Novel dissociative electron transfer photoniniciators for free radical polymerization

Abstract

Radical polymerization of 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate (TMPTA), photoinduced with redox system: electron donor-absorber, has been presented. Xanthene dyes: Rose bengal ditetrabutyl-ammonium salt [RBTBAS - Formula (I)] and 5,7-diiodo-3-pentoxy-6--fluorone [DIPF - Formula (II)] were used as absorbers. Electron donors in the system investigated were: (phenylthio)acetic acid (PTAA), (phenylthio)acetic acid tetrabutylammonium salt (PTAA AS), ethyl (phenylthio)acetate (PTAA EE) or n-butyltriphenyl borate (BuPh 3 B + ). Photopolymerization mechanism has been investigated using laser flash photolysis method. It has been found that photoreduction with PTAA or PTAA AS goes with electron transfer from sulfur atom to dye in triplet state (Schemes A and B). In case when RBTBAS is used as electron acceptor the anionic radicals of the dye [RB_ 3 - -Formula (III) and RB_ 2 - Formula (IV)] are obtained. The presence of these anionic radicals shows that after electron transfer the carboxylic group exists in an ionic form what let intramolecular electron transfer from carboxylate group to sulfur cationic radical, followed with rapid decarboxylation. As a result of decarboxylation the neutral thiomethylene radicals (Ph-S-CH 2 _) are formed which, after escape from solvent cage, take part in photoinitiation of the polymerization. It has been stated that transformation of sulfur(II) containing carboxylic acids into their tetrabutylammonium salts significantly increases the sensitivity of the photoinitiating system (Fig. 6 and 7). It also increases photopolymerization rate (R p ) (Table 1, Fig. 10), which is a function of square root of the quantum yield of decarboxylation process (Φ c o ) (Fig. 9).