Kinetics and mechanism of pyrrole chemical

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Synthetic Metals 175 (2013) 183–191

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Kinetics and mechanism of pyrrole chemical polymerization Yang Tan, Khashayar Ghandi ∗ Department of Chemistry and Biochemistry, Mount Allison University, NB, Canada

a r t i c l e

i n f o

Article history: Received 5 April 2013 Received in revised form 7 May 2013 Accepted 11 May 2013 Available online 12 June 2013 Keywords: Kinetics Polypyrrole Conductive polymers Chemical polymerization

a b s t r a c t Proton NMR spectroscopy was used to study the kinetics of the chemical polymerization of pyrrole in water using ferric chloride as an oxidant. A kinetics model was proposed in which the consumption of pyrrole is due to a fast oxidation reaction, and a relatively faster reaction with the oxidized oligomers of pyrrole. This suggests the commonly used mechanism for polymerization of pyrrole to polypyrrole, which is an important conductive polymer, is not correct for polymerization in water. Instead the polymerization is based on a mechanism where the pyrrole monomer is attacked by a radical cation, leading directly to the pyrrole reacting with the oxidized oligomer. We conducted kinetics studies as a function of temperature and determined the activation energy, activation entropy and enthalpy for each step of the polymerization process for the first time. The activation enthalpy and activation entropy are respectively 77,091 ± 17 J and 24.25 ± 0.06 J/K for the initial step, and 70,971 ± 14 J and 24.25 ± 0.06 J/K for the propagation step. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Conjugated conducting polypyrrole (PPy) has been well known for many years [1–4]. It is one of the promising conducting polymers applied in the industry and new technology with unique properties, such as relatively high conductivity, easy synthesis and stability [2]. The kinetics study of pyrrole chemical polymerization at different temperatures contributes to developing our understanding of polymerization mechanism, optimizing the synthesis method and designing reaction processes of pyrrole oligomers with other chemical reagents. For example, if we know the mechanism and the temperature dependence of the kinetics of the polymerization, a desired chemical reagent can be added at a proper time and with the choice of proper temperature to have a reaction with the

monomer or oligomers in order to modify the polymer. The yield and rate constants which are obtained at different temperatures for this chemical polymerization would benefit both laboratory and industrial applications. The polymerization of pyrrole is proposed to involve complex reactions including oxidation, deprotonation and crosslinking [5]. Synthesis of polypyrrole by electropolymerization is an easily controlled process, so the kinetics of pyrrole electropolymerization has received more attention than chemical polymerization [5–8]. The mechanism of PPy formation by chemical method is controversial [9–11]. The most widely accepted polymerization mechanism of PPy is the coupling between radical cations [12]. In the initiation step, the oxidation of a pyrrole monomer yields a radical cation.

Coupling of the two generated radical cations then deprotonation produces a bipyrrole [13]. The bipyrrole is oxidized again and couples with another oxidized segment.

∗ Corresponding author. Tel.: +1 506 961 0802; fax: +1 506 364 2313. E-mail addresses: [email protected], [email protected] (K. Ghandi). 0379-6779/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2013.05.014

In the propagation step, re-oxidation, coupling, and deprotonation continue to form oligomers and finally PPy.

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On the other hand, another mechanism for the chain growth of polypyrrole has been proposed: (1) The radical cation reacts with a neutral monomer [11,14,15] (reaction (6)). (2) This will be followed by oxidation and deprotonation to yield a dimer.

(3) This dimer will be oxidized immediately, and form a dimeric radical cation (reaction (8)). (4) The dimeric radical cation will attack another neutral monomer leading to the formation of a trimer (reaction (9)). By this repeated process, the polymer chains grow and finally lead to the polymer. The polymerization of pyrrole can be described by the following scheme:

In general,

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Polymerization of pyrrole occurs steadily in the presence of an oxidant, such as FeCl3 [16] or ammonium persulfate [17], therefore these oxidants are usually used in chemical polymerization. Stanke et al. [18] found that the polymerization of polypyrrole is very slow in nitromethane in the presence of FeCl3 , so they studied the conversion of pyrrole under different conditions, including varying the temperature and the water/FeCl3 ratio. Bjorklund [19] reported the kinetics of chemical oxidation of pyrrole by FeCl3 in aqueous solutions of methylcellulose. He monitored the decrease of pyrrole by measuring the absorption at 800 nm, and found that the kinetics curves exhibited behavior characteristic of autocatalytic reactions. The proposed mechanism is also dependent on methylcellulose. Cavallaro et al. [20] used a reaction calorimeter to determine the enthalpy at three different temperatures for the polymerization of pyrrole in acetonitrile, by using FeCl3 as the oxidative agent. They found the reaction to be first order with respect to pyrrole. Rate constants and enthalpy of reaction were determined. High performance liquid chromatography was used to monitor the rate of monomer depletion when using ammonium persulfate as oxidant [17] and FTIR was employed to monitor the chemical conversion of PPy/Au particles [21]. Other techniques such as combined in situ Raman spectroscopy, potentiometry, and calorimetry measurements [20] were also used to study the rate of pyrrole polymerization in acetonitrile. However, the kinetics of chemical polymerization of pyrrole in water has never been studied to our knowledge. Chemical polymerization of pyrrole in water is a good method for industry because it can be scaled up and it is less harmful to the environment, comparing with methods that use organic solvents such as nitromethane and acetonitrile. Research on the rate constants, activation energy, activation entropy and enthalpy of chemical polymerization of pyrrole is also scarce [19]. In this paper we address the following questions: (1) What is the mechanism of chemical polymerization of pyrrole in water using the oxidant FeCl3 ? This is investigated using our detailed integrated rate law kinetics studies to differentiate the two proposed mechanisms. The criteria for selection of the mechanism are based on the best fit of the associated models to our experimental data. The mechanism is described in Section 3.2. (2) How does the temperature influence the polymerization? (3) What are the activation parameters, such as activation energy, activation enthalpy and activation entropy of this polymerization in water? These parameters were obtained from analysis of the rate constants at different temperatures. These results are elaborated in Section 3.3.

D2 O (99.9 at.% deuterium), acetone-d6 and deuterated dimethyl sulfoxide (DMSO-d6) were all purchased from Aldrich and were used as received. The second tubes, filled with acetone-d6 and DMSO-d6 respectively, were prepared in advance. 2.2. NMR measurements All the experiments were performed with a 270 MHz JEOL Delta NMR spectrometer. Pyrrole and FeCl3 were added to D2 O. D2 O was the solvent for both polymerization and NMR. The polymerization of pyrrole was carried out at different temperatures to study the effects of temperature on rate of polymerization. In order to measure the depletion of pyrrole quantitatively, it is necessary to use a reference compound. The required conditions for the reference compound are as follows. (1) The proton NMR signal of the compound should be separate from pyrrole aromatic proton signal and H2 O (in the D2 O solvent signal). (2) The compound should be liquid under our experiment temperatures conditions. (3) The compound should not react with any other chemicals in the system, and must not be affected by factors like pH changes during polymerization, so that it gives a constant proton signal. (4) The reference chemical should not affect the polymerization. To meet the above requirements, an in-house made set-up was employed with a smaller tube inside the NMR tube (Fig. 1). This apparatus isolated the reference compound from the reaction physically, thus preventing any interference effects on the polymerization. DMSO-d6 was chosen to be the reference solvent when the temperature was higher than 293 K because the melting point of DMSO is 292 K. When the experiments were conducted at temperatures smaller than 293 K, acetone-d6 was used as the reference

2. Experimental 2.1. Materials The pyrrole was obtained from Aldrich, and stored in the fridge after distillation under vacuum. Anhydrous ferric chloride (FeCl3 ),

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Fig. 1. NMR apparatus for kinetics study of pyrrole polymerization.

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Fig. 2. The conversion of pyrrole during polymerization at 303 K. The polymerization started very quickly, indicating that there is no induction period at high temperatures.

solvent. The sealed second tube with DMSO and acetone were prepared prior to use. We also confirmed that the external reference solvents do not affect the kinetics of polymerization by running the sample in two reference solvents at the same temperature. As the polymerization process is fast, the concentration of pyrrole and anhydrous ferric chloride were very low. In this research, 0.0002 g FeCl3 was added to 100 ␮L D2 O to make a solution, and then 0.5 ␮L pyrrole was added to the solution. The polymerization mixture was quickly placed into the NMR probe. The concentration of remaining pyrrole in the solution can be obtained by the NMR integral of aromatic pyrrole protons. We measured ratio of concentration of remained pyrrole to the initial concentration of pyrrole as a function of time, to investigate the polymerization rate. 3. Results and discussion 3.1. Monomer depletion To make sure the decrease in the NMR peak integrals are not due to the aromatic pyrrole protons exchanging with D2 O, which leads to an artificial reduction in the NMR peak integral of the pyrrole monomers, we conducted an exchange test (by NMR) and a pH test during polymerization. In our research, the experiments without oxidants did show a very small rate of exchange between pyrrole protons and D in D2 O which our experimental data were corrected for this effect. The pH remains fairly steady throughout the polymerization for the concentrations used in our studies. As polypyrrole is insoluble, pyrrole proton NMR peaks are the only important peaks to be detected. Fortunately, it was observed that PPy precipitation did not affect the measurements of the other peaks. The polymerization started so quickly that there was almost no induction period. This is clear at higher temperatures (Fig. 2). 3.2. Kinetics analysis The radical coupling mechanism is the most popular mechanism of polymerization of pyrrole. Several studies reported the chemical polymerization of pyrrole as first order with respect to the pyrrole. Planche et al. [22] made a kinetics study of pyrrole disappearance by high-performance liquid chromatography with various FeCl3 concentrations. They found the polymerization to be first order with respect to the pyrrole. Bjorklund [19] reported the kinetics of the chemical oxidation of pyrrole by iron (III) chloride in aqueous solutions of methylcellulose, and found the polymerization to be first order with respect to pyrrole and FeCl3 . In our work, we also tried to fit the data to a first order rate law, but it cannot fit properly. Fig. 3 shows the experimental conversion of pyrrole and the fitted first order plot at 288 K, which shows that the polymerization is not a first order reaction with respect to pyrrole.

Fig. 3. The experimental conversion of pyrrole at 288 K, compared with the fitted single exponential plot based on a first order reaction (red curve), indicating that the chemical polymerization of pyrrole with oxidant FeCl3 is not a first order reaction. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)

We then tried to fit the experimental conversion with another model, and the theoretical conversion fits the experimental conversion very well (Fig. 4). In this model, we assumed the consumption of pyrrole would be via a fast homogeneous reaction with oxidant and a relatively faster heterogeneous reaction with oligomers. In the initiation reaction pyrrole is oxidized by an oxidant (Eq. (6)). Then the pyrrole radical cation is polymerized by adding a neutral pyrrole or radical–radical coupling based on the two different mechanisms described in Section 1 (Eq. (7)). The dimer will be oxidized immediately and the above process will be repeated (Eqs. (8) and (9)). This is a propagation reaction or chain growth. In electrochemical polymerization, it is found that oxidation of a oligomer or polymer with a conjugated chain is easier than that of a monomer, because the oxidation potential of polypyrrole is ∼0.2 V [23] while the oxidation potential of the pyrrole monomer is ∼0.7–0.8 V [24]. The longer the length of the polypyrrole chain is the lower its oxidation potential and the easier to oxidize [24]. Hence, after generating the oligomer, the oligomer will be oxidized preferentially (Eqs. (10) and (11)). Because the oxidation process of pyrrole oligomers (Eqs. (8) and (10)) is very fast, this reaction is assumed to have negligible effects on the rate of the reaction with respect to reactants. Therefore this step is not considered as rate determining step in the propagation process and it is not involved in the rate equation. We assumed that the oxidized oligomer reacted with the pyrrole monomer (Eq. (9)). The concentration of the oxidized oligomer can be considered to be equivalent to concentration of the oligomer. Therefore the following rate equation is proposed: −

d[Py] = k1 [Fe3+ ][Py] + k2 [Py][P] dt

(12)

where [Py], [Fe3+ ] and [P] are the concentrations of pyrrole, FeCl3 , and the growing oligomer, respectively. Both k1 and k2 are rate

Fig. 4. The experimental conversion of pyrrole at 288 K, compared with the theoretical conversion obtained from our proposed kinetics model. The theoretical conversion curve fits the experimental data very well.

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constants, in which k1 is the rate constant of the initiation step (ki1 in Eq. (6)) while k2 is assumed to be the rate constant of chain growth (kp1 , kp2 ,. . .kpm in Eqs. (7), (9) and (11)). With this mechanism the majority of pyrrole monomer would be oxidized by FeCl3 during the early stage of polymerization and the oligomers would grow fast to insoluble oligomers with high molecular weight. This is consistent with the experimental data in Fig. 4 that the concentration of pyrrole decreases dramatically very early on. Along with the depletion of oxidant, the conversion becomes slower than the early stage. It is reported that the reaction stoichiometry is 2.25 mol Fe(III) per mole of pyrrole polymerized [19]. The concentration of FeCl3 at any time can be defined as: [Fe3+ ] = [Fe3+ ]0 − 2.25([Py]0 − [Py])

(13)

[Fe3+ ]

Substituting for in Eq. (12), two variables defined as ˇ1 and ˇ2 were introduced: ˇ1 = 2.25 k1 − k2

ˇ2 = k1 [Fe3+ ]0 − ˇ1 [Py]0

(14)

Fig. 5. A comparison of the experimental conversion rate and the fitted conversion rates.

The integrated rate equation is given by: [Py] ˇ2 = [Py]0 (ˇ1 [Py]0 (exp(ˇ2 t) − 1 + ˇ2 exp(ˇ2 t)))

(15)

We tried to fit the experimental conversion curves into the rate equation with two adjustable parameters ˇ1 and ˇ2 . Then the rate constants were calculated using Eq. (14) from the best-fit parameters ˇ1 and ˇ2 . Fig. 5 compares the conversion obtained by NMR spectroscopy and the conversion calculated from fitted Eq. (4) to the experimental data at different temperatures. It is clear that the kinetics model fits well to the experimental data at all temperatures. Although the mechanism of polymerization (reactions (6)–(11)) has been proposed before, it is the first time that this kinetics model has been applied to fit to the experimental kinetics data for the chemical polymerization of pyrrole [11,25,26]. The rate constants at temperatures ranging from 278 K to 303 K are listed in Table 1. From the summary of rate constants obtained at different temperatures, it can be concluded that temperature is an important factor that influences the rate of polymerization dramatically. Both rate constants k1 and k2 increase significantly

with temperature from 278 K to 303 K, whereas the influence of temperature on the rate constant of the initiation reaction is greater than that of the propagation reaction. In the electrochemical polymerization of pyrrole, the formation of the pyrrole radical cation seems to be a rate determining step. Thus, the mechanism of pyrrole polymerization by chemical oxidation may be similar to that by electrochemical polymerization [27]. We have proposed a kinetics model for the oxidative chemical polymerization of pyrrole based on our experimental results. Our work helps to improve the understanding of the mechanism of chemical polymerization in water. Based on the results, the interaction occurs between a radical cation and monomer or oligomer chain. The polymerization process would be (or include) a → b → d → e → f → g (Fig. 6). This result is different with the widely used mechanism, which holds that the polymerization is via the coupling between radical cations. However the work of [16] is consistent with our results. Another interesting point is that at low temperatures, the rate constant k1 decreases more than the rate constant k2 . As it is

Fig. 6. The schemes for the two main possible different mechanisms of polymerization of pyrrole.

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Table 1 Summary of rate constants for pyrrole polymerization. Temperature (K)

ˇ1

278 283 288 293 298 303

−0.165 −0.133a −0.396a −0.237 ± 0.001 −0.089a 0.237a

a

k1 (M−1 min−1 )

ˇ2 a

a

0.016 0.015a 0.043a 0.042a 0.050a 0.063a

a

0.355 0.421 ± 0.001 1.153 ± 0.004 2.051 ± 0.009 3.525a 6.561 ± 0.032

k2 (M−1 min−1 ) 0.965a 1.081 ± 0.002 2.989 ± 0.010 4.852 ± 0.019 8.020a 14.526 ± 0.071

These numbers have an uncertainty smaller than 0.001.

mentioned above, the process of radical coupling takes an extremely short amount of time, so this process does not affect the rate law. The initiation step is the rate determining step at the lower temperature.

The Eyring–Polanyi equation has been used to calculate the standard enthalpy of activation and the standard entropy of activation: ln

3.3. Activation energy, entropy and enthalpy To the best of our knowledge, this is the first study that investigates the activation parameters (e.g. activation entropy and enthalpy) for the chemical oxidation of pyrrole in water. In the Arrhenius equation, k = Ae−Ea /RT A is the pre-exponential factor for the reaction, R is the universal gas constant 8.3145 J/mol K, T is the temperature (in Kelvin), and k is the reaction rate coefficient. The units of the pre-exponential factor A are identical to those of the rate constant, and can change depending on the order of the reaction. For the initiation step of polymerization, the ln (k) versus 1/T is shown in Fig. 7. The slope can be used to determine the activation energy. Based on our data, the activation energy of the initiation step is 79,485 ± 17 J/mol. For the propagation step of the polymerization, the activation energy was calculated in the same way as for the initiation step and was 73,365 ± 14 J/mol. The Arrhenius plot is depicted in Fig. 8.

Fig. 7. Arrhenius plot for the initiation step of the pyrrole polymerization.

Fig. 8. Arrhenius plot for the propagation step of pyrrole polymerization.

S ‡ kB −H ‡ 1 k + = · + ln T R T R h

where H‡ is enthalpy of activation, kB is the Boltzmann constant, h is the Planck’s constant and S‡ is entropy of activation. The plot of ln (k/T) versus 1/T gives a straight line, with the slope determining the enthalpy of activation and the intercept the entropy of activation. For the initiation of the polymerization, we found that the enthalpy of activation H‡ was 77,091 ± 17 J and the entropy of activation S‡ was 24.25 ± 0.06 J/K. The plot is shown in Fig. 9. For the propagation of polymerization, the activation enthalpy H‡ was 70,971 ± 14 J and the activation entropy S‡ was 10.55 ± 0.05 J/K. The Eyring–Polanyi plot is presented in Fig. 10. The activation energy and enthalpy of the propagation step is slightly smaller than the activation energy and enthalpy of initiation step. The two reactions are of different nature so probably there are several effects at play. It should be pointed that the propagation step as the oligomer grows becomes a heterogeneous reaction, and the transition state could be stabilized by potential catalytic behavior of this particulate, thus the activation enthalpy and energy in the propagation step would be smaller than the initiation step. At the same time the transition state for the first step (electron transfer reaction) could be more stabilized by solvent effects.

Fig. 9. The plot of ln (k/T) versus 1/T for the initiation step of polymerization.

Fig. 10. The plot of ln (k/T) versus 1/T for the propagation step of polymerization.

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Fig. 11. The hydrogen-numbered in pyrrole, and the possible structures of the pyrrole dimer.

The activation entropy reflects an indication of the nature of the transition state [28]. A positive value of the activation entropy means that the entropy of the transition state is greater than the entropy of the reactants. Since we obtained positive activation entropies, the transition state is more disordered than the reactants. In the first step, the intermediate complex pyrrole–FeCl3 possesses a donor/acceptor complex between the ␲-pyrrole system and FeCl3 . Moving forward along the reaction coordinate, the intermolecular hydrogen bonds and coordination structure of D2 O become relaxed. The solvent has more degrees of freedom of translation, rotation or vibration, which increase the activation entropy. We also noticed the activation entropy in the first step is much larger than that in the second step. It is probably because of the coordination bonding between FeCl3 or FeCl2 and D2 O in the initiation step, which increases the solvent effect. By understanding the solvent effects on the activation parameters, additives could be added to change the polymerization rate. The chemical agents such as HCl, NaF and NH4 F that have strong hydrogen bonds with water and monomers would increase the activation entropy of the polymerization, leading to a faster polymerization. While for those chemical agents which can reduce the hydrogen bonds such as potassium iodide in the solution would help to slow down the polymerization. Addition of chemical agents to change the kinetics of polymerization would be useful for the applications in optimization of polymerization process.

3.4. The crosslinking of PPy Like many other polymers, PPy has a high degree of cross-linking [29–31]. During the kinetics study of pyrrole polymerization, it was noted that the proton NMR peak of pyrrole at the 3 and 4 positions (see Fig. 11) also decreased as a function of time.

Fig. 12. A comparison of the experimental and theoretical conversion of crosslinking of PPy at 288 K.

Fig. 12 shows a representative plot that compares the experimental and theoretical conversions of the crosslinking of PPy at 288 K. To make sure this is not due to the aromatic pyrrole protons exchanging with D2 O, which leads to an artificial reduction in the NMR peak integral of the pyrrole monomers, we conducted an exchange test (by NMR) and a pH test during polymerization. In our research, the experiments without oxidants did only show a small rate of exchange between pyrrole protons and D2 O which had a small effect on the rate of reactions. The pH remains fairly steady throughout the polymerization. We therefore propose that during polymerization, the PPy polymers formed a high degree of crosslinking. Thus while pyrrole was oxidized and polymerized at the 2 and 5 positions as a linear chain propagation, the protons at the 3 and 4 positions of pyrrole were oxidized and cross-linked. 4. Conclusions This is the first kinetics study of the polymerization of pyrrole by NMR, including the studies of the temperature influence, activation parameters, and cross-linking. We have used 1 H NMR spectroscopy to study the chemical polymerization of pyrrole. The 1 H NMR spectroscopy exhibits the consumption of pyrrole clearly. We monitored the polymerization process with NMR, and studied the conversion of pyrrole as a function of time. The proposed theoretical conversion fits the experimental conversion well, indicating that the pyrrole did polymerize based on a mechanism where the pyrrole monomer is attacked by a radical cation, leading directly to the pyrrole reacting with the oxidized oligomer. Knowing the right reaction mechanism allows further improvements of the reaction conditions, such as proper temperature selection, or selection of possible catalysts, which can assist the potential industrial production of the polymer. The experiments were carried out at different temperatures. The activation energy, activation enthalpy, and entropy were obtained in our research. As far as we know, this is the first research that measures the activation enthalpy and activation entropy for this chemical reaction in water. It is found that the activation energy of the propagation step is a little bit smaller (about 6 kJ/mol) than the activation energy of initiation step. This is probably because the propagation step is a heterogeneous reaction, and the produced oligomer is potentially in the form of particulate dispersed in the solution. The positive values of our entropy of activation suggest a significant role of the water on the chemical polymerization. In particular the structure of water is reduced as the polymer is being formed and this causes an increase in activation entropy. This suggests a likely structure breaking for hydrogen bonding solvents as the polymerization progress. Such positive activation

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Fig. A.1. Typical proton NMR spectrums of the products of the pyrrole polymerization in two reference solvents. Spectra a is a proton NMR spectrum of the products of the pyrrole polymerization with DMSO-d6 in the second tube, and Spectra b is a proton NMR spectrum of the products of the polymerization with acetone-d6 in the second tube.

entropies suggest faster polymerization reactions in the solvents with stronger hydrogen bonding or in the water in the presence of structure making solutes. The cross-linking reaction was observed, and the degree of crosslinking is increased with temperature based on our preliminary data. Hence the temperature is a predominant parameter in the polymerization. The choice of temperature should be dependent on desired products. If PPy with high ordered structure is desired, the degree of crosslinking should be low so the low temperature is preferred. The proposed mechanism and solvent effects in this system are expected to be applied for the similar polymers like polythiophene and polyaniline.

Acknowledgments This research was financially supported by the Natural Sciences and Engineering Research Council of Canada, the New Brunswick Innovation Foundation, and ChemGreen Innovation. We thank Dan Durant from Mount Allison University for his kind help in the preparation and use of the NMR instrument.

Appendix A. Fig. A.1 shows a representative proton NMR spectrum of the products of the polymerization. The peaks at ı 5.8–6.8 are related to the aromatic pyrrole protons. The H2 O in D2 O solvent exhibits a signal at ı 4.6. The peak at ∼ı 3.4 is attributed to the reference solvent of DMSO-d6. Another reference solvent acetone-d6 gives multiple peaks at ∼ı 3.2. Integration of the area under these peaks gives the concentration of the chemicals in each product mixture.

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