13 Pulse Radiolysis of Styrene and
Addition and Condensation Polymerization Processes Downloaded from pubs.acs.org by WESTERN SYDNEY UNIV on 01/31/19. For personal use only.
α-Methylstyrene CHRISTEL SCHNEIDER Institute for Physical Chemistry, University of Cologne, 5 Cologne, West Germany
Information
can be obtained
the nature and reactions
by pulse
of transient
radiolysis
species
rene and α - m e t h y l s t y r e n e . A very short-lived ing
an
absorption
identified
about
styrene
aqueous
mµ
owing
by addition
to
a
350-400
molecule. was
identified CHC H .
showed
an additional
hydroxy
6
5
been elec
the anion, as the
Pulsing
a at
poly of
an
absorption
cyclohexadienyl
of OH to the benzene
show
maximum
~CH
2
has
through
Besides
on
in sty
species mµ
with an absorption
which
radical
solution of styrene
345
formed
species
320 mµ appeared
range
anion radical formed
by a monomer
longer-lived
merizing at
the
as the monomer
tron capture second
in
studies
formed
ring of the
radical styrene
molecule.
T o u r i n g the past few years pulse radiolysis has yielded valuable information about excited molecules, ionic species, and free radicals produced i n irradiated systems (3, 4). However, it has been applied mainly to aqueous systems where results of great general interest have been obtained—e.g., the detection of the solvated electron (6, 13). Only since about 1965 has this technique been applied to polymerizing systems where, although much is known about the reaction kinetics, there is still little direct information on the nature of the initiating species and on the growing radicals. U p to now, styrene and a-methylstyrene are the mono mers which have been studied under pulsed irradiation. These monomers are well suited to such studies because of the profound knowledge of their polymerization kinetics which exists and because they can be polymerized by anions or cations as well as by free radicals; this has been proved by using various conventional catalysts and by initiating poly merization through ionizing radiation under extreme conditions.
219
220
ADDITION
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POLYMERIZATION
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Pulse radiolysis studies on styrene and α-methylstyrene have been carried out independently by three research groups using comparable facilities and optical detection methods. There is good agreement among the authors' results with regard to the general pattern on the nature and even on the possible mechanism of formation of the transient species while such agreement is often lacking i n the kinetic data obtained or i n the effects produced by adding other reactants. This paper summarizes information obtained by pulse radiolysis on the nature and reactions of transient species formed in styrene and α-methylstyrene. Anions
The formation of the styrene and α-methylstyrene anion under pulsed irradiation of monomers i n the pure state and i n dilute solutions has been clearly indicated from the absorption spectra obtained. Katayama et al. (10, 11, 12) reported on the appearance of absorp tion maxima at about 350 and 546 τημ with a shoulder at about 420 τημ in the pulsing of extremely dry α-methylstyrene sealed in a quartz cell. The absorptions have been shown to be sensitive to traces of water, and the kinetics of the absorption at 3663 A . showed two kinds of intermedi ates, one of which (A1/2 — 25 /xsec. ) was regarded as the radical anion. Metz and co-workers (16), using ultrapure styrene observed a very short-lived transient absorbing at 370 πΐμ. The sensitivity of this species to water indicated strongly that it was an ion, which the authors think is partially responsible for the kinetics of polymerization observed under extremely dry conditions (19, 20). Oxygen decreased the initial height of the absorption and slowed the decay rate of this species, while addition of N 2 0 showed no effect at all. Both authors tentatively identified the species observed as the styrene anion radical formed through electron capture by a styrene molecule. The species decayed according to firstorder kinetics with a half-life of about 4 /xsec. obtained at 370 m/χ by Metz compared with 25 /msec, reported by Katayama for all three absorp tion maxima. The formation of a styrene and α-methylstyrene anion, absorbing at about 390 τημ, was shown by Schneider and Swallow (23, 24, 25), even for conventionally purified monomers which had not been treated to remove traces of water. The nature and reactions of this transient were studied i n more detail in dilute solutions of styrene i n different solvents. A solution (10" 3 M) of styrene in cyclohexane showed, immediately after the pulse, a spectrum with a distinct absorption band at 390 ηΐμ ( Figure 1), similar to that observed by Keene et al. (14). However, the intensity of the absorption was irreproducible during a series of experiments, probably because of the varying moisture content of the air and of the
13.
SCHNEIDER
Pulse
Radiolysis
221
0D10
3
10
300
350
400
450
Figure 1. Absorption spectrum of a deaerated 10~3M solu tion of styrene in cyclohexane, taken immediately after a 2-μ8βο. pulse of —5000 rads
glassware used. The influence of impurities on the 390 τημ absorption was checked by adding small amounts of water, methanol, chloroform, carbon tetrachloride or n-butylamine or by saturating the solution with N 2 0 . A l l additives considerably reduced or even removed the 390-m/x absorp tion, the strongest effects being exerted by the halogenated compounds and N 2 0 , both of which act as electron scavengers (Figure 2 ) . Pulsing of dilute solutions of styrene in hexane showed similar results. Schneider and Swallow concluded that the species absorbing at 390 m/x was the styrene anion. This finding was supported by the fact that pulsed dilute solutions of styrene (5 Χ 10"4 to 5 X 10" 3 M) in aliphatic alcohols and water no longer show the broad absorption band of the solvated electron observed for the irradiated pure solvents i n the visible region of the spectrum (J, 22). However, no absorption at about 390 τ η μ , as would be expected for the anion formed through capture of the solvated electron by the styrene could be observed. This is explained by rapid protonation of the anion i n solvents like water or aliphatic alcohols.
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ADDITION
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POLYMERIZATION
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0D'10
J
12
Λ
10
300
350
400
Figure 2. Influence of small amounts (10~SM) of addi tives on the absorption spectrum of a 10~SM solution of styrene in cyclohexane; dose rate: ^5000 rads/pulse φ Ο X A •
Cyclohexane (aerated) Cyclohexane (dried) Water added Methanol added CCI added
The styrene anion was extremely short lived when cyclohexane and hexane were used as solvents, but contrary to the findings of Katayama et al. and of Metz et ah, Schneider and Swallow stated that the decay of the anion followed second-order kinetics with a first half-life of about 3—4 /asec. for styrene ( 10" 3 M ) i n cyclohexane and hexane. Evaluation of the decay curves at 390 rmt led to values for k/e ^ 7 Χ 106 cm. sec."1 for styrene i n the pure state and k/e = 3.8 ± 0.6 Χ 107 cm. sec."1 for a 10" 3 M solution of styrene i n cyclohexane. The decay kinetics were not influ enced by varying the dose rate of the pulse by a factor of about 6, thus confirming that the decay is second order. If a G value of 0.2 is assumed for the formation of free ions i n hydrocarbons, an extinction coefficient
13.
SCHNEIDER
Tulse
Radiolysis
223
of about 6 X 1 0 3 M _ 1 c m . 1 for pure styrene and 8 ± 3 X 1 0 3 M _ 1 c m . ' 1 for the cyclohexane solution was calculated from the dose rate (5000 rads/pulse) and the optical density obtained immediately at the end of a pulse. Thus, decay constants of k = 6 X 1 0 1 0 M _ 1 sec."1 for styrene i n the pure state and k = 3 X 1 0 1 1 M " 1 sec."1 for styrene in cyclohexane were obtained, which are not inconsistent with a diffusion-controlled ion-ion recombination reaction (5, 9). α-Methylstyrene behaved like styrene with respect to the absorption spectra and the decay kinetics. F r o m the findings of the different authors obtained under different conditions one must conclude that ions are always produced i n a substantial number, regardless of the purity of the monomers, but the successful propagation of these ions requires extreme purity.
Cations
The role of cations i n the pulse radiolysis of styrene and a-methylstyrene is not yet clear. Using frozen glasses, Shida and H a m i l l (26) and Williams (27) have seen absorptions with peaks at 350 and 650 τημ for styrene and α-methylstyrene. Bands have also been seen previously ( 24 ) at 460 or 475 τημ. Perhaps the species responsible for the shoulders seen by Schneider and Swallow between 340 and 370 τημ is a cation, whose absorption overlaps that of the anion radical. This could explain the diminution of the absorption intensity round 390 τημ when n-butylamine was added. However, there was no evidence for an absorption band at longer wavelengths.
Radicals
Katayama et al. have reported that the species absorbing at 350, 420, and 546 τημ i n α-methylstyrene in the pure state is sensitive to both water and D P P H , both of which were also found to suppress the polymerization reaction. They suggest, therefore, that it is the anion radical which is responsible for the polymerization of a-methylstyrene. In addition, they observed a transient which decayed very slowly (ti/2 >— 5 msec.) and which was regarded to be probably the free radical. In addition to the anion, Metz and co-workers, when pulsing ultrapure styrene, observed a second, longer-lived species with an absorption maximum at 320-330 τημ. It was unaffected by addition of water or oxygen and decayed according to first-order kinetics with a half-life of
224
ADDITION
AND CONDENSATION
POLYMERIZATION
PROCESSES
about 220 ttsec. B y comparison with work done on the γ-irradiation of bromoethylbenzene by H a m i l l et al. (8) and on the flash photolysis of ethylbenzene and benzyl compounds by Porter and co-workers (17, 18), the American authors tentatively identify this species as the styrene radi cal C 6 H 6 ' C H C H 3 . Based on their dosimetry value and the initial optical density obtained and taking a G value for radical formation i n styrene of 0.35 (2), they calculate an extinction coefficient at 320 τημ for this species of c = 7 X 1 0 3 M - 1 cm." 1 . Metz et al, however, note that two of the results are difficult to understand if the species is indeed a radical— i.e., the first-order decay and the apparent insensitivity of the species to oxygen. Schneider and Swallow showed that repetitive pulsing of styrene in the pure state and i n different solvents using 10 4 pulses (2 /xsec./pulse, « 3 χ 10 1 7 e.v./gram pulse) at a frequency of 50 pulses/sec. yielded polystyrene i n each case. The amount of polymer formed and its molecu lar weight were determined. The effect of dose rate variation on the amount and molecular weight as well as addition of radical scavengers and studies on copolymer formation showed results similar to those found in the conventional free radical polymerization of styrene. F r o m the degree of polymerization and the rate of polymer formation a rate con stant for mutual chain termination of kt = 1.1 X 1 0 6 M _ 1 sec."1 was calcu lated. Correspondingly, the polymerizing radical showed a mean lifetime τ of about 0.5 sec. Besides the absorption of the anion at about 390 τημ, the absorption spectra of pulsed styrene and α-methylstyrene, taken by Schneider and Swallow show broad and rather intense absorptions round 320 τημ. These absorptions are influenced only slightly by water but are eliminated i n the presence of iodine. Oxygen decreases the initial height of the absorp tion and also causes an apparent change i n the decay kinetics (Figure 3 ) . The decay i n the deaerated solution d i d not fit first- or second-order kinetics, but if the decay after about 0.5 msec, is regarded as second order, a value of k/e < 1.3 Χ 10 3 cm. sec."1 is obtained for styrene. Combination with a molar extinction coefficient ε = 1.2 X 1 0 4 M _ 1 cm." 1 (7) led to a decay rate constant i ^ 1.6 X 1 0 7 M - 1 sec."1, which is i n reasonable agreement with values given i n the literature for the termina tion rate constant kt of styrene polymerization and the value of 1.1 X 1 0 6 M - 1 sec."1 derived from the polymerization experiments under pulsed conditions. F r o m the effect of additives on the 320-m/x absorption, the kinetic data obtained and considerations already mentioned i n connection with Metz's work the authors attribute the absorption i n large part to a resonance-stabilized growing polymer radical of the benzyl type: where R' is the initiating group and R " is Η for styrene and C H 3 for α-methylstyrene.
225
R'-
CHo-
CH.,
Both groups—Metz, Potter, and Thomas and Schneider and Swallow —observed an additional very long-lived absorption at 310 to 320 τημ which may be caused by more than one species. The absorption decays, in the dark, over many hours while it is photolyzed by light i n seconds or minutes, and its nature is not yet clear. c .o WO
i : 90 Q>
jHISESfiSB
80 200 /usee per large division
200jasec per large division Figure 3. Oscilloscope traces of pulsed deaerated styrene at 315 πιμ after a 2^sec. pulse of ^2000 raids (top) and ^5000 rods (bottom) in the presence of oxygen
W h e n an aqueous solution of styrene or α-methylstyrene ( 5 Χ 10~4 to 5 X 10" 3 M) was pulsed, Schneider and Swallow observed a group of intense and distinct absorption bands obviously caused by different radi cal species, while the absorption of the hydrated electron could no longer be observed (Figure 4 ) . The absorption obtained at 345 τημ for styrene and 350 τημ for α-methylstyrene is caused mainly b y a hydroxy—cyclohexa-
226
ADDITION
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POLYMERIZATION
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125 0D1(?\
100
75.
50.
25
250
300
350
40C
λ^μ]
Figure 4. Absorption spectrum of an acidified aqueous solution of styrene (5 X 10~*M, pH = 1.26), taken im mediately after a 2-^sec. pulse of —5000 rads
Table I. Pulse Radiolysis Results Sensitive to Short Lived
Species
Katayama et al.
Metz et al. Schneider and Swallow Long Lived
Metz et al.
Species
[m/x]
DPPH
H20
N20
—350 —420 546
yes
370
yes
yes
no
yes [yes]
little
yes [yes]
—390 [390] 320
no
yes
no
no
yes yes [yes] 1 Values in brackets refer to dilute solutions of styrene in cyclohexane.
no
Schneider and Swallow
—320 [320]
no [no]
13.
SCHNEIDER
Pulse
227
Radiolysis
dienyl radical formed by addition of O H to the benzene ring of the styrene molecule. This was shown by saturating the solution with N 2 0 ( 1 0 _ 1 M ) which converts all hydrated electrons to O H , thus doubling the intensity of the absorption. O n the other hand, adding formate or ali phatic alcohols almost eliminated the 345-πΐμ absorption completely by capturing the O H radicals. Pulsing of acidified aqueous styrene solutions ( p H = 2.76 and p H = 1.26) gave absorption spectra which d i d not differ significantly from that i n neutral solution, either with respect to the wavelengths of the absorptions or to the intensities. The species observed is rather short lived and decays according to a second-order reaction with a rate constant i n the range 7.4 Χ 10 8 to 2.9 X 10 9 Af _ 1 sec."1; the exact value, however, depends on the acidity of the solution. In the ultraviolet region, two absorption maxima, one at 320 τημ and a weaker one at 305 τημ were observed; their intensities were increased slightly by N 2 0 and were decreased by about half when formate or alcohols were added. The absorption, which is rather long lived, is again attributed to the growing substituted benzyl radical. The splitting of the absorption, which could not be detected i n the organic monomer solu tions, is suggested by the authors to support this assignment because the benzyl radical itself is known to have absorptions at 306 and 317 τημ (15). It is assumed that the benzyl type radical is formed partly through electron capture by the styrene molecule followed by rapid protonation i n the side chain and partly by addition of H and O H to the double vinyl bond. Table I summarizes some of the results on the pulse radiolysis of styrene and α-methylstyrene obtained by the different authors. Although Obtained on Styrene and α-Methylstyreneα Kinetics of Decay, Order
Half-Life,
first
-25
first
—4
second second first second second
Decay 6
220
X n i a x
1 X 10 (G —0,15) —6 Χ 10 3 [—8 Χ 10 3 ] 4
Constant,
k
Assignment
-4.0 Χ 104
(styrene)
-2.5 X 10*
( styrene ) "
—6 Χ 10 1 0 [3 Χ 10 1 1 ]
(styrene)"
7 Χ 10 3
4.5 Χ 10 3
C6H5'CHCH3
—1.2 Χ 10 4
1.6 Χ 10
CGH5CHCH3
7
ADDITION A N D CONDENSATION P O L Y M E R I Z A T I O N PROCESSES 228 there is already a great deal of information on the nature and reactions of the transient species, there are still many questions which need to be clarified.
Acknowledgments
The author thanks his colleagues, especially A . J. Swallow, for helpful discussions. Literature
Cited
(1) Adams, G . E., Baxendale, J. H., Boag, J. W . , Proc. Roy. Soc. (London) A277, 549 (1964). (2) Chapiro, Α., "Radiation Chemistry of Polymeric Systems," Interscience, New York, 1962. (3) Dorfman, L . M . , Matheson, M . S., Progr. Reaction Kinetics 3 (1965). (4) Ebert, M . , Keene, J. P., Swallow, A . J., Baxendale, J. H., Eds., "Pulse Radiolysis," Academic Press, New York, 1965. (5) Freeman, G. R.,J.Chem. Phys. 46, 2822 (1967). (6) Hart, E . J., Boag, J. W., J. Am. Chem. Soc. 84, 4090 (1962). (7) Hagemann, R. J., Schwarz, Η. Α., J. Phys. Chem. 71, 2694 (1967). (8) Hamill, W . H., Guarino, J. P., Ronayne, M . R., Ward, J. Α., Discussions Faraday Soc. 36, 169 (1964). (9) Hummel, Α., Allen, A . O., J. Chem. Phys. 44, 3426, 3431 (1966). (10) Katayama, M . , Hatada, M . , Hirota, K., Yamazaki, H., Ozawa, Y., Bull. Chem. Soc. (Japan) 38, 851 (1965). (11) Katayama, M . , Bull. Chem. Soc. (Japan) 38, 2208 (1965). (12) Katayama, M . , Yamazaki, H., Ozawa, Y., Hatada, M . , Hirota, K., Nippon Kagaku Zasshi 87, 37 (1966). (13) Keene, J. P., Nature 197, 47 (1963). (14) Keene, J. P., Land, E . J., Swallow, A . J., J. Am. Chem. Soc. 87, 5284 (1965). (15) McCarthy, R. L., MacLachlan, Α., Trans. Faraday Soc. 56 (1960). (16) Metz, D . J., Potter, R. C., Thomas, J. K., J. Polymer Sci., Pt. A-1 5, 877 (1967). (17) Porter, G., Strachan, E., Trans. Faraday Soc. 54, 1595 (1958). (18) Porter, G., Windsor, M . W., Nature 180, 187 (1957). (19) Potter, R. C., Johnson, C. L . , Metz, D . J., Bretton, R. H., J. Polymer Sci., Pt. A-1 4, 419 (1966). (20) Potter, R. C., Bretton, R. H., Metz, D . J., J. Polymer Sci., Pt. A-1 4, 2295 (1966). (21) Ronayne, M . R., Guarino, J. P., Hamill, W . H., J. Am. Chem. Soc., 84, 4230 (1962). (22) Sauer, M . C., Arai, S., Dorfman, L . H., J. Chem. Phys. 42, 708 (1965). (23) Schneider, C., Swallow, A. J., Proc. Tihany Symp. Radiation Chem., 2nd, Akademiai Kiado, Budapest, 1967. (24) Schneider, C., Swallow, A. J., J. Polymer Sci., Pt. Β 4 (1966). (25) Schneider, C., Swallow, A . J., Makromol. Chem. 114, 155, 172 (1968). (26) Shida, T., Hamill, W. H., J. Chem. Phys. 44, 4372 (1966). (27) Williams, F., private communication. R E C E I V E D March 25,
1968.
