Examples of Type I Photoinitiators

Some examples of type I and type II photoinitiators are shown in Tables 4.1 and 4.2, respectively. 

Kinetics of the Cleavage Process in Type I Photoinitiators Working now on the picosecond timescale (with a pump-probe laser setup) shows the shortlived transient absorptions observed upon light excitation [250], For example [251], the cleavage process of DMPA (2,2-dimethoxy -2 phenyl-acetophenone) occurs... 

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]. 

There are large differences in the reactivity of the free radicals generated from type I photoinitiators towards unsaturated monomers. For example, the rate constants for the addition of diphenylphosphinoyl radicals to vinyl monomers are of the order of 10 to 10 mol 1 s -that is, several orders of magnitude larger than those for benzoyl or other carbon-centered radicals [18b]. 

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 photoinitiator makes the resin formulation UV-curable. Only the development of thermally stable but photolabile compounds enabled the development of UV curing. For an efficient UV curing, the absorption of the photoinitiator should match, at least partly, the emission spectrum of the light source used. Two main types of photoinitiators are used photoinitiators based on an a-deavage process (Norrish type I), and photoinitiators based on an electron transfer followed by a hydrogen abstraction process (Norrish type II). a-Cleavage photoinitiators are mono-molecular initiators, of which a-hydroxy or a-alkoxy ketones and benzoyl phosphine oxides are well known examples shown in Scheme 16.33. 

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