Type II free radical photoinitiators

Table 10.3 Chemical structures of amines functioning as co-initiators for type II free radical photoinitiators.

Free-radical photoinitiators are classified by their chemical nature as type I and type II however, there are a few systems with different chemistry, e.g., borate salt initiators that depend on inter-/intramolecular electron transfer,i that do not fit into either category. 

All UV curable systems have four basic components which must be included in order to develop a successful coating. They are the photoinitiator(s), oligomer(s), monomer(s), and additive(s). Table II lists the properties of each component which make it essential to the UV curable formulation. In the next few pages, we summarize each of the components and their properties. The photoinitiator section covers both radical and cationic type photoinitiators while the oligomer and monomer sections are restricted to components used in free radical systems. 

Two types of compounds are employed as photoinitiators of free radical polymerizations, which differ in their mode of action of generating reactive free radicals. Type I initiators undergo a very rapid bond cleavage after absorption of a photon. On the other hand, type II initiators form relatively long-Hved excited triplet states capable of undergoing hydrogen-abstraction or electron-transfer reactions with co-initiator molecules that are deliberately added to the monomer-containing system. 

Yagci, Tnrro and coworkers also reported a mechanistic study of photoinitiation of free radical polymerization with thioxanthone thioacetic acid as a one-component, Type II, photoinitiator. 

On the basis of the mechanism by which initiating radicals are formed, photoinitiators are generally divided into two classes Type I photoinitiators nndergo a unimolecular bond cleavage upon irradiation to yield free radicals. Type II... 

TAS 07a] Tasdelen M.A., Demirel A.L., Yagci Y., Poly(propylene imine) dendrimers as hydrogen donor in Type II photoinitiated free radical polymerization , European Polymer Journal, vol. 43, pp. 4423-4430, 2007. 

Type II Photoinitiators undergo a bimoiecuiar reaction where the excited state of the photoinitiator interacts with a second molecule (a coinitiator) to generate free radicals. 

There are two groups of photoinitiators that differ in the mechanism of radical formation. Type I photoinitiators generate radicals via unimolecular bond cleavage after irradiation, similar to thermal initiators. Type II initiators show no bond cleavage directly after irradiation. Instead, they enter a bound excited state which reacts with a so-called co-initiator molecule to generate free radicals, mostly via H-abstraction. Most visible light active photoinitiators belong to type II. 

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