Benzophenone−Phenylthioacetic Acid Tetraalkylammonium Salts as Effective Initiators of Free-Radical Photopolymerization of Vinyl Monomers, Mechanistic Studies

Abstract

The mechanism of the photooxidation of tetraalkylammonium salts of phenylthioacetic acid, C6H5−S−CH2−COO-N+R4, was investigated in detail in order to understand why they turned out to be efficient co-initiators of free-radical polymerizations. The photosensitizer was benzophenone (BP), and the alkyl substituents (R) were n-butyl, n-propyl, or ethyl. The techniques used were steady-state and nanosecond flash photolysis. It was shown that electron transfer from the sulfur atom to the benzophenone triplet state was the primary photochemical step followed by a decarboxylation reaction leading to CO2, the C6H5−S−CH2 • radical, and a [BP•-···N+R4] ion pair. The latter underwent a Hofmann elimination (unexpected in these mild experimental conditions) leading to alkene-1 and trialkyl amine. The quantum yields of all the observed transients and the stable products were determined. The mechanism of primary and secondary photochemical reactions was quantitatively described, and it was shown to be similar for all of the alkyl derivatives used. The results of photochemical studies were compared with respect to the polymerization studies where the systems consisting of benzophenone and tetraalkylammonium salts of phenylthioacetic acid (and phenylthioacetic acid for comparison) were used as photoinitiating couples for free-radical polymerizations using 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate as the monomer. A linear correlation was found for the polymerization rates with respect to the square root of the CO2 quantum yields. This justified the hypothesis that the C6H5SCH2 • radical is the initiator of the free-radical polymerizations for the BP/C6H5−S−CH2−COO-N+R4 photoredox pairs studied in this work.

Formation of cation–radical anion pairs derived from carboxybenzophenone–tetrabutylammonium salts. Pulse radiolysis studies

“…The lifetimes (30 ls, [3]) of the ion pairs [BP •-ÁÁÁN ? R 4 ] (Chart 2) were an order of magnitude longer than those (4.6 ls; Table 1) The 2-carboxy-substituted benzophenone offers the possibility for assessing the competition between intramolecular proton transfer and ion pair formation.…”

“…Photosensitized oxidation reactions of sulfur-containing carboxylic acids and their salts have been investigated for many years due to their importance in chemistry, e.g., in free-radical photopolymerization [1][2][3]. The mechanism of the photooxidation of tetraalkylammonium salts of (phenylthio)acetic acid, (C 6 H 5 S)CH 2 COOH, was recently investigated in detail [3].…”

“…The mechanism of the photooxidation of tetraalkylammonium salts of (phenylthio)acetic acid, (C 6 H 5 S)CH 2 COOH, was recently investigated in detail [3]. The photosensitizer was benzophenone (BP), and the alkyl substituents (R) were n-butyl, n-propyl, or ethyl.…”

“…It was shown that electron transfer from the sulfur atom to the benzophenone triplet state was the primary photochemical step followed by a decarboxylation reaction leading to CO 2 , the • CH 2 SC 6 H 5 radical, and a [BP •-ÁÁÁN ? R 4 ] ion pair (decaying monoexponentialy with a lifetime of 30 ls) [3][4][5].…”

Visible Light Induced Emulsion Polymerization: An Improved Photoinitiation System Based on Amphiphilic Tertiary Amine N‐Oxides

“…The excited‐state electron transfer process in light‐induced polymerizations often leads to the formation of free radicals and its efficiency is closely related to the precise nature of the reacting partners taking part in the electron transfer process. These systems include aromatic ketone amine combinations and dye eco‐initiator combinations, as well as certain organometallic photoinitiators …”