856
NAN-LOHYANGAND GERALDOSTER
Dye-Sensitized Photopolymerization in the Presence of Reversible Oxygen Carriers1*
by Nan-Loh Yang and Gerald Oster'b Department of Chemistry, Polytechnic Institute of Brooklyn, Brooklyn, New Y O T ~11201
(Received July 1 1 , 1969)
Addition of cobalt(I1) ion to dye-sensitized photopolymerization systems changes the kinetic characteristics of the system. The induction period can be eliminated. The rate is higher than that in the absence of cobalt and remains constant throughout prolonged irradiation. Furthermore, the monomer conversion is much greater than for the conventionalsystems. The cobalt ion forms a reversible oxygen carrier with the electron donor and thus serves as an oxygen buffer. It is demonstrated that oxygen is necessary for the initiation process in dyesensitized photopolymerization. The reversible oxygen carrier also enhances the rate of photoreduction of dye in the absence of monomer.
Introduction Since the discovery of dye-sensitized photopolymerization2 considerable effort has been expended to improve its quantum efficiency (for review see Section J of ref 3). The polymerization system consists of a vinyl monomer in aqueous solution and is sensitized with a photoreducible dye in the presence of an electron donor for the light-excited dye. The dye may be a member of the acridine, xanthene, or thiazine families. The electron donor may be a mild reducing agent such as ascorbic acid or may be a secondary or tertiary nitrogen compound such as ethylenediaminetetraacetic acid. The reaction is sensitive to visible light in the spectral region which the dye absorbs. The light-excited dye is converted into a metastable species, presumably the triplet state, which thereupon abstracts an electron from the electron donor.4 The reduced dye, as the semiquinone, may react with ambient oxygen to give the initiating radical and the dye is regenerated.2" An alternative suggestion is that the semiquinone and the oxidized electron donor serve as the initiating However, in the rigorous absence of oxygen, the dye is photoreduced to its leuco form but no polymer is produced.2 Some authors' contend oxygen is not necessary for dye-sensitized photopolymerization since the oxygen level was reduced to lo-' M by flushing the system with helium and still the reaction proceeded. In the present paper it will be shown, however, that oxygen is a necessary component for the reaction. The photopolymerization reaction, like all radicalinitiated chain reactions, is inhibited by oxygen. The induction period is reduced by lowering the concentration of oxygen. If oxygen were not necessary for the initiation process, then one should remove as much of the oxygen as possible to improve the over-all efficiency. If, on the other hand, oxygen is necessary for initiation, there should exist an optimum range of oxygen concenT h e Journal of Physical Chemistry
tration for the greatest efficiency. The optimum range would then be where the oxygen level is low enough to eliminate the induction period and yet is sufficient to maintain the initiating processes. The best dyesensitized photopolymerization system will be the one where the oxygen is maintained at the optimum level even as oxygen is being consumed during the course of the reaction. This would require a system in which there is a continuous supply of free oxygen in just the right amount to compensate for the oxygen being consumed in the initiation of polymerization. In the present work the proper level of oxygen is maintained with a reversible oxygen carrier. A reversible oxygen carrier is a complex which combines with molecular oxygen when its environment is rich in oxygen and releases it when the oxygen concentration is low.* In our system, the reversible oxygen carrier is a cobalt(I1) complex. Many of the ligands for cobalt-oxygen carrier complexation are also electron donors for light-excited dyes.g Thus, such substances may serve in dye-sensitized photopolymerization both as an electron donor and as a cobalt-complexing agent (1) (a) Taken in part from the Ph.D. dissertation of N. L. Yang, Polytechnic Institute of Brooklyn, June 1969. Work was supported by the National Institute of Health on Grant GM 13823. (b) Address correspondence to the author a t Mount Sinai School of Medicine of the City University of New York, New York, N. Y. 10029. (2) (a) G. Oster, Nature, 173, 300 (1954); (b) G.K. Oster, G. Oster, and G. Prati, J . Amer. Chem. Soc., 79,595 (1957). (3) G.Oster and N. L. Yang, Chem. Rev., 68,125 (1968). (4) G. Oster and A. H. Adelman, J . Amer. Chem. Soc., 78, 913 (1956). (5) S.Toppet, G. Delzenne, and G. Smets, J. Polym. Sci., A2, 1539 (1964). (6) C. S.H.Chen, ibid., A3, 1807 (1965). (7) S. Chaberek and R. J. Allen, J . Phys. Chem., 69, 647 (1965); S. Chaberek, A. Shepp, and R. J. Allen, ibid., 69,641 (1965). (8) E.Bayer and P. Sohretamann, Strucl. Bonding (Berlin), 2, 181 (1967). (9) G. Oster and N. Wotherspoon, J . Amer. Chem. SOC.,79, 4836 (1967).
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DYE-SENGITIZED PHOTOPOLYMERIZATION
for reversible oxygen binding. Since the ligand which is complexed with metal loses its electron-donating
I
character,lOJ1the ligand must be in excess of the cobalt(I1) ion concentration. The purpose of the present work is to demonstrate the advantages of the use of reversible oxygen carriers in dye-sensitized photopolymerization. The second purpose is to show the role of oxygen in the initiating process.
~~
”
Results and Discussion As a model system for dye-sensitized photopolymerization in the presence of reversible oxygen carriers, we have chosen methylene blue as the sensitizer, acrylamide as the monomer, and triethylenetetramine (TETA) as both the electron donor and the ligand for Co(I1). Two molecules of CoI’(TETA) combine with one molecule of oxygen to form the oxygenated species ( T F I T A ) C O ~ ~ O ~ C O ~ ~ (with T E Tan A )association constant KO,given by:
Koe
[(TETA)Co1102C01’(TETA)] [(TETA)CO’I]~[O~]
(1)
and is observedI2 to be equal to log. Other chelating agents such as diethylenetriamine, glycylglycine, and histidine whose Co(1I) complexes are known to be reversible oxygen carriers,8 have likewise been found by us to serve as electron donors when present in excem of the cobalt concentration. Methylene blue was chosen because its absorption spectrum does not overlap with that of the oxygenated form of the oxygen carrier which is yellowish. Other dyes such as rose bengal are also sensitizing dyes but, because of their color, their use complicates the photometry of the system. The photopolymerization reactions were carried out in two different conditions of oxygen control. I n one series (the “closed system”), before Co(I1) is added the solution is saturated with air and sealed off from the ambient atmosphere, both during the introduction of Co(I1) and during the entire course of the photopolymerization. I n the other series (the “aerobic system”), the solution is saturated with air after Co(I1) is added. In the aerobic system the air stream also serves to oxygenate the Co’I(TETA) complex. I n the closed system the total oxygen concentration [O2]t is the solubility of oxygen in the aqueous solution under 1 atm of air and is taken as 2.5 X M . After Co(II), of final concentration C, is introduced, the total oxygen concentration [Ot]t is the sum of free oxygen (Oz]t and bound oxygen [ 0 2 ] b , from which, using eq 1
Here we have assumed that all of the cobalt ions are complexed to the ligand which is in large excess. In
10
20
30
40 50 Time, sec
60
70
80
90
Figure 1. Monomer conversion curve for closed system. M , acrylamide 3.8 M , Methylene blue 5.5 X triethylenetetramine 2.0 X M a t pW 9.6. Intensity of red light absorbed initially, 5.45 X 10-8 einstein cm-a sec-1. Numbers on curves indicate molar concentration of Co(I1). See text for conditions for curve e.
the aerobic system [O2]r is taken as being 2.5 X 10-4M; thus = 2.5
x
io-4
+ [02]b
(3) Closed System Photopolymerization. The course of photopolymerization of the cobalt-containing system with varying amounts of the metal ion under closedsystem conditions is illustrated by curves a, b, c of Figure 1. These curves differ considerably from those for the conventional ( i e , , Co(I1) absent) system, curves d and e of Figure 1. The conventional system when not deoxygenated (curve d) exhibits a long induction period of 35 see and even after 50 sec of irradiation achieves only 2% monomer conversion, When Co(I1) is added to the conventional system, the induction period is shortened and the rate of polymerization is increased (curves a, b, and c). At a cobalt concentration of 10-3M (curve c) the induction period is practicdly eliminated and after 50 sec of irradiation the monomer conversion is 20%, i.e., tenfold that for the conventional system under the same conditions (curve d). For the conventional system which has been deoxygenated by passing a stream of helium through the solution for 30 min just prior to the irradiation, there is also no induction period (curve e) but the rate of polymerization quickly levels off. Here after 35 sec of irradiation the extent of conversion reaches its limiting value, namely, 13%. This is in contrast to the cobaltcontaining system where the polymerization proceeds with a relatively constant rate over an extended period of time of irradiation. Thus for a Co(I1) concentration of 1 X M (curve c) in 35 sec the monomer conversion is 1601, and at 90 sec the monomer conversion is 27% and continues upward thereafter. Only in the [&It
(10) G.K.Oster and G. Oster, J. Amer. Chern. Soc., 81,5543 (1959). (11) J. Joussot Dubien and G. Oster, Bull. SOC. Chim. France, 343 (1960). (12) 0.Bekiiroglu and S . Fallab, Hela. Chim. Acta, 46, 2125 (1963).
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February 19,1970
858 very earliest stages of the reaction is the rate of conversion for the deoxygenated conventional system higher than that for the cobalt-containing system. For the examples given in Figure 1 the ligand concentration is more than an order of magnitude greater than the Co(I1) concentration. When Co(I1) and triethylenetetramine are at comparable concentrations, polymerization does not take place since then the ligand ceases to function as an electron donor. The differences in the kinetic behavior between the conventional system and the cobalt-containing system can be explained in terms of the action of oxygen in the initiation process. The conventional system which has not been subjected to deoxygenation (curve d) exhibits a long induction period due to the inhibitory effect of the excess oxygen. The conventional system which has been deoxygenated by passing helium through it (curve e) immediately polymerizes an irradiation due to the low level of oxygen initially present. The polymerization ceases, however, when all available traces of molecular oxygen are consumed in the initiating step. The cobalt-containing system, on the other hand, is equipped with an in situ oxygen reservoir, so to speak. Now there is no or only a short induction period (curves ajb, and c) since most or part of the total oxygen, [02]t, is bound to the CoII(TETA) complex. As free oxygen [Oa]t is being depleted in the initiating step, the oxygenated complex releases its bound oxygen to the system for participation in further initiation. The oxygen carrier functions as an oxygen buffer. In the context of the present work we define the oxygen buffer capacity as the number of moles of oxygen released from the complex per mole of free oxygen consumed in the polymerizatlion process or, from eq 2
Hence as [Oz]r decreases there is an increasing tendency to release oxygen. For dilute systems the buffer capacity is approximately proportional t o the concentration of the oxygenated carrier. Aerobic Photopolymerization. In the aerobic series (Figure 2) each run has a different concentration of bound oxygen which depends upon the Concentration of Co(I1) the higher the limiting value of monomer conversion and the greater the steady-state rate of polymerization (curves a-d of Figure 2). I n all cases, there is induction period due to the high initial free oxygen concentration. On irradiation the free oxygen is consumed in the course of the induction period and in the initiation, however, the bound oxygen is concurrently being released. The reaction ceases when all the oxygen, whether free or stored in the complex, has been consumed. The higher the cobalt concentration, the higher is the limiting monomer conversion since more oxygen is stored. Since the buffer capacity is T h e Journal of Physical Chemistry
NAN-LOHYANGAND GERALD OSTER
2
4
6 a Time, min.
10
12
14
Figure 2. Monomer conversion curve for aerobic system. Concentrations and intensity of light as in Figure 1.
greater at more concentrated cobalt solutions, the steady-state rate will likewise be higher. In the absence of cobalt, the helium-purged system (curve e) exhibits no induction period. For the airsaturated case (curve d), however, there is, of course an induction period, which has a lower rate in the early stages of the polymerization but eventually overtakes the former system since there is more available oxygen and reaches a higher monomer conversion. Here again oxygen appears to be necessary for dye-sensitized photopolymerization. Photobleaching. In order to learn more of the role of the initiation process in photopolymerization, studies on the photobleaching (Le., photoreduction) of methylene blue in the absence of monomer were carried out. In the closed system the addition of cobalt not only greatly accelerates the initial rate of fading but also eliminates the induction period (compare curves a, b, and c with curves d of Figure 3). The rate increases with increasing amount of Co(I1) when TETA is in large excess but, parallel t o the polymerization studies, the reaction does not proceed when the metal ion exceeds the concentration of the ligand.’O The elimination of the induction period and the enhancement of the rate on addition of Co(I1) to the closed system are due t o the abilityof the cobalt complex to bind free oxygen which would otherwise convert the leuco (reduced) dye back t o the blue form. Indeed, the CoI’(TETA) complex is a very effective oxygen remover in that, for example, Co(I1) at a concentration of 5 X 10-3 M lowers the concentration of free oxygen to less than 10-7 M (see eq 2). In comparison, flushing the air-saturated system with helium for 40 min is
DYE-SENSITIZED PHOTOPOLYMERIZATION 1.o
859 1.0 r
e. 5.0 Y 10-3
0.9
0.8 0.7
2
0.6
0 t
*E
0.5
v)
0
$ 0.4
0.3 Time, rnin.
Figure 3. Photobleaching of methylene blue in closed system. M, triethylenetetramine 7.50 X Methylene blue 2.75 X 10-2 M at pH 7.3. Numbers on curves indicate molar concentration of Co(I1). For comparison, see the Co(I1)-free system purged with helium for 30 min (curve e).
0.2 0.1
0.0
0
5
10
15
20
25
30
Time, rnin.
two orders of magnitude less effective in removing oxygene6 This is manifested in photobleaching as illustrated in curves c and e of Figure 3. The hypothesis that the cobalt complex functions as an oxygen buffer is further borne out from observations of the spectral changes of this complex during both photobleaching and photopolymerization. The yellow color associated with the oxygenated carrier bleaches concurrently with the photobleaching of methylene blue. This is an indication of the release of oxygen from the carrier. If, as has been contended516r7the semiquinone species of the sensitizing dye and the oxidized electron donor are the initiating species, then the course of photopolymerization should parallel the course of photobleaching since these species are the primary photochemical products. That this is not the case can be readily seen by comparing the aerobic photopolymerization (Figure 2) with the aerobic photobleaching (Figure 4). For aerobic photobleaching, the addition of cobalt decreases both the rate and the limiting extent of reaction whereas the reverse is the case for photopolymerization. Increasing the cobalt concentration increases the oxygen buffer capacity and hence provides more oxygen for the reoxidation of the leuco dye; thus the rate of photobleaching is decreased. With photopolymerization, on the other hand, the reaction is enhanced since there is more oxygen available for reacting with the leuco dye to produce the initiating species.
Conclusions The new dye-sensitized photopolymerization system is unique in that the free oxygen concentration is maintained in an optimum range for an extended period during the course of the reaction. The closed system polymerizes immediately upon irradiation without the need for usual cumbersome degassing procedures. The reaction proceeds with a constant high rate and the
Figure 4. Photobleaching of methylene blue in aerobic system. Concentrations (but no monomer) and intensities as in Figure 1.
conversion is greater than for the conventional system.
A constant rate over long irradiation time implies that if our new system is used for photography the image produced is a faithful replication of the exposure (Le., dosage of light). Still further, the new system under aerobic conditions yields much higher monomer conversions than does the conventional system. The behavior of the cobalt-containing systems clearly demonstrates that oxygen is essential for the dye-sensitized photopolymerization system described in this paper and may be the case for similar systems.
Experimental Section Materials. Technical grade triethylenetetramine obtained from Fisher Scientific Co. was purified by vacuum distillation over sodium metal. The purified TETA was standardized by conductometric titration with standard perchloric acid. Acrylamide from Eastman Kodali Co. was recrystallized from chloroform and stored in a refrigerator. Methylene blue (zinc free) and cobalt(II1) chloride were of reagent grade obtained from Merck Co. A stock solution of Co(I1) was standardized by titration with disodium ethylenediaminetetraacetate (disodium EDTA, 0.1 M , Fisher Certified reagent) using murexide as the indicator. Helium was purified by passing the gas through a chromous chloride solution to remove trace amounts of oxygen. All other chemicals were analytical reagent grade. Procedwe. A 500-W tungsten lamp in a slide projector was used as the light source. To eliminate light below about 475 mp, a pale yellow glass filter was interposed and a heat glass filter was used to reduce radiation of wavelength above about 700 mp. To ensure uniform Volume 74, Number 4
February 19, 1970
860
S.J. JORIS, K. I. ASPILA,AND C. L. CHAKRABARTI
illumination of the sample cell a ground-glass plate was inserted in the usual slide position of the projector. The sample was fixed 10 cm from the front of the projector. The lamp was stabilized with a constant-voltage transformer. The intensity of light falling on the sample was determined with a calibrated thermopile (Epply Laboratories). The polymerization was followed by the thermal-rise method2and the adiabatic heat rise was followed with a thermistor unit (Yellow Spring Llodel 73) in conjunction with a recorder. The temperature rise was converted into monomer converted into polymer by means of a conversion factor predetermined gravimetrically. For a 20-ml aqueous solution in a cylindrical test tube 2
cm in diameter a rise of 1" corresponds to a convermol. For the aerosion of acrylamide of 1.25 X bic system where the time of irradiation is long the sample cell was insulated to maintain adiabatic conditions. The rate of photobleaching was followed photometrically using a red-sensitive silicon solar cell (Hoffman), the same light beam being used for the radiation and for the transmission measurements. An interference filter which transmitted maximally at 665 mfi, corresponding to the absorption maximum of methylene blue, was interposed between the sample and the light detector. An optical cell with optical path of 1 cm was used as the reaction cell.
On the Mechanism of Decomposition of D i t h i o c a r b a r n a t e s 1 a . b by Serge J. Joris,lo Keijo I. Aspila, and Chuni L. Chakrabartild Department of Chemistry, Carleton Unioersity, Ottawa 1, Ontario, Canada
(Received July 7 , 1969)
The titration of the dithiocarbamates and the kinetics of their decomposition indicate that an intramolecular hydrogen bond exists between the sulfur and the nitrogen atoms of the dithiocarbamic (DTC) acid molecule. This intramolecular hydrogen bond is formed as soon as the sulfur atom of the dithiocarbamateion protonates. The fractional charges resulting from the intramolecular hydrogen bond introduce a high internal energy in the DTC acids and are responsible for their great instability. This is proven by the kinetics of decomposition in mixed solvents, which also reveals the existence of a large steric effect on the solvation and on the related stability of the DTC acids. Specific solvation of the DTC acids was also detected in mixed solvents. It is also shown that the activated complex for decomposition cannot be very different from the acid form of the dithiocarbamates. Introduction I n a previous paper2 some erroneous conclusions found in the literature concerning the acidic properties of the dithiocarbamic (DTC) acids were corrected. It was proven that these acids are definitely monobasic (with the proton located on the sulfur atom) and differ therefore from the usual amino (or dithioamino) acids which are dibasic. This difference was explained2 by the delocalization of the free electrons of the nitrogen in the DTC anion (model I) and by the possible formation of an intramolecular hydrogen bond in the acid molecule (see model 111).
I
I1
I11
The kinetic profiles for the decomposition of the dithiocarbamates indicate t'hat only the acid form of the T h e Journal of Physical Chemistry
molecule decomposes. The overall equation for the decomposition is
RlRzNCSS-
+ 2H+
--t
RlRzNHz'
+ CSz
(1)
The proximity of the fractional positive charges on the nitrogen and carbon atoms in model I11 is obviously responsible for the decomposition of the molecule into carbon disulfide and an amine (this amine further protonates to form an ammonium ion). The zwitterion IV, which is an extreme case of model 111,was initially proposed by Zahradnik and Zuman to represent the DTC acids in solution and the reactive form for their de(1) (a) This paper was partly presented at the 52nd Canadian Chemical Conference, Montreal, Canada, May 25-28, 1969. (b) This paper will constitute part of the Ph.D. thesis of K. I. A. ( 0 ) Postdoctorate research fellow. (d) Author t o whom all correspondence should be addressed. (2) S. J. Joris, K. I. Aspila, and C. L. Chakrabarti, Anal. Chem., 41, 1441, 1969.
