4898
Langmuir 1997, 13, 4898-4905
Polymerization of N-Vinyl-2-Pyrrolidone under Anodic Polarization: Characterization of the Modified Electrode and Study of the Grafting Mechanism C. Doneux* and R. Caudano Laboratoire Interdisciplinaire de Spectroscopie Electronique, FUNDP, 61 rue de Bruxelles, B-5000 Namur, Belgium
J. Delhalle Laboratoire de Chimie The´ orique des Surfaces et des Interfaces, FUNDP, 61 rue de Bruxelles, B-5000 Namur, Belgium
E. Le´onard-Stibbe, J. Charlier, C. Bureau, J. Tanguy, and G. Le´cayon Commissariat a` l’Energie Atomique, DSM-DRECAM-SRSIM, F-91191 Gif-sur-Yvette Cedex, France Received February 26, 1997. In Final Form: June 9, 1997X The application of single step or multiple anodic potential steps to highly concentrated solutions of N-vinyl-2-pyrrolidone leads to the formation of poly(N-vinyl-2-pyrrolidone) (or PVP) films on platinum electrodes. These films are insoluble in a good solvent of the commercial polymer. Multiple potential steps yield thicker films than a single potential step. Moreover, the PVP film thickness is maximum for a working potential amplitude of 1.7 V. Above this potential, the polymer film deteriorates by oxidation. In situ quartz crystal microbalance measurements show that the formation mechanism of polymer films cannot be attributed to an initiation in solution followed by precipitation of the polymer on the surface but rather to a gradual growth of the polymer on the electrode. This result is confirmed by the analysis of the electrochemical medium by different techniques which does not reveal the presence of polymeric chains in solution but only of dimer molecules. The molecular structure and the origin of these molecules are precisely established. From these results, we conclude that the polymer is chemically bonded to the platinum at the interface and grows progressively on the electrode surface. Both a radical and a cationic mechanism are examined to explain the above observations. A detailed theoretical analysis of plausible reaction intermediates suggests that only the cationic mechanism is able to account for all present experimental results and, in particular, for the occurence of a voltammetric current during the polymer growth.
1. Introduction The grafting of polymer chains onto solid substrates is of major interest for the design of materials with specific surface and structural properties. These modified materials are of technological interest. For instance, substrates coated with poly(N-vinyl-2-pyrrolidone) (or PVP) films have great potential for applications in the medical domain.1 Indeed, PVP is a bio- and hemocompatible polymer with very low toxicity. Therefore, materials coated with this polymer can be used as medical devices intended for either transient or permanent implant in the human body. The formation of a dense and homogeneous distribution of chemical bonds between the substrate and the polymer would resist mechanical stress and chemical attack from the environment and oppose microbial colonization. An approach toward this goal is to create reactive centers at the surface substrate which can react with functional groups of the polymer or initiate the polymerization of the monomer. This last possibility has been exploited by Tsubokawa and his co-workers, who studied the polymerization of N-vinyl-2-pyrrolidone (or NVP) initiated by carboxylic groups grafted onto carbon surfaces and the chemical bonding of PVP chains to these substrates.2-4 In these studies, they claimed that the propagation and the grafting mechanism are cationic, X
Abstract published in Advance ACS Abstracts, August 1, 1997.
(1) Fan, Y. L. Polym. News 1992, 17, 70. (2) Tsubokawa, N.; Takeda, N.; Kanamaru, A. J. Polym. Sci.: Polym. Lett. Ed. 1980, 18, 625.
S0743-7463(97)00212-6 CCC: $14.00
though it is known that the cationic polymerization of NVP in the bulk or in solution leads to oligomers only and has no technical importance.5,6 Recently, we have shown that the application of an anodic potential to a highly concentrated solution of NVP (5 mol‚L-1) under recycled argon atmosphere leads to the grafting and polymerization of the monomer onto platinum or gold electrodes.7-12 X-ray, ultraviolet photoelectron, and infrared reflection-absorption spectroscopies have revealed the presence of thin PVP films on these surface. (3) Tsubokawa, N.; Maruyama, H.; Sone, Y. J. Macromol. Sci.-Chem. 1988, A25, 171. (4) Tsubokawa, N.; Yoshihara, T. Polym. J. 1991, 23, 177. (5) Haaf, F.; Sanner, A.; Straub, F. Polym. J. 1985, 17, 143. (6) Biswas, M.; Mishra, P. K. Polymer 1975, 16, 621. (7) Le´onard-Stibbe, E.; Viel, P.; Younang, E.; Defranceschi, M.; Le´cayon, G.; Delhalle, J. In Polymer-Solid Interfaces, Proc. of the First Int. Conf., Namur; Pireaux, J. J., Bertrand, P., Bre´das, J. L., Eds.; Adam Hilger: Bristol, 1992; p 93. (8) Younang, E.; Le´onard-Stibbe, E.; Viel, P.; Defranceschi, M.; Le´cayon, G.; Delhalle, J. Molec. Engin. 1992, 1, 317. (9) Le´onard-Stibbe, E.; Defranceschi, M.; Delhalle, J.; Le´cayon, G.; Legeay, J. Le VidesLes Couches Minces 1993, suppl. 268, 117. (10) Le´onard-Stibbe, E.; Le´cayon, G.; Deniau, G.; Viel, P.; Defranceschi, M.; Legeay, J.; Delhalle, J. J. Polym. Sci.: Part A, Polym. Chem. 1994, 32, 1551. (11) Doneux, C.; Riga, J.; Verbist, J. J.; Charlier, J.; Le´onard-Stibbe, E.; Deniau, G.; Le´cayon, G.; Delhalle, J. Organic Coatings, AIP Conf. Proc. 354, 53th Int. Meeting of Physical Chemistry, Paris; Lacaze, P. C., Ed.; AIP: New-York, 1996; p 59. (12) Doneux, C.; Riga, J.; Delhalle, J.; Charlier, J.; Bureau, C.; Tanguy, J.; Le´cayon, G. Polymer-Solid Interfaces, Proc. of the 2th Int. Conf., Namur; Pireaux, J. J., Ed.; in press.
© 1997 American Chemical Society
Polymerization of N-Vinyl-2-pyrrolidone
In analogy with the electrochemical polymerization under cathodic polarization of poly(acrylonitrile),13,14 poly(2methyl-2-propenenitrile),14-16 and poly(p-chlorostyrene)17 films, we have proposed an ionic mechanism (cationic) to explain the grafting and the growth of PVP films. Similar experiments have recently been performed by Je´roˆme and his co-workers in which they suggest that NVP polymerizes in solution and that the polymer precipitates on the electrode surface without any chemical bonding.18 They explain the insolubility of the PVP films in the solvent by a secondary reaction rather than by a grafting reaction. They argue that ionization of the pyrrolidone substituent might occur in addition to polymerization so that interand/or intramolecular interactions (radical couplings, dipolar interaction) can trigger the polymer insolubility. In this paper, we study the influence of different electrochemical parameters (anodic polarization shape and working potential amplitude) on the adhesion and the quality of the PVP films formed on a platinum electrode. In order to obtain further insight into the formation mechanism of polymer films, we have followed the frequency response of a metallized quartz crystal used as the working electrode during the electrochemical process and studied the intermediate species formed in the electrochemical medium. 2. Experimental Section 2.1. Preparation of the Reactive Medium. All the experiments (reactant purification and electrochemical processes) were carried out in gloveboxes under recycled argon atmosphere. The amounts of residual water vapor and oxygen were continuously monitored and kept at levels of 1 and 5 ppm, respectively, by flow through various purifiers. The reaction medium was prepared by dissolving the monomer (5 mol‚L-1) in an electrolytic solution consisting of the purified solvent, acetonitrile, and the supporting electrolyte, tetraethylammonium perchlorate (5 × 10-2 mol‚L-1). N-vinyl-2-pyrrolidone (Aldrich, 99%) and acetonitrile (Fluka, >99.5%) were dehydrated by storage on molecular sieves (4 Å) and purified by fractional distillation under reduced pressure. Before dehydration, the solvent was treated by stirring 24 h with potassium permanganate in order to oxidize residual traces of unsaturated impurities such as acrylonitrile and allylic alcohol. The amount of water in both monomer and solvent was determinated by the Karl Fisher method and found to be less than 5 × 10-4 mol‚L-1. The supporting electrolyte (Fluka, 99%) was dried by permanent storage at 110 °C under reduced pressure. 2.2. Electrolysis. For the formation of polymer films, we performed the electrolysis in a Teflon cell in which cathodic and anodic compartments were not separated. On the other hand, for the study of the intermediate species formed in the electrochemical medium, we used a glass cell where anolyte and catholyte were separated by a sintered glass diaphragm. The latter avoids undesirable reactions between species generated at the anode and at the cathode. The electrolysis was carried out with a standard three-electrode arrangement. The working electrodes were 6-cm2 glass plates coated with 1 µm of platinum produced by cathodic sputtering. The auxiliary electrode was a platinum foil, and the reference electrode was based on the electrochemical Ag/Ag+ couple (Ag+ 10-2 mol‚L-1). The polymerization technique consisted of applying a single or a sequence of potential steps to the NVP solution. The potential steps were defined by their initial potential (0.0 V), working potential (0.5 (13) Le´cayon, G.; Bouizem, Y.; Le Gressus, C.; Reynaud, C.; Boiziau, C.; Juret, C. Chem. Phys. Lett. 1982, 91, 506. (14) Deniau, G.; Le´cayon, G.; Viel, P.; Hennico, G.; Delhalle, J. Langmuir 1992, 8, 267. (15) Bureau, C.; Deniau, G.; Viel, P.; Le´cayon, G.; Delhalle J. J. Adhes. 1996, 58, 101. (16) Deniau, G.; Viel, P.; Le´cayon, G.; Delhalle, J. Surf. Int. Anal. 1992, 18, 443. (17) Deniau, G.; Le´cayon, G.; Viel, P.; Zalczer, G.; Boiziau, C.; Hennico, G.; Delhalle, J. J. Chem. Soc., Perkin Trans. 1990, 2, 1433. (18) Je´roˆme, R.; Mertens, M.; Martinot, L. Adv. Mater. 1995, 7, 807.
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Figure 1. Representation of a single step and multiple potential steps application modes. f 2.1 V), and electrolysis duration (900 s). In order to follow the evolution of the intermediate species formed in solution, we applied a single potential step of 1.5-V amplitude of 2-h duration. The electrochemical experiments were performed with a PAR 273A potentiostat-galvanostat coupled to a personal computer. A Seiko EGRG Model QCA917 quartz crystal microbalance (QCM) allowed us to monitor the frequency shift of the working electrode during the electrochemical synthesis of the PVP film. 2.3. Characterization of the Electrode Surface. After electrolysis, the samples were rinsed with acetonitrile before analyses by X-ray photoelectron spectroscopy (XPS). The XPS spectra were recorded with a Scienta-300 spectrometer using a high-speed rotating anode (4100 rpm) and monochromatized Al KR radiation (1486.6 eV). The polymer film thickness was determinated with a Veeco Dektak 3030 profilometer. 2.4. Analysis of the Electrochemical Medium. The composition of the electrochemical solution in the anodic compartment after electrolysis was determinated by gel permeation chromatography (GPC) and mass spectrometry (MS). The GPC analysis were carried out on Knauer equipment with differential refractometry detection and Shodex OH-pak columns (KB 806, 805, 804, and 803). The eluant consisted of a 1:1 mixture of water/acetonitrile with 0.1 mol‚L-1 NaNO3, and the standards were commercial poly(N-vinyl-2-pyrrolidone) solutions. The MS analyses were performed with a Dani 3800 vapor-phase chromatograph coupled to a Vacuum Generator 70E spectrometer. The molecular structure of the isolated product was determinated by infrared spectroscopy (IR) and nuclear magnetic resonance (NMR) of the 13C isotope. The IR spectra were recorded by a transmission technique with a Bruker IFS 66 spectrometer. The 13C NMR spectra were collected with a Bruker AC200 spectrometer (50.3 MHz) using proton decoupling and distortionless enhancement by polarization transfer (DEPT) techniques.
3. Results 3.1. Formation of PVP Films on a Pt Electrode. 3.1.1. Influence of the Anodic Polarization Shape. In order to study the influence of the anodic polarization shape on the quality of the formed polymer film, we have applied to an NVP solution a single step or a series of potential steps varying between 0.0 and 1.7 V. The electrolysis duration for both modes is 900 s. A representation of both potential application modes is depicted in Figure 1. After electrolysis, the platinum electrodes have been analyzed by XPS and IRAS. Figure 2 compares the XPS C 1s core level spectra of platinum electrodes obtained after the application of a single (2A) step and multiple (2B) potential steps to that of a platinum electrode coated by dipping in a commercial PVP solution (2C). Four components, characteristic of the reference polymer, are needed to simulate the C 1s core level spectra of the polarized surfaces.19 The first one is due to carbon atoms in a CHx environment (a) (285.0 eV), while the second one is attributed to carbon atoms next to the carbonyl function (CsCdO) (b) (285.4 eV). The carbons singly bonded to nitrogen (CsN) appear in the third component (c) located at 286.2 eV and the carbon atoms doubly bonded to oxygen (CdO) in the fourth one (d) (287.8 eV). The XPS analysis (19) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers; John Wiley & Sons: Chichester, 1992; p 192.
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Figure 2. Simulation of the XPS C 1s core level spectra of Pt electrodes obtained after application of a single step (A) or multiple (B) potentials steps and a Pt electrode coated with a commercial PVP film (C).
allows us to conclude that the anode surface is covered by a PVP film and that the structure of the polymer is the same for both potential application modes. These conclusions are corroborated by IRAS spectroscopy. The films obtained after the application of a single step or a series of potential steps present the same absorption bands, all being characteristic of the molecular structure of PVP.12 From these results, it appears that the chemical and molecular structures of the PVP films obtained after the application of a single step or multiple potential steps are identical. Furthermore, these PVP films are insoluble in acetonitrile which is a good solvent for the commercial polymer. However, the thickness of these polymer films, determinated by profilometry, depends on the potential application mode. If the potential is applied as a single potential step, the PVP film thickness reaches 30 nm. In contrast, the thickness is of the order of 85 nm after a sequence of shorter potential steps. This observation can be explained by the fact that the sequence of potential steps favors the diffusion of the monomer toward the reactive sites at the surface of the anode and thus the polymer growth. This mode will be used for all the following measurements. 3.1.2. Influence of the Working Potential Amplitude. The PVP film thickness is also influenced by the working potential amplitude (Table 1). A film starts to form at a potential of 1.1 V. Its thickness increases with the amplitude of the working potential and reaches a maximum value of 85 nm at 1.7 V. These working potential values are situated in the electroactivity range of the monomer. Above 1.7 V, the thickness of the film decreases, reflecting degradation under oxidation. Indeed, the current-potential curve of a commercial PVP solution presents an oxidation peak at 2.0 V corresponding to the oxidation of the polymer.12
Doneux et al.
Figure 3. Evolution of the electrochemical current (A) and of the quartz oscillator frequency (B) as a function of the applied potential (cyclic voltammetry, sweep rate ) 5 mV s-1). Table 1. Influence of the Working Potential Amplitude on the PVP Film Thickness working potential amplitude vs Ag/Ag+, V
PVP film thickness, nm
0.5 1.1 1.3 1.5 1.7 1.9 2.1
0 10 30 55 85 70 30
3.1.3. PVP Film Growth Monitored by the Quartz Crystal Microbalance Technique. In order to obtain further information on the formation mechanism of PVP films, we have monitored the changes in the eigenfrequency of the working electrode by means of a QCM during the cyclic voltammetry measurements. A triangular potential with a sweep rate of 5 mV/s has been applied to a highly concentrated NVP solution. Figure 3 presents the measured electrochemical current and the variation of the quartz oscillator frequency as a function of the applied potential. The quartz oscillator frequency depends on the weight of the working electrode and/or on the density of the electrochemical medium close to the electrode.20 In both cases, the variation of the quartz oscillator frequency can be related to the formation of polymer or oligomer in the vicinity of the electrode surface. On Figure 3, we observe that the variation of the quartz oscillator frequency follows the evolution of the current. A large negative frequency shift (adsorption) is observed during the direct scan followed by a positive shift (desorption). The negative frequency shift is due to the adsorption of low molecular weight species and the polymer film growth. On the other hand, the positive (20) Ivanov, D. V.; Yelon, A. J. Electrochem. Soc. 1996, 143, 2835.
Polymerization of N-Vinyl-2-pyrrolidone
frequency shift can be explained by the desorption of the low molecular weight species and, possibly, by the degradation of the polymer film which has been observed by profilometry. The net quartz frequency change ∆ν measured at the end of the reverse scan can be attributed mainly to the presence of a stable polymer layer on the electrode. This behavior is not consistent with the mechanism proposed by Je´roˆme et al.18 Indeed, if the PVP films were formed by a simple precipitation of polymer chains formed in solution, one would rather expect a sudden rise of the quartz crystal frequency with an amplitude corresponding at least to the change ∆ν measured at the end of the cyclic voltammetry. From these observations, we suggest that the polymer grows continuously and progressively on the electrode during the cyclic voltammetry. The analysis of the electrochemical medium after electrolysis will allow us to confirm this suggestion. 3.2. Analysis of the Electrochemical Medium. In order to study the composition and the evolution of the reactive medium after electrolysis, we applied an anodic polarization at 1.5 V to an NVP solution for 2 h. The amount of current is about 4 C, which corresponds to the oxidation of 0.2% of the monomer molecules. After this electrolysis, the anolyte has been analyzed by gel permeation chromatography, mass spectrometry, and infrared and nuclear magnetic resonance spectroscopies. 3.2.1. Determination of the Anolyte Composition by Gel Permeation Chromatography and Mass Spectrometry. The anolyte GPC analysis reveals the presence of low molecular weight products but not of polymeric chains in solution. MS displays the presence of NVP (M ) 111) and of a dimer (M ) 222). 3.2.2. Determination of the Molecular Structure of Low Molecular Weight Species by Infrared and Nuclear Magnetic Resonance Spectroscopies. If we compare the infrared spectra of the electrochemical medium before and just after the electrolysis, we observe a decrease of the absorption band intensity located at 1629 cm-1 corresponding to the CdC stretching mode and a broadening of the band due to the CdO stretching mode (1704 cm-1). Assuming the validity of the Lambert-Beer law, the evolution of the CdC stretching band corresponds to a monomer consumption of about 10%. This value is higher than that predicted by electrochemistry (0.2%). This observation implies that monomer molecules are consumed not only by the electrochemical oxidation but also by a secondary reaction which probably leads to the formation of a dimer detected by mass spectrometry. In order to further check this interpretation, we have followed the evolution of the anolyte composition at the end of the polarization by infrared spectroscopy. Figure 4 presents the evolution of absorption bands corresponding to the CdO and the CdC stretching modes as a function of time. After 96 h, the CdC stretching band has completely disappeared. This clearly shows that all NVP molecules have been consumed and confirms the existence of secondary reactions. The broadening of the CdO stretching band arises from the emergence of two new peaks located at 1660 and 1680 cm-1, characteristic of the dimer molecules formed. The molecular structure of the isolated dimer has been determined by nuclear magnetic resonance spectroscopy of the 13C isotope using the proton decoupling technique (Figure 5). We can observe on the general spectrum presented in Figure 5A two peaks located at 173.6 and 172.7 ppm which correspond to two CdO groups in different surroundings. In Figure 5B and C are presented the 13C NMR spectra recorded with the DEPT technique, respectively DEPT(135) and DEPT(90). The latter shows
Langmuir, Vol. 13, No. 18, 1997 4901
Figure 4. Evolution of the infrared spectra (CdO and CdC stretching absorption bands) of the electrochemical medium after electrolysis as a function of time.
the presence of two vinylic CH groups (125.7 and 111.3 ppm) and of one aliphatic CH group (46.6 ppm). Six CH2 groups and one CH3 group are also detected on the DEPT(135) spectrum. These results have allowed us to establish the molecular structure of the dimer formed in the electrochemical medium (Figure 5).21,22 The trans conformation of the dimer has been confirmed by infrared spectroscopy. Indeed, the IR spectrum of the isolated dimer (which is not presented in this paper) shows an intense absorption band located at 960 cm-1 corresponding to the out-of-plane wagging of vinylic CH groups in trans conformation.23 The two IR absorption bands located at 1660 and 1680 cm-1 are respectively attributed to the stretching vibration of the RCdCR bond and to the stretching vibration of the C(1)dO bond.23 The formation mechanism of this compound will be discussed in the next section (paragraph 4.2). The analysis of the electrochemical solution clearly supports the conclusions drawn from the QCM measurements. The PVP films detected on the electrode surface cannot be due to precipitated polymer chains since GPC analysis reveals that no polymer is present in solution. 4. Discussion 4.1. Search for the Polymerization Mechanism of PVP Films. The application of a single step or multiple potential steps to an NVP solution leads to the formation of PVP films which are insoluble in acetonitrile. The (21) Czerwinski, W. K. Makromol. Chem. 1991, 192, 1297. (22) Landis, W. R.; Perrine, T. D. Appl. Spectrosc. 1968, 22, 161. (23) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic: San Diego, 1991; p 74.
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Figure 5. 13C NMR spectra of the isolated dimer (solvent: CDCl3): (a) general spectrum, (b) DEPT(135) spectrum, and (c) DEPT(90) spectrum.
adhesion of the electroinitiated PVP films to the Pt electrode, while in contact with a good solvent of the commercial polymer, can be explained either by chemisorption of the polymer at the surface or by precipitation of the cross-linked polymer formed in solution. As shown above by QCM and GPC, precipitation can be excluded, and we therefore conclude that the polymer films are grafted and grow progressively from the electrode surface. On the basis of the published literature, a cationic7-10 or a radical mechanism18,24 can be proposed to explain electropolymerization reactions and, in particular, the grafting and the growth of the PVP films. Let us first consider the cationic mechanism (Scheme 1A): the application of an anodic potential to an NVP solution implies the transfer of one electron from the monomer to the working electrode (A1). This transfer leads to the formation of a radical cation adsorbed on the electrode surface which is represented in Figure A2. Let us show that A2 is indeed the Lewis structure of an adsorbed radical cation. In the spirit of what was done in the case of the electropolymerization of acrylonitrile, we can give the major electronic configurations describing A2 and stemming from the interface.25 These are shown in Scheme 2: (A) The interface bond is dissociated so that one electron of the single C/surface bond is kept on the molecule and one on the surface. The system correlates at infinite separation with an organic radical cation and a surface in the same electronic configuration as before the electron transfer. One electron is transferred from the monomer molecule to the electrode, and this process gives rise to an anodic current which has been measured experimentally by the voltammetry technique. (24) Mertens, M.; Calberg, C.; Martinot, L.; Je´roˆme, R. Macromolecules 1996, 29, 4910. (25) Bureau, C.; Deniau, G.; Viel, P.; Le´cayon, G. Macromolecules 1997, 30, 333
Doneux et al.
(B) The interface bond is dissociated so that the two electrons of the single C/surface bond are kept on the molecule. The resulting fragments are a mesomeric form of the neutral NVP molecule and a positive charge on the surface. This configuration is of higher energy than configuration A by an amount A1(+) - µ (which is always positive), where µ (0) is the first electron affinity of the NVP radical cation.15 No electron transfer has occurred. Therefore, this process does not give rise to a voltammetric current. (C) The interface bond is dissociated so that the two electrons of the single C/surface bond are kept on the surface. In this case, the system correlates with a dicationic monomer and a locally anionic site on the surface. The energy of this configuration is I1(+) + µ (which is a positive value) above that of configuration A, where µ () -5.6 eV) is the chemical potential of the polarized electrode and I1(+) ()16.2 eV, UHF/3-21G) is the first ionization potential of the NVP radical cation.15 From this analysis, it appears clearly that configuration A is the most stable one. Thus, A2 describes an adsorbed radical cation (oxidized NVP) and is in agreement with the presence of a voltammetric current. This radical cation can desorb and react in solution to give rise to species of low molecular weight (A3). These reactions will be described in paragraph 4.2. Alternatively, if the lifetime of A2 is long enough compared to the mean time for a NVP molecule to diffuse toward the electrode, then the NVP molecules can add on via a cationic mechanism at the charged ends of the adsorbed oxidized NVP (A4). This process by propagation leads to the formation of a grafted polymer. This is all the more valid as the concentration of monomer increases. Let us now consider a radical mechanism (Scheme 1B), which is thought to prevail in nonelectrochemical media.5 The radical counterpart of A2 is B2. First, we show that the chemisorbed radical depicted in B2 cannot be the result of an electrochemical process. In analogy with Scheme 2, we have applied the dissociation procedure to the chemisorbed radical species, and the three electronic configurations are presented in Scheme 3: (A) The interface bond is cleaved so that one electron of the single C/surface bond is kept on the molecule and one on the surface. The resulting organic moiety is a mesomeric form of the neutral NVP molecule, and the surface is in the same electronic configuration as before interaction. No electronic transfer has occurred, and this gives rise to no voltammetric current. (B) In the case where the two electrons of the single C/surface bond are kept on the molecule, the system correlates with an organic radical anion and a positive charge on the surface. There has been no electronic transfer from the monomer molecule to the surface, but rather from the surface to the molecule. This configuration corresponds to a highly excited situation where the charge transfer is forced to be opposite to what the polarization would impose. (C) The interface bond is dissociated so that the two electrons of the single C/surface bond are kept on the surface. The electron transfer does give rise to an anodic current, and the resulting fragments at infinite separation are an organic radical cation and a locally anionic site on the surface. However, the radical cation has a charge distribution which is opposite to that expected for its ground state (positive charge on the substituted carbon) and thus also corresponds to a highly excited state.8 Among these configurations, structure A is the lowest energy configuration and thus the most stable one. Thus, the chemisorbed radical depicted in B2 describes an
Polymerization of N-Vinyl-2-pyrrolidone
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Scheme 1. Proposed Cationic (A) and Radical (B) Mechanisms for the Grafting and the Growth of PVP Films
Scheme 2. Major Mesomeric Forms Lying under the Lewis Structure of the Adsorbed Reaction Intermediate in the Cationic Mechanism (A2)a
a
Only excitations stemming from the molecular orbitals describing the interface bond have been taken into account.
adsorbed neutral NVP molecule (in its diradical form) and does not involve an electrochemical process. This is in contradiction to the observed voltammetric current accompanying the polymer growth.
Scheme 3. Major Mesomeric Forms Lying under the Lewis Structure of the Adsorbed Reaction Intermediate in the Radical Mechanism (B2)a
a Only excitations stemming from the molecular orbitals describing the interface bonds have been taken into account.
If we assume that the grafting step is disconnected from the electrochemical process, we are nevertheless faced with the fact that the radical mechanism is not particularly favored under the present electrochemical conditions.
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Scheme 4. Dimerization Reaction of the Radical Cation NVP Formed in Solution and Deprotonation Reaction of the Dicationic Species
Scheme 5. Mechanism of the Dimerization Reaction of NVP in Acid Medium
Indeed, the radical mechanism proceeds via a homolytic scission of the vinylic double bond (Scheme 1B). However, homolytic rupture of the CdC bond is highly improbable considering the initial polarization of the bond (due to the electron-donating effect of the pyrrolidone substituent) which is further enhanced under the field created by the electric double layer.26,27 Moreover, this electric field induces a reorientation of the molecule.28 This all together makes the radical mechanism rather unlikely to account for the grafting and the growth of PVP films under anodic electrochemical conditions. 4.2. Search for the Formation Mechanism of Low Molecular Weight Products in Solution. In the following, we will try to give evidence of the reactions taking place in solution and leading to the formation of the dimer which has been evidenced by MS, IR, and NMR spectroscopies. The oxidation of NVP molecules leads to the formation of a radical cation adsorbed on the surface (Scheme 1(A2)). This radical cation can desorb and react (26) Hennico, G.; Delhalle, J.; Younang, E.; Defranceschi, M.; Le´cayon, G.; Boiziau, C. Int. J. Quant. Chem. Symp. 1991, 25, 507. (27) Raynaud, M.; Reynaud, C.; Ellinger, Y.; Hennico, G.; Delhalle, J. J. Chem. Phys. 1990, 142, 191. (28) Geskin, V. M.; Lazzaroni, R.; Mertens, M.; Je´roˆme, R.; Bre´das, J. L. J. Chem. Phys. 1996, 105, 3278.
in solution. A highly probable reaction is the dimerization reaction, which gives rise to dicationic species (Scheme 4A). On the basis of the GPC results, these species cannot propagate the cationic polymerization of NVP in solution. Indeed, theoretical calculations have shown that these species are very stable and thus have a poor reactivity.29 A possible reaction would be the deprotonation of the dicationic species (Scheme 4B). However, the structure of the resulting dimer is not consistent with the IR and NMR results. The existence of the dimer evidenced in the anolyte by IR and NMR spectroscopies has already been mentioned by Breitenbach in his pioneering work on the polymerization of NVP.30 In an anhydrous and acid solution, NVP molecules yield this product. The mechanism of this reaction is described in Scheme 5 and is in agreement with our experimental results. However, the electrochemical synthesis is carried out in an aprotic anhydrous medium. The absence of protons in the initial solution implies that they are produced after the oxidation of NVP molecules. In order to verify the presence of protons in the anolyte and to elucidate their origin, we have performed a linear voltammetry in the cathodic range on (29) Younang, E. Personal communication. (30) Breitenbach, J. W. J. Polym. Sci. 1957, 23, 949.
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5. Conclusions
Figure 6. Current-potential curves in the cathodic range recorded with a sweep rate of 100 mV s-1 for the electrolytic solution (without monomer) before (a) and after (a′) the electrolysis, the electrochemical solution (with monomer) before (b) and after (b′) the electrolysis, and a hydrochloric acid solution (c).
the electrolytic (without monomer) and electrochemical (with monomer) media before and after the application of an anodic potential. The voltammetry curves are presented in Figure 6. No electroactive species are detected in the electrochemical solution that is not oxidized beforehand (curve 6b). In contrast, a large peak around -1.4 V is recorded for the electrochemical solution which has been oxidized beforehand at 1.5 V for 1 h (curve 6b′). This peak is attributed to the reduction of H+ ions. Indeed, an analogous peak is also observed for a hydrochloric acid solution (curve 6c). The absence of electroactive species in the electrochemical solution before the application of an anodic potential (curve 6b) and in the electrolytic solution (curves 6a and 6a′) shows that protons result from the electrochemical oxidation of NVP. As mentioned above, the oxidation of NVP leads to the formation of an adsorbed radical cation which can desorb and dimerize in solution. The deprotonation of the dicationic species formed could explain the presence of protons in the anolyte (Scheme 4).
In the present paper, we have shown that the application of a single step or multiple anodic potential steps to a highly concentrated solution of NVP leads to the formation of PVP films which are insoluble in a good solvent of commercial polymer. Films are thicker for the multiple potential steps than for the single one. If a working potential larger than 1.7 V is applied, the polymer film deteriorates by oxidation. In situ quartz crystal microbalance measurements have shown that the formation mechanism of polymer films cannot be attributed to an initiation in solution followed by precipitation of the polymer onto the surface but strongly suggest that the polymerization is initiated at the surface of the working electrode. This result has been confirmed by the analysis of the electrochemical solution after electrolysis by gel permeation chromatography which finds no polymer in solution. From these results, we have concluded that PVP films are grafted on the electrode surface and grow progressively from the electrode. The grafting and the growth of the PVP films can be explained either by a cationic or by a radical mechanism. However, a detailed theoretical analysis indicates that the radical mechanism is highly improbable on two grounds. First, this mechanism is unable to explain the observed voltammetric current, and second, it is incompatible with the conditions under which the CdC bond cleavage operates. Consequently, the cationic mechanism is most probable. The analysis of the electrochemical solution after electrolysis by different techniques has shown not only the absence of polymeric chains in solution but also the presence of dimer molecules. The molecular structure of this dimer has been determined. The dimerization reaction is initiated by protons coming from the oxidation products of NVP. This reaction is autocatalytic and thus can follow the electrolysis. These results indicate that the cationic growth of PVP is impossible in solution, while we find that it is the only plausible mechanism for the formation of PVP films accounting for the occurrence of our voltammetric currents. More work is thus needed in order to elucidate this apparent peculiar role of the electrochemical interface and to detail the geometrical growth of the PVP chains from the surface. Acknowledgment. C. Doneux acknowledges the “Fonds pour la formation a` la Recherche dans l’Industrie et l’Agriculture” (FRIA, Belgium) and the “Fonds National pour la Recherche Scientifique” (FNRS, Belgium) for their financial support. This work has been funded by the Belgian National Program of Interuniversity Research Project on Materials Science initiated by the Belgian State Minister Office (Science Policy Programming). LA970212I
