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Videos de Conceptos Relacionados

Free-Radical Chain Reaction and Polymerization of Alkenes02:35

Free-Radical Chain Reaction and Polymerization of Alkenes

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The conversion of alkenes to macromolecules called polymers is a reaction of high commercial importance. The structure of the polymer is defined by a repeating unit, while the terminal groups are considered insignificant. The average degree of polymerization represents the number of repeating units in the polymer molecule and is denoted by the subscript n.
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Olefin Metathesis Polymerization: Acyclic Diene Metathesis (ADMET)00:53

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Acyclic diene metathesis polymerization or ADMET polymerization involves cross-metathesis of terminal dienes, such as 1,8-nonadiene, to give linear unsaturated polymer and ethylene. As ADMET is a reversible process, the formed ethylene gas must be removed from the reaction mixture to complete the polymerization process.
Similar to cross-metathesis, ADMET also involves the formation of metallacyclobutane intermediate by [2+2] cycloaddition of one of the double bonds of a terminal diene with...
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Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

Cycloaddition Reactions: MO Requirements for Photochemical Activation

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Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.
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Olefin Metathesis Polymerization: Overview01:13

Olefin Metathesis Polymerization: Overview

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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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Radical Chain-Growth Polymerization: Chain Branching01:17

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The skeletal structure of polymers synthesized via radical polymerization is always branched. For example, the polymerization of ethylene by radical polymerization results in a low-density grade of polyethylene with a heavily branched skeletal structure. Here, the radical site abstracts hydrogen from the growing chain, and the radical site shifts from the end (a primary carbon center) to anywhere within the growing chain (a secondary carbon center). Consequently, the part of the chain from the...
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Anionic Chain-Growth Polymerization: Mechanism01:04

Anionic Chain-Growth Polymerization: Mechanism

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The mechanism for anionic chain-growth polymerization involves initiation, propagation, and termination steps. In the initiation step, a nucleophilic anion, such as butyl lithium, initiates the polymerization process by attacking the π bond of the vinylic monomer. As a result, a carbanion, stabilized by the electron‐withdrawing group, is generated. The resulting carbanion acts as a Michael donor in the propagation step and attacks the second vinylic monomer, which acts as a Michael...
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Ethylene Polymerizations Using Parallel Pressure Reactors and a Kinetic Analysis of Chain Transfer Polymerization
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Visualización de la polimerización de etileno en la superficie mediante la inserción de etileno

Weijun Guo1,2, Junqing Yin3, Zhen Xu2

  • 1SynCat@Beijing, Synfuels China Technology Co., Ltd., Beijing 101407, China.

Science (New York, N.Y.)
|March 10, 2022
PubMed
Resumen

Este estudio visualiza la polimerización del etileno mediante microscopía de túnel de barrido. Revela un mecanismo de autoiniciación en las superficies de carburo de hierro, confirmando la vía de inserción de etileno a nivel molecular.

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Área de la Ciencia:

  • Catálisis
  • Química de los polímeros
  • Ciencias de la superficie

Sus antecedentes:

  • La polimerización catalítica del etileno es una piedra angular de la industria química, produciendo polietileno.
  • El mecanismo ampliamente aceptado de Cossee-Arlman describe el crecimiento de la cadena a través de la inserción de etileno en enlaces metal-carbono.
  • La confirmación experimental a nivel molecular de este mecanismo ha estado ausente.

Objetivo del estudio:

  • Proporcionar evidencia experimental directa, microscópica y espacial para el mecanismo de polimerización del etileno.
  • Para visualizar el proceso de polimerización del etileno a nivel molecular.
  • Para aclarar los pasos de iniciación y propagación en la polimerización de etileno en una superficie catalítica específica.

Principales métodos:

  • Visualización in situ de la polimerización del etileno mediante microscopía de túnel de barrido (STM).
  • Utilizando una superficie de un solo cristal de hierro carburado como catalizador.
  • Observación de los intermedios de polimerización y de la dinámica de crecimiento de la cadena.

Principales resultados:

  • Se observó que la polimerización del etileno ocurría en sitios triangulares específicos de hierro en el límite de los dominios de carburo.
  • Se identificó una nueva vía de autoiniciación, que involucra un intermediario de etilideno anclado en la superficie.
  • Se visualizó la posterior inserción de etileno en este intermedio, confirmando el crecimiento de la cadena.

Conclusiones:

  • La evidencia experimental directa confirma la vía de polimerización del etileno a nivel molecular.
  • Los hallazgos validan y refinan la comprensión del mecanismo Cossee-Arlman.
  • El estudio pone de relieve el papel de los sitios de superficie específicos y los intermedios en la polimerización catalítica.