In chemistry, initiation is a chemical reaction that triggers one or more secondary reactions. Initiation creates a reactive centre on a molecule which produces a chain reaction. The reactive centre generated by initiation is usually a radical, but can also be cations or anions. Once the reaction is initiated, the species goes through propagation where the reactive species reacts with stable molecules, producing stable species and reactive species. This process can produce very long chains of molecules called polymers, which are the building blocks for many materials. After propagation, the reaction is then terminated. There are different types of initiation, with the two main ways being thermal initiation and photo-initiation (light).
Thermal initiation
Thermal initiation involves initiating a reaction in the presence of heat, usually at very high temperatures. Heating a reaction can result in radical initiation of the substrate(s). In the presence of heat, a monomer can self-initiate and react with other monomers or pairs of monomers. This process is called spontaneous polymerization and requires a lot of heat to occur (up to 200°C). This type of initiation can happen at much lower temperatures, mainly room temperature, then thermal initiation.