Controlled synthesis of low-molecular-weight polyethylene waxes by titanium–biphenolate–ethylaluminum sesquichloride based catalyst systems

2007 ◽  
Vol 104 (3) ◽  
pp. 1531-1539 ◽  
Author(s):  
Prashant S. Umare ◽  
Karumanchi Rao ◽  
Gopal L. Tembe ◽  
Deepa A. Dhoble ◽  
Bhavna Trivedi
Keyword(s):  
Molecular Weight ◽  
Low Molecular Weight ◽  
Controlled Synthesis ◽  
Catalyst Systems

scholarly journals Synthesis of a Chiral Meso-Like Ansa-Zirconocene Complex and its Use for the Catalytic Formation of Low Molecular Weight Polypropylene

1995 ◽  
Vol 50 (6) ◽  
pp. 982-989 ◽  
Author(s):  
Sven Thiele ◽  
Gerhard Erker ◽  
Cornelia Fritze ◽  
Christian Psiorz ◽  
Roland Fröhlich
Keyword(s):  
Crystal Structure ◽  
Molecular Weight ◽  
Double Bond ◽  
Ambient Temperature ◽  
Organic Synthesis ◽  
Crystal Structure Analysis ◽  
Low Molecular Weight ◽  
X Ray ◽  
Catalyst Systems ◽  
Double Bond Shift

Reaction of lithium(neomenthylcyclopentadienid) 3 with dimethyldichlorosilane gives dimethylbis[3-(neomenthyl)cyclopentadienyl]silane 4 regioselectively as a mixture of double bond shift isomers (57% isolated). Deprotonation with 2 equiv. of butyllithium followed by treatment with zirconiumtetrachloride furnished a mixture of the three {dimethylsilylenbis[η5-3-(neomenthyl)cyclopentadienyl][zirconium dichloride diastereomers from which the pure meso-like isomer 6e [p -R (1′S ,3 ′R ,4 ′R),p -S (1′S ,3 ′R ,4 ′R)] was isolated by fractional crystallization. Complex 6 c was characterized by an X-ray crystal structure analysis. Its D 1 - Zr - D 2 angle of 126.8° is very similar as observed in bis(η-cyclopentadienyl) zirconium dichloride (D 1 and D 2 denote the centroids of the cyclopentadienyl rings). The homogeneous 6c/methylalumoxane Ziegler-type catalyst produces polypropylene of low molecular weight (Mη ≈ 670 at ambient temperature). Related catalyst systems may find interesting applications in organic synthesis.

A Rigid C1-Bridged Ansa-Zirconocene-Derived Catalyst System Suited for Stereoselective Low Molecular Weight Polypropylene Formation

1995 ◽  
Vol 50 (4) ◽  
pp. 469-475 ◽  
Author(s):  
Gerhard Erker ◽  
Christian Psiorz ◽  
Roland Fröhlich
Keyword(s):  
Molecular Weight ◽  
Fractional Crystallization ◽  
Isotactic Polypropylene ◽  
Catalyst System ◽  
Low Molecular Weight ◽  
X Ray Diffraction ◽  
X Ray ◽  
Tert Butyl ◽  
Catalyst Systems

2,5-Hexanedione was converted into the bisfulvene 2, then treated with two molar equivalents of methyllithium to yield the [4-cyclopentadienylidene-4,7,7-trimethyl-4,5,6,7-tetrahydroindenyl] dilithio compound 4. Hydrolysis, followed by treatment with acetone/pyrrolidine, gave the corresponding fulvene system 5. Reaction of 5 with methyllithium followed by treatment with ZrCl4 furnished the ring-annulated C1-bridged ansa-metallocene 8, bearing a tert-butyl substituent at the Cp ring, as a 1:1 mixture of two diastereoisomers. Treatment of the fulvene 5 with LiAlH4 followed by ZrCl4 yielded the respective isopropyl-substituted ansa-metallocene diastereomers 9a and 9b. Complex 9b was separated by fractional crystallization and characterized by X-ray diffraction. Complexes 8 and 9 provide active homogeneous Ziegler-type catalyst systems upon activation with excess methylalumoxane producing low molecular weight isotactic polypropylene with high catalyst activities.

scholarly journals Controlled Synthesis of Water-Soluble NaYF4:Yb3+, Er3+Nanoparticles with Surfactant Dependent Properties

2014 ◽  
Vol 2014 ◽  
pp. 1-7
Author(s):  
Junwei Zhao ◽  
Tiekun Jia ◽  
Xiaofeng Wang ◽  
Xianggui Kong
Keyword(s):  
Molecular Weight ◽  
Upconversion Luminescence ◽  
Excitation Power ◽  
Water Soluble ◽  
Solvothermal Reaction ◽  
Low Molecular Weight ◽  
Controlled Synthesis ◽  
Mixed Solution ◽  
Branched Polyethylenimine ◽  
Chain Lengths

Water-soluble NaYF4:Yb3+, Er3+nanoparticles (NPs) are successfully prepared by a solvothermal reaction using branched polyethylenimine (PEI) with different chain lengths as the surfactants in a water/diethylene glycol (DEG) mixed solution. It is shown that the size of NaYF4NPs prepared with high molecular weight PEI (HPEI) is smaller than that of the NPs prepared with low molecular weight (LPEI), while small-sized NPs exhibit more intense upconversion luminescence intensities than large-sized NPs in the same excitation power of 980 nm. It is found that HPEI is conducive to the formation of smaller NP with high crystallinity. Small-sized NaYF4:Yb3+, Er3+NPs with intense upconversion luminescence and improved crystallinity were related to their growth process. A possible growth mechanism of the samples is proposed. The results of this study can provide new insights into the controlled synthesis of novel NPs.

A high-performance oil-free backing pump for electron microscopes

1978 ◽  
Vol 36 (1) ◽  
pp. 110-111
Author(s):  
G.K.W. Balkau ◽  
E. Bez ◽  
J.L. Farrant
Keyword(s):  
Molecular Weight ◽  
Electron Microscope ◽  
Hot Spots ◽  
High Performance ◽  
Carbonaceous Material ◽  
Contamination Rate ◽  
Low Molecular Weight ◽  
Principal Source ◽  
Rotary Pumps ◽  
Electron Microscopes

The earliest account of the contamination of electron microscope specimens by the deposition of carbonaceous material during electron irradiation was published in 1947 by Watson who was then working in Canada. It was soon established that this carbonaceous material is formed from organic vapours, and it is now recognized that the principal source is the oil-sealed rotary pumps which provide the backing vacuum. It has been shown that the organic vapours consist of low molecular weight fragments of oil molecules which have been degraded at hot spots produced by friction between the vanes and the surfaces on which they slide. As satisfactory oil-free pumps are unavailable, it is standard electron microscope practice to reduce the partial pressure of organic vapours in the microscope in the vicinity of the specimen by using liquid-nitrogen cooled anti-contamination devices. Traps of this type are sufficient to reduce the contamination rate to about 0.1 Å per min, which is tolerable for many investigations.

Low-molecular-weight heparin in the prevention and treatment of thromboembolic disorders

1998 ◽  
Vol 1 (5) ◽  
pp. 166-174 ◽  
Author(s):  
Evelyn R Hermes De Santis ◽  
Betsy S Laumeister ◽  
Vidhu Bansal ◽  
Vandana Kataria ◽  
Preeti Loomba ◽  
...  
Keyword(s):  
Molecular Weight ◽  
Low Molecular Weight Heparin ◽  
Low Molecular Weight ◽  
Prevention And Treatment
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