New insights about diazonium salts as cationic photoinitiators

4-Hexyloxysubstituted diazonium salts with complex anions are thermostable compounds in several solvents (dioxane: 12 days; 1,2-dichloroethane: 410 da ys; 40 degreesC; salt as SbF6-). These salts initiate efficiently the photocrosslinking of vinyl ethers and epoxides. Interestingly, oxygen influences the efficiency of this cationic process. EPR-experiments prove that radicals possess a key function for the production of the initiating species. alpha -Ether radicals induce a secon dary radical-induced cation formation. Such reactions are always possible i f E-red of the onium salt is lower than -1 V. Oxygen inhibits this radical- induced cation formation, On the other hand, the decay of peroxides results in a branched radical reaction. The reaction rate is faster under air with respect to inert conditions. The high thermostability of the used salt decreases by addition of a small amount of monomer. A bimolecular dediazoniation mechanism explains the obse rved effects. This mechanism produces directly initiating cationic species, which start the cationic polymerization. The monomer and its byproducts are the cause of the poor thermal stability of the diazonium salts and not the own thermal instability of the salt used . (C) 2001 Elsevier Science B.V. All rights reserved.