A Novel Approach to the Preparation of Dissociative Electron Transfer Photoinitiators for Free Radical Polymerization

A new approach to the design of the electron transfer free radical photoinitiating system (ETPS) is presented in the paper. The system applying a light absorber (dye), and an electron donor (sulfur-containing aromatic carboxylic acid, SCCA), possessing the structure allowing the formation of the leaving group that forms a neutral free radical, is described. The experimental results show that after the transformation of the SCCA into its ammonium salt a substantial increase of the polymerization photoinitiation ability of the system is observed. The mechanism of the photoinitiated polymerization for the tested photoredox pairs is clarified on the basis of the laser flash photolysis experiments obtained from the neutral dye (5,7-diiodo-3-pentoxy-6-fluorone, DIPF) serving as electron acceptor and (phenylthio)acetic acid (Ph−S−CH2−COOH, PTAA) and its tetrabutylammonium salt (PTAA AS) as electron donors in MeCN solution. It is documented that the photoreduction of DIPF in the presence of (phenylthio)acetic acid and its tetrabutylammonium salt occurs via the photoinduced electron transfer process. On the basis of the known photochemistry of sulfur-containing aromatic carboxylic acids, it is postulated that the existence of the carboxyl group in an ionic form allows a rapid decarboxylation, yielding a neutral α-alkylthio-type radical (R−S−CH2 •). The system described in this paper applies the dissociative electron transfer process for an effective production of very reactive free radicals able to initiate a radical polymerization.