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Thermal degradation behaviour of two methacrylate polymers with side chain amide groups
Polymer Degradation and Stability 78 (2002) 49–55 www.elsevier.com/locate/polydegstab
Thermal degradation behaviour of two methacrylate polymers with side chain amide groups . Mehmet Cos¸ kun*, Ibrahim Erol, M. Fatih Cos¸ kun, Kadir Demirelli Department of Chemistry, University of Firat, Faculty of Science and Arts, Elazıg, Turkey Received 22 March 2002; accepted 9 April 2002
Abstract 2-Oxo-2-phenylamino)ethylene methacrylate (PAMA) and (2-oxo-2-benzylamino)ethylene methacrylate (BAMA) were prepared by reaction of sodium methacrylate with N-phenyl chloroacetamide and N-benzyl chloroacetamide, respectively, which were obtained from reaction of chloroacetyl chloride with aniline or benzylamine. The thermal degradations of poly(PAMA) and poly(BAMA) prepared by a free-radical procedure were investigated by thermogravimetric analysis (TG) and by programmed heating of the polymer from ambient temperature to 500 C under vacuum, followed by product collection, and using IR spectra of partially degraded polymer. The products volatile at degradation temperature but not at ambient temperature were collected on the cooled upper part of the degradation tube [cold ring fraction, CRFP for poly(PAMA), CRFB for poly(BAMA)]. Product analysis studies showed that only N-benzyl hydroxyacetamide formed as major product in thermal degradation of poly(BAMA) and the monomer did not, whereas in the case of poly(PAMA) N-phenyl hydroxyacetamide and the monomer both formed as major products. For poly(BAMA), the activation energy of cyclization is calculated as 47.4 kJ mol1; for poly(PAMA) the activation energy of the depolymerization is calculated as 58 kJ mol1. The mechanism of thermal degradation including formation of the major products is discussed. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Methacrylate; Amide side chain; Thermal degradation
1. Introduction In polymer degradation, side groups on the main chain are very important to understand how decomposition takes place. Some of the side groups cause degradation without main chain scission. Dehydrochlorination from 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] and, recently, the cyclic imidation reaction giving 2-aminopyridine in the degradation of poly(2-methacrylamidopyridine) [5], may be given as a few typical examples for these processes. Polymers bearing ester side chains, such as poly(methacrylic esters) show different behaviour in thermal degradation depending on the alkyl side chain of the ester. Some poly(methacrylates) degrade relatively easily to the monomer on heating [6–9]. In contrast, some poly(methacrylates) undergo ester decomposition without giving the corresponding mono* Corresponding author. Fax: +90-4242-330062. E-mail address: mcoskun@firat.edu.tr (M. Cos¸ kun).
mer [10,11]. On the other hand, many poly(methacrylates), including ones degrading to the monomer, undergo ester decomposition at some higher temperature followed by elimination of some small molecules and formation of cyclic anhydride structures [8,9,12,13]. The present paper describes thermal degradation behaviours of two poly(methacrylic ester)s having an amide group in the side chain, poly [(2-oxo-2-phenylamino)ethylene methacrylate], poly(PAMA), and poly [(2-oxo-2-benzylamino)ethylene methacrylate], poly (BAMA).
2. Experimental 2.1. Materials Aniline, benzylamine, acetonitrile, dimethyl sulfoxide (DMSO), sodium methacrylate, anhydrous MgSO4 (Aldrich), were used as received. Benzene was dried over anhydrous MgSO4 and then distilled freshly before use. K2CO3 was dried by heating at 110 C. The chloroacetyl chloride (Aldrich) was freshly distilled prior to use.
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2.2. Preparation of the monomers The amide structures discussed here were prepared by aid of a method adapted from the literature [14]. Chloroacetyl chloride (5.5 mmol) was added dropwise to a mixture of aniline (or benzylamine) (5.5 mmol) and K2CO3 (5.5 mmol) in 100 ml dried benzene at 0 C and the reaction stirred at room temperature for 12 h. The precipitate was filtered off, and washed with water, and then dissolved in diethyl ether. After the benzene was evaporated, the residue was added to the ether solution, and it was dried over anhydrous MgSO4. After the ether was evaporated, the solid products were crystallized from ethanol. Yield: above 90% for each product. N-Phenyl chloroacetamide: IR (cm1): 3230 (–NH–), 1680 (N–CO–), 763 (C–Cl) N-Benzyl chloroacetamide: IR(cm1): 3282 (–NH–), 1650 (N–CO–), 788 (C–Cl) 2-Oxo-2-phenylamino)ethylene methacrylate, (PAMA), and (2-oxo-2-benzylamino)ethylene methacrylate, (BAMA), were prepared from the reaction of sodium methacrylate with the corresponding chloroacetamides, as reported previously for a similar reaction of the chloroacetyl group [15]. Both monomers were solid, and they were crystallized before use. Yield: about 80% for each monomer. PAMA, IR (cm1): 3260 (–NH–), 3100–3000 (¼C–H), 1730 (C¼O in ester), 1684 (C¼O, amide-I), 1632 (C¼C), 1600 (C¼C in aromatic ring), 1549 (N–H bending, amide-II) 1 H NMR (ppm, CDCl3): 8.10 (–NH–), 7.10–7.60 (aromatic protons), 6.25 and 5.68 (CH2¼C), 4.75 (–COOCH2), 2.03 (¼C (CH3)–) BAMA, IR (cm1): 3297 (–NH–), 3100–3000 (¼C– H), 1720 (C¼O in ester), 1661 (C¼O, amide-I), 1627 (C¼C, as a shoulder), 1605 (C¼C in aromatic ring), 1529 (N–H bending, amide-II) 1 H NMR (ppm, CDCl3): 7.10–7.80 (–NH– and aromatic protons), 6.15 and 5.60 (CH2¼C), 4.60 (–COOCH2–), 4.35 (Ph–CH2–N), 1.95 (¼C (CH3)–). 2.3. Polymerization Poly(PAMA) and poly(BAMA) were prepared by free-radical polymerization of the corresponding monomers in dimethyl sulfoxide, in a sealed tube in the presence of AIBN under argon atmosphere at 65 C for 12 h. The polymers were precipitated in excess ethanol. After reprecipitating from acetone solution using ethanol, the polymers were dried under vacuum at 50 C for 18 h.
plate and drying in a vacuum oven at 50 C for 12 h 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 and a rotary pump. The polymers were heated at a rate of 10 C/min under vacuum from ambient to 500 C. Products volatile at degradation temperature but not at ambient temperature were collected for each polymer on the cooled upper part of the degradation tube (cold ring fraction, CRF). The CRFs were examined by IR, 1H-NMR and 13C-NMR, and gas chromatography–mass spectrometry (GC–MS). Thermogravimetric measurements were carried out on 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 and a Varian Gemini 200 MHz spectrometer.
3. Results and discussion 3.1. Characterization of the polymers While the IR spectrum of poly(BAMA) shows characteristic bands at 3350 (N–H), 1740 (C¼O), 1660 and 1680 cm1 (as a shoulder) (amide-I) , that of poly (PAMA) has characteristic bands at 3330 (N–H), 1737 (C¼O), 1690 cm1 (amide-I) (Fig. 1). 1H-NMR assignments of the polymers, for poly(BAMA) (CDCl3, 200 MHz spectrometer, ppm): 7.1 (N–H), 7.2–7.4 (aromatic protons), 4.4 (–CO–O–CH2–), 4.1 (Ph–CH2–N), 1.2–2.0 (–CH2– and –CH3), for poly(PAMA) (d6-acetone, 90 MHz, ppm): 9.3 (N–H), 7.0–7.7 (aromatic protons), 4.7 (–CO–O–CH2–), 1.2–2.2 (–CH2– and –CH3). 3.2. Thermogravimetric study The thermogravimetric (TG) curves (Fig. 2) of poly (BAMA) and poly(PAMA), obtained at a heating rate of 10 C/min under nitrogen flow, show some differences from each other. Firstly, it is noticeable that poly(PAMA) is more stable than poly(BAMA) in degradation up to about 400 C, and poly(BAMA) has a decomposition with two stages of which the first is below 340 C with a weight loss of 26%, and the second is above 340 C. Some of the degradation characteristics are summarized in Table 1. 3.3. Changes in IR spectra during degradation of the polymers
2.4. Thermal degradation studies Polymer films were made by dissolving about 5 mg of each polymer in acetone, casting a thin film on a salt
The polymer films prepared on a salt plate were partially degraded under N2 flow at 10 C min1 heating rate to 260, 280, 300, 320, 350, 400 and 425 C. The IR
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Fig. 2. TGA curves of (a) poly(BAMA) and (b) poly(PAMA).
ture are shown in Fig. 4. This study shows that the formation of cyclic anhydride structures begins together with weight loss in case of poly(BAMA), but in case of poly(PAMA) it begins after a weight loss of about 4%. Whilst the IR spectrum of the reside at 425 C of poly(BAMA) still shows bands characteristic of the cyclic anhydride structures, that of poly(PAMA) does not. Fig. 1. IR spectra of (a) poly(BAMA) and (b) poly(PAMA).
3.4. Product analysis studies spectra of the polymer films heated to 260, 280, 320, 425 C are shown in Fig. 3. There is no change the in the IR spectrum of poly(PAMA) at 260 C from that of the original polymer although decomposition starts according to the TG curve, but at the same temperature the IR spectrum of poly(BAMA) with a weight loss of about 2.5% shows new bands at 1805 and 1018 cm1. At 280 C, the IR spectrum of poly(BAMA) with a weight loss of about 5.8% shows very important changes such as increased absorbance at 1805, 1763 and 1018 cm1 while the spectrum of poly(PAMA) with a weight loss of about 3% is still the same as that of the original polymer. IR spectra (Fig. 3) of both poly(PAMA) and poly(BAMA) heated to 320 C, with a weight loss of 8.1 and 21.2%, respectively, have developed intense bands at 1805, 1763 and 1018 cm1, which are characteristic of cyclic anhydride structures [8,11]. Plots of absorbance of the bands at 1018, 1805 and 1685 cm1 versus tempera-
The degradation was carried out in a system consisting of a degradation tube, with a condenser for product collection a rotary pump. The polymer was heated at 10 C/min under vacuum from ambient temperature to 500 C. The products volatile at degradation temperature but not at ambient temperature were collected on the cooled upper part of the degradation tube [cold ring fraction, CRFB for poly(BAMA), and CRFP for poly (PAMA)] . These fractions were examined by 1H and 13 C-NMR, IR and GC–MS techniques. While there are signals at 8.5; 8.2; 7.9 (–CO–NH–), 7.0–7.6 (aromatic protons), 5.7; 6.3 (CH2¼C) , 4.75 (–COO–CH2–CO–N), 4,70; 4.5; 4.3 (–COO–CH2–), 4.1 (HO–CH2–CO–N–), 1.0–2.2 ppm (aliphatic protons) in 1H-NMR of the CRFP, there are signals at 7.5–7.2 (aromatic protons), 7.1; 6.9 (–CO–NH–), 4.4 (HO–CH2–CO–N–), 4.1 (N– CH2–phenyl), 4.9; 4.6 (COO–CH2– in various posi-
Table 1 Degradation characteristics of poly(BAMA) and poly(PAMA) Polymer
IDTa
Weight loss (%), at 320 C
10% Weight loss, at ( C)
50% Weight loss, at ( C)
Residue at 500 C
Poly(BAMA) Poly(PAMA)
230 250
21.2 8.1
290 328
402 400
7.3 10.6
a
IDT; initial decomposition temperature.
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tions), 1.0–2.4 ppm (aliphatic protons) in that of the CRFB (Fig. 5). It is striking that no olefinic proton signals are observed in the CRFB as they are present in that of the CRFP. 13C-NMR signals (50.4 MHz, solvent: CDCl3), for the CRFB: 173.97 (–CO–N–), 139.80;
131.08; 130.70–129.57 (aromatic and/or olefinic carbons), 64.15 (–OCH2CO–) and 45.02 ppm (–CH2– phenyl). For the CRFP: 172.35; 172.02 (–CO–N– and –COO–), 139.94; 131.04; 126.75; 122.08 (aromatic carbons),
Fig. 3. IR spectra of partially degraded (a) poly(PAMA) and (b) poly(BAMA) after heating to 260, 280, 350 and 425 C.
Fig. 4. Variation of absorbance with temperature of the bands at 1018, 1685, 1805 cm1 of (a) poly(PAMA) and (b) poly(BAMA).
M. Cos¸kun et al. / Polymer Degradation and Stability 78 (2002) 49–55
138.60; 127.20 (olefinic carbons), 65.04; 64.36 (–O– CH2CO–), 20.60 ppm (–CH3). The characteristic bands, in IR spectra of both the CRFP and the CRFB, are at 3200–3350 (O–H and N– H stretching), 3030–3100 (¼C–H stretching), 1605– 1765 (–CO– stretching, in various positions), 1543 (–N– H bending), 763 and 702 cm1 (¼C–H out-of-plane bending in mono substituted aromatic). In the spectrum of CRFP, there are also some different bands at 3140 (– N–H), 1205 (–COO–CH2– stretching), 980; 947; 905 and 860 cm1 (probably, ¼C¼H out-of-plane bending, in vinyl–vinylidene structures). GC–MS investigation of CRFB showed that N-benzyl hydroxyacetamide [retention time, r.t.: 20.3 min, GC peak area: 76.8%, m/e: 165 (M+), 147, 106, 91 (base peak)] formed as a major product, and benzaldimine, N-benzyl acetamide, N-phenyl hydroxyacetamide as minor products, and no monomer was observed. About twenty products whose sum is below 20% could not be
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identified. In the case of poly(PAMA), the GC–MS investigation of the CRFP indicated that N-phenyl hydroxyacetamide [retention time, r.t.: 19.9 min, GC peak area: 70.2%, m/e: 151 (M+), 120, 93 (base peak), 77] and the monomer [retention time, r.t.: 22.0 min, GC peak area: 19.1%, m/e: 219 (M+), 127, 93 (85%), 77, 69 (base peak), 41] are major products of the degradation. Other products are in minor amounts (> 1%).
4. Mechanism of degradation This study shows that the polymers being discussed behave differently in thermal degradation, especially in the beginning. In degradation of poly(BAMA), the monomer is not detected among the degradation products. The cyclic anhydride structures are observed from temperatures close to the beginning ( 2% weight loss at 260 C) to above 400 C. In contrast, in the case
Fig. 5. 1H-NMR spectra of (a) the CRFB and (b) the CRFP (200 MHz, solvent: CDCl3).
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of poly(PAMA), formation of the cyclic anhydride structures starts at around 290 C ( 4% weight loss). The CRFP collected at 310 C ( 6.5% weight loss) contains 65% of the monomer, although there is 19.08% in total CRFP (weight losses have been given according to the TG curves, but the CRFs and the cyclic anhydride structures have been created in a oven). This means that thermal degradation of poly(PAMA) starts by depolymerization and continues together with formation of the cyclic anhydride structures after about 290 C for some time. Depolymerization followed by formation of cyclic anhydride structures has been observed in thermal degradation of many poly(methacrylate esters) [8,13,16,17]. That thermal degradation starts by formation of cyclic anhydride structures, which is observed in case of poly(BAMA), is a new result. Tautomerization of the amide group of one unit is followed by an intramolecular chain reaction with side chain participation, and this process results in formation of a cyclic anhydride structure and N-benzyl hydroxyacetamide as follows. In the case of poly(PAMA), since a nonbonding pair of electrons on nitrogen combined with the ring may also be moved from the nitrogen to the ring, this decreases tautomerization probability of the amide group of one unit and increases the cyclization temperature. For this reason, thermal degradation of
poly(PAMA), similar to many other poly(methacrylic esters), starts by depolymerization and then continues by cyclization [8,13,16,17]. To estimate the activation energy of thermal decomposition of the polymers at time intervals near the start, isothermal heating at 270, 280, 290 and 300 C for poly(PAMA), and at 230, 240 and 250 C for poly (BAMA), was carried out for 60 min. We assume that the decomposition 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 and the decomposition order, respectively. The weight loss% (%)time (t) plot and d%/dt% plots are shown in Figs. 6 and 7, respectively. For n=1, k values can be estimated from the slope of the linear part of the curves in Fig. 7 for time intervals near the start of the decomposition at just right of the maximum for both polymers. The activation energy Ea of the part region of the decomposition can be calculated from an Arrhenius plot (ln k1/T) since k=A exp(Ea/RT). The value obtained for poly(PAMA) is 58 kJ mol1 which is the activation energy of the depolymerization. This value is comparable to the activation energy reported for the depolymerization of disubstituted vinyl polymers,
M. Cos¸kun et al. / Polymer Degradation and Stability 78 (2002) 49–55
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may easily capture the hydrogen atom on OH in the tautomeric structure of the amide.
5. Conclusion
Fig. 6. Plots of % versus time (min) for poly(PAMA) and poly (BAMA) during isothermal heating.
The thermal degradations of poly(PAMA) and poly (BAMA) were investigated by thermogravimetric analysis (TG) and by programmed heating of the polymer from ambient temperature to 500 C. CRFs, products volatile at degradation temperature but not at ambient temperature, were identified by FT–IR, NMR and GC–MS. The CRFP showed that the monomer and N-phenyl hydroxyacetamide were major products except for very small signals. The CRFB contains only considerable amount of N-benzyl hydroxyacetamide as major product, and there is no monomer in it. In thermal degradation of poly(PAMA), the activation energy of the depolymerization is calculated as 58 kJ mol1, and in that of poly(BAMA) the activation energy of the cyclization is estimated as 47.4 kJ mol1.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] Fig. 7. Plots of d%/dt versus % for isothermal heating of poly (PAMA) and poly(BAMA).
including poly(methyl methacrylate) [1], and polystyrene and substituted polystyrenes [18]. The value estimated as 47.4 kJ mol1 for poly(BAMA) is the activation energy of the cyclic anhydride formation. This value is very much lower than those given for some poly(methacrylate esters) [13,19]. This may be explained by the suggestion that the ester oxygen in the next unit
[12] [13] [14] [15] [16] [17] [18] [19]
Kelen T. Polymer degradation. V.N. Reinhold; 1983. McNeill IC, Cos¸ kun M. Polym Degrad Stab 1987;18:213. McNeill IC, Cos¸ kun M. Polym Degrad Stab 1989;23:1. Rafalco JJ. J Polym Sci Polym Phys 1984;22:1211. Coskun M, Temu¨z MM, Demirelli K. Polym Degrad Stab [in press]. Razga J, Petranak J. Eur Polym J 1975;11:805. Solomon DH. J Macromol Sci-Chem 1982;17:337. Coskun M, Soykan C, Ahmedzade M, Demirelli K. Polym Degrad Stab 2001;72:69. Demirelli K, Coskun M, Kaya E. Polym Degrad Stab 2001;72:75. Hurley SL, Mittleman ML, Wilkie CA. Polym Degrad Stab 1993; 39:345. Coskun M, Erten H, Demirelli K, Ahmedzade M. Polym Degrad Stab 2000;69:245. Grant DH, Grassie N. Polymer 1996;1:125. Penich C, Zaldivar D, Bulay A, Roman JS. Polym Degrad Stab 1989;23:299. Peet NP, Sunder S, Barbuch RJ. J Heterocyc Chem 1982;19:747. O¨zdemir E, Soykan C, Cos¸ kun M, Ahmedzade M. J Macromol Sci-Pure Appl Chem 1997;A34:551. Coskun M, Demirelli K. Polym Degrad Stab 1997;58:187. Demirelli K, Coskun M, Dogru M. Polym Degrad Stab 2000; 69:237. Boinon B, Ainad-Tabet D, Montheard JP. Polym Degrad Stab 1990;28:197. Lin SY, Liao CM, Hsiue GH. Polymer 1996;37(2):269.
Thermal degradation behaviour of two methacrylate polymers with side chain amide groups . Mehmet Cos¸ kun*, Ibrahim Erol, M. Fatih Cos¸ kun, Kadir Demirelli Department of Chemistry, University of Firat, Faculty of Science and Arts, Elazıg, Turkey Received 22 March 2002; accepted 9 April 2002
Abstract 2-Oxo-2-phenylamino)ethylene methacrylate (PAMA) and (2-oxo-2-benzylamino)ethylene methacrylate (BAMA) were prepared by reaction of sodium methacrylate with N-phenyl chloroacetamide and N-benzyl chloroacetamide, respectively, which were obtained from reaction of chloroacetyl chloride with aniline or benzylamine. The thermal degradations of poly(PAMA) and poly(BAMA) prepared by a free-radical procedure were investigated by thermogravimetric analysis (TG) and by programmed heating of the polymer from ambient temperature to 500 C under vacuum, followed by product collection, and using IR spectra of partially degraded polymer. The products volatile at degradation temperature but not at ambient temperature were collected on the cooled upper part of the degradation tube [cold ring fraction, CRFP for poly(PAMA), CRFB for poly(BAMA)]. Product analysis studies showed that only N-benzyl hydroxyacetamide formed as major product in thermal degradation of poly(BAMA) and the monomer did not, whereas in the case of poly(PAMA) N-phenyl hydroxyacetamide and the monomer both formed as major products. For poly(BAMA), the activation energy of cyclization is calculated as 47.4 kJ mol1; for poly(PAMA) the activation energy of the depolymerization is calculated as 58 kJ mol1. The mechanism of thermal degradation including formation of the major products is discussed. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Methacrylate; Amide side chain; Thermal degradation
1. Introduction In polymer degradation, side groups on the main chain are very important to understand how decomposition takes place. Some of the side groups cause degradation without main chain scission. Dehydrochlorination from 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] and, recently, the cyclic imidation reaction giving 2-aminopyridine in the degradation of poly(2-methacrylamidopyridine) [5], may be given as a few typical examples for these processes. Polymers bearing ester side chains, such as poly(methacrylic esters) show different behaviour in thermal degradation depending on the alkyl side chain of the ester. Some poly(methacrylates) degrade relatively easily to the monomer on heating [6–9]. In contrast, some poly(methacrylates) undergo ester decomposition without giving the corresponding mono* Corresponding author. Fax: +90-4242-330062. E-mail address: mcoskun@firat.edu.tr (M. Cos¸ kun).
mer [10,11]. On the other hand, many poly(methacrylates), including ones degrading to the monomer, undergo ester decomposition at some higher temperature followed by elimination of some small molecules and formation of cyclic anhydride structures [8,9,12,13]. The present paper describes thermal degradation behaviours of two poly(methacrylic ester)s having an amide group in the side chain, poly [(2-oxo-2-phenylamino)ethylene methacrylate], poly(PAMA), and poly [(2-oxo-2-benzylamino)ethylene methacrylate], poly (BAMA).
2. Experimental 2.1. Materials Aniline, benzylamine, acetonitrile, dimethyl sulfoxide (DMSO), sodium methacrylate, anhydrous MgSO4 (Aldrich), were used as received. Benzene was dried over anhydrous MgSO4 and then distilled freshly before use. K2CO3 was dried by heating at 110 C. The chloroacetyl chloride (Aldrich) was freshly distilled prior to use.
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2.2. Preparation of the monomers The amide structures discussed here were prepared by aid of a method adapted from the literature [14]. Chloroacetyl chloride (5.5 mmol) was added dropwise to a mixture of aniline (or benzylamine) (5.5 mmol) and K2CO3 (5.5 mmol) in 100 ml dried benzene at 0 C and the reaction stirred at room temperature for 12 h. The precipitate was filtered off, and washed with water, and then dissolved in diethyl ether. After the benzene was evaporated, the residue was added to the ether solution, and it was dried over anhydrous MgSO4. After the ether was evaporated, the solid products were crystallized from ethanol. Yield: above 90% for each product. N-Phenyl chloroacetamide: IR (cm1): 3230 (–NH–), 1680 (N–CO–), 763 (C–Cl) N-Benzyl chloroacetamide: IR(cm1): 3282 (–NH–), 1650 (N–CO–), 788 (C–Cl) 2-Oxo-2-phenylamino)ethylene methacrylate, (PAMA), and (2-oxo-2-benzylamino)ethylene methacrylate, (BAMA), were prepared from the reaction of sodium methacrylate with the corresponding chloroacetamides, as reported previously for a similar reaction of the chloroacetyl group [15]. Both monomers were solid, and they were crystallized before use. Yield: about 80% for each monomer. PAMA, IR (cm1): 3260 (–NH–), 3100–3000 (¼C–H), 1730 (C¼O in ester), 1684 (C¼O, amide-I), 1632 (C¼C), 1600 (C¼C in aromatic ring), 1549 (N–H bending, amide-II) 1 H NMR (ppm, CDCl3): 8.10 (–NH–), 7.10–7.60 (aromatic protons), 6.25 and 5.68 (CH2¼C), 4.75 (–COOCH2), 2.03 (¼C (CH3)–) BAMA, IR (cm1): 3297 (–NH–), 3100–3000 (¼C– H), 1720 (C¼O in ester), 1661 (C¼O, amide-I), 1627 (C¼C, as a shoulder), 1605 (C¼C in aromatic ring), 1529 (N–H bending, amide-II) 1 H NMR (ppm, CDCl3): 7.10–7.80 (–NH– and aromatic protons), 6.15 and 5.60 (CH2¼C), 4.60 (–COOCH2–), 4.35 (Ph–CH2–N), 1.95 (¼C (CH3)–). 2.3. Polymerization Poly(PAMA) and poly(BAMA) were prepared by free-radical polymerization of the corresponding monomers in dimethyl sulfoxide, in a sealed tube in the presence of AIBN under argon atmosphere at 65 C for 12 h. The polymers were precipitated in excess ethanol. After reprecipitating from acetone solution using ethanol, the polymers were dried under vacuum at 50 C for 18 h.
plate and drying in a vacuum oven at 50 C for 12 h 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 and a rotary pump. The polymers were heated at a rate of 10 C/min under vacuum from ambient to 500 C. Products volatile at degradation temperature but not at ambient temperature were collected for each polymer on the cooled upper part of the degradation tube (cold ring fraction, CRF). The CRFs were examined by IR, 1H-NMR and 13C-NMR, and gas chromatography–mass spectrometry (GC–MS). Thermogravimetric measurements were carried out on 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 and a Varian Gemini 200 MHz spectrometer.
3. Results and discussion 3.1. Characterization of the polymers While the IR spectrum of poly(BAMA) shows characteristic bands at 3350 (N–H), 1740 (C¼O), 1660 and 1680 cm1 (as a shoulder) (amide-I) , that of poly (PAMA) has characteristic bands at 3330 (N–H), 1737 (C¼O), 1690 cm1 (amide-I) (Fig. 1). 1H-NMR assignments of the polymers, for poly(BAMA) (CDCl3, 200 MHz spectrometer, ppm): 7.1 (N–H), 7.2–7.4 (aromatic protons), 4.4 (–CO–O–CH2–), 4.1 (Ph–CH2–N), 1.2–2.0 (–CH2– and –CH3), for poly(PAMA) (d6-acetone, 90 MHz, ppm): 9.3 (N–H), 7.0–7.7 (aromatic protons), 4.7 (–CO–O–CH2–), 1.2–2.2 (–CH2– and –CH3). 3.2. Thermogravimetric study The thermogravimetric (TG) curves (Fig. 2) of poly (BAMA) and poly(PAMA), obtained at a heating rate of 10 C/min under nitrogen flow, show some differences from each other. Firstly, it is noticeable that poly(PAMA) is more stable than poly(BAMA) in degradation up to about 400 C, and poly(BAMA) has a decomposition with two stages of which the first is below 340 C with a weight loss of 26%, and the second is above 340 C. Some of the degradation characteristics are summarized in Table 1. 3.3. Changes in IR spectra during degradation of the polymers
2.4. Thermal degradation studies Polymer films were made by dissolving about 5 mg of each polymer in acetone, casting a thin film on a salt
The polymer films prepared on a salt plate were partially degraded under N2 flow at 10 C min1 heating rate to 260, 280, 300, 320, 350, 400 and 425 C. The IR
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M. Cos¸kun et al. / Polymer Degradation and Stability 78 (2002) 49–55
Fig. 2. TGA curves of (a) poly(BAMA) and (b) poly(PAMA).
ture are shown in Fig. 4. This study shows that the formation of cyclic anhydride structures begins together with weight loss in case of poly(BAMA), but in case of poly(PAMA) it begins after a weight loss of about 4%. Whilst the IR spectrum of the reside at 425 C of poly(BAMA) still shows bands characteristic of the cyclic anhydride structures, that of poly(PAMA) does not. Fig. 1. IR spectra of (a) poly(BAMA) and (b) poly(PAMA).
3.4. Product analysis studies spectra of the polymer films heated to 260, 280, 320, 425 C are shown in Fig. 3. There is no change the in the IR spectrum of poly(PAMA) at 260 C from that of the original polymer although decomposition starts according to the TG curve, but at the same temperature the IR spectrum of poly(BAMA) with a weight loss of about 2.5% shows new bands at 1805 and 1018 cm1. At 280 C, the IR spectrum of poly(BAMA) with a weight loss of about 5.8% shows very important changes such as increased absorbance at 1805, 1763 and 1018 cm1 while the spectrum of poly(PAMA) with a weight loss of about 3% is still the same as that of the original polymer. IR spectra (Fig. 3) of both poly(PAMA) and poly(BAMA) heated to 320 C, with a weight loss of 8.1 and 21.2%, respectively, have developed intense bands at 1805, 1763 and 1018 cm1, which are characteristic of cyclic anhydride structures [8,11]. Plots of absorbance of the bands at 1018, 1805 and 1685 cm1 versus tempera-
The degradation was carried out in a system consisting of a degradation tube, with a condenser for product collection a rotary pump. The polymer was heated at 10 C/min under vacuum from ambient temperature to 500 C. The products volatile at degradation temperature but not at ambient temperature were collected on the cooled upper part of the degradation tube [cold ring fraction, CRFB for poly(BAMA), and CRFP for poly (PAMA)] . These fractions were examined by 1H and 13 C-NMR, IR and GC–MS techniques. While there are signals at 8.5; 8.2; 7.9 (–CO–NH–), 7.0–7.6 (aromatic protons), 5.7; 6.3 (CH2¼C) , 4.75 (–COO–CH2–CO–N), 4,70; 4.5; 4.3 (–COO–CH2–), 4.1 (HO–CH2–CO–N–), 1.0–2.2 ppm (aliphatic protons) in 1H-NMR of the CRFP, there are signals at 7.5–7.2 (aromatic protons), 7.1; 6.9 (–CO–NH–), 4.4 (HO–CH2–CO–N–), 4.1 (N– CH2–phenyl), 4.9; 4.6 (COO–CH2– in various posi-
Table 1 Degradation characteristics of poly(BAMA) and poly(PAMA) Polymer
IDTa
Weight loss (%), at 320 C
10% Weight loss, at ( C)
50% Weight loss, at ( C)
Residue at 500 C
Poly(BAMA) Poly(PAMA)
230 250
21.2 8.1
290 328
402 400
7.3 10.6
a
IDT; initial decomposition temperature.
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tions), 1.0–2.4 ppm (aliphatic protons) in that of the CRFB (Fig. 5). It is striking that no olefinic proton signals are observed in the CRFB as they are present in that of the CRFP. 13C-NMR signals (50.4 MHz, solvent: CDCl3), for the CRFB: 173.97 (–CO–N–), 139.80;
131.08; 130.70–129.57 (aromatic and/or olefinic carbons), 64.15 (–OCH2CO–) and 45.02 ppm (–CH2– phenyl). For the CRFP: 172.35; 172.02 (–CO–N– and –COO–), 139.94; 131.04; 126.75; 122.08 (aromatic carbons),
Fig. 3. IR spectra of partially degraded (a) poly(PAMA) and (b) poly(BAMA) after heating to 260, 280, 350 and 425 C.
Fig. 4. Variation of absorbance with temperature of the bands at 1018, 1685, 1805 cm1 of (a) poly(PAMA) and (b) poly(BAMA).
M. Cos¸kun et al. / Polymer Degradation and Stability 78 (2002) 49–55
138.60; 127.20 (olefinic carbons), 65.04; 64.36 (–O– CH2CO–), 20.60 ppm (–CH3). The characteristic bands, in IR spectra of both the CRFP and the CRFB, are at 3200–3350 (O–H and N– H stretching), 3030–3100 (¼C–H stretching), 1605– 1765 (–CO– stretching, in various positions), 1543 (–N– H bending), 763 and 702 cm1 (¼C–H out-of-plane bending in mono substituted aromatic). In the spectrum of CRFP, there are also some different bands at 3140 (– N–H), 1205 (–COO–CH2– stretching), 980; 947; 905 and 860 cm1 (probably, ¼C¼H out-of-plane bending, in vinyl–vinylidene structures). GC–MS investigation of CRFB showed that N-benzyl hydroxyacetamide [retention time, r.t.: 20.3 min, GC peak area: 76.8%, m/e: 165 (M+), 147, 106, 91 (base peak)] formed as a major product, and benzaldimine, N-benzyl acetamide, N-phenyl hydroxyacetamide as minor products, and no monomer was observed. About twenty products whose sum is below 20% could not be
53
identified. In the case of poly(PAMA), the GC–MS investigation of the CRFP indicated that N-phenyl hydroxyacetamide [retention time, r.t.: 19.9 min, GC peak area: 70.2%, m/e: 151 (M+), 120, 93 (base peak), 77] and the monomer [retention time, r.t.: 22.0 min, GC peak area: 19.1%, m/e: 219 (M+), 127, 93 (85%), 77, 69 (base peak), 41] are major products of the degradation. Other products are in minor amounts (> 1%).
4. Mechanism of degradation This study shows that the polymers being discussed behave differently in thermal degradation, especially in the beginning. In degradation of poly(BAMA), the monomer is not detected among the degradation products. The cyclic anhydride structures are observed from temperatures close to the beginning ( 2% weight loss at 260 C) to above 400 C. In contrast, in the case
Fig. 5. 1H-NMR spectra of (a) the CRFB and (b) the CRFP (200 MHz, solvent: CDCl3).
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of poly(PAMA), formation of the cyclic anhydride structures starts at around 290 C ( 4% weight loss). The CRFP collected at 310 C ( 6.5% weight loss) contains 65% of the monomer, although there is 19.08% in total CRFP (weight losses have been given according to the TG curves, but the CRFs and the cyclic anhydride structures have been created in a oven). This means that thermal degradation of poly(PAMA) starts by depolymerization and continues together with formation of the cyclic anhydride structures after about 290 C for some time. Depolymerization followed by formation of cyclic anhydride structures has been observed in thermal degradation of many poly(methacrylate esters) [8,13,16,17]. That thermal degradation starts by formation of cyclic anhydride structures, which is observed in case of poly(BAMA), is a new result. Tautomerization of the amide group of one unit is followed by an intramolecular chain reaction with side chain participation, and this process results in formation of a cyclic anhydride structure and N-benzyl hydroxyacetamide as follows. In the case of poly(PAMA), since a nonbonding pair of electrons on nitrogen combined with the ring may also be moved from the nitrogen to the ring, this decreases tautomerization probability of the amide group of one unit and increases the cyclization temperature. For this reason, thermal degradation of
poly(PAMA), similar to many other poly(methacrylic esters), starts by depolymerization and then continues by cyclization [8,13,16,17]. To estimate the activation energy of thermal decomposition of the polymers at time intervals near the start, isothermal heating at 270, 280, 290 and 300 C for poly(PAMA), and at 230, 240 and 250 C for poly (BAMA), was carried out for 60 min. We assume that the decomposition 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 and the decomposition order, respectively. The weight loss% (%)time (t) plot and d%/dt% plots are shown in Figs. 6 and 7, respectively. For n=1, k values can be estimated from the slope of the linear part of the curves in Fig. 7 for time intervals near the start of the decomposition at just right of the maximum for both polymers. The activation energy Ea of the part region of the decomposition can be calculated from an Arrhenius plot (ln k1/T) since k=A exp(Ea/RT). The value obtained for poly(PAMA) is 58 kJ mol1 which is the activation energy of the depolymerization. This value is comparable to the activation energy reported for the depolymerization of disubstituted vinyl polymers,
M. Cos¸kun et al. / Polymer Degradation and Stability 78 (2002) 49–55
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may easily capture the hydrogen atom on OH in the tautomeric structure of the amide.
5. Conclusion
Fig. 6. Plots of % versus time (min) for poly(PAMA) and poly (BAMA) during isothermal heating.
The thermal degradations of poly(PAMA) and poly (BAMA) were investigated by thermogravimetric analysis (TG) and by programmed heating of the polymer from ambient temperature to 500 C. CRFs, products volatile at degradation temperature but not at ambient temperature, were identified by FT–IR, NMR and GC–MS. The CRFP showed that the monomer and N-phenyl hydroxyacetamide were major products except for very small signals. The CRFB contains only considerable amount of N-benzyl hydroxyacetamide as major product, and there is no monomer in it. In thermal degradation of poly(PAMA), the activation energy of the depolymerization is calculated as 58 kJ mol1, and in that of poly(BAMA) the activation energy of the cyclization is estimated as 47.4 kJ mol1.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] Fig. 7. Plots of d%/dt versus % for isothermal heating of poly (PAMA) and poly(BAMA).
including poly(methyl methacrylate) [1], and polystyrene and substituted polystyrenes [18]. The value estimated as 47.4 kJ mol1 for poly(BAMA) is the activation energy of the cyclic anhydride formation. This value is very much lower than those given for some poly(methacrylate esters) [13,19]. This may be explained by the suggestion that the ester oxygen in the next unit
[12] [13] [14] [15] [16] [17] [18] [19]
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