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Videos de Conceptos Relacionados

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

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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Polymer Classification: Stereospecificity01:26

Polymer Classification: Stereospecificity

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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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Anionic Chain-Growth Polymerization: Overview01:20

Anionic Chain-Growth Polymerization: Overview

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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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Ziegler–Natta Chain-Growth Polymerization: Overview01:17

Ziegler–Natta Chain-Growth Polymerization: Overview

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Ziegler–Natta polymerization is another form of addition or chain‐growth polymerization used for synthesizing linear polymers over branched polymers. The catalyst used for polymerization is the Ziegler–Natta catalyst, named after Karl Ziegler and Giulio Natta, who developed it in 1953. This catalyst is an organometallic complex of titanium tetrachloride and triethyl aluminum, with the active form of the catalyst being an alkyl titanium compound. Using the Ziegler–Natta...
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Polymers02:34

Polymers

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The word polymer is derived from the Greek words “poly” which means “many” and “mer” which means “parts”. Polymers are long chains of molecules composed of repeating units of smaller molecules, known as monomers. They either occur naturally, such as DNA and proteins, or can be constructed synthetically, like plastics. They have varied structural characteristics, such as linear chains, branched chains, or complex networks, that contribute to the...
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Synthesis of Cyclic Polymers and Characterization of Their Diffusive Motion in the Melt State at the Single Molecule Level
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Personalización de polímeros mediante el control de la dinámica de conmutación catiónica en la polimerización no viva

Thi V Tran1, Eryn Lee1, Yennie H Nguyen1

  • 1Department of Chemistry, University of Houston, 4800 Calhoun Road, Houston, Texas 77004, United States.

Journal of the American Chemical Society
|September 7, 2022
PubMed
Resumen

Los investigadores desarrollaron una nueva estrategia para controlar la polimerización no viva utilizando catalizadores metálicos y cationes metálicos secundarios. Este método permite una regulación precisa del crecimiento de la cadena de polímeros, superando las limitaciones de la terminación rápida de la cadena en la polimerización del etileno.

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Área de la Ciencia:

  • Química de los polímeros
  • Química organometálica

Sus antecedentes:

  • El control de la polimerización no viva es un desafío debido a la terminación rápida de la cadena.
  • Los métodos existentes luchan para aplicar disparadores externos antes de que ocurra la terminación.

Objetivo del estudio:

  • Desarrollar una nueva estrategia para regular las polimerizaciones no vivas.
  • Aprovechar los equilibrios químicos entre los catalizadores metálicos y los cationes metálicos secundarios para el control de la polimerización.

Principales métodos:

  • Síntesis de dos variantes de níquel fenoxifosfina y polietileno glicol (Ni1 y Ni2) con diferentes sustituyentes de fosfina.
  • Estudios de polimerización del etileno utilizando estos complejos en presencia de varias sales alcalinas (Li+, Na+, Cs+).
  • Investigación de los efectos de la polaridad del disolvente (mezclas de tolueno y éter dietílico) en los modos de polimerización (no conmutación frente a conmutación dinámica).
  • Análisis de los productos de reacción mediante espectroscopia de RMN para elucidar los mecanismos.

Principales resultados:

  • El crecimiento de la cadena es sensible a los efectos electrónicos, mientras que la terminación depende tanto de factores estéricos como electrónicos.
  • El ajuste de la polaridad del disolvente permite controlar los modos de polimerización sin conmutación o con conmutación dinámica.
  • El polietileno bimodal se produjo con Ni1/Li+/Na+, con fracciones dependientes de la relación de cationes.
  • El polietileno monomodal con peso molecular controlado y baja dispersión (< 2,0) se logró con Ni2/Cs+.

Conclusiones:

  • La estrategia desarrollada regula efectivamente la polimerización no viva mediante el uso de equilibrios entre catalizadores metálicos y cationes.
  • El mecanismo de conmutación dinámica, facilitado por el intercambio de cationes rápido, está respaldado por evidencia experimental.
  • Este enfoque ofrece un control preciso sobre las propiedades del polímero, como el peso molecular y la dispersión.