- Study of alkyl radicals fragmentation from 2-alkyl-2-propoxyl radicals
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The reaction of a series of 2-alkyl-2-propoxyl radicals 2 in cumene has been studied. Alkoxyl radicals 2 were generated from the thermolysis of the corresponding tert-alkyl peroxypivalates, and underwent several modes of unimolecular reactions, that is, β-scission to give methyl radical, β-scission to give alkyl radicals and 1,5-H shift, which competed with hydrogen abstraction from cumene. The ratios of the rate constant for alkyl radicals elimination to that for methyl radical elimination, kβ(R)/kβ(Me), were determined by using the radical trapping method employing TEMIO as the scavenger. The logarithm of the relative rate was satisfactorily correlated with the heat of formation of the leaving alkyl radicals containing the steric parameters of neopentyl radical 2d. Then the plot of ln [kβ(R)/kβ(Me)] vs. the SOMO energies of the corresponding alkyl radicals showed a linear relationship with a slope of -7.6 eV-1, which is comparable to that of formolysis of alkyl bromides. The mechanism involving a fairly polar transition state of the alkyl radicals fragmentation from tert-alkoxyl radicals 2 is discussed in terms of the MO diagrams and the solvent effects.
- Nakamura, Tomoyuki,Watanabe, Yasumasa,Suyama, Shuji,Tezuka, Hiroshi
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p. 1364 - 1369
(2007/10/03)
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- Thermal decomposition mechanisms of tert-alkyl peroxypivalates studied by the nitroxide radical trapping technique
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The thermolysis of a series of tert-alkyl peroxypivalates 1 in cumene has been investigated by using the nitroxide radical-trapping technique. tert-Alkoxyl radicals generated from the thermolysis underwent the unimolecular reactions, β-scission, and 1,5-H shift, competing with hydrogen abstraction from cumene. The absolute rate constants for β-scission of tert- alkoxyl radicals, which vary over 4 orders of magnitude, indicate the vastly different behavior of alkoxyl radicals. However, the radical generation efficiencies of 1 varied only slightly, from 53 (R = Me) to 63% (R = Bu(t)), supporting a mechanism involving concerted two-bond scission within the solvent cage to generate the tert-butyl radical, CO2, and an alkoxyl radical. The thermolysis rate constants of tert-alkyl peroxypivalates 1 were influenced by both inductive and steric effects [Taft-Ingold equation, log(rel k(d)) = (0.97 ± 0.14)Σσ* - (0.31 ± 0.04)ΣE(s)(c), was obtained].
- Nakamura,Busfield,Jenkins,Rizzardo,Thang,Suyama
-
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- Reaction of acyclic hydrocarbons towards t-butoxy radicals. A study of hydrogen atom abstraction by using the radical trapping technique
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The reaction of 3-methylpentane and 2,4-dimethylpentane toward t-butoxy radicals has been investigated, in neat and benzene solutions, by using the radical trapping technique. Abstraction occurs principally from the tertiary and secondary C-H reaction sites of 3-methylpentane and the tertiary position of 2,4-dimethylpentane. The tertiary and in particular secondary C-H reaction sites of 2,4-dimethylpentane are shown to be considerably less susceptible towards t-butoxy radical facilitated abstraction compared with the equivalent reaction sites of 3-methylpentane. As a result, the latter is three times as reactive as 2,4-dimethylpentane as a neat hydrocarbon solution and seven times as reactive in a diluted mixture of benzene. Differences in selectivity and rate of hydrogen abstraction, between the substrates, are interpreted in terms of non-bonding interactions retarding t-butoxy radicals from approaching sterically demanding C-H reaction sites. The selectivity from 3-methylpentane is solvent-insensitive whereas abstraction from 2,4-dimethylpentane is modified in benzene. Further, the rate of hydrogen abstraction, from either substrate, to t-butoxy radical β-scission is considerably smaller in benzene. Both observations are interpreted in terms of t-butoxy radical solvation by the aromatic solvent.
- Dokolas, Peter,Loffler, Steven M.,Solomon, David H.
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p. 1113 - 1120
(2007/10/03)
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- Thermal decomposition of 1-cyclohexyl-1-methylethyl peroxypivalate
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The thermal decomposition of a novel peroxyester, 1-cyclohexyl-1-methylethyl peroxypivalate 1a in cumene has been studied using the radical trapping technique employing 1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-yloxyl T, a stable aminoxyl radical, as
- Nakamura, Tomoyuki,Busfield, W. Ken,Jenkins, Ian D.,Rizzardo, Ezio,Thang, San H.,Suyama, Shuji
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p. 965 - 966
(2007/10/03)
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- Free radical initiation mechanisms in the polymerization of methyl methacrylate and styrene with 1,1,3,3-tetramethylbutyl peroxypivalate: Addition of neopentyl radicals
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The reactions of 1,1,3,3-tetramethylbutyl (tert-octyl) peroxypivalate (1) with methyl methacrylate (MMA) and styrene in the presence of the free radical scavenger 1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-yloxyl (2) have been studied at 60°C. tert-Butyl and tert-octyloxyl radicals (3) were generated from the thermolysis of 1. The predominant unimolecular reactions of 3, that is, β-scission to form neopentyl radicals (14b) and a 1,5-H shift to form 4-hydroxy-2,2,4-trimethylpentyl radicals (14c), were observed in both monomer systems. The resulting alkyl radicals underwent selective addition to the two monomers. The relative reactivities of the alkyl radicals toward addition to the monomers were obtained from competitive addition/trapping reactions. The absolute rate constants for the addition of alkyl radicals 14b and 14e to the two monomers at 60°C were estimated to be 9.5 x 105 and 2.6 x 105 M-1 s-1 to MMA and 4.5 x 105 and 0.7 x 105 M-1 s-1 to styrene, respectively. The low reactivities of 3 and 14e toward addition to MMA and styrene were attributed to steric effects. Steric effects were also responsible for the low rate of the 1,5-H shift in 3.
- Nakamura, Tomoyuki,Busfield, W. Ken,Jenkins, Ian D.,Rizzardo, Ezio,Thang, San H.,Suyama, Shuji
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p. 10987 - 10991
(2007/10/03)
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- Reaction of tert-alkoxyl and alkyl radicals with styrene studied by the nitroxide radical-trapping technique
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The reactions of tert-alkyl peroxypivalates 1 (R = methyl, ethyl, and n- propyl) with styrene in the presence of the free-radical scavenger (1,1,3,3- tetramethyl-2,3-dihydro-1H-isoindol-2-yl)oxyl (2) have been studied at 60 °C. tert-Butyl and tert-alkoxyl radicals (tert-butoxyl, tert-pentyloxyl, and tert-hexyloxyl radicals) were generated from the thermolysis of 1, and the derivative alkyl radicals (methyl, ethyl, n-propyl, and 4-hydroxy-4- methylpentyl radicals) were formed by subsequent unimolecular reactions (β- scission and 1,5-H shift) of the corresponding tert-alkoxyl radicals. The extent of the unimolecular reactions of the tert-alkoxyl radicals (versus addition to styrene) and the relative reactivity of alkyl radicals toward addition to styrene were obtained from the competitive addition/trapping reactions. The absolute rate constants for the addition of tert-butyl, ethyl, methyl, and n-propyl radicals to styrene at 60 °C were estimated to be (7.4, 4.7, 5, and 5.4) x 105 M-1 s-1, respectively.
- Nakamura, Tomoyuki,Busfield, W. Ken,Jenkins, Ian D.,Rizzardo, Ezio,Thang, San H.,Suyama, Shuji
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p. 5578 - 5582
(2007/10/03)
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- Initiation Processes in Copolymerization Studied by the Nitroxide Radical-Trapping Technique: Ethyl Vinyl Ether and Acrylonitrile
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The nitroxide free-radical trapping technique has been applied to an investigation of the initiation mechanism of the copolymerization of ethyl vinyl ether and acrylonitrile initiated by t-butoxyl radicals.In addition to a range of products normally produced from reactions with individual monomers, four new trapped products each involving both monomers have been observed.These arise because the strongly electron-accepting acrylonitrile reacts so fast with the strongly nucleophilic ethyl vinyl ether radical end groups that the reaction competes successfully with radical trapping. t-Butoxyl radicals react 3-6 times faster with ethyl vinyl ether than with acrylonitrile depending on solvent, illustrating the strong electrophilic nature of the t-butoxyl radicals.Reactions carried out in non-olefinic solvents show that polarity is not a major factor in the solvent effect.It is more likely to be due to selective interaction of one monomer with the radical end enhancing its electrophilic nature.A similar effect is caused by a hydrogen-bonding solvent.
- Busfield, W. Ken,Jenkins, Ian D.,Monteiro, Michael J.
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- A novel organic peroxyester as an exclusive source of tert-butyl radicals
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A novel peroxyester, 1,1,2,2-tetramethylpropyl peroxypivalate 1b, has been synthesized and thermolyzed in cumene. The kinetic data show that the decomposition rate constant of 1b is about three times higher than that of tert-butyl peroxypivalate. The radical trapping technique employing 1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-yloxyl has been used to study the decomposition mechanism of 1b. The results show that the thermolysis of 1b in cumene generates tert-butyl radicals exclusively.
- Nakamura, Tomoyuki,Watanabe, Yasumasa,Tezuka, Hiroshi,Busfield, W. Ken,Jenkins, Ian D.,Rizzardo, Ezio,Thang, San H.,Suyama, Shuji
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p. 1093 - 1094
(2007/10/03)
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- Initiation mechanisms for radical polymerization of methyl methacrylate with tert-butyl peroxypivalate
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The reaction of tert-butyl peroxypivalate (2) with methyl methacrylate (3) has been studied by the radical trapping technique employing 1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-yloxyl (1) as a scavenger. Thermolysis of 2 generated tert-butoxyl, tert-butyl, and methyl radicals in the ratios of 48:50:2 at 60 °C in 3. Both alkyl radicals underwent selective tail addition to 3. tert-Butyl radicals reacted about twice as fast as methyl radicals with 3. The absolute rate constant for addition of tert-butyl radicals to 3 was estimated to be 2.3 x 106 M-1 s-1 at 60 °C. The overall ratio of addition to H abstraction in the reaction of 2 with 3 was 5:1.
- Nakamura, Tomoyuki,Busfield, W. Ken,Jenkins, Ian D.,Rizzardo, Ezio,Thang, San H.,Suyama, Shuji
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p. 10824 - 10828
(2007/10/03)
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- The Reaction of Organophosphorus Radicals with Vinyl Acetate and Acrylonitrile in the Presence of an Aminoxyl Radical Scavenger
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The radical-trapping technique employing 1,1,3,3-tetramethyl-1,3-dihydro-2H-isoindol-2-yloxyl (1) as a radical scavenger has been used to study the reaction of diphenylphosphinoyl (2) and dimethoxyphosphinoyl (3) radicals with vinyl acetate and acrylonitrile.The phosphorus-centred radicals were generated by hydrogen abstraction from diphenylphosphine oxide and dimethyl phosphite respectively.Diphenylphosphine oxide was approximately three times as reactive as dimethyl phosphite towards hydrogen abstraction by t-butoxyl radicals and four times as reactive as tetrahydrofuran (towards abstraction of an α-hydrogen).Diphenylphosphinoyl radicals were found to be relatively nucleophilic and, in competition experiments, reacted about an order of magnitude faster with acrylonitrile than with vinyl acetate.Dimethoxyphosphinoyl radicals were rather less nucleophilic and reacted only twice as fast with acrylonitrile as they did with vinyl acetate.In the presence of excess aminoxyl (1), both diphenylphosphinoyl and dimethoxyphosphinoyl radicals were efficiently scavenged to produce stable phosphinic and phosphate esters respectively.The rate of scavenging was close to diffusion-controlled (c. 1.8E9 l.mol-1s-1).
- Busfield, W. Ken,Grice, I. Darren,Jenkins, Ian D.
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p. 625 - 634
(2007/10/02)
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- Unexpected Products from the Reaction of tert-Butoxyl Radicals with Acetylenes in the Presence of an Aminoxyl Radical Scavenger
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The radical trapping technique employing 1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-yloxyl (1) as a scavenger has been used to study the reaction of tert-butoxyl radicals with phenylacetylene, methyl acetylenecarboxylate and dimethyl acetylenedicarboxylate.With phenylacetylene, an aromatic ketone (5) was formed in which both the tert-butoxy group and the radical trap were bound to the same carbon, whereas the acetylenecarboxylates gave vinylamine and vinyloxyamine products.The mechanism of formation of these unexpected products is discussed.Acetylene and diphenylacetylene did not appear to react with tert-butoxyl radicals.
- Bottle, Steven,Busfield, W. Ken,Jenkins, Ian D.,Skelton, Brian W.,White, Allan H.,et al.
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p. 1001 - 1007
(2007/10/02)
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- Photolysis of Dioxiranes in the Presence of a Nitroxide Radical Scavenger: The Intermediacy of Radical Anion and Diyl Species in the Production and Trapping of Methyl and Trifluoromethyl Radicals
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Electron transfer between dioxiranes 1 and nitroxide 2 affords the dioxirane radical anion-oxoammonium cation pair, which generates methyl radicals that are trapped by the nitroxide.
- Adam, Waldemar,Bottle, Steven E.,Mello, Rossella
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p. 771 - 773
(2007/10/02)
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- Kinetics of the Coupling Reactions of the Nitroxyl Radical 1,1,3,3-Tetramethylisoindoline-2-oxyl with Carbon-Centered Radicals
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Radical clocks have been used to study the kinetics of the coupling of the nitroxyl radical 1,1,3,3-tetramethylisoindoline-2-oxyl (T) in cyclohexane or benzene with a variety of carbon-centered radicals including simple unhindered primary, secondary, and
- Beckwith, Athelstan L. J.,Bowry, Vincent W.,Moad, Graeme
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p. 1632 - 1641
(2007/10/02)
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- Initiation Mechanisms in Radical Polymerization: Reaction of t-Butoxy Radicals With Allyl Acrylate and with Diallyl Ether
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The radical trapping technique employing 1,1,3,3-tetramethyl-1,3-dihydroisoindol-2-yloxyl (1) as a scavenger has been used to study the reaction of t-butoxy radicals with allyl acrylate and with diallyl ether.With allyl acrylate, extensive H-abstraction as well as addition to both allyl and acryloyl double bonds was observed whereas with diallyl ether, the only products obtained were derived exclusively from H-abstraction.
- Busfield, W. Ken,Jenkins, Ian D.,Thang, San H.,Rizzardo, Ezio,Solomon, David H.
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p. 485 - 490
(2007/10/02)
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- The Addition of t-Butoxy Radicals to Halo-olefins
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A radical trapping technique employing 1,1,3,3-tetramethylisoindolin-2-yloxyl (1) as scavenger has been used to examine the regioselectivity for tail or head addition (kT/kH) by t-butoxy radicals to chloroethylene (5), fluoroethylene (6), 1,1-difluoroethylene (7), and 1,1,2-trifluoroethylene (8) at 30-75 deg C.Addition rates relative to that for ethylene (4) have also been evaluated.An atypical preference for head addition was observed for olefins (7) and (8).In the case of (7) the regioselectivity was shown to vary with temperature according to A comparison of the selectivity with that of other radicals indicates that t-butoxyl exhibits nucleophilic character in its addition to halo-olefins.
- Cuthbertson, Matthew J.,Rizzardo, Ezio,Solomon, David H.
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p. 315 - 324
(2007/10/02)
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- REACTION OF t-BUTOXY RADICALS WITH NORBORNADIENE
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95 percent of the reaction of t-butoxy radicals with norbornadiene occurs by radical addition followed by rearrangement to nortricyclyl and 7-t-butoxynorbornenyl products; the remainder includes a novel radical rearragement involving a 1,3-H shift and some radical abstraction observed for the first time.
- Busfield, Ken W.,Jenkins, Ian D.,Thang, San H.,Rizzardo, Ezio,Solomon, David H.
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p. 5081 - 5084
(2007/10/02)
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- The Reactions of t-Butoxyl with Unsaturated Hydrocarbons: Structure and Reactivity of Allylic Radicals
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A radical trapping technique employing 1,1,3,3-tetramethylisoindolin-2-yloxyl (1) as scavenger has been used to study the reactions of t-butoxy radicals with propene, 2-methylpropene, but-1-ene, (E)- and (Z)-but-2-ene, 3-methylbut-1-ene, buta-1,3-diene and 2-methylbuta-1,3-diene.Relative rates of double bond addition and of allylic hydrogen abstraction have been measured and are discussed, as are the relative stabilities of the products formed.The allylic radicals generated in these systems react with (1) mainly by coupling at the more substituted terminus of the radical ? system.Substituent effects on this regioselectivity may be explained in terms of the electrophilicity of (1).
- Cuthbertson, Matthew J.,Rizzardo, Ezio,Solomon, David H.
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p. 1957 - 1973
(2007/10/02)
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- Solvent Effects on the Reaction of t-Butoxy Radicals with Methyl Methacrylate
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Examination of the title reaction in a range of solvents shows that the ratio of hydrogen abstraction to t-butoxy radical addition increases with increasing solvent polarity.In several cases there is a competition between solvent and methyl methacrylate for reaction with t-butoxy radicals.The implications of these findings for polymer structure are discussed.
- Grant, Richard D.,Griffiths, Peter G.,Moad, Graeme,Rizzardo, Ezio,Solomon, David H.
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p. 2447 - 2454
(2007/10/02)
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- QUANTITATIVE STUDIES ON FREE RADICAL REACTIONS WITH THE SCAVENGER 1,1,3,3-TETRAMETHYLISOINDOLINYL-2-OXY
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Previously unobserved reaction pathways for the attack of tert-butoxy radicals on vinyl monomers have been found by a technique which employs the title nitroxide as radical scavenging agent.
- Griffiths, Peter G.,Rizzardo, Ezio,Solomon, David H.
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p. 1309 - 1312
(2007/10/02)
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- Initiation Mechanisms in Radical Polymerizations: Reaction of Cumyloxy Radicals with Methyl Methacrylate and Styrene
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Cumyloxy (1-methyl-1-phenylethoxy) radicals have been generated by thermolysis (60 deg C) of dicumyl hyponitrite in methyl methacrylate and styrene.The carbon-centred radicals formed by interaction of cumyloxyl with the respective monomers were trapped as stable adducts of 1,1,3,3-tetramethylisoindolin-2-yloxyl.Extensive hydrogen atom abstraction and methyl radical generation as well as double-bond addition were observed in methyl methacrylate.Styrene underwent only double-bond addition by both cumyloxy and methyl radicals.Some possible implications of these results for polymer structure are discussed.A kinetic study of the decomposition of dicumyl hyponitrite in cyclohexane at various temperatures gave k=7.7*1014exp(-13600/T) s-1 for the rate constant.Rate constants for the addition of cumyloxyl to methyl methacrylate (k ca.2*104 dm3mol-1s-1) and styrene (k ca. 2*105dm3mol-1s-1) at 60 deg C have been estimated
- Rizzardo, Ezio,Serelis, Algirdas K.,Solomon, David H.
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p. 2013 - 2024
(2007/10/02)
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