–Fox model (or a kinetic model), allows more reliable estimates of fundamental parameters, especially when the microstructure is a weak function of one of these variables at a constant value of T (e.g., [M]). © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1797–1810, 2005
    No preview · Article · Mar 2005 · Journal of Polymer Science Part A Polymer Chemistry
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    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.
    No preview · Article · Jul 2003 · Journal of the American Chemical Society
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    Marcio Nele · Muqtar Mohammed · Shixuan Xin · Scott Collins
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    ABSTRACT: A kinetic model to describe the propylene polymerization behavior of ansa-metallocene catalysts was derived. The model can predict n-ad stereosequence distributions, polymer crystallinity, and related properties as well as distinguish between extremes in kinetic behavior expected for such catalysts. In particular, where polymer microstructure is sensitive to changes in [C3H6], the model can provide reliable estimates of kinetic parameters of interest, including ratios between rates of some of the significant reaction steps involved in polymer microstructure formation. The model is applied to a description of the polymerization behavior of some simple symmetrical [Me2C(Cp)(Flu)MCl2; M = Zr, Hf] and unsymmetrical [Me2Y(Cp)(Ind)MCl2; M = Zr, Hf; Y = C, Si] ansa-metallocene catalysts, activated with methyl aluminoxane. With the former two catalysts, the Zr catalyst operates very close to the kinetic quenching limit where chain inversion (or chain back-skip) is slow compared to monomer insertion, while for the Hf analogue, these two processes have more comparable rates. In the more complicated, unsymmetrical systems, both Zr- and Hf-based systems (Y = Si) operate under conditions where inversion is much faster that propagation, whereas for the Hf catalyst (Y = C), intermediate behavior is observed, and the corresponding Zr complex (Y = C) produces poly(propylene) where propagation is faster than inversion.
    Full-text · Article · May 2001 · Macromolecules

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