Azoinitiators

Azoinitiators, or azo initiators, are organic compounds that contain an azo group (–N=N–) and are used to generate free radicals that start radical polymerization. They are employed to initiate the polymerization of vinyl monomers such as acrylates, methacrylates, styrene, and related compounds. Initiation occurs when the azo bond cleaves under appropriate conditions, producing radical species that add to monomer double bonds and propagate chain growth.

Mechanistically, thermal initiation typically involves homolytic cleavage of the azo bond, releasing nitrogen gas and yielding two radical fragments. The rate of decomposition and radical production depend on the substituents on the azo molecule and the temperature. Some azo initiators can also be activated by light, though many are primarily used for thermal initiation. The usefulness of azo initiators lies in their ability to provide a steady supply of radicals and to influence polymerization kinetics and molecular weight distribution.

The most widely used example is 2,2'-azobis(isobutyronitrile) (AIBN), a solid that decomposes at moderate temperatures to form initiating radicals. Azo initiators are available in various solubilities, including oil-soluble and water-soluble forms, to suit different polymerization media such as bulk, solution, emulsion, or aqueous systems. They are often selected for their thermal stability in storage and their predictable decomposition behavior under reaction conditions.

Safety and handling considerations include the potential for hazardous decomposition under high heat or confinement. They should be stored away from heat, oxidizers, and reducing agents, and handled with appropriate precautions to mitigate risks of rapid exothermic release or pressure buildup.