DYE-SENSITIZED PHOTOPOLYMERIZATION PROCESSES
64 1
Dye-Sensitized Photopolymerization Processes.'a I. The Thionine-Nitrilotripropionamide-Acrylamide Systemlti
by S. Chaberek, A. Shepp, and R. J. Allen Technical Operatwns Research, Budington, Massachusetts
(Received September 18, 1964)
A reinvestigation of the anaerobic, thionine-sensitized photopolymerization of acrylamide reported earlier showed that this is indeed an amine-activated process, in which traces of nitrilotripropionamide present in the nionomer served as the weak reducing agent. Experimental data on the rates of dye fading and of polymerization are in accord with a reaction mechanism in which the polymerization-initiating free radical is semithionine. Quan_turnyields of polymerization as high as 0.12 polymer molecules/photon were obtained.
Introduction In 1962 we reported the thionine-sensitized photopolymerization of acrylamide2 by a process differing from that described by Oster and co-workers3~* in two respects; naniely, neither oxygen nor weak reducing agents such as tertiary amines and amino acids were required for polymer formation. On the basis of data available a t that time, we postulated a reaction mechanism in which the polymerization-initiating free radical was seniithionine and possibly hydroxyl radical formed by an oxidation-reduction reaction between the light-excited dye and hydroxyl ion. Further work on this systeni, as part of a general program to develop rapid , dye-sensitized photopolymerization processes for photographic purposes, showed that our conclusions were untenable. The acrylamide used in the prior investigation contained traces of nitrilotripropionaniide in ainounts insufficient to be detected by elemental analysis but sufficient to function as a weak reducing agent for the light-excited dye to cause dye bleaching and polymerization. Our system, therefore, is an amine-activated one, similar in soiiie respects to those described by Oster and c o - ~ o r k e r s ~but * ~ diff ~ ererit in that oxygen is not required for the process. However, since its quaiituiii efficiency for polymerization was the highest reported to date, we considered it desirable to reinvestigate its reaction mechanism. The results of this study are summarized in this paper.
Experimental The acrylamide used in this investigation was a pure saiiiple 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. A pure sample of nitrilotripropionamide (STP) was obtained from the American Cyanamid Co. Thioiiine was purified by two recrystallizations from water, followed by chromatography on alumina. A h terial purified in this way shows no impurities upon subsequent repetition of the chromatographic step. The absorption coefficient of this purified dye was found to be 6.2 X lo41. cm.-' mole-' a t 5980 1. The experimental procedures used in this study were substantially the same as those described previously.2 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 tiiiie intervals, precipitating the polyacrylamide in methanol, filtering it, arid drying the residue to constant weight. Polymer molecular weights were determined viscomet rically. (1) (a) These studies were performed under Contract No. AF33-
(657)-8754, Itec-otinaissanceDivision, Aeronautical Systems Division, Wright-Patterson Air Force Base, Ohio; (h) presented in part at the Photochemistry Symposium, University of Rochester, Rochester, N. Y.,March 27-29, 1963. (2) A. Shepp, S. Chaherek, and I t . AIacNeil, J. P h y s . Chem., 66, 2563 (1962). (3) (a) G. Oster, .Vaticre, 173, 300 (1954); (b) Phot. Eng., 4, 153 (1953). (4) G. K . Oster. G. Oster, and G . Prati, J. A m . Chem. Soc., 79, 595 (1957).
Voliime 69.h'irmher 0 Febricary 1966
642
S. CHABEREK, A. SHEPP,AND R. J. ALLEN
THIONINE = IO-%
Figure 1.
5
0
zz 5-
‘0 6-
w
0
z4-
I-
32-
0 TIME, min
NTP = 3 . 3 ~ IO-~M
The ./ournnl
OJ’
Physical C h n n i s l r u
0
I .o
2.o
3.0
TIME,min ACRY LAMI DE ~ 5 (.704M) %
._ 4.0
DYE-SENSITIZED PHOTOPOLYMERIZATION PROCESSES
643
polymerization increases. Moreover, the rate is linear up to a reaction time of about 2 min., after which the rate falls off. This 2-min. reaction time corresponds to a conversion of about 10%. Figure l a shows the rate of polymerization as a function of the concentration of the activator K T P with the monomer held constant a t 5% (0.075 mole). Here, again, there is an increase in the rate of polymerization with the concentration of the activator. Figure 2 summarizes the effects of STP and monomer levels on the dye-bleaching reaction. Figure 2a shows that the maximum rate of dye fading is obtained in the absence of monomer and that Rf progressively decreases with an increase in the acrylamide concentration. This trend must be interpreted as a quenching of light-excited thionine by the monomer. Figure 2a also summarizes the effect of thionine concentration on photo bleaching. The dashed lines indicate the bleaching rates a t three dye concentrations, 0.5 X lo-5,l.O X and 1.2 X M , a t the same activator and niononier levels. The initial Rr values are independent of the thionine levels. Figure 2b shows the variation of Rt with STP level. I t is seen to increase with an increase in STP. Efeci of Solution p H . Both the dye-bleaching and polymerization reactions are sensitive to solution pH. Table I summarizes the variation of Rr and R, with solution acidity for anaerobic solutions containing lops M thionine, 0.704 91 acrylamide, and 3.3 X M STP. It is seen that R, increases in the pH range 5.55 to 7.6 and then falls off a t higher pH levels; Rt appears to follow the same trend although it is not as well defined. We believe that the pH dependence in the range of 3.5;i to 7.6 must be predominantly related to the acid-base properties of STP and that the free base inust be considerably more reactive than the protonated species. This is because, all other reaction parameters being equal, changes in the acidity alter only the relative proportions of these forms and not the total amount. Thus, if both species had the same reactivities and if hydrogen ions were not involved directly in the formation of free radicals, this initiator system would be insensitive to changes in solution pH. Let us compare, therefore, the Rt and R, values in Table I with the percentage of free base, B, existing a t equi1ibrium.j At a p1-T of 5.55, NTP exists predominantly in the protonated form. As the pH is increased, the concentration of free base increases rapidly i n the 6.61-7.90 interval and at higher alkalinities approaches inore slowly toward 100% conversion. The R, values parallel the increase i l l the amount of B. For exaniple, a pH increase from 6.61 to 7.6 doubles the amount of free base, and R, is about twice as great. The trend i n
Rr is not as clear-cut although a large increase in re activity is obtained by increasing the pH above 5.55. Both rates decrease, however, at p H levels exceeding 8.0. We believe that this loss of activity, so characteristic of thionine-containing systems, is the result of a change in the absorption characteristics of the dye. Absorption spectra show that the onset of spectral change begins a t pH levels of about 8 to 9.
Table I : Effect of pH on the Anaerobic Thionine-NTPAcrylamide System R f x 108, PH
mole I.-' set.-'
5 55 6 61
7 62 7 90
Very slow 8 33 8 33 8 33
8 50
6 94
R,
Free base ( B ) ,
X 104,
%
mole I.-' see.-'
None 3 5 5 4
6 2 43 1 88 6 93 6 98 3
0 9 4 5
Efect of Oxygen. Oxygen profoundly affects the behavior of the thionine-STP-acrylainide systems in that it introduces an induction period prior to the onset of photopolymerization. In Figure 3 are plotted the degrees of dye bleaching and polymerization of soluM thionine, 3.3 X J l NTP, tions co:itaining and 0.704 A// acrylamide a t two oxygen levels. Also plotted are dashed curves showing the degrees of bleaching and polymerization of a siinilar system containing no oxygen. It is seen that the introduction of oxygen produces an induction period during which the dye is slowly bleached, but no polymer is produced. The initial bleaching rates decrease as the oxygen concentration increases. At the conclusion of the induction period, fading of thioiiine proceeds at a rate comparable to that of the anaerobic control, and polymer is formed. The relation between the length of the induction period and the oxygen level is apparent froin Figure 3. The induction tiines were obtained both by extrapolating the polymerization curves to zero conversion arid by detern~iningthe tiiiie corresponding to the intersection of the slopes of the two different dye-fading rates during and after the inhibition reaction. These data show conclusively that the length of the induction period is proportional to the amount of oxygen i n the system and that the onset of polynierization coincides with the terinination of the induction as measured by changes in the dye-bleaching rate. ~~
~~~
+
( 5 ) T h e equilibrium cotistarit for the reac*tion H B e H B was calrulated from poteiitiotiietric~titration data a t 25' 111 0.1 .If KCI and has a value of 10 -6.73 +
+
S. CHABEREK, A. SHEPP,AND R. J. ALLEN
644
DYE BLEACHING
POLY MERlZATlON
I
I
I
18 I6 -
IDS
I
I
I
I
1
I
I
I
DYE= IXIO’b ACRY LAMID€=0.704 M N T P = 3 . 3 ~ 1 0 -M~
I
CD
2 x
I
z
:
U
K I-
z W 0
z
0
4
\
4r
\
Q
*
c
A-
I
ANAEROBIC
’
d Q
+
0 W
zz
2x10-5 M O ~
I 0.
1 . 1
.5
1.0
I
1.5
I
2.0
2
1
2.5
I
30
I 3.5
/
/ I 11
I
I
4.5
4.0
0
TIME, MIN
Figure 3.
/
/
41-7’ / 1/
0 ANAEROBIC
-0 S
/
I
2
I I!.
3
/
-
/
0 4
I
5
-
I
6
1
7
I
8
J
9
TIME,MIN
Effect of oxygen on the thionine-NTP-aorylamide system.
The Photopolymerization Mechanism. A photopolymerization mechanism consistent with the experimental data is represented by the sequence of reactions
T -+T*
--
+ heat rr* + M T +M T* + N T P I_ .ST + R. 2(.ST) -+- T + LT .ST + +M. T* -+ T
J* (4 kc
(b)
k,
(4
ki, kz
(4
kd
(e)
k.
(f)
ks’
(g>
k3
(h)
+ M +M *
k,
(i)
2M. --+ polymer
kt
(9
R.
+ M +M.
2R. -+ product ?\/I.
In reaction a, T* represents the triplet state of lightexcited thionine, and 4 is the efficiency of the formation of the triplct state when thionine has absorbed the quantity of light I,. Reaction b represents the ther. mal deactivation of T* to the ground state. The rate constant for this reaction has been measured by Hatchard and Parker6 to be 5 X lo4set.-'. Reaction c denotes the quenching of light-excited dye by the monomer 31 The Journal of Physical Chemktry
The reaction of excited thionine with NTP to form semithionine, .ST, and a radical, R . , is shown by (d). The exact structure of R . or its ultimate fate during the photoreaction is not known a t this time. Reaction d shows both forward and backward reaction steps, denoted by rate constants kl and ICz, respectively. As in most kinetic schemes of this sort, kz is probably comparable in magnitude to kl, but rapid removal of .ST and R . by other reactions suppresses this reversal reaction. Reaction e is the disniutation reaction that forms thionine and leucothionine (LT). In our mechanism it is the only reaction by which LT is formed. Hatchard and Parker6 report k d to be 2 X lo91. mole-‘ set.-'. Reaction f represents the initiation of polymerization by the reaction of .STwith the monomer to form the initiating radical -\I.. The rate constant IC, for this reaction cannot be measured by ordinary vinyl polymerization because it cancels out in the mathematical analysis of the reaction. However, because of our measurements of the regeneration of LT, to be described later, we will be able to evaluate k,. Reactions g and h are possible reactions of the photoproduct R. formed in reaction d. Reactions
(6) G. G. Hatchard and C. A. Parker, Trana. Faraday Soc., 5 7 , 1093 (1961).
DYE-SENSITIZED PHOTOPOLYMERIZATION PROCESSES
i and j are the usual polymer propagation and polymer termination steps, respectively. Verification of the Reaction Mechanism with Respect to Rf. The following three equations form the basis of the mathematical analysis of this reaction mechanism with respect to Rr dt
d ( ,ST) dt
=
0
-
=+la
[kc
+ kl(NTP) + kq(M)](T*) (1)
645
We must now solve eq. 2 for 'ST. Since this cquation contains terms in both .ST arid (.ST)', a n exact solution cannot be obtained simply, and an approximation must be made. Referring to eq. 3 , we see that in eq. 2 we have 2kd('ST)* = 1.2Rf,while k,(.ST)(AI) = 0.4Rf. Thus, we may neglect the latter term. Then, if we assume that kq(.ST)(R.) is less than kl(T*)* ( S T P ) , eq. 2 beconies 2kd(.ST)*
=
O = kl(T*)(NTP) - kz(.ST)(R.) -
R f = - -d(T) - kl(T*)(NTP) - Icz(.ST)(R.) dt kd(.ST)' Combination of eq. 2 and 3 gives Rr
=
ka(*sT)(hI)-k kd(.ST)'
By putting eq. 6 into the dominant term of eq. 4, we obtain
Rr (3)
.ST)'
k,(.ST)(AI)
=
0.4Rt
Therefore, to a first approximation, dominant term in eq. 4.
(7)
2
Rr
=
kl(NTP) 2 [k,
+la
+ kl(NTP) + kq(\5]
For data analysis, eq. 8 is written in reciprocal f o r m _ -
+
1 - 2_-[kc ki(KTP) ] Rr ki( NT P)+Za -
+ ~ ~ (2kqS T P ) $(AI)J I ~ (8b) ____
Thus, eq. 8a and 8b predict linear plots of 1/Rr us. l/(NTP) and (M), respectively. Figure 4 shows the plots of these equations, using the data of Figures 1 and 2 . The piot,s of these equations are both linear and, within experimental error, have the same intercept. ( I I N T P ) I03(MOLES/LITER)-'
.2
-
.4
.6
.8
1.0
1.2
1.4
1.6
1.8
2.0
-
7
r
(54
0.6Rr
=
k1(T*)(NTP)
Finally, solution of eq. 1 for T * and its substitution into eq. 7 gives
(4)
This equation shows that Rf is made up of two terms: the first is k,( -ST)(M),which represents the formation of polymer by reaction f; the second is kd(.ST)', which represents the formation of leucothionine by reaction e. The solution of eq. 4 for .ST is complex; to simplify the solution, a series of thioriirie regeneration experiments was performed on typical anaerobic photopolymerization runs to determine which of the two terms was more important. By cutting off the irradiation (thus terminating the reaction at 1- and 2-niin. intervals) and then introducirig oxygen, we could estimate how much of the dye fading resulted from the formation of leucothionine and how much resulted from the formation of polymer by semithionine radicals. The results of two regeneration experiments are shown in Figure 2b. In the first case, the reaction was stopped a t 30 sec., oxygen was introduced, and the thionine concentration was increased froin 0.8 X lo-* to 0.94 X M. In the second case, the reaction was stopped after 60 sec., and the same regeneration ratio was obtained. Approximately 60% of the faded dye was restored in both cases. We, therefore, concluded from this experiment that 60% of the fading of thionine results in forination of leucodye and the remaining 40% of fading results in forniation of polymer. This is expressed by the equations kd(
(6)
kl(T*)(NTP)
(5b) kd(
.ST)' is the
0.1
0.2
0.5 0.6 0.7 0.8 MONOMER, MOLES 1 LITER
0.3 0.4
0.9
1.0
I
Figwe 4. Confirmation of Rf dependence on NTP and acrylamide coitcentratioim.
Volume 69. .\'limber
2
Fehriiary 1066
S. CHABEREK, A. SHEPP,AND R. J. ALLEN
646
Verification of Reaction Mechanism with Respect to R,. Let us now derive the expression for the rate of polymerization 8,. From reaction i it follows that
R,
=
kp(NI)(iVt*)
(9)
We must now obtain an expression for M , assuming a steady state in 11.. This, in turn, will require a steady-state assumption for R
2/7~t(M.)~(10) d(R.) - 0 dt
=
kl(T*)(NTP) - kz(R*)(*ST)2k3(R.I2 - ka'(R.1 (MI (11)
There are no simple solutions for eq. 10 and 11 unless simplifying assumptions are made. In the case of .ST, we know that kd(.ST)' is slightly greater than k,(.ST)(;\I), and, by analogy, we expect k3(R.)' to be slightly greater than ka'(R*)(lI). Therefore, neglecting the term ks in eq. 11, we obtain
+ ka'(R.)(AI) = kl(T*)(NTP)
2k3(R.)%
(12)
As a first approximation, let us assume that k,'(R.)(hI)
kl(T*)(NTP)
(13)
Figure 5. Confirmation of R, dependence on monomer concentration.
data of Figures 1 and 2. A linear plot is obtained. However, the scatter of the data points is probably due to the approximations leading to eq. 12 through 13. Evaluation of Rate Constants. Reaction rate constants and steady-state concentrations of the transient species were calculated from the slopes and intercepts of the data plots of Figures 4 and 5 , using the following values?: I , = 8 X lo-' einstein I.-' set.-'; k, = 5 X lo4 1. mole-' sec.-'; kd = 2.4 X lo9 1. mole-' set.-'. These data are sumniarized in Table TI.
Equation 10 then reduces to 2k+,(JI.)'
A
kl(T*)(NTP)
+ k(.ST)(W
=
2Rt
(14)
We believe that the functional dependence of eq. 14 is correct, but that the approximations involved make the coefficient of 2Rf incorrect. Using eq. 14 in eq. 9, we arrive a t the final equation
R,
=
kpkt-"zR~"z(M)
(15)
Figure 3 shows a plot of R, us. ( M ) f i f , using the
Tab,e 111: Quantum Yield Data 4th
0 0.350
0.15
0 490 0 704 1 060
0.10 0.077 0.055
Rate constant"
12, k, k, kd ka
i;;&
Steady-atste* values
Value of constant
Species
2 . 7 X lo1 1 . 7 X lo6 5 x 104 B 4 x 109 7.5
I. T* .ST
8 X lo-' 9 x lo-'%
...
... ...
... 2.5
.. ..
...
4
x
10-9
All rate constants have the dimensions 1. mole-' see.-'. Values are computed for monomer = 0.704 ICI, S T P = 3.3 X 10 - 3 J I , and for 'STand T* at steady-state values.
Th'e Journal of Physical Chemistry
1 X 10-5
0.55 1.1 2.2 3.3
IM N T P
0 0 023 0.027 0.047 0,057
..
NTP, M X 103
Table 11: Rate Constant Data for Thionine-NTPAcrylarnide System
QP
1 X 10-5 M dye, 3.3 X 10-2
Acrylarnide, M
M dye, 0.704 M monomer
0.02 0.04 0.06 0.08
0.018 0.019 0.028 0.048
Quantum Yields. Experimental quantum yields for polymerization 4, and for dye fading 4 t h were calculated with the equations 4
R
niononier mol. wt,.
= I p, x polymer inol. wt.
(16)
(7) Values for k , and k,, are those determined hy Hatchard and Parker, ref. 6 .
DYE-SENSITIZED PHOTOPOLYMERIZATION PROCESSES
4th =
Rr
-
I.
(17)
Table I11 summarizes 4p and 4 t h values as a function of niononier and N T P levels. I n these calculations the polymer molecular weight was taken as 106. These
647
data show that 4p increases with an increase in both monomer and "I'P levels. However, 4 t h increases with an augmentation of the N T P concentration but decreases with increasing nionotiier owing to dye quenching by reaction c. These quantum yields of 0.05 niolecule/photon of light absorbed are the highest reported on a cont,rolled, visible light-induced system
Dye-Sensitized Photopolymerization Processes." 11. A Comparison of the Photoactivities of Thionine and Methylene Blue
by S. Chabereklb and R. J. Allen Technical Operations Research, Burlington, Massachusetts
(Received September 12, 1964)
The mathematical analysis of the experimental data on the anaerobic thionitie- and methylene blue-TEA-acrylaniide s y s t e m is consistent with the postulation that the polymerization-initiating radical is seniithionine (or senii(niethy1ene blue)) produced by the reaction of the light-excited dye with TEA. The predominant reactivity of TEA lies in its free base form, and the rate constant for this reaction is four to five tiines greater with thionine than with methylene blue. However, methylene blue is a better photosensitizer a t pH levels exceeding 9. The presence of oxygen in these systems causes an iiiduvtion period during which no polymer is formed, but the dyes are slowly bleached. The length of the induction period is proportional to the oxygen level. Systems containing met hyletie blue appear to be about twice as sensitive to oxygen content as those containing thionine. The quarituiii yields for polymerization for both dyes can be as high as 4 photorq'polymer molecule and are the highest obtained to date with aiiiiiie-containirig systems.
Introduction During studies of dye-sensitized free radical photopolymerization processes, it became desirable to increase the pH-operating range of photoinitiator systenis. 1Iost of our studies involved red-light-absorbirig thionine. Although this dye has many attractive its properties, it also has a disadvantage-naniely, photoinitiating efficiency at pH values exceeding 8.5 is greatly reduced. Exploratory. experiments showed that the structurally siniilar phenothiazine dye, niethylene blue, was apparently superior to t hionine in this respect. However, the latter appeared to be more sen-
sitive to oxygen. In view of the differences in the behavior of these two closely related dyes, it was considered desirable to make a more quantitative assessment of their relative efficiencies. This paper sunimarizes studies on the photopolymerization of acrylamide by thionine and methylene blue-triet hanolamine (TEA) initiator systems.
(1) (a) This study was performed under Contrart No. AF33(657)11553, I'hotographir Branch, Reconnaissance Division, Air Force Avonirs Laboratory, Wright-Patterson Air Force Base, Ohio; (b) to whom inquiries should be sent.
Voliime 69, Number 2
Fehricary 1966
