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J. Phys. Chem. B 2008, 112, 6976–6987
Chemical Oxidative Polymerization of Aminodiphenylamines ´ iric´-Marjanovic´,*,† Miroslava Trchova´,‡ Elena N. Konyushenko,‡ Petr Holler,‡ and Gordana C Jaroslav Stejskal‡ Faculty of Physical Chemistry, UniVersity of Belgrade, Studentski trg 12-16, 11158 Belgrade, Serbia, and Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic ReceiVed: NoVember 17, 2007; ReVised Manuscript ReceiVed: March 10, 2008
The course of oxidation of 4-aminodiphenylamine with ammonium peroxydisulfate in an acidic aqueous ethanol solution as well as the properties of the oxidation products were compared with those of 2-aminodiphenylamine. Semiconducting oligomers of 4-aminodiphenylamine and nonconducting oligomers of 2-aminodiphenylamine of weight-average molecular weights 3700 and 1900, respectively, were prepared by using an oxidant to monomer molar ratio of 1.25. When this ratio was changed from 0.5 to 2.5, the highest conductivity of oxidation products of 4-aminodiphenylamine, 2.5 × 10-4 S cm-1, was reached at the molar ratio [oxidant]/[monomer] ) 1.5. The mechanism of the oxidative polymerization of aminodiphenylamines has been theoretically studied by the AM1 and MNDO-PM3 semiempirical quantum chemical methods combined with the MM2 molecular mechanics force-field method and conductor-like screening model of solvation. Molecular orbital calculations revealed the prevalence of Nprim-C10 coupling reaction of 4-aminodiphenylamine, while Nprim-C5 is the main coupling mode between 2-aminodiphenylamine units. FTIR and Raman spectroscopic studies confirm the prevalent formation of linear Nprim-C10 coupled oligomers of 4-aminodiphenylamine and suggest branching and formation of phenazine structural units in the oligomers of 2-aminodiphenylamine. The results are discussed with respect to the oxidation of aniline with ammonium peroxydisulfate, leading to polyaniline, in which 4-aminodiphenylamine is the major dimer and 2-aminodiphenylamine is the most important dimeric intermediate byproduct. Introduction Aromatic diamine polymers have been extensively studied during the past decade1,2 because they have many novel and advanced functions in comparison with the polyaniline (PANI) and other similar conducting polymers of aromatic monoamines. Aminodiphenylamines, the most important dimeric intermediates of the oxidative polymerization of aniline,3 are aromatic diamine monomers of great theoretical interest. Mechanistic studies of various pathways of aniline polymerization are of fundamental importance for the understanding and control of PANI properties.4 Virtually all proposed mechanisms of the oxidative polymerization of aniline are based on the fact that 4-aminodiphenylamine (4-ADPA, p-semidine, Scheme 1) is the major dimeric product. These mechanisms differ only in the assumption whether the formation of 4-ADPA involves the radical recombination reactions between aniline cation radicals or the electrophilic aromatic substitution reactions of aniline nitrenium cations with neutral aniline molecules. Whether the oxidation of 4-ADPA would lead to PANI, similar to the oxidation of aniline, is thus the question of fundamental interest. Despite several studies on this topic, especially the electrochemical oxidation of 4-ADPA,5–11 the unambiguous answer is not known. To the best of our knowledge, only a few studies of the chemical oxidative polymerization of 4-ADPA have been performed.12,13 The probable first synthesis of 4-ADPA oligomers dates back to 1930 when Yoffe et al.12 synthesized various * To whom correspondence should be addressed. E-mail: gordana@ ffh.bg.ac.yu. † University of Belgrade. ‡ Academy of Sciences of the Czech Republic.
oligoaminodiphenylamines by the oxidation of 4-ADPA with ferric chloride followed by reduction with phenylhydrazine. Ding et al.13 performed polymerization of 4-ADPA with ammonium peroxydisulfate (APS), using [APS]/[4-ADPA] molar ratio of ∼2.8. In the present contribution, we investigate the oxidation of 4-ADPA with APS, as well as conductivity and structure of oxidation products, including the dependence on the oxidant-to-monomer molar ratio. The N-C para-linked aniline molecules are assumed to constitute typical PANI chains. Only a few reports on the oxidative polymerization of aniline consider also the possibility of the N-C ortho-coupling of aniline molecules in the dimerization step, and its potential role in the further growth of PANI chains.3,14–17 It has recently been revealed that both types of aminodiphenylamines, i.e., 4-ADPA formed by N-C paracoupling and 2-aminodiphenylamine (2-ADPA; o-semidine, Scheme 1) produced by N-C ortho-coupling of aniline molecules, are generated in the course of aniline oxidation with APS in water without added acid.3 It was proved that the formation of 4-ADPA is significantly preferred over the formation of 2-ADPA, which was further shown to be a much more important byproduct than benzidine and hydrazobenzene. That is why, we have decided to compare the oxidations of 4-ADPA and 2-ADPA, leading to poly(4-aminodiphenylamine) (P(4ADPA)) and poly(2-aminodiphenylamine) (P(2-ADPA)), respectively, under the conditions that are currently used in the preparation of PANI.18 As in the case of P(4-ADPA) and other polymers of aromatic diamines, only a deeper insight into the electrochemical oxidation of 2-ADPA was reported.19,20 There is no study devoted to the chemical oxidative polymerization of 2-ADPA.
10.1021/jp710963e CCC: $40.75 2008 American Chemical Society Published on Web 05/20/2008
Polymerization of Aminodiphenylamines SCHEME 1: 4-Aminodiphenylamine and 2-Aminodiphenylamine
P(4-ADPA) and P(2-ADPA) were characterized by gel permeation chromatography, conductivity measurements, and with FTIR and Raman spectroscopies. For the first time, the mechanism of the oxidative polymerization of aminodiphenylamines is computationally modeled. Semiempirical quantum chemical computational methods are seen to be well balanced; they are accurate enough to have useful predictive powers and yet fast enough to allow large systems such as aniline oligomers to be studied. The semiempirical modified neglect of diatomic overlap-parametric method 3 (MNDO-PM3) quantum chemical computational method is a distinct improvement over Austin model 1 (AM1), because overall errors in the heat of formation are reduced by about 40% relative to AM1.21,22 In a recent computational study focused on the torsion angle between rings, known to affect strongly the electronic and optical properties of conjugated polymers, Da´vila et al. suggest that the AM1 method is preferred over the PM3 for ring-structured oligomeric chains in the gas phase.23 Therefore, we performed comparative MNDO-PM3/AM1 study for dimeric hydrated intermediate species. The AM1 and MNDO-PM3 semiempirical quantum chemical methods, proved to be reliable tools when applied to oligomers of aromatic amines,2,3,23–31 have been combined with MM2 molecular mechanics force-field method32 and conductorlike screening model33 of solvation (COSMO) in the present computational study of the early stages of the chemical oxidative polymerization of aminodiphenylamines in aqueous ethanol solution. The results of quantum mechanical calculations are confronted with the experimental analysis of molecular structure of oxidation products based on FTIR and Raman spectra. The observed differences in molecular weights, conductivities, and molecular structures of P(4-ADPA) and P(2-ADPA) in comparison with PANI are explained on the basis of substantial differences in polymerization mechanism of aminodiphenylamines vs aniline. Experimental Section Oxidation of Aminodiphenylamines. The comparative oxidations of 4-ADPA and 2-ADPA (0.2 M, Fluka, Switzerland) have been carried out with APS (Lach-Ner, Czech Republic) at an oxidant-to-monomer molar ratio of 1.25 in aqueous ethanol (50 vol %) solution of 0.1 M sulfuric acid at room temperature, ∼20 °C. The oxidations were started by mixing the 0.4 M solution of 4-ADPA or 2-ADPA in ethanol with equal volume of the aqueous solution of APS in 0.2 M sulfuric acid. Temperature of reaction mixture was monitored to follow the progress of this exothermic reaction. The precipitates were collected after 24 h on the filter, rinsed with 0.1 M sulfuric acid, and dried in air. 4-ADPA was also oxidized at various molar ratios of the oxidant to monomer, 0.5-2.5. The products were treated as above. Characterization of oxidation products. Molecular weights were assessed with a gel permeation chromatography (GPC)/ SEC apparatus using a 8 × 600 mm PLMixedB column (Polymer Laboratories, UK) operating with N-methylpyrrolidone and calibrated by polystyrene standards using a spectrophoto-
J. Phys. Chem. B, Vol. 112, No. 23, 2008 6977 metric detection at the wavelength of 546 nm. The samples were dissolved in N-methylpyrrolidone containing 0.025 g cm-3 triethylamine, for deprotonation of samples, and 0.005 g cm-3 lithium bromide, to prevent potential aggregation. Infrared spectra of the samples dispersed in potassium bromide were recorded at 2 cm-1 resolution with a Thermo Nicolet NEXUS 870 FTIR spectrometer with a DTGS TEC detector. Raman spectra with HeNe 633 nm laser excitation were collected on the Renishaw inVia Reflex Raman microscope using a 50× objective and 10 s accumulation times. Conductivity of polymers was measured with a four-point van der Pauw method on pellets compressed at 700 MPa with a manual hydraulic press using a current source SMU Keithley 237 and a multimeter Keithley 2010 Vmeter with a 2000 SCAN 10-channel scanner card. Density was determined with a Sartorius R160P balance by weighing the pellets in air and immersed in decane. A scanning electron microscope (SEM) JEOL 6400 has been used to characterize the morphology of the samples. Computational Methods. The semiempirical AM1 and MNDO-PM3 models21,22 (included in molecular orbital package34 MOPAC 97, part of the Chem3D Pro 5.0 package, CambridgeSoft Corporation), with full geometry optimization by the EigenFollowing procedure35,36 were used to obtain the molecular orbitals, ionization energy (Ei), heat of formation (∆Hf), and spin density of individual species. The conformational analysis of aminodiphenylamines and their dimeric intermediates was performed. Input files for the semiempirical computations were the most stable conformers of the investigated molecular structures with minimized steric energy using the MM2 molecular mechanics force-field method.32 The COSMO technique has been applied during the geometry optimization to approximate the effect of a solvent surrounding the molecule.33 Dielectric constant in COSMO calculations (EPS), for the experimentally used water-ethanol mixture, was taken into account (EPS ) 53.5).37 The restricted Hartree-Fock method was used for the molecular structures, and the unrestricted Hartree-Fock method was used for radical species. Results and Discussion Comparative Oxidation of 4-ADPA and 2-ADPA. The solubility of aminodiphenylamines in water is low (3.3 × 10-3 mol dm-3 at 20 °C for 4-ADPA), and under the conditions comparable with the aniline oxidation, the monomer is not completely soluble in the reaction medium. That is why, the oxidations of 4-ADPA and 2-ADPA with APS have been carried out in the aqueous ethanol (50 vol %) solution of 0.1 M sulfuric acid, where both the monomers and oxidant were completely dissolved. We have compared the oxidations of 4-ADPA and 2-ADPA at the same [APS]/[monomer] molar ratio of 1.25, which is used in the “standard” preparation of PANI.18 The progress of the exothermic reaction was monitored by recording the temperature of the medium (Figure 1). In both oxidations, there was no induction period, which is typical for the oxidation of aniline in acidic media where anilinium cations are significantly prevalent over neutral aniline molecules.18,38 This is due to the considerably lower ionization energy of solvated monoprotonated forms of 4-ADPA (Ei AM1 ) 8.88 eV) and 2-ADPA (Ei AM1 ) 8.97 eV) in comparison with solvated anilinium cation (Ei AM1 ) 10.22 eV) in an aqueous ethanol solution. This is in accordance with the observation that the induction period of the oxidative polymerization of aniline in an acidic aqueous solution was found to be significantly reduced when small amounts of 4-ADPA were added to the reaction mixture.39 The
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TABLE 1: Properties of the Oxidation Products of 4-Aminodiphenylamine and 2-Aminodiphenylamine oxidation producta of property
4-ADPA
2-ADPA
yield, % conductivity, S cm-1 density,d g cm-3 molecular weight,e Mw polydispersity,e Mw/Mn
96.8 7.1 × 10-5b 1.34 3700 1.3
49.0
