Type I free-radical photoinitiators

Table 3.5 Chemical structures of typical type I free-radical photoinitiators.

With regards to the hydrogen-abstraction method referred to in Table 3.13, it should be noted that triplet nitrene and benzoyl radicals can be formed by the photodecomposition of azides (Scheme 3.19), and of type I free radical photoinitiators (see Table 3.5), respectively. 

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. 

Free radical photoinitiator development has had considerable impact on the growth of UV. Several advancements have opened up new applications. One of the first classes developed was benzoin ethers which form radicals via a Norrish I type mechanism. These had stability problems and were very oxygen sensitive. Oxygen sensitivity was addressed through several developments. 

In fact, alkylated succinamides were isolated in some cases, though in very poor yields, and result from radical combination, which is a chain termination step. The experimental observations, i.e. the formation of (a) 1 1 adducts, (b) telomeric products, (c) alkylated succinamides, and (d) oxamide (when an olefin is absent), are consistent with a free radical mechanism. The telomeric products obtained support the assumption that we deal here with a chain reaction, because they are characteristic products of this type of reaction. Another proof for the chain reaction mechanism is the fact that when benzophenone is used as a photoinitiator (vide infra), the amount of benzpinacol formed is smaller than the amount of the 1 1 addition product of formamide and olefin (16). Quantum yield determinations will supply extra evidence for the validity of a chain reaction mechanism for this photoaddition reaction. 

The classical photochemical free-radical source is a compound that is photoexcited and then undergoes bond cleavage to yield active radicals. Benzoin and its ethers are efficient radical sources and are the most commonly employed photoinitiators in industrial photopolymerization processes (42,43). The reaction is a simple Norrish type I cleavage ... 

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. 

In spite of the large number of available photoinitiators [4], the search for new initiators is ongoing. For example, S-(4-benzoyl)phenylthiobenzoate, BpSBz, has been found to be a type I photoinitiator. Upon exposure to light it is cleaved into free radicals (quantum yield 0.45), which initiate the polymerization of methyl methacrylate. In contrast, BpOBz (see Chart 10.1) is not cleaved. It forms a long-lived triplet state rather than free radicals [43]. 

I photoinitiators such as azobisisobutyronitrile (AIBN) and benzoin methyl ether (BME), typically used to generate free radicals during monolith formation, can play the very same role as type... 

If the monomer itself cannot form free radicals by this process, then a photoinitiator must be added. Bisulfides form two free radicals RS on irradiation. The azo group of azobisisobutyronitrile absorbs light at 350 nm and then forms free radicals [reaction (20-3)]. Certain aliphatic ketones decompose according to a Norrish type I mechanism into two free radicals ... 

Two classes are known based on free radicals and hydrogen abstraction techniques, respectively. Free residual types are receptive to UV light by absorbing radiation energy such that free radicals result and these produce a chain polymerization reaction and eventually a solid polymer matrix. An example of the photoinitiation reaction sequence, which follows a Norrish I-type cleavage, is given in Fig. 12.10. Another photoinitiator would be benzoin butyl ether shown in Fig. 12.11. 

The methods used to prepare NC gels are simple and versatile, i.e., injection of reaction solutions into closed vessels followed by polymerization at ambient temperature. Hence, NC gels can be readily formed in various shapes and sizes, such as films, sheets, rods, spheres, hollow tubes, etc. (Fig. 4a) [21,29]. NC gels can also be prepared by photoinitiated free-radical polymerization using very low concentrations (e.g., 0.1 wt% relative to the monomer) of a hydrophobic photoinitiator in aqueous systems (Fig. 4b) [48], Furthermore, the other type of NC gel, i.e., tetra-PEG-based NC gels, with good biocompatibility can be prepared by incorporating clay nanoparticles into the tetra-PEG network [30],... 

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... 

Type I Photoinitiators undergo a unimoiecuiar bond cleavage upon irradiation to yield free radicals. 

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