Mechanistic Studies of Photoinitiated Free Radical Polymerization Using a Bifunctional Thioxanthone Acetic Acid Derivative as Photoinitiator

Abstract

A bifunctional photoinitiator for free radical polymerization, thioxanthone catechol-O,O 0 -diacetic acid, was synthesized, characterized, and compared to photoinitiator parameters of the monofunctional analogue, 2-(carboxymethoxy)thioxanthone. Photophysical studies such as fluorescence, phosphorescence, and laser flash photolysis in addition to photopolymerizations of methyl methacrylate show that the bifunctional photoinitiator is more efficient in polymer generation than the monofunctional derivative. These studies suggest that initiator radicals are generated from a π-π* triplet state in an intramolecular electron transfer, followed by proton transfer and decarboxylation to generate alkyl radicals, which initiate polymerization. The initial electron transfer is faster for the bifunctional photoinitiator than the monofunctional derivative, which is based on laser flash photolysis studies. Because of the relatively fast intramolecular radical generation from the triplet state (triplet lifetime = 490 ns), quenching by molecular oxygen is insignificant and polymerization of methyl methacrylate proceeds efficiently without deoxygenation. At higher concentrations of initiator (∼5 mM) intermolecular electron transfer competes with intramolecular electron transfer. Both processes, inter-and intramolecular processes, yield initiating alkyl radicals.

Synthesis and application of dyes derived from benz[cd]indol‐2(1H)‐one as visible‐light‐absorbing polymerisation photoinitiators

“…Thioxanthone [19][20][21][22] and benzophenone [23][24][25][26] and, more recently, novel erythrosine B [27] and naphthoquinone [28] derivatives linked with appropriate groups have been synthesised and applied as initiators for free radical polymerisation. There are very few chromophores that can be used to design onecomponent initiator systems.…”

Shining a light on an adaptable photoinitiator: advances in photopolymerizations initiated by thioxanthones

“…113 The versatility of the synthesis route for TX derivatives has made it possible to incorporate desired functionalities into the structure of TX using functional aromatic compounds as starting materials. 118 Arsu and co-workers investigated the influence of the substitution pattern and their position on the photoinitiation activity of carboxylic acid-functionalized TX derivatives. 114 Acting as onecomponent Type II photoinitiator, the mechanism of the formation of initiating free radicals in carboxylic acidfunctionalized TX is through the abstraction of the acidic hydrogen by the triplet excited state TX core, which further results in a decarboxylation process to evolve carbon dioxide yielding initiating radicals.…”

Naphthoylenebenzimidazolone dyes as one‐component photoinitiators

“…In the last decade many papers have described the synthesis of initiators. Only thioxanthone [3][4][5][6][7][8][9], benzophenone [10][11][12][13][14][15][16], erythrosine B [17], and, more recently, naphthoquinone [18] and benz[cd]indol-2(1H)-one [19] linked with appropriate groups have been applied as chromophores to obtain one-component initiator systems. There are two different ways to create initiators sensitive to visible radiation.…”