Thermal Behavior, Stability, and Decomposition Mechanism of Poly(N

Article pubs.acs.org/Macromolecules

Thermal Behavior, Stability, and Decomposition Mechanism of Poly(N‑vinylimidazole) Csaba Fodor,† János Bozi,‡ Marianne Blazsó,‡ and Béla Iván*,† †

Department of Polymer Chemistry, Institute of Organic Chemistry, and ‡Department of Environmental Chemistry, Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, H-1525 Budapest, Pusztaszeri út 59-67, P.O. Box 17, Hungary S Supporting Information *

ABSTRACT: Homo- and copolymers of N-vinylimidazole belong to a rapidly emerging class of polymeric materials. Because of the fact that these materials can be utilized in several high-temperature processes and applications, such as catalysis, fuel cells, polymeric ionic liquids (PIL), precursors for new materials by thermolysis, etc., and because fundamental details on the thermal behavior of such polymers are lacking, systematic investigations have been carried out to reveal the stability and the mechanism of thermal decomposition of poly(N-vinylimidazole) (PVIm) by using a variety of techniques, such as differential scanning calorimetry (DSC), thermogravimetry (TG), thermogravimetry−mass spectrometry (TG-MS), and pyrolysis−gas chromatography/mass spectrometry (Py-GC/MS). The investigated PVIm was obtained by free radical polymerization initiated by AIBN in benzene at 70 °C. By the unique combination of the applied methods to investigate the thermal decomposition mechanism of PVIm, it was found that the thermal decomposition of PVIm takes place in one main step in the temperature range 340−500 °C. An initial mass loss of 4% occurs before the main endothermic decomposition step due to the evaporation of water and acetone physically bound to the polymer during purification. The major products of the thermal decomposition of PVIm are 1H-imidazole and 1-vinylimidazole accompanied by several minor products, such as benzene and several alkyl aromatics. The relative ratios between imidazoles and aromatics, i.e., the 2 orders of higher amounts of imidazoles, indicate that in contrast to other polymers with heteroatom pendant groups, e.g., poly(vinyl chloride) (PVC), poly(vinyl acetate), (PVAc) and poly(vinyl alcohol) (PVA), not zip-elimination of 1H-imidazole but homolytic scission of the carbon−nitrogen bond is the main reaction of its formation. 1-Vinylimidazole is formed by main chain scission followed by depolymerization. Both 1H-imidazole and 1-vinylimidazole formation lead in part to macroradicals and short conjugated double bond sequences (polyenes) in the chain, the thermolytic cyclization, isomerization, and aromatization of which result in the low amounts of aromatics. These findings served for the basis of formulating the mechanism of the thermal decomposition of PVIm, which can be utilized in the course of further investigations with this unique polymer.

1. INTRODUCTION Imidazole is one of the most important heterocyclic aromatic compounds. The imidazole ring plays a crucial role in primary biomacromolecules, such as amino acids (histidine), nucleic acids (e.g., purine ring of adenine and guanine), proteins, such as hemoglobin, metalloproteins, etc., hormones (e.g., histamine), and certain vitamins. The imidazole moiety is also of paramount importance in many pharmaceutical and agrochemical compounds. Synthetic macromolecules with the imidazole group, especially poly(N-vinylimidazole) (PVIm), and its derivatives belong to rapidly rising sector of polymers.1 Imidazole-containing polymers are components of novel amphiphilic conetworks and gels,1o−q useful materials in fuel cells,1j,2 membranes for metal ion complexing and removal,3 catalysts,4 and catalyst supports at higher temperatures,5 anticorrosion coatings,6 gene delivery vectors,1g,7 in wine production,8 and biomaterials with various pharmacological activities.9 In recent years, there has also been an increasing interest in these materials as proton conducting membranes in © 2012 American Chemical Society

proton exchange membrane fuel cells (PEMFC) in order to substitute water by N-heterocycles as strong proton acceptors at higher temperatures.2c,10 On the other hand, it should also be noted that imidazole-based salts as ionic liquids (IL) have attracted significant interest as “green” reaction media and/or to improve the efficiency of various processes due to their favorable properties (e.g., thermal and chemical stability, high ionic conductivity).11 Recently, imidazole-containing polymeric ionic liquids (polyionic liquids, PIL) have also become very attractive materials for a variety of applications,2d,11b,12 including even as precursors for novel, specialty carbon materials12d obtained by thermolysis of such PILs. One of the most critical limiting factors of the application of polymeric materials is their limited thermal stability. However, in spite of the growing interest in PVIm-based materials and Received: August 13, 2012 Revised: October 25, 2012 Published: November 6, 2012 8953

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Fourier transform infrared spectra (FTIR) of the PVIm homopolymer with background correction were obtained in KBr pastille in the 4000−400 cm−1 range, using 128 scans at a nominal resolution of 4 cm−1 by a Nicolet Avatar spectrophotometer. The PVIm homopolymer sample was annealed under a nitrogen atmosphere at 200 °C in a vacuum oven for 2 h before thermal analysis. The temperature was increased stepwise, and after the heat treatment, the sample was allowed to slowly cool down. The calorimetric measurement of the annealed PVIm homopolymer was made with a Mettler TG50 instrument differential scanning calorimeter (DSC) in a dry nitrogen atmosphere. The polymer was first heated to 200 °C from −120 °C at a rate of 10 °C/min (first heating scan). Subsequently, the sample was cooled at a rate of −10 °C/min (cooling scan). Following the cooling scan, the PVIm homopolymer was second heated to 200 °C from −120 °C at the same heating rate as in the first scan. The inflection point of the specific heat increase in the transition region during the second heating is reported as the glass transition temperature (Tg). The thermogravimetry−mass spectrometry (TG-MS) instrument was built from a PerkinElmer TGS-2 thermobalance and a Hiden HAL 3F/PIC mass spectrometer and controlled by a computer. For the examination of thermal stability and decomposition of PVIm, 0.5 mg of polymer was placed into a platinum sample pan. The sample was heated at 20 °C min−1 up to 800 °C in a flowing argon atmosphere. The flowing rate of argon was 140 mL min−1. A portion of the volatile products was introduced into the ion source of the mass spectrometer through a glass-lined metal capillary held at 300 °C. The quadrupole mass spectrometer was operated at 70 eV electron energy. The ion intensities were normalized to the sample mass and to the intensity of the 38Ar isotope of the carrier gas. Pyrolysis−gas chromatography/mass spectrometry (Py-GC/MS) measurements were carried out in a Pyroprobe 2000 (Chemical Data System) pyrolyser equipped with a platinum coil and quartz sample tube, coupled to Agilent 6890 GC-5973 MSD (Agilent Technologies) instrument. The platinum coil was heated up by 20 °C ms−1 heating rate. Helium carrier gas at a flow rate of 20 mL min−1 purged the pyrolysis chamber held at 280 °C. Successive pyrolysis experiments were carried out for 20 s from 300 to 600 °C with 50 °C temperature steps. The mass of the sample was about 1 mg. Flash pyrolysis method was applied at 500 °C. Approximately 0.2 mg polymer sample was pyrolyzed for 20 s. The GC separation was performed on a HP-5MS capillary column (30 m × 0.25 mm × 0.25 mm) (Agilent Technologies); the temperature after 1 min isotherm period at 50 °C was programmed to 300 °C at 10 °C min−1 heating rate and held at 300 °C for 4 min. The temperature of the transfer line of GC/MS and the source of the mass spectrometer were 280 and 230 °C, respectively. The mass spectrometer was operating in electron-impact mode (EI) at 70 eV in the scan range of m/z 14−500 u. The GC/MS identification of the pyrolysis products was carried out by using mass spectral library and mass spectrometric identification principles.

their high-temperature applications, only sporadic reports1d,h,10,11b can be found on its thermal degradation, and according to the best of our knowledge, systematic detailed investigations on the thermal decomposition of poly(vinyl imidazole)s have not been reported yet. Most of the publications dealing with the thermal stability of imidazole containing polymers describe cross-linked11e,13 and modified PVIm14 or copolymers of VIm.15 Some of these studies report on certain thermal data of PVIm homopolymer as well, such as the glass transition temperature (Tg = 175 °C)14a,15d and thermogravimetry (TG) curves indicating a low amount of mass loss between 100 and 200 °C,14b,15a,b,d a few percent of further loss up to 350 °C,15a,b,d and main decomposition of the polymer between 350 and 500 °C.14b,15a,b,d However, the thermal decomposition mechanism of PVIm has not been revealed at all yet. Typically, most of the vinyl polymers decompose by free radical decomposition to monomer and some dimer and trimer as described in details for polystyrene (PS) by Kruse et al.16 Thermal scission or elimination of the side groups from the main chain of vinyl polymers takes place only when the chemical bond of the substituent to the chain is weaker than the C−C bond in the main chain. For instance, dehydrochlorination by a chain reaction, the so-called zipelimination, is initiated in poly(vinyl chloride) (PVC)17 at around 300 °C. At the same time, the saturated hydrocarbon chain of PVC is transformed to conjugated polyenes which decompose to aromatic, alkylaromatic, and polyaromatic compounds between 400 and 500 °C.17 Herein, we report on our findings obtained in the course of a series of detailed and systematic investigations on the thermal stability and degradation process of PVIm. In order to reveal the thermal stability and decomposition mechanism of this pendant N-heterocycle-containing polymer, calorimetric and thermogravimetric studies were applied in conjunction with thermogravimetry−mass spectrometry (TG-MS) and pyrolysis−gas chromatography/mass spectrometry (Py-GC/MS) at different temperatures to analyze and identify the pyrolyzates.

2. EXPERIMENTAL SECTION 2.1. Materials. N-Vinylimidazole (VIm, Aldrich) was vacuumdistilled from CaH2 (95%, Aldrich) at 72 °C and kept under nitrogen until use. 2,2′-Azobis(2-methylpropionitrile) (AIBN, Aldrich) was recrystallized from methanol before use. Freshly distilled benzene (Spektrum 3D) was used as solvent for the homopolymerization. 2.2. Preparation of PVIm Homopolymer. The PVIm homopolymer was synthesized by radical polymerization of Nvinylimidazole (VIm) in benzene with AIBN as initiator. The desired amount of monomer (1.92 mL, 21.2 mmol) was dissolved in benzene, and then the initiator stock solution (18.5 mg, 0.11 mmol) was added to the reaction mixture. Oxygen was removed by a freeze−thaw process. The reaction mixture in a glass reactor tube was kept in an oil bath under nitrogen with constant stirring at 70 °C for a period of 48 h. Then the polymer was dissolved in methanol (30 mL) and precipitated in acetone. The precipitated polymer was filtered and dried first in air and then in vacuum oven at 60 °C. The yield was 67%. The polymer was analyzed by 1H NMR and Fourier transform infrared spectroscopy. 2.3. Methods. 1H NMR spectroscopy was used to obtain the chemical composition and the purity of the monomer and polymer used in this research. 1H NMR spectra were obtained on a Mercury Plus Varian VRX-200 spectrometer operating at 1H frequency of 200 MHz. Samples were dissolved in appropriate deuterated solvents (deuterated chloroform and deuterium oxide). In the case of deuterated chloroform tetramethylsilane (TMS) at 0 ppm was used as internal reference for 1H NMR.

3. RESULTS AND DISCUSSION Revealing the thermal properties, especially the stability and decomposition processes of polymers, is of paramount importance for a wide range of applications and also for understanding and improving the properties of macromolecular materials. Imidazole-containing polymers, especially poly(Nvinylimidazole) (PVIm) and derivatives, are widely used at higher temperatures (fuel cells, catalysts and catalyst supports, polyionic liquids, etc.) in processes in which the thermal stability and the decomposition products are critical from several points of view. In order to study the thermal behavior, stability, and decomposition of PVIm, radical polymerization of VIm was performed to prepare pure PVIm homopolymer. After free radical homopolymerization and purification by precipitation a white powder was obtained with 67% yield. This polymer was characterized by 1H NMR and FTIR spectroscopy. Figure 1 shows the 1H NMR spectrum of PVIm in 8954

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Figure 1. 1H NMR spectrum of the PVIm homopolymer, recorded in deuterium oxide at room temperature (acetone is denoted with an asterisk).

Figure 4. Selected ion peak areas of 1H-imidazole (m/z 68) and 1vinylimidazole (m/z 94) (A) and benzene (m/z 78) and alkylbenzene compounds (m/z 91) (B) at 300, 350, 400, 450, and 500 °C pyrolysis temperatures.

corresponds to the inclusion of acetone, the precipitating solvent, in the polymer. The FTIR spectrum of the PVIm polymer (Figure S1 in the Supporting Information) exhibits the following characteristic bands: 3110 cm−1 (C−H ring stretching mode), 2950 cm−1 (CH and CH2 the main chain stretching modes), 1660 cm−1 (CC ring stretching mode), between 1500 and 1515 cm−1 (C−C, CN ring stretching modes), 1456 cm−1 (C−H main chain in-plane bending mode), between 1080 and 1284 cm−1 (C−H ring in-plane bending and C−N ring stretching modes), 914 cm−1 (ring stretching and in-plane bending modes), between 740 and 821 cm−1 (C−H and ring out-of-plane bending modes), 359 cm−1 (C−N ring stretching mode), and 633 cm−1 (ring out-of-plane bending mode). The infrared spectrum is dominated by a broad region between 3700 and 3200 cm−1, which is assigned to the stretching vibration mode of physically bound water, indicating polymeric association through intermolecular hydrogen bonding. The FTIR spectrum of this PVIm is in good accordance with that of literature spectra,18 indicating the purity of the PVIm sample prepared by us. The DSC thermograms for the PVIm homopolymer are displayed in Figure 2. The first heating scan was used to remove the thermal history and to detect the loss of solvents, namely water and acetone, from the homopolymer sample. An endothermic peak appearing at higher temperatures can be identified with the loss of physically bound acetone and water closed in inclusions.19 The reheating (second heating) scan is clearly different from the first heating cycle. The thermal history and the physically bound solvents are not present, which allows to detect the glass transition temperature (Tg) of the polymer.

Figure 2. DSC thermograms (first heating−cooling−second heating) of PVIm homopolymer (glass transition temperature: Tg(PVIm) = 171 °C).

Figure 3. Thermogravimetric (TG), differential thermogravimetric (DTG), and TG-MS molecular ion curves of PVIm homopolymer. Mass loss and DTG curves of PVIm: full line; TG-MS molecular ion curve of 1H-imidazole (m/z 68): red dotted line; molecular ion of benzene (m/z 78): blue long dashed line; molecular ion of 1vinylimidazole (m/z 94): green short dashed line.

deuterated water. In this spectrum, the signals corresponding to the backbone protons appear between 1.8 and 3.7 ppm, while the signals around 6.4 and 7.2 ppm belong to the three protons in the imidazole ring.5c,11d,e,12a,13,14a The signal at 2.2 ppm 8955

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Scheme 1. Main Thermal Decomposition Reactions of PVIma

a

The compound identification numbers are as in Table 1.

Figure 5. Py-GC/MS total ion chromatogram of PVIm homopolymer at 500 °C; the peak identifications are given in Table 1.

(TG) and the differential thermogravimetric (DTG) curves of the polymer, and the MS molecular ion profiles of 1Himidazole (m/z 68), benzene (m/z 78), and 1-vinylimidazole (m/z 94). The thermogravimetric analysis of the synthesized PVIm resulted in observations similar to published results.13,14a,b,15b−d,18b,19 According to the data in Figure 3, the thermal decomposition of PVIm takes place in one major step in the temperature range of 340−500 °C. However, the temperature domain of the most intensive decomposition is between 400 and 500 °C. The maximum of the decomposition temperature according to the DTG curve is at 455 °C. By 500 °C, the thermal decomposition of PVIm can be considered as complete. A mass loss of about 4% can also be detected up to 340 °C, and further 1−2% mass loss occurs between 500 and 800 °C. In order to determine the composition of the volatile products of the decomposition process, detailed analyses were carried out by us with coupled techniques. The TG-MS experiments revealed that the mass loss of 3% up to 200 °C is due to the physically bound (adsorbed) water in the PVIm sample (Figures S2 and S3). In the temperature range of 200− 340 °C, further 1% mass loss can be detected which is caused by the liberation of included acetone. These results, in

Table 1. Identified Thermal Decomposition Products of PVIm Homopolymer at 500 °Ca GC peak no.b 1 2 3 4 5 6 7 8 9 10

compound name benzene toluene 1-vinylimidazole (monomer) 1H-imidazole α-methylvinylimidazolec naphthalene 2-methyl-1H-benzimidazolec biphenyl 1-phenylimidazole 2,6-di(imidazol-1-yl)-hex-1enec

M (u)

retention time (min)

78 92 94 68 108 128 132 154 144 216

3.10 3.80 5.91 6.87 7.52 9.54 11.40 12.25 12.92 19.06

a The structures of these compounds are shown in Scheme 2. bPeak numbers refer to the indications in Figure 5. cTentatively identified.

The observed Tg of the investigated PVIm is in good agreement with reported Tg values (Tg(PVIm) = 171 °C).13,14a,15d,18a,b,19 The thermal stability and decomposition of PVIm were first investigated by TG-MS. Figure 3 shows the thermogravimetric 8956

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Scheme 2. Structures of the Identified Thermal Decomposition Products of PVIm Homopolymer at 500 °Ca

a

The compound identification numbers are as in Table 1.

Scheme 3. Thermal Decomposition Reactions of PVIm Leading to Minor Productsa

a

The compound identification numbers are as in Scheme 2 and Table 1.

accordance with the NMR and FTIR spectra in Figure 1 and Figure S1, respectively, clearly prove the presence of physically bound water and acetone in the PVIm homopolymer as inclusions even after drying. The TG-MS ion profile at m/z 68 in Figure 3 indicates that formation of 1H-imidazole starts at 400 °C. It has a sharp peak between 400 and 500 °C, while the tailing ends at 650 °C. Benzene formation also starts at 400 °C. However, the maximum and the peak end of the ion curve at m/z 78 coincides with that of the DTG curve at 452 and 500 °C, respectively. The ion curve of 1-vinylimidazole (m/z 94) shows that the formation of 1-vinylimidazole starts at about 30 °C higher temperature than that of 1H-imidazole and benzene, and its rate is the highest at 457 °C. The ion curve of 1vinylimidazole has also a tailing that ends at 550 °C.

The ion curve profiles lead us to the assumption that 1Himidazole, benzene, and 1-vinylimidazole formation are due to different decomposition processes. For the profound understanding of the thermal decomposition process, successive PyGC/MS experiments were carried out in the temperature range 300−600 °C. The relative amounts of the most characteristic thermal decomposition products of PVIm formed under successive pyrolysis experiments are shown in Figure 4A,B. The area values of the corresponding peaks are measured from extracted ion chromatograms at m/z 68 and 94 for 1Himidazole and 1-vinylimidazole (Figure 4A) and moreover at m/z 78 and 91 for benzene and alkylbenzene compounds (Figure 4B), respectively. It should be noted that the intensity scale in Figure 4A is 2 orders of magnitude larger than in Figure 4B, indicating the prominence of 1H-imidazole and 18957

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higher temperature causing tailing of the m/z 68 ion curve in Figure 3. Correspondingly, we may assume that the crosslinked structure hinders benzene and 1-vinylimidazole formation. Side group cleavage and free radical depolymerization are nearly simultaneous during thermal decomposition of PVIm. However, the formation of 1H-imidazole starts at about 30 °C lower temperature than that of 1-vinylimidazole according to the TG-MS measurements. For clarifying the considerable difference of PVIm thermal decomposition from that of other major vinyl polymers with pendant carbon−heteroatom side groups, such as poly(vinyl chloride) (PVC), poly(vinyl acetate) (PVAc), and poly(vinyl alcohol) (PVA), we have to take into consideration that these latter polymers have similar TG curves with two distinct thermal decomposition steps. Significant side group elimination and simultaneous formation of conjugated polyenes is the first step for PVC,17,20,21a PVAc,21a,b and PVA21a,c in the 250−370, 320−380, and 250−330 °C ranges, respectively. During the second step, mainly between 400 and 530 °C, decomposition of polyenes to significant amounts of aromatic compounds occurs in the case of these polymers.17b−e,21 According to Starnes,17a thermal HCl elimination from PVC producing conjugated polyene sequences in the main chain likely occurs by ion-pair formation or by a quasiionic process. In PVAc, similar ionic process leads to acetic acid and polyene sequences at slightly higher temperatures. In contrast, side group cleavage takes place at a considerably higher temperature from PVIm than that from PVC and PVAc, on the one hand. On the other hand, this decomposition temperature for PVIm overlaps with the range of free radical depolymerization leading to a one-step weight loss according to our results shown in Figure 3. This observation together with the formation of minor amounts of aromatic compounds among the thermal decomposition products indicate that side group cleavage by ionic autocatalytic zip-elimination from PVIm cannot be the determining decomposition process leading to 1H-imidazole. As a consequence, the formation of 1H-imidazole should occur by a radical C−N bond scission. The difference of the onset temperatures of 1H-imidazole and 1-vinylimidazole formation can be explained by the different bond energy values of the Calkyl−N bond (305 kJ mol−1) and the backbone C−C bond (346 kJ mol−1).22 Therefore, a similar ionic process that occurs during HCl elimination from PVC and acetic acid elimination from PVAc as a consequence of the acidity of the eliminated molecules cannot be expected to take place for the formation of 1H-imidazole from PVIm. It has to be noted that Hofmann elimination, which may take place during the thermal decomposition of ionic liquids, especially quaternary ammonium containing ionic liquids,23 can also be taken into account for 1H-imidazole and simultaneous doublebond formation in the polymer chain during thermal decomposition of PVIm. However, this process requires the presence of a nucleophile, which exists for ionic liquids in the form of the negatively charged counterions, but such nucleophiles are absent in the case of the neutral poly(Nvinylimidazole), on the one hand. On the other hand, such mechanism would lead to larger amounts of conjugated polyenes in the main chain and, as a consequence, to much larger amounts of aromatics than that observed by us during thermolysis of PVIm. In this context, it has to be noted that the thermal stability of imidazole-based ionic liquids is in the range of 200−400 °C,23a which is significantly lower than that of PVIm, and the major decomposition products of such

Figure 6. MS spectra of tentatively identified pyrolysis products of PVIm homopolymer: α-methylvinylimidazole (5), 2-methyl-1Hbenzimidazole (7), and 2,6-di(imidazol-1-yl)-hex-1-ene (10). The peak numbers are as in Table 1.

vinylimidazole formation; that is, ∼2 orders of smaller amounts of aromatic compounds are formed in this process than 1Himidazole and 1-vinylimidazole. As these charts show, the product distribution as a function of pyrolysis temperature is in good accordance with the thermogravimetric results. The formation of 1-vinylimidazole and alkylaromatic compounds become significant at above 400 °C, at higher temperature than that observed for the formation of 1H-imidazole and benzene. The highest intensity of each peak is observed at 500 °C. Above this temperature, the amounts of 1-vinylimidazole and benzene decrease rapidly, but that of the alkylaromatic compounds changes only slightly by 550 °C, showing that not only the appearance but also the most intensive formation of these compounds occurs also at a higher temperature than that of the other compounds. The results of TG-MS and successive pyrolysis measurements indicate that there are two main decomposition processes of PVIm: side group elimination and free radical depolymerization leading to 1H-imidazole and 1-vinylimidazole formation, respectively. However, macroradicals formed by side group elimination and by homolytic chain scission may partly be combined, leading to temporary cross-linking of macromolecules from which further imidazole groups can split off at 8958

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missing one or two side groups. The mass spectra of these minor products are presented in Figure 6.

compounds are usually not olefins but substituted alkyl compounds, such as alkyl halides.23a−e Recently, Antonietti et al.23f found that Hofmann elimination is also absent during decomposition of dialkylimidazolium ionic liquids with tetracyanoborate and dicyanamide counterions. Hofmann elimination as decomposition mechanism for imidazolecontaining polymeric ionic liquids was also excluded even under strong alkaline conditions on the basis of steric reasons and the aromaticity of the imidazole rings by Ye and Elabd.2d Based on these considerations and the observed TG-MS and Py-GC/MS results, the major elementary reactions of the thermal decomposition of PVIm in inert atmosphere are displayed in Scheme 1. As shown in this scheme, homolytic side group cleavage and main chain scission followed by depolymerization are the most determining steps in the course of the thermal decomposition of PVIm. The main products are 1H-imidazole and 1-vinylimidazole. Scission of the pendant C− N bond followed by hydrogen abstraction yields 1H-imidazole. In case of homolytic scission of the main chain, depolymerization occurs, and particularly 1-vinylimidazole and lower amounts of α-methylvinylimidazole are formed beside oligomers. The pyrolysis gas chromatogram of a flash pyrolysis experiment carried out at 500 °C is displayed in Figure 5, and the peak identifications are shown in Table 1, while the corresponding structures of the identified thermal decomposition products are displayed in Scheme 2. In addition to the two main decomposition products of PVIm, that is, 1vinylimidazole (3) and 1H-imidazole (4), there are several other minor and trace compounds in the pyrolysate as the data indicate in Figure 5. Among the minor pyrolysis products aromatic and alkylaromatic hydrocarbons of one and two rings and N-containing aromatic compounds (containing imidazole and phenyl groups) were identified. The observation that aromatic compounds are minor pyrolysis products confirms that zip-elimination of 1H-imidazole producing significant amounts of main chain polyene sequences is negligible during thermal decomposition of PVIm. Isothermal degradation of PVIm corroborated this conclusion, because neither 1Himidazole formation nor the presence of conjugated double bonds in the thermally treated PVIm was observed. However, successive 1H-imidazole elimination according to the decomposition mechanism in Scheme 1 leads to short polyenic structures in the main chain. Because of the fact that 1Himidazole evolves in the temperature range where cyclization and aromatization of conjugated polyenes occur in the course of thermal decomposition of other vinyl polymers, such as PVC, PVAc, and PVA, a variety of cyclization, isomerization, and aromatization reactions of the low amounts of polyenes formed simultaneously with 1H-imidazole elimination are expected to occur resulting in a variety of aromatic compounds in line with the results of Montaudo and Puglisi.17e Thus, as shown in Scheme 3, main chain scission accompanied by side group cleavage yields 2,6-di(imidazol-1-yl)-hex-1-ene (10), while chain scission and side group elimination results in short polyenic structures the aromatization of which leads to 1phenylimidazole (9) or to cyclization followed by aromatization giving benzene (1) and 1H-imidazole (4). It has to be noted that the character of the imidazole side group influences the products of free radical decomposition of PVIm as well. Compounds corresponding to peaks 5, 7, and 10 in Figure 5 are probably formed from dimer or trimer units

4. CONCLUSION Poly(N-vinylimidazole) (PVIm) is a versatile polymer, and it possesses a variety of unique properties and provides several new advanced application possibilities. In this work, the thermal behavior and decomposition mechanism of PVIm were investigated in details by the combination of different techniques, such as differential scanning calorimetry, thermogravimetry−mass spectrometry, and pyrolysis−gas chromatography/mass spectrometry. Our findings indicate that free radical depolymerization and side group elimination, typical thermal decomposition reactions of several vinyl polymers, simultaneously govern the formation of main products during thermal decomposition of PVIm. The onset temperature of side group elimination is at 400 °C. This reaction leads to 1Himidazole and small amounts of benzene. Free radical main chain scission is initiated at 430 °C, resulting mainly in 1vinylimidazole, i.e., the monomer by depolymerization. Above this temperature, the two main reactions occur simultaneously; hence, a number of minor and trace products are also formed through the combination of side group cleavage and intramolecular radical transfer reactions. The polyene segments of the main chain left behind after side group elimination decomposes to alkyl- and polyaromatic compounds as in the analogous reaction occurring in other vinyl polymers with heteroatom pendant linkages (e.g., PVC, PVAc, etc.), but these are only minor products of PVIm pyrolysis, indicating that the major step in 1H-imidazole formation is the homolytic C−N bond cleavage and not zip-elimination as for the other mentioned heteroatom-containing polymers.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed analysis by FTIR and TG; DTA and TG-MS curves of PVIm homopolymer. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] or [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the thermal analyses to Ms. J. Szauer. Partial support of this research by the Hungarian Scientific Research Fund (OTKA K81592) and by the Nanomedicine Thematic Program of the Chemical Research Center of the Hungarian Academy of Sciences is also acknowledged.

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REFERENCES

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dx.doi.org/10.1021/ma301712k | Macromolecules 2012, 45, 8953−8960