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The radical chain-growth polymerization mechanism consists of three steps: initiation, propagation, and termination of polymerization. The polymerization initiates when a free radical generated from the radical initiator adds to the unsaturated bond in the monomer. The unpaired electron of the free radical and one π electron in the unsaturated bond creates a σ bond between the free radical and the monomer. As a result, the other π electron in the unsaturated bond converts this species into...
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Chemical reactions often occur in a stepwise fashion, involving two or more distinct reactions taking place in a sequence. A balanced equation indicates the reacting species and the product species, but it reveals no details about how the reaction occurs at the molecular level. The reaction mechanism (or reaction path) provides details regarding the precise, step-by-step process by which a reaction occurs.
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Shortly after de Broglie published his ideas that the electron in a hydrogen atom could be better thought of as being a circular standing wave instead of a particle moving in quantized circular orbits, Erwin Schrödinger extended de Broglie’s work by deriving what is now known as the Schrödinger equation. When Schrödinger applied his equation to hydrogen-like atoms, he was able to reproduce Bohr’s expression for the energy and, thus, the Rydberg formula governing hydrogen spectra.
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Observación ultrarrápida de un mecanismo de reacción fotorredóxica: fotoiniciación en la polimerización orgánica por

Daisuke Koyama1, Harvey J A Dale1, Andrew J Orr-Ewing1

  • 1School of Chemistry, University of Bristol , Cantock's Close, Bristol BS8 1TS, U.K.

Journal of the American Chemical Society
|January 5, 2018
PubMed
Resumen

La espectroscopia ultrarrápida reveló el mecanismo de polimerización orgánica por transferencia de átomos (O-ATRP). La transferencia de electrones fotoinducida por catalizadores excitados inicia la polimerización, controlando las propiedades del polímero.

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Área de la Ciencia:

  • Química orgánica
  • Química Física
  • Ciencias de los Polímeros

Sus antecedentes:

  • La catálisis fotorredóxica es una herramienta poderosa en la síntesis orgánica, pero la comprensión mecanicista detallada de las reacciones sintéticamente útiles es limitada.
  • La polimerización orgánica por transferencia de átomos (O-ATRP) es una aplicación clave, sin embargo, su mecanismo de iniciación requiere una mayor aclaración.
  • Los intermediarios transitorios en las reacciones fotorredóxicas son a menudo de corta duración, lo que dificulta su observación directa.

Objetivo del estudio:

  • Observar directamente la dinámica ultrarrápida de las reacciones fotorredóxicas organocatalizadas bimoleculares.
  • Elucidar el mecanismo de iniciación en la polimerización por transferencia atómica organocatalizada (O-ATRP) utilizando espectroscopia de resolución temporal.
  • Investigar el papel de la transferencia de electrones fotoinducida (PET) en el control de los resultados de la polimerización.

Principales métodos:

  • Se utiliza la espectroscopia de absorción vibratoria y electrónica de resolución ultra rápida con una resolución inferior a un picosegundo.
  • Se han estudiado dos organocatalizadores complementarios de N,N-diarilo-5,10-dihidrofenasina fotorredóxicas.
  • Monitoreo de la fotoexcitación del catalizador, el PET y la formación intermedia en tiempo real.

Principales resultados:

  • Se observó una transferencia directa de electrones bimoleculares desde el estado excitado de los fotocatalizadores al metil 2-bromopropionato.
  • Rastreó los estados excitados del fotocatalizador, los estados básicos, los cationes radicales y el fragmento radical desbromado en tiempo real.
  • Identificó el PET de estados singlet excitados de corta duración como un paso clave de iniciación, desafiando los mecanismos anteriores.

Conclusiones:

  • El estudio proporciona evidencia espectroscópica directa para el mecanismo de iniciación en O-ATRP.
  • El PET de estados singlet excitados de corta duración influye en el peso molecular del polímero y la dispersión mediante el control de la concentración de radicales.
  • Demuestra la utilidad de la espectroscopia de absorción ultrarrápida para la observación de intermedios transitorios en la catálisis fotorredóxica.