34 Pulse Radiolysis of Monomers and Polymers A. J. S W A L L O W
Radiation Chemistry Downloaded from pubs.acs.org by YORK UNIV on 12/02/18. For personal use only.
Paterson Laboratories, Christie Hospital and Holt Radium Institute, Manchester, 20, England
In aqueous solution the primary radiolysis products react rapidly at the vinyl group of monomers, giving ions or radicals. For styrene and α-methylstyrene, appreciable ring addition occurs also. Rate constants for reactions of the radicals formed are tabulated. Styrene and α-methylsty rene in the pure state give rise to short-lived anions and long-lived radicals on irradiation, and the results are corre lated with those obtained from measurements on the poly mer which is formed. Ionic reactions are important in the irradiation of styrene and α-methylstyrene in organic sol vents. Studies are reported on the mechanism of protection of polymers by sulfur-containing compounds. When irradi ated in the solid state, poly(methyl methacrylate) gives some radicals which disappear over tens or hundreds of micro seconds, leaving behind further radicals which remain trapped.
*"phe study of radiation-induced polymerization is one of the most successful in radiation chemistry but is still incomplete. For example, little is known about the initiation stages, and many aspects of the mecha nisms are still in dispute. Pulse radiolysis offers a means of studying monomers and polymers in a new way and provides certain advantages over flash photolysis, which has hardly been applied to monomers or polymers. Apart from the relevance to the chemistry of synthetic poly mers, such studies should help in understanding the effect of radiation on macromolecules of biological significance. F o r these and other rea sons, several groups of workers have pulsed polymeric systems, and, as so often in pulse radiolysis, have often worked along similar lines, thus providing an opportunity to compare results. This paper attempts to review the work which has been published in the pulse radiolysis of synthetic polymeric systems. 499
500
RADIATION CHEMISTRY II
Monomers in Dilute Aqueous Solution Reactions with the Primary Species. In aqueous solution the mono mers are exposed to the action of the species formed from the water i n the primary act. The rate constants published for the reactions of the hydroxyl radicals and hydrated electrons are included in Table I. Most of the hydroxyl rate constants were measured using thiocyanate and are therefore subject to the usual uncertainties of this method (5). N o rate constants appear to have been published for the reactions of the hydrogen atoms. Table I.
Rate Constants for Formation of Monomer Radicals or Ions in Aqueous Solution
Reaction
Rate Constant, Mr sec.' 1
OH + CH2=CH2
1 X 10»
2
2.0 Χ 10
S 0 - + CH =CHCN
S 0 " + CH2=CHOCOCH ( H 0 ) - + CH2=CH ( H 0 ) - + CH =CH—CH=CH (H 0) ~ + CH2=CHCONH 4
3
2
2
n
n
2
2
n
2
( H 0 ) - + CH2=C(CH )COO(H 0) - + CHo=CHC H 2
n
2
w
3
e
5
( H 0 ) - + CH =CHC H N ( H 0 ) - + CH2=CHC H NH 2
2
n
n
2
6
e
4
4
+
2
1.1 C H O H — C H · 2
2
(1)
2
It has been found that the radicals formed by reaction of O H radicals with ethylene do not react with ferric ions to produce acetaldehyde i n the yield which would be expected if the only reactions occurring were Reaction 1 followed by Reaction 2. C H O H — C H - + Fe 2
2
3+
- » C H C H O + Fe 3
+ H
2+
+
(2)
It was originally thought that some O H radicals must abstract hydrogen atoms from ethylene to form C H radicals (14), but it is now concluded that all O H radicals add to ethylene (Reaction 1) and that the /S-hydroxyethyl radicals enter into reactions, not a l l of which give rise to acetalde hyde (13). Other recent evidence also supports this view (4). The absorption spectrum of the β-hydroxyethyl radicals is similar to that of the α-hydroxyethyl radicals which can be prepared from ethyl alcohol (13). 2
3
In the pulse radiolysis of acrylamide, methacrylamide, and some other monomers, Chambers et al. have observed broad absorption bands with peaks near 370 τημ (9), which can plausibly be attributed to O H adducts at the β position—e.g., O H + C H 2 = C H C O N H -> C H O H — C H C O N H 2
2
2
(3)
W i t h styrene and a-methylstyrene, nitrous oxide-saturated solutions exhibit several sharp peaks after a 5000-rad pulse (30). One sharp peak is seen with a maximum at 345 τημ for styrene (optical density i n 1.6-cm. cell = 0.14) and 350 τημ for a-methylstyrene (optical density = 0.1). The position of the absorption maximum is consistent with that expected for an adduct of the hydroxyl radical at the benzene ring (7). F o r many aromatic compounds, the extinction coefficient of such adducts at the maximum is about 3700M" cm." (8). Using this figure we can calculate that 9 0 % of the O H radicals add to the ring for styrene and 60% for α-methylstyrene. These high percentages are consistent with the high rate constants (2-5 X 1 0 M sec." ) for reaction of O H radicals with benzene and other aromatic compounds (3). 1
1
9
_ 1
1
Nitrous oxide-saturated solutions of styrene and a-methylstyrene also give sharp peaks at about 305 and 320 τημ, after pulse radiolysis. These resemble the characteristic double peaks of benzyl at 306 and 317 τημ (16). The peaks are therefore attributed mainly to adducts of O H radicals at the ^-position in the vinyl group, to give radicals resembling benzyl—e.g., O H + C H 2 = C H C H -> C H O H — C H C H e
5
2
e
5
(4)
502
RADIATION CHEMISTRY
H
The most recent value for the extinction coefficient of the benzyl radical (in cyclohexane solution) is 1.2 χ 10 M c m . ' at 317 m/x and 6 X 1 0 M " c m . at 360 m/x (16). Other recent values are 1.9 X 1 0 M cm." at 318 m/x (20) and 1.8 X 1 0 M " cm." at 320 m/x (17). If we assume that similar extinction coefficients apply in aqueous solution, that the hydroxyl adducts of styrene and α-methylstyrene have the same extinction coeffi cient as benzyl, and that no other species absorb at these wavelengths, we would estimate that about 20-40% of the O H radicals add to the β-position of the vinyl group for styrene and 15-30% for a-methylstyrene. 4
1
1
3
-1
4
4
1
_ 1
1
1
1
Products Formed by Hydrated Electrons and Hydrogen Atoms. W h e n hydrated electrons react with uncharged vinyl monomers, they may in principle give rise directly to hydrogen atom adducts: ( Η 0 ) - + Μ -> ΜΗ + O H " + ( H 0 ) . ! 2
Λ
2
(5)
n
Alternatively, anions may be formed in the first case, which might or might not be protonated by reaction with water or hydrogen ions. Chambers et al. find that pulse radiolysis of neutral aqueous acrylamide (10" M) gives rise to absorptions with peaks at 275 and 370 m/x (9). The peak at 275 m/x does not appear when an excess of N 0 is present, whereas the peak at 370 m/x is almost unaffected by N 0 . It may be therefore that the absorption at 275 m/x and half the absorption at 370 m/x are caused by an anion formed by reaction of the hydrated electron with acrylamide (9) or else by a hydrogen atom adduct formed directly as in Reaction 5 or by protonation of an anion. The peak at 275 m/x is absent in a solution which is sufficiently acid for all hydrated electrons to give hydrogen atoms, so that the species responsible for the peak could not be a hydrogen atom adduct, unless hydrogen atoms react with acrylamide to give a radical with a structure different from the hydrogen adduct formed from hydrated electrons. [In a recent communication ( K . Chambers, E . Collinson, F . S. Dainton, Joint Annual Meetings of Chemical Society, Institute of Chemistry of Ireland and Royal Institute of Chemistry, Dublin, A p r i l 1968) it is concluded that the species ab sorbing at 275 and 370 m/x in the pulse radiolysis of aqueous acrylamide is in fact formed by protonation of the anion, and is C H — C H = C ( O H ) N H , whereas the species absorbing at 370 m/x only, formed by hydrogen atom attack, is C H — C H C O N H . Although the absorption at 275 m/x is no longer attributed to the anion, an absorption at 290 m/x can be seen at p H 11-12, and this is attributed to the anion. This work does not neces sitate a revision of Table II of this paper, but it contains useful additional 2
2
2
2
3
2
2
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503
information, including valuable new rate constants.] Methacrylamide and N-terf-butylacrylamide behave like acrylamide, but acrylonitrile and acrylic acid show some differences. It would be interesting to establish the reactions of anionic species in such cases by using electrical conduc tivity in conjunction with pulse radiolysis. Among other experiments, it would be interesting to see whether the rate of protonation of the anion could be observed. Styrene (5 X 10" M) has been given 2-/xsec. pulses in neutral solu tion (30), and after reaction with the hydrated electron it shows no sign of an absorption which could be attributed to the styrene anion. This could be because reaction with hydrated electrons occurs according to Reaction 5. If the anion is an intermediate, its reaction with hydrogen ions would be too slow to account for the absence of the anion absorp tion; hence, perhaps the anion is formed in the first case and then reacts with water. If we assume that this reaction occurs within a microsecond, the rate constant would be > 1 0 M sec." . A n absorption with a peak at — 400 m/x is seen in the pulse radiolysis of styrene dissolved in cyclo hexane (24, and see below), but is not seen when water is added before irradiation. The absence of the anion is attributed to rapid protonation by the water, and the rate constant for the reaction may be estimated at greater than about 1 0 M sec." . Further experiments could give a better value. α-Methylstyrene behaves similarly to styrene. 3
4
9
- 1
_ 1
1
1
Styrene (or a-methylstyrene) could accept a hydrogen atom at the a- or ^-position in the side chain or at several positions in the benzene ring. However, the α-position in the side chain is energetically unlikely. Incorporation of a hydrogen atom at the ^-position in the side chain would give a methyl-substituted benzyl radical, C H — C H C H (or C H — C ( C H ) C H ) , which might be expected to have a spectrum like that of benzyl itself, which has characteristic peaks at 306 and 317 τημ. (16). Aqueous solutions of styrene (or α-methylstyrene) exhibit peaks at about 305 and 320 π\μ after the hydrated electrons have reacted (31), but the intensity is only about one-seventh of that expected if all hydrated electrons (and hydrogen atoms) gave the substituted benzyl radical and if its extinction coefficient in aqueous solution were the same as for benzyl in cyclohexane solution. Hence, it may be that 15% of the hydrogen atoms appear at the β-position in the side chain with the remainder appearing at some position in the benzene ring. It might have been expected that the hydrogen atom ring adduct would exhibit a similar absorption to the O H radical ring adduct. Yet for both styrene and α-methylstyrene, the optical density at — 350 π\μ in the presence of nitrous oxide is twice that in the absence of nitrous oxide—not the same, 3
3
3
e
5
e
5
504
RADIATION CHEMISTRY
Table II.
II
Products Formed by Reaction OH CH OH—CH -
CHo^CH.
^2
2
2
CH =CHCONH CH =C(CH )CONH CH =CHCONH-tert-Bu
CH OH—CHCONH2 C H O H — C (CH ) C O N H CH OH—CHCONH-ter*-Bu
CH =CHC H
80% C H 2 = C H C e H O H 20% C H O H — C H C H
2
2
2
3
2
2
2
e
2
2
3
2
2
5
5
2
CH =C (CH ) C H 2
3
e
5
e
5
70% C H 2 = C ( C H ) C H O H 30% C H O H — C ( C H ) C H 3
2
e
e
3
e
5
as would be expected from this view. Perhaps the discrepancy is con nected with the fact that various sites in the ring are available for addition. In acid solutions, the spectrum after the pulse of both styrene and α-methylstyrene is the same as in neutral solution. This contrasts with acrylamide and related compounds whose spectrum in acid solutions resembles that in nitrous oxide-saturated solutions. These effects are consistent with the view that the reaction of the electron with styrene and α-methylstyrene gives a hydrogen atom adduct which is the same as that formed by hydrogen atom attack, whereas with acrylamide and related compounds the anion is formed and is stable enough to be seen. In the experiments reported by Chambers et al (9) the disappearance of the anion by reaction with water might have been expected as a firstorder reaction if the rate constant were greater than about 1 0 M " sec." . Hence, this figure may be regarded as an upper limit for the reaction (surprisingly low) (see bracketed note above). Tentative identification of the products of the reaction of hydroxyl radicals, hydrated electrons, and hydrogen atoms with monomers on the basis of the discussion given here is summarized in Table II. M a n y of the products can exist in various mesomeric forms. For styrene and α-methylstyrene the percentages discussed in the text have been adjusted to make the totals add to 100%. The identification seems reasonably sound for the O H radical products but less so for the others. Apart from the need to identify the products from other monomers, more work is needed on the monomers already studied to test the speculations made and to quantify any conclusions. Reactions of the Monomer Ions and Radicals. The rate constants for the reactions of monomer radicals (or ions) i n aqueous solution are given in Tables III and IV. Mutual interactions of radicals (Table III) all 2
1
1
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Monomers and Polymers
H No reaction so far detected
CH3—CH2*
CH "—CHCONH2 CH —C(CH )CONH CH —CHCONH-terf-Bu
CH —CHCONH CH —C(CH )CONH
85% C H 2 = C H C H 15% C H — C H C H
85% C H ^ H C e H e 15% C H — C H C H
2
2
3
3
2
e
6
e
3
5
85% C H 2 = C ( C H ) C H 15% C H — C ( C H ) C H 3
3
3
2
e
e
e
2
CHo—CHCONH-tert-Bu
2
3
2
3
e
85% C H 2 = C ( C H ) C H 15% C H — C ( C H ) C H 3
5
5
3
3
e
6
e
5
proceed with a rate constant of about 1 0 M sec." (to within a factor of about 2), and this value is substantially above the usual rate constant for termination, as might be expected. There is little indication of appre ciable difference between the reaction rates for different species. Reaction of monomer radicals with oxygen also proceeds rapidly, as expected (Table I V ) . The peroxy radicals formed from ethylene react more rapidly with each other than either H 0 radicals or 0 " radicals do (15). Their reaction with ferrous ions also seems to be somewhat faster than the latest value for the corresponding reaction of H 0 (21). The reaction of the acrylamide radicals with ferricyanide is about eight times faster than the reaction of polyacrylamide radicals with ferricyanide (12). 9
_ 1
1
2
2
2
Monomers in the "Pure State and in Organic Solution Pure Monomers. The first pulse radiolysis study of a polymerizing system was with isobutylene, which gave an absorption with a peak at 297 τημ, which disappeared in a fast first-order reaction (6). The absorp tion may be attributed to the trimethylcarbonium ion, which, from independent work, appears to have an absorption maximum at 292 τημ and an extinction coefficient close to 6.3 X 1 0 M cm." (28). Most of the work with pure monomers has been with styrene and α-methylstyrene, which have been investigated separately by different groups. These two monomers appear to behave similarly. Katayama and colleagues were the first to study one of these monomers (23). They found that α-methylstyrene gave rise to a broad absorption with an apparent peak at 350 τημ and a shoulder at about 420 τημ. In the presence of water or D P P H the absorptions were substantially diminished. They attributed the absorption to the anion of α-methylstyrene, which can be produced independently of radiation by the action of sodium metal on 3
_ 1
1
506
RADIATION CHEMISTRY
II
α-methylstyrene. Later additional absorption was seen at 546.1 m/x, decaying at the same rapid rate as the other, and equally affected by D P P H or water (22). This result fully substantiated the earlier conclu sion. Schneider and Swallow saw a similar rapidly decaying absorption in both styrene (half-life quoted as about 5 /xsec.) (29) and α-methyl styrene (31). The absorption tailed off beyond about 550 m/x, but a larger absorption was seen i n addition, with a peak at 315 m/x for styrene and 320 m/x for α-methylstyrene. The absorption at this wavelength was much longer lived, and i n fact, part of it persisted for seconds at least (29). Metz, Thomas, and Potter worked with styrene and analyzed their data i n terms of three species: (1) a rapidly decaying species (half-life 4 /xsec.) with a broad absorption centering at about 370 m/x; this species was removed by water; (2) a more intensely absorbing longer-lived species with a peak at about 320 m/x; (3) a stable species with a peak i n a similar region. This stable species could also be observed i n styrene after y-irradiation. It was very sensitive to photolysis (27). Table III.
Rate Constants for Mutual Interaction of Monomer Radicals or Ions in Aqueous Solution
Reaction (R = H or OH) 2CH OH—CH 2CH R—CH 2 CH R—CHCONH 2CH R—CHCONIV 2
2
2
2 C H R — C (CH ) C O N H 2 C H R — C (CH ) C O N H 2
3
2
6.3 Χ 10 7 Χ 10 1.3 X 10» 1.8 X 10» 4.2 X 10» 1.1 X 10 1.6 Χ 10 4.5 Χ 10 6 Χ 10 7 Χ 10 1.1 X 10 7.6 Χ 10 8 Χ 10
(13)
8
2
2
Reference
J
8
2
2
Rate Constant 2k, M ' sec.'
3
9
2
2
β
9
9
2 CH R—CHCONH-terf-Bu 2 CH R—CHCONH-terf-Bu° 2
2
8
8
9
2HO(CH =CHC H ) 2HO(CH =C(CH )C H ) 2
2
β
e
5
3
e
5
8
8
1
(13) (9) (β) (9) (9) (9) (9) (9) (9) (9) (31) (31)
R = er or OH.
Comparison of the data obtained by the three groups reveals many points of agreement. A l l three agree about the existence of the short-lived species, and above about 350 m/x the shapes of the spectra observed immediately after the pulse are consistent with each other, especially since the absorption "peaks" are broad ones and the absorptions obtained are not large. The tentative attribution of this species to an anion is now (31) agreed on by all. Below 350 m/x it is tempting to suppose that
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Monomers and Polymers
the difference between the Japanese work on the one hand and the American and European work on the other might be caused by instru mental factors, especially in view of the difficulties often encountered with spectral inhomogeneity i n pulse radiolysis and the sensitivity of the "stable" species to light. The long-lived transient with the peak near 320 τημ (seen by Katayama and colleagues as a long-lived component of the absorption at higher wavelengths) is attributed for the main part to a radical with a benzyl-type structure (31). It is noted that such radicals would retain their structure if they add to monomer—e.g., for styrene R C H 2 — C H C e H ~J~ C H = C H C e H 5
• RCH2—CH(CeH )—CH —CHCeHg (6) Investigations are still being done on the "permanent" absorption(s) with peak (s) near 320 τημ (27). 5
Table IV.
2
CHgR—CH - + 0 CH R—CHCONH + 0 2 CrLjR—CH 0 C H R — C H 0 - + Fe * 2
Rate Constant, M " sec.' J
6.0 1.9 1.4 2.4
2
2
2
2
2
2
e
2
2
2
C H R — C H C O N H + Fe(CN) 32
β
2
Rate Constants for Reactions of Monomer Radicals or Ions in Aqueous Solution
Reaction (R = H or OH)
2
5
2
e
X Χ Χ X
10» 10 10 10 9
8
e
6.8 X 10*
1
Reference (13) (9) (13) (13) (9)
R = e~ or OH.
The kinetics and effect of additions on the absorptions are not yet clear. According to Katayama and colleagues the most rapidly decaying species disappears in a first-order reaction with a half-life of 25 tisec. (22). Metz et al. find a first-order reaction with a half-life of 4 /xsec. (27). Schneider and Swallow, however, assert that the species disappears i n a second-order reaction (31). Perhaps the variability is connected with the effect of traces of impurities, especially water. The Japanese and American groups had used elaborately drieçl styrene, whereas the European group had not. However, on this basis, we might expect the Japanese and American results to agree. Also the European styrene could not have contained much water; otherwise the short-lived absorption would not have been seen at all. Similarly, the kinetics of the decay of the long-lived transient are not easily understood. Water does not seem to affect this species (27). Oxygen affected it in the work of Schneider and Swallow (31) but not in that of Metz, Potter, and Thomas (27). Attempts have been made to study the formation of polymer under pulse radiolysis conditions. Metz et al. have examined the ionic polymerization which should predominate with their very dry styrene (26).
508
RADIATION CHEMISTRY
H
Schneider and Swallow have produced evidence to show that with their styrene the polymerization was free radical in nature (29). This is con sistent with the likely presence of traces of water, and it would be expected that much less water would suffice to prevent ionic polymeriza tion than would be needed to interfere with the reactions of the "primary" species. It can be deduced from Schneider and Swallow's polymer measurements that the termination rate constant is about 1 0 M sec." (31), which is lower than the values which may be deduced from experi ments at low dose rates (10) (around 2 X 1 0 M sec." ). However, all experiments may perhaps require some reinterpretation in view of the stable product absorbing at 320 m/x. The value of 1 0 M sec." is con sistent with literature values and is not inconsistent with the rate of disappearance of the transient absorption at 320 m/x, especially since ter mination rate constants must be average values, and in pulse radiolysis many rates should be observable (depending on the length of the poly merizing radical) and should vary downward from the rate of mutual reaction of the monomer radicals. In the absence of measured values, this latter rate may be taken to be k = 2 X 1 0 M sec." as for benzyl (16), a value close to those given in Table III for monomeric radicals in aqueous solution. According to Schneider and Swallow's calculations polymerization had occurred with an initiation rate corresponding to G = 0.95 (31). Hence, most of the benzyl-type radicals had initiated polymerization rather than reacted with each other. F r o m the conditions used in the experiments this means that the rate constant for addition of monomeric radicals to monomer must be greater than about 1 0 M " sec." , probably much greater, and this is entirely reasonable. e
7
- 1
1
1
e
9
_ 1
_ 1
- 1
1
1
2
1
1
Styrene and α-Methylstyrene in Organic Solvents. Pulse radiolysis studies have been made on styrene and α-methylstyrene dissolved i n methanol, benzene, carbon tetrachloride, dioxane, tetrahydrofuran, hex ane, and cyclohexane (9, 24, 29, 30, 31). The results are easiest to under stand for the aliphatic hydrocarbons and especially for the styrene i n cyclohexane, which has been studied the most (31). For such solutions, two absorption bands were seen after the pulse by Keene, L a n d , and Swallow (24) and Schneider and Swallow (30) with peaks at 320 and 390 m/x. The absorption at 320 m/x disappeared slowly by complex kinetics, and the 390-m/x absorption was very short lived, decaying by second-order kinetics with k/e = 4-7 X 10 cm. sec." . The relative intensities of the two peaks were quite variable. Chambers et al. saw the long lived absorption at 320 m/x but d i d not see a separate peak at 390 m/x, although it was observed that the absorption at 375 m/x decayed rapidly with k/e = 2.6 X 10 cm. sec." . e
e
1
1
It seems likely that the absorption at 320 m/x is caused largely by the substituted benzyl radical. Since the species is too long lived to be
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Monomers and Polymers
the monomeric radical, it seems to be the growing polymer radical. The species at 390 τημ may be the anion as seen with the pure monomer. Adding traces of impurities (including water) markedly lowered the absorption at 390 τημ, which may explain the ^reproducibility at this wavelength. The yield of the absorption at 320 τημ was found to rise with concen tration in a way suggesting that it is formed by anionic process—e.g., by protonation of a styrene anion by its geminate partner (31). If it is a polymerizing radical, then the rate constant for addition of the benzyl-type radical to styrene must be at least 1 0 - 1 0 M " sec." . The yield of the observable anion seemed much less dependent on concentra tion, which is consistent with the view that it is formed by those electrons ( G — 0.2) which escape from the spur. Absorptions with peaks close to 320 τημ are seen for all the other solvents above, but the component at the longer wavelength is seen only with the aliphatic hydrocarbons. For methanol solutions this may be because of rapid protonation. 3
6
1
1
Polymers A pulse radiolysis study has been reported on the mechanism of "protection" of a polymer in aqueous solution by sulfur compounds ( I I ) . Hydrated electrons d i d not react with polyethylene oxide ( P E O ) , but it was presumed that O H radicals must react. W h e n thiourea is present in an aqueous solution containing P E O , the transient thiourea absorption peak at 400 τημ does not appear, presumably because the O H radicals responsible for it are now reacting with P E O instead. B y analogy with the work on sulfur compounds reported by Adams et al. (2) the peak at 400 τημ in the absence of P E O would now be attributed to a product formed by addition of the oxidized thiourea radical to thiourea. Various second-order reactions occur after a pulse has been delivered to the system containing thiourea, and the reaction of P E O radicals with re duced thiourea radicals could lead to "protection" of the polymer from radiation damage. Under low dose rate conditions the more likely pro tection mechanism would be reaction of P E O radicals with thiourea to abstract a hydrogen atom, but this reaction could not be demonstrated in the earlier work ( I I ) . Work using cysteamine ( I ) which is now better understood than thiourea, has succeeded in establishing Reaction 7. Ρ · + NH CH CH SH -» PH + NH CH CH S · 2
2
2
2
2
2
(7)
A t p H 7.5 the rate constant is ca. 5 X 1 0 M sec." , which is about one-tenth that for the corresponding reaction of - C H O H radicals with cysteamine ( I ) . e
_ 1
1
2
510
RADIATION CHEMISTRY
]
Figure 1. Spectrum after the pulse (— 5000 rads) for poly(methyl methacrylate) containing unchanged monomer. Optical path length, 0.1 cm. Pulse radiolysis experiments have been done with solid poly (methyl methacrylate) (32). The material had been prepared in blocks (3 X 1 X 1 cm. or 3 X 1 χ 0.1 cm.) by γ-irradiation of methyl methacrylate and contained unchanged monomer at unknown concentration. Marked transient absorptions were seen after 5000-rad pulses, which decayed over a few tens or hundreds of microseconds to give a permanent species, stable for hours and which had a spectrum resembling that of the tran sient. A solution of methyl methacrylate i n cyclohexane gave a similar broad absorption spectrum. The absorption spectrum immediately after the pulse for a block with a light path of 0.1 cm. is shown i n Figure 1. Within experimental error there was no change i n transient intensity between the first and 31st pulse. The decay kinetics were complex, varied from sample to sample, and were not the same at high wavelengths (e.g., 600 m/x) as at low wavelengths (e.g., 340 m/x). The permanent absorption exhibited by the irradiated polymer is quite different from that shown by the polymer itself, which absorbs only weakly at 300-350 m/x. Absorptions below 300 m/x are difficult to interpret because of the monomer absorption. It is suggested that for the most part both transient absorption and permanent absorption are caused by the radical: R—CH,—C (CH ) —COOH 3
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511
which is probably also responsible for the E S R spectrum of irradiated poly(methyl methacrylate) containing traces of monomer. Some such radi cals may be formed during the pulse by fission of the polymer chain, and more would be formed by reaction of radiation-produced radicals with unchanged monomer. Where the radicals are i n favorable positions i n the polymer matrix they recombine after the pulse, but a certain fraction (perhaps 25-50%) remain for hours after the pulse as trapped radicals. The difference between the kinetics at 600 and 340 τημ shows that other species are present at certain stages in addition to the one discussed.
Acknowledgments The author thanks his colleagues, especially B . Cercek, M . Ebert, E . J. Land, and Christel Schneider for helpful discussions. H e also thanks those who have sent private communications about their work, especially M . Katayama and D . J. Metz.
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