Abstract
Cationic photopolymerization is attracting much attention in many applications, owing to its technical advantages such as freedom from oxygen inhibition, low shrinkage during polymerization and, as a result of the latter, good adhesion to the substrates. Cationic photoiniators are important raw material for this technology and comprise mainly onium salts with non-nucleophilic anions. During the past 10 years or so, research on the former has centered around iodonium, sulfonium, and ammonium ions, aiming at initiators with longer wavelength UV absorption, higher photo-activity, and better storage stability. Multi-functional initiators, which can trigger thermal, photocationic, and photoradical polymerization, are also reported. On the other hand, study on anions has focused on development of substitutes for SbF6-. The latter has long been the best performing anion but its application has been limited because of its possible toxicity. As a substitute, B(C6F5)4- anion has already been introduced in the market as an iodonium salt. Following this, sulfonium and ammonium salts of the same anion are reported. Also reported are two new anions : one is Ga(C6F5)4- and another, perfluoroalkyl fluorophosphate, PFnRf(6-n)-. Onium salts of these anions showed excellent properties as SbF6- substitutes.
