JOURNAL OF RESEARCH of the National Bureau of Standards-A. Physics and Chemi stry Vol. 64A, No. 4, July- August 1960

Gamma Irradiation of Hexafluorobenzene1 R. E. Florin, 1. A. Wall, and D. W. Brown r, (March 27, 1960) Mi xtures of hexafluorobenze ne a,nd benzene were irradi ated in li quid phase by means of a C060 cram ma ource at 20° a nd a t '2 18° C. P crfiuorohep tane a nd vari ous binar.v mixt ures involvi~ g perfluorohepta nc, hexaflu or obe ll ze ne, be nze ne, and cyclo hexane were a lso in'adi­ ated at 20° C. H exafluorobenzene rese mbled benzene very closely in its behavior upon r adio\ysis. Generall y t he fluorocarbon-hydrocarbon mixt ures evolved much mo rc SiF4 (indicatin g the formation of J-IF, which r eacts with the glass vessel) than the pure flu oro­ carbon components. The polymer from hexaflu o robenzene-benzrne mixtures was p r oba~ly rich in cyclohexadiene and cyclohexene un its, I' csembiing t hat from pu re benzene, a nd Its co mposition ratio exhibited a strong "altern atin g" tendency. The res ults a re discussed in terms of free-radical and excited-state mechanisms. r\ t 218 0 C hexaflu orobenzene a nd a lso its mixtures with benze ne showed qualitative difl'erences from their behavior a t 20° C, a ltho ugh t he G va lues for SiF4 and polymer rema ined moderate. 1. Introduction more resistant than hydrocarbons to heat, and also ftromatic compounds more resistant than aliphatic. Fully fluorinated aromatic compounds have onl y U nder ionizing radiation, ali phatic hydrocarbons recentlv become accessible. Because of their com­ give off relatively large amounts of hydrogen; bination of C- F bonds and aromatic reso nan ce aromatic hydrocarbons lose very l,ittle hy~roge~, structure the\- Iln.ve attrn.cted iuterest as possible but form polymers in moderate ywld . Ahphat!c heat-res istanl; mnterials. It has beC'n observed thn.t fluorocarbons su fl' er breaks in the carbon cham certain polyphenyls (0 6F4)X are stable at hi gh and lose slig ht to moderate amounts of fluorine in temperatures [1] 2 }wel that the mass . spectrum ionic form f7 to 9]. The arOlllcarbons have been studied f16 to 18] . In radiation of the liquid and characterized by infrared spectra chemical studies in general, atom and free-radical and elemental analysis. The hexafiuorobenzene mechanisms have bee n fairly well accepted f19], used was from a specially purified center cut of though seldom unequivocally proven. Besides the material synthesized in our laboratory l30] and great wealth of data on hydrocarbon radical re­ su bj ected to repeated fractional freezing. Its con­ actions f20], there have recently been a numb er~f stants were bp= 80.5° C/759 mm, nf3 = 1.3781. The studies on reactions of aliphatic fluorocarbon radI­ benzene used was the commercial product (c.p.) , cal [21 to 29]. not further purified. The perfluoroheptane used The present situation is that fluorocarbons seem was the product of Minnesota Mining & Manu­ 1 This wo rk was spo nsored by the Aeron autical Research Laboratory, "\'right facturing Co. and was used as received. The Air D evelopment Center, U.S. Ail' Force. . ' Figures in brackets indicate t he literature references at t he end of thiS pa per. cyclohexane used was an NBS standard sample, 269 For the irradiations at room temperature, the from ref [31] . Typical factors in ev g-INIr-l X 10- 20 liquids were dried several weeks over P 20 S in break­ were: C nH 2n, 0.623; C6H 6 , 0.589 ; CnF 2n) 0.526; and seal tubes and distilled on the vacuum line into C6F 6, 0.530 . weighing bulbs of 30-ml capacity. These bulbs were After irradiation, which required several weeks, fitted with standard taper joints and Hoke bellows the samples were brought to the mass spectrometer valves connected via copper or Kovar glass seals. whenever the design of the vessel was appropriate, Samples were made up by distilling in vacuo into and mass spectra taken with contents at -80° and irradiation vessels of several kinds, then degassing, + 25° C . Where the container was not adapted for cooling with liquid nitrogen , opening for about ] this, the contents were first transferred on an minute to a measured pressure of a few centimeters auxiliary vacuum line. :YIany of the Kovar-seal of argon, and sealing or closing off. 'iVeights of com­ containers in which hydrocarbon-fluorocarbon mix­ ponents of the mixture, obtained by difference, were tures were e(Cposed to high doses were broken during checked against the total weight of the radiation irradiation or subsequ ent handling because of cor­ vessel. Discrepancies of a few milligrams were rosion at the glass-metal seals. To calculate the common, probably because of absorption of the mass spectrometer results for gaseous products, volatile liquids in stopcock grease. reliance was placed upon the argon in troduced as The radiation vessels for use at 218° C were 3-mm an internal standard. For liquid products, the thicl,-walled glass tubes. At room temperature examination at 25 ° C yielded only relative values , nickel capsules of about 10- to 15-ml capacity were distorted by fractionation effects. The opened used. They were silver-soldered to 1. 5-mm x 40-mm tubes were emptied by suction, and the liquid con­ nickel tubes and then through 4.7-mm o.d. Kovar tent was frozen and sublimed at reduced preSSUTe to seals to long 3-mm o.d. Pyrex end sections which isolate the nonvolatile residue. The empty tubes were designed for openin g into the mass spectrometer wer e rinsed with benzene repeatedly, yielding small inlet system . In one irradiation experimcn t under addition al amounts of residue. The residues were a high pressure of hydrogen , a third type of radiation analyzed 3 for C, H , and F , and infrared spectra 4 vessel was used, consistin g of a Monel bomb, 12-mm were also taken in film s and Nujol mulls. o.d. x 6-mm i.d. x 300 mm in length, closed by a In the mass spectrometric procedure, bulbs con­ Hoke bellows valve. The simple capillary tube had taining a large liquid sample were connected to the the advantages of strength and small size but had a inlet system of the mass spectrometer before break­ large potentially reactive wall surface and also could ing the seal. Anal~7 ses of volatiles were then made not be opened for analysis without some exposure to with the sample bulb first cooled to - 80° C and then air. The composite capsules were mostly corrosion warmed to 25° C. The analysis of material volatile resistant, vacuum tight, and easily fittcd to the mass at - 80° C should give a reasonably accurate esti­ spectrometer but were very fragile after use. The mate of gaseous products for all samples in whi ch the existence of a partial internal glass surface had some liquid was compl etel ~7 frozen at that temperature. advantages as well as drawbacks. Any HF formed This includes all samples containing hexafluoro­ was converted to the easily measured SiF4, while any benzene, benzene, and cyclohexane only . However , reactive fluorocarbon intermediates of short life perfluorohep tane, although the reported freezin g probably underwen t further reaction in the liquid point is - 55 °, is often still liquid at - 80° C and system before they could diffuse to the glass surJace. therefore can hold large quantities of gases in solu­ It was noticed that corrosion was especially severe in tion. For samples containing perfluorohepLane, the special graded glass of the Kovar seal. The therofore, the mass spectrometric analyses at -80° C Monel bomb vessel had the advantage of strength may seriously underestimate the yields of gases. and all-metal construction but probably was subject This error will be greatest for the samples rich in to slow leaks over long periods. I t is un certain perftuoroheptane and for the higher boiling gases whether it would have remained tight "vith the valve such as CF4 (bp - 128° C), SiF4 (subl _ 95° C), exposed to high temperatures. C2F 6 (bp - 76.3° C), and CF3H (bp -84.4 ° C). The sample tubes to be irradiated were placed in a For similar reasons, the volatiles at 25 ° C have can and lowered into the uniform central r egion of a merely qu alitative interest. Products less volatile 2,000-curie cobalt-60 source consisting of upright than the starting material will appear in greatly re­ rods arranged in a ciTCle and shielded by water. duced concentration. The results will be especially Heat, when needed, was provided within the can by un certain for products of intermediate volatility, a thermostat furn ace capable of reaching 500 ° C. which are depleted in supply because of the previous Electrical leads were carried through a pipe leading analysis at - 80° C, but favored by high rela tive to the surface of the water. The exposure dose rate volatility at 25 ° C. was determined by the ferrolls sulfate dosimeter Polymeric residues in irradiated samples were using G= 15.5, and by a time correction for decay analyzed by combustion methods. In many in­ of the cobalt. The dose rate was 0.576 Mr/hr on stances, the sum of C, H , an d F is low by several D ecember 11 , 1956. The variations with geometry percent. The defi cit may be attributed either to poor accuracy of fluorin e analyses or to oxygen abo (mainly vertical extension) and container wall sorbed during the period between opening of samples shielding were significant only for the Monel bomb and all alysis. The polymeric product from irradi- containers. The absorbed dose was calculated 3 Combustion analyses were by E. R. Deardorff, N 13S. with the aid of best values for the elemen ts, derived , Spectra were obtaineci by John J. Comeford and W . J . Pummer, NBS. \ 270 .< atecl benzene is hi ghl:Y rea ct ive wi Lh oxygen [16], and carbon systems, unles equivalent amoun ts of CO or (,he relaLed material from fluorocarbons couldl'eact CO2 are observe d. The CO and CO2 were usuall y not only with oxygen bu t wiLh moistUl'e as well . obser ved only in very small amounts relative to The e r eactions should have been minimized in the SiF 4 , The reaction involvill g HF should produ('(\ pre en t work by th e facL th aL the longes t storage of water as a by-product: samples was in the crys talline or glassy form. IL > will therefore b e a sum ed in calculations that the 4HF + Si 0 2---? 2H2 0 + SiF 4. (5 ) toLal defic it is due to low flu orin e analy es. A nearly self-con is Le nL accounL call be given of Conversion of BF to SiF 4 is pres um ed to be nearly the composition of C6F6-C6H 6 residues by postu­ co mplete, although some fIxation in th e form of la ting combina tion of molecules and elimination of fluoride may possibly have 218° C. been present, but this could not contribute importanL 3 . Results and Discussion errors at the doses used in this "'Iork. All of the systems containing some flu orocarbon 3.1 . Hexafluorobenzene and some glass produced SiF4 • Froln pure hexa Ouorobenze ll e the observed prod­ In the capillary tubes containi ng C6F6 aL 21 8° C , ucts were a non volatile residue (L he so-called "poly­ this product ma:y have been formed by dir ec LreacL ion mer," table 2) a nd SiF4 (tables 3 an d 4) wiLh a little of excited mol ecules wi Lh Lh e wall. In Lhe composi tc CO and CO2 , which may have bee n derived from metal bulb reactors, the long diffusion path makes flu orin e a toms or molecules or unstable fluorocarbon it very unlikely th at shod-lived in Lermecliates of any intermedi ates. At 20 ° the release of fluorine was kind could r each the glass parts in significant qu an­ ) alm ost negli gible, G(SiF4)= 0.01 , but at 218° it be­ tity. Among possible agents attacbng the gla ss arc came 0.2 1 moiec ul es/ IOO ev. The y ield of polymer F 2, HF, and perh aps e peciaDy reactive fluorocarbon was abou t the same at both temperatures within molecules. the large ex perim enLal elTor

is produce d: T A B I, E 2. FolYllle?' f rom in-adiated hexajl!wroben zene and ben zene al 20° C · I I I I - C- F +-Si- O- -+ - C - O-+-Si- F . (2) Cli PS in feed , m ole fract ion 1. 00 0.667 0. 114 I I I I ------.------1----·-- 0 61" 6 in pol ymer, mole Crac iiol1 ______0. 922 0.527 0. 422 Polymer woight. ______. g._ . 705 l . 6822 .860 R eactions not producing oxides of carbo n arc also %0 ______. ______.. _. ______40.2 55. 3 6 t. 2 poss ible: % Il. ... ____ ._ .. ____ .. _. ______b O. 3 2. 2. 7 %F'c ______55. 38.4 31. 2 B ase 111 0I es . __ .... __ . ______.. ______. 00394 .00524 . 00731 llF 1110les lost per 1110Ic. ______. o .230 .296

u Exposure close 275 lvir. b Pres lI lll ed absent from fresh polymer. The rettction of F 2 with glass would apparently , For further calculations, F is taken as 100-0- 11. produce oxygen as a byproduct: T ABLE 3. R adiation yields from hexctjluoroben zene and ben­ zene at 20° C • (4) C II E' e in feecl, mole fraction 1. 00 0. 667 0. 114 0' This oxygen could react wi th radicals to form oxides ------1------0 61?6 in pol ymer, mole fr action _____ 0. 922 0.527. ____ . 0. 422 o of carbon or oxygenated fluorocarbon compounds. GCpolylTler)b ______.... _...... _. __ 2.01 2. i3 ______2. 79 '. 93 D espite the known slown ess of the glass-fluorine re­ GCC, /', units) .. __ .... _...... ___ ._. 1. 85 1.44. ______1. 18 GCIl l' lost)d .. _._ ...... _. _ .... ______0.628 ______. 826 action , it appears likely that it should occur in ap­ 4G CS iF.) •. _...... _.... _.. ___ ._... . 045 Presen t __ _ 1. 334 2G CI L,)_.. __ ._ . _.... _...... _. ____ . 0014 Lost. .. __ _ . J26 '. 088 ( preciable aillou n ts over the long radiation t im es. Even in view of the uncertainties implied by the • Exposure dose 275 J\ lr; close fa ctors for C, II" 0.589X I0" evjg·i\lr; for O,l<" 0.530 X I0" evjg- M r. a bo ve reac tions, it seems reason able to consider b C61L 6 and C6F6 wl its. each SiF4 molecule as derived from 4 HF in mixtures ' From Gordon et al. . , ref. [17]. d From polym er anulysis. with hydrocarbons, and from 2F2 in pure fJu oro- e V'rom gas an alys is. 271 l,,; TAB I ,}~ 4. Radiation yields fm1n hexajluorobenzene and ben­ are consistent with a C- F bond adjacent to either zene at 2180 C· an aromatic or an olefinic carbon atom. The C 6F 6 mole frac tion 1. 00 0.256 infrared absorption offers no reliable basis for a ------1---·------disti nction. 4G(SiF.) ...... __ ._ ...... _.. _. 0.84 0.105 o There appear to be no small fluorocarbon mole­ O(CO)._ . • _.... _.. _..... _._ •.• _.. . . . 054 .0063 o O(CO,) . .•. _...• __ ..... _._._._ ..••.. . 022 .0048 o cules analogous to t he C 2H 2 and CH4 found with G(1[,) .. _ • . _ •..• __..•...... _....•.• . o .00i5 . 0022 benzene. The similarity of C6F6 to C6H6 was strik­ G(C,I1,) . •. _..•• __ ...... • _••.. o .0048 . 017 O(polymer) b •• • • __ • •••••••• _ •• • ••••• 1. 3±0.5 1± 0.5 13 Ing- very low yields of volatile products, and a moderate yield of polymer; G(polymer) =2.01 for • Exposure dose 350 Mr. b C 611 6 and C6F6 tWitS. C6F6, and 0.93 for C6H6. that of the parent compound (table 2). The defi cit, It is recognized t hat llealh t he total effect of lOO- C- F - H , and H content may represent con­ ionizing radiation on organic 'matter is due to the tamin ations in handling, difficul t i e~ or If UeW ti tative secondary elect,ron;;; . 1'lwir first· p fl'p('t i s to form fluorine determination, or in the case of the deficit positive ions which can be important intermediates possibly oxygen absorption during s torage [16, 17]. in t hc gas'ph~se l32] but are more likely to recapture The quantity of the 218 0 C polymer was not sufficient electrons In hqllld phase and form neutral radicals for a llalysis. a nd atoms. Although the number of excited mole­ Infrarcd spectra of the polymer and a synthetic cul es (singlet and triplet) ma~' considerablv exceed perfluoropolyphenyl are compared in figure 1. Both the numbe.r of unex cit~d radicals formed l'19], it is have strong peaks at 6.6 a nd at 10.15 }J. , but the often pOSSIble to restnct attention to atoms and radiation polymer has a broader absorption generally radicals as the effective chemical intermediates. and numrrolls additional peaks at 5.7, 7.5, 8.8, Feng l1 3, 14] has briefly considered ionic in ter­ 12.8, 13.3, and 13.7 iJ.. .l\i(ost of the absorption bands mrdiates, pointing out that atom formation is

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FIGURE 1. Infrared absorption spectm of hexajluorobenzene mdialion polym er and oj perjluoropolyphenyl. a. C,F , radiation polymer. b. I·CC,F.),,·I. 272 encrgctically less favorab]c for carbon-fluorine than J n summar.\', it t herefore seems unlikely that for othcr carbo n-halogc n bonds, a nd other investi­ fl uorin e will be abstracted from either C6F 6 01' C 7F 16, gators [16 , 17] havc inLrociuccci cxcitcd states in thc exccpt p Cl'hops by " bot" atoms 01' radicals. Thus, eli cu sio n of lllc r aciioly is of bcnzcnc. th ere l'emn in s a radical mechanism with dis ocifttion, T.h c radiolysis of h'cxafiuorobcnzc nc offcrs fcw addit ion , ,wd rccombination steps, yielding It poly­ novel ti cs bcyond i Ls hy ciro ca rbo n analoo.; both lhc mer lnl'gcly nonaromatic. The very low yield of frcc-racii ca l mechanism s a nd cxcilcd-slatc mccha­ SiF4 requ ires t hat C 6F 6 s llOuld b e a very effi cient nism s [16 , 17] secm admissiblc wiLhliLtle choi cc. tnl p for Ii' atom s. If the F atoms are formed in an The o ll t ~ illC S 01' a frcc-radical mcchanism , following effic ient cnge of C6F 6 molecules, r eactions (7) and Burlon's [16] trcatmcnt of ('6l-T6' would be ( 10) Cl1 n preciom i nate over reaction (13)

(6 ) (13) l wi t110lJ t r equiring ~\lly great iJleq uali ty of rate C6F 6+ F·- C6F 7·, (7) cons l;wts. Gordon a nd others [17 , 40] IhLVe written ~L mech­ RCoF 6· (R = C6F ;·, C6F 7·, C6F 6C6F ;.), (8) a ni sm for C6H s ntdiolysis involving excitcd sbttes R + R RR, (9) only. The Sl1me mec1Janism C'ft n be wri ttcn for C6F 6: (14) R + F· RF. (10 ) (15) Rcactions (7) and (8), by 2na logy with lhc hydrogc n aLom-bcnzene J'caction, pro ba.bly have an activation Hydrocarbon analogs of the dimer have been re­ e ncrgy of sevcr al kiloca lories at m ost. The activa­ ported. An advantage of t he excited-state m echa­ Lion energ,v of reaction (8) would be reduced for ni sm is that the nearly complete abscnce of SiF4 C6:B-\ in a n exciled state. Thc low yicld (G= 2.01 ) (derived from corrosive fragm ents) is explained docs not )'cquirc a chain reaction ; however, the impl~~ when fragmen ts arc not formed. struct lire of the polymer (less volatile til a n bi phen)'l , At hi gh er temperatures (218° C) the arg ument melling below 100 0 to a modcrately yiseo ll liq ll id) 11gn in s t the ]'adic~ll mccha ni sm docs not apply , as requires a fcw addition stcps like reaction (8) . Xo C'o ll sid embly m ore SiF4 is formed. The ~t ct u,tl b e­ abstraclion reaction has bccn introduce d: hl1 vior of C6F 6 at hig her tcmpeJ'

Presumably, the fluorine atoms of aromatic fluoro­ T AB L E 5. R adialion yields j1'01l1 hexajluo1'obenzen e and hyd1'o­ carbons are likewi se resistant. For the abstrilction gen 20° C" reaction HF+ CF3· (12) C,l',

4G (SiF ,) ______0.045 0.440 E > 17 k cal, and the r eaction is not observed up to G(CO,) ______0 .004 .004 400 C [36]. G(CO) ______~ _____ ~ ___ ~ ______~ ______.0012 o G(poJ y mcr ) b ______~ _____ ~ ______2.01 2.30 For C 2F 4 [38J and C6H SF [39] r eacting with H atoms, th e eviden ce is for an effi cient addition rather th a n IL Exposure dose 319 Mr; hydrogen pressure 34 atm. abstraction . b 0 6 1;'6 units . 273 The dose was 319 Mr. Much of the vapor was in a The formation of HF can also be accounted for less intense radiation fi eld. For the calculations in by excited-state mechanisms such as in reactions table 5, it is assumed that all radiation was absorbed (20 ) and (21 ); there is, unfortunately, no explicit by C6F6 as liquid. literature for comparison. The attempt to acco unt The value of G(SiF4) is 0.11 as against 0.01 for for HF without C6F5H remains somewhat uncon­ pure C6F 6 and 0. 33 for a mixture of C6F 6 and C6H 6. vin cing, as reaction (2 1) could be followed b!T a Conceivable steps producing the HF may be: combina tion of C6Fs and H .

F· + H z----+HF+ H ·, (16) 3 .3. Hexafluorobenzene and Benzene

The data for these mixtures are shown ill tables 2 and 3. For comparison , results on C6H 6 are repro­ duced from the work of other investigators [17] . The polymer analyses are low by 4.3 to 4.9 percent. Since fluorin e analyses tend to be low, it is assumed C6F 6* + H 2----+ H 2* + C6F 6, for calculation that the true fluorine valu e is that '" (20) obtained from the difference between (C plus H ) 2H- and 100. Some of the defiCit may, however , have arisen from oxidation of th e polymer prior to analy­ C6F 6*+ H 2----+HF+ H, + C6F5' (2 1) sis, which could have lowered the C, F , and H, and simulLaneously introduced 0 and some H. To These reactions must compete with addition reaction arrive at O(polymer) valu es, it was assumed formally l (8 ) and must be roughly comparable with reactions that polymer is produced b y withdrawing x mol es of for HF production in hydrocarbon mi xtures , e.g., C6F 6 and y moles of C6H 6 from the liquid and reject­ ing z moles of HF. If Hand F are lost in other forms (22) and unequal amounts, small inconsistencies arise. Any I-I atoms produced in reactions (16) to (2 0) On the basis proposed, one can calculate yields of may react by addition or abstraction: each type of unit in th e polymer and of HF lost per unit. H· + C6F 6---+C6F 6H., (23 ) moles C %C X weigh t H · + C6F r,---+C6F f, + HF. (24) Base moles poly mer = x+ y = 6 = 100 X 6 X 12 ' I The raciiatioll received in the vapor is relatively I Moles HF lo st ( unimportant, estimated at 16 X 1020 ev as compared with 554 X I020 ev in the liquid. In the liquid, the Base mole polymer H2/C6F6 mole ratio is about 0.01, as against much =z=3 {mOles C - moles F - moles higher ratios in the mixtures with CBH 6 and C6I-112. H1. , For comparable HF production in the two cases moles C ) this would require that k16 be considerably gr eater than k22 • For reactions (16) and (22) the activation Moles C6F 6 :r energies may be near 6 !ccal/mole or less. Some­ Base mole polymer x + y what analogous reactions 'with chlorine are: = ! {moles C + moles F - moles H} . CI·+ H 2---+HCI+ H .; E "",, 6 !ccal [41], (25) 2 moles C

CI· + CH4---+CH 3·+ HCI ; E < 8 1<:cal [42]. (26 ) For all Lh e mixtures, G(polymer) is higher than for Of th!.' other reactions, (17 ) should be discounted either pure component, and G(SiF4 ) is very much because of the failure to find C6FSH experimentally . larger than in pure C6F6 (see fig. 2). The SiF4 was I Activation energies for reactions related to (17 ) and almost certainly formed from HF. The color of the (18) have been es timated [28]: polymer solu tio n became darker with increasin g C6H 6 < content, and the polymer during frozen benzene (2 7) evaporation remained stiffer and spongier, n ever collapsing to a clear glass . For the various hydrocarbons RH the aetivfttion The comuosition of the polymer remained ncar energies of the reaction in kilocalories per mole are 1 : 1 for C6H 61C6F6, even for wide variations in feed C2H 6, 7. 5; C6H 6, 7.7; H z, 8.8 ; and CH 4 , 10.3. Al­ ratio (table 2, fig. 3). At increasing C6H 6 feed though the basis of the estimates has been criticized content, the H/C and F/C ratios of polymer gradu­ [2 0], comparable work [23 to 29] is consistent with ally declined, refl ectin g the increased removal of H values somewhere near these. The abstraction and F men tioned in connection with the SiF4 yields. reactions arc generally expected to be slower than As with the pure C6F6, the mixture did not show any the additions to C6F 6, as diseussed earlier for pure CF4 or C2F 2, and only the mixture with the high C6F 6• C6H6 content, 0.886 mole fraction, showed CH4 or 274 4.------,------,------,------, (30)

3 PolariLy difl'erenccs often fa VOl' addi tion of unlike u lli ts. III parLial suppor-L , Szwarc [21 ] finds the >­ U m ethyl affl1l iLics of C6F6 and C 2F 4 to be 14 and 10 :J o o 2 times greater than Lhe meth yl affinities of C6H 6 and a: 0.. C2H 4 • The co rrcspo nding CF3 • affinities do not C> scem Lo be known. For Lhese low-molecular weight polymers, favorable crossed Lerminations could also influ ence the polymer co mposition. There are several pos ible m echanisms involving exciLed states. Formally, a "crossed" generation of excited molecules co uld be favored by an energy transfer mechanism [43]. Against this concept is t he fact t hat the overall form ation of po l~' m e r is .2 5 . 50 .75 1.0 n ea rl~r independent of chn,nging co mposition. I t MOLE FRA CTIO N C6 F6 has n.lso been pointed out in criticism that n, sym­ F ie U RE 2. Radiation yields .f1'0 1II C6F6+ C6II6• metrical mutual energy transfer should not OCCLlI" • • G(Dolymer. total). O. G(polymer C,Il, units) • ..... G(pol ymer C, F, unils). very gell crally [40] . Thus, an initial formation of tc, ,4G(SiF.). triplet excited states or radicals of t he two species ill equivalenL ailloun t by mutual transfer of excita­ I.OO.------,------,------r------, tion is unlikel:". However, t he cbemical reactivity o of triplet excited states may depend on sO llle of the same co nsiderations whi ch apply to free radicals, l\ mong which polarity differences arc included. As in t he other cases, a decision between t he t riplet .7 5 state and radical lllcc hanism is difficult . The ob­ ~ vious qualitative difference is thaL molecules arc d is­

O o------~------~------~------~ At low co nversions, Lhere are mil.l1:V more C 6 mole­ o .25 .50 .75 1.0 MOLE FRACTION C.F. cules t ban an ~ ' other species. The fact that the polymer formed is mainly higher than Cl2 indicates FIe UFtE 3. Composition of polymer from C6F6+ C6H 6• that the C 12 species once formed must retain chemical activity. A radical-addition mechanism allows this ( C 2H 2 . Surprisingly, neither C6FSH nor C6H5F was to occur in a self-evident way: found. The behavior of polymer composition is reminis­ cent of vinyl copolymerization with a strong alter­ nating tendency. By analogy, a mechanism can be In a pure triplet-state m echanism, it is not evident written in wh ich large " crossed" propaga tion ra te t hat the C 12 species would remain in an excited state constants are responsible for thc composi tion , i.e., with a long lifetime. Possibly excited C6 could where k 29 , 1;: 31> k 28, 1;: 30. transfer excitation preferentially to ground-state C12 molecules. Both the relative absence of fragments and the growth of larger species could be explained if excited molecules initially combine to form a bi­ radical which subsequently grows b~- ordinary' radical addition. 275 The high G(HF) in mixtures could be formallv is not very different from that of C6F6 and benzene. accounted for by a Bagdassarian [4 3] or Magat [40] The polymer composition can be discussed by an excitation mechanism . An atom and radical mech­ extension of the method used for C6F6 and C61'16, if anism can account for this feature with emphasis on C- C chain scission products can be neglected. Let the steps: each C6 unit of polymer be b uild from} moles of C6F6 C6F6---->C6F 5+ F·, (6) and (1-}) moles of C6H 12, rejecting i atoms of F and j atoms of H. Then F · + C6H 6---->C6H5' + HF, (22) F i F· + C6H6---->C6H6F ·, (34) 1=0+6'

F· + C6F 6---->C6F 7'. (7) F 1>­ - 0 J H ere k 22< k7< k 34' but all are of substantial magni­ t ude. To Illustrat e, HF+ C6I-Ill·, (35) Fz or HF. Thus, the G(SiF4 ) suggests no change from mixtures at 20 0 C, whIle the insolubility and R + C6F6---->HF+ C6Fs" (24 ) color suggest more HF elimination. The dark color .c1 of the polymer from mixtures does suggest som e As discussed earlier for hexafluoro benzene and h vdro­ conjugated unsaturation even at 20 0 C. The prin­ gen , the evidCll ee for reaction (2 4) or any abstraction cipal remaining anomaly may then be the rela tively from C6F6 is weak ; for example, in the failure to high G(SiF4) from pure C6F 6 a t 21 80 C. This could detect C6FsI-I here. For the same reasons, radicals be a ttributed to union of F atoms as Fz, to more from C6H 1Z are not likely to abstract flu orin e to form efficient escape from a C6F6 cage to the wall, or to alkyl and cyclohexyl fluorides. The faillll'c to find the onset of C- C eleavage like that which produced RF is no t quite conclusive because C6H Il F is rather C ZH2 from benzene. The CZF2 and similar fragments unstable and small er scission products of any one could be reactive with the walls of the apparatus. kind would be small in amount. On the whole re­ action (35) is preferred as th e source of HF. 'The 3.4. Hexafluorobenzene and Cyclohexane mechanism of protection may still be either of the sponge type or by addition, as with C6H 6. From C6F6 and cyelohexane, the principal products were SiF4 (G= 0.322 ), H z (G= 1.92 ), and polymer 3.5. Per£luoroheptane (G=3 to 5) (see table 6). The G value for the SiF4 The data of table 7 are rather uncertain because T ABLE 6. R adiation yields from hexajlu01'obenzene and of the large number of possible products and the cyclohexane at 20° Cs general ambiguity of mass spectrometer analyses for saturated fluorocarbons. All of these compounds C6F6, 11lOle fraction Ob 0.192 0.234 0.655 furnish larg.e amounts ?f fragmen t ions, especially

4G~iF. ) ______0 ______(0) ______1. 29______(') . CF3+, and htt.le paren t lOn. However, the G values O( ,)--______5.2,5.9 __ __ (0) ______1. 92______(' ). for Si];-\, CF 4 , and perhaps C2F6. are more reliable, as O(C H .)-- --- ______0.09, O. 02 (,) ------0.005______(' ) . O(C,H .)------0. 21,0. 14 __ (,) ------0.007..______(') . thes e compon e ~ ts were determll1ed at -80 0 , where O(polymer) d ______1. 66 b __ __ _ 4.9 ______6. 1______3. 1. 0. 336 ______0.368______0. 479. most of the higher fluoro carbons are no t vola tile I. 279 ______1.1 48 ______0.925. enough to interfere. The reported G value may, Poly~~l~ m~er,~ ~ n~mole :-:~~===== fraction _ ~ ======:::: C6F 6______0 ______0. 34 to O. 36.. 0.37 to O. 43.. 0. 48 to O. 54. however, be low because of differen tial solu bility of gases in the liquid a t -80 0 ; by contrast, bo th a Exposure dose 174.5 M r. d 0 6 units. C6F6 and C6H 6 have high freezing points and at -80 0 b Rcf[191, pp. 18 a nd 20 . e M ole ratio. e Container fai led before analysis. would have crystallized, expelling dissolved gases. 276 T AB IJI, 7. Radial'ion yields fr-om hexajll101'obenzene and R ccO Ill bina tion : )J eljll1 01'ohe)Jlane al 20° C • (13) e ll 10' 6, 1II0\(' fraction 0.3R 0 .i4 1.00 (39) ,W (S i l",), . " 0. H(! ~ 1. 202 " ..' 0. 052. 0.0'10 GCCO,) .... h. O:lH 0.025 b •. _.. 0.000. .0012 GCC I·',).. . 125 0. 131 • 0. 085 o R t' + Rr (40) G(C, I', . h, 08 1 0. 122" _.. 0. 008.. . o GCpolYlIler) ' . . 2. 0 0.2 to 0.8 _ 0.0 to 3.5 ... 2.01 Poly mer FiCd.... 2. 102 I. (;24 • I. o()l .93 Dis propol'tionation : P olymcr moll' fr action o < 0.55 . _'. < 0. i 5. C, I', . (41 ) • " xposurc dose 030 to ·108 .\1 1' ; d ose faclor for C, I'" 0.530X I0" e vir. M r; for C; 1' " , 0.,126. b .\l ay be low becH llse of solu hility . c A s (;6 Or C7 units. (42) d ~ I ole ratio.

Thc ch aracLcl' Or thc polymcr was similar to that Radical attack on F 2 : repo rted b~ ' ol h('l' \vo l'kers. :Mos t boil ed in th e range 179° to 250 °, and the material was a viscous liquid Hc + F2-->R i F + F · . (43) at 4° C. For comparison , n - C I ZF Z6 boils at 175° 'l' ransfcl': and fl'cczcs aL 42°, and n - C' 6F34 boils at 240° ancl (44) freezes at 11 50. Thc failure of thc sample to fl' (, czc may indicaLc branchcd s tructure, but i not VC I'~T The transfcr J'C' iCet ion (44) is unlikel,Y bccausc of conclusiyc in a mixtu)'c. c ncrge ti cs, as prc"iousl," cl iscu sed . Dis proporliona­ In addition to thc data of table 7, the analy es Lion (4 l ) and (42 ) wou ld gc neratc olcfinic molccules of liquid in tablc 8 ar c of qualitativc interest , although a nd so should bc unimportant hcrc ; m orcovcr, t hcre boUt Lll e amounts a nd idcn tities quoted arc subject is co nvi ncing expcrimc ntal cvidcnec against rcaetion to largc uncertaintics of int(']'prelati on. Pl'c s um abl~ ' , (41) [2:3 to 29]. Thc ]'cma.i ning ]'cactio ns, cou pled wi th ma,t('l'ial balance, all rcq uirc CO Jl scrvalio n of TAB I.E 8. Liqll iri phase f rolll radiolysis of h f.ra/ill oroben zene Lotal numbcr of molcculcs, a nd t hus a compcnsation anri perjlll oroheplan e • bctwccn product molcculc largcr a nd smallc]' than C\. Largc]' molccuies wC]'C found in thc distilla tion C7 F 16 prr parNL mole frac tion. 1. 00 0. 62 C;F ., found _ m olc %_ 9.9 12.3 rcsidue although no t b," thc mass s pccLromctcr. C, F, fo und . mole 0/,. o 9.0 C 6F 14 fO lll1(L _ mole 0/,... i .O 10 . 2 To cxtcnd thc co nclusion about co nscrvation of Cj P 12 b found __ mole 0/, (i. 0 5. 7 numb('l' of moleculcs from ( '71"16 to hi g h polymcrs C,F, found . mole 0/, 9. 0 1. 5 C , F, found . . m ole 0/,-- 2. 19 ~O a ppcars inCOll sistcnL with thc familial' rapid dcgrada­ tion or pol,l'tl'lra([uoroc tity ien c a nd pol,"cltlorotri­

Il Sec ta ble 7, foo t n OLl' 0., for radi a l ion C'o ndit ions. Salllpies a nalyzed hy BlaSS (iuorocthylc nc. Thc diffi culty could be 111Ct b\' as­ s pe-c tra of va por at 25° C. b Or eye lo·C,l riO. scrt ing that suffi cicnt oxygcn 01' h.wlrocal'bon m a­ Lcrial was pl'C'scntin all polymcr cxpcl'imcnLs to com­ fractionation cFreels would wcigitt und ul,'" tite lowcs L bi nc or u nd('l'go H absLraction wi t h lltc polymer boiling co mponrllts a nd rninimizc tilO SC abovc ( '7. radicals a nd thus lowcr Lhc molcc ul ar wcight ; or that The valucs of G(CF4 )= O,19 5 and of G(SiF.)=0.167 tite radicals wcrc trappcci in thc solid matrix and fol' C7F I6 arc high rclativc to thosc for C6F 6 (table 7). co uld then ulldcrgo various othcr reactions ordinarily Thc main products ar c thus SiF4 a nd hig itcr and of low probabilit.\' ; or thaI, a relatively Ycr," smaIl in­ , n lowcr aLurated Duo]'ocarbons. The mass spcctrom­ crcase in thc llumber of molecules a nd doublc bonds eter find almost no olefilJic molecules. It is occurs in all cases but is easily noticcd only in Lh e possible that more sensitive and reliable indica tions p olymcr. In this connection it h as becn ]lo'Li ccd re­ wo Id b e given by infrared or b~' bromine or pe1'­ ccntly that pol.Ytetl'afluoro e Lh~'l e n c irradiated in manganate titl'ations . Stoichiometr.,T rcquires that vacuo is degraded ver y muc h lcss rapidly than in air the extra HUOl'inc co n tent of the C, Lo C6 perfiuoro­ [45]. Nevertheless, a . thorough sear ch for olefinic paraffins a nd the loss to SiF4 b e compensated either molecules in il'J'adiatecl perfluoroheptane would be by equ ivalen t condcnsation to highcr p erfl uoropar­ desirable. affins 01' formation of double bonds. The following The set of possible reactions is still too complicated r eactions arc to be consid ered, where ratcs can val',V for easy trcatment, even when simplified by assump­ with the s ize of th c radicals R i ancl Hj : tions such as ra ndom splitting and equal reactivity of all radicals. Substantial amounts of all perfluoro­ C- C scission: paraffins C 1 to C I4 co uld be expected. By assuming (36) t hat all C- C and C- F bonds split with equal prob­ ability , a nd that aU lower radicals disappear onl~~ by C- F atom splitting: reacLion with F atoms with equall'ate constants, one arri ves at a n j ni Lial fragmcn t distribu tion:

F : prim- ('7 : &ec- C, : all lower alk~' l : CF3 = 16:6:10:12 :2

548228- 00--2 277

-~------n nd a product distribution in which all lowcr per­ fluoroalkanes occur equally and G(CF in styn'l1e was found to promote polymerization wi th a rartial elec­ F ·-I- F · (13 ) tron-fraction G valu e of 20 ± 10[46]. The present C7F J6 environmen t should be in so me ways similar. F ·-I- CF3·-->CF 4 (for i = l ), (39a) If the high estima te of G(CF 3H ) of aboll t 2 is correct for C 7F I6 in benzene, it ma ~T be consisten t 2C7F Js,-->C14F30[ (for i, j = 7), (40a) with the polymerization G= 20 for C7F I6 in s t ~7 J' en e, for th e various modes of C- C and C- F sci ssion 2C6FI 3,-->CI ZF z6 (for i, j = 6). (41a) can form many other radicals as well as the CF3. It can be seen tha t these reactions account for much If the low valu e in table S is more n ea rl~T right, then the high results wi th styrene may require a of the product SiF 4 , CF 4 , and hi gh-boiling residue. spec ial energy-transfer effect. In the radiolysis of 3.6. Perfluoroheptane and Benzene mixtures of C6H 6 wi th C6H 5CF3, CF 4, and chloro­ fluoro carbons, F eng [1 3, 14] has repo rted C(radi cais) The mixture of perfluorohep tan e and benze ne WH,S of the order of 1 by the DPPH di sappearance method h eterogeneou s bu t of considera.bl e qualitative in­ in contrast with the high G(l'adicals) usually ob­ terest. The prin cipal products from irradiation served foJ' other halo carbons [40]. Although F eng's were SiF4 (at a G valu e abou t twice that of pure CF 4 eff ects were observed at unknown high dilution C 7F J6), polymer , numerous lower fluorocarbons, and and the difference from pure C6H6 was cl ose to the CF3H (table 9). The last named compound had a experimen tal enol', his a valu es for C6H 5CF3 and G va.hle of 0.15S by calculation from analyses at CF 4 in any even t were J10 t large, and thus differ - SO°C ; however, th e sample, when later warmed to from the case of th e chloro carbons. Considering 25 ° C, con tained large amounts of CF3H in the vapor all the evidence, it seems best to suppose that our phase, and a rough estimate from the 25 ° C analysis true G(CF 3H ) is considerably less than 2, that suggested a G(CF 3H ) in the region of 2. G(radi ca.ls) is usually low for fluorocarbons, a nd

T A BLE 9. R adiation yields f rom hydrocarbon-perfluoroheplane that the high G(radicals) = 20 for C7F I6 in styrene mixtures at 20° C • polymerization may be due to special energy transfer eff ects, valid for styrene but not benzene. C,F ", mole fr action .. 0. 71 6. ___ _ 0.560 __ .. _ 0.685 .. ___ 0.188 __ . __ 0.213. It was further renortecL by Feng that the irradia­ Second com ponen t. __ C, Ho____ _ CoH o_____ c-CoH I' ___ c-CoH I' __ _ c·C,lfl ,. Dose ______Mr __ 339 ______174 ______339 ______339 ______li4. tion of CF 4 and C6H6 produced C6HSF and C6H 5CF3, 4G(SiF.) ______(b) ______2.62 __ . ___ (b). __ ___ (b) ______(b) . G(H ,) ______0.09 ______detected by infrared, with G valu es risin g to 1.5 G(CF 3H ) .. _ ------O.I 58 d ___ ------[1 3, 14]. In th e presen t mixture, C6HSF was not G(CF,) ______0.189 __ . ______G(C,F.) ______0.021 d ______found, al though its formation by combination of G(C,F ,). ______0.017 d ___ ------. G(CH.) .. ______0 ______F + C6H 5 is no t unreasonable. In the presen t G(C,I-I,) .. _. ______0 ______... __ _ study, th e C6H5F would have been associated with G(pol ymer) 0 ______4.2 to 4.9_ 5.8 to 6.8 2.7 to 3.L 2.8 to 3.2_ 4.7 to 5.4 .' P olymer H /C . _. _____ 0.340. _. __ 0.305_ . . __ 0.866 _____ l.326 ___ __ large proportions of unchanged liquid and perhaps Polymer F/C ______1.337 ____ _ 1.357. __ __ 0.951. ____ 0502 ____ _ Polymer, mole frac- no t detected with high sensitivity. The ref- orted tion C,1'I. ______0.55 to 0.56 to 0.38 to 0.1 9 to high G value of C6HsF from so dilute a solution of 0.62. 0.66. 0.53. 0.3e. CF 4 is surprisin g. Possibly other compounds, • All samples have two liquid phases. such as polymer stru ctures formed by addit,ion, b Large; failure through glass seal corrosion . could have absorbed at C- H and C- F frequencies , In C. or C, nnits, from weight and carbon analysis. d May be m uch too low; large content rem ains in room temperature analysis. close to those of C6HsCFa and C6HSF , with a very • From weigh t, assuming analysis of preceding col umn . large absorp tion coeffi cien t.

Inasmuch as CFaH boils at - S4°, and C 7F J6, the major component of the mLxture, is sometimes liquid 3.7. Perfluoroheptane and Cyclohexane at - 80°, it is no t unreasonable that most of the CF3H present a t - SO° would be h eld in solution in The mixtures of perfluorohep tane and cyclo­ the C 7F 16. For the same reason , G values for C2F6 and CZF4 should probably also be much higher. The h exane were heterogeneous. The only da ta ar e CF3H indica tes a clear-cu t abstraction reaction from those upon polymer, table 9, as all con tainers wer e -C 6H 6: broh:en by corrosion of glass seals, even after rcla- 278 tively small doses of the order of 70 Mr. Presum­ in the gas phase would be interestin g for comparison · ably the yield of HF Wf\,S con idC'rably higher than Even less C- F scission than that found here is any reported in the tables. Comparison of the .ll ggested by the faeL that Simons and T aylor [5], polymer yields in column 4 and 5 suggest that lJ'l'adlatmg perfLuoroahphatlC co mpounds in all­ G(polymC'l' ) declin e wilh in el' easC'd dose. ahllnl·lum containel' , found no evidCllce whatever of corrosiv e fluorine. 3.8. Perfluorobenzene and Perfluoroheptane A ~ id e from differ ences of purity 0 1' analytical senSitivity, both se ts of observations appear con­ 'With perfluorobenzen e and pel'fluoroheptan e (table sistent with the exis Lenc e of a small steady-state , 7) there appears to be a n initial ri e in G(C 2F6 ) concen tration of F 2 which d isappeared in Olle in­ as the C6F6 cont ent is increased. This is believC' d sta,11c e by diffu sion to the glass parts of lhe apparatus to be all a rtifact due to sob.! biliLy, as the freezin g or a.net conversion to SiF4, and in the other by attack of C6F6 WIU concentrate the gaseous solutes in the J'C'­ fluoro carbon radicals to form lower perfluoroalkalles. maining liquid and vapor. The data on SiF4 and Some minimal C- F sc ission seems necessary to CF4 are subject less strongly to the same kind of accoun t for the considerable amOli ll t of C13 ancl C 14 errol', which may serioll ly underestimate Lhese couplin g products from C 7F16. Irntdiated poly­ products where much C 7F 16 is nrescnt. There is at t e tl'afl~loroethyl e n e. seems to undergo C- F scissions least no ll'On g "protective" efJ'ect. Sin ce F atoms excluslvely, accordm.g to electron reSonallC(' obseJ'Ya­ should be prC'sent, it appears that they combine lions [48]. This behavior is again consisten t with a with each oLh er and wi th alipha tic raciicals more cage efreet, as F from a C- F sc is ion can d ifl'use r e ad i l~- than lhe.\T add to C6F 6. 011 the other hand away, while the radical pail' from a C- C sc ission a l'fwicl addition of F to C6F 6 is needed ill the radical is held more rigidly and recombines. mechanism for C6F6 radio lysis to explain the low So me of the early indications of fluol'ocarbon SiF4 yield: This inconsisteJl cy may cal] into question scnsitivity \~ ' e r e du e to the p J'e S{' 11 ce of oxygen . any purely radical mechanism for the radiolYsis Recent stuclws of the tensile strength ofirradia.ted of C6F 6 and favor a triplet-sLate mechanism Lh·e re. poly tetrafluoroethylcl1e show th e loss of ten sil e is A similar argument may appl.v to th e r a diol~Ts i s of ver~T I' an idin th e prcsence of oxygen and hardly 7 C,JI6, which exhibits a Pl'otective efrect less drasti c perccptible for long p criods in its absence [45] . The than might be exp ected from th e radioh-sis of the strong oxygen eff ect is rem ini scen t of th e cl egracla tion pure componenl. The very low hy dn)gen yields of ve ry pure chlorinated comrounds exposed to from pure C6H 6 may be due maillly to fa ilul'e to light, a il' , and moisture. For fluorocarbons tmder snli t off h .nll'ogen atoms, I'ather tha1:' to rapid reac­ irradiaLion it may b e speculated Lhat th e radical tion with them . The yield of radicals from C6f1 6 recombination rate is somewhat slower than for detected bv ordinary methods is somewhat low ' hydrocarbons, allowing more efl' eetive competition C= 0. 33- 0.89 b)' ioclim [47] a nd DPPH di sap ~ by oxygen reaction . pearance [40]. ])ossibl y it is also of about this same Aromatic fluorocarbons have th e Sfllll e kind of magni tude in mixtures where C6II6 exll i bi Ls a pro­ rC' sisLan cc to ionizing r H.C\i atioll ItS the aromatic teclive efrect. The "sponge" mecha ni sm of p]'o­ h.\'dl'ocarbons, yiclding vel')Tlittl e gas and a moderate ~ectio n ill C6H6 ~lix tur es has been di scussed r ecell tly HIl1 0un t of low polymer. O(polymer) is 2.01 for In terms of relatIOns between excited states [40]. C6F 6, ~LS ag~in st O. 93 for C6H6. The polymers from Th (' failure of pro Lee Lion in the presen t mixture both matenals are close to the s t~trtill g m~tteJ'i~LI in suggests that protection, whel'e it occU)'S, is of Lhe elemental analysis. There has been some specul ation sponge type and ]lot due to extreme reactivity of th e in the literature conccrning tbe degree of arom atic aromatic )'ing with atoms alld. radicals, and that the c h fL r ~Lcte l' present in perhaloaromfttic compounds c ~l aracte ri st i cs of aromatic radiation chemistry (con­ [49]. At least those aspects of aromatic character Siderable polymer, very little hydrogen or halogen ) concerned with radiatioH resistance seem to rcmain denend more upon reactions which proceed via in the totally fluorin ated analog. triplet states than upon atom and radical reaction s. R ecalli ng the consid erable resistance of poly­ styrene to radiation, one might predict a similar 4 . Conclusions resistance in polymers containing perfluoroaromatic I groups. The d.ata presented here show that repJ'esentfLt ive Experimentally, poly(2,3,4,5,6-pentafluorostyrene) I pure liquid fluorocarbons are no t especially sensi tive ha.s fL G value for free mdicals observed by electron spm resonance, almost as low as polystyrene itself toward ionizing radiation. In the paraffini c ser ies [50], which suggest that the general radiation resist­ the indicated C- C scissions are a bou t equal i 1~ ance might be similar. Studies on mechanical n-C7F 16 and n-C7H , as judged by the r espective I 16 and solution properties of large samples would be of G(CF4 ) and G(CH 4 ); and the indicated C- F scission s in terest, as would studies on poly(perfluorostyrene) of the fluorocarbon arc much less than the C- H sciss ions of the hrdl'ocarbon . The low vield of C- F if it should become fLva ilable. Presum ably, polymers with perfluoroaromatic rings in the main chain, scission products' (SiF4 ) may be a cage eff ect phenom­ enon . The diffusion away of th e hydrogen atom of rather than a side chain, would show fL better com­ a C- H pair must be an easier process than the bined resistance to heat and mdiation than any correspon din g diffu sion of a fluorine atom . R esults styrene derivative. Polyphenyls and perfluoro- 279 polyphenylene ethers are the obvious structural [15] L . Bouby, A. Chapiro, M. Magat, E. Migirdieyan, A. possibilities of this kind. Prevot-Bernas, L. Reinisch, and J. Sebban, Geneva Conference on Peaceful Utilization of N uclear Energy, It is not surprising that mixtures of fluorocarbons Vol. 7, p. 612 (1955). with hydrocarbons are usually less stable to radiation [16] "IV. N. Patrick and M . Burton, J. Am. Chem. Soc. 76, than the pure components themselves, for the pro­ 2626 (1954). duction of hydrogen fluoride is now possible. I t is [17] S. Gordon, A. R . van Dyken, and T . F . Doumani, J . Phys. Chem. 62, 20 (1958). likely that most partially fluorinated compounds [18] E. L. Colichman a nd n. Fish, Pyrolytic and radiolytic would have the same weakness. In spite of the decomposition rate studies o n Ortho~ , m eta~ , a nd para~ general tendency toward increased sensitivity, hexa­ terphenyls, U.S. AEC R e port NAA- SR- 1287 (1955). [19] M. Burton, Radiolyse de Liquides Organiques, Chapter fluorobenzene appears to repress somewhat the 1 in M . H aissinksy (editor) Actions Chimiques et production of hydrogen from cyclohexane. The Biologiques des Radiations, 2me Seri e, Masson et Cie, increased polymer production in mixtures is in most Paris (1958). cases a complicated phenomenon, but in the benzene­ [20] E. W . R . Steacie, Atomic and Free R adical R eactions, hexafluorobenllene mixtures it exhibits a strong 2d ed. (R einhold Publishing Co., New York, N.Y., 1954) . tendency toward eq ual numbers of benzene flnd [21] M. Szwarc, private communication. hexaHuorobenzene units, as in alternating copol.vm er­ [22] J. Smid and M. Szwarc, J . Am. Chem. Soc. 79, 1534 ization . A likely reason for this behavior is the (1957). [23] P . Ayscough and E. W . n. Steacie, Proc. Roy. Soc. enhancement of radical or triplet-state reactivities A234, 476 (1956). by polarity differences. [24] P . B . Ayscough, J . C. Polanyi, and E. W. R. Steacie, At ordinary tempcratures, atom and radici\,l Can. J. Chern. 33, 743 (1955). mechanisms modified by cage effects seem able to [25] P . B. Ayscough, Can. J. C hem. 33, 1566 (1955) . [26] P. B. Aysco ugh, J. CheJJ1. Phys. 24, 944 (1956). acco unt for the results. NIechi\,ni slll s involving [27] G. G iacometLi a nd E. W. R . Steaeie, Can. J. Chem. 36, triplet states, as outlined by other authors for 1493 ( 1958). benzene, are perhaps preferable for the perfluoro­ [28] G. O. PriLchard, H . O. Pritchard, I-I. I. Schiff, and A. F . aromatic systems, especially because of the very Trotman~Dick e n so ll , Trans. Faraday Soc. 52, 849 (1955). slight occurrence of fragmentation. Ionic mech­ [29] G. H . MilicI', C. O. Pritchard, and E. W. R. Steacie, Z. anisms, proposed by Feng for cer tain h.vclro cal'bon physik . Chem. 15, 262 (1958). fluorocarbon mixtures, have not been co nsirlered [30] M. H ellman, E. Peters, W . J . Pummel', a nd L. A. Wall, \ J . Am. Chem. 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