Mossbauer Spectroscopy of Fe +-complexed Pol y( N-pro p ylp yrrole- sul f onate)

P. AURIC, Equipe de Magnktisme et Diffusion par Interactions Hyperfines, Semice de Physique (and USTMG), and G. BIDAN,

Equipe d'Electrochimie Molkclrlaire, Laboratoires de C h i d , Dkpartement de Recherche Fond.umntale, Centre #Etudes NucGaires

de Grenoble, 85 X , 38041 Grenoble Cedex, France

Synopsis

The polymer obtained by chemical oxidation of N-propylpyrrole-sulfonate (PPS) with E'e(CIO,), is studied through elemental analysis and Mijssbauer spectroscopy. From elemental analysis data this polymer appears to be composed of a polypyrrolic backbone with side chains of al- kylsulfonate pendant groups. These sulfonate groups are mainly in the form of iron salts that act s a dopant of the poly(PPS). Mijssbauer spectra of iron-complexed poly(PPS) has been analyzed as functions of temperature, water content, and applied magnetic field in order to characterize the iron environment. Nearly 95% of the iron ions are ferrous ions in different environments. The spectra of a dried sample or a fully hydrated one, which are recorded at a given temperature, were fitted by using identical isomer-shift values but two different distributions or quadrupole doublets ( the Q.S. evolution with temperature was the same). The Debye temperature of the two samples has the same value, but the recoil-free fraction falls to zero somewhere above 340 K for a dried sample and near 290 K for the fully hydrated material. These results suggest that the additional water molecules that act as a solvate layer mainly change the lattice properties and that the Fe2 ' ions are probably linked to the SOT-terminated side chains of the backbone. These high-spin

ions are in the fast relaxation limit a t 4 K without external field and have anisotropic magnetic properties under high applied fields. No clusters or dimers have been detected. Fe2+ '

INTRODUCTION

Coulomb interactions between the polymer and the dopant seem responsible for the abnormal voltammetric behavior of conducting polymers such as polypyrrole.' Genies et a1.2 have shown by a spectroelectrochemical study that electrooxidation of polypyrrole involves the double movement of inser- tion of anions and exclusion of cations. The understanding of this ionic diffusion in polypyrrole is of interest from a fundamental point of view and for battery applications. In order to facilitate the modeling of the process of ionic diffusion in polypyrrole, we wished to eliminate one movement by immobilizing the doping anion.

Polypyrrole composites have been prepared using polymeric dopants such as polystyrene sulfonate3~r N a f i ~ n ~ . ~ - ~ In these cases, the quasi-immobiliza- tion of the sulfonate dopants (bonded to a polymer matrix) enforces the single movement of a counter cation during the electrochemical cycling7 of the polypyrrole-composite membrane.

Recently, we developed a new approach to functionalization of polypyrrole. Polymer films containing an electroactive center8 can be obtained by direct electropolymerization of a pyrrole covalently bonded to this electroactive

Journal of Polymer Science: Part B: Polymer Physics, Vol. 25, 2239-2252 (1987) 0 1987 John Wiley & Sons, Inc. CCC 0098-1273/87/112239-14$04~OO

2240 AURIC AND BIDAN

center. Using this method as an alternative to the insertion of polysulfonate dopant, we have prepared the poly( N-propylpyrrole-sulfonate) [poly(PPS)] in which the sulfonate anions are linked to the polypyrrolic skeleton by an alkyl chain. The chemical synthesis involves the use of the oxidant Fe(C104)3 (see Experimental), and elemental analysis data indicate an unusually high con- tent of iron as compared with the unsubstituted polypyrrole synthesized by the Same chemical method.g

Consequently, as a preliminary to the electrochemical study of the poly( PPS), we present in this paper a Mossbauer spectroscopic investigation of this compound with the iron ions acting as an internal probe of the poly(PPS) structure.

EXPERIMENTAL

Chemical Experiments

Pyrrole (Fluka, purum) was distilled under argon and stored over alumina in a dry box. Iron perchlorate (Fe(ClO,), . 6 H,O, Ventron) and the 1,3-pro- pane sultone (Aldrich) was used as received. THF was distilled in the dry box over the sodium salt of the benzophenone dianion. All chemical experiments were performed in an argon dry box.

Chemical Synthesis of 1 -(Pyrrol-1 -yl)-prop-3-yl-sulfonate potassium salt

The reaction path is the following:

SO, K'

and, to our knowledge, has never been described. This reaction can be easily extended; for instance, replacing the 1,3-propane sultone by the 1,4-butane sultone gives the l-(pyrrol-l-yl)but-4-yl-sulfonate potassium salt in similar conditions.

The pyrrolyl potassium salt was prepared in the dry box by the reaction of potassium with pyrrole (5% excess) in THF. The white precipitate was filtered and dried under vacuum. A solution of 5.2 g (5 x 10 mol) of pyrrolyl potassium salt in 50 cm3 THF was slowly added to 6.1 g (5 x mol) of 1,3-propane sultone in 10 cm' THF. A white-cream colored precipitate ap- peared with heat evolution. After stirring overnight, the solution was poured into 200 cm' diethyl ether and then filtered, yielding 10.6 g (94%) of PPS as a white-cream colored powder that melts and decomposes a t T = 220°C.

Chemical analyses (elemental and high resolution mass spectroscopies) were correct. IR (KBr): Y = 3100-3120 cm ' (pyrrolic C-H), 2880-2930 cm '

MOSSBAUER SPECTROSCOPY 2241

(alkyl C-H and C-H,), 1190 and 1063 cm-' (sulfonate). 'H-NMR (d6 DMSO used as a solvent, TMS as internal standard): 6 (ppm) 2.03(qi, 7 Hz, 2 H), 2.53(t, 7.5 Hz, 2 H), 3.97(t,6.8 Hz,2 H), 5.!36(t,2.2 Hz,2 H), 6.7(t, 2.2 Hz, 2 H).

Chemical Polymerization and Treatments of poly(PPS)

The electrochemical polymerization of PPS failed.* The basic reaction, similar to those previously described for the chemical synthesis of p~lypyrrole,~ is the following:

x(2 + 6)Fe(CIO,),

- 2xHCI04

> -x(2 +8)Ft$CIO,), -0- N

\ \

i ) SO;,K'

qL+, sc10; N

i SO,, K '

X

6 = doping level of the polymer

In fact, i t is very likely (and is confirmed by elemental analysis) that the doping is mainly ensured by -SO; , so (KCIO,), when solubilized, is expelled by Coulombic interaction. Moreover, the doping counter cation K ' may exchange with Fe2+ and Fe", which are better complexing agents of sulfonate ions than K'.

mol) of PPS was dissolved in 20 cm3 H,O, then 50 cm:' CH,CN was added. There was 21.5 g (4.7 x 10 mol) of Fe(ClO,), . 6 H,O in 50 cm3 CH,CN were quickly poured into the vigorously stirred PPS solution. A black precipitate was separated by filtra- tion. After washing with CH,CN in a Soxhlet apparatus and drying overnight under vacuum at room temperature, 2.243 g ( p = 30% from elemental analy- sis) of poly(PPS) was obtained and labeled sample A .

To the organic solution coming from the filtration an additional amount 9.3 g (2 X mol) of Fe(C10,), . 6 H,O was added in powder form. A second fraction of 3.59 g ( p = 53% from elemental analysis data) poly(PPS) was obtained. This behavior is quite different from the usual chemical synthesis of polypyrrole that gives the polymer in almost 100% yield in a single stoichio- metric step.9 A portion of this second sample (1.78 g) was set aside, washed with water, and then washed overnight with CH,CN in a Soxhlet apparatus in the dry box. There was 1.193 g of compound (labelled sample B) obtained.

The procedure is the following: 4.54 g (2 X

'It is known that electropolymerization of pyrrole in the presence of alkyl or aryl sulfonate ions requires a high concentration (about 0.1 M ) of pyrrole. This is probably due to the inhibition of the coupling of the pyrrole radical cations by sulfonate ions. In the case of PPS, internal inhibition must be enhanced in a way similar to the internal inhibition in electropolymerization of N-(hydroxypropyl) pyrrole.'" However electrocopolymerization of pyrrole and PPS in a mixture of DMSO and water is possible and gives a film than can be handled" ( o = 3 X 10 c m - ' ).

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MOSSBAUER SPECTROSCOPY 2243

From elemental analysis i t was concluded that washing with water eliminates perchlorate anions, mostly in the potassium salt form [0.587 g by weight compared with (338.8-218.9) X 1.78/338.8 = 0.630 g from elemental analysis (see Table I)].

Elemental analysis, formula, doping level, and conductivity data are listed in Table I.

A 100 mg pellet of sample A was exposed for 10 days to saturated aqueous vapor and protected from diffusion of air. The water content of this fully hydrated sanpk A was determined by weighmg and corresponds approxima- tively to 10 mol H,O for one SO; group.

Conductivity Measurements

Four-probe-method measurements were performed on 13 mm diameter pellets obtained by a 8 x lo3 kg cm-' pressure on about 100 mg of polypyr- role powder in the dry box.

MGssbauer Experiments

The Mijssbauer spectra were recorded at various temperatures in a cryostat equipped with a superconducting magnet. Parallel magnetic fields of up to 8 T were applied to the sample. The emitter (a 100 mCi 57C0 source in a Rhodium matrix) was maintained a t 4 K. All isomer shifts are given relative to a metallic iron absorber a t T = 300 K, which was used for calibration.

MOSSBAUER EXPERIMENTS AND DISCUSSION

Mossbauer Experimenta on Dried Sample

M b b a u e r spectra of sample A, recorded a t different temperatures, are

The main features are the following: shown in Figure 1.

At T = 4 K, the mean quadruple splitting is 3.10 mm/s and decreases when the temperature increasea [quadrupole splitting (QS) = 2.20 mm/s at T = 340 K]. These values and the isomer shift (IS) values (see Fig. 2) are typical of Fe2+ in octahedral sites.

There is no evidence of magnetic hyperfine structure a t low temperature. The shape of the peaks is asymmetric, and this asymmetry increases with

increasing temperature. Since i t was not possible to fit the experimental spectra with only one doublet, all spectra were computer-fitted with a distribution of environments, more precisely a distribution of quadruple doublets P(QS). The quadrupole splittine were fixed at predetermined values, the linewidth at half-maximum was 0.28 mm/s, the isomer shifts and the relative intensities of 14 different doublets were free parametem.

The fits were performed allowing a Fe3+ contribution that was of about 5 f 3%. The best fits were obtained when the isomer shift values of the different iron sites were identical within the experimental error. These values plotted in Figure 2 are close to thme generally given for Fe(H,O)g+ (1.36-1.42

2244 AURIC AND BIDAN

100.0

99.0

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98.0

200.0

.\*

z c3 - 98.0 '3 cn E L3 2 4 96,O a

100.0

I

c

96.0

92.0

,20 -2.10 0. 2.10 L o 2 0 VELOCITY ( M R / S )

Fig. 1 . MGsbauer spectra of poly(PPS); sample A in the dried state at different temperatures.

rn~n/s'~-'~) and those for aFe(CH,SO,), (1.45 mm/s14-16) where the sulfonate is supposed to adopt layered polymeric structures in which each RSO, group bridges three different Fe2+ and where each Fe2+ is surrounded by a nearly octahedral arrangement of oxygen atoms from six different RSO, anions.

The P(QS) curves at different temperatures from 4 to 340 K were plotted; two are presented in Figure 3a. At high temperature, the site distribution is broadened, indicating a large nonequivalence of the electric field gradient (EFG) for the iron sites. The QS values corresponding to the maximum of the P(QS) curves are plotted in Figure 2; these values are lower than the QS values of Fe(H,O)i+ or aFe(CH,SO,), reported in Table I1 and more temper- ature dependent. From these IS and QS values relative to a dried sample, it is

MOSSBAUER SPECTROSCOPY 2245

2.0 I- - ? 1.40 -

X E - E - c/i 4 1.20 -

L 1.30 -

- I I I

7

? E E L 3.0

I I I

dried sample A

A hydrated sample

Fig. 2. Temperature dependence of Mkbauer parameters of poly(PPS) in the dried and hydrated states deduced from fits assuming a P(QS) distribution. The QS values correspond to the maximum of the P(QS) curves presented in Figure 3.

not possible to know if Fe2+ ions interact only with RSO; ions, or only with H,O molecules, or with both. Whatever the case, the QS values are typical of an orbital singlet ground state and low-lying excited states.

To get more information about the Fe2+ interactions, we studied another material (sample B) with a different composition. The Mossbauer spectra of sample B were identical to the spectra of sample A. Since the main difference between the two samples is the concentration in ClO; ions (see Table I), we concluded that these anions do not interact with Fez ’ ions.

Influence of Water

When comparing the Mossbauer spectra of the swollen and fully hydrated sample A (H,O/SO, = 10, see Experimental) with those recorded with the dried polymer (H,O/SO; = 1, see Table I), we observed that the former sample has narrower lines, larger quadrupole splittings, and the same isomer- shift values. The spectra were fitted as described above. A narrowing of the distribution curve P(QS) and a displacement of QS to higher values is observed in Figure 3b. These results can be understood if one assumes that the QS distribution is partly due to a distribution of the water in the hydration shell of the ferrous ions, and that large high concentrations reduced the P(QS) distribution; however, it is not possible to establish a quantitative correlation between the hyperfine parameters and the numbers of water molecules in the vicinity of the iron atoms. But, since a P(QS) distribution is still present in the hydrated sample and easily seen on high temperature spectra, we can

2246 AURIC AND BIDAN

I I I I I

Q. S. (mm/s) Fig. 3. Histograms showing temperature dependence of P(QS) curves for (a) dried and (b)

hydrated poly(PPS). The solid and dotted lines correspond to experiments carried out at 4 and 250 K, respectively.

TABLE I1 '' Fe M6esbauer Parameters

Dried

Hydrated sample 4 1.39 3.12

sample 4 1.39 3.36

aFe(CH3So3 )2 85 1.45 3.37

-24.5 40 1.39 -2.8 1 -3.0 -3.0 -28.0 60

4 1.39 +3.4 0 -Haw - H'PP d

e 295 1.34 2.95

80 1.42 3.60 Fe(H2Oy 20 1.36 3.30

.Relative to iron metal at 7' = 3W K. bTheee values are only indicative. ' Parameters notation from Ref. 26.

cData from Ref. 14 and 15. 'Data from Ref. 12.

Evaluated from experiments cnrried out with external fielda

MOSSBAUER SPECTROSCOPY 2247

suppose that the backbone takes a prominent part in the broad distribution of the ligand fields a t the Fe2+ ions because of interactions between iron and the SOT-terminated side chains of the polymer. More information can be ob- tained from Figure 2, where we note that the evolution of the hyperfine parameters with increasing temperature is the same for the two samples. This evolution can be accounted for by some remarks concerning the EFG. As first approximation, this parameter arises from two contributions: (1)

the electronic contribution that results from anisotropic temperature-depen- dent charge distribution of the open-shell electrons of the iron ion under consideration and (2) the lattice contribution, which is largely temperature independent except through crystallographic changes or lattice thermal ex- pansion effects.

Since the QS evolution with temperature is the same for the dried sample or the hydrated one, one can suppose that the insertion of water molecules into the compound mainly modifies the lattice contribution of the QS Mossbauer parameter and creates few perturbations in the open-shell electrons of the iron ions. This assumption is supported by the calculated isomer shift values, which are the same for the dried sample or the hydrated one. Additional water molecules seem to act as a solvate layer modifying the lattice surround- ing the iron ions; these ions are still in a range of environments.

Dynamic Properties of the Lattice

The absorber recoilless fraction f , which is proportional to the area A under the absorption spectrum when the absorber thickness is small, is given by the expression f = exp( - k '( x ')) where k is the wave vector of the y ray, and ( x ' ) is the mean square atomic displacement of the iron atom along the direction of the gamma ray emission. If the vibrational amplitude of this atom arises only from the vibrational modes of the polymer matrix, the excitation is acoustic and can be related to the bulk properties of the solid."*'8 A Debye model should give a reasonable first approximation for the f factor. The lattice vibrations are simulated with a frequency distribution of harmonic oscillators. This predicts a linear variation of In( f ) = f (T) , and the slope of this curve gives the Debye temperature OD, which is a lattice characteristic.

For several p~lymers '~ a strong correlation between the glass transition temperature Tg and the temperature a t which there is a sudden fall in In( f ) versus T has been established. Above Tg a rapid decrease of f is due to the onset of cooperative motions of large parts of the main polymer chainm involving intermolecular interactions. In dilute frozen solutions, f falls to zero at the melting point, where ( x ' ) diverges.

From the experimental data, the YA(T)/A(4.2 K)]* curves relative to sample A in the dried and hydrated states are plotted and shown in Figure 4. In the dried sample A, the ln[A(T)/A(4.2 K)] values are higher than in the hydrated sample A, probably because of stronger bonds betwee macromole- cules.

' A ( T ) is the area under the Massbauer spectrum at T (K). The ratio [A(T)/A(4.2 K)] is proportional to the f factor.

2248

0.5 Cls Ft L

Y

-0.5

-1.5

AURIC AND BIDAN

I I I

dried sample A hydrated sample

0 100 200 300 400

Fig. 4. ln[A(T)/A(4.2 K)] versus T for dried and hydrated poly(PPS).

One can see that, for 95 K < T < 220 K the temperature dependence of In[ A( TyA(4.2 K)] is linear for the two samples with nearly the same slope, hence they have the same OD. This observation indicates that the same frequency distribution of allowed harmonic modes is involved. The 8, value can be estimated2’ from

The effective mass (illefl) giving rise to the observed phonon spectrum can be extracted from the second-order Doppler shift,” which is given, as a first approximation, by a relationship of the form

3kE, - - - -___ d I S d T 2Me,, c2

which represents the temperature dependence of the isomer shift for T >

The Me, value (75 + 8 g mol-’), which is identical for the dried and the hydrated .sample, is smaller than values expected for a macromolecule2’ but significantly larger than values corresponding to a “ bare”57 Fe atom.

This excess mass relative to the bare atom could be understood as a reflection of covalency in the bonding interaction between the iron atom and its nearest-neighbor oxygen atoms. We can suppose that the observed dynami- cal harmonic behavior of the lattice is due to motions of the same small side group as well for the dried sample than the hydrated one. This result confirms our previous conclusions that additional water molecules are not directly linked to iron atoms.

( & , P I (Fig. 2).

MOSSBAUER SPECTROSCOPY 2249

The Debye temperature was estimated as 8 , = 185 f 3 K; this value is higher than those corresponding to hydrated Nafion membrane.^^^*^^ neutral- ized with ferrous chloride, in which SO,-terminated side chains are also present. The difference in the 8, , values of these two polymers could be accounted for by the difference in the iron environment: in the hydrated Nafion membranes, iron ions were supposed to be present as Fe(H,O)g' complexes, whereas in the polypyrrole polymer, we have now evidence that iron forms complexes with the SOY-terminated side chains, even in the presence of a water excess.

The temperature To a t which the f factor falls to zero has not been evaluated for the dried sample; a t T = 340 K, the In( f ) versus T curve for this material is still linear. This result is not surprising, since we know that this polymer does not exhibit a glass transition before its thermal decomposi- tion a t T > 680 K. For the hydrated sample To is approximately 290 K; this temperature is near the melting point of ice, indicating that addition of water molecules tends to decrease the rigidity of the lattice, i.e., the intermolecular interactions.

Magnetic Hyperfine Interaction

As noted above, a t T = 4 K in zero magnetic field, the Mossbauer spectrum is a doublet, which means that the electronic spin relaxation is fast relative to the nuclear precession frequency.

Experiments with applied longitudinal magnetic fields of 4.0 and 8.0 T were carried out at T = 4.2 K. Figure 5 shows the 4.2 K spectrum of the dried sample A under 8.0 T. The central part consists of a poorly resolved hyperfine pattern, and one can notice the external lines corresponding to the magnetic pattern of Fe3+ ions already detected in previous experiments.

The hyperfine splitting of the two outermost lines corresponds to a 49.4 T field. Since the hyperfine field for Fe3+ ions is negative, the magnitude of the field measured at the nucleus decreases in an external field by exactly the right amount ( - 57.4 T + 8.0 T = 49.4 T), assuming that we are dealing with the I - 5/2 ) level of the Kramers doublets of the isolated Fe3+ ions in the slow relaxation limit.24.25 The spectrum was fitted with a four-line hyperfine pattern, because two lines out of the usual six are suppressed by the applica- tion of a parallel magnetic field. The I - 3/2 > level was detected, thanks to the expected hyperfine field (H,,, = 34.5 T), but i t was not possible to identify the pattern corresponding to the I - l/2) level. We concluded that this material contains isolated Fe3+ ions (5 f 3%).

The central part of the spectra, which corresponds to Fe2+ ions, was very difficult to simulate. We are dealing here with paramagnetic ions with aniso- tropic properties in large fields applied along the y beam direction. The theoretical Mossbauer spectra were calculated with the same formalism as for Fe2+ in fluosilicates.26 The effective field He, acting on the nucleus can be expressed as

where H,, is the hyperfine field.

2250 AURIC AND BIDAN

39.0

.~ 98.0

z 0

m m 200.0

98.0

Fig. 5. Mijssbauer spectra of poly(PPS) caried out at 4.2 K with Happ = 8 T. The lower solid line corresponds to the theoretical pattern obtained adding two Fe3' contributions shown in the lower part ( -5/2 and -3/2 levels) and two Fez+ contributions shown in the upper part (see Table 11).

Calculations of the theoretical Mossbauer spectrum corresponding to one ion site were performed by summing up 36 simulated spectra corresponding to 36 applied field directions distributed over an octant. In these conditions, i t was not possible to introduce either a P(QS) distribution with a positive and/or negative sign or an q distribution.

First we observed that only small changes near +(2-4) mm/s occur when the applied field increases from 4 to 8 T. This indicates that He, does not strongly depend on the applied field values if Happ 2 4 T. Preliminary simula- tions assuming a hypefine field independent of Happ gave poor agreement with experimental data.

Our most successful attempt was obtained assuming two iron sites with anisotropic magnetization effects. The two calculated QS values are consistent with experiments carried out without an external field and can be understood if we suppose that they correspond to the mean QS values of two P(QS) distributions with opposite sign. A broad linewidth (0.7 mm/s) was required, indicating the inequivalence of the iron sites. The main component of the hyperfine fields ( - 24.5 T and - 28.0 T) is along the V,, axis, and we can assume that i t corresponds to magnetic saturation along this axis. A sizeable hyperfine field has been estimated in the OXY plane in the EFG axes, nearly constant for one iron site but almost compensating the applied field for the other one.* No accurate determination of the asymmetry parameter q and of the quadrupole splitting signs could be done, and the reported values derived with a complex model from a many-parameter fitting of powder spectra are only indicative.

'The corresponding parameters are listed in Table 11.

MOSSBAUER SPECTROSCOPY 2251

From these results we have no evidence of any iron belonging to dimers giving diamagnetic spectra or of any iron belonging to magnetically ordered clusters that involve hundreds of ferrous ions. The iron ions associated with the polymer backbone seem to behave as isolated paramagnetic ions with anisotropic magnetic properties.

CONCLUSIONS

Mossbauer spectra of poly(PPS) indicate that nearly all the iron ions are ferrous ions located in various environments, which gives rise to a distribution of quadruple splittings. The addition of water molecules slightly alters this distribution and does not modify its temperature dependence; the isomer shifts values are not changed. These observations indicate that water mole- cules are not directly associated with iron ions and that they act as a solvate layer. From In( f ) versus T curves, we can see that the addition of water tends to decrease the rigidity of the lattice but does not change the Debye tempera- ture. These experiments suggest that iron ions are associated mainly with SO;-terminated side chains in an amorphous solid phase. From experiments with applied magnetic fields, we observed that the Fe2+ ions behave as isolated paramagnetic ions with anisotropic magnetic properties.

All these Miissbauer spectroscopic data support a structural model of hexacoordinated Fe2+ complexed with N-propylpyrrole sulfonate ions and surrounded by an outer sphere of water molecules.

This structure differs from iron-exchanged Nafion in which iron ions are involved in the octahedral Fe(H,O)i+ ~ o m p l e x . ~ . ~ ~ “he origin of this kind of inverted aflinity of iron could lie in the difference between the synthetic procedures for the preparation of the polymer and for the insertion of iron ions. On one hand, after hydrolysis of SO,-Cl groups, Nafion is produced in its final structure with clusters of water and sulfonate ions. Iron ions are t h n introduced in this somewhat fixed polymer structure. On the other hand, the polymerization of PPS by Fe(C10,), seems to involve a first step of complexa- tion Fe3+ by PPS, accounting for the large excess of Fe(C104), needed for the synthesis of poly(PPS) (see Experimental). It is likely that the “preforming” of the polymer structure around the iron ions greatly promotes the sulfonate complexation of iron ions in poly(PPS). The presence of iron in the polymer almost exclusively as Fe2+ indicates that the complexed Fe2+ produced during the redox polymerization process is not oxidized by Fe(H,O):+ ions still in solution.

We are indebted to B. Rodmacq and J. Gaillard for helpful comments and discusai~n~ and to 1. Sert for typing the manuscript.

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2252 AURIC AND BIDAN

7. P. Aldebert, P. Audebert, M. Armand, G. Bidan, and M. Pineri, J. Chem. Soc., Chem. Commun., 1636 (1986).

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paper, the 3-pyrazoline cycle in Table 1, p. 150 must be corrected by a pyrrole cycle.

20, 603 (1982).

Received September 5 , 1986 Accepted March 30, 1987