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01:13Olefin 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.
Ruthenium-based Grubbs catalyst is the most commonly used catalyst for olefin metathesis polymerization. Grubbs catalyst consists...
Ruthenium-based Grubbs catalyst is the most commonly used catalyst for olefin metathesis polymerization. Grubbs catalyst consists...
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01:16Olefin Metathesis Polymerization: Ring-Opening Metathesis Polymerization (ROMP)
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Ring-opening metathesis polymerization or ROMP involves strained cycloalkenes as starting materials. The mechanism of ROMP proceeds by reacting cycloalkene with Grubbs catalyst to give metallacyclobutane intermediate which undergoes a ring-opening reaction to form new carbene. The new carbene reacts with another molecule of cycloalkene. Repetition of these steps leads to the formation of an unsaturated open-chain polymer product. All these steps are reversible, however, relieving the ring...
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01:16[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction
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The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred to as a dienophile. The reaction involves a concerted movement of six π electrons, four from the diene and two from the dienophile, forming an unsaturated six-membered ring. As a result, these reactions are classified as [4+2] cycloadditions.
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00:53Olefin Metathesis Polymerization: Acyclic Diene Metathesis (ADMET)
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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...
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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02:35Regioselectivity of Electrophilic Additions-Peroxide Effect
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In the presence of organic peroxides, the addition of hydrogen bromide to an alkene yields the isomer that is not predicted by Markovnikov’s rule. For example, the addition of hydrogen bromide to 2-methylpropene in the presence of peroxides gives 1-bromo-2-methylpropane. This addition reaction proceeds via a free radical mechanism, which reverses the regioselectivity. The free radical reaction mechanism involves three stages: initiation, propagation, and termination.
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01:17Thermal Electrocyclic Reactions: Stereochemistry
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The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
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Descifrando la Paradoja de la Isomerización de Olefinas y la Polimerización del Paladio (II) Catalizadores de
Dung Nguyen1, Shengguang Wang2,3, Lars C Grabow2,3
1Department of Chemistry, Center of Excellence in Polymer Chemistry (CEPC), University of Houston, 3589 Cullen Boulevard, Houston, Texas 77204, United States.
Journal of the American Chemical Society
|April 18, 2023
Resumen
Este estudio revela un nuevo mecanismo para los catalizadores de paladio (Pd) (II) -diimina, que explica la isomerización simultánea de la olefina y la polimerización en vivo. Este descubrimiento permite la síntesis de copolímeros avanzados de bloque de poliolefina.
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Área de la Ciencia:
- Química de los polímeros
- Química organometálica
- Catálisis
Sus antecedentes:
- Los catalizadores de paladio (Pd) (II) dimina presentan paradójicamente tanto la isomerización de la olefina como la polimerización en vivo.
- Este comportamiento dual a menudo se ignora como error experimental en los modelos mecanicistas clásicos.
Objetivo del estudio:
- Para aclarar la paradoja inexplicable de la isomerización simultánea y la polimerización viva de las alfa-olefinas utilizando catalizadores Pd (II) -diimina.
- Proponer y validar una nueva vía mecanicista para estos sistemas catalíticos.
Principales métodos:
- Investigación mecanicista integral mediante estudios experimentales y computacionales.
- Análisis de los complejos neutros y catiónicos de Pd (II) -diimina.
- Exploración de los efectos del ácido de Lewis en la actividad catalítica.
Principales resultados:
- Los complejos neutros de Pd (II) -diimina muestran actividad catalítica para la isomerización de la olefina a través de la disociación parcial del ligando.
- Los catalizadores catiónicos de Pd (II) -diimina facilitan las vías de isomerización y polimerización independientes.
- Los ácidos de Lewis aceleran la isomerización al debilitar los enlaces de paladio-nitrógeno.
Conclusiones:
- Una nueva vía mecanicista que implica la disociación parcial del ligando explica el comportamiento catalítico dual observado.
- Esta comprensión permite la síntesis controlada de arquitecturas complejas de poliolefinas, incluidos los copolímeros de bloque.