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Polymerization of acrylamide photosensitized by the tris(2,2′-bipyridine)chromium(III) ion in aqueous solution
0277-5387193 S6.00+ .oO 01993 Pergamon Press Ltd
fotykedron Vol. 12, No. 8, PP. W-8858, 1993 Printed in Gnat Britain
POL~~RIZATION
OF’ ACRYL~~E
PHOTOSENSITIZED
BY THE TRIS(2,2’-B~YR~~E)CHROM~M(~) AQUEOUS SOLUTION CHRISTINE
and MICHELE
PIZZOCARO
ION IN
BOLTEt
Laboratoire de Photochimie Moltculaire at Macromol~~aire, URA CNRS 433, UFR de Recherche Scientifique et Technique, Universiti: Blaise Pascal, 63 177-Aubiere Cedex, France and MORTON
Z. HOGAN
Department of Chemistry, Boston University, Boston, MA 02215, U.S.A. (Received 23 November 1992 ; accepted 6 January 1993) Abstract-The 365 nm continuous irradiation of acidic aqueous solutions of Cr(bpy)33+ and acrylamide yields polyacrylamide ; the Cr(bpy)z(OH2)23+ aquation product is also produced. Upon pulsed-laser excitation, no chromium(I1) product is observed, likely due to the low rate constant for the quenching of thermally-equilibrated ‘TJ2E excited states of the photosensitizer by acrylamide and the low-cage escape yield of redox products. Nevertheless, Cr(bpy)33f is a suitable photosensitizer even for excitation with visible light up to 450 nm. The positive dependence of the rate of polymerization on the intensity of absorbed light and [monomer]’ and the inverse dependence on [Cr(bpy)33+] suggest that the termination step of the chain reaction involves the oxidative scavenging of the macroradicals by the photosensitizer.
The use of light to initiate the generation of free radicals has been extensively used in polymerization processes. In addition to providing on-off control at ambient temperature, the photochemical method permits the selection of the wavelength of light, and thus the nature of the excited states and radicals to be generated. Unfortunately, monomeric species generally absorb at short wavelengths and their polymerizations only occur with the excitation of high energy (1~ 270 nm). Through the use of a photosensitizer the selective formation of radicals can be achieved with light of lower energy. The polymerizations of vinyl monomers photoinitiated by coordination complexes, such as those of iron( ‘** cobalt(III),3-6 ruthenium(II)7 and chromium(W), 8-l’ have already been reported. Chromium(II1) as Cr(H20),3+ also appears to be
t Au&or to whom correspondence should be addressed.
an efficient photoinitiator of vinyl monomer polymerization.” In order to elucidate the mechanism of the photoinitiation process, and because of our previous work on the photochemistry and photophysics of polypyridyl complexes of chromium (III), ’ 2 we chose to investigate the polymerization of acrylamide (AA) photosensitized by Cr(bpy)33+ (bpy = 2,2’-bipyridine), the therapy-eq~~brated 2T1/2E excited states of which are luminescent, long-lived (z = 70 ps) in the absence of O2 and strongly oxidizing (*E” = 1.44 V).13 EXPERIMENTAL Materials Cr(bpy)33f (as the C104- salt) was synthesized according to literature procedures. l4 Acrylamide (Fluka puriss) was used without further purification. The solutions were prepared with doubly-
'l
II
I I
I Illl
C. PIZZOCARO et al.
856
distilled water and deaerated by bubbling with argon for 45 min. The p H of the solutions was adjusted with HCIO4 to +0.02 units; the ionic strength was not controlled.
0.10
Procedures The irradiations at 365 nm were performed with a high pressure mercury lamp (Osram H B O 200W) equipped with a grating monochromator (Bausch and Lomb). The beam was parallel and the reactor was a square cuvette with a 1 cm pathlength. The incident light intensity was determined by ferrioxalate actinometry ( I o ~ 1-2x1015 photon cm 2 s--l). In order to study the effect o f the absorbed light intensity on the polymerization rate neutral density filters were placed in front of the cell. Absorption spectra were recorded on a Cary 118C spectrophotometer; emission spectra were obtained with a Jobin Yvon JY 3C spectrofluorimeter equipped with a red-sensitive H a m a matsu detector. The extent of quenching of *Cr(bpy)33+ by acrylamide was determined by monitoring the emission intensity at 727 nm as a function of [AA]. The polymer was precipitated by pouring the irradiated solution into methanol; the polymer was separated and weighed, which offered a measure of the fractional conversion of acrylamide into polymer. The rate of polymerization, Rp, was taken from the slope of the linear part of the plot of per cent of m o n o m e r conversion vs irradiation time ; the precision of the values of Rp is estimated to be _ 10%. RESULTS
Quenching and photochemistry A linear Stern-Volmer plot from emission intensity measurements (Io/Ivs [AA]) yields kq = 3 × 103 M - ~ s -1 in the presence and absence of air. Irradiation of a deaerated solution at 365 nm, containing 0.11 m M Cr(bpy)33+ and 1 M AA at p H 2.24, resulted in spectral changes that indicated the formation of the aquation product Cr(bpy)2 (OH2)23+ (Fig. 1). The quantum yields of photoaquation (~pa) were virtually the same in the absence (7.0x 10 -4) and presence (8.0x 10 -4) of 1 MAA.
2 O
0.05
.B .< 1 -
I 4OO
300
x (nm) Fig. 1. Spectral changes upon irradiation at 365 nm m the absence of air at pH 2.24. [Cr(bpy)33+] = 0.11 raM; [AA] = 1 M.
time; a steady state was attained without any detectable induction period and the polymerization rate was constant over a large extent of the photoreaction. Since the formation of a gel at high conversion ( > 50%) made the solution difficult to handle, the influence of the various parameters on Rp was studied at conversions usually lower than 25%. Influence of [Cr(bpy)33+]. The variation of Rp as a function of [Cr(bpy)33+] is shown in Fig. 3. In the studied domain the rate of polymerization decreased with an increase in the concentration of the complex. Influence of pH. The variation of Rp with p H is complex (Fig. 4), with Rp first rising to a plateau with increasing pH, then falling off around p H 4. Due to the p H dependence all other experiments were performed at a p H in the plateau range. 80
,
,
,
,
,
,
i
,
,
i
. . . .
,
,
I
'
'
'
'
i
,
,
,
,
I
'
i
,
,
,
,
i
,
,
,
,
i
,
70-
60> o
5040-
®
30-
~-
2010-
Rate of polymerization
0
# ,
0
All experiments were performed on carefully deaerated solutions ; in aerated solutions only traces of polymer were observed. Figure 2 shows a plot of the per cent of m o n o m e r conversion vs irradiation
0.5
1 1.5 2 irradiation lime (hr)
Fig. 2. Plot of per cent conversion of monomer vs irradiation time in the absence of air at pH 2.70. [Cr(bpy)33+1 = 0.12 mM ; [AA] = 1 M.
857
Polymerization of acrylamide
1
1.5
[
2
2.5
3
1
3.5
4
[AA]’(M2)
Cr(bpy):+ x10' (M)
Fig. 6. Plot of
R, vs [AA]* at pH 2.20. [Cr(bpy)33+] = 0.14 mM.
Fig. 3. Plot of Rp vs [Cr(bpy),3+] at pH 2.75. [AA] = 1 M.
DISCUSSION From the results we can propose the following simplified scheme of acrylamide polymerization photosensitized by Cr(bpy) 33+. (1) Photophysics andphotochemistry 2lo
,,1,,,,,,,,,,,,,,,,1,,,,,,,,,,,,,, 1.5
2
2.5
3
3.5
4
4.5
5
PH
Fig. 4. Plot of R, vs pH. [Cr(bpy)33+]= 0.12 mM; [AA] = 1 M.
Influence of absorbed light intensity and [AA]. Rp is directly proportional to 1, (Fig. 5) and shows a square dependence on [AA] (Fig. 6). Influence of excitation wavelength. A few experiments were performed at 400, 425 and 450 nm. Polymerization was observed with values of R, of the order 10-5mol dm-3 s-l.
Reactions (l)-(3) describe the formation and disappearance of *Cr(bpy)33+. The lifetime of the excited state in the absence of quencher is determined by the rates of the various decay modes ; z, = l/(k,,+k,,). Reaction (3) is a complex multistep process, presumably involving the formation of a seven-coordinate intermediate from the attack of Hz0 at the metal centre and subsequent ligand loss and solvent substitution. Cr(bpy), 3+ hv
*Cr(bpy)3 3+
(1)
*Cr(bpy), 3+ - k0 Cr(bpy) 33+( + hv’)
(2)
*Cr(bpy)33++2Hz0”e, Cr(bpy)@W23+
+bpy
(3)
(2) Initiation -6-
w
-Iw5z4% x 3&
*Cr(bpy)33+ +AA~Cr(l~py)~~+
2lo 0
Reaction (4) describes the electron-transfer quenching of *Cr(bpy) 33+ by acrylamide :
,,,,,,,,,,,,,,(,. 0.5
1
1.5
1,x 1O-'5 (photonctn-3s-')
Fig. 5. Plot of Z$ vs Z, at pH 2.93. [Cr(bpy)33+]= O.l4mM;[AA]= 1M.
+M,
(4)
where M’ is the radical arising from the oxidation of the monomer. Due to the value of k4, the extent of quenching, even with 1 M AA, is quite low. Furthermore, the formation of Cr(bpy),*+ is not observed in pulsed-laser flash photolysis ; the quantum yield of redox products is estimated to be
858
C. PIZZOCARO et al.
@ is the quantum yield of M" formation in reaction (4); qb is given as kq[AA]/(ka+kq[AA]) with kd = ko+kp~. Due to the low extent of quenching and the low value of ~p,, Ri = (kq/ko)[AA]Ia. (3) Propagation M'+AA
kl ) M 2"
M2"+AA
k2
>M3"
an inverse first-order dependence on Cr(bpy)33+ at constant [AA] and Ia. Inasmuch as Ia is a function of [Cr(bpy)33+] via Beer's Law, a plot of Rp/I, vs 1/[Cr(bpy)33+] from the data in Fig. 3 should be linear with a y-intercept of zero. Such a plot is shown in Fig. 7, although we are at a loss to account for the non-zero intercept. Nevertheless, we feel that linear behavior demonstrates, in a general way, the efficacy of the mechanism. CONCLUSION
M._ l" + A A
4r, M,"
The rate of propagation (Rpr) is equal to kpr[P'][AA], where [P'] is the total concentration of radicals. (4) Termination It is proposed that in the presence of the photosensitizer the radicals (probably carbon-centred radicals adjacent to an amide moiety) react with Cr(bpy)33+ ( E ° = - 0 . 2 5 V), according to redox reaction (5) ; the rate of termination (Rt) is equal to kt[P'][Cr(bpy)3 3+]. p.+Cr(bpy)33+
k, ,polymer+Cr(bpy)32+
Cr(bpy)3 3+ is a suitable photosensitizer for the polymerization of acrylamide in acidic aqueous solution by near-UV and visible light. However, the low value of the rate constant for the quenching of *Cr(bpy)33+ by acrylamide and the low cage escape of the redox products are features that severely limit the overall efficiency of the process. Acknowledgements--This research was supported in part by the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy. M.B. and M.Z.H. also acknowledge support from the NATO Collaborative Research Grants Programme (CRG 900037). The authors thank Hai Sun for assistance with the pulsed-laser flash photolysis experiments.
(5) REFERENCES
(5) Overall kinetics For the overall process, [P'] = (kq/kokt)[AA]I~/ [Cr(bpy)33+] ; the rate of polymerization is given by equation (6), where Rp = Rpr. Fkqkpr] [nA]2Ia Rp = L kokt _] [Cr(bpy)33+]
(6)
Thus, the proposed mechanism accounts for a first-order dependence of Rp on/~ (Fig. 5), a secondorder dependence on [AA] (Fig. 6) and predicts i
i
i
r
i
i
.y
5040~5020100 2000
l
4000
i
i
6000
l
1
q
8000
Fig. 7. Plot of Rp/I~ vs 1/[Cr(bpy)33+].
10000
1. T. Okimoto, Y. Inaki and K. Takemoto, J. Macromolec. Sci. Chem. 1973, A7(8), 1537. 2. A. Abbas and I. Tadjudin, J. Photochem. 1986, 35, 87. 3. H. Kothandaraman and M. Santappa, J. Polym. Sci. Polym. Chem. Ed. 1971, 9, 1351. 4. K. Kaeriyama and Y. Shimura, Makromolec. Chem. 1973, 167, 129. 5. R. Bhaduri and S. Aditya, Makromolec. Chem. 1977, 178, 1385. 6. G. A. Delzenne, J. Polym. Sci. Part C. 1967, 16, 1027. 7. K. Iwai, M. Uesugi and F. Takemura, Polym. J. 1985, 17, 1005. 8. P. Fageol, M. Bolte and J. Lemaire, J. Phys. Chem. 1988, 92, 239. 9. P. Fageol and M. Bolte, Makromolee. Chem. 1989, 190, 367. 10. P. Fageol and M. Bolte, Makromolec. Chem. Rapid Commun. 1989, 10, 533. 11. T. Galcera, X. Jouan and M. Bolte, J. Photochem. Photobiol. A Chem. 1988, 45, 249. 12. G. Neshvad, M. Z. Hoffman, M. Bolte, R. Sriram and N. Serpone, Inorg. Chem. 1987, 26, 2984 and refs therein. 13. M. A. Jamieson, N. Serpone and M. Z. Hoffman, Coord. Chem. Rev. 1981, 39, 121. 14. B. R. Baker and B. D. Hehta, Inorg. Chem. 1965, 4, 848.
fotykedron Vol. 12, No. 8, PP. W-8858, 1993 Printed in Gnat Britain
POL~~RIZATION
OF’ ACRYL~~E
PHOTOSENSITIZED
BY THE TRIS(2,2’-B~YR~~E)CHROM~M(~) AQUEOUS SOLUTION CHRISTINE
and MICHELE
PIZZOCARO
ION IN
BOLTEt
Laboratoire de Photochimie Moltculaire at Macromol~~aire, URA CNRS 433, UFR de Recherche Scientifique et Technique, Universiti: Blaise Pascal, 63 177-Aubiere Cedex, France and MORTON
Z. HOGAN
Department of Chemistry, Boston University, Boston, MA 02215, U.S.A. (Received 23 November 1992 ; accepted 6 January 1993) Abstract-The 365 nm continuous irradiation of acidic aqueous solutions of Cr(bpy)33+ and acrylamide yields polyacrylamide ; the Cr(bpy)z(OH2)23+ aquation product is also produced. Upon pulsed-laser excitation, no chromium(I1) product is observed, likely due to the low rate constant for the quenching of thermally-equilibrated ‘TJ2E excited states of the photosensitizer by acrylamide and the low-cage escape yield of redox products. Nevertheless, Cr(bpy)33f is a suitable photosensitizer even for excitation with visible light up to 450 nm. The positive dependence of the rate of polymerization on the intensity of absorbed light and [monomer]’ and the inverse dependence on [Cr(bpy)33+] suggest that the termination step of the chain reaction involves the oxidative scavenging of the macroradicals by the photosensitizer.
The use of light to initiate the generation of free radicals has been extensively used in polymerization processes. In addition to providing on-off control at ambient temperature, the photochemical method permits the selection of the wavelength of light, and thus the nature of the excited states and radicals to be generated. Unfortunately, monomeric species generally absorb at short wavelengths and their polymerizations only occur with the excitation of high energy (1~ 270 nm). Through the use of a photosensitizer the selective formation of radicals can be achieved with light of lower energy. The polymerizations of vinyl monomers photoinitiated by coordination complexes, such as those of iron( ‘** cobalt(III),3-6 ruthenium(II)7 and chromium(W), 8-l’ have already been reported. Chromium(II1) as Cr(H20),3+ also appears to be
t Au&or to whom correspondence should be addressed.
an efficient photoinitiator of vinyl monomer polymerization.” In order to elucidate the mechanism of the photoinitiation process, and because of our previous work on the photochemistry and photophysics of polypyridyl complexes of chromium (III), ’ 2 we chose to investigate the polymerization of acrylamide (AA) photosensitized by Cr(bpy)33+ (bpy = 2,2’-bipyridine), the therapy-eq~~brated 2T1/2E excited states of which are luminescent, long-lived (z = 70 ps) in the absence of O2 and strongly oxidizing (*E” = 1.44 V).13 EXPERIMENTAL Materials Cr(bpy)33f (as the C104- salt) was synthesized according to literature procedures. l4 Acrylamide (Fluka puriss) was used without further purification. The solutions were prepared with doubly-
'l
II
I I
I Illl
C. PIZZOCARO et al.
856
distilled water and deaerated by bubbling with argon for 45 min. The p H of the solutions was adjusted with HCIO4 to +0.02 units; the ionic strength was not controlled.
0.10
Procedures The irradiations at 365 nm were performed with a high pressure mercury lamp (Osram H B O 200W) equipped with a grating monochromator (Bausch and Lomb). The beam was parallel and the reactor was a square cuvette with a 1 cm pathlength. The incident light intensity was determined by ferrioxalate actinometry ( I o ~ 1-2x1015 photon cm 2 s--l). In order to study the effect o f the absorbed light intensity on the polymerization rate neutral density filters were placed in front of the cell. Absorption spectra were recorded on a Cary 118C spectrophotometer; emission spectra were obtained with a Jobin Yvon JY 3C spectrofluorimeter equipped with a red-sensitive H a m a matsu detector. The extent of quenching of *Cr(bpy)33+ by acrylamide was determined by monitoring the emission intensity at 727 nm as a function of [AA]. The polymer was precipitated by pouring the irradiated solution into methanol; the polymer was separated and weighed, which offered a measure of the fractional conversion of acrylamide into polymer. The rate of polymerization, Rp, was taken from the slope of the linear part of the plot of per cent of m o n o m e r conversion vs irradiation time ; the precision of the values of Rp is estimated to be _ 10%. RESULTS
Quenching and photochemistry A linear Stern-Volmer plot from emission intensity measurements (Io/Ivs [AA]) yields kq = 3 × 103 M - ~ s -1 in the presence and absence of air. Irradiation of a deaerated solution at 365 nm, containing 0.11 m M Cr(bpy)33+ and 1 M AA at p H 2.24, resulted in spectral changes that indicated the formation of the aquation product Cr(bpy)2 (OH2)23+ (Fig. 1). The quantum yields of photoaquation (~pa) were virtually the same in the absence (7.0x 10 -4) and presence (8.0x 10 -4) of 1 MAA.
2 O
0.05
.B .< 1 -
I 4OO
300
x (nm) Fig. 1. Spectral changes upon irradiation at 365 nm m the absence of air at pH 2.24. [Cr(bpy)33+] = 0.11 raM; [AA] = 1 M.
time; a steady state was attained without any detectable induction period and the polymerization rate was constant over a large extent of the photoreaction. Since the formation of a gel at high conversion ( > 50%) made the solution difficult to handle, the influence of the various parameters on Rp was studied at conversions usually lower than 25%. Influence of [Cr(bpy)33+]. The variation of Rp as a function of [Cr(bpy)33+] is shown in Fig. 3. In the studied domain the rate of polymerization decreased with an increase in the concentration of the complex. Influence of pH. The variation of Rp with p H is complex (Fig. 4), with Rp first rising to a plateau with increasing pH, then falling off around p H 4. Due to the p H dependence all other experiments were performed at a p H in the plateau range. 80
,
,
,
,
,
,
i
,
,
i
. . . .
,
,
I
'
'
'
'
i
,
,
,
,
I
'
i
,
,
,
,
i
,
,
,
,
i
,
70-
60> o
5040-
®
30-
~-
2010-
Rate of polymerization
0
# ,
0
All experiments were performed on carefully deaerated solutions ; in aerated solutions only traces of polymer were observed. Figure 2 shows a plot of the per cent of m o n o m e r conversion vs irradiation
0.5
1 1.5 2 irradiation lime (hr)
Fig. 2. Plot of per cent conversion of monomer vs irradiation time in the absence of air at pH 2.70. [Cr(bpy)33+1 = 0.12 mM ; [AA] = 1 M.
857
Polymerization of acrylamide
1
1.5
[
2
2.5
3
1
3.5
4
[AA]’(M2)
Cr(bpy):+ x10' (M)
Fig. 6. Plot of
R, vs [AA]* at pH 2.20. [Cr(bpy)33+] = 0.14 mM.
Fig. 3. Plot of Rp vs [Cr(bpy),3+] at pH 2.75. [AA] = 1 M.
DISCUSSION From the results we can propose the following simplified scheme of acrylamide polymerization photosensitized by Cr(bpy) 33+. (1) Photophysics andphotochemistry 2lo
,,1,,,,,,,,,,,,,,,,1,,,,,,,,,,,,,, 1.5
2
2.5
3
3.5
4
4.5
5
PH
Fig. 4. Plot of R, vs pH. [Cr(bpy)33+]= 0.12 mM; [AA] = 1 M.
Influence of absorbed light intensity and [AA]. Rp is directly proportional to 1, (Fig. 5) and shows a square dependence on [AA] (Fig. 6). Influence of excitation wavelength. A few experiments were performed at 400, 425 and 450 nm. Polymerization was observed with values of R, of the order 10-5mol dm-3 s-l.
Reactions (l)-(3) describe the formation and disappearance of *Cr(bpy)33+. The lifetime of the excited state in the absence of quencher is determined by the rates of the various decay modes ; z, = l/(k,,+k,,). Reaction (3) is a complex multistep process, presumably involving the formation of a seven-coordinate intermediate from the attack of Hz0 at the metal centre and subsequent ligand loss and solvent substitution. Cr(bpy), 3+ hv
*Cr(bpy)3 3+
(1)
*Cr(bpy), 3+ - k0 Cr(bpy) 33+( + hv’)
(2)
*Cr(bpy)33++2Hz0”e, Cr(bpy)@W23+
+bpy
(3)
(2) Initiation -6-
w
-Iw5z4% x 3&
*Cr(bpy)33+ +AA~Cr(l~py)~~+
2lo 0
Reaction (4) describes the electron-transfer quenching of *Cr(bpy) 33+ by acrylamide :
,,,,,,,,,,,,,,(,. 0.5
1
1.5
1,x 1O-'5 (photonctn-3s-')
Fig. 5. Plot of Z$ vs Z, at pH 2.93. [Cr(bpy)33+]= O.l4mM;[AA]= 1M.
+M,
(4)
where M’ is the radical arising from the oxidation of the monomer. Due to the value of k4, the extent of quenching, even with 1 M AA, is quite low. Furthermore, the formation of Cr(bpy),*+ is not observed in pulsed-laser flash photolysis ; the quantum yield of redox products is estimated to be
858
C. PIZZOCARO et al.
@ is the quantum yield of M" formation in reaction (4); qb is given as kq[AA]/(ka+kq[AA]) with kd = ko+kp~. Due to the low extent of quenching and the low value of ~p,, Ri = (kq/ko)[AA]Ia. (3) Propagation M'+AA
kl ) M 2"
M2"+AA
k2
>M3"
an inverse first-order dependence on Cr(bpy)33+ at constant [AA] and Ia. Inasmuch as Ia is a function of [Cr(bpy)33+] via Beer's Law, a plot of Rp/I, vs 1/[Cr(bpy)33+] from the data in Fig. 3 should be linear with a y-intercept of zero. Such a plot is shown in Fig. 7, although we are at a loss to account for the non-zero intercept. Nevertheless, we feel that linear behavior demonstrates, in a general way, the efficacy of the mechanism. CONCLUSION
M._ l" + A A
4r, M,"
The rate of propagation (Rpr) is equal to kpr[P'][AA], where [P'] is the total concentration of radicals. (4) Termination It is proposed that in the presence of the photosensitizer the radicals (probably carbon-centred radicals adjacent to an amide moiety) react with Cr(bpy)33+ ( E ° = - 0 . 2 5 V), according to redox reaction (5) ; the rate of termination (Rt) is equal to kt[P'][Cr(bpy)3 3+]. p.+Cr(bpy)33+
k, ,polymer+Cr(bpy)32+
Cr(bpy)3 3+ is a suitable photosensitizer for the polymerization of acrylamide in acidic aqueous solution by near-UV and visible light. However, the low value of the rate constant for the quenching of *Cr(bpy)33+ by acrylamide and the low cage escape of the redox products are features that severely limit the overall efficiency of the process. Acknowledgements--This research was supported in part by the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy. M.B. and M.Z.H. also acknowledge support from the NATO Collaborative Research Grants Programme (CRG 900037). The authors thank Hai Sun for assistance with the pulsed-laser flash photolysis experiments.
(5) REFERENCES
(5) Overall kinetics For the overall process, [P'] = (kq/kokt)[AA]I~/ [Cr(bpy)33+] ; the rate of polymerization is given by equation (6), where Rp = Rpr. Fkqkpr] [nA]2Ia Rp = L kokt _] [Cr(bpy)33+]
(6)
Thus, the proposed mechanism accounts for a first-order dependence of Rp on/~ (Fig. 5), a secondorder dependence on [AA] (Fig. 6) and predicts i
i
i
r
i
i
.y
5040~5020100 2000
l
4000
i
i
6000
l
1
q
8000
Fig. 7. Plot of Rp/I~ vs 1/[Cr(bpy)33+].
10000
1. T. Okimoto, Y. Inaki and K. Takemoto, J. Macromolec. Sci. Chem. 1973, A7(8), 1537. 2. A. Abbas and I. Tadjudin, J. Photochem. 1986, 35, 87. 3. H. Kothandaraman and M. Santappa, J. Polym. Sci. Polym. Chem. Ed. 1971, 9, 1351. 4. K. Kaeriyama and Y. Shimura, Makromolec. Chem. 1973, 167, 129. 5. R. Bhaduri and S. Aditya, Makromolec. Chem. 1977, 178, 1385. 6. G. A. Delzenne, J. Polym. Sci. Part C. 1967, 16, 1027. 7. K. Iwai, M. Uesugi and F. Takemura, Polym. J. 1985, 17, 1005. 8. P. Fageol, M. Bolte and J. Lemaire, J. Phys. Chem. 1988, 92, 239. 9. P. Fageol and M. Bolte, Makromolee. Chem. 1989, 190, 367. 10. P. Fageol and M. Bolte, Makromolec. Chem. Rapid Commun. 1989, 10, 533. 11. T. Galcera, X. Jouan and M. Bolte, J. Photochem. Photobiol. A Chem. 1988, 45, 249. 12. G. Neshvad, M. Z. Hoffman, M. Bolte, R. Sriram and N. Serpone, Inorg. Chem. 1987, 26, 2984 and refs therein. 13. M. A. Jamieson, N. Serpone and M. Z. Hoffman, Coord. Chem. Rev. 1981, 39, 121. 14. B. R. Baker and B. D. Hehta, Inorg. Chem. 1965, 4, 848.