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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

2.1K
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.
Selection Rules: Photochemical Activation
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Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

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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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Structure of Conjugated Dienes01:16

Structure of Conjugated Dienes

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Introduction
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

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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Monitoring the Effects of Illumination on the Structure of Conjugated Polymer Gels Using Neutron Scattering
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Charge Transport in Sequence-Defined Conjugated Oligomers.

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
PubMed
Summary
This summary is machine-generated.

The sequence of monomers in synthetic polymers significantly impacts charge transport. Specific sequences in conjugated oligomers enhance molecular conductance over 10-fold by creating unique charge pathways.

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Area of Science:

  • Polymer Chemistry
  • Materials Science
  • Molecular Electronics

Background:

  • Understanding the relationship between monomer sequence and material properties is crucial for synthetic polymers.
  • Charge transport in conjugated oligomers is a key factor in molecular electronics.

Purpose of the Study:

  • To investigate how the primary monomer sequence affects charge transport in single molecule junctions.
  • To synthesize and characterize sequence-defined conjugated oligomers.

Main Methods:

  • Iterative synthesis using the van Leusen reaction to create sequence-defined oligomers (2-7 units).
  • Characterization of charge transport properties using scanning tunneling microscope-break junction (STM-BJ) technique.
  • Measurement of molecular conductance for various oligomer lengths.

Main Results:

  • Specific monomer sequences were found to enhance molecular conductance by over 10-fold.
  • Sequence-defined pentamers with imidazole or pyrrole groups facilitated multiple conductance pathways.
  • Steric hindrance and heterocycle directionality play critical roles in charge transport.

Conclusions:

  • Monomer sequence is a critical design parameter for controlling charge transport in conjugated oligomers.
  • Tailoring molecular structure can lead to enhanced performance in molecular electronic devices.
  • This study provides insights for designing advanced molecular electronic components.