2842
S. CHABEREK, R. J. ALLEN,AND A. SHEPP
Dyesensitized Photopolymerization Processes.lS IV.
Kinetics and
Mechanism of ThioninepDiketone-Aerylamide Systems
by S . Chaberek,'b R. J. m e n , and A. Shepp Technkl O p e ~ a t ~ o&seuTch, n~ BuTl>on, Maaaaehusett8
(Re&ved
January 4,1966)
A detailed study of the kinetics and reaction mechanism of thionine- (and methylene blue) sensitized photopolymerization of acrylamide in the presence of 2,4-pentanedione and dimedon has been made. Experimental data are consistent with a mechanism involving the formation of a light-excited complex between dye and p-diketone and its subsequent reaction with monomer to give dye bleaching and polymerization. As with amine-activated processes described in the first two papers in this series, the polymerization-initiating free radical appears to be semithionine (or semi(methy1ene blue)).
Introduction p-Diketones such as 2,4-pentanedione and dimedon have recently been found to function as photoreducing agents in conjunction with thionine and methylene blue for the photopolymerization of acrylamide.2 Studies into the relation between photoactivity and molecular structure showed that the active compounds are those capable of undergoing keto-enol tautomerism, and that the enol forms are responsible for reactivity. Nevertheless, even the limited data reported previously showed that there are distinct differences in the photoactivity of the @diketones compared to that of tertiary amines and amino acids. The most significant contrast is in the dye bleaching reactions. @-Diketonesrequire the presence of monomer to bring about dye fading; the tertiary amines and amino acids photobleach the dyes in the absence of monomer. Thus, in view of the differences in the behavior of the p-diketones, a more detailed investigation into their reaction mechanism was desirable. This paper summarizes, then, our studies on the anaerobic photopolymerization of acrylamide by free radicals formed by reaction of light-excited thionine (or methylene blue) with 2,4-pentanedione (AA) and dimedon (DM).
Experimental The acrylamide used in this investigation was a pure sample obtained from the American Cyanamid
Co. Preliminary screening of this sample for anaerobic thionine-sensitized photopolymerization showed no polymer formation in the absence of added activators and only a trace of dye bleaching over periods of 10 to 15 min. Thionine and methylene blue were purified by three recrystallizations from water. The absorption coefficients were found to be 5.8 X lo4 1. cm.-l mole-' for thionine (at 5980 8.)and 6.4 X lo41. cm.-l mole-' for methylene blue (at 6620 B.). Dimedon and 2,4pentanedione were obtained from the Aldrich Chemical co. The general experimental procedures used in this study were the same as those described previously.238 The rate of dye bleaching Rr was calculated directly from spectrophotometric measurements. The polymerization rate R, was determined by taking aliquots of the reaction solution at several time intervals, precipitating the poIyacrylamide in methanol, filtering it, and drying the residue to constant weight. Polymer molecular weights were determined viscometrically.
(1) (a) This study was performed under Contract No. AF 33(657)8754 and AF 33(647)-11553, Photographic Branch, Reconnaissance Division, Air Force Avionics Laboratory, WrightrPatterson Air Force Base, Ohio,J. R. Pecqueux, Project Engineer: (b) to whom inquiries should be sent at Cowles Chemical Go., Skksneatelea Falls,N. Y. (2) S. Chaberek, R. J. Men, and G. Goldberg, J . Phya. Chem., 69, 2834 (1965). (3) A. Shepp, 8. Chaberek, and R. MacNeil, ibid., 66, 2563 (1962).
KINETICS AND MECHANISM OF THIONINE-p-DIKETONE-ACRYLAMIDE SYSTEMS
Experimental Data on Dye Bleaching and Polymerization Rates as a Function of Reaction Parameters Activator Concentration. Table I summarizes the rates of dye bleaching and polymerization for systems M thionine or methylene blue, 0.704 M containing acrylamide, and varying concentrations of AA or DM M . Solution pH values between 8.0 and 40.0 X were 8.55 for thionineAA, 8.60 for methylene blue-AA, and 7.0 for thionine-DM. The general trend for both DM and AA parallels that observed with amineptivated systems such as triethanolamine4 (TEA) in that both Rf and R, increase with an increase in the activator concentration. In contrast to the TEAcontaining systems, however, similar Ri and R, dependences on the AA level were found for both thionine and methylene blue. For TEA, amine concentrations had to be about five times greater in the presence of methylene blue to obtain comparable rates with thionine-containing systems. Finally, Rr values for DM are comparable with those for AA; however, R, is 40 to 50% smaller for DM. Table I: Effect of Acetylacetone and Dimedon Concentrations on R f and R, for the Anaerobic Thionine- and Methylene Blue-Acrylamide Systems" AA (or DM) ooncn., M X IOs
2.0 4.0 8.0 12.0 16.0 24.0 40.0 80.0
--Thionine -AA-DMRr X 10'
0.397 0.701 1.212 1.550 1.688 2.339
...
...
systems Rp
X 108 R f X
0.40 0.58 0.92 0.99 1.17 1.36
... ...
lo7
Methylene blue SYEtemE A -AR, R p X 108 Rf X I O 7 X IO'
.
...
...
0.667 1,102
0.35 0.54
1.515
0.70
2.381 2.720
0.87 1.06
...
...
...
...
...
... ...
...
1.140
0.99
1.667 2.204 2.469
1.33 1.33 1.77
...
...
2843
in two respects. First, in the absence of monomer, no significant dye bleaching occurs with both AA and DM; in the amine systems extensive photobleaching is obtained in the absence of monomer. Second, while Rf decreases with an increase in monomer level in the amine systems, Table I1 shows that the rate is independent of monomer concentration for AA and actually increases with an increase in monomer level for DM in the range 0.141 to 1.265 M. At lower acrylamide AA also concentrations (1.4 X 10-1 to 1.4 X parallels the trend with DM.
Table 11: Effect of Acrylamide Concentration on Rf and R, for the Thionine- and Methylene Blue-AA- and -DM-Acrylamide Systems"
AcVlamide conon.,
M
1.4 X 2.8 X 1.41 x 1.41 x 2.82 x 3.52 X 4.22 X 4.93 x 7.04 X 10.56 X 12.65 X
10-a 10-8 10-2
10-1 lo-' 10-l 10-1 10-1 10-l lo-' 10-l
Y T h i o n i n e syatemeA -A-DM& Rp Rt x 107 x io: x 107
... ...
0.258 0.443 0.874 1.212 0.091 1.212 0.28 1.212 ...
...
...
...
1.212 0.51 1.212 0.85 1.212 1.23
...
...
Methylene blue systems
Rp
x
101
...... ...... ...... 0.730 . . . ...... ...... 1.149 0.28 ...... 1.299 0.56 1.360 1.12 1.429 1.39
-AARf
x
107
Rp 10'
x
... . . , . . . ... ...... 0.995 0.094
...
*..
0.995 0.29
... ...
... ...
1.042 0.65 0.995 1.15
...
..,
" All systems contain M dye, 8 X 10-6 M AA (or DM) at pH 8.55 for AA and pH 7.00 for DM; T = 24 f 1'; Rf and R, are expressed aa moles 1.-' aec.-l.
...
...
All systems contain 10-6 M dye, 0.704 M acrylamide at pH 8.55 for thionine-AA, pH 7 for thionine-DM, and pH 8.60 for methylene blue-AA; T = 24 d= 1'; Rf and R, are expressed as moles 1.-1 sec.-l. a
Monomer Concentration. Table I1 summarizes Rf and R, data for systems containing lom5M dye and M AA or DM. These data indicate both a 8 X similarity and a difference in the behavior of these substances with the amine-activated systems. The rates of polymerization are similar with both classes of activators in that they increase with an increase in the monomer concentration. However, the Rr dependences for the two classes of compounds differ
Light Intensity. Calibrated neutral density filters were used to determine the effect of light intensity on Rr and R, for the thionine and methylene blue-AAacrylamide systems. Experimental results are listed in Table 111. The reaction rates for both dyes decrease with a decrease in light intensity. Dye Concentration. Dye fading and polymerization rates were found to be independent of dye concentration over the range of 0.5 to 1.5 X M , except for the dependence of the reactions on the light absorbed I,. As the dyes fade, Ia decreases and thus the reaction rates decrease. Solution p H . A detailed qualitative discussion of the pH effect on the thionine-DM and thionine-AA systems was given in paper I11 of this series.2 However, measurements of Ri and R, were extended to (4)
S. Chrtberek and R. J. AUen, J. Phys. C h . ,69, 647 (1966).
Volume 60, Number 9 September 1966
2844
S. CHABEREK, R. J. ALLEN,AND A. SHEPP
THIONINE-IX IO-'M PH. 8.56 OXYGEN CONC.2Y16'M MONOMER.5X
Table III: Effect of loon R fand R, for the Thionineand Methylene Blue-AA-Acrylamide Systems" N D filterb
None (100%) 0.28 (52.5%) 0.48 (33.lY0)
-Thionine Rr X 10'
AACONC:
-4-SXld'M -X-I.SXld*U --0--4.0
X lO-'M
systems-Methylene blue systemsR, X 108 Rf X 107 R, X 10:
1.294 0.644 0.426
0.80 0.66 0.47
1.170 0.614 0.341
0.79 0.55 0.43
All systems contain 10-6 M dye, 8 X 10-6 M AA, 0.704 M acrylamide a t pH 8.55; T = 24 i 1'; Rfand R, are expressed aa moles 1.-l sec.-l. *Value in parentheses denotes relative per cent transmission for filter. Or
other pH values to provide data for mathematical analysis and to include the methylene blue-.Uacrylamide system. Results are summarized in Figure 1. The most striking aspect of the DM-containing system is the shift of photoactivity to lower pH leveh. This shift arises from the greater acidity of the enol form of DM. Although the data plots for thionine-DM and thionine-AA are similar in shape, maximum photobleaching is obtained at a pH of about 5.0. to 5.5 for DM and at a pH of about 9 for AA. The polymerization data show the greater range of activity for the DM system. Finally, the superiority of methylene blue-AA over thionine-AA at pH levels exceeding 9 is clearly seen. Both dyes are comparable in activity at pH levels between 7 and about 8.55, but at higher alkalinity the thionine Rt and R, rapidly decrease, while those for methylene blue remain essentially constant.
IO
The Journal of Physical Chemistry
-
A &-
-+,
ki
(e>
KE
(f)
k2
(g)
kd
01)
ka
(9
d('ST) - - 0 = kZ(TE*)(M) - ka(.ST)(M) dt 2kd(*ST)' (3)
kat
(j)
Rr = kz(TE*)(M) - kd(.ST)'
k,
(k)
To simplify eq. 4, dye regeneration experiments were performed as described previously.6 Approximately 60% of the faded dye was restored in all cases. Thus
41s
T+T*
2845
2M. +polymer kt 0 In reaction a r T* represents the triplet state of the light-excited dye and 4 is the eEciency of the formation of the triplet state when the dye has absorbed a quantity of light;I,. Reaction b represents the thermal deactivation of T* to the ground state. The rate constant for this reaction has been measured by Hatchard and Parker6 to be 5 X 104 sec.-l for thionine. The key step of this mechanism is the formation of an excited dye-activator complex (TE)* by interaction of the excited dye with either the enolate ion E(reaction c) or with enol E accompanied by the displacement of a hydrogen ion (reaction d). The activated complex can now undergo collisional deactivation and dissociation to the dye and E or E- by reaction e, where the relative concentrations of enol and enolate ion are controlled by acidity constant KE (reaction f). Alternately, (TE)* reacts with monomer to form semithionine (or semi(methy1ene blue)), .ST, and a radical R . (reaction g). The structure of Re or its ultimate fate during the photoreaction is not known at this time. Reaction h is the dismutation reaction that forms dye and leucodye LT; it is the only reaction in our mechanism by which LT is formed. Hatchard and Parker6 report hd to be 2 X lo9 1. mole-l sec.-l for thionine. Reaction i represents the initiation of polymerization by the reaction of .ST with the monomer to form the initiating radical Me. A similar possible reaction involving the photoproduct R - is denoted by 6).
d(T*) dt
-0
=
$Ia - k,(T*)
- ~E-(T*)(E-)-
+
d(TE*) -- 0 = ~E-(T*)(E-) ~E(T*)(E)dt
(4)
60% of the fading of the dye results in the formation of leucodye and 40% results in polymer formation. We see in eq. 3 that 2 / ~ d ( . s T )=~ 1.2Rf, while k,(.ST)(M) = 0.4Rr. As a fist approximation, therefore, we may neglect the latter term, and eq. 3 now becomes 2kd(~ST)~ kz(TE*)(M)
(5)
and
Finally, solving eq. 1 and 2 for T* and TE* and substituting in eq. 6 gives r
1.
/u+\ 1
For data analysis, eq. 1 is written in reciprocal forms
(6) G. G. Hatohard and C. A. Parker, Trans. Faraday SOC.,57, 1093
(1961).
Volume 69,Number 9 September lg66
S. CHABEREK, R. J. ALLEN,AND A. SHEPP
2846
5
I
I
I
I
I
I
20
1
I
(7b)
6 st
Equations 7a and 7b predict a linear relationship between 1/Rr and l/(E-) or l/(M), respectively. To test eq. 7a requires experimental values for (E-). These values were calculated by eq. 8
--A
o
02
.
O4
3
f XIO-elFOR
0
OTHIONINE-AA THIONINE-OM
- 6
06
5
5
5
:
A A l i i X I O o (FOR DM1,LITER-MOLE"
Figure 3. Rf dependence on AA and DM levels.
where CAdenotes the total concentration of AA or DM added, and
20 I8 16
y'
I4 0
t
12
Y L7 .
&
The following values of KT and KE reported by Schwarzenbach and Felder' were used in these calculations
AA
DM
KT
KE
0.184
10-8.13
20.3
10
5
'9' :
8
e
-la-
--DM
D
THlONlNE
2
I
'0
4
6
0
(5x10 IFOR AAI; E-
10-ma
IO
12
14
j X I 0 FOR
E-)
I6
20
I8
OM, LITER
22
24
26
- MOLE-'
Figure 4. Rf dependence on monomer level.
Figures 3 and 4 are plots of the data of Tables I and I1 using eq. 7a and 7b, respectively. These figures show that good linear relations are obtained for both p-diketones. Furthermore, eq. 7 predicts that Rf should be directly proportional to Ia,if all other reaction parameters are held constant. The experimental results in Figure 5 are in accord with prediction. To check the pH depeadence of the mechanism, the rate equation involving Rr was redefined in terms of enol (E) instead of enobte ion (E-). Rearrangement of terms in eq. 7 gives
II 12
-
(9) Equation 9 predicts that a plot of x us. 1/(H+) should be a straight line whose intercept is the rate constant kE and whose slope is related to k ~ - . All terms in x are either known or may be calculated from the data, plots of Figures 3 or 4. The validity of eq. 9 is obvious from the linear nature of the plots in Figure 6. The Journal of Physical Chemistry
0
0.2
0.4
0.6
0.6
I, f FOR R, IJf,lFo"
I.o
Rpl
Figure 5. Rf dependence on I.. (7) G. Schwarzenbach and E. Felder, Helv. Chim. Acta, 27, 1701 (1944).
2847
KINETICSAND MECHANISM OF THIONINE-b-DIKETONE-ACRYLAMIDE SYSTEMS
7
*-
-8
8-
8
' Ta '-t w
E-
;
a'
X lo6 to 7.5 X lo6 sec. for (M) = 0.14 to 1.2 M. There, according to eq. 15, the R, data for DM should be
PTHIONINE-DM
R, = constant (M)f
y,
G
3-
SLOPE ISEC-'li kE-KE*I.IIX $(OM) I.S~X~IAA)
2-
'0
10
40
30
20
50
60
'/a
(16)
and so a plot of Rf vs. M"' should be linear with a falloff of the straight line at greater (M), where b(M) > a. Equation 16 is confirmed in Figure 7 by a plot of the DM data for R, vs. (M)'la. The rate of fade Rr for AA is independent of monomer concentration in the range 0.14 to 1.0 M , as shown in Table 11; therefore in eq. 14, b(M) is much greater than a, and the equation for R, reduces to the form
A METHYLENE BLUE-FA
- 4
] a+ 1 b(M)
78.
Figure 6. Rf dependence on pH,
Verificationof the Reaction Mechanism with Respect to R,. From mechanism reaction k, it follows that the rate of polymerization R, is given by R, To solve for M
e ,
=
k,(M.)(M)
(10)
we can set up eq. 11 for all radicals
A plot of R, us. (M) in this concentration range for the AA system is also shown in Figure 7. Here the approximate linear relationship is again demonstrated. 14
I
I
I
I
I
I
- d (radicals) = 0 = 2kz(TE*)(M) dt
2kd(.ST)' - 2kt(M*)2 (11) Rearranging eq. 11together with eq. 4 gives kt(M*)2= kz(TE*)(M)
- kd(8T)'
(12)
Since the right-hand term of eq. 12 equals Ri (according to eq. 4) then combining eq. 12 with eq. 10 gives eq. 13, expressing R, in terms of Rf and (M)
For a constant concentration of enolate ion (E-), the expression for Rf as a function of (M) is given by eq. 7b, which can be rendered to the simple form of eq. 14
where a and t) are constants defined by eq. 7b. It follows that eq. 13 for R, can be expressed in terms of (M) for constant (E-) by
For DM, the linear relationship between R f and (M) given by eq. 14 is shown in Figure 4. We see that a = 1.05 X lo6 sec., while b ( M ) varies from 0.88
6
M OR
,"'.
Figure 7. R, dependence on monomer level.
Figure 4 shows that the linear relationship between Ri and (M) given by eq. 7a or 14 does hold for AA in the low monomer concentration range of 0.014 to 0.0014 M . Because R, data could not be gathered at these low AA concentrations, we cannot check the expected ( M ) dependence of R,. Finally, eq. 7 shows that Rr vanes directly with I,, and therefore, by eq. 13, R, should vary with Ia'/'. The dashed line of Figure 5 indicates that this squareroot dependence holds for the thionine- and methylene blue-AA systems, and presumably should hold for thionine-DM systems. Volunae 69,Number 9 September 1966
S. CHABEREK, R. J. ALLEN,AND A. SHEPP
2848
Evaluation of Rate Constants The data plots of Figures 3, 4,and 6, together with the following values for I , and k,, were used to estimate some of the rate constants
I,
= 8 X lo-'
einstein I.-' see.-'
kc = 5 X lo4 1. mole-' sec.-] The most reasonable values are summarized in Table IV. These data show that the efficiencies of formation of the light-excited dyes and the magnitudes of kE- for AA and DM are comparable in magnitude. Table IV : Rate Constants for the Thionine- and Methylene Blue-AA- and -DM-Acrylamide Systems Parameter
@
AA system
DM system
0.90
0.96 1.9 x 100
kE
2 . 1 x 109
