J. Phys. Chem. 1981, 85,1365-1368
(e.g., in electrochemical reactions) and is useful in identifying intermediates. Note also that parallel "reductive oxidations" occur, for example, during the oxidation of oxalate, where the strong reductant COz-. is formed and ecl is observed.29 Similar mechanisms have also been
1365
proposed for chemiluminescence of diphenoyl peroxide and related compounds.30 Acknowledgment. The support of this research by the Army Research Office is gratefully acknowledged.
Flash Photolysis Study on Initiation of Radical Polymerization. Addition Rates of Benzothiazole-2-thiyl Radical to Vinyl Monomers Osamu Ito," Kbjl Nogaml, and Mlnoru Matsuda Chemical Research InsfHUte of Nonaqueous Solutions, Tohoku UnlversnV. Katahira-2, Semihi, 980 Japan (Received: M y 16, 1980: In Final Form: October 29, 1980)
Rate constants for addition (k,) of benzothiazole-2-thiylradical (BS-)to vinyl monomers (CH2=CHY) have been determined by means of flash photolysis. Reversibility of the reaction has been shown to be important in the decay process of BS.. Rate constants for the reverse reaction (k-2 and equilibrip constants (K = k,/k-J have been estimated as ratios of the rate constants for the reaction between BSCH2CHYand oxygen; oxygen was used as a scavenger for BSCH&HY because of the low reactivity of BS. toward oxygen. The k, values estimated in this work were in the range of 6.3 X lo5 (vinyl acetate)-2.5 X lo8 M-' s-l (styrene); the addition rates depend on both the resonance effect and the polar effect of vinyl monomers. On the other hand, the reactivity of the dimethylthiuramthiylradical toward vinyl monomers is too low to be determined by flash photolysis.
Introduction Some organic disulfides have been found to serve as excellent photoinitiators for radical polymerization of vinyl monomers.' The mechanism of the initiation reaction has been investigated by the rotating sector method2Band the spin trapping m e t h ~ d .Recently ~ ~ ~ we have applied the flash photolysis technique to evaluate kinetic parameters of the addition reaction of thiyl radicals to vinyl monomers.- An advantage of this method is that it allows one to determine the absolute rate constants for the initiation reactions. The compounds 2,2'-dibenzothiazolyl disulfide (BSSB) and tetramethylthiuram disulfide (TSST)were studied as thermal or photochemical initiators for polymerization of vinyl monomers.lyg Upon photolysis, radical polymerization of acrylonitrile or methyl methacrylate is initiated by BSSB but not by TSSTSgThe flash photolysis method should be useful to clarify this difference. The photochemistry of BSSB has been studied; the benzothiazole-2-thiyl radical (BS-) was suggested as an
BS.
TS.
intermediate in the photolysis of BSSB in ethanol yielding (1) W. A. Pryor, "Mechanism of Sulfur Reactions", McGraw-Hill,New York, 1962,p 42. (2)M. Onyszchuk and C. Sivertz, Can. J . Chem., 33, 1034 (1955). (3)C. Sivertz, J.Phys. Chem., 63,34 (1959). (4)T.Sato, M. Abe, and T. Otsu, Makromol. Chem., 178,1951(1977). (5)T.Sato, M. Abe, and T.Otsu, Makromol. Chem., 180,1165(1979). (6)0.Ito and M. Matsuda, Bull. Chem. SOC.Jpn., 51, 427 (1978). (7)0.It0 and M. Matsuda, J. Am. Chem. SOC.,101, 1815 (1979). (8)0.Ito and M. Matsuda, J. Am. Chem. SOC.,101, 5732 (1979). (9)T.Otsu, K. Nayatani, I. Muto, and M. Imai, Makromol. Chem., 27, 142 (1958). 0022-3654/81/2085-1365$01.25/0
the corresponding mercaptan (BSH).1° A transient absorption band around 350 nm was observed by flash photolysis and was attributed to BS..'O However, analysis of decay kinetics of the transient band was not reported because of the overlap with the band of BSSB. In this work, another transient band attributable to BS. was found in the visible region; thus the rates of addition of BS. toward vinyl monomers were estimated. A similar experiment was carried out for dimethylthiuramthiyl radical (TS.). Experimental Section Materials. 2,2/-Benzothiazolyldisulfide (BSSB) was recrystallized from benzene below 40 "C. Bis(2-benzothiazolyl) sulfide (BSB) was prepared by the method described in the literature." Tetramethylthiuram disulfide (TSST) and the monosulfide (TST) were used after purification. Vinyl monomers were distilled in the usual way before use. Cumene was distilled under reduced pressure over a stable free radical, l,l-diphenyl-2-picrylhydrazyl. The cyclohexane wed as solvent was of spectrophotometric grade. Methods. The flash photolysis apparatus was of standard design; the half-duration of the xenon flash lamps (Xenon Corp. N-851) was ca. 10 ps, and first-order rate constants less than 5 X lo4s-l were measurable. The flash photolysis measurements were performed in a cylindrical cell (opticalpath = 10 cm)at room temperature controlled at 23 f 1 "C. The oxygen concentration in cyclohexane was determined in the following way. After the solution was degassed up to ca. lo4 torr, oxygen was dissolved in the solution under an appropriate pressure of oxygen which (IO) H. Shizuka, K. Kubota, and T. Morita, Mol. Photochem., 3,335 (1972). (11) J. J. D'Amica, R. H. Campbell, S. T. Webster, and C. E. Twine, J. Org. Chem., 30,2625 (1965).
0 1981 American Chemical Soclety
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The Journal of Physical Chemistty, Vol. 85,No. 10, 198 1
It0 et al. -1
0.08
0.5
I
-2
z
C
-3 Wavelength, nrn
F b r e 1. Transient absorption spectra produced by the flash photolysis of BSSB and BSB In cyclohexane. Both aerated and degassed solutions gave slmilar spectra. The absorbance was plotted 22 ~s after M) and (b) BSB (1.5 X lo4 M). each flash: (a) BSSB (6.7 X Insert: second-order plot of the 580-nm band formed from BSSB in degassed cyclohexane (optical path length of flash cell is 10 cm).
was measured by a mercury manometer, and the oxygen concentration was calculated from Henry’s law by using the reported oxygen concentration in aerated cyclohexane (2.3 x 10-3 ~ ) . 1 2 Results and Discussion A transient absorption band was observed in the visible region upon irradiation of BSSB in cyclohexane with the flashlight between 250 and 400 nm (Figure 1). A similar transient band was observed from BSB, though the yield from BSB was considerably lower than that from BSSB. Since the transient absorption band at 580 nm was observed even in aerated solution, the carbon-centered radical formed by C-S bond dissociation is not attributable to this transient band; rate constants of 108-109M-’s-’ were reported for reactions between carbon-centered radicals and oxygen,13so that such a radical would not be observed with the xenon flash photolysis of aerated solution. Therefore, the transient absorption band shown in Figure 1 was ascribed to BS.. Upon irradiation of TSST with the flashlight, a transient absorption band was observed around 600 nm. A similar band was observed from TST. This absorption band was ascribed to TS., since the band was no sensitive to oxygen. In degassed solution the transient absorption band of BS- disappeared with second-orderkinetics as depicted in the insert in Figure 1. This suggests that BS. decays with recombination. From the slope, 2 k , / (k, ~ refers ~ ~ to the rate constant for recombination and 6 to the molar extinction coefficient) was obtained as 7 X lo5cm s-l. The recombination of BS. may be a diffusion-controlledreaction because 2kr/tmm increased with a decrease in solvent viscosity; k, can be calculated to be 6 X lo9 M-’s-l in cyclohexane from Debye’s eq~ati0n.l~As was discussed in detail elsewhere,14J6however, the real rate constant must be estimated by dividing this value by an empirical factor of ca. 2. Thus, cWnm is estimated to be ca. 8500 cm-l M-’ from the latter k, value, and the concentration of BS. immediately after the flash (Figure la) is calculated as ca. 4.4 X lo-’ M from Lambert-Beer’s law. On the addition of oxygen to solution, the decay rate of BS. increased slightly, and decay kinetics deviated from second-order kinetics. This is attributable to the reaction
-
(12) S. I. Murov, “Handbook of Photochemistry”, Mercel Dekker, New York, 1973, p 89. (13) J. A. Howard in “Free Radicals”, Vol. 11, J. K. Kochi, Ed., Wiley, New York, 1973, p 1. (14) K. U. Ingold in “Free Radicals”, Vol. I, J. K. Kochi, Ed., Wiley, New York, 1973, p 37. (15) 0. Ito and M. Matauda, Can. J. Chem., 58, 1080 (1978).
-4 time, p s
Flgure 2. Flrst-order plots for the decay of BS. at 580 nm: (a) In degassed solution, (b) In the presence of vinyl acetate (1.2 X 10-1 M) In the degassed solutlon, and (c) in the presence of both vinyl acetate (1.2 X 10-1 M) and oxygen (2.3 X 10“ M).
between BS. and oxygen, which may show pseudo-firstorder kinetics since the oxygen concentration ( N 10-s-10-2 M)is larger than that of BS-. Graphic methods and a computer simulation method were applied to separate the first-order rate constants from mixed-order decay c ~ r v e s However, . ~ ~ ~ ~since the contribution of first-order kinetics was small even in oxygen-saturated solution, an accurate first-order rate constant could not be estimated. Thus, the rate constant for the reaction between BS- and O2 was estimated as a limiting value, lo4 M-’s-l, by dividing the pseudo-first-orderrate constants by the oxygen concentrations. On the addition of a vinyl monomer to degassed solution, decay of BS- was not accelerated; an example for vinyl acetate is shown in Figure 2a and b. In the presence of both vinyl acetate and oxygen, the decay rate of BS- increased (Figure 2c) and the decay curve changed into first-order kinetics. These findings suggest that the addition of BS. to vinyl monomers (CH2=CHY)occurs reversibly and that oxygen reacts with the resulting carbon-centered radical; this radical can be shown as BSCH&HY since the addition of thiyl radicals is known as anti-Markovnikov.20 A similar phenomenon was found for other vinyl monomers, and the above findings are reasonably explained by Scheme I. The reaction of Scheme I BSSB BS. + CH2=CHY
5 2BS. kr
.& BSCH2CHY k,
ko
02
(1)
peroxy radical
(2) BSCH2CHYwith O2may yield a peroxy radical (probably, BSCH2C(O0.)HY);the subsequent reactions have been (16) E. F. Zwicher and L. I. Grossweiner, J. Phys. Chem., 67,649 (1963). (17)G. L. Closs and B. E. Rabinow, J. Am. Chem. SOC.,98,8190 (1976). (18) Both of the methods need the initial concentration of the thiyl radical or the initial absorbance,which contains uncertainty because of the emission of the flash lamp. In our preceding paper,’g we proposed a method to analyze mixed-order kinetics without using the initial quantity. (19) M. Nakamura, 0.Ito, and M. Matauda, J. Am. Chem. SOC.,102, 698 (1980). (20)G.Sosnovsky, “Free Radical Reactions in Preparative Organic Chemistry”, Macmillan Co., New York, 1964.
The Journal of Physical Chemlstiy, Vol. 85, No. 10, 1981 1367
Initiation of Radical Polymerization
TABLE I: Rate Constants k,, k-,lk,, and K k , for the Addition of BS. t o Various Vinyl Monomers" vinyl monomer e Q k,, M-I s-l k-alko, M K k , , M-* s-l St -0.8 1.0 2.5 X lo8 MMA 0.4 0.74 2.8 x 107 ~9.4 x 10-4 >3.0 x lolo AN 1.2 0.6 1.2 x 106 ~ 2 . x4 10-4 >4.9 x 109 IBVE -1.77 0.023 3.7 x 106 2.3 x 10-3 1.6 x 109 IPA -0.5 0.045 2.1 x 106 7.1 x 10-3 3.0 X lo8 VAc -0.22 0.026 6.3 X lo3 2.1 x 10-3 2.9 X 10' Estimation errors are ca. i20% except k-,/ko and K h , of MMA and AN which are limiting values. Oxygen concentration: 1.4-12 mM. Abbreviations of vinyl monomers and their concentrations: St, styrene (0.04-0.3mM);MMA, methyl methacrylate (0.2-3mM); AN, acrylonitrile (4- 50 mM); IBVE, isobutyl vinyl ether (4-20 mM); IPA, isopropenyl acetate (10-100mM);VAc, vinyl acetate (10-100 mM).
,
'
-
-2
-
'
2 C
OO
-3.
10
20
[IBVE] , m M
Figure 4. Plots of k , vs. [IBVE] under various concentrations of oxygen. [O,]: (a) 1.2 X lo-*, (b) 3.8 X lo4, (c) 2.3 X IO3, and (d) 1.4 X lo-' M. Insert: plot of l/kawvs. 1/[02]. -4
0
100 t i m e , JJS
200
Figure 3. First-order plots for the decays of BS. at 580 nm in the presence of isobutyl vinyl ether (IBVE) in aerated solution. [IBVE]: (a) 4.3 X lo4, (b) 8.6 X lo4, (c) 1.3 X lo-*, and (d) 1.7 X lo-* M. Insert: plot of k l (slopes In Figure 3) vs. [IBVE].
investigated in detail by Oswald et al.21 In the case of TS-,decay of TS-was not accelerated in the presence of both vinyl monomer and oxygen. This suggests that the reactivity of TS. toward vinyl monomers is too low to be detected with flash photolysis. This may be a reason that photoinduced radical polymerization of vinyl monomers was not initiated with TSST. From Scheme I, decay of BS. is expressed as in eq 3, -d[BS*]/dt = 2k,[BS*I2 + ke[BS*][ CH,=CHY] - k,[BSCH&HY] (3) where the reaction of BS*with oxygen has been eliminated because of its small contribution. Under our experimental conditions, given in Table I, [CH,=CHY] is larger than [BS.] by a factor of -102-10s; thus [CH2=CHY] may be considered to be kept constant during the reaction. Decay rates of BS. increased with an increase in [CH,=CHY]; this behavior is shown in Figure 3 for isobutyl vinyl ether (IBVE) as an example. The fmt-order rate constants were estimated from the slopes. When [IBVE] is small, deviation from first-order kinetics was observed (Figure 3a) because of mixing with second-order kinetics. In this case, the first-order rate constant must be estimated by the computer simulation method or graphical The first-order rate constants are linearly proportional to [IBVE] (insert of Figure 3). Under our experimental conditions, [O,] may be kept constant during the reaction since [O,] is in large excess compared with [BSCH&HY]
which may be smaller than [BS.]. Thus, from eq 3 the first-order part (kI) with respect to [BS.], which is proportional to [CH,=CHY], can be expressed as eq 4 by kI = k,(l - k,/(k, + ko[O,]))[CH2=CHY] (4) applyin the steady-state approximation with respect to [BSCH2 HY].,, Division of kI by [CH2=CHY] gives the second-order rate constant (kap), and the relation between keppand [O,] is clearly shown in eq 5. Figure 4 shows that (5) 1/kapp = l/ka + k-a/(kak~[od)
%
slopes of plots of kI vs. [IBVE] increased with an increase in [O,]. A fairly good linear correlation may be obtained for a plot of l/ka vs. 1/[02]; the intercept and slope yield l/ka and k,/(kago), respectively. From these values, rate constants k, and k-,/k0 could be calculated. The equilibrium constant, K = k,/k,, may be obtained in the form of Kko. These values for the reactions with other vinyl monomers were obtained in a similar manner and are summarized in Table I. Errors inherent in the flash photolysis method are of the order of ca. &lo%,but in the process of the estimation the errors may increase up to ca. *20%. Since the slope of the plot of eq 5 for styrene was negligibly small, k,/ko could not be estimated. The slopes for methyl methacrylate (MMA) and acrylonitrile (AN) were small but appreciable, so each upper limiting value of k,/ko was obtained. In order to estimate the reactivity of BS. in hydrogen abstraction, we used cumene as a hydrogen donor. The decay curve of BS. in cumene was very close to secondorder kinetics even in oxygen-containing solution and was similar to that in the mixed solution (cyclohexane and benzene) adjusted to the same viscosity as cumene. From the small difference in the first-order plots, which may be (22) The validity of the steady-state approximationis supported since
(21) A. A. Oswald, K. Griesbaum, and B. E. Hudson, Jr., J. Org. Chern., 28, 2361 (1963).
k,[O*]>> k, [CHdHY], in which k 0 [ 0 2 ] 22 X le 8-l in aerated solution from the literaturela and k, [CH,=CHY] 5 6 X 10' 8-l under our experimental conditions.
1368
The Journal of Physlcal Chemistry, Vol. 85, No. 10, 1981 101
\
91
-
5
I
,
-2
-1
0
1
e value
Flgure 5. Plot of log k, vs. e value for vinyl monomers (abbreviations of vinyl monomers refer to Table I).
attributable to the hydrogen abstraction reaction, the rate constant was found to be less than ca. lo2M-I s-l. In the case of tert-butoxyl radical, it is reported that the hydrogen-donor ability of the methyl group located at the a position of the double bond is similar to that of benzylic hydrogen.23 If this is also the case for BS., the rate constant for hydrogen abstraction from such a vinyl monomer as MMA may be less than lo2M-l s-l, and lower hydrogen-donor abilities are assumed for other vinyl monomers. Thus, it is evident that the 12, values in Table I virtually exclude the rate process for hydrogen abstraction. Rate constants for reactions between thiyl radicals and vinyl monomers have been reported, and the order of the rate constanb with styrene h the following: 24 n-butylthiyl radical (8 X lo8 M-l ~4-9~ > BS. (2.5 X lo8 M-' s-l) > p-chlorobenzenethiylradical (5.1 X lo7 M-' s-l) > TS-. If the activation energy of the reaction is proportional to the difference in the thermodynamic stabilities between the product and reactant, the above order may correspond to an opposite order of the thermodynamic stability of the thiyl radicals since the carbon-centered radicals in the products may have a similar stability. On the other hand, if the reactivity is controlled by the polar character of the thiyl radicals, the above order of the reactivity may be that of the electrophilicity of the thiyl radicals since styrene has an electron-rich double bond. However, p-chlorobenzenethiyl radical, having an electron-withdrawing substituent, seems to be the most electrophilic radical among them; thus the stability of the thiyl radicals may determine the reactivity more efficiently than the polar effect.2s According to the theory proposed by Alfrey and Price,* the reactivity of vinyl monomers is determined by both (23) T. Sato and T. Otau, Makrornol. Chern., 178,1941 (1977). (24) The value of the n-butylthiyl radical is the rate constant estimated by the rotating sector method*J and other values are of flash photolysis.? Since no reaction system investigated by both methods has been reported, we should be careful in comparing small differences in the rate constants estimated by the different methods. In this caw, however, the differencemay be large enough to be compared since the rate constant of n-butylthiyl radical is 3 times larger than that of BS.. (25) The order of radical absorption maxima at the longest wavelength is as follows: n-butylthiylradical (330 nm)< pchlorobenzenethiylradical (510 nm)' < BS. (580 nm) < TS. (600 nm). In general, a bathochromic shift may be caused by the delocalization of an unpaired electron in the free radical, which may thermodynamically stabilize the free radical. Thus, the order in the radical absorption maxima might be opposite to that in the rate constanta. This is realized except for BS.. It is suggested that a large bathochromic shift is caused in BS. because of the presence of the heteroatoms near the radicd center. (26) J. Alfrey and C. C. Price, J. Polym. Sci., 5, 101 (1947).
It0 et al.
the resonance factor and the polar factor; the Q value is a measure of the former factor, and the e value of the latter (Table I). Figure 5 shows a plot of log k, vs. e value. The points in this figure are clearly divided into two groups: the conjugated vinyl monomers, having large Q values belong to the upper group, and nonconjugated vinyl monomers to the lower one. Comparison at similar e value reveals that the k, value for conjugated vinyl monomers is larger than that for nonconjugated ones; an increase in the conjugation of ?r electrons in vinyl monomers accelerates the addition rates. Each group in Figure 5 is correlated with a line having a negative slope, which indicates that the polar effect also affects the addition rates. The negative slope suggests the contribution of polar resonance structures (Le., [BS-, CHpCHY+*])to the transition state, which increases with an increase in electron-donating power of Y; this reduces the activation energy and increases the addition rate. In Figure 5, the slope for conjugated vinyl monomers is steeper than that for nonconjugated 0nes.2~ It can be presumed that the polar effect for the former group is more efficient than that for the latter one. Rate constants for reactions between carbon-centered radicals and oxygen have been reported to be close to diffusion ~0ntrolled.l~ From a general principle that the higher the reactivity the lower the selectivity, it can be assumed that the variation in ko upon replacing Y in BSCH&HY is small. On the basis of this assumption, the meanings of k-,/k0 and Kko may become clear; k,/ko is the relative rate of the reverse reaction and Kk is a measure of the thermodynamic Stability of BSCH2bHY. In Table I, there is a tendency for Kko to increase with an increase in k, or Q value. This suggests that an increase in the stability of BSCH2CHY, which increases with the conjugation power of Y, reduced the activation energy and accelerates the addition rate. This is one of the possible explanations why the addition rates for conjugated vinyl monomers are faster than those for nonconjugated ones, as seen in Figure 5. By assuming ko to be 108-109 M-' s-l Is we can calculate k, to be 104-108s-' and K to be l&-102 M-' from Table I. The k, values suggest that the reverse reactions are considerably fast unimolecular reactions; thus the presence of the reverse reaction may reduce the initiation efficiency in polymerization induced by BS-. The K valuea in the range of ~ 1 0 - l - 1 0M-l, ~ however, suggest that [BSCH2CHY]becomes large enough to propagate polymerization if the reaction is taking place in the degwed bulk monomer system upon steady-light photolysis of BSSB. On the other hand, in the case of TS- the initiation step itself does not occur because of low reactivity of "73. toward vinyl monomers. These estimations are supported by the experimental results reported by Otsu et ale9 In conclusion, the rate constants for the addition reactions of BS. toward vinyl monomers were determined by means of flash photolysis, and high reactivity of BS. toward vinyl monomers was confirmed. The addition rates are controlled by both the resonance effect and the polar effect of the vinyl monomers. Low reactivity of "73. toward vinyl monomers was also confirmed.
-
N
-
(27) A similar tendency was reported for the relative rate constanta of the benyylthiyl radical reactions toward vinyl monomers by Sato, Abe, and Otau.
