Amino ketone photoinitiators

Many pigmented eoating formulations make use of a-amino ketone photoinitiators.The effeetiveness of these materials is further enhaneed by addition of photosensitizers, sueh as methyl or isopropyl thioxanthene-9-one. Some typieal a-amino-ketones are shown in Table 2.4. 

In the electron transfer mechanism, which is more common, the photosensitizer becomes excited upon illumination and forms an excimer with the photoinitiator. Electron transfer from the photoinitiator to the photosensitizer then occurs, with a subsequent proton transfer. This results in the formation of two radicals. The photosensitizer radical is capable of initiating polymerization, while the photoinitiator radical may not unless it undergoes further reaction (3,13,17). Figure 3 demonstrates this for a representative a-amino ketone photoinitiator and a thioxanthone being used as a photosensitizer. 

Morpholino- and Amino-Ketones These classes of photoinitiators present a good activity [Is], The cleavage mechanisms ((10.16) and (10.17)) and the reactivity of the excited states have been fully reported [2i,30], Excellent representatives of morpholino ketones are 2-methyl- l-(4-methylthiobenzoyl)-2-morpholino-propan-1-one TMPK (10.16) and 2-methyl-l-(benzoyl)-2-morpholino-propan-l-one MPK. In amino ketones, the introduction of a morpholino moiety on a benzoyl group such as in 2-benzyl-2(diethylamino)-4,-morpholinobutyrophenone MPPK (10.17) red shifts the absorption. In both cases, efficient initiating radicals are generated. 

Tertiary amines with an a-hydrogen are among the most effective electron donors other electron donors include alcohols, amides, amino acids, and ethers. A third process, direct hydrogen atom transfer from RH to the ketone, is not common hut does occur with some photoinitiators. The overall result is the same as the electron-transfer process. Although two radicals are produced by photolysis of the photoinitiator, only one of the radicals is typically active in initiation—the aroyl and amine radicals in Eqs. 3-48 and 3-49, respectively. The other radical may or may not initiate polymerization, hut is active in termination. The decrease in photoinitiator concentration during polymerization is referred to as photo-bleaching. 

The primary reaction of Type 2 photoinitiators is a hydrogen abstraction from the tertiary amine by a triplet excited ketone. The amino radical thus formed is sufficiently active to initiate the polymerization of vinyl monomers Scheme 2. 

Photoinitiators are generally aryl alkyl ketones or diaryl ketones (Table 2.19). For aryl alkyl ketones two free radicals are produced by homolytic scission of a C-C bond (Eq. (2.96)). Diaryl ketones are usually mixed with a tertiary amine the mechanism of production of free radicals involves H abstraction from the tertiary amine by the excited state PI, via a charge-transfer stabilized exciplex (Eq. (2.97)). The a-amino alkyl radical formed is very reactive and is in fact the true initiator because the cetyl radical disappears rapidly through a coupling reaction (formation of pinacol). 

Amine containing ketone based photoinitiators bearing an amine moiety in the benzophenone or thioxanthone series [162] as well as a new thioxanthone derivative (where the coinitiator such as a thiol [163], an acetic acid derivative [164,165] is grafted) have been developed (10.51) Covalently bonded camphorquinone/ amine [166] or amino alkyl ketone/thiol [167] were also recently reported. 

Benzyl dialkyl amino morpholinyl ketone Figure 7 Typical photoinitiators. 

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