2-Mercaptothioxanthone (TX-SH), a hydrogen abstraction type photoinitiator for free radical polymerization, is synthesized and characterized. Its capability to act as an initiator for the polymerization of methyl methacrylate (MMA), styrene (St), and multifunctional monomers is examined. The relative efficiencies of TX-SH, the parent thioxanthone (TX), and their combination with an amine synergist such as N-methyldiethanolamine (MDEA) are compared in the photopolymerization. The postulated mechanism is based on the intermolecular reaction of triplet, 3 TX-SH*, with the thiol moiety of ground-state TX-SH. Thus, when TX-SH is irradiated in the presence of a monomer, it can serve as both a triplet photosensitizer and a hydrogen donor. The resulting thiyl radicals initiate the polymerization. Incorporation of TX-S groups into polymers was demonstrated by spectroscopic methods.

The compounds 2‐thioxanthone‐thioacetic acid and 2‐(carboxymethoxy)thioxanthone, bimolecular photoinitiators for free radical polymerization, are synthesized and characterized. Their capability to act as initiators for the polymerization of methyl methacrylate was examined. The postulated mechanism is based on the intermolecular electron‐transfer reaction of the excited photoinitiator with the sulfur or oxygen atom of the ground state of the respective photoinitiator followed by decarboxylation. The resulting alkyl radicals initiate the polymerization.Structures of the photoinitiators.magnified imageStructures of the photoinitiators.

A novel thioxanthone-anthracene (TX-A) photoinitiator, namely 5-thia-pentacene-14-one, possessing the respective photochromic groups was synthesized. TX-A is an efficient photoinitiator for free radical polymerization of acrylic and styrenic type monomers in the presence of oxygen. UV-vis, FT-IR, and fluorescence spectroscopic and polymerization studies revealed that photoinitiation occurs through anthracene chromophore. In contrast to thioxanthone-based photoinitiators, TX-A does not require an additional hydrogen donor for the initiation.

A mechanistic study concerning photoinitiated free radical polymerization using thioxanthone thio-acetic acid (TX-S-CH 2-COOH) as one-component Type II photoinitiator was performed. Steady-state and time-resolved fluorescence and phosphorescence spectroscopy, as well as laser flash photolysis was employed to study the photophysics and photochemistry of TX-S-CH 2-COOH. The initiator undergoes efficient intersystem crossing into the triplet state and the lowest triplet state posseses π-π* configuration. In contrast to the unsubstituted thioxanthone, TX-S-CH 2-COOH shows an unusually short triplet lifetime (65 ns) indicating an intramolecular reaction. From fluoroscence, phosphorescence, and laser flash photolysis studies, in conjunction with photopolymerization experiments, we propose that TX-S-CH 2-COOH triplets undergo intramolecular electron transfer followed by hydrogen abstraction and decarboxylation producing alkyl radicals, which are the active initiator radicals in photoinduced polymerization. At low initiator concentrations (below 5 × 10 -3 M) this intramolecular reaction is the dominant path. At concentrations above 5 × 10 -3 M, however, the respective intermolecular reactions may be operative.

Two thioxanthone derivatives with fluorene additional chromophoric groups, namely thioxanthone-fluorenecarboxylic acid (TX-FLCOOH) and thioxanthone-fluorene sodium carboxylate (TX-FLCOONa), as visible light absorbing oil-and water-soluble photoinitiators, respectively, were synthesized and characterized. Their ability to initiate photopolymerization of methyl methacrylate (MMA), 2-(2-phosphonoethoxymethyl)acrylic acid ethyl ester (EAEPA), and a multifunctional monomer, trimethylolpropane triacrylate (TMPTA), were examined. The initiation efficiency of TX-FLCOOH was compared with the parent thioxantone (TX) both in the presence and in the absence of a co-initiator, and TX-FLCOOH was found to be more effective in all cases. Photopolymerization and laser flash photolysis studies revealed that depending on the concentration initiation by TX-FLCOOH occurs by intra-and intermolecular H-abstraction of the triplet 3 (TX-FLCOOH)*. No polymerization was observed when TX-FLCOONa was used as photoinitiator in the absence of a co-initiator. However, TX-FLCOONa acts as an efficient initiator in the visible range in the presence of H donors in water.

Macrophotoinitiators containing thioxanthone (TX) moieties as side chains were synthesized by using "double click chemistry" strategy; combining in-situ 1,3-dipolar azide-alkyne [3 + 2] and thermoreversible Diels-Alder (DA) [4 + 2] cycloaddition reactions. For this purpose, thioxanthone-anthracene (TX-A), N-propargyl-7-oxynorbornene (PON), and polystyrene (PS) with side-chain azide moieties (PS-N 3 ) were reacted in N,N-dimethylformamide (DMF) for 36 h at 120 °C. In this process, PON acted as a "click linker" since it contains both protected maleimide and alkyne functional groups suitable for 1,3-dipolar azide-alkyne and Diels-Alder click reactions, respectively. This way, the aromacity of the central phenyl unit of the anthracene moiety present in TX-A was transformed into TX chromophoric groups. The resulting polymers possess absorption characteristics similar to the parent TX. Their capabilities to act as photoinitiator for the polymerization of monoand multifunctional monomers, namely methyl methacrylate (MMA) and 1,1,1-tris(hydroxymethyl)propane triacrylate (TPTA) were also examined.

A polystyrene copolymer (PS-B-DMAB) possessing both benzophenone and dimethylamino moieties in the side chain was synthesized by a combination of nitroxide-mediated radical polymerization (NMRP) and simultaneous double "click" reactions. First, a random copolymer of styrene (S) and chloromethylstyrene (CMS) with 32 mol % CMS content was prepared by NMRP process. Then, chloromethyl groups were converted to azide groups by reacting with NaN 3 in DMF. The other two click components, namely, propargyl benzophenone (Pr-B) and propargyl 4-(dimethylamino)benzoate (Pr-DMAB), were prepared independently by the etherification and esterification reactions, respectively. Then, in the final stage, the obtained alkyne functional chromophoric (Pr-B) and hydrogen-donating (Pr-DMAB) molecules were anchored to azide-modified polystyrene (PS-N 3 ) in one-step by "click chemistry". The final polymer (PS-B-DMAB) and the intermediates were characterized in detail by spectral analysis and laser flash photolysis studies. The resulting polymer possesses absorption characteristics similar to bare bezophenone. The one-component photoinitiating nature of PS-B-DMAB was demonstrated by photopolymerization of mono-and multifunctional monomers, namely, methyl methacrylate (MMA) and trimethylolpropane triacrylate (TMPTA), respectively.

SummarySide chain thioxanthone-containing polymer (PSt-TX) was synthesized and characterized by modification of polystyrene prepared by Atom Transfer Radical Polymerization. PSt-TX exhibits absorption characteristics similar to that of the unsubstitued thioxanthone. Its capability to act as initiator for the polymerization of methyl methacrylate was also examined.