Photoinitiators compound chemistries

Photochemically-generated radicals are encountered as reactive intermediates in many important systems, being a major driving force in the photochemistry of ozone in the upper atmosphere (stratosphere) and the polluted lower atmosphere (troposphere). The photochemistry of organic carbonyl compounds is dominated by radical chemistry (Chapter 9). Photoinitiators are used to form radicals used as intermediates in the chain growth and cross-linking of polymers involved in the production of electronic circuitry and in dental treatment. 

I found that the philosophy behind making the TCM-HABI—Could we use less costly ingredients, shift spectra, increase stability, etc.—required an approach toward chemistry that depended on synthesis to solve problems. Most industrial photochemists are better at formulating around deficiencies, and so it was at DuPont Once o-Cl-HABI was available, the formulators worked around it. Only when a special problem arose, such as the need for a less colored photoinitiator, was an effort made to develop new compounds After all, one cannot readily add a compound that would make a yellowish compound white Hence, the work that resulted in the 2-(alkyloxy)-4,5-diphenylimidazole dimers. 

Vinyl ether and epoxy chemistry established more than 50 years ago finally found its way into radiation curing technology. These compounds have been demonstrated to be reactive monomers in curing processes photoinitiated by iodonium and sulfonium salts. In particular, vinyl ethers are known to be among the most reactive monomers in photocuring chemistry. 

By virtue of their absorption characteristics, many of the compounds listed in Table 10.4 can be employed in conjunction with visible light sources. As the research in organometallic chemistry gained momentum, the potential advantages of organometallic complexes as photoinitiators were also explored, and two such compounds, a ferrocenium salt and a titanocene, were commercialized (see Chart 10.3). 

In macroscopic chemistry, the experimental procedures for the bromination of aromatic compounds depend greatly on the nature and reactivity of the starting material. Activated aromatics such as phenol and aniline can be brominated to the tri-and tetrabrominated derivatives by using dilute aqueous solutions of bromine, whereas a controlled monobromination is very challenging and often requires cryogenic conditions. On the other hand, thermally controlled brominations of less activated aromatics such as toluene are rather slu ish reactions. Th often require photoinitiation and the use of Lewis adds as catalysts. 

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