Onium salt cationic photoinitiator

Another example is thermally depolymerizable polycarbonates that are sensitized with onium salt cationic photoinitiators (139). As discussed previously (Section 3.2.3), one of the positive-negative resists based on polystyrene with pendant tertiary butyl carbonate protecting groups undergoes acid-catalyzed thermolysis to generate poly( p-vinylphenol), carbon dioxide, and isobutene. Therefore, if the tertiary butyl carbonate moiety is incorporated in the polymer backbone as a repeating unit, such polymers will... 

H. Ito and E. Flores, Evaluation of onium salt cationic photoinitiators as novel dissolution inhibitor for novolac resin, J. Electrochem. Soc. 135,2322 (1988) H. Ito, Aqueous base developable deep UV resist systems based on novel monomeric and polymeric dissolution inhibitors, Proc. SPIE 920, 33 (1988). T. Aral, T. Sakamizu, K. Katoh, M. Hashimoto, and H. Shiraishi, A sensitive positive resist for 0.1 p,m electron beam direct writing hthography, J. Photopolym. Sci. Technol. 10, 625 (1997). 

I J.V. Crivello, The Discovery and Development of Onium Salt Cationic Photoinitiators, J. Polym. Sci. Part A, Polym. Chem. 37 (1999) 4241. 

These materials all undergo acid catalyzed thermolysis and thus, a resist may be formulated by combining such polymers with an onium salt cationic photoinitiator that is known to generate strong acid upon exposure. 

Figure 2.1. Relationship between the wavelength sensitivity and application for onium salt cationic photoinitiators...

A second photochemical process called electron-transfer photosensitization is, in reality, a photoinduced redox reaction [EBE 87, PAP 84a, PAP 84b] and this method of photosensitization has been much more successful for the extension of the spectral sensitivity of onium salt cationic photoinitiators into the long wavelength UV and visible spectral regions. Electron-transfer photosensitization is a well-understood process and a general mechanism for this process as exemplified for diaryliodonium salts is shown in Diagram 2.2. 

CRI 99] Crivello J.V., The discovery and development of onium salt cationic photoinitiators . Journal of Polymer Science Part A Polymer Chemistry, vol. 37, pp. 4241 254, 1999. 

Many of the above-described defidts surrounding the use of Bronsted superadds as initiators for cationic polymerization have been offset through the use of onium salt cationic photoinitiators. 

Having presented the chemistry of the main classes of onium salt cationic photoinitiators, this section will be used to further elaborate on their use in cationic polymerizations. Those onium salts that generate Bronsted superacids on photolysis are technically capable of initiating the cationic polymerizations of all known polymerizable electron-rich vinyl and heterocyclic monomers. Figure 4 provides an overview of the photopolymerizations of some of the more commonly used classes of monofimctional vinyl and heterocyclic monomers and is not intended to be inclusive. 

Onium salt cationic photoinitiators present many unique and interesting opportunities for basic studies of cationic ring-opening polymerizations. Since they are latent photochemical sources of strong Bronsted adds, they can be dissolved in the subject monomers and then precisely tri ered on demand by the application of light. Mixing problems and the use of complex stopped-flow devices and other apparatuses required to overcome them are thus avoided. Only the rate of initiation is different in a photoinitiated cationic polymerization as compared to a conventional thermally initiated polymerization. The rate of initiation for an onium salt-photoinitiated cationic potymerization (eqn [68]) is... 

In principle, onium salt cationic photoinitiators are capable of photochemically generating any desired Bronsted add by simply selecting the appropriate anion of the salt. If individual members of a series of onium salts in which only the anion is varied are irradiated at a constant light intensity, the same quantity of Bronsted acid per unit time will be produced in every case. When the photolysis is carried out in a suitable heterocyclic monomer, a Bronsted acid can then be ranked... 

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