D08. Define initiator and free radicals and describe three ways that a molecule can be activated.

Free radicals are compounds with a free electron. Such compounds can be formed when a bigger compound is split into two smaller compounds. During such a reaction, a bond is split, and when that split occurs, the smaller segments retain one free electron each from the bond that was broken.

Free radicals are formed when certain molecules interact with electromagnetic fields. Such fields are formed by electromagnetic waves such as infra red (heat) light, visible light, ultra violet light or x-ray. Certain compounds, such as tertiary amines, can also interact with larger compounds and cause them to produce two free radicals

For example, benzoyl peroxide, C6H6-C(O)-O-O-C(O)-C6H6, when exposed to heat or tertiary amines (different activators), decomposes into two C6H6-C-O* units, where * represents an electron. These two units are two free radicals, I*, that will initiate the first step of the polymerization process. This mechanism is used for so called "cold-curing."



Activation can occur after exposure to heat, x-rays, light and certain chemicals. In all cases, an exchange of electromagnetic wavelengths occurs during the activation process. A chemical activator that has been used in self-curing dental resins is N,N-dimethyl-p-toluidine. However, because this amine tends to cause discolorations, other more color stable tertiary amines have been introduced. 




Initiator is the chemical compound that is capable of forming free radicals. Benzoyl peroxide is an example. Another example includes campheroquinone, which is used in light-activated composites.

Benzoyl peroxide


Free radical

A free radical is a compound that has a free electron that is available for bond formation.


Free radical


When the tertiary amine interact with the benzoyl peroxide, two free radicals are from from each benzoyl peroxide molecule.



Where are the activator and the initiator kept before the self-curing material is mixed?  

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© 1999, Karl-Johan M. Söderholm