How to Polymerize Ethylene in a Highly Controlled Fashion?

Kempe, Rhett

doi:10.1002/chem.200601842

Cited by 257 publications

(220 citation statements)

References 67 publications

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“…water). MAO, or the incorporated trimethylaluminum (Me 3 Al) always present in MAO mixtures, could also act as a chain-transfer agent allowing for an efficient "Aufbaureaktion" at the Al center [18] or shuttling the growing polymer chain between different catalytic centers (Scheme 1). [19][20][21][22][23][24][25] This interplay between group 4 and group 13 metal centers in olefin polymerization catalysis is well established [15] [a] Inorganic and Organometallic Chemistry, University of Erlangen-Nürnberg, and has led to the design of some interesting heterobimetallic catalysts.…”

Section: Introductionmentioning

confidence: 99%

Oxide‐Bridged Heterobimetallic Aluminum/Zirconium Catalysts for Ethylene Polymerization

2015

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Protein‐based pharmaceuticals typically display high selectivity and low toxicity, but are also characterized by low oral availability, mainly because of enzymatic degradation in the gastrointestinal tract and poor permeability across the intestinal wall. One way to increase the proteolytic stability of peptides and proteins is by intramolecular crosslinking, such as the introduction of disulfide bridges. However, disulfide bridges are at risk of thiol‐disulfide exchange or reduction during production, purification, and/or therapeutic use, whereas thioether bridges are expected to be stable under the same conditions. In this study, thioether crosslinking was investigated for a 46 aa albumin‐binding domain (ABD) derived from streptococcal protein G. ABD binds with high affinity to human serum albumin (HSA) and has been proposed as a fusion partner to increase the in vivo half‐lives of therapeutic proteins. In the study, five ABD variants with single or double intramolecular thioether bridges were designed and synthesized. The binding affinity, secondary structure, and thermal stability of each protein was investigated by SPR‐based biosensor analysis and CD spectroscopy. The proteolytic stability in the presence of the major intestinal proteases pepsin (found in the stomach) and trypsin in combination with chymotrypsin (found in pancreatin secreted to the duodenum by the pancreas) was also investigated. The most promising crosslinked variant, ABD_CL1, showed high thermal stability, retained high affinity in binding to HSA, and showed dramatically increased stability in the presence of pepsin and trypsin/chymotrypsin, compared to the ABD reference protein. This suggests that the intramolecular thioether crosslinking strategy can be used to increase the stability towards gastrointestinal enzymes.

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Section: Introductionmentioning

confidence: 99%

Oxide‐Bridged Heterobimetallic Aluminum/Zirconium Catalysts for Ethylene Polymerization

2015

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“…[1][2][3][4] However, melt processing of such polyolefins with narrow MWDs is rather difficult because of lack of shear thinning during processing. Therefore it is an important objective in polyolefin development to tailor MWDs and to carefully balance the content of low and high molecular weight polyethylene fractions.…”

Section: Introductionmentioning

confidence: 99%

Mesoporous Silica Supported Multiple Single‐Site Catalysts and Polyethylene Reactor Blends with Tailor‐Made Trimodal and Ultra‐Broad Molecular Weight Distributions

Kurek

Mark

Enders

et al. 2010

Macromol. Rapid Commun.

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A ternary blend of the bisiminopyridine chromium (III) (Cr-1) with the bisiminopyridine iron (II) (Fe-2) post-metallocenes with the quinolylsilylcyclopentadienyl chromium (III) halfsandwich complex (Cr-3) was supported on mesoporous silica to produce novel multiple single-site catalysts and polyethylene reactor blends with tailor-made molecular weight distributions (MWDs). The preferred cosupporting sequence of this ternary blend on MAO-treated silica was Fe-2 followed by Cr-1 and Cr-3. Cosupporting does not impair the single-site nature of the blend components producing polyethylene fractions with $\overline M _{\rm w}$ = 10(4) g · mol(-1) on Cr-1, $\overline M _{\rm w}$ = 3 × 10(5) g · mol(-1) on Fe-2, and $\overline M _{\rm w}$ = 3 × 10(6) g · mol(-1) on Cr-3. As a function of the Fe-2/Cr-1/Cr-2 mixing ratio it is possible to control the weight ratio of these three polyethylenes without affecting the individual average molecular weights and narrow polydispersities of the three polyethylene fractions. Tailor-made polyethylene reactor blends with ultra-broad MWD and polydispersities varying between 10 and 420 were obtained. When the molar ratio of Fe-2/Cr-1 was constant, the ultra-high molecular polyethylene (UHMWPE, $\overline M _{\rm w}$ > 10(6) g · mol(-1) ) content was varied between 8 and 16 wt.-% as a function of the Cr-3 content without impairing the blend ratio of the other two polyethylene fractions and without sacrificing melt processability. When the molar ratio Fe-2/Cr-3 was constant, it was possible to selectively increase the content of the low molecular weight fraction by additional cosupporting of Cr-1. Due to the intimate mixing of low and ultra-high molecular weight polyethylenes (UHMPEs) produced on cosupported single-site catalysts a wide range of melt processable polyethylene reactor blends was obtained.

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“…[10] In this study, to circumvent these problems, we have analyzed by NMR techniques low molecular weight sPS prepared by chain-growth polymerization. [18] In a similar fashion to that we recently described for the synthesis of allyl end-capped syndiotactic oligostyrenes, [19] low molecular weight (M n = 2 000-10 000 g mol À1 ) sPS terminated by a nbutyl end-group were prepared with high efficiency by polymerizing styrene in the presence of [{Cp'CMe 2 Flu'}Nd( 2 (50 equiv vs. Nd). [20,21] The resulting materials are fully soluble in common organic solvents such as aromatic hydrocarbons, THF and chloroform, which allowed their full characterization using size exclusion chromatography (SEC) and NMR techniques.…”

mentioning

confidence: 98%

On the Initiation Mechanism of Syndiospecific Styrene Polymerization Catalyzed by Single‐Component ansa‐Lanthanidocenes

Perrin

Sarazin

Kirillov

et al. 2009

Chemistry A European J

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The mechanism of the initiation step of styrene polymerization promoted by single-component ansa-lanthanidocene catalysts [{Cp'XMe(2)Flu'}Ln(R)(ether)(n)] (Cp' = C(5)H(4); Flu' = 9-C(13)H(8); X = C or Si; R = allyl = CH(2)CHCH(2) or alkyl = CH(2)SiMe(3); ether = THF or Et(2)O, n = 0,1) has been studied from a combined experimental/theoretical perspective. First, detailed (13)C NMR studies conducted on syndiotactic oligostyrenes prepared by chain-growth polymerization with [{C(5)H(4)CMe(2)Flu'}Nd(eta(3)-1,3-C(3)H(3)(SiMe(3))(2))]/Mg(nBu)(2) (1:50) have shown that the insertion of styrene in these lanthanidocene systems proceeds selectively in a secondary (2,1) fashion, both at the initiation and propagation steps. Next, DFT studies of styrene insertion have been carried out on three model compounds, [{Cp'CMe(2)Flu'}Eu(eta(3)-allyl)(thf)] (I), [{Cp'SiMe(2)Flu'}Eu(eta(3)-allyl)(thf)] (II), and [{Cp'CMe(2)Flu'}Eu(eta(1)-CH(2)SiMe(3))(thf)] (III), in order to rationalize the influence of the ansa bridge (CMe(2) vs. SiMe(2)) and that of the "active" R ligand (eta(3)-allyl vs. eta(1)-CH(2)SiMe(3)), previously noticed in styrene polymerization experiments. Dissociation of the THF molecule in precursor I appears not to be a rate-limiting step and proceeds readily. The steric hindrance between the phenyl ring of the incoming styrene monomer and the ancillary ligands (Cp', Flu'), induced by the change of either the bridge or the "active" R ligand, is proposed to control the reactivity of the complexes. In all cases, orientation of the styrene phenyl ring toward the most sterically opened Cp' ligand (as compared to Flu') and 2,1-insertion of styrene have been found to be the most thermodynamically and kinetically favorable pathway. Also, in all cases, insertion of the first styrene unit proceeds directly in the eta(3)-coordinated allyl group. The lack of activity of the ansa-dimethylsilylene allyl complex and of the ansa-isopropylidene alkyl complex appears to be mainly due to the thermodynamics of the insertion reaction rather than the height of the activation barrier.

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How to Polymerize Ethylene in a Highly Controlled Fashion?

Cited by 257 publications

References 67 publications

Oxide‐Bridged Heterobimetallic Aluminum/Zirconium Catalysts for Ethylene Polymerization

Oxide‐Bridged Heterobimetallic Aluminum/Zirconium Catalysts for Ethylene Polymerization

Mesoporous Silica Supported Multiple Single‐Site Catalysts and Polyethylene Reactor Blends with Tailor‐Made Trimodal and Ultra‐Broad Molecular Weight Distributions

On the Initiation Mechanism of Syndiospecific Styrene Polymerization Catalyzed by Single‐Component ansa‐Lanthanidocenes

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