ISSN 1560-0904, Polymer Science, Ser. B, 2006, Vol. 48, Nos. 1–2, pp. 11–17. © Pleiades Publishing, Inc., 2006. Original Russian Text © A.V. Orlov, S.Zh. Ozkan, G.P. Karpacheva, 2006, published in Vysokomolekulyarnye Soedineniya, Ser. B, 2006, Vol. 48, No. 1, pp. 134–141.
Oxidative Polymerization of Diphenylamine: A Mechanistic Study A. V. Orlov1, S. Zh. Ozkan, and G. P. Karpacheva Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninskii pr. 29, Moscow, 119991 Russia Received June 9, 2005; Revised Manuscript Received August 16, 2005
Abstract—The mechanism of oxidative polymerization of diphenylamine is considered. The kinetic study of diphenylamine polymerization and of the structure and molecular-mass characteristics of the reaction products has shown that the degree of oxidation of intermediates plays the key role in polyrecombination. The relationship between the polymerization procedure and the molecular mass of polydiphenylamine was revealed. DOI: 10.1134/S1560090406010039
INTRODUCTION Numerous studies on the oxidative polymerization of aniline led researchers to assume that this reaction proceeds according to the cation-radical mechanism and involves polyrecombination of cation-radical intermediates arising in the course of oxidation [1–7]. However, many issues concerning the mechanism of chain propagation remain unclear. These are primarily as follows: (1) How does a growing polyconjugated chain preserve the activity of the radical center until the end of reaction? (2) To what extent may the mechanism of oxidative polymerization of aniline be extended to the polymerization of other aromatic amines? (3) Can high-molecular-mass products be derived from aniline derivatives? In this paper, on the basis of the comparative study of the kinetics of polymerization of aniline and its derivative—diphenylamine, some conclusions regarding the above issues are made.
H2SO4 (1/4 based on the total volume) were prepared; for polymerization in a H2SO4–tert-butanol mixture, a 0.2 M solution of diphenylamine in a mixture of 4 M H2SO4 and tert-butanol (equal volumes) and a 0.25 M solution of ammonium persulfate in the same solvent (1/4 based on the total volume) were prepared; and for the interfacial polymerization, a 0.2 M solution of diphenylamine in toluene and a 0.25 M solution of ammonium persulfate in the same volume of a 1 M solution of HCl were prepared.
EXPERIMENTAL The monomer and reagents were prepared as described in [8]. The kinetic experiments on the polymerization of diphenylamine were performed in the following manner. Solutions of the monomer and oxidizer were prepared separately. For polymerization in a solution of sulfuric acid, a 0.1 M solution of diphenylamine in 5 M H2SO4 and a 0.125 M solution of ammonium persulfate in 5 M
The UV spectra of polydiphenylamine samples in N,N'-dimethylformamide were measured on a Specord UV-VIS spectrophotometer in the 260–800 mm range.
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In order to start polymerization, solutions of the monomer and oxidizer were first cooled to –2–0°C and then mixed momentarily. The reaction was carried out under continuous intense stirring, and the temperature of the reaction mixture was maintained at a level of −2−0°C. At a certain time interval, the reaction mixture was precipitated into either a 5-fold excess of ice water (polymerization in H2SO4 and the H2SO4–tert-butanol mixture) or a 5-fold excess of isopropyl alcohol cooled to –2−0°C (the interfacial polymerization), filtered, and washed many times with distilled water until the reaction was neutral. The product was treated as described in [8].
The IR spectra of polydiphenylamine were recorded in the 4000–400 cm–1 range on a Specord M-82 spectrophotometer using samples prepared as KBr pellets. The spectra were processed using the Soft-Spectra program. The molecular mass of polymers was estimated by GPC on a Milton Roy chromatograph equipped with PL gel columns having pore sizes of 100, 500, and 105 Å. A DMF–0.1 M LiBr mixture was used as an elu-
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exceed (5–7) × 103 [8]. In the case of aniline, the analogous value achieves several hundred thousands [12, 13]. The most probable reason behind this effect is that the structures of both monomers and intermediates are different. An important characteristic of the monomer is its oxidation potential since this parameter determines the rate of oxidation and the induction period at the onset of the process. As is known, the oxidation potential for diphenylamine (6.9 V) is much lower than that for aniline (7.7 V) [14]. During the polymerization of diphenylamine, the one-electron oxidation of aromatic amine to the cation radical proceeds initially. This cation radical is stabilized in several resonance forms.
ent. The elution rate was 0.5 ml/min. Calibration was made relative to poly(ether sulfone). RESULTS AND DISCUSSION Figure 1 shows the kinetic curves for the polymerization of polydiphenylamine under homogeneous conditions (curves 1 and 2 refer to the sulfuric acid solution and H2SO4–tert-butanol mixture, respectively). As can be seen, the kinetic curves do not exhibit the S-shaped pattern that is typical of the autocatalytic polymerization of aniline [9–11]. A rather low rate of polymerization is accompanied by a short, compared to the polymerization of aniline, induction period, and the molecular mass of the products of diphenylamine oxidation conducted under homogeneous conditions does not
H
+
•
H
III
+
•
+
N
H
H
I
II +
N
N
H
H
IV
Owing to a reduction in the oxidation potential, the rate of oxidation at the initial stage increases by an order of magnitude and the induction period shortens. The recombination between cation radicals I–III is
N
+
N
+
•
N
•
+•
(NH4)2S2O8
N
N
H
V
hampered because of steric reasons. Therefore, the dimerization of cation radicals of diphenylamine occurs via C–C addition in the para-position of phenyl rings of IV and V (the so-called tail-to-tail addition).
+
•
•
–2H+
H
N
N
H
H
VI
Already at this stage, the process differs from the oxidative polymerization of aniline. In the case of
.
+•+
NH2 +
•
aniline, the dimerization of cation radicals occurs as a N–C addition (head-to-tail).
+
NH2
–2H+
N
NH2
H
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Since the oxidation potentials for dimers VII and oligomers of aniline are much lower than that of the monomer [1, 15, 16], they are primarily involved in oxi-
N
NH + 2
NH2
dation. The resulting quinodiimine structures of the aniline dimer are able to oxidize the monomer to the cation radical.
+2H+
N
The rate of this two-step oxidation (oxidizer–oligomer–monomer) turns out to be much higher than that of the one-step process (oxidizer–monomer). The autocatalysis phenomenon observed in the polymerization of aniline relies on this effect. Up to complete consumption of the oxidizer, polyaniline exists in the form of the pernigraniline salt and is reduced to the emeraldine state only at the end of reaction [9–11]. As for diphenylamine, a difference in the oxidation potentials of the diphenylamine dimer VI and the monomer is not as appreciable. As a consequence, in the course of polymerization of diphenylamine, the role of autocatalysis is less pronounced. Actually, when the oxidation of diphenylamine is carried out in sulfuric acid, the color of the reaction mixture typical of pernigraniline structures changes from blue to green (inherent to emeraldine structures) already during the induction period. If the reaction is performed in the H2SO4–tert-butanol mixture and via the interfacial process, the color typical of pernigraniline structures is absent. This indicates that the degree of oxidation of polydiphenylamine remains small throughout polymerization. UV spectroscopy studies provide support for these observations. The electronic absorption spectra taken for the reaction products at the beginning and end of the process correspond to the spectra of emeraldine structures in the doped form (λmax = 810 nm).
•+
NH2
jointed phenyl rings between amino groups formed via C−C addition (VI). For diphenylene compounds, the state in which the planes of neighboring rings take orthogonal positions is known to be thermodynamically more favorable [17, 18]. The occurrence of oxidized diquinodiimine units XI presumes a strictly coplanar position of phenyl rings. Therefore, the oxidation of dimers and oligomers of diphenylamine to the pernigraniline form requires sufficiently severe reaction conditions. Moreover, diquinodiimine units are primarily involved in oxidative hydrolysis. This offers another argument against the occurrence of autocatalysis in the polymerization of diphenylamine. Subsequent chain growth proceeds through recombination involving cation radicals of dimer IX and affords X and the cation radical of the monomer. Yield of polydiphenylamine, % based on theory 45
30
15
2
The low degree of oxidation of polydiphenylamine and the finding that a certain excess of the oxidizer with respect to the monomer facilitates an increase in the molecular mass of the polymer lead us to assume that, even though diphenylamine undergoes easy oxidation to the cation radical, oxidation of its dimers and oligomers to the pernigraniline form is hampered. This effect is most likely related to the structure of the monomer unit of polydiphenylamine that is composed of two Vol. 48
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Thus, the above evidence provides explanation why no S-shaped pattern (typical for the polymerization of polyaniline) is observed for the kinetic curves of the polymerization of diphenylamine and why the rate of oxidation of diphenylamine at the initial stage tends to increase.
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NH 2+ 2
H
VIII
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Fig. 1. Time variation in the yield of polydiphenylamine prepared by polymerization in (1) 5 M H2SO4 and (2) H2SO4–tert-butanol mixture. T = –2–0°C; [diphenylamine] = (1) 0.1 and (2) 0.2 mol/l; [(NH4)2S2O8] = (1) 0.125 and (2) 0.25 mol/l; and [H2SO4] = 2 mol/l. 2006
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VI
(NH4)2S2O8
+
N
N
H
–2H+
N
N
H
H
•
H
IX
X
N
N
H
H
A further oxidation of the leucoemeraldine form (IX) yields the polaron structure of the emeraldine form of polydiphenylamine.
•+
N
N
H
H
XI
+
N
N
H
H
•
Its further oxidation to dicationic pernigraniline structures is hindered owing to the above reasons. +
+
N
N
H
H
+
XII
Due to a high level of delocalization of π-electrons along the conjugation chain of polaron structures, the
+
N
N
H
H
stabilization of the radical center takes place shortly after formation of low-molecular-mass products.
Evolution of the molecular-mass characteristics of polydiphenylamine in the course of time (the synthesis temperature is –2–0°C) τ, h
Mw × 10–3
Mw /Mn
[Monomer], mol/l
[Oxidizer], mol/l
[Acid], mol/l
Polymerization in H2SO4 solution 0.25
6.7
1.6
0.1
0.125
5.0
1.5
7.1
1.4
0.1
0.125
5.0
4.0
6.7
1.4
0.1
0.125
5.0
6.0
6.1
1.7
0.1
0.125
5.0
Polymerization in H2SO4–tert-butanol solution 0.75
4.8
3.9
0.2
0.25
2.0
1.5
5.3
3.1
0.2
0.25
2.0
4.0
5.5
3.5
0.2
0.25
2.0
6.0
5.4
3.6
0.2
0.25
2.0
Interfacial polymerization in HCl 0.75
4.9
1.9
0.2
0.25
1.0
1.5
7.7
1.8
0.2
0.25
1.0
4.0
9.3
1.1
0.2
0.25
1.0
6.0
8.2
1.6
0.2
0.25
1.0
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In the polymerization of aniline, the level of oxidation of the growing chain corresponds to the pernigraniline form, which in the doped state has a dicationic structure. N
N
+
+•
H
H
H
+
N
Yield of polydiphenylamine, % based on theory 45
30
•+
NH2
XIII
15
In this case, the efficiency of conjugation is small, as is evident from the low conductivity of pernigraniline [16, 19, 20]. The activity of the radical center does not decrease up to the end of reaction, when the coloration of the reaction mixture changes from blue to green. This corresponds to a reduction of the pernigraniline form to the emeraldine one. Different activities of radical centers in the polymerization of diphenylamine and aniline are also responsible for different molecular masses of the polymers throughout the synthesis. In the polymerization of diphenylamine, the molecular mass of the products arising at the beginning of the process should not change in the course of polymerization, while in the polymerization of aniline, molecular mass should increase almost up to the end of the process. The experimental data convincingly confirm this assumption. When the polymerization of polydiphenylamine is conducted under homogeneous conditions (in sulfuric acid solution or in the H2SO4–tert-butanol mixture), the molecular mass of the products remains virtually unchanged in the course of synthesis. Only at the final stage, molecular mass somewhat decreases owing to the occurrence of oxidative hydrolysis (table). However, as is seen from the table, in the case of interfacial polymerization, molecular mass grows in the course of time and its value is higher. In the interfacial process, the degree of oxidation for oligomeric products of polymerization formed on the interphase is much higher than that in the organic phase. Owing to interaction with the aqueous solution of ammonium persulfate in the absence of the monomer, their polaron structure transforms into the pernigraniline form. This situation promotes conservation of the activity of radical centers owing to a lower stabilization by the conjugation chain. At the moment when oligomeric cation radicals occur in the reaction zone on the interphase, they undergo recombination and propagation of macromolecules take place. After polyaddition events, reaction products return to the organic phase, where the competing hydrolytic degradation processes are minimized. This also promotes an increase in the molecular mass of products arising from the interfacial polymerization. In the organic phase, oligomers of diphenylamine are reduced to the emeraldine form via the redox reaction involving the monomer. As a consequence, the POLYMER SCIENCE
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6 Time, h
Fig. 2. Time variation in the yield of polydiphenylamine prepared by interfacial polymerization. T = −2–0°C; [diphenylamine] = 0.2 mol/l; [(NH4)2S2O8] = 0.25 mol/l; [HCl] = 1 mol/l; and the toluene : HCl solution = 1 : 1 (vol/vol).
interfacial autocatalysis makes itself evident. With this, the curve of the yield of polydiphenylamine versus time acquires an S-shaped pattern (Fig. 2), characteristic of the autocatalytic process. Thus, as follows from the green color of the reaction mixture, the degree of oxidation of reaction products resulting from the interfacial synthesis carried out under homogeneous conditions is low. Nevertheless, in the reaction zone on the interphase, their degree of oxidation is significantly higher. As a result, the molecular mass of the products may increase before the end of reaction, when oxidative hydrolysis processes show themselves. The above-discussed hypothesis regarding stabilization of the radical center is justified by the facts that the copolymerization of diphenylamine with benzidine [21, 22], aniline [23], and anthranilic acid [24, 25] and the homopolymerization of its substituted derivatives, such as N-(alkyldiphenylamine) [26], 3-(methyldiphenylamine) [27], 3-(methoxydiphenylamine) [27], 3-(chlorodiphenylamine) [28], and diphenylamine-4sulfonic acid [29, 30], yield sufficiently high-molecular-mass products. In both cases, homogeneity of the polaron structure is deteriorated and this structure stabilizes the radical center via either insertion of allied units into a polymer chain or owing to steric factors. At the final stage of polymerization of diphenylamine, independently of the synthetic procedure, the yield and molecular mass of reaction products decrease and their degree of polydispersity increases. Similar results were reported for the polymerization of aniline [31–33]. In the course of time, oxidative hydrolysis processes begin to involve diquinodiimine units XIV and give rise to low-molecular-mass fragments containing quinone XV and amine XVI end groups. 2006
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N
N
NH
XIV H2O, H+
O + H2N
NH2 + O
XVI
XV
The presence of these groups is confirmed by the emergence of IR absorption bands at 1700 cm–1 (C=O) and 3520 and 3432 cm–1 (NH2). Simultaneously, the molecular mass and the degree of polydispersity of polydiphenylamine drop. In the interfacial synthesis, oxidative hydrolysis reactions are minimized since polymerization products are removed from the aqueous phase into the organic one. CONCLUSIONS Thus, in accordance with the cation-radical mechanism, the oxidative polymerization of diphenylamine includes the C–C dimerization of the monomer in contrast to the N–C dimerization operative in the case of aniline owing to steric hindrances arising at the nitrogen atom. A lower, compared to aniline, oxidation potential of diphenylamine ensures shortening of the induction period. However, the oxidation of polydiphenylamine to the pernigraniline form is hampered by the presence of two jointed aromatic rings in the monomer unit. This fact, in combination with the hydrolytic instability of the diquinodiimine group, is responsible for the inefficiency of catalysis and a decrease in the reaction rate. This is the reason the characteristic blue color of the reaction mixture is absent in the course of synthesis and the pattern of kinetic curves of homogeneous polymerization of diphenylamine differs from S-shaped. Owing to the low oxidation degree of diphenylamine oligomers, stabilization of the radical center upon conjugation with the polaron structure of the growing chain in the course of reaction occurs shortly after formation of these oligomers. A drop in the activity of the radical center accompanied by the hydrolytic chain degradation leads to formation of low-molecularmass products (Mw = (5–7) × 103) [8] under homogeneous conditions. In the interfacial process, oligomeric intermediates occur in the reaction zone—on the interphase—for a short time and, during this time period, they undergo oxidation to the pernigraniline form. No decrease in the activity of radical centers on the oxidized chain is observed in the course of chain growth. Therefore, oligomeric cation radicals can undergo unhindered recombination.
XVI
When oligomers return to the organic phase, the emeraldine structure of oligomers recovers under the action of the monomer in the course of interfacial autocatalysis. Thus, the molecular mass of polydiphenylamine increases throughout the process and this effect is assisted by weakened oxidative hydrolysis in the organic phase. As a result, the kinetic curves of interfacial polymerization acquire an S-shaped pattern typical of autocatalysis and the molecular mass of reaction products increases to (15–20) × 103 [8]. REFERENCES 1. D. M. Mohilner, R. N. Adams, and W. S. Angersinger, Jr., J. Am. Chem. Soc. 84, 3618 (1962). 2. Y. Wei, Y. Sun, and X. Tang, J. Phys. Chem. 93, 4878 (1989). 3. J. Bacon and R. N. Adams, J. Am. Chem. Soc. 90, 6596 (1968). 4. R. L. Hand and R. F. Nelson, J. Am. Chem. Soc. 96, 850 (1974). 5. Y. Wei, Y. Sun, S. Patel, and X. Tang, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 30, 228 (1989). 6. A. Guenbour, A. Kacemi, A. Benbachir, and L. Aries, Prog. Org. Coat. 38, 121 (2000). 7. L. R. Sharma, A. K. Manchanda, G. Singh, and R. S. Verma, Electrochim. Acta 27, 223 (1982). 8. A. V. Orlov, S. Zh. Ozkan, G. N. Bondarenko, and G. P. Karpacheva, Vysokomol. Soedin., Ser. B 48, 126 (2006) [Polymer Science, Ser. B 48, 5 (2006)]. 9. W.-S. Huang, B. G. Humphrey, and A. G. MacDiarmid, J. Chem. Soc., Faraday Trans. 1 82, 2385 (1986). 10. K. Tzou and R. V. Gregory, Synth. Met. 47, 267 (1992). 11. G. Zotti, S. Cattarin, and N. Comisso, J. Electroanal. Chem. 239, 387 (1988). 12. P. N. Adams, P. J. Laughlin, A. P. Monkman, and A. M. Kenwright, Polymer 37, 3411 (1996). 13. P. N. Adams, P. J. Laughlin, and A. P. Monkman, Synth. Met. 76, 157 (1996). 14. V. L. Beloborodov, S. E. Zurabyan, A. P. Luzin, and N. A. Tyukavkina, Organic Chemistry: A Manual for Institutes (DROFA, Moscow, 2002) [in Russian].
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