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Radical Chain-Growth Polymerization: Overview01:10

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Chain-growth or addition polymerization is successive addition reactions of monomers with a polymer chain. In radical chain-growth polymerization, the reaction proceeds via a free-radical intermediate. The free radical is formed from radical initiators, which spontaneously generate free radicals by homolytic fission. Organic peroxides (such as dibenzoyl peroxide, as shown in Figure 1) or azo compounds are popular radical initiators. A low concentration ratio of radical initiator to monomer is...
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The presence of electron-donating, electron-withdrawing, or conjugating groups adjacent to a radical center, imparts electronic stabilization to the radicals. Examples of such electronically-stabilized radicals are triphenylmethyl, tetramethylpiperidine‐N‐oxide, and 2,2‐diphenyl‐1‐picrylhydrazyl. These radicals are remarkably stable and are known as persistent radicals. Some of the persistent radicals can even be isolated and purified.
Along with electronic...
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Radical Halogenation: Stereochemistry01:33

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Stereochemistry is the study of the different spatial arrangements of atoms in a given molecule. The stereochemistry of radical halogenations can be understood from three different situations:
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Radicals: Electronic Structure and Geometry01:07

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This lesson delves into the geometry of a radical, which is influenced by the electronic structure of the molecule. The principle is similar to that of a lone pair, where the unpaired electron influences the geometry at the radical center.
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Radical Formation: Elimination00:51

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Another method of radical formation is the elimination process. It is the opposite of the addition route and is driven by the instability of the radical. For example, as depicted in Figure 1, dibenzoyl peroxide yields a pair of unstable radicals upon homolysis. Given its instability, this radical spontaneously undergoes elimination via a C–C bond cleavage to form a relatively more stable phenyl radical. The mechanism involves cleavage of the bond between the α and β positions with respect...
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Radical Formation: Homolysis00:54

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A bond is formed between two atoms by sharing two electrons. When this bond is broken by supplying sufficient energy, either two electrons can be taken up by one atom forming ions by the cleavage called heterolysis, or the two electrons are shared by two atoms, with one each creating radicals by the cleavage called homolysis.
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Isolating Free Carbenes, their Mixed Dimers and Organic Radicals
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Toward Benzobis(thiadiazole)-based Diradicaloids.

Yi Liu1, Hoa Phan1, Tun Seng Herng2

  • 1Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore, Singapore.

Chemistry, an Asian Journal
|June 1, 2017
PubMed
Summary

Acetylene-bridged benzo[1,2-c;4,5-c']bis[1,2,5]thiadiazole (BBT) oligomers exhibit increased diradical character with chain length. Synthesized BBT diradicaloids show paramagnetic activity and intense near-infrared absorption, especially with electron-donating groups.

Keywords:
benzobis(thiadiazole)diradicaloidsdonor-acceptor systemsmagnetic propertynear infrared dye

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Area of Science:

  • Organic Chemistry
  • Materials Science
  • Photophysics

Background:

  • Diradical character in organic molecules is crucial for unique electronic and magnetic properties.
  • Acetylene-bridged benzo[1,2-c;4,5-c']bis[1,2,5]thiadiazole (BBT) systems are promising candidates for novel materials.
  • Understanding the relationship between molecular structure and diradical character is key for designing functional materials.

Purpose of the Study:

  • To theoretically predict and experimentally validate the increasing diradical character in BBT oligomers with chain length.
  • To synthesize and characterize stable BBT-based diradicaloids.
  • To investigate the effect of electron-donating substituents on diradical properties and optical absorption.

Main Methods:

  • Theoretical prediction of diradical character.
  • Synthesis of six stable BBT-based diradicaloids.
  • Characterization using X-ray crystallography and various spectroscopic techniques (e.g., EPR, UV-Vis-NIR).

Main Results:

  • Experimental validation of increased diradical character with BBT oligomer chain extension.
  • Three synthesized compounds exhibited paramagnetic activity due to thermal population of triplet states.
  • Triphenylamine substitution enhanced diradical character, reduced the singlet-triplet energy gap, and induced intense near-infrared absorption.

Conclusions:

  • BBT oligomers are effective platforms for tuning diradical character through structural modification.
  • The synthesized diradicaloids possess interesting magnetic and optical properties relevant for materials applications.
  • Electron-donating groups play a significant role in modulating the electronic and photophysical behavior of these systems.