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Cationic Chain-Growth Polymerization: Mechanism00:57

Cationic Chain-Growth Polymerization: Mechanism

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The cationic polymerization mechanism consists of three steps: initiation, propagation, and termination. In the initiation step of the polymerization process, the π bond of a monomer gets protonated by the Lewis acid catalyst, which is formed from boron trifluoride and water. The protonation of the π bond generates a carbocation stabilized by the electron‐donating group. In the propagation step, the π bond of the second monomer acts as a nucleophile and attacks the...
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Anionic Chain-Growth Polymerization: Mechanism01:04

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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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Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

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The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
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Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
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Conjugated dienes are compounds characterized by the presence of alternating double and single bonds. In a conjugated system like 1,3-butadiene, the unhybridized 2p orbital on each carbon overlaps continuously, allowing the π electrons to be delocalized across the entire molecule. In contrast, this type of overlap does not occur in cumulated and isolated dienes, such as 2,3-pentadiene and 1,4-pentadiene, respectively. Instead, the π electrons remain localized between the double...
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Anionic Chain-Growth Polymerization: Overview01:20

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The polymerization process that involves carbanion as an intermediate is called anionic polymerization. It is also a type of addition or chain-growth polymerization. Anionic polymerization gets initiated by a strong nucleophile such as an organolithium or a Grignard reagent. The most commonly used initiator for anionic polymerization is butyl lithium. Monomers involved in anionic polymerization must possess a vinyl group bonded to one or two electron-withdrawing groups. For instance,...
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Transporte de carga en oligómeros conjugados definidos por secuencia

Hao Yu1, Songsong Li2,3, Kenneth E Schwieter4

  • 1Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.

Journal of the American Chemical Society
|February 19, 2020
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Resumen

La secuencia de monómeros en polímeros sintéticos tiene un impacto significativo en el transporte de cargas. Secuencias específicas en oligómeros conjugados mejoran la conductividad molecular más de 10 veces al crear vías de carga únicas.

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Área de la Ciencia:

  • Química de los polímeros
  • Ciencias de los materiales
  • La electrónica molecular

Sus antecedentes:

  • Comprender la relación entre la secuencia de monómeros y las propiedades del material es crucial para los polímeros sintéticos.
  • El transporte de carga en oligómeros conjugados es un factor clave en la electrónica molecular.

Objetivo del estudio:

  • Investigar cómo la secuencia monomérica primaria afecta el transporte de carga en las uniones de una sola molécula.
  • Sintetizar y caracterizar los oligómeros conjugados definidos por secuencia.

Principales métodos:

  • Síntesis iterativa utilizando la reacción de van Leusen para crear oligómeros definidos por secuencia (2-7 unidades).
  • Caracterización de las propiedades de transporte de carga mediante la técnica de unión de ruptura de microscopio de túnel de barrido (STM-BJ).
  • Medición de la conductividad molecular para varias longitudes de oligómero.

Principales resultados:

  • Se encontró que las secuencias específicas de monómeros mejoran la conductividad molecular en más de 10 veces.
  • Los pentámeros definidos por secuencia con grupos de imidazol o pirrol facilitaron las vías de conducción múltiple.
  • El obstáculo estérico y la direccionalidad del heterociclo juegan un papel crítico en el transporte de carga.

Conclusiones:

  • La secuencia de monómeros es un parámetro de diseño crítico para controlar el transporte de carga en oligómeros conjugados.
  • La adaptación de la estructura molecular puede conducir a un mayor rendimiento en los dispositivos electrónicos moleculares.
  • Este estudio proporciona información para el diseño de componentes electrónicos moleculares avanzados.