PHOTOINITIATORS FOR FREE-RADICAL-INITIATED PHOTOIMAGING SYSTEMS

Polymer photoimaging systems are an important application of photochemical technology. Imagewise exposure of these systems either forms a polymer or modifies the properties of a preformed polymer to form a latent image in the exposed region. These latent images can be converted into useful images by appropriate techniques. Polymer photoimaging systems have numerous important applications, primarily in the printing and electronics industries. Several excellent reviews of these topics are available. The terms photopolymer and polymer imaging system are often used to describe any of a number of systems in which a light-initiated change in a monomer or polymer is produced. Several types of imaging systems, each of which employs a distinctly different type of imaging reaction, are grouped together under these headings. An important class of polymer imaging systems employs photoinitiated polymerization. In these systems relatively low molecular weight monomers undergo light-initiated free-radical or cationic polymerization to form higher molecular weight materials. Polymerization is initiated by photoactivation of an added low molecular weight component, known as the photoinitiator. In cationic-initiated systems, irradiation of the photoinitiator produces a Lewis acid or a protonic acid that initiates polymerization. In free-radical-initiated systems, irradiation of the photoinitiator produces free radicals that initiate polymerization. Initiation of polymerization of a number of monomer molecules by the absorption of one photon is a form of chemical amplification. Imaging systems of high sensitivity can be developed. This review emphasizes photoinitiators used in photoimaging systems. In order to present a comprehensive review of free-radical-initiated photoimaging systems, cationic-initiated photoimaging systems will not be discussed. This rapidly developing area has been reviewed. A brief introduction to free-radicalinitiated photoimaging systems has been included. A comprehensive review of both cationic and freeradical photoinitiators, especially those used in radiation curable coatings, appeared after completion of this manuscript. This review is an excellent source for those seeking more detailed information, especially about coating applications, than is presented here. Initiators for radiation-curable coating have also been discussed elsewhere. Since many of the systems used in free-radical-initiated photoimaging systems, especially the earlier systems, were originally developed for radiation curable coatings, there is some overlap between these reviews and the material presented here. The compositions of most of the commercial photoimaging systems have not been disclosed. Thus, it can often not be stated with certainty whether a particular photoinitiator system is, or has been, used in a commercial product. We will indicate those systems whose properties make them generally suited for use in commercial products. However, systems that have not been discussed, or that we have indicated are generally unsuitable for use in photoimaging systems, may still have limited applications in commercial products. Mechanisms will not be discussed in detail. This aspect of photoinitiator chemistry has been reviewed. In addition, many of the commercially important photoinitiator systems have been described primarily in the patent literature. Little or no mechanistic work has been published on these systems. We have organized this article by the type of reaction that takes place: fragmentation, electron transfer, or hydrogen abstraction. This division, however, is far from clear-cut. In some systems the mechanistic details are unknown. In others, more than one type of reaction may occur (electron transfer followed by hydrogen abstraction, electron transfer followed by fragmentation, fragmentation followed by hydrogen abstraction, etc.). The type of reaction may also vary with the conditions. For convenience, organometallic systems have been treated as a separate class. © 1993, American Chemical Society. All rights reserved.