Free Radical Photoinitiators

Free Radical Photoinitiator Mechanism

We’ve talked about the types of photoinitiators in previous post, and know that in general we can categorize photoinitiators into two types: Free Radical Photoinitiators and Cationic Photoinitiators. In this post, we’ll go to more details about free radical photoinitiators, because they are the most widely used ones.

Free radical photoinitiators produces free radicals when exposed to certain wavelength UV light, the reactive radical then setup the polymerization reaction. It can be divided into two classes: Norish type I and Norish type II photoinitiators.

Norish type I photoinitiator: Upon absorbing certain UV light, a specific bond of its molecular undergoes homolytic cleavage to produce free radicals. This types of photoinitiator are typically compounds containing benzoyl groups. The carbonyl group of the initiator absorbs a photon and becomes activated, which leads to homolytic cleavage of the excited α-carbon bond and produces two radical fragments. One of the typical type I photoinitiator is TPO. Below figure shows the process of homolytic cleavage of the excited α-carbon bond.

photoinitiator tpo molecular homolytic cleavage
The highlighted bond undergoes homolytic cleavage upon absorbing a photon to create free radicals

Except TPO, other type I photoinitiator including Irgacure 184, Irgacure 1173, Irgacure 369, Photoinitiator 819, Irgacure 651, Irgacure 907, etc.

Norish Type II photoinitiators require a co-initiator, which can donate a an electron or a hydrogen atom to the photoinitiator. This kind of donors (also called ‘synergist’) are usually an alcohol or amine functional groups that can readily have hydrogens abstracted. The mechanism is that, upon absorbing UV light energy, type-II photoinitiators become reactive then abstract a hydrogen from the co-initiator, meantime, splitting a bonding pair of electrons.

Benzophenone and benzophenone-type photoinitiators are the most common Type II photoinitiators. An example mechanism is shown below:

Norish type II photoinitiator mechanism

Once the free-radicals are generated, the polymerization mechanism is no different than type I photoinitiator polymerization process. Mechanistically, it is only the initiation process to generate free radicals that separates UV polymerization apart.

Except benzophenone, other typical type II photoinitiators include 2-Isopropylthioxanthone, 4-Methylbenzophenone, Ethyl 4-dimethylaminobenzoate, etc.

Free Radicals are the most widely used photoinitiators and have a strongly developed product line with many different applications. They are usually used within styrene-based or acrylate-based formulations for polymerization.