After learning about ‘what’s the photoinitiator?‘, we know that UV photoinitiator can produce reactive fragments after absorbing certain light energy. These active species may be free radicals or cations, which are the trigger to set off polymerization. And according to the different initiation mechanism, UV photoinitiators can be divided into free radical photoinitiators and cationic photoinitiators, among which radical polymerization photoinitiators are the most widely used.

Radical Photoinitiators

Free radical photoinitiators produce free radicals when exposed to UV light, then setup the polymerization. Depends on whether it needs a co-initiator (the synergist), it can be classified into Norish type I and Norish type II initiators.

Norish type I Photoinitiators 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. For instance, the cleavage of 2,2-dimethoxy-1,2-diphenyl-ethan-1-one largely yields a methoxybenzyl and benzoyl radical. The benzoyl radical initiate free radical polymerization whereas the methoxybenzyl radical decomposes to give the more stable methyl radical and methyl benzoate.

Other very common Norish type I photoinitiators include: Irgacure 1173, Photoinitiator TPO, Irgacure 184, Irgacure 369, Irgacure 651, Irgacure 907, etc.

Norish type II Photoinitiators absorb UV light to form excited molecules which then abstract an electron or hydrogen atom from a donor molecule (synergist). The donor molecule then reacts with a monomer to initiate polymerization.

Common type II photoinitiator systems include Benzophenone, 2-Isopropylthioxanthone, 4-Methylbenzophenone, Ethyl 4-dimethylaminobenzoate, 4-Chlorobenzophenone, etc.

Cationic Photoinitiators

The mechanism of a photoinduced cationic initiation is shown below for a triarylsulfonium salt. When exposed to UV radiation the initiator produces a strong Lewis / Broensted acid which then initiates a ring-opening polymerization of epoxy resins

Ar₃SX + hν (UV) → [Ar₃S⁺ X^–]*

[Ar₃S⁺ X^–]* + Y-H → Ar₂S + HX + Ar· + Y·

HX + M → HM⁺ X^–

HM⁺ X^– + n M → H(M)_nM⁺ X^–

where HX is a Lewis acid such as HBF₄, HPF₆, HAsF₆, HSbF₆ and Y is a hydrogen donor (solvent). 

Common cationic photoinitiators include triphenylsulfonium salts, diazonium salts, diaryliodonium salts and ferrocenium salts as well as various metallocene compounds, etc.

P.S. : The knowledge about Radical and cationic photoinitiator in this post is abstracted from plymerdatabase.com, please go there for more information.