Lithium-and magnesium phenyl-2,4,6-trimethylbenzoylphosphinates (TMPPL and TMPPM) are effective water-soluble photoinitiators for the free-radical polymerization of appropriate monomers such as acrylamide (AA) and methacrylamide (MAA) in aqueous solution. They are also capable of initiating the polymerization of other olefinic compounds such as styrene (St), methyl methacrylate (MMA) or acrylonitrile (AN) in water-containing solvent mixtures such as 1 : 1 water-acetonitrile mixtures. This is due to the fact that TMPPL and TMPPM undergo ascission with a rather high quaqtum yield ($(a) = 0,35) resulting in the formation of 2,4,6-trimethylbenzoyl radicals and O = P (C6H5)(0 -) radical anions. The latter are very reactive toward olefinic monomers. Bimolecular rate constants kR+M/(L/(mol . s)) were determined by flash photolysis at room temperature, e.
Cationic organic dyes (phenosafranine, safranine T (ST + ), thiopyronine, and methylene blue) are efficient quenchers of the triplet states of aromatic ketones. The triplets of benzophenone (BP), xanthone, thioxanthone, benzil, and N-methylacridone are quenched by the dyes via energy transfer, generating triplet excited dyes (typically k q ) 5.5 × 10 9 M -1 s -1 for 3 BP * + ST + in acetonitrile solution). Regarding Michler's ketone, p-(dimethylamino)benzophenone, and 3,6-bis(dimethylamino)thioxanthone, the mechanism of triplet quenching is solvent-dependent. Electron transfer occurs in polar solvents like acetonitrile, and energy transfer is the dominant reaction pathway in less polar solvents, such as dichloromethane. Delayed fluorescence of the dye caused by heterotriplet-triplet annihilation involving ketone and dye was detected upon studying the reaction of the dyes with triplet excited ketones (BP, xanthone, thioxanthone, and benzil). By applying two successive laser pulses (λ ex ) 532 and 308 nm, respectively), the rate constant of the reaction 3 BP * + 3 (ST + ) * was determined: k ht ) 1.1 × 10 10 M -1 s -1 .
Rate constants for addition reactions of diphenylphosphonyl radicals (Ph2p=0) to vinyl monomers were directly estimated by time-resolved electron spin resonance (ESR) spectroscopy. The rate constants were on the order of 106-107 s_1 NT1, being 1 or 2 orders of magnitude larger than those of carboncentered radicals, which were obtained by photolysis of benzoin, benzoin methyl ether, or l-phenyl-2-hydroxy-2-methylpropanone-1. The fact that (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TMDPO) is a more effective initiator than a,a'-azobis(isobutyronitrile) (AIBN) was made clear in the radical polymerizations of vinyl pivalate at -30 and -70 °C.
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