Characterization and thermal degradation of poly(2

Polymer Degradation and Stability 77 (2002) 371–376 www.elsevier.com/locate/polydegstab

Characterization and thermal degradation of poly(2-methacrylamidopyridine) Mehmet Cos¸ kun*, M. Mu¨rs¸ it Temu¨z, Kadir Demirelli University of Firat, Department of Chemistry, Faculty of Science and Arts, Elazıg-Turkey Received 8 January 2002; accepted 30 January 2002

Abstract 2-Methacrylamidopyridine (MAPy) was prepared by reaction of methacryloyl chloride with 2-aminopyridine. The MAPy was polymerized in the presence of 2,20 azobisisobutyronitrile (AIBN) as an initiator in acetonitrile. The thermal decomposition behaviour of poly(2-methacrylamidopyridine) [poly(MAPy)] was investigated by thermogravimetric analysis (TG) and by programmed heating of the polymer from ambient temperature to 300, then from 300 to 500 C under vacuum, and was followed by product collection, and by using IR spectra of partially degraded polymer. The products volatile at degradation temperature but not at ambient temperature were separately collected, after each heating, stage on the cooled upper part of the degradation tube (cold ring fraction, CRF1 for between ambient temperature and 300 C, CRF2 for between 300 and 500 C). The 1H-NMR spectrum showed that almost all of the light yellow coloured CRF1 is 2-aminopyridine, not including some very small signals. On the other hand, the CRF2 showed that 2-aminopyridine and 2-methacrylamidopyridine formed as main products. The activation energy of thermal degradation of poly(MAPy), in the first stage of decomposition, was calculated as 110 kJ mol1, and the pre-exponential factor as 5.421011 s1. The mechanism of thermal degradation including formation of the major products is discussed. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Methacrylamidopyridine; Thermal degradation

1. Introduction A very important type of polymer degradation is the processes in which side groups participate in the reaction. Many different reactions may take place corresponding to the variety of possible side groups. Some polymers may give these processes basically without scission of the main polymer chain. Dehydrochlorination of PVC, acetic acid elimination from PVA [1], dehydrohalogenation from chain halogenated PS [2,3], reaction between nitrile groups and HCN elimination in polyacrylonitrile degradation [4], may be given as a few examples for these processes. There are a few studies of thermal degradation of poly(meth)acrylamides and their copolymers [5–7]. On the other hand, since aminopyridines are widely used as binding materials [8–10], 2-aminopyridine, as an example, is introduced into polymer structures [11–13]. Thermal degradation of poly(2-acrylamidopyridine) and * Corresponding author. E-mail address: mcoskun@firat.edu.tr (M. Cos¸ kun).

some of its polymeric metal complexes were investigated in the literature [14]. In this study the characterization and thermal degradation of poly(2-methacrylamidopyridine) has been investigated.

2. Experimental 2.1. Materials 2-Aminopyridine (Merck), acetonitrile, tetrahydrofuran (THF), triethylamine [(Et)3N], (Aldrich) were used as received. Benzene (Fluka) was dried on anhydrous MgSO4 and freshly distilled prior to use. Methacryloyl chloride (Aldrich) was freshly distilled under vacuum prior to use. 2.2. Preparation of the monomer 2-Methacrylamidopyridine(MAPy) was prepared by a method adapted from the literature [14,15].

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M. Cos¸kun et al. / Polymer Degradation and Stability 77 (2002) 371–376

Methacryloyl chloride (12 mmol) in 30 ml dried benzene was added dropwise to a solution of 10 mmol 2-aminopyridine and 20 mmol (Et)3N in 250 ml dried benzene, over a 30 min period, at 0–5 C, and the reaction contents were stirred for an additional 1 h at the same temperature. Then at room temperature ( 20 C), they were kept continuously stirred for about 12 h. The precipitate was filtered off, and the benzene removed by a rotary evaporator, The red-coloured viscous residue was fractionally distilled to yield 10.0 g (62%) of the monomer, b.p. 112–117 C (at 1–2 mm-Hg). IR(cm1): 3300, 3246, 3175 (N–H);3090, 3060, 3020(=C–H stretching in aromatic and olefinic structures); 1682 (C=O, amide I); 1623 (C=C) ; 1508 (N–H bending, amide II), 1587, 1438, 1310 ( pyridine ring stretchings); 929 (=C–H, out of plane bending). 1 H NMR(ppm, in CDCl3): 9.2 (1H, NH); 8.2 (2H, protons on C5 and C6 in pyridine ring); 7.8 (1H, proton on C4 in pyridine ring); 7.1 (1H, proton on C3 in pyridine ring); 5.9 and 5.5 (2H, =CH2) and 2.0 (3H, CH3). 13 C NMR(ppm, in CDCl3): 167.5 (C=O); 152.0, 152.5, 138.0, 120.6 and 114.2 ( carbons in pyridine ring); 141.0 (=C); 119.5 (=CH2); 17.2 (CH3).

heated at a rate of 10 C/ min under vacuum from ambient to 300 C, and then its residue from 300 to 500 C. Products volatile at degradation temperature but not at ambient temperature were separately collected after each heating stage on the cooled upper part of the degradation tube (cold ring fraction, CRF1 for between ambient temperature and 300 C, CRF2 for between 300 and 500 C). Products volatile at ambient temperature were collected at 196 C (in liquid nitrogen). While this fraction of the degradation products was examined by gas-phase IR spectroscopy, the CRFs were examined by IR, 1 H-NMR and 13C-NMR, and the CRF2 was additionally examined by gas chromatography–mass spectrometry (GC–MS). Thermogravimetric measurements were carried out in a Shimadzu TGA-50 thermobalance under nitrogen flow. A Mattson 1000 FTIR spectrometer was used for all IR spectra. NMR spectra were recorded on a Jeol FX-90Q spectrometer.

2.3. Polymerization of the monomer

The bands at 3020, 1623 and 929 cm1 in the IR spectrum of the polymer, corresponding to the olefinic structure disappeared, bands related to N–H were seen at 3430 and 3360 cm1, and the carbonyl band shifted slightly to 1688 cm1. The 1H-NMR spectrum (Fig. 1a) showed the broad signals at 9.2–6.5 ppm ( aromatic CH, and NH) and 2.8–0.8 ppm ( aliphatic protons). The 13CNMR spectrum (Fig. 1b) agrees well with the polymer structure.

Poly(MAPy) was prepared by refluxing in the presence of AIBN as an initiator in acetonitrile for 5 h with a 55% conversion, and the resulting polymer was precipitated into a mixture of water and ethanol (v/v: 70:30). The polymer was purified by reprecipitating in water+ ethanol from THF solution.

3. Results and discussion 3.1. Characterization of poly(MAPy)

2.4. Thermal degradation studies 3.2. Thermogravimetric study Polymer film was made by dissolving about 5 mg of poly(MAPy) in THF, casting a thin film on a salt plate and drying in a vacuum oven at 50 C for 24 h in order to investigate changes in IR spectra during degradation. For identification of thermal degradation products, the degradation was carried out in a system consisting of a degradation tube, with a condenser for product collection, gas phase IR cell and a rotary pump. The polymer was

Fig. 1. (a) 1H-NMR spectrum and (b) poly(MAPy).

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C-NMR spectrum of

The thermogravimetric (TG) curve (Fig. 2a) of poly(MAPy) indicates two main stages of decomposition, except

Fig. 2. Thermogravimetric curve of (a) poly(MAPy) and (b) poly(AAPy) [14].


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Fig. 4. 1H-NMR spectrum of the CRF1. Fig. 3. IR spectra of partially degraded polymer from film deposited on a salt plate from THF solution.

for a weight loss of 2.4% at the beginning of heating probably from solvent or non-solvent, starting at about 170 and 380 C. After the first stage (between 170–300 C) of breakdown with a weight loss of about 25.5%, there is a plateau with very small weight loss (about 1.8%) between 300 and 380 C, and the last stage of decomposition between 380 and 500 C shows a weight loss of 57.5%. The residue at 500 C is about 13.2%. The TG curve is quite different from that (Fig. 2b) of poly(acrylamidopyridine), Poly(AAPy) [14]. In case of poly(AAPy) the degradation starts at about 120 C, and is completed at about 370 C, and the residue is lower (about 4%) than that of poly(MAPy). It is clear that poly(MAPy) behaves distinctly from poly(AAPy) in terms of thermal degradation. 3.3. Changes in IR spectra during degradation of poly(MAPy) The polymer film on a salt plate was partially degraded under N2 flow at 10 C min1 heating rate to 175, 270 and 380 C. The IR spectrum was recorded for each heating stage (Fig. 3). There was no change in the IR spectrum at 175 C from that of original polymer although decomposition started according to the TG curve. At 270 C, the IR spectrum of partially degraded polymer showed many changes such as the disappearance of the bands at 3430 cm1(N–H stretching), 1515 cm1 (N–H bending) and 670 cm1 (out-of-plane N–H wagging), and the appearance of 1725, 1350 and 1200 cm1, and the shift to 1682 cm1 of the band at 1688 cm1. All these spectroscopic changes suggested disappearance of the amide structure and appearance of cyclic six-ring imides. The band at 1725 and 1682 cm1 are asymmetric and symmetric stretching of C=O in the imide, respectively, and those at 1350 and 1200 cm1 are ring stretching in the imide [16]. The IR spectrum at 380 C is nearly similar to that at 270 C.

3.4. Product analysis studies The degradation was carried out in a system consisting of a degradation tube, with a condenser for product collection, gas phase IR cell and a rotary pump. The polymer was heated at 10 C/min under vacuum from ambient to 300 C, and then its residue from 300 to 500 C. The products volatile at degradation temperature but not at ambient temperature were separately collected after each heating stage on the cooled upper part of the degradation tube (cold ring fraction, CRF1 for between ambient temperature to 300 C, CRF2 for between 300 and 500 C). The 1H-NMR spectrum (Fig. 4) showed that almost all of the light yellow coloured CRF1 is 2-aminopyridine, not including some very small signals. The 1H-NMR spectra of volatile materials trapped at 196 C while heating to 300 C, show that it contains small amounts of 2-aminopyridine, and water and ethanol. The latter are non-solvents of the polymer. The CRF2 is dark brown-coloured, and its 1H-NMR spectrum shows some characteristic peaks at 11.3 ppm (small, COOH), 10.2 ppm (small, CHO), 9.3, 8.6, 8.3, 8.0, 7.6, 7.3, 7.0, 6.7, 6.5 ppm (all aromatic ring protons), 4.6–6.0 ppm (NH, NH2 and olefinic protons) and 0.8–3.0 ppm (aliphatic protons in various positions). The IR spectrum and the APT 13C-NMR spectrum of the CRF2 are shown in Fig. 5. The some characteristics of the IR bands (cm1): 3446, 3347 and 3196 (N–H, O–H); 3060, 3020 (=C–H, olefinic, aromatic); 1730, 1702 (as shoulder, C=O), 1687 (– CO–N), 1655 (C=C), 1527 (N-H bending), 1620, 1486, 1446 (aromatic ring stretching). In the APT 13C-NMR spectrum of the CRF2 the negative signals belong to primary(CH3) and tertiary(CH) carbons, and the positive signals to quaternary (C) and secondary (CH2) carbons. Some of the signals in this spectrum at 168.8 ppm (C=O), between 160.6 and 110.8 ppm ( aromatic and olefinic carbons in various positions) and 50.0–18.6 ppm (aliphatic carbons).

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Fig. 6. Mass spectra of the products with retention time of (a) 13.820 min and (b) 20.054 min in the gas chromatogram.

Fig. 5. (a) IR spectrum and spectrum (b) APT CRF2.

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C-NMR of the

4. Mechanism of degradation In the first stage of the degradation, tautomerization of the amide group of one unit is followed by an intra-

The GC–MS investigation of the CRF2 showed that 2-aminopyridine (retention time, r.t.: 13.82 min, GC peak area (%):73.8) and 2-methacrylamidopyridine (r.t.:20.05 min., GC peak area (%):10.1) formed as main products in the degradation between 300 and 500 C. The MS spectra of these two products, and some fragmentations in them, are presented in Fig. 6. The IR, 1Hand 13C-NMR spectra of the CRF2 confirm that these two main products are present. Some other products which might be detected are N-alkyl-2-aminopyridines such as N-isopropyl-2-aminopyridine (r.t.:16.49 min., m/e:136(M+), 121 (base peak), 94 (small), 91, 39); N-npropyl-2-aminopyridine (r.t.:18.09 min., m/e:136(M+), 121 (small), 94 (base peak), 67, 43,39); N-tert-butyl-2aminopyridine (r.t.: 19.34 min, m/e:150 (M+), 135, 121, 94 (base peak) 67, 57, 39. The total of these three products is 2.5%. About ten products (total 14%) which have molecular weight above 200 could not be identified.

Fig. 7. Weight loss versus time during the isotermal heating in region of the first stage degradation.


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Fig. 8. Rate of weight loss versus weight loss for the isothermal heating.

molecular chain reaction with side chain participation, and this process results in formation of a cyclic imide structure and 2-aminopyridine as follows.

If this kind of reaction occured completely along the polymer chain, the weight loss would have been 29% in the degradation between 170 and 300 C. In this range the TG curve shows a weight loss of 25.5%. The IR spectrum of the polymer partially degraded at 380 C is nearly similar to that at 270 C. This means that the cyclization continues along the plateau with a weight loss of about 2% (at temperature between 300 and 380 C), and this cyclic imidation reaction take place without scission of the main polymer chain, like dehydrochlorination from PVC,

acetic acid elimination from PVA [1] and dehydrohalogenation from chain halogenated PS [2,3]. In the second stage of the degradation (above 380 C) the imide structure decomposes by giving 2-aminopyridine and 2-methacrylamidopyridine as major products, as follows:

Isothermal heating at 180, 190, 200 and 210 C for 60 min have been recorded to calculate the kinetic parameters of the cyclic imidation reaction. We assume that the cyclization process is described by the equation: d%=dt ¼ kð1 % %Þn where 1% is the weight lost at infinite time and % the weight lost at time t. k And n are the rate constant

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and the decomposition order, respectively. The weight loss% (%)time (t) plots and the d%/dt % plots are shown in Figs. 7 and 8, respectively. k Can be obtained from the slope of the straight line in Fig. 8 for which n=1. The activation energy Ea and pre-exponential factor A of the cyclic imidation reaction can be calculated from an Arhenius plot (ln k1/T) since k=A exp(Ea/RT). The k values are 10.2102, 21.7102, 28.5102 and 10.2102 min1 for 180, 190, 200 and 210 C, respectively. The activation energy is calculated as 110 kJ mol1, and the pre-exponential factor as 51011. This value of the activation energy is very much higher than that of given for activation energy (33.4 kJ mol1) of thermal degradation of poly(AAPy) [14], and it is comparable to the activation energies reported for the formation of cyclic anhydride with the partial depolymerization ( 86.9–153.4 kJ mol1 up to weight loss of 59%) [17] and the random chain scission of poly(vinyl acetate) in solution(131.7 kJ mol1) [18] and the decomposition of the poly(orthochloromethyl styrene) [19]. But the activation energy of the cyclic imidation is lower than those of formation of six membered-cyclic anhydrides in thermal degradation of Eudragit L-100 polymer (214.2 kJ mol1) [20] and b-elimination from monosubstituted vinyl polymers ( 167.2–250.8 kJ mol1) [1].

idine and 2-methacrylamidopyridine formed as main products. The activation energy of thermal degradation of poly(MAPy) was calculated as 110 kJ mol1, and the preexponential factor as 5.01011 s1.

Acknowledgements The authors wish to thank The Fırat University Research Fund for financial support of this project (FU¨NAF-525).

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5. Conclusions 2-Methacrylamidopyridine(MAPy) was synthesized and polymerized via free radical polymerization. The thermal degradation of poly(MAPy) was investigated by thermogravimetric analysis (TG) and by programmed heating of the polymer from ambient temperature to 300, then from 300 to 500 C. CRFs as products volatile at degradation temperature but not at ambient temperature were identified by FT–IR, NMR and GC–MS (only for CRF2). The CRF1 showed that 2-aminopyridine was major product except for very small signals. Also, the CRF2 showed that 2-aminopyr-

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