Radical initiator

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In chemistry, radical initiators are substances that can produce radical species under mild conditions and promote radical reactions.[1] These substances generally possess weak bonds—bonds that have small bond dissociation energies. Radical initiators are utilized in industrial processes such as polymer synthesis. Typical examples are halogen molecules, azo compounds, and organic and inorganic peroxides.[2]

Major types of initiation reaction[edit]

  • Azo compounds (R-N=N-R') can be the precursor of two carbon-centered radicals (R• and R'•) and nitrogen gas upon heating and/or by irradiation. For example, AIBN and ABCN yield isobutyronitrile and cyclohexanecarbonitrile radicals, respectively.
AIBN initiator
  • Inorganic peroxides function analogously to organic peroxides. Many polymers are often produced from the alkenes upon initiation with peroxydisulfate salts. In solution, peroxydisulfate dissociates to give sulfate radicals:[3]
[O3SO-OSO3]2− \overrightarrow{\leftarrow} 2 [SO4]

The sulfate radical adds to an alkene forming radical sulfate esters, e.g. .CHPhCH2OSO3, that add further alkenes via formation of C-C bonds. Many styrene and fluoroalkene polymers are produced in this way.

  • In atom transfer radical polymerization (ATRP) carbon-halides reversibly generate organic radicals in the presence of transition metal catalyst.
General ATRP Reaction. A. Initiation. B. Equilibrium with dormant species. C.Propagation


Some radical initiators such as azo compounds and peroxides are often unstable. They are often stored cold.


  1. ^ March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.), New York: Wiley, ISBN 0-471-85472-7 
  2. ^ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN 0-471-72091-7 
  3. ^ Harald Jakob, Stefan Leininger, Thomas Lehmann, Sylvia Jacobi, Sven Gutewort (2005), "Peroxo Compounds, Inorganic", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a19_177.pub2