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Preparation of Epoxides03:00

Preparation of Epoxides

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Epoxides result from alkene oxidation, which can be achieved by a) air, b) peroxy acids, c) hypochlorous acids, and d) halohydrin cyclization.
Epoxidation with Peroxy Acids
Epoxidation of alkenes via oxidation with peroxy acids involves the conversion of a carbon–carbon double bond to an epoxide using the oxidizing agent meta-chloroperoxybenzoic acid, commonly known as MCPBA. Since the O–O bond of peroxy acids is very weak, the addition of electrophilic oxygen of peroxy acids to...
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Recently, the development of olefin metathesis polymerization advanced the field of polymer synthesis. Simply put, the reorganization of substituents on their double bonds between two olefins in the presence of a catalyst is known as the olefin metathesis reaction. The use of metathesis reaction for polymer synthesis is called olefin metathesis polymerization.
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Polymer Classification: Stereospecificity01:26

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Polymerization generates chiral centers along the entire backbone of a polymer chain. Accordingly, the stereochemistry of the substituent group has a significant effect on polymer properties. Polymers formed from monosubstituted alkene monomers feature chiral carbons at every alternate position in the polymer backbone. Relative to the predominant orientation of substituents at the adjacent chiral carbons, the polymer can exist in three different configurations: isotactic, syndiotactic, and...
Structure and Nomenclature of Ethers02:28

Structure and Nomenclature of Ethers

Structure and Bonding
Ethers are organic compounds with an ether functional group which is characterized by an oxygen atom connected to two — identical or different — alkyl, aryl, or vinyl groups. The C–O–C linkage in dimethyl ether — the simplest ether — has an approximately tetrahedral bond angle of 110.3 degrees. The oxygen atom is sp3- hybridized, with the C–O distance being about 140 pm.
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Alcohols are one of the most important functional groups in organic chemistry. The name of alcohol comes from the hydrocarbon from which it is derived. Alcohols are organic molecules containing the functional hydroxyl or –OH group directly bonded to carbon. Phenols have an OH group directly attached to a benzene ring. While alcohols are colorless, phenol is a white crystalline compound with a characteristic "hospital smell" odor.
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ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH3

All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...

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Stereospecific octahedral group 4 bis(phenolate) ether complexes for olefin polymerization.

Elizabeth T Kiesewetter1, Sören Randoll, Madalyn Radlauer

  • 1Department of Chemistry, Stanford University, Stanford, California 94305-5080, USA.

Journal of the American Chemical Society
|April 3, 2010
PubMed
Summary
This summary is machine-generated.

Hafnium and Zirconium bisphenolate ether complexes are highly active alpha-olefin polymerization catalysts. Hafnium catalysts exhibit superior stereoselectivity, producing high-quality polypropylene with specific structural features.

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Area of Science:

  • Organometallic Chemistry
  • Polymer Science
  • Catalysis

Background:

  • Octahedral group 4 bisphenolate ether complexes are known catalysts for alpha-olefin polymerization.
  • Methylaluminoxane (MAO) is a common activator for these types of catalysts.

Purpose of the Study:

  • To investigate the catalytic activity and stereospecificity of group 4 bisphenolate ether complexes.
  • To compare the performance of hafnium (Hf) and zirconium (Zr) based catalysts.
  • To understand the influence of ligand structure on catalyst stereoselectivity.

Main Methods:

  • Synthesis and characterization of octahedral group 4 bisphenolate ether complexes.
  • Activation of complexes with methylaluminoxane (MAO).
  • X-ray crystallographic analysis to determine structural similarities and differences.
  • Alpha-olefin polymerization studies to evaluate catalyst activity and stereospecificity.

Main Results:

  • Both Zr and Hf complexes, when activated by MAO, demonstrated high activity and stereospecificity in alpha-olefin polymerization.
  • X-ray crystallography showed Zr and Hf complexes to be isostructural, with slightly shorter bond lengths in the Hf complex.
  • Hf complexes yielded more stereoselective catalysts compared to Zr complexes, despite structural similarity.
  • Bis-tert-butyl phenyl substituted Hf and Zr complexes produced high molecular weight polypropylene (>97% isotactic, melting point up to 165°C).

Conclusions:

  • Group 4 bisphenolate ether complexes activated by MAO are effective and stereospecific catalysts for alpha-olefin polymerization.
  • Hafnium-based catalysts offer enhanced stereoselectivity over zirconium-based catalysts.
  • Ligand structure plays a significant role in dictating the stereospecificity of these polymerization catalysts.