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Preparation of Epoxides03:00

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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...
Olefin Metathesis Polymerization: Overview01:13

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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...
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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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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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Complejos de éter estereospecíficos del grupo octaédrico 4 bis (fenolato) para la polimerización de olefinas.

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
Resumen

Los complejos de éter de hafnio y bisfenolato de zirconio son catalizadores de polimerización de alfa-olefinas muy activos. Los catalizadores de hafnio exhiben una estereoselectividad superior, produciendo polipropileno de alta calidad con características estructurales específicas.

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Área de la Ciencia:

  • Química organometálica Química orgánica de los metales.
  • La ciencia de los polímeros es la ciencia de los polímeros.
  • La catálisis de la catálisis.

Sus antecedentes:

  • Los complejos de éter bisfenolato del grupo octaédrico 4 son catalizadores conocidos para la polimerización de alfa-olefinas.
  • El metilaluminoxano (MAO) es un activador común para este tipo de catalizadores.

Objetivo del estudio:

  • Para investigar la actividad catalítica y la estereospecificidad de los complejos de éter de bisfenolato del grupo 4.
  • Para comparar el rendimiento de los catalizadores basados en hafnio (Hf) y zirconio (Zr).
  • Comprender la influencia de la estructura del ligando en la estereoselectividad del catalizador.

Principales métodos:

  • Síntesis y caracterización de complejos de éter bisfenolato del grupo octaédrico 4.
  • Activación de los complejos con el metilaluminoxano (MAO).
  • Análisis cristalográfico de rayos X para determinar similitudes y diferencias estructurales.
  • Estudios de polimerización de alfa-olefina para evaluar la actividad del catalizador y la estereospecificidad.

Principales resultados:

  • Ambos complejos Zr y Hf, cuando se activan por MAO, demostraron una alta actividad y estereospecificidad en la polimerización de alfa-olefina.
  • La cristalografía de rayos X mostró que los complejos Zr y Hf son isostruturales, con longitudes de enlace ligeramente más cortas en el complejo Hf.
  • Los complejos Hf produjeron más catalizadores estereoselectivos en comparación con los complejos Zr, a pesar de la similitud estructural.
  • Los complejos Hf y Zr sustituidos por bis-tert-butilo-fenila producen polipropileno de alto peso molecular (>97% isotáctico, punto de fusión de hasta 165°C).

Conclusiones:

  • Los complejos de éter bisfenolato del grupo 4 activados por MAO son catalizadores efectivos y estereospecíficos para la polimerización de alfa-olefinas.
  • Los catalizadores basados en hafnio ofrecen una mayor estereoselectividad sobre los catalizadores basados en zirconio.
  • La estructura del ligando juega un papel importante en la determinación de la estereospecificidad de estos catalizadores de polimerización.