Photoreduction of triplet thioxanthone by amines: charge transfer generates radicals that initiate polymerization of olefins

Abstract

3349tions,laa additions of thiyl radicals to styrenes ( p --0.4 vs. and additions of C13G to styrenes ( p = -0.42 vs. a+)% and aliphatic olefins (p* = -0.2 to -0.4 vs. 8 or a*).% Table 111, the substituent effects for additions of PhCO2-, MeOO., and t-BuOO. to styrenes also resulted in small negative p values (p = -0.1 to -0.3 vs. a+) with poor correlations. That is, these oxy radicals add to styrenes as a weak electrophile. In contrast, acylperoxy radicals afford large negative p values ( p = -1 vs. a+), indicating the importance of polar effects in the radical additions to olefins.% The coincidence of p values between the reactions of Reo3., RC03H, and '02 is interesting. It is also interesting to recall that similar polar effects (Le., p = -0.3 As shown into -1.1 vs. a ' )are known in benzylic hydrogen abstract i o n~~' where benzyl radicals are being formed.In conclusion, a-diketones are photooxidized to 2 equiv of acylperoxy radical, which add to olefins 105-fold faster than alkylperoxy radicals and lead to effective epoxidations. Selective epoxidation of double bonds is possible since Reo3. is not reactive to sulfides, sulfoxides, and pyridine. The photoepoxidation proceeds at low temperature or in the presence of pyridine; then is application to acid-sensitive or unstable epoxides may be possible.2e The accompanying C-C cleavage of olefins proceeds via the addition of Reo2. or ROO.. Experimental SectionGLC analyses were performed with a Yanagimoto G180 gas (22) (a) Cadogan, J. I. G.; Sadler, I. H. (25) (a) The observed preference of the addition of Reo8. to C=C over H abstraction for c clohexene has been explained by ita highly U.

Tetraalkylammonium Salts of Amino Acids and Sulfur‐Containing Amino Acids as Effective Co‐Initiators of Free Radical Polymerization in the Presence of Aromatic Ketones

“…Earlier, Yip [29] reported that the absorption maximum of thioxanthone triplet is located at 630 nm in acetonitrile solution. Similar observations were reported by Schuster [30] and Encinas, [31] who observed the same product with the maximum at 620 nm. The triplet-triplet spectroscopic properties of all thioxanthones under the study are compiled in Table 1.…”

“…Thioxanthones [1][2][3] (TX, Scheme 1) as well as other aromatic ketones such as benzophenone, [4,5] benzyls, [6,7] xanthene dyes [8][9][10] and many others [4] in the presence of an electron donor have acquired importance as light absorbers in the formulation of photoinitiating polymerization acrylates. Reduction of the excited state of a ketone by an amine leads to radical formation, which initiates polymer chain reaction.…”

Mechanistic Study of Tetrahydrofuran Polymerization Photoinitiated by a Sulfonium Salt/Thioxanthone System

“…Photoreduction of TX triplets by other reductants, like amines, involving electron transfer are also known. [37,43] Photolysis Experiments 425 nm occurs at the same wavelength as the transient spectrum of the TX + 9 cation radical. No changes are observed in the absorption spectrum of TX when it is photolysed in the presence of THF alone.…”

Synthesis and characterization of poly(vinylchloride) type macrophotoinitiator comprising side-chain thioxanthone via click chemistry

“…Second, although radical polymerizations are not sensitive to the polarity of the solvent [32][33], triplet-state lifetime of photoinitiators which involve electron transfer such as TX species, may depend on some polarity effects. The electron transfer mechanism of TX has been extensively investigated by spectroscopic and laser flash photolysis techniques in the literature [34][35].…”