Curing behavior of epoxy resin initiated by S-alkylsulfonium salts of aromatic sulfides as thermal latent cationic initiators

The curing behavior of bisphenol-A-type epoxide oligomers (E-P) was evaluat ed by differential scanning calorimetry in the presence of S-alkylsulfonium salts of dibenzothiophene, phenoxathiin, thianthrene, thioanisole, and tet rahydrothiophene as thermal latent initiators. These initiators dissolved h omogeneously in E-P, except for 2,8-dimethoxy-5-methyldibenzothiophenium te trafluoroborate, and the curing reaction of the resulting mixtures occurred on heating, except for S-methyltetrahydrothiophenium tetrafluoroborate. Th e initiation activity of these salts was controlled by the character of the substituents on the benzene ring, the leaving sulfide group, and the S-alk yl group. Presumably, the electron density on the sulfide moieties and the stability of the carbocation released from the sulfonium salts affected the initiating temperature. A good correlation was obtained between the initia ting temperature and the electron density of the sulfur atom of the corresp onding sulfides, estimated from ab initio molecular orbital calculations in which the initiating temperature became higher as the electron density of the sulfur atom increased. (C) 2001 John Wiley & Sons, Inc.