Initiatorcoinitiator

Initiatorcoinitiator is a term used to describe a system composed of a chemical initiator and a co-initiator that work together to generate radicals and start polymerization or other radical reactions. In these systems, the initiator provides the energy or chemical potential to initiate the process, while the co-initiator participates in radical generation, often by transferring electrons or hydrogen atoms.

Two common modes of action dominate initiator–co-initiator systems. In photoinitiation, especially Type II photoinitiation, a photoinitiator absorbs light to reach an excited state and then interacts with a co-initiator (often a tertiary amine) to form radicals that start polymerization. In redox initiation, an oxidizing initiator is paired with a reducing co-initiator; their redox reaction creates radicals that initiate the process. The choice of pair influences initiation rate, wavelength sensitivity, and the overall efficiency of curing.

Typical examples illustrate the range of applications. A classic dental and coating system uses camphorquinone as the photoinitiator with an amine co-initiator such as an ethyl-dimethylaminobenzoate compound. In aqueous or gel systems, redox pairs such as ammonium persulfate with TEMED or similar reducing agents generate radicals upon mixing.

Applications of initiator–co-initiator systems span UV-curable coatings, dental resins, 3D printing resins, adhesives, and various polymerization processes where rapid initiation and controlled curing are desirable. Advantages include efficient radical generation at moderate light intensities or ambient temperatures and tunable curing behavior. Limitations can involve odor, color formation, oxygen inhibition, toxicity of components, and sensitivity to formulation conditions.

See also: radical polymerization, photoinitiator, redox initiation.