J . Phys. Chem. 1988, 92, 239-243
239
Initiation Mechanism of Acrylonitrile Polymerization Photoinitiated by Chromium(V1) Patrick Fageol, Michele Bolte, and Jacques Lemaire* Laboratoire de Photochimie MolPculaire et MacromolPculaire, U.A. C.N.R.S.433, UniversitP de Clermont II, B.P. 45, 63170 Aubicre, France (Received: November 24, 1986; In Final Form: July 22, 1987)
Analytical and kinetic aspects of the photoinitiated polymerization of acrylonitrile in the presence of chromium(V1) salt in aqueous solution are reported. The proposed mechanism includes a photoredox process that takes place in the excited state. A new complex, formed from the excited [HCrO,-]* anion and acrylonitrile, was inferred from the UV absorption spectra of irradiated solutions of the components at pH 3.12. The protonated form of this complex is written as [HCrO,-.-acrylonitrile] without any assumption about the stoichiometry or the site of complexation. This complex may dissociate to yield a chromium(V) complex and an organic radical, and the radical initiates the polymerization of acrylonitrile. The effect of various parameters on the initial quantum yields of chromium(V1) reduction is studied. a0is proportional to the monomer concentration and decreases when the pH of the solution is increased, with a pseudoplateau at pH 2.5-3.5. The final products formed in the redox phenomena are chromium(II1) and polyacrylonitrile. The CrV1 Cr"' photoreduction involves three H+ ions per chromium reduced. The mechanism of the initiation is governed by two acid-base equilibria between HCr0,- and Cr04,- on the one hand and between the protonated and deprotonated forms of the complex on the other.
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Introduction The polymerization of a vinyl monomer photoinitiated by coordination complexes has received particular interest. Iron(II1) salts,'v2 cobalt(II1) salt^,^-^ and manganese carbonyl complexes6 were proved to be efficient photoinitiators of the polymerization, but, while the kinetics of polymerization have been well studied for long time, only a few papers deal with the initiation mechanism in terms of coordination photochemistry. Natarajan7 reported the photoredox phenomenon involved in acrylamide and acrylonitrile polymerization photoinitiated by diacidobis(ethy1enediamine)cobalt(III) complexes. Delzennes described the mechanism of acrylamide polymerization photoinitiated by pentaamminecobalt(II1) complexes. To account for the second-order dependence of the rate of polymerization on the monomer concentration, Delzenne assumed the substitution of the coordinative bonded water by a monomer molecule. In an earlier paper, we reported acrylamide polymerization photoinitiated, in aqueous solution, by chromium(V1) as HCr0,- and by chromium(II1) as Cr(H20)2+? But we were not able to establish whether the redox phenomenon proceeds through the formation of a complex between HCr0,- and acrylamide. Very recently, acrylamide was found to be polymerized upon irradiation by visible light in the presence of R ~ ( b p y ) ~ ,and + an electron donor, triethylamine.1° In this paper, we report results regarding the mechanism of acrylonitrile (AN) polymerization photoinitiated by HCr0,- in aqueous solution. We are only considering the coordination aspect of the phenomenon; a further paper will deal with the polymerization parameters. Experimental Section Materials. (NH4),CrZO7was of commercial origin (Fluka analytical grade) and used without further purification. Acrylonitrile (Fluka puriss.) was freed from inhibitor by distillation under atmospheric pressure; the boiling fraction between 77 and 79 OC was collected. The solutions were deaerated with argon ( I ) Okimoto, T.; Inaki, Y.; Takemoto, K. J . Mucromol. Sei., Chem. 1973, A 7 ( 8 ) , 1537-1553. (2) Dainton, F. S.; James, D. G. L. J . Polym. Sci. 1959, 39, 299-312. (3) Kothandaraman, H.; Santappa, M. J . Polym. Sci., Polym. Chem. Ed. 1971, 9, 1351-1361. (4) Kaeriyama, K.; Shimura, Y. Makromol. Chem. 1973, 167, 129-137. (5) Bhaduri, R.; Aditya, S . Mukromol. Chem. 1977, 178, 1385-1401. (6) Bamford, C. H.; Mullik, S. U. Polymer 1973, 14, 38-39. (7) Natarajan, L. V.; Santappa, M. J . Polym. Sci., Polym. Chem. Ed. .. 1968, 6 , 324513257. (8) Delzenne, G. A. J . Polym. Sci., Part C 1967, 16, 1027-1036. (9) Robert, B.; Bolte, M.; Lemaire, J. J . Chim. Phys. 1985,82, 361-367. (10) Iwai, K.; Uesugi, M.; Takemura, F. Polym. (Tokyo) 1985, 17, 1005-1011.
0022-3654/88/2092-0239$01.50/0
for 30 min at 22 O C . The ionic strength was not controlled, and the pH was adjusted with HC1 or HClO, and was controlled to 10.02 pH unit. Apparatus and Procedures. A high-pressure mercury lamp with a Bausch and Lomb grating monochromator was used for the irradiations at 365 nm. The beam is parallel and the reactors are cylindrical quartz cells of 1- or 2-cm pathlength. The light intensity was measured by ferrioxalate actinometry (Io N 4 X 1015 photons-s-'.cm-2). A 365-nm irradiation setup delivering higher incident intensities ( I o N 1OI6 photons.s-1.cm-2) was used to study simultaneously chromium(V1) and H + consumptions. The reaction cell was surrounded by six fluorescent tubes (type TLD 15 W/05 Philips) emitting at 365 nm with a 50-nm bandwidth. UV-vis spectra were recorded on a Cary 118C and a PerkinElmer 554 double-beam spectrophotometer. Precipitated polyacrylonitrile was centrifuged down before the measurements. ESR spectra were recorded on a Brucker ER 200 D spectrometer at 9.30 GHz with a modulation field of 100 Hz. A Xe-Hg Hanovia lamp was used for irradiations in the ESR spectrometer cavity. DPPH was used as an internal standard. The HPLC chromatograms were carried out by using a 420 Beckman chromatograph with an ultrasphere ODS Altex column of 25-cm length. The eluent was a methanol/water mixture (30/70 v/v), and the flow rate is 0.8 cm3.min-'. The UV detection analysis was monitored at 360 and 220 nm.
Results Chromium(V1) exists in different forms in aqueous solution. In our experimental conditions, three species were present in the solution: HCrO,, Cr20+- in acidic medium, and CrO,,- in basic medium:" HCr0,2HCr0,-
%
CrO,,-
+ H+
* Cr207,- + H,O
pK = 6.49 Kd = 48 mo1-I.L
(1) (2)
mo1-L-l According to eq 1 and 2, in a solution of 5 X chromium(V1) at pH 3, the ratio HCrO,-/CrO,*- was equal to 3090 and Cr2072-represented 4.6% of the total chromium(V1); during the reaction, the chromium(V1) concentration decreased and the percentage of Cr2072-became less and less important. Moreover, we checked if the photoreactivity was affected in a significant manner when the chromium(V1) concentration was increased from lo-, to mol-L-'. (Cr207,- representing re(1.1) Pourbaix, M. Atlas d'Zquilibres Zlectrochimiques d 25 O C ; Gauthier-Villars: Paris, 1963; p 258. Handbook of Chemistry and Physics, 64th ed.; CRC: Cleveland, 1983-1984; p D169. Linge, H. G.; Jones, A. L . Ausf. J . Chem. 1968, 21, 2189.
0 1988 American Chemical Society
240
The Journal of Physical Chemistry, Vol. 92, No. 1, 1988 Abs.
t
300
Fageol et al.
B
u-
500
400
Figure 1. (A) Changes in UV spectrum of chromium(V1) (c = 5 X lo4 M) in the presence of acrylonitrile (0.9 M) at pH 3.0, A,,, = 365 nm. 15; (- X -) 25; (- -) Irradiation time (min): (-) 0; (- - -) 3; (-) 8; 35. (B) Time dependence of the absorbance at 350 nm. (-e-)
--
spectively 0.95% and 8.4% of the total chromium(VI).) So in a first approach HCr0; was considered as the only chromium(V1) species at pH 8.0, Cr042-was the only species present in the solution. The UV-vis spectrum of C r 0 2 - showed two intense bands at 273 and 373 nm. In acidic solution, the H C r 0 4 spectrum showed two less intense bands at 260 and 350 nm, and a plateau appeared at 440 nm, which was characteristic of the acidic form. The absorption of a 0.9 mol-L-' acrylonitrile solution in water was negligible at wavelength longer than 240 nm. The UV-vis mo1.L-I chromium(V1) was spectrum of a solution of 5 X not modified by the addition of acrylonitrile. Acrylonitrile did not affect the acid-base equilibrium between HCr04- and Cr04*-; the pK remained constant. HCr04- at pH 8.0 were considered as the only absorbing species. When kept in the dark, a solution of chromium(V1) ( 5 X mo1.L-I) and acrylonitrile (0.9 mol.L-') at 1.5 < pH < 9.0 was quite stable. The photoconversion of chromium(V1) into chromium(II1) was measured by UV-vis spectrophotometry from the decrease in absorbance at 350 nm, the chromium(II1) absorbance being negligible at this wavelength. Photoredox Phenomenon in Chromium( VI)-Acrylonitrile System. As shown in Figure lA, upon irradiation at 365 nm, the absorbance at 350 and 440 nm of an aerated solution ([chromol.L-', [acrylonitrile] = 0.9 mo1.L-l at mium(VI)] = 5 X pH 3.0) decreased and the original yellow color disappeared. When irradiated to completion, solutions of higher concentration of chromium(V1) ( c > mol-L-') turned light blue; two very weak absorption bands appeared at 420 and 560 nm. The initial quantum yields of chromium(V1) reduction were calculated from the slope of the absorbance decrease at 350 nm versus irradiation time (Figure 1B). They were measured at various pHs, the chromium(V1) and acrylonitrile concentrations being held constant at 5 X and 0.9 mol.L-', respectively. We did not use any buffer to avoid the eventual complexation with chromium(V1) or chromium(II1). The quantum yields were determined with a 5% conversion, so that the change of pH was negligible during the experiment. The experiments were repeated with and without oxygen (Figure 2). Duplicate and triplicate runs yielded values of a0 within &5%. pH adjustment by HCI or HC104 gave identical results. The quantum yield of chromium(V1) reduction was very low at pH >6.0 (ao< lo4 at pH 8.0); it increased when the pH was decreased, with a pseudoplateau at 2.5 < pH < 3.5. In that range of pH oxygen did not affect the kinetics of the photooxidoreduction. At lower pH (pH < 2.5), 9 O in deaerated solution remained constant and equal to the value obtained at the plateau. On the contrary, in the presence of oxygen, an increase of a0 was detected: at pH 1.8, 9 O is 40% higher than in the absence of oxygen. It has to be pointed out that, at any pH, in the presence of oxygen, no polymer precipitation was observed, and the absorbance at 420 and 560 nm due to chromium(II1) was detected.
O
2
5 p H
4
3
Figure 2. pH dependence of reduction quantum yield a0for CrVi Cr"'. [CrV1]= 5 X lo4 M and [acrylonitrile] = 0.9 M (-with 0,; - - - without 02).
Figure 3. Plots of the ratio H+/reduced chromium(V1) versus reduction percentage of chromium(V1) at different starting pH (pH,).
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TABLE I: Dependence of CrV1 Cr"' Quantum Yield on the Acrylonitrile Concentration
[AN],
mo1.L-l
-
Oo X lo3 for
CrV1 Cr"'
0
0
0.25 0.50
3.4 6.4
[AN], mol.L-' 0.75 0.90
a0X
Cr"'
-
lo' for
Cr"' 9.2
11.8
In degassed solution, a fast polymerization took place and it was very difficult to detect chromium(II1) absorption in the solution. The study by ESR spectroscopy revealed that the major part of chromium(II1) was immobilized in the polymer matrix. From this point of the work, the experiments were carried out at 2.5 < pH < 3.5 and in the absence of oxygen. Quantum yield measurements were also performed at constant pH (pH 3.0) and constant chromium(V1) concentration (c = 5 X lo4 mo1.L-I) with increasing acrylonitrile concentration in the range 0.2-1 mo1.L-l (the maximum of acrylonitrile solubility in water being 1.2 mo1.L-I). The results are summarized in Table I: a0increased linearly with acrylonitrile concentration. In the earlier experiments performed at pH ~ 5 . 0 we , noted that the photoreduction process induced the transformation of HCr0,- into CrO,*- for prolonged exposure: there was an increase of pH during the process. H+ consumption was measured at different starting pH, and the ratio (H' consumed)/(chromium(VI) reduced) was calculated. In the investigated domain (2.5 < pH C 3 . 9 , as shown in Figure 3, the ratio was constant and equal to 3 provided the conversion was low. But for longer irradiations and according to the pH, the behavior was different. At pH
