J . Phys. Chem. 1987, 91, 937-941
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the case of scrambling, an isotropic label distribution could result even if the monomer residence time were much shorter than the chain residence time. In this case, the majority of the active carbon would be comprised of growing chains. Further evidence against rapid and complete scrambling comes from the iron data in the present work. If the entire product scrambled, then there should be a statistical distribution for the molecular weights, and we show this is not the case.24
V. Concluding Remarks The estimates of residence times and concentrations ignore the possible contributions of very unreactive intermediates which contribute little to the overall rate. Such material would appear in the long tails in the transient. Because of the long residence time on the surface, this could represent an appreciable amount of carbon, even though it does not contribute appreciably to the rate. The chain growth rates derived here are much faster than previously thought. While in qualitative agreement with earlier conclusions of Biloen,6 they place an order of magnitude more stringent limit on the rate of chain growth. However, Zhang and Biloen have recently reported measurably slow chain growth on a cobalt catalyst at conditions comparable to those here.8 The time constants for chain growth which they report are 20-30 times slower than the limit set in Table 11. They base their analysis on sequential appearance of 13C in successively longer hydrocarbons (C, through C3). Their data are clearly at odds with our data in Figures 8 and IO, which show simultaneous infusion of
-
937
13C into C, and C3 C6 products. The reason for this disagreement is unclear. However, Zhang and Biloen did not use GCMS and the details of how the very complicated reduction of the mass spectrometric data was accomplished have not been published. Alternatively, subtle differences in catalyst properties and/or operating conditions might make a large difference in chain growth rates. The modeling calculations require the lifetime of the chain building monomer to be long compared to the time for the chain growth step. This longer residence time masks the kinetics of chain growth and precludes the possibility of learning more about the mechanism of chain growth on these catalysts. The chemical identity of this precursor bath is undetermined. Any precursor to chain growth could be the species with the long residence time. Possibilities include surface carbidic carbon, methylene groups, or a minority state of chemisorbed CO. (The majority of C O is reversibly adsorbed with a lifetime of less than 1 s, as in Figure 7 . ) The fact that the activation energy of the exchange rate of the monomer matches that of C O conversion rate implies that the monomer exchange rate is governed by the rate of product formation. This in turn implies that this precursor bath is irreversibly adsorbed.
Acknowledgment. We acknowledge the help of M. Melchior, R. Pabst, and K. Rose for the N M R measurements. We also thank J. Krajewski for technical assistance and D. Jocelyn for manuscript preparation. Registry No. CO, 630-08-0; Co, 7440-48-4; Fe, 7439-89-6.
Reaction of Solvated Electron with Poly(methy1 methacrylate) and Substituted Poly(methy1 methacrylate) in Hexamethylphosphoramide Studied by Pulse Radiolysist Masaaki Ogasawara,* Migaku Tanaka, and Hirosi Yoshida Faculty of Engineering, Hokkaido University, Sapporo 060, Japan (Received: July 9, 1986; In Final Form: October 3, 1986)
Short-lived reaction intermediatesproduced by pulse radiolysis of solutions of poly(methy1methacrylate), poly(ethy1 methacrylate), poly(n-butyl methacrylate), and poly(isobuty1methacrylate) in hexamethylphosphoramidehave been investigated. Immediately after an electron pulse of 20-11s duration, transient absorption spectra which have absorption bands in the UV and in the longer wavelength side of the visible (>600 nm) were observed in all the solutions examined. The former and the latter bands are assigned to the anion radical of the polymers and the solvated electron, respectively. Reactions between the solvated electrons and the polymers and the decay reactions of the polymer anions were studied.
Introduction There is growing interest in the reaction of polymers in solution. Different from the low-molecular-weight molecules, the kinetic behavior of polymers is not always straightforward, since the reactions are much more affected by a diffusional process as well as the conformational motion of polymers in solution which has not fully been understood yet. A number of experiments have been performed to investigate the effects of polymers,' yet there have been only a few reports concerning the reactions in which polymer ions are involved.2 In the present paper solutions of poly(methy1 methacrylate) (PMMA) and substituted PMMA in hexamethylphosphoramide (HMPA) have been studied by pulse radiolysis in the hope of clarifying the kinetics of the fast electron-capturing reaction by the polymers. As far as the authors know, this is the first attempt to measure the reaction rate of solvated electrons with polymers in nonaqueous solution. Another aim of this paper is to provide spectroscopic informations about ionic species produced from PMMA and its ana'Dedicated to Professor Leon Dorfman on the occasion of his retirement.
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logues. It seems that ionic intermediates play important roles in radiation-induced degradation of solid PMMA. Torikai et al. studied ESR and absorption spectra of y-irradiated solid PMMA at 77 K and suggested that trapped electrons or PMMA anions were produced in the Recently, PMMA films were investigated by Tabata et al. by nanosecond pulse radiolysis at various temperatures, and by ESR after y-irradiation at 77 K.8 (1) (a) Behzadi, A., Borgwardt, U., Henglein, A,, Schamberg, E., Schnabel, W. Ber. Bunsenges. Phys. Chem. 1970, 74,649. (b) Matheson, M. S., Mamou, A., Silverman, J., Rabani, J. J . Phys. Chem. 1973, 77, 2420. (c) Niki, E., Kamiya, Y. J . Chem. Soc., Perkin Trans. 2 1975, 1221. (2) Washio, M.; Tagawa, S.; Tabata, Y. Polym. J . 1981, 13, 935. (3) Torikai, A,; Asai, T.; Suzuki, T. J . Polym. Sci. Chem. Ed. 1975, 13, 797. (4) Torikai, A,; Kato, H.; Kuri, Z. J . Polym. Sci. Polym. Chem. Ed. 1976, 14, 1065. (5) Torikai, A.; Kato, R. J . Polym. Sci. Chem. Ed. 1978, 16, 1487. (6) Torikai, A,; Okamoto, S. J . Polym. Sci. Polym. Chem. Ed. 1978, 16, 2689. (7) Torikai, A,; Mishina, H. J . Polym. Sci., Polym. Chem. Ed. 1981, 19, 2297. ( 8 ) Tabata, M.; Nilsson, G.; Lund, A,; Sohma, J. J . Polym. Sci., Polym. Chem. Ed. 1983, 21, 3257.
0 1987 American Chemical Society
938 The Journal of Physical Chemistry, Vol. 91, No. 4, 1987
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Short-lived transients with optical absorptions at 440 and 725 nm observed were assigned to cations and anions of PMMA, respectively. However, the assignments of the absorption bands by these two groups are not consistent with each other and, in our view, the formation of PMMA ions has not experimentally been confirmed yet.
Experimental Section Pulse Radiolysis Apparatus. The source of the electron pulse was Hokkaido University 45-MeV linear accelerator. The pulse was changeable in its duration to as short as 10 ns, but usually a 100-ns pulse with a dose of 140 Gy/pulse was used. The optical system was described e l ~ e w h e r e . ~The electron signals from a Hamamatsu Photonics R446 photomultiplier were digitized by an Iwatsu TS-8 123 storage scope and the data were processed by a NEC PC-9801 personal computer. Materials. HMPA was dried over calcium hydride for 24 h, refluxed with fresh calcium hydride for 2 h, and distilled at a reduced pressure before use. PMMA and substituted PMMA from Science Products SP3Co. Ltd. were purified by repeating the following procedures: the polymer was dissolved in THF, precipitated by adding methanol into the solution, filtered, and dried under reduced pressure. Occasionally, the PMMA samples that were synthesized by radiation-induced polymerization were used, but there was no difference in experimental results. The purified polymers were frozen by liquid nitrogen and smashed into a powder by a hammer, and a weighted amount of polymer powder was dissolved in HMPA with stirring of the solution for several hours in vacuo. The solution was transferred from a vessel to an optical cell in vacuo. Cell. Rectangular cells made of Supracil with a single entry tube connected to a vacuum line for filling the solutions and for degassing the sample solutions were used. The inside dimensions of the cell were 45 X 10 X 10 mm with the light path being 10 mm. The cell was sealed off under vacuum with the solution frozen in liquid nitrogen. Calculation. The ab-initio SCF MO calculation was made with the JAMOL3 program.I0 For the open-shell doublet of anions, restricted Hartree-Fock (ROHF) approaches and minimal basis sets at the STO-3G level were used. Results and Discussion Absorption Spectra of Reaction Intermediates. Figure 1 shows the absorption spectra obtained in solutions of 0.2 unit mol % PMMA and its substituted analogues (the polymer concentration is expressed in monomer units throughout this paper) in HMPA by electron pulses of IO-ns duration. Immediately after a pulse, a strong absorption in the UV and a broad absorption in the visible and the near-IR were observed for all the solutions examined. With time, the absorption on the longer wavelength side of the visible (>600 nm) decreased and the absorption on the UV (300-350 nm) increased. At 320 nm the absorption rose rapidly after the pulse, being accompanied with a rapid decay of the near-IR band, and then it decayed slowly. The spectrum at 200 ns after the end of the pulse consists of an absorption band in the UV which increases continuously toward the shorter wavelength side and a broad shoulder or a long tail extending to the near-IR. At 2 w s after the end of the pulse, the absorption bands in the UV and on the shorter wavelength side of the visible still survived, but the absorption in the near-IR disappeared completely. The shape of the shoulder or the tail in the range 400-600 nm is different depending on the kind of ester alkyl group in the polymers. In the spectrum for PMMA, no distinct maximum is discernible in this range but only a smooth, long terrace is shown (Figure 1A). The spectra for poly(ethy1 methacrylate) (PEMA) and poly(isobuty1 methacrylate) (PIBMA) show a broad peak at ca. 440 nm and a kind of terrace extending from 400 to 500 nm, (9) Ogasawara, M.; Kajimoto, N.; Izumida, T.; Kotani, K.; Yoshida, H J . Phys. Chem. 1985, 89, 1403 (10) Program J A M O L ~written for the Program Library at the Hokkaido University Computing Center by Kashiwagi, H.; Tanaka, T.; Miyosi, E.; Obara, S
Ogasawara et ai.
0.1 0.15[ 0
0.05
tI
0’15 O.l0t
a Di
0.05
400
600
500
700
800
A Inm
Figure 1. Transient absorption spectra observed in pulse radiolysis of solutions of (A) poly(methy1 methacrylate), (B) poly(ethy1 methacrylate), (C) poly(n-butyl methacrylate),and (D) poly(isobuty1methacrylate) in HMPA. The polymer concentration is 0.2 unit mol % for all cases: -e-, 60 ns; ---, 200 ns, -0-, 2 ps after the end of an electron pulse of IO-ns duration.
respectively (Figure 1, B and D). In the absence of polymer a very broad absorption band in the visible which increases continuously toward the near-IR region was observed together with a rather weak, long-lived absorption in the UV. The former band has already been assigned to solvated electron, es-, in HMPA by Shade et al.:” the absorption band of e[ has a maximum at 2200 f 100 nm with very long tail extending to the visible and even to the UV. The weak band in the UV, which evidently does not belong to the e; band, may be due to radicals or cations from the solvent. The time-dependent spectra shown in Figure 1 can be interpreted in terms of the reaction between the solvated electrons and the polymers. In the solution of PMMA, for example, the following reactions may take place: HMPA e,-
---
HMPA+ + e,-
-
+ HMPA+ e,-
e,,,- 4- PMMA e,- 4- PMMA
HMPA*
e,-
-
(1) (2)
(3)
PMMA-
(4)
PMMA-
(5)
(11) Shade, E. A.; Dorfman, L. M.; Flynn, G. J.; Walker, D. C. Can. J . Chem. 1973, 51, 3905.
The Journal of Physical Chemistry, Vol. 91, No. 4, 1987 939
Substituted P M M A in HMPA
h
c; .-
c
C
A
3
2
c.
0
Y
.-
Et
0" 600 800 Alnm Figure 2. Absorption spectra observed in a y-irradiated MTHF matrix containing 0.01 mol dm-, methyl isobutyrate at 77 K ---, immediately after y-irradiation; -, after photobleaching by light >690 nm. Vertical bars indicate electronic transitions determined theoretically by use of ab-initio SCF-MO calculations.
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200
B
400
where e,,,- denotes a mobile electron prior to solvation. The absorption band on the longer wavelength side of the visible (>600 nm) observed in the solutions of polymers immediately after the pulse is mostly due to e; which are produced by process 3. The absorption band in the UV together with the shoulder or tail in the visible is assigned to polymer anion radicals produced by reactions 4 and 5 for the following reasons. (1) HMPA is known to be a good solvent for observing the anion, since it is a liquid of moderately high dielectric constant which solvates the positive ion very strongly to prevent a fast recombination reaction with the anion." (2) The absorption grew at the expense of the e; band and the decay of e; was accelerated by an increase of the polymer concentration. (3) Addition of carbon tetrachloride, an efficient electron scavenger, suppressed the absorption completely. (4) A similar spectrum was obtained at 77 K by use of a 2methyltetrahydrofuran (MTHF) matrix which has been known to be a good matrix for observing anions at low tet~peratures.~ The following ketyl-type anion is the most probable structure for the PMMA anion: wCH,-CH
(CH~)W
I _ .co
This structure is confirmed, either experimentally and theoretically, by using methyl isobutyrate as a model compound. The y-irradiated MTHF matrix containing methyl isobutyrate at 77 K shows an absorption spectrum with two bands, in the UV and in the near-IR, as indicated by the dashed line in Figure 2. The near-IR band is due to the well-known trapped electron in the M T H F matrix. On photobleaching the electron, the band in the UV increased and a broad shoulder became discernible at ca. 450 nm as shown by the solid line in Figure 2. The spectrum after photobleaching should be responsible for the methyl isobutyrate anion radical produced by electron capture of a solute molecule. It should be noted that the spectrum is similar to that observed 2 p s after the end of the pulse in polymer solutions. The ab-initio SCF-MO calculation of the molecular anion of methyl isobutyrate suggests allowed a-a* transitions that absorb light at 210-240 nm and a forbidden n-a* transition at 420 nm as indicated by the vertical bars in Figure 2. Therefore, a strong absorption band at COO, "Criegee intermediate", analogous to the corresponding solution-phase reaction,*-I0 i.e.
7\0
0
(1)
However, considerable uncertainties exist concerning the mechanism of the subsequent fate of the biradicals. For instance, in the O3reactions of ethylene, propylene, and cis- or trans-2-butene under atmospheric conditions, less than 40% of the ensuring biradicals H 2 C O 0 and C H 3 C H O 0 have been shown to react with aldehydic compounds to yield the corresponding secondary ozon i d e ~ , ~ ,Le., ~ " reaction 2. Presumably, the remaining fraction 0-0
thermochemically accessible isomerization and dissociation channels," e.g.
dissociation products
At present, details of these unimolecular processes are not well established.' The present long-path FTIR study of the 03-tetramethylethylene (TME) reaction was made upon mixtures typically containing part-per-million ("Torr) concentrations of these reactants in 700 Torr of Nz-02 in an attempt to probe various reaction channels operative for the dimethyl-substituted biradical (CH3)2CO0under atmospheric conditions. Among the potentially important reaction channels suggested by previous theoretical studies' and experimental results in the gas-3 and solution-phases9 are the bimolecular reaction with aldehydes (reaction 2) and the following unimolecular processes: CH3\*
CH3,
dissocilltion
c=o
/c-oo CH3
of the biradicals formed from these light olefins decays unimolecularly to molecular and free radical products via numerous
0022-3654/87/2091-0941$01.50/0
+
0 ~ 3 ~ )
CH3/
-
CH3
isomerization
0
0.
\ /
c
CHI/
II
-CH~-C-OCH~'
OOH H migration
-
\Os
products (CH3;CH30;CH3CO;CO;COz)
(1)Wadt, W.R.;Goddard, W. A., 111 J . Am. Chem. Soc. 1975,97,3004. (2)Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Chem. Phys.
Lett. 1977, 46, 327. (3)Martinez, R. I.; Herron, J. T.; Huie, R. E. J. A m . Chem. Soe. 1981, 103, 3807. (4) Su, F.; Calvert, J. G.; Shaw, J. H.J . Phys. Chem. 1981,84, 239. (5) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. J . Phys. Chem. 1981,85,1024. (6)Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Emiron. Sei. Teehnol. 1983,17, 312A. (7)Atkinson, R.;Carter, W. P. L. Chem. Rev. 1984,84, 437. (8)Criegee, R.Angew. Chem. 1977,49, 13. (9)Bailey, P.S.Ozonation in Organic Chemistry; Academic: New York, 1978;Vol. 1. (10)Kuczkowski, R.L. Ace. Chem. Res. 1983,16, 42.
-
(4)
-
I
-CH~=C-CH,~
0 products
*
II
(CH2-C-CH3
; HO)
(5)
The intermediate species indicated by the thermodynamic symbol (1 1) "Chemical Kinetic Data Needs for Modeling the Lower Troposphere", NBS Spec. Publ. (US.) 1979,557.
0 1987 American Chemical Society
