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Volumn 129, Issue 25, 2007, Pages 7831-7840

Mechanism of activation of a hafnium pyridyl-amide olefin polymerization catalyst: Ligand modification by monomer

Author keywords

[No Author keywords available]

Indexed keywords

ALKYL CATIONS; EPIMERIZATION; NAPHTHYL GROUP; OLEFIN POLYMERIZATION;

EID: 34347244137     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja070718f     Document Type: Article
Times cited : (129)

References (71)
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    • For reviews of stereocontrol in α-olefin polymerization, see: a
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    • For reviews of late-metal catalysts for olefin polymerization, see: a
    • For reviews of late-metal catalysts for olefin polymerization, see: (a) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414.
    • (1995) J. Am. Chem. Soc , vol.117 , pp. 6414
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    • (b) Macchioni, A.; Boone, H. W.; Busico, V.; Stevens, J. C.; Zuccaccia, C. Abstracts of Papers, 231st ACS National Meeting, Atlanta, GA, March 26-30, 2006; American Chemical Society: Washington, DC, 2006; BMGT-021.
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    • In calculations discussed further in this paper, if multiple rotamers are possible, only the lowest energy one is quoted; further details can be found in the Supporting Information
    • In calculations discussed further in this paper, if multiple rotamers are possible, only the lowest energy one is quoted; further details can be found in the Supporting Information.
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    • Interestingly, this binding is stronger than for many metallocene and constrained-geometry catalysts and, for 2a, more closely resembles binding to late transition metal catalysts. Many group IV metallocenes have olefin binding energies in the 6-8 kcal/mol range relative to the β-agostic intermediate, and the inclusion of entropic effects at temperatures in the 100-200°C range renders the olefin complex higher in energy than the initial metal-alkyl, olefin separated pair. Ultimately, this situation leads to activation parameters with low ΔH‡ and large negative ΔS‡. Late transition metal catalysts possess stronger olefin binding energies and the activation parameters with larger ΔH‡ and smaller negative ΔS‡ indicate the resting state is a bound olefin complex. In this case of the pyridyl-amide complexes 1, the nature of the resting state is
    • ‡ indicate the resting state is a bound olefin complex. In this case of the pyridyl-amide complexes 1, the nature of the resting state is not known, but the large olefin binding energy is unusual for an early transition metal complex.
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    • For examples of η2-benzyl interactions in zirconium cations, see: (a) Jordan, R. F, LaPointe, R. E, Bajgur, C. S, Echols, S. F, Willett, R. J. Am. Chem. Soc. 1987, 109, 4111-13
    • 2-benzyl interactions in zirconium cations, see: (a) Jordan, R. F.; LaPointe, R. E.; Bajgur, C. S.; Echols, S. F.; Willett, R. J. Am. Chem. Soc. 1987, 109, 4111-13.
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    • Using ChemDraw Ultra, these chemical shifts are predicted at 14.9 and 26.2 ppm
    • Using ChemDraw Ultra, these chemical shifts are predicted at 14.9 and 26.2 ppm.
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    • For a catalyst system claimed to exhibit a higher reactivity for α-olefins than for ethylene, see
    • For a catalyst system claimed to exhibit a higher reactivity for α-olefins than for ethylene, see: Irwin, L. J.; Reibenspies, J. H.; Miller, S. A. J. Am. Chem. Soc. 2004, 126, 16716-16717.
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    • The trend in catalyst activity in Table 2 is not clear, but we consider analysis of the GPC data to be more reliable.
    • The trend in catalyst activity in Table 2 is not clear, but we consider analysis of the GPC data to be more reliable.
  • 48
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    • Additional details of the GPC deconvolutions are given in the Supporting Information
    • Additional details of the GPC deconvolutions are given in the Supporting Information.
  • 49
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    • For examples of poly(α-olefins) from related pyridylamido procatalysts with broad molecular weight distributions and multiple melting peaks, see refs 10c,d
    • For examples of poly(α-olefins) from related pyridylamido procatalysts with broad molecular weight distributions and multiple melting peaks, see refs 10c,d.
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    • Several other potential active species have been considered to explain these homopolymerization results. One possibility involves β-elimination of the aryl-ethyl to give a styryl species followed by reinsertion in a 2,1-fashion to give a benzylic ligand. The same site could also result from hydrogenolysis of the aryl-ethyl moiety followed by C-H activation at the benzylic carbon this proposed mechanism was inspired by a reviewer's comments, We are investigating these possibilities, but preliminary DFT calculations and experimental work do not support them
    • Several other potential active species have been considered to explain these homopolymerization results. One possibility involves β-elimination of the aryl-ethyl to give a styryl species followed by reinsertion in a 2,1-fashion to give a benzylic ligand. The same site could also result from hydrogenolysis of the aryl-ethyl moiety followed by C-H activation at the benzylic carbon (this proposed mechanism was inspired by a reviewer's comments). We are investigating these possibilities, but preliminary DFT calculations and experimental work do not support them.
  • 51
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    • s- symmetric pyridyl-amide catalyst that forms isotactic polypropylene. This observation supports our proposed mechanism of ligand modification by monomer. Domski, G. J.; Lobkovsky, E. B.; Coates, G. W. Macromolecules 2007, 40, ASAP article.
    • s- symmetric pyridyl-amide catalyst that forms isotactic polypropylene. This observation supports our proposed mechanism of ligand modification by monomer. Domski, G. J.; Lobkovsky, E. B.; Coates, G. W. Macromolecules 2007, 40, ASAP article.
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    • Gaussian, Inc, Wallingford, CT, For the complete reference, see the Supporting Information
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* 이 정보는 Elsevier사의 SCOPUS DB에서 KISTI가 분석하여 추출한 것입니다.