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 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.
A new photoinitiator for free radical
photopolymerization, belonging to the thioxanthone derivatives, was
synthesized and characterized. It was found that the compound absorbs
over the whole UV–vis spectrum with relatively high absorption
coefficients. Fluorescence studies reveal that three different singlet
excited states are responsible for this absorption. Phosphorescence
and laser flash photolysis evidence the formation of a triplet state
from which a photoreduction can occur. Consequently, initiating radicals
were formed which are able to initiate the photopolymerization of
methyl methacrylate in DMF. Finally, the photopolymerization of acrylates
was performed in film at different wavelengths such as 392, 476, 532,
and 632 nm, emphasizing the possibility to use this novel photoinitiator
when panchromatic irradiation is required.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.