Investigation of a fine-grain MgCl2-supported Ziegler catalyst by stopped-flow propene polymerization: Model for the formation of active sites induced by catalyst fragmentation during polymerization

1997 ◽  
Vol 198 (10) ◽  
pp. 3207-3214 ◽  
Author(s):  
Hideharu Mori ◽  
Masaki Yoshitome ◽  
Minoru Terano
Keyword(s):  
Active Sites ◽  
Stopped Flow ◽  
Fine Grain ◽  
Propene Polymerization

New Evidence on the Nature of the Active Sites in Heterogeneous Ziegler−Natta Catalysts for Propene Polymerization

1997 ◽  
Vol 30 (16) ◽  
pp. 4786-4790 ◽  
Author(s):  
Vincenzo Busico ◽  
Roberta Cipullo ◽  
Giovanni Talarico ◽  
Anna Laura Segre ◽  
John C. Chadwick
Keyword(s):  
Active Sites ◽  
Propene Polymerization ◽  
New Evidence ◽  
Ziegler Natta Catalysts

scholarly journals Studies by electron-paramagnetic-resonance spectroscopy and stopped-flow spectrophotometry on the mechanism of action of turkey liver xanthine dehydrogenase

1976 ◽  
Vol 153 (2) ◽  
pp. 297-307 ◽  
Author(s):  
M J Barber ◽  
R C Bray ◽  
D J Lowe ◽  
M P Coughlan
Keyword(s):  
Electron Paramagnetic Resonance ◽  
Redox Potential ◽  
Active Sites ◽  
Electron Paramagnetic Resonance Spectroscopy ◽  
Xanthine Dehydrogenase ◽  
Resonance Spectroscopy ◽  
Stopped Flow ◽  
Paramagnetic Resonance ◽  
Flow Measurements ◽  
Electron Paramagnetic

Studies by e.p.r. (electron-paramagnetic-resonance) spectroscopy and by stopped-flow spectrophotometry on turkey liver xanthine dehydrogenase revealed strong similarities to as well as important differences from the Veillonella alcalescens xanthine dehydrogenase and milk xanthine oxidase. The turkey enzyme is contaminated by up to three non-functional forms, giving molybdenum e.p.r. signals designated Resting I, Resting II and Slow. Slow and to a lesser extent Resting I signals are like those from the Veillonella enzyme, whereas Resting II is very like a resting signal described by K. V. Rajagopolan, P. Handler, G. Palmer & H. Beinert (1968) (J. Biol. Chem. 243, 3784-3796) for aldehyde oxidase. Another non-functional form that gives the Inhibited signal is produced on treatment of the enzyme with formaldehyde. Stopped-flow measurements at 450 nm show that, as for the milk enzyme, reduction by xanthine is rate-limiting in enzyme turnover. The active enzyme gives rise to Very Rapid and Rapid molybdenum(V) e.p.r. signals, as well as to an FADH signal. That these signals are almost indistinguishable from those of the milk enzyme, confirms the similarities between the active sites. There are two types of iron-sulphur centres that give signals like those in the milk enzyme, though with slightly different parameters. Quantitative reduction titration of the functional enzyme with xanthine revealed two important differences between the turkey and the milk enzymes. First, the turkey enzyme FADH/FADH2 system has a redox potential sufficiently low that xanthine is incapable of reducing the flavin completely. This finding presumably explains the very low oxidase activity. Secondly, whereas the Fe/S II chromophore in the milk enzyme has a relatively high redox potential, for the turkey enzyme the value of this potential is lower and similar to that of its Fe/S I chromophore.

scholarly journals Sequential Immobilization of Ansa-Hafnocene Complexes for Propene Polymerization

2019 ◽  
Author(s):  
Marius Arz ◽  
Tim Kratky ◽  
Sebastian Günther ◽  
Katia Rodewald ◽  
Thomas Burger ◽  
...  
Keyword(s):  
Active Sites ◽  
Poor Performance ◽  
Reaction Conditions ◽  
Continuous Polymerization ◽  
Propene Polymerization ◽  
Close Proximity ◽  
Catalytically Active

We report the immobilization of the ultrarigid <i>ansa</i>-hafnocene complexes [Me<sub>2</sub>Si(Ind*)<sub>2</sub>HfCl<sub>2</sub>] (Ind* = 7,(3',5'-Di-<i>tert</i>-butylphenyl)-4-methoxy-2-methylindenyl) on silica as heteregeneous catalysts for propene polymerization. A sequential three-step synthesis on the siliceous surface led to pre-catalysts of the generalized structure SiO<sub>2</sub>-Si(Ind*)<sub>2</sub>HfCl<sub>2</sub>, which possess the silylene bridge of the substituted bis(indenyl) ligand directly attached to the surface. The immobilized pre-catalysts show very poor performance in the polymerization of propene, independent on the reaction conditions and the employed silica. Based on the results, we suggest that the close proximity of the catalyst to the surface combined with the steric congestion provoked by the ligand prevents a continuous polymerization, most likely due to a blockage of the catalytically active sites with growing polymer.<br>

scholarly journals Development of Large-Scale Stopped-Flow Technique and its Application in Elucidation of Initial Ziegler–Natta Olefin Polymerization Kinetics

Polymers ◽  
2019 ◽  
Vol 11 (6) ◽  
pp. 1012 ◽  
Author(s):  
Ashutosh Thakur ◽  
Toru Wada ◽  
Patchanee Chammingkwan ◽  
Minoru Terano ◽  
Toshiaki Taniike
Keyword(s):  
Active Sites ◽  
Large Scale ◽  
Olefin Polymerization ◽  
Polymerization Kinetics ◽  
Propylene Polymerization ◽  
Time Range ◽  
Stopped Flow ◽  
Chemical Transformations ◽  
Polymerization Catalysis ◽  
Short Period

The stopped-flow (SF) technique has been extensively applied to study Ziegler–Natta (ZN) olefin polymerization kinetics within an extremely short period (typically <0.2 s) for understanding the nature of the active sites as well as the polymerization mechanisms through microstructure analyses of obtained polymers. In spite of its great applicability, a small amount of polymer that is yielded in a short-time polymerization has been a major bottleneck for polymer characterizations. In order to overcome this limitation, a large-scale SF (LSF) system has been developed, which offers stable and scalable polymerization over an expanded time range from a few tens milliseconds to several seconds. The scalability of the LSF technique has been further improved by introducing a new quenching protocol. With these advantages, the LSF technique has been effectively applied to address several unknown issues in ZN catalysis, such as the role of physical and chemical transformations of a catalyst on the initial polymerization kinetics, and regiochemistry of ZN propylene polymerization. Here, we review the development of the LSF technique and recent efforts for understanding heterogeneous ZN olefin polymerization catalysis with this new system.

scholarly journals Studies on alkaline phosphatase. Phosphorylation of calf intestinal alkaline phosphatase by 32P-labelled pyrophosphate

1968 ◽  
Vol 107 (2) ◽  
pp. 279-283 ◽  
Author(s):  
H N Fernley ◽  
Sylvia Bisaz
Keyword(s):  
Alkaline Phosphatase ◽  
Intestinal Mucosa ◽  
Active Sites ◽  
Stopped Flow ◽  
Serine Residue ◽  
Intestinal Alkaline Phosphatase ◽  
Calf Intestinal Alkaline Phosphatase ◽  
Substrate Hydrolysis

1. A purified preparation of alkaline phosphatase from calf-intestinal mucosa was phosphorylated by 32P-labelled PPi at a serine residue on the enzyme. Under the conditions employed, up to 0·15μm-labelled sites were obtained from 1μm-[32P]PPi. 2. The phosphorylated enzyme was labile, the rate of dephosphorylation being similar to the overall rate of substrate hydrolysis. 3. A stopped-flow technique was used to determine the number of phosphomonoesterase active sites, which agreed with the number of 32P-labelled sites. 4. It is concluded that calf-intestinal alkaline phosphatase is both a phosphomonoesterase and a pyrophosphatase.

scholarly journals Escherichia coli alkaline phosphatase. An analysis of transient kinetics

1971 ◽  
Vol 125 (1) ◽  
pp. 319-327 ◽  
Author(s):  
S. E. Halford
Keyword(s):  
Escherichia Coli ◽  
Alkaline Phosphatase ◽  
Inorganic Phosphate ◽  
Active Sites ◽  
Substrate Binding ◽  
Equilibrium Mixture ◽  
Stopped Flow ◽  
Transient Kinetics ◽  
Substrate Complex ◽  
Catalytically Active

1. The hydrolysis of 2,4-dinitrophenyl phosphate by Escherichia coli alkaline phosphatase at pH5.5 was studied by the stopped-flow technique. The rate of production of 2,4-dinitrophenol was measured both in reactions with substrate in excess of enzyme and in single turnovers with excess of enzyme over substrate. It was found that the step that determined the rate of the transient phase of this reaction was an isomerization of the enzyme occurring before substrate binding. 2. No difference was observed between the reaction after mixing a pre-equilibrium mixture of alkaline phosphatase and inorganic phosphate, with 2,4-dinitrophenyl phosphate at pH5.5 in the stopped-flow apparatus, and the control reaction in which inorganic phosphate was pre-equilibrated with the substrate. Since dephosphorylation is the rate-limiting step of the complete turnover at pH5.5, this observation suggests that alkaline phosphatase can bind two different ligands simultaneously, one at each of the active sites on the dimeric enzyme, even though only one site is catalytically active at any given time. 3. Kinetic methods are outlined for the distinction between two pathways of substrate binding, which include an isomerization either of the free enzyme or of the enzyme–substrate complex.

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