Catalyst Site Epimerization during the Kinetic Resolution of Chiral α-Olefins by Polymerization

Min, Endy Y.-J; Byers, Jeffery A.; Bercaw, John E.

doi:10.1021/om700778e

Cited by 26 publications

(20 citation statements)

References 38 publications

(26 reference statements)

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“…13 C NMR spectroscopy is one of the few methods that can provide quantitative information about polymer topology and microstructure, and has been widely used for the determination of branch content and tacticity of polyolefins 34-38. The sensitivity of solution-state NMR spectroscopy is limited due to the low concentration of 13 C nuclei, and bulk samples typically suffer severe line broadening.…”

Section: Resultsmentioning

confidence: 99%

Ring-Expansion Metathesis Polymerization: Catalyst-Dependent Polymerization Profiles

et al. 2009

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Ring-expansion metathesis polymerization (REMP) mediated by recently developed cyclic Ru catalysts has been studied in detail with a focus on the polymer products obtained under varied reaction conditions and catalyst architectures. Depending upon the nature of the catalyst structure, two distinct molecular weight evolutions were observed. Polymerization conducted with catalysts bearing 6-carbon tethers displayed rapid polymer molecular weight growth which reached a maximum value at ca 70% monomer conversion, resembling chain-growth polymerization mechanism. In contrast, 5-carbon tethered catalysts lead to molecular weight growth that resembled a step-growth mechanism with a steep increase occurring only after 95% monomer conversion. The underlying reason for these mechanistic differences appeared to be ready release of 5-carbon tethered catalysts from growing polymer rings, which competed significantly with propagation. Owing to reversible chain transfer and the lack of end groups in REMP, the final molecular weights of cyclic polymers was controlled by thermodynamic equilibria. Large ring sizes in the range of 60 -120 kDa were observed at equilibrium for polycyclooctene and polycyclododecatriene, which were found to be independent of catalyst structure and initial monomer/catalyst ratio. While 6-carbon tethered catalysts slowly incorporated into the formed cyclic polymer, the incorporation of 5-carbon tethered catalysts was minimal, as revealed by ICP-MS. Further polymer analysis was conducted using meltstate magic-angle spinning 13 C NMR spectroscopy of both linear and cyclic polymers, which revealed little or no chain ends for the latter topology.

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Section: Resultsmentioning

confidence: 99%

Ring-Expansion Metathesis Polymerization: Catalyst-Dependent Polymerization Profiles

et al. 2009

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“…Figure 27 demonstrates schematically the ligand-induced site epimerization with the 13/MAO catalyst system. Metallocene molecule 13 is the first example in a series of C 1 symmetric molecules that form the basis for a new important class of highly isospecific metallocene catalysts [32,[178][179][180][181][182][183][184][185].…”

Section: Other Types Of C S Symmetric Metallocene Catalysts With Syndmentioning

confidence: 99%

Syndiotactic Polypropylene: Discovery, Development, and Industrialization via Bridged Metallocene Catalysts

Razavi¹

2013

Advances in Polymer Science

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This chapter covers the discovery that enabled, for the first time, the manufacturing of high molecular weight, highly stereoregular syndiotactic polypropylene polymers with narrow molecular weight distribution via the new class of metallocene molecules with bridged cyclopentadienyl-fluorenyl ligands. First, the salient crystal structural features of the neutral metallocene molecules and the crystal structure of the corresponding mono-alkyl-zirconocenium cation is introduced. Then, by employing the structural and geometrical data, a realistic working model for the active site precursor is developed and presented. The stereochemistry of the catalytic propylene poly-insertion reaction and the formation of syndiotactic polypropylene according to the enantiomorphic site control mechanism at the enantiotopic cationic active sites, formed after the activation of the prochiral metallocene dichloride with methylaluminoxane, MAO, is reviewed and further delineated. In the following sections, the impact of the introduction of substituents with proper size and composition at some selected positions of the ligand is discussed and the methods for increasing the molecular weight and stereoregularity of syndiotactic polypropylene polymers are elaborated. With the aid of computational calculations on real and hypothetical catalyst systems with substitutionally altered ligand structures, it is shown that the degree of enantioselectivity of syndiospecific catalyst systems can be determined and/or predicted for these systems quantitatively and with high precision. The calculations confirm that the introduction of substituent(s) reinforcing the preferential conformational orientation of the polymer chain enhances the overall enantioselectivity of the active site, and to certain extends stereoselectivity in general, by affecting the site epimerization processes. The computational calculations reveal that to account correctly for the frequency of the site epimerization-dependent m-type stereo-errors it is essential to include the counter-ion, as an integrated part of the A. Razavi (*) 35 Domaine de la Brisee, 7034 Obourg, Belgium e-mail: abbasrazavi@skynet.be catalyst system, into the calculations. The relevance of the symmetry and stereorigidity of the metallocene structure for the syndiospecificity of the final catalyst is debated by presenting a few "non-conforming" catalyst examples.Finally, the challenges involved in the manufacture of syndiotactic polypropylene in commercial scale continuous production processes are highlighted as well as the polymer's unique structural, physical, mechanical and rheological properties.

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“…Reaction of 4 with N-bromosuccinimide (NBS) in the presence of 2,2Ј-azo-bis(isobutyronitrile) (AIBN) as described in a related case, [13] gave allylic bromide 5, which, on reaction with quinoline at high temperature, [14] gave the desired diacetate (i.e., 6). Treatment of this iodide with potassium cyclopentadienide in DMF in the presence of 18-crown-6 (5 mol-%) [15,16] gave polycycle 12, as a result of nucleophilic substitution of the neopentyl-type iodide by the cyclopentadienide anion, followed by intramolecular Diels-Alder reaction, in good yield. gave the corresponding Diels-Alder adduct 7 in good yield (Scheme 2).…”

Section: Resultsmentioning

confidence: 99%

“…The X-ray intensity data were measured with a D8 Venture system equipped with a multilayer monochromator and a Mo microfocus (λ = 0.71073 Å). [b] Absolute structure parameter: 0.44 (15). The integration of the data using a triclinic unit cell yielded a total of 15320 reflections to a maximum θ angle of 26.45°( 0.80 Å resolution), of which 3007 were independent (average redundancy 5.095, completeness: 99.6 %, R int = 2.06 %, R sig = 1.39 %), and 2847 (94.68 %) were greater than 2σ(F 2 ).…”

Section: (B) Mixture Of Mesylate 10 Its C-7 Epimer and Dimethyl (1rmentioning

confidence: 99%

Synthesis of Polycycles by Single or Double Domino Nucleophilic Substitution/Diels–Alder Reaction

2015

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Catalyst Site Epimerization during the Kinetic Resolution of Chiral α-Olefins by Polymerization

Cited by 26 publications

References 38 publications

Ring-Expansion Metathesis Polymerization: Catalyst-Dependent Polymerization Profiles

Ring-Expansion Metathesis Polymerization: Catalyst-Dependent Polymerization Profiles

Syndiotactic Polypropylene: Discovery, Development, and Industrialization via Bridged Metallocene Catalysts

Synthesis of Polycycles by Single or Double Domino Nucleophilic Substitution/Diels–Alder Reaction

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