Radical Polymerization Initiators

Azo Radical Initiators

An azo polymerization initiator is a compound having an azo group (R-N=N-R') which is decomposed by heat and/or light, and forms carbon radical. The formed carbon radical is excellent in reactivity, and progresses polymerization and halogenation reaction of different types of vinyl monomers.

Introduction

We develop a large variety of products taking advantage of our original organic synthesis technology, production technology, and refining technology accumulated in our long experience in the manufacturing of reagents. Azo polymerization initiators are used as reaction initiators in the synthesis of polymers. They are used mainly as radical polymerization initiators in a wide range of industries, such as acryl resins for paints, water absorbent resins, polymer coagulants, adhesives, and paper finishing agents. We have approximately 20 types of azo polymerization initiators of organic-solvent soluble type and water-soluble type.

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What is Radical Polymerization?

Radical polymerization is initiated by the formation of free radicals. Free radicals are formed by thermic energy, light, or radioactivity. Radical polymerization is mainly used for the polymerization of vinyl monomers. In addition to radical polymerization, cationic polymerization and anionic polymerization are common polymerization methods. In the case of radical polymerization, initiators are generally used. The typical initiators are azo polymerization initiators and peroxides.

Characteristics of Radical Polymerization Initiators

  • Radical polymerization initiator shows an effect even in a small amount.
  • It is not polar-sensitive, and a large number of solvent is available, a wide range of monomers can be polymerized.
  • Polymerization at a low to high temperature ranges is possible.
  • Reaction with simple facility and equipment is possible.

 

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Characteristics

Characteristics of Azo Polymerization Initiators
  • Azo polymerization initiators can be used safely as they do not decompose by induction and there is no risk of explosion.
  • They decompose at a constant rate regardless of the solvent used, so they can be used with different solvents..
  • Unlike the case of peroxides, the resulting carbon radical does not cause a hydrogen abstraction reaction, but forms linear polymers..

Radical Formation Mechanism

Radical Formation Mechanism of Azo Polymerization Initiators (Thermal Decomposition)
  1. Formation of radical: Azo polymerization initiators decompose with heat or light, and form nitrogen gas and carbon radicals.
    *The decomposition rate (in solution) follows first-order reaction rate kinetics due to structure differences.
  2. Initiation reaction/Propagation reaction: Azo polymerization initiator addition-polymerizes with vinyl monomers and forms a polymer.
    *Since a section of the azo polymerization initiator is introduced at the end of polymer, the effect of end group is expected.
    The efficiency of common azo polymerization initiators is approximately 0.5-0.7, and the remaining results in (3) recombination or (4) disproportionation
  3. Recombination: The carbon radicals which did not engage in polymerization recombine.
  4. Disproportionation: The carbon radicals which did not engage in polymerization abstracts hydrogen of other carbon radicals.
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Characteristics2

Characteristics of Azo Polymerization Initiators and Comparison with Peroxides

Peroxides are used as an initiator for radical polymerization, in addition to azo polymerization initiators.

 

Characteristics of Azo Polymerization Initiators

  • Azo polymerization initiators can be used safely as they do not decompose by induction and there is no risk of explosion.
  • They decompose at a constant rate regardless of the solvent used, so they can be used with different solvents.
  • Unlike the case of peroxides, the resulting carbon radical does not cause a hydrogen abstraction reaction, but forms linear polymers.
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Reaction examples

Examples of radical reactions using Azo Polymerization Initiators

Azo polymerization initiators are used as catalyst and foaming agent in organic synthesis, in addition to in polymer synthesis.

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Polymer Synthesis

Radical polymerization of styrene

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Radical polymerization of acrylic ester

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Copolymerization of styrene and acrylic ester

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Catalyst for Organic Synthesis

Addition reaction to olefins
Azo polymerization initiator can be used for additional reactions of HBr, H2S, etc. to olefins. Especially, when using styrenes, bromine or mercapto groups may be selectively introduced to the β position.

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Mitsunobu reaction*3

Azo compounds*4 can be used as reagents for Mitsunobu reaction.

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Foaming Agent

Azo polymerization initiators are used as foaming agents for vinyl chloride or other plastics, taking advantage of the property of generating nitrogen gas.

 

*3 Mitsunobu reaction: Sn2 (reaction) that activates hydroxyl group of alcohol with azo carboxylic acid ester and triphenylphosphine.

*4 Azo compounds: Bis(2-methoxyethyl)azodicarboxylate, etc.

Industrial-scale azo initiator

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Author: N. Shimamura

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