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Oxidative polymerization of o-phenylenediamine and pyrimidylamine
Polymer Degradation and Stability 71 (2001) 31±38
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Oxidative polymerization of o-phenylenediamine and pyrimidylamine Mei-Rong Huang a, Xin-Gui Li a,*, Yuliang Yang b a
Department of Polymer Materials Science and Engineering, College of Materials Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, People's Republic of China b Department of Macromolecular Science, Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, People's Republic of China Received 10 April 2000; accepted 18 May 2000
Abstract Oxidative polymerization of o-phenylenediamine and 2-pyrimidylamine (MA) was performed by using three molar ratios of ammonium persulfate as an oxidant in boiling glacial acetic acid. The polymerization yield decreases signi®cantly but the intrinsic viscosity of the polymers increases slightly with increasing the feed content of 2-pyrimidylamine. The resulting polymers were characterized by 1H-nuclear magnetic resonance and Fourier transform Infrared spectroscopies, wide-angle X-ray diraction, and thermogravimetry. The results suggested that the polymers are amorphous and exhibit thermal decomposition temperatures higher than 560 C; the maximum weight-loss rate was lower than 3.5%/min, char yield larger than 17 wt.% at 700 C in nitrogen and air. The activation energy of thermal decomposition for the copolymers was 43±53 kJ/mol in nitrogen and air. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Poly(o-phenylenediamine); 2-Pyrimidylamine copolymer; High-temperature oxidative polymerization; NMR spectrum; FTIR spectrum; TG analysis
1. Introduction Chemical oxidative polymerization has aorded various functional polymers. Typical are polyaniline, polytoluidine, polypyrrole, polyaminopyridine, polythiophene, and polyphenylenediamine, showing high conductivity. Among them, polyphenylenediamine homopolymer has attracted attention recently because it has been reported to be a highly aromatic polymer containing 2,3-diaminophenazine or quinoxaline repeat unit and exhibits unusually high thermostability [1±3]. However, the conductivity and solubility of the phenylenediamine homopolymer are low [1±3]. To improve the solubility and solution processability of the o-phenylenediamine (PDA) polymer, copolymerization of o-phenylenediamine(PDA) with 2-pyrimidylamine (MA) might be one of the best methods. However, no report about the copolymerization has been found so far. Investigation of the X-ray * Corresponding author. E-mail address: [email protected] (X.-G. Li).
diraction and thermal degradation of the PDA homopolymer and copolymer with MA has not been also reported. The purpose of this work was to synthesize an easily soluble copolymer through introducing aminopyrimidyl units into the rigid poly(o-phenylenediamine) main chain, to illustrate the variations in yield, intrinsic viscosity, crystallinity, thermostability with monomer ratio, and to elaborate the structural and property improvement produced by copolymerization of PDA with MA in boiling glacial acetic acid with ammonium persulfate as an oxidant. 2. Experimental 2.1. Reagents o-Phenylenediamine, 2-pyrimidylamine, ammonium persulfate, glacial acetic acid, dimethylsulfoxide, and all solvents were commercially obtained and used as received.
0141-3910/01/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(00)00137-3
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M.-R. Huang et al. / Polymer Degradation and Stability 71 (2001) 31±38
2.2. Copolymerization
3. Results and discussion
Polymers were prepared by oxidative polymerization by using a previously described method [1]. A typical procedure for the preparation of the copolymer of ophenylenediamine (PDA)/2-pyrimidylamine (MA) (90/ 10) is as follows: to 150 ml of glacial acetic acid at 40 C was added 1.98 g (0.018 mol) PDA and 0.19 g (0.002 mol) MA in a 500 ml single-neck glass ¯ask. 13.68 g (0.024 mol) ammonium persulfate [(NH4)2S2O8] was dissolved separately in 14 ml water to prepare an oxidant solution. The monomer solution at 40 C was then stirred and treated with the oxidant solution added dropwise at a rate of one drop every 3 s over 30 min ( the total molar ratio: monomer/oxidant=1/3). Immediately, after the ®rst few drops the reaction solution turns blue-violet. The reaction mixture was re¯uxed at 118 C for 72 h. The copolymer acetate was isolated from the reaction mixture by ®ltration and washed with an excess of distilled water to remove the oxidant and oligomers. A blackish violet solid powder was left to dry in ambient air for 1 week. The copolymer of 1.4 g was obtained with the yield of ca. 65%. This copolymer exhibits the following general structure:
3.1. Synthesis of polymers from o-phenylenediamine and 2-pyrimidylamine The copolymerization of PDA and MA with ammonium persulfate as an oxidant in boiling glacial acetic acid aorded black precipitates. PDA and MA monomers with three molar ratios can polymerize at 118 C in 72 h. It was found that the polymerization yield decreased from 96 to 48% with an increase in feed MA content from 0 to 30 mol%, as shown in Table 1. It appears that PDA easily homopolymerizes under the same reaction condition with the highest yield of 96% and PDA is more polymerization reactive than MA. These results suggest that 2-pyrimidylamine and oligo (2-pyrimidylamine)s act as an anticatalyst or deactivator for the polymerization of o-phenylenediamine. This situation is nearly the same as that for the copolymerization of o-phenylenediamine with 2,3-xylidine [4]. Intrinsic viscosity of the copolymers increased with an introduction of MA monomer, as listed in Table 1. The PDA/MA (90/10) copolymer shows the highest intrinsic viscosity of 0.26 dl/g. 3.2. Solubility of the polymers from o-phenylenediamine and 2-pyrimidylamine
2.3. Measurements The intrinsic viscosity for the copolymers in DMSO was measured with an Ubbelodhe viscometer at 25 C. The solubility of the polymers was evaluated using the following method: polymer powder of 5 mg was added to 0.5 ml of solvent and dispersed thoroughly. After the mixture was agitated continuously for 24 h at room temperature, the solubility of the polymers can be characterized semi-quantitatively. 1H-NMR spectra were obtained in deuterated DMSO using a Bruker MSL-300 spectrometer operated at 300.13 MHz. IR spectra were recorded on a Nicolet Magna 550 FT-IR spectrometer made in USA at 2 cmÿ1 resolution on KBr pellets. Wide-angle X-ray diractograms were recorded using Rigaku RAX-10 diractometry with monochromatized CuKa (l=0.1541 nm) radiation operated at 30 kV and 20 mA in a re¯ection mode. The scanning rate was 8 / min. The precision for the measurement of Bragg angle (2) is 0.01 . Thermogravimetry testing was carried out at a heating rate 10 C/min in a nitrogen ¯ow (40 ml/ min) and in a static air with the sample size of 0.9± 1.0 mg using a Netzsch-Geratebau GmbH Thermal Analyzer TG 209.
PDA/MA polymers were almost completely soluble in NMP, DMSO, and tri¯uoroacetic acid at room temperature, but their solubility in other solvents exhibited an apparent composition dependency, as listed in Table 2. Among them, PDA/MA (90/10) copolymer is mainly soluble in tetrahydrofuran and PDA/MA (70/30) mainly soluble in benzene. In fact the solubility of the copolymers in most of the solvents in Table 2 gets better with introducing MA unit in the poly(o-phenylenediamine). Apparently, a good solubility of the copolymers results from the breaking of regular chain structure containing 2,3-diaminophenazine repeat unit. PDA/MA copolymers exhibit both higher intrinsic viscosity and better solubility than PDA homopolymer, suggesting that the solubility is primarily determined by molecular structure rather than intrinsic viscosity of the polymers [4,5]. In addition, better solubility was evidence that the polymerization products were indeed copolymers containing the two monomer units rather than a simple mixture of two homopolymers [6]. 3.3. 1H-NMR spectra of the polymers from o-phenylenediamine and 2-pyrimidylamine 1
H-NMR spectra of the PDA/MA polymers in deuterated DMSO are characterized by three main signals, which should correspond to the three types of protons on the polymer chains, as shown in Fig. 1. The 1H-NMR spectra
M.-R. Huang et al. / Polymer Degradation and Stability 71 (2001) 31±38 33 Table 1 Copolymerization of o-phenylenediamine (PDA) and 2-pyrimidylamine (MA) with monomer:ammonium persulfate of 1:3 in glacial acetic acid at 118 C for 72 h and their thermal stable parameters in a nitrogen ¯ow at sample weight 0.9±1.0 mg Feed PDA/MA (mol%)
Polym. yield (%)
Intrinsic viscosity (dl/g)
Td ( C)
Tdm ( C)
(d /dt)m (%/min)
Char yield at 700 C (wt.%)
E (kJ/mol)
n
ln Z (minÿ1)
In nitrogen 100/0 90/10 70/30
96 65 48
0.12 0.26 0.22
562
677
2.7
39
43
1.0
2.6
567
681
3.4
22
48
0.8
3.5
In air 100/0 70/30
96 48
0.12 0.22
554 540
670 667
2.8 3.5
32 17
53 43
1.3 0.8
4.3 3.1
Table 2 Solubility and solution color of o-phenylenediamine (PDA) and 2-pyrimidylamine (MA) polymers prepared at 118 C in organic solventsa PDA/MA (mol)
NMP
DMSO
TFA
Benzene
THF
Acetic acid
CHCl3
Ethylene chloride
Acetone
CCl4
Ethanol
100/0 90/10 70/30
S S S
S(BR) S(BR) S(BR)
MS(BR) S(BR) S(BR)
PS(B) PS(B) MS(B)
SS(G) MS(B) SS(B)
PS(B) PS(B) SS(B)
SS(B) SS(B) PS(B)
IS SS(G) SS(B)
IS SS(B) IS
SS(G) PS(B) IS
IS IS IS
a
IS, insoluble; MS, mainly soluble; PS, partially soluble; S, soluble; SS, slightly soluble. The letters in the parentheses indicate the color of bipolymer solution in the solvents. B, black; BR, brown; G, grey. The color of the three solid polymers is black.
of PDA/MA (100/0) and (70/30) polymers exhibit the strongest and sharpest three peaks at 6.917±6.926, 7.089, and 7.259±7.266 ppm due to ÿNH2 and ÿNHÿ groups on phenazine unit, the second strongest broad peaks centered at 8.001 and 8.136 ppm ascribed to the aromatic protons on phenazine and pyrimidyl units, and very weak peaks at 1.910± 1.912 ppm due to methyl groups on acetic acid (dopant) because the polymer products have not been dedoped in ammonium hydroxide. There are also two very weak broad peaks centered at 8.9 and 9.8 possibly due to 1,2,4-trisubstituted benzene ring without connecting ÿNH2 groups in a quinoid phenazine unit. The assignments of the NMR peaks of the copolymers are shown in Fig. 1 on the basis of a comparison of integrated peak areas and chemical shift of dierent protons. Although the calculation of the actual PDA/MA molar ratio in PDA/MA (70/30, feed ratio) copolymer seems to be dicult based on its NMR spectrum because this spectrum exhibits low resolution between phenazine and pyrimidyl units, the spectrum of PDA homopolymer is much informative on the evaluation and calculation of number-average degree of polymerization of the PDA homopolymer. According to the results reported previously [1], the macromolecular chain structure could be assumed as follows:
Through a comparison of the integrated area of aromatic ring protons to ÿNH2 and ÿNHÿ protons, the number-average degree of polymerization of PDA homopolymer could be roughly calculated by the following two equations: DPn 2 10 m
1
aromatic proton area NH2 NH proton area 6 50 m 4 21 m: 2 The (DP)n calculated through these two equations is 39 for PDA homopolymer. This (DP)n value agrees substantially with the molecular weight of the PDA homopolymer determined by GPC [1]. Additionally, the 1H-NMR spectrum of PDA homopolymer reported in this article is dierent from that in Ref. [1] because two weak peaks centered at 8.9 and 9.8 ppm were not observed in the spectrum in previous work [1]. In contrast, two small peaks centered at 7.57 and 7.87 ppm reported in Ref. [1] were not observed in the spectrum in this work. Possible explanation is that the molecular weight of the PDA homopolymer prepared in this work is higher than that in Ref. [1], thus causing lower resolution and larger chemical shift.
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M.-R. Huang et al. / Polymer Degradation and Stability 71 (2001) 31±38
Fig. 1. 1H-NMR spectra of polymers with the o-phenylenediamine (PDA)/2-pyrimidylamine (MA) molar ratios of 70/30 [lower, numbers of summation (NS=464) and 100/0 (upper, NS=128)] in DMSO-d6 at 300.13 MHz.
3.4. FT-IR spectra of the polymers from o-phenylenediamine and 2-pyrimidylamine Representative FT-IR spectra for the copolymers with PDA/MA molar ratios of 100/0 and 70/30 are shown in Fig. 2. A broad band centered at 3390 cmÿ1 due to the characteristic free NÿH stretching vibration suggests the presence of secondary amino groups (ÿNHÿ) [3]. A shoulder band at 3200 cmÿ1 is corresponding to the hydrogen-bonded NÿH vibration. A very weak shoulder peak at about 3060 cmÿ1 might be due to CÿH stretching on aromatic phenazine and pyrimidine rings. The two sharp IR adsorptions in 1474±1622 cmÿ1 are associated with aromatic ring stretching. Generally, the peak at 1616± 1622 cmÿ1 is assigned to the following quinoid ring
and 1474±1483 cmÿ1 to the following benzenoid ring.
The peak at 1616±1622 cmÿ1 is sharper but the peak at 1474±1483 cmÿ1 is broader. The fact that the peak at 1616±1622 cmÿ1 has almost the same area as the peak at 1474±1483 cmÿ1 suggests the same quinoid and benzenoid unit contents in the copolymers. A weak peak at 1352 cmÿ1 is attributable to the CÿN stretching vibration in quinoid imine units. Two peaks at 1232±1236 and 612±614 cmÿ1 are ascribed to the CÿN stretching in the benzenoid unit and out-of-plane bending vibration of the CÿH bonds of the 1,2,4-trisubstituted benzene nuclei respectively and get weaker with introducing MA unit. With an increase in MA unit content from 0 mol% to 30 mol%, the IR absorption of the polymers exhibits a reduced intensity at 1114±1117 cmÿ1 and 858±862 cmÿ1 which are due to the CÿH in-plane bending vibration of 1,2,4-trisubstituted benzene ring and CÿH out-of-plane bending vibration of the 1,2,4,5-tetrasubstituted benzene ring respectively. These results indicate that the abovementioned four peaks are indeed attributable to the CÿN and CÿH vibrations of a substituted phenazine ring from PDA monomer. Another two peaks at 1188±1189 and 764 cmÿ1 might be ascribed to the CÿN stretching in the quinoid unit and the CÿH out-of-plane bending of aromatic nuclei respectively and become stronger with an introduction of MA unit. Therefore, these two peaks
M.-R. Huang et al. / Polymer Degradation and Stability 71 (2001) 31±38
35
Fig. 3. Wide-angle X-ray diraction diagrams of the copolymer powders with the o-phenylenediamine (PDA)/2-pyrimidylamine (MA) molar ratios of 100/0 (upper), 90/10 (middle), and 70/30 (lower).
Fig. 2. FT-IR absorption spectra of the copolymers with the o-phenylenediamine (PDA)/2-pyrimidylamine (MA) molar ratios of 100/0 (upper) and 70/30 (lower).
should be due to both aminophenazine unit and aminopyrimidylene unit. 3.5. Wide-angle X-ray diractograms of ophenylenediamine/2-pyrimidylamine polymers Wide-angle X-ray diractograms of three PDA/MA polymer powders are shown in Fig. 3. The broad peak of three diagrams is characteristic of the diraction by an amorphous polymer. With increasing MA content, the PDA/MA polymers exhibit an increased Bragg angle of the major diraction peak and narrowed diffraction peak. Note that the intensity of the diraction peak at low Bragg angle of ca. 3 changed from the strongest to the lowest with introducing MA unit content from 0 to 30 mol%. These results might demonstrate a quite dierent crystalline structure between the PDA homopolymer and PDA/MA copolymers, suggesting that the polymers are actual copolymers of PDA and MA monomer rather than a mixture of two homopolymers. A strongest diraction peak at a low Bragg angle indicates that there is larger crystalline size in the PDA homopolymer, but an introduction of MA unit makes the crystalline size smaller signi®cantly and crystalline order better based on the narrowest major diffraction exhibited by PDA/MA (70/30) copolymer. Introducing MA unit into PDA polymer will decrease both intermolecular chain spacing and the amorphous-
ness as compared with PDA homopolymer because MA unit has smaller spatial size and more regular shape than aminophenazine unit from PDA monomer. 3.6. Thermogravimetry of the polymers from ophenylenediamine and pyrimidylamine Figs. 4 and 5 show the thermogravimetry (TG) and derivative thermogravimetry (DTG) curves of PDA/ MA polymer powders in ¯owing nitrogen and static air respectively. The polymers exhibit two smaller weightloss processes at 60 C and 280±340 C and a single major decomposition starting at temperature of above 500 C in nitrogen and air. The ®rst two losses should be due to the water evaporation and thermal dedoping of the polymers (evolution of acetic acid) respectively. The last larger loss is attributable to the thermal degradation of polymer chains. Table 1 shows the kinetic parameters of the thermal degradation of the polymers. It is found that two thermal decomposition temperatures (Td and Tdm) both increase with introducing MA unit in nitrogen but Td and Tdm decrease slightly in air. These suggest that MA unit is more thermostable in nitrogen but less stable in air than aminophenazine unit. The polymers exhibit an enhanced maximum weight-loss rate (d /dt)m but reduced char yield at 700 C in both atmospheres, indicating MA unit behaves faster degradation rate and lower carbon-forming tendency than aminophenazine unit. It is easily understood that aminophenazine unit possesses higher aromatics than MA unit. The kinetic parameters of the thermal degradation for the polymers are calculated [7±11] through the data in Fig. 6 by the Friedman technique, through Eq. (3)
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M.-R. Huang et al. / Polymer Degradation and Stability 71 (2001) 31±38
Fig. 4. Thermogravimetry (TG) and derivative thermogravimetry (DTG) traces of the polymer powders with the o-phenylenediamine (PDA)/2pyrimidylamine (MA) molar ratios of 70/30 (.....) and 100/0 (Ð) at a heating rate of 10 C/min in a ¯owing nitrogen.
Fig. 5. Thermogravimetry (TG) and derivative thermogravimetry (DTG) curves of the polymer powders with the o-phenylenediamine (PDA)/2pyrimidylamine (MA) molar ratios of 70/30 (.....) and 100/0 (Ð) at a heating rate of 10 C/min in a static air.
ln da=dt ln Z n ln 1 ÿ a ÿ E=RT:
3
It can be seen that there is no regular dependence of activation energy E, decomposition reaction order n, and frequency factor ln Z values on PDA/MA ratio. It should be noticed that the polymers possessing higher decomposition temperatures generally exhibit lower activation energy and frequency factor of decomposition. The acti-
vation energy and frequency factor of thermal decomposition for the copolymers are larger in nitrogen than in air. 3.7. Predicted isothermal thermogravimetric thermograms of the polymers Fig. 7 is the isothermal TG curves at 400 C (673 K) predicted by the following Eqs. (4) and (5) on the basis
M.-R. Huang et al. / Polymer Degradation and Stability 71 (2001) 31±38
of the kinetic parameters listed in Table 1 for the nonisothermal degradation [12±15]. Heating time 1 ÿ 1 ÿ a1ÿn exp E=RT=Z 1 ÿ n n 6 1
4
Heating time ÿln 1 ÿ a exp E=RT=Z n 6 1:
5
The heating time is the lifetime of polymer to failure at the weight loss of . It is found that the predicted TG curves are dierent as expected. The PDA/MA (100/0) polymer exhibits the highest and second highest iso-
37
thermal stability in air and nitrogen, respectively. PDA/ MA (70/30) copolymer shows the third highest and lowest isothermal stability in nitrogen and air respectively. PDA/MA (100/0) polymer will lose 80 wt.% after an isothermally heating time of 340 min in air but PDA/ MA (70/30) copolymer loses 80 wt.% only after a heating time shorter than 125 min in air. These results might suggest that the PDA/MA (100/0) polymer is more isothermally stable in air than in nitrogen. However, a complete contrary situation appeared for the PDA/MA (70/30) copolymer. This obvious dierence of the isothermal stability for the two polymers could be due to their dierence of molecular structure. 4. Conclusions O-phenylenediamine and 2-pyrimidylamine can polymerize to amorphous copolymers with a high yield by an oxidation polymerization method for 72 h at 118 C in boiling glacial acetic acid. The polymers consist of aminophenazine units with quinoid and benzenoid formula. The thermostability and decomposition rate of the polymers appear to increase in nitrogen with introducing MA unit. The decomposition temperatures of the polymers decrease slightly in air with adding MA unit. The PDA/MA copolymer shows higher decomposition temperatures, larger char yield at 700 C, slower maximum weight-loss rate, and higher activation energy of decomposition in nitrogen than in air.
Fig. 6. Friedman method (plot of d /dt vs 1/T) for the calculation of the activation energy of the thermal degradation of polymer powders with o-phenylenediamine (PDA)/2-pyrimidylamine (MA) molar ratios of 100/0 (*) in nitrogen, 100/0 (~) in air, 70/30 (!) in nitrogen, and 70/30 (&) in air.
Acknowledgements The project is supported (1) by the National Natural Science Foundation of China (29804008), (2) by the Foundation of Key Teachers in the University of Chinese Ministry of Education, (3) by the Phosphor Plan of Science Technology of Young Scientists of Shanghai China (98QE14027), (4) by the Foundation for Visiting Scholars of the Key Laboratory of Molecular Engineering of Polymers at Fudan University of Chinese Ministry of Education, and (5) also by the State Key Laboratory for Modi®cation of Chemical Fibres and Polymer Materials at East China University in Shanghai China. References
Fig. 7. Calculated isothermal TG curves for the copolymers with the o-phenylenediamine (PDA)/2-pyrimidylamine (MA) molar ratios of 100/0 (*) and 70/30 (~) in nitrogen, and 100/0 (*) and 70/30 (~) in a static air based on the kinetic parameters listed in Table 1.
[1] Premasiri AH, Euler WB. Macromol Chem Phys 1995;196:3655. [2] Chan HSO, Ng SC, Hor TSA, Sun J, Tan KL, Tan BTG. Eur Polym J 1991;27:1303. [3] Cataldo F. Eur Polym J 1996;32:43. [4] Li X-G, Huang M-R, Yang Y. Polymer, in press. [5] Ichinohe D, Muranaka T, Sasaki T, Kobayashi M, Kise H. J Polym Sci Part A Polym Chem 1998;36:2593.
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M.-R. Huang et al. / Polymer Degradation and Stability 71 (2001) 31±38
[6] Conklin JA, Huang SC, Huang SM, Wen T, Kaner RB. Macromolecules 1995;28:6522. [7] Friedman HL. J Polym Sci Part C 1964;6:183. [8] Li X-G. Polym Degrad Stab 1999;65:473. [9] Li X-G, Huang M-R. J Macromol Sci Pure Appl Chem 1999; A36:859.
[10] [11] [12] [13] [14] [15]
Li X-G, Huang M-R, Bai H. J Appl Polym Sci 1999;73:2927. Li X-G, Huang M-R, Yang Y. Polym Int 1999;48: 1277. Huang M-R, Li X-G. J Appl Polym Sci 1998;68:293. Li X-G, Huang M-R. J Appl Polym Sci 1999;73:2911. Chang WL. J Appl Polym Sci 1994;53:1759. Li X-G, Huang M-R, Yang Y. Polym J 2000;32:348.
www.elsevier.nl/locate/polydegstab
Oxidative polymerization of o-phenylenediamine and pyrimidylamine Mei-Rong Huang a, Xin-Gui Li a,*, Yuliang Yang b a
Department of Polymer Materials Science and Engineering, College of Materials Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, People's Republic of China b Department of Macromolecular Science, Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, People's Republic of China Received 10 April 2000; accepted 18 May 2000
Abstract Oxidative polymerization of o-phenylenediamine and 2-pyrimidylamine (MA) was performed by using three molar ratios of ammonium persulfate as an oxidant in boiling glacial acetic acid. The polymerization yield decreases signi®cantly but the intrinsic viscosity of the polymers increases slightly with increasing the feed content of 2-pyrimidylamine. The resulting polymers were characterized by 1H-nuclear magnetic resonance and Fourier transform Infrared spectroscopies, wide-angle X-ray diraction, and thermogravimetry. The results suggested that the polymers are amorphous and exhibit thermal decomposition temperatures higher than 560 C; the maximum weight-loss rate was lower than 3.5%/min, char yield larger than 17 wt.% at 700 C in nitrogen and air. The activation energy of thermal decomposition for the copolymers was 43±53 kJ/mol in nitrogen and air. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Poly(o-phenylenediamine); 2-Pyrimidylamine copolymer; High-temperature oxidative polymerization; NMR spectrum; FTIR spectrum; TG analysis
1. Introduction Chemical oxidative polymerization has aorded various functional polymers. Typical are polyaniline, polytoluidine, polypyrrole, polyaminopyridine, polythiophene, and polyphenylenediamine, showing high conductivity. Among them, polyphenylenediamine homopolymer has attracted attention recently because it has been reported to be a highly aromatic polymer containing 2,3-diaminophenazine or quinoxaline repeat unit and exhibits unusually high thermostability [1±3]. However, the conductivity and solubility of the phenylenediamine homopolymer are low [1±3]. To improve the solubility and solution processability of the o-phenylenediamine (PDA) polymer, copolymerization of o-phenylenediamine(PDA) with 2-pyrimidylamine (MA) might be one of the best methods. However, no report about the copolymerization has been found so far. Investigation of the X-ray * Corresponding author. E-mail address: [email protected] (X.-G. Li).
diraction and thermal degradation of the PDA homopolymer and copolymer with MA has not been also reported. The purpose of this work was to synthesize an easily soluble copolymer through introducing aminopyrimidyl units into the rigid poly(o-phenylenediamine) main chain, to illustrate the variations in yield, intrinsic viscosity, crystallinity, thermostability with monomer ratio, and to elaborate the structural and property improvement produced by copolymerization of PDA with MA in boiling glacial acetic acid with ammonium persulfate as an oxidant. 2. Experimental 2.1. Reagents o-Phenylenediamine, 2-pyrimidylamine, ammonium persulfate, glacial acetic acid, dimethylsulfoxide, and all solvents were commercially obtained and used as received.
0141-3910/01/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(00)00137-3
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M.-R. Huang et al. / Polymer Degradation and Stability 71 (2001) 31±38
2.2. Copolymerization
3. Results and discussion
Polymers were prepared by oxidative polymerization by using a previously described method [1]. A typical procedure for the preparation of the copolymer of ophenylenediamine (PDA)/2-pyrimidylamine (MA) (90/ 10) is as follows: to 150 ml of glacial acetic acid at 40 C was added 1.98 g (0.018 mol) PDA and 0.19 g (0.002 mol) MA in a 500 ml single-neck glass ¯ask. 13.68 g (0.024 mol) ammonium persulfate [(NH4)2S2O8] was dissolved separately in 14 ml water to prepare an oxidant solution. The monomer solution at 40 C was then stirred and treated with the oxidant solution added dropwise at a rate of one drop every 3 s over 30 min ( the total molar ratio: monomer/oxidant=1/3). Immediately, after the ®rst few drops the reaction solution turns blue-violet. The reaction mixture was re¯uxed at 118 C for 72 h. The copolymer acetate was isolated from the reaction mixture by ®ltration and washed with an excess of distilled water to remove the oxidant and oligomers. A blackish violet solid powder was left to dry in ambient air for 1 week. The copolymer of 1.4 g was obtained with the yield of ca. 65%. This copolymer exhibits the following general structure:
3.1. Synthesis of polymers from o-phenylenediamine and 2-pyrimidylamine The copolymerization of PDA and MA with ammonium persulfate as an oxidant in boiling glacial acetic acid aorded black precipitates. PDA and MA monomers with three molar ratios can polymerize at 118 C in 72 h. It was found that the polymerization yield decreased from 96 to 48% with an increase in feed MA content from 0 to 30 mol%, as shown in Table 1. It appears that PDA easily homopolymerizes under the same reaction condition with the highest yield of 96% and PDA is more polymerization reactive than MA. These results suggest that 2-pyrimidylamine and oligo (2-pyrimidylamine)s act as an anticatalyst or deactivator for the polymerization of o-phenylenediamine. This situation is nearly the same as that for the copolymerization of o-phenylenediamine with 2,3-xylidine [4]. Intrinsic viscosity of the copolymers increased with an introduction of MA monomer, as listed in Table 1. The PDA/MA (90/10) copolymer shows the highest intrinsic viscosity of 0.26 dl/g. 3.2. Solubility of the polymers from o-phenylenediamine and 2-pyrimidylamine
2.3. Measurements The intrinsic viscosity for the copolymers in DMSO was measured with an Ubbelodhe viscometer at 25 C. The solubility of the polymers was evaluated using the following method: polymer powder of 5 mg was added to 0.5 ml of solvent and dispersed thoroughly. After the mixture was agitated continuously for 24 h at room temperature, the solubility of the polymers can be characterized semi-quantitatively. 1H-NMR spectra were obtained in deuterated DMSO using a Bruker MSL-300 spectrometer operated at 300.13 MHz. IR spectra were recorded on a Nicolet Magna 550 FT-IR spectrometer made in USA at 2 cmÿ1 resolution on KBr pellets. Wide-angle X-ray diractograms were recorded using Rigaku RAX-10 diractometry with monochromatized CuKa (l=0.1541 nm) radiation operated at 30 kV and 20 mA in a re¯ection mode. The scanning rate was 8 / min. The precision for the measurement of Bragg angle (2) is 0.01 . Thermogravimetry testing was carried out at a heating rate 10 C/min in a nitrogen ¯ow (40 ml/ min) and in a static air with the sample size of 0.9± 1.0 mg using a Netzsch-Geratebau GmbH Thermal Analyzer TG 209.
PDA/MA polymers were almost completely soluble in NMP, DMSO, and tri¯uoroacetic acid at room temperature, but their solubility in other solvents exhibited an apparent composition dependency, as listed in Table 2. Among them, PDA/MA (90/10) copolymer is mainly soluble in tetrahydrofuran and PDA/MA (70/30) mainly soluble in benzene. In fact the solubility of the copolymers in most of the solvents in Table 2 gets better with introducing MA unit in the poly(o-phenylenediamine). Apparently, a good solubility of the copolymers results from the breaking of regular chain structure containing 2,3-diaminophenazine repeat unit. PDA/MA copolymers exhibit both higher intrinsic viscosity and better solubility than PDA homopolymer, suggesting that the solubility is primarily determined by molecular structure rather than intrinsic viscosity of the polymers [4,5]. In addition, better solubility was evidence that the polymerization products were indeed copolymers containing the two monomer units rather than a simple mixture of two homopolymers [6]. 3.3. 1H-NMR spectra of the polymers from o-phenylenediamine and 2-pyrimidylamine 1
H-NMR spectra of the PDA/MA polymers in deuterated DMSO are characterized by three main signals, which should correspond to the three types of protons on the polymer chains, as shown in Fig. 1. The 1H-NMR spectra
M.-R. Huang et al. / Polymer Degradation and Stability 71 (2001) 31±38 33 Table 1 Copolymerization of o-phenylenediamine (PDA) and 2-pyrimidylamine (MA) with monomer:ammonium persulfate of 1:3 in glacial acetic acid at 118 C for 72 h and their thermal stable parameters in a nitrogen ¯ow at sample weight 0.9±1.0 mg Feed PDA/MA (mol%)
Polym. yield (%)
Intrinsic viscosity (dl/g)
Td ( C)
Tdm ( C)
(d /dt)m (%/min)
Char yield at 700 C (wt.%)
E (kJ/mol)
n
ln Z (minÿ1)
In nitrogen 100/0 90/10 70/30
96 65 48
0.12 0.26 0.22
562
677
2.7
39
43
1.0
2.6
567
681
3.4
22
48
0.8
3.5
In air 100/0 70/30
96 48
0.12 0.22
554 540
670 667
2.8 3.5
32 17
53 43
1.3 0.8
4.3 3.1
Table 2 Solubility and solution color of o-phenylenediamine (PDA) and 2-pyrimidylamine (MA) polymers prepared at 118 C in organic solventsa PDA/MA (mol)
NMP
DMSO
TFA
Benzene
THF
Acetic acid
CHCl3
Ethylene chloride
Acetone
CCl4
Ethanol
100/0 90/10 70/30
S S S
S(BR) S(BR) S(BR)
MS(BR) S(BR) S(BR)
PS(B) PS(B) MS(B)
SS(G) MS(B) SS(B)
PS(B) PS(B) SS(B)
SS(B) SS(B) PS(B)
IS SS(G) SS(B)
IS SS(B) IS
SS(G) PS(B) IS
IS IS IS
a
IS, insoluble; MS, mainly soluble; PS, partially soluble; S, soluble; SS, slightly soluble. The letters in the parentheses indicate the color of bipolymer solution in the solvents. B, black; BR, brown; G, grey. The color of the three solid polymers is black.
of PDA/MA (100/0) and (70/30) polymers exhibit the strongest and sharpest three peaks at 6.917±6.926, 7.089, and 7.259±7.266 ppm due to ÿNH2 and ÿNHÿ groups on phenazine unit, the second strongest broad peaks centered at 8.001 and 8.136 ppm ascribed to the aromatic protons on phenazine and pyrimidyl units, and very weak peaks at 1.910± 1.912 ppm due to methyl groups on acetic acid (dopant) because the polymer products have not been dedoped in ammonium hydroxide. There are also two very weak broad peaks centered at 8.9 and 9.8 possibly due to 1,2,4-trisubstituted benzene ring without connecting ÿNH2 groups in a quinoid phenazine unit. The assignments of the NMR peaks of the copolymers are shown in Fig. 1 on the basis of a comparison of integrated peak areas and chemical shift of dierent protons. Although the calculation of the actual PDA/MA molar ratio in PDA/MA (70/30, feed ratio) copolymer seems to be dicult based on its NMR spectrum because this spectrum exhibits low resolution between phenazine and pyrimidyl units, the spectrum of PDA homopolymer is much informative on the evaluation and calculation of number-average degree of polymerization of the PDA homopolymer. According to the results reported previously [1], the macromolecular chain structure could be assumed as follows:
Through a comparison of the integrated area of aromatic ring protons to ÿNH2 and ÿNHÿ protons, the number-average degree of polymerization of PDA homopolymer could be roughly calculated by the following two equations: DPn 2 10 m
1
aromatic proton area NH2 NH proton area 6 50 m 4 21 m: 2 The (DP)n calculated through these two equations is 39 for PDA homopolymer. This (DP)n value agrees substantially with the molecular weight of the PDA homopolymer determined by GPC [1]. Additionally, the 1H-NMR spectrum of PDA homopolymer reported in this article is dierent from that in Ref. [1] because two weak peaks centered at 8.9 and 9.8 ppm were not observed in the spectrum in previous work [1]. In contrast, two small peaks centered at 7.57 and 7.87 ppm reported in Ref. [1] were not observed in the spectrum in this work. Possible explanation is that the molecular weight of the PDA homopolymer prepared in this work is higher than that in Ref. [1], thus causing lower resolution and larger chemical shift.
34
M.-R. Huang et al. / Polymer Degradation and Stability 71 (2001) 31±38
Fig. 1. 1H-NMR spectra of polymers with the o-phenylenediamine (PDA)/2-pyrimidylamine (MA) molar ratios of 70/30 [lower, numbers of summation (NS=464) and 100/0 (upper, NS=128)] in DMSO-d6 at 300.13 MHz.
3.4. FT-IR spectra of the polymers from o-phenylenediamine and 2-pyrimidylamine Representative FT-IR spectra for the copolymers with PDA/MA molar ratios of 100/0 and 70/30 are shown in Fig. 2. A broad band centered at 3390 cmÿ1 due to the characteristic free NÿH stretching vibration suggests the presence of secondary amino groups (ÿNHÿ) [3]. A shoulder band at 3200 cmÿ1 is corresponding to the hydrogen-bonded NÿH vibration. A very weak shoulder peak at about 3060 cmÿ1 might be due to CÿH stretching on aromatic phenazine and pyrimidine rings. The two sharp IR adsorptions in 1474±1622 cmÿ1 are associated with aromatic ring stretching. Generally, the peak at 1616± 1622 cmÿ1 is assigned to the following quinoid ring
and 1474±1483 cmÿ1 to the following benzenoid ring.
The peak at 1616±1622 cmÿ1 is sharper but the peak at 1474±1483 cmÿ1 is broader. The fact that the peak at 1616±1622 cmÿ1 has almost the same area as the peak at 1474±1483 cmÿ1 suggests the same quinoid and benzenoid unit contents in the copolymers. A weak peak at 1352 cmÿ1 is attributable to the CÿN stretching vibration in quinoid imine units. Two peaks at 1232±1236 and 612±614 cmÿ1 are ascribed to the CÿN stretching in the benzenoid unit and out-of-plane bending vibration of the CÿH bonds of the 1,2,4-trisubstituted benzene nuclei respectively and get weaker with introducing MA unit. With an increase in MA unit content from 0 mol% to 30 mol%, the IR absorption of the polymers exhibits a reduced intensity at 1114±1117 cmÿ1 and 858±862 cmÿ1 which are due to the CÿH in-plane bending vibration of 1,2,4-trisubstituted benzene ring and CÿH out-of-plane bending vibration of the 1,2,4,5-tetrasubstituted benzene ring respectively. These results indicate that the abovementioned four peaks are indeed attributable to the CÿN and CÿH vibrations of a substituted phenazine ring from PDA monomer. Another two peaks at 1188±1189 and 764 cmÿ1 might be ascribed to the CÿN stretching in the quinoid unit and the CÿH out-of-plane bending of aromatic nuclei respectively and become stronger with an introduction of MA unit. Therefore, these two peaks
M.-R. Huang et al. / Polymer Degradation and Stability 71 (2001) 31±38
35
Fig. 3. Wide-angle X-ray diraction diagrams of the copolymer powders with the o-phenylenediamine (PDA)/2-pyrimidylamine (MA) molar ratios of 100/0 (upper), 90/10 (middle), and 70/30 (lower).
Fig. 2. FT-IR absorption spectra of the copolymers with the o-phenylenediamine (PDA)/2-pyrimidylamine (MA) molar ratios of 100/0 (upper) and 70/30 (lower).
should be due to both aminophenazine unit and aminopyrimidylene unit. 3.5. Wide-angle X-ray diractograms of ophenylenediamine/2-pyrimidylamine polymers Wide-angle X-ray diractograms of three PDA/MA polymer powders are shown in Fig. 3. The broad peak of three diagrams is characteristic of the diraction by an amorphous polymer. With increasing MA content, the PDA/MA polymers exhibit an increased Bragg angle of the major diraction peak and narrowed diffraction peak. Note that the intensity of the diraction peak at low Bragg angle of ca. 3 changed from the strongest to the lowest with introducing MA unit content from 0 to 30 mol%. These results might demonstrate a quite dierent crystalline structure between the PDA homopolymer and PDA/MA copolymers, suggesting that the polymers are actual copolymers of PDA and MA monomer rather than a mixture of two homopolymers. A strongest diraction peak at a low Bragg angle indicates that there is larger crystalline size in the PDA homopolymer, but an introduction of MA unit makes the crystalline size smaller signi®cantly and crystalline order better based on the narrowest major diffraction exhibited by PDA/MA (70/30) copolymer. Introducing MA unit into PDA polymer will decrease both intermolecular chain spacing and the amorphous-
ness as compared with PDA homopolymer because MA unit has smaller spatial size and more regular shape than aminophenazine unit from PDA monomer. 3.6. Thermogravimetry of the polymers from ophenylenediamine and pyrimidylamine Figs. 4 and 5 show the thermogravimetry (TG) and derivative thermogravimetry (DTG) curves of PDA/ MA polymer powders in ¯owing nitrogen and static air respectively. The polymers exhibit two smaller weightloss processes at 60 C and 280±340 C and a single major decomposition starting at temperature of above 500 C in nitrogen and air. The ®rst two losses should be due to the water evaporation and thermal dedoping of the polymers (evolution of acetic acid) respectively. The last larger loss is attributable to the thermal degradation of polymer chains. Table 1 shows the kinetic parameters of the thermal degradation of the polymers. It is found that two thermal decomposition temperatures (Td and Tdm) both increase with introducing MA unit in nitrogen but Td and Tdm decrease slightly in air. These suggest that MA unit is more thermostable in nitrogen but less stable in air than aminophenazine unit. The polymers exhibit an enhanced maximum weight-loss rate (d /dt)m but reduced char yield at 700 C in both atmospheres, indicating MA unit behaves faster degradation rate and lower carbon-forming tendency than aminophenazine unit. It is easily understood that aminophenazine unit possesses higher aromatics than MA unit. The kinetic parameters of the thermal degradation for the polymers are calculated [7±11] through the data in Fig. 6 by the Friedman technique, through Eq. (3)
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M.-R. Huang et al. / Polymer Degradation and Stability 71 (2001) 31±38
Fig. 4. Thermogravimetry (TG) and derivative thermogravimetry (DTG) traces of the polymer powders with the o-phenylenediamine (PDA)/2pyrimidylamine (MA) molar ratios of 70/30 (.....) and 100/0 (Ð) at a heating rate of 10 C/min in a ¯owing nitrogen.
Fig. 5. Thermogravimetry (TG) and derivative thermogravimetry (DTG) curves of the polymer powders with the o-phenylenediamine (PDA)/2pyrimidylamine (MA) molar ratios of 70/30 (.....) and 100/0 (Ð) at a heating rate of 10 C/min in a static air.
ln da=dt ln Z n ln 1 ÿ a ÿ E=RT:
3
It can be seen that there is no regular dependence of activation energy E, decomposition reaction order n, and frequency factor ln Z values on PDA/MA ratio. It should be noticed that the polymers possessing higher decomposition temperatures generally exhibit lower activation energy and frequency factor of decomposition. The acti-
vation energy and frequency factor of thermal decomposition for the copolymers are larger in nitrogen than in air. 3.7. Predicted isothermal thermogravimetric thermograms of the polymers Fig. 7 is the isothermal TG curves at 400 C (673 K) predicted by the following Eqs. (4) and (5) on the basis
M.-R. Huang et al. / Polymer Degradation and Stability 71 (2001) 31±38
of the kinetic parameters listed in Table 1 for the nonisothermal degradation [12±15]. Heating time 1 ÿ 1 ÿ a1ÿn exp E=RT=Z 1 ÿ n n 6 1
4
Heating time ÿln 1 ÿ a exp E=RT=Z n 6 1:
5
The heating time is the lifetime of polymer to failure at the weight loss of . It is found that the predicted TG curves are dierent as expected. The PDA/MA (100/0) polymer exhibits the highest and second highest iso-
37
thermal stability in air and nitrogen, respectively. PDA/ MA (70/30) copolymer shows the third highest and lowest isothermal stability in nitrogen and air respectively. PDA/MA (100/0) polymer will lose 80 wt.% after an isothermally heating time of 340 min in air but PDA/ MA (70/30) copolymer loses 80 wt.% only after a heating time shorter than 125 min in air. These results might suggest that the PDA/MA (100/0) polymer is more isothermally stable in air than in nitrogen. However, a complete contrary situation appeared for the PDA/MA (70/30) copolymer. This obvious dierence of the isothermal stability for the two polymers could be due to their dierence of molecular structure. 4. Conclusions O-phenylenediamine and 2-pyrimidylamine can polymerize to amorphous copolymers with a high yield by an oxidation polymerization method for 72 h at 118 C in boiling glacial acetic acid. The polymers consist of aminophenazine units with quinoid and benzenoid formula. The thermostability and decomposition rate of the polymers appear to increase in nitrogen with introducing MA unit. The decomposition temperatures of the polymers decrease slightly in air with adding MA unit. The PDA/MA copolymer shows higher decomposition temperatures, larger char yield at 700 C, slower maximum weight-loss rate, and higher activation energy of decomposition in nitrogen than in air.
Fig. 6. Friedman method (plot of d /dt vs 1/T) for the calculation of the activation energy of the thermal degradation of polymer powders with o-phenylenediamine (PDA)/2-pyrimidylamine (MA) molar ratios of 100/0 (*) in nitrogen, 100/0 (~) in air, 70/30 (!) in nitrogen, and 70/30 (&) in air.
Acknowledgements The project is supported (1) by the National Natural Science Foundation of China (29804008), (2) by the Foundation of Key Teachers in the University of Chinese Ministry of Education, (3) by the Phosphor Plan of Science Technology of Young Scientists of Shanghai China (98QE14027), (4) by the Foundation for Visiting Scholars of the Key Laboratory of Molecular Engineering of Polymers at Fudan University of Chinese Ministry of Education, and (5) also by the State Key Laboratory for Modi®cation of Chemical Fibres and Polymer Materials at East China University in Shanghai China. References
Fig. 7. Calculated isothermal TG curves for the copolymers with the o-phenylenediamine (PDA)/2-pyrimidylamine (MA) molar ratios of 100/0 (*) and 70/30 (~) in nitrogen, and 100/0 (*) and 70/30 (~) in a static air based on the kinetic parameters listed in Table 1.
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