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pyromellitic acid
European Polymer Journal 39 (2003) 2161–2165 www.elsevier.com/locate/europolj
Reverse atom transfer radical polymerization of methyl methacrylate with FeCl3/pyromellitic acid Gang Wang, Xiulin Zhu *, Zhenping Cheng, Jian Zhu School of Chemistry and Chemical Engineering, Suzhou University, 1 Shizi Street, Suzhou 215006, PR China Received 3 April 2003; received in revised form 29 May 2003; accepted 30 May 2003
Abstract IronIII chloride coordinated by pyromellitic acid was successfully used as the catalytic system in reverse atom transfer radical polymerization of MMA. Well-defined poly(methyl methacrylate) with narrow molecular weight distribution was synthesized in N,N-dimethylformamide at 80–110 C. Chain extension was performed to confirm the living nature of the polymer. The presence of the end chloride atom on the resulting PMMA was demonstrated by 1 HNMR spectroscopy. This catalyst system is effective for reverse ATRP of methacrylates but not for acrylates. 2003 Elsevier Ltd. All rights reserved. Keywords: Reverse atom transfer radical polymerization; Methyl methacrylate; Pyromellitic acid
1. Introduction Interest in living/controlled free radical polymerization has been stimulated to a great extent by the impressive progress made in several methods such as atom transfer radical polymerization (ATRP), nitroxyl radical mediated polymerization (NMP), reversible addition fragmentation transfer polymerization (RAFT), and in the field of synthetic well-defined polymers. Research groups such as those of Matyjaszewski [1], Sawamoto [2], Percec [3], Rizzardo [4,5] have successfully introduced these approachs into polymerization chemistry as a novel ÔlivingÕ/controlled free radical polymerization. By far the most successful catalysts have been those based upon electron-donating compounds such as bipyridine (bipy) [1], N-alkyl-2-pyridylmethanimines (NPMI) [6], N,N,N0 ,N0 -tetramethylethylenediamine (TMEDA) [7] and triphenylphosphine (PPh3 ) [8] as metal complex ligands. However, the catalyst system of ATRP is generally toxic and expensive and the catalyst such as CuCl is unstable under air and moisture. To overcome these
*
Corresponding author. Tel./fax: +86-512-65112796. E-mail address: [email protected] (X. Zhu).
shortcomings, a so-called reverse ATRP with 2,20 -azobis-isobutyronitrile (AIBN)/CuBr2 /bipy [9] and AIBN/ CuBr2 /dNbipy [10] as the initiating systems have been developed. In this process, a higher oxidation state transition-metal species Mtnþ1 X =Lx and a conventional radical initiator were used instead of a lower oxidation state Mtn =Lx and halide species RX, respectively. Teyssie and coworkers [11] reported the reverse ATRP of MMA using FeCl3 and AIBN in the presence of PPh3 . The ligands in iron-mediated reverse ATRP are usually bipy [12] and PPh3 [11]. Recently, Matyjaszewski and coworkers [13] reported that the reverse ATRP initiated by AIBN/FeBr3 /onium salts, which led to a controlled polymerization of both MMA and MA, while for styrene uncontrolled molecular weights and high polydispersities were obtained. Yan and coworkers [14] reported the reverse ATRP of MMA initiated by AIBN/FeCl3 / isophthalic acid, the polymerization was controlled up to a molecular weight of 50,000, and the polydisperity index is 1.4, the observed molecular weight was higher than the theoretical value, which indicates the relatively low initiator efficiency. In this work we investigated a new catalytic system based on iron complexes with pyromellitic acid for reverse ATRP of MMA in the presence of a conventional
0014-3057/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0014-3057(03)00137-X
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G. Wang et al. / European Polymer Journal 39 (2003) 2161–2165
radical initiator, AIBN. Pyromellitic acid is a quaternary carboxylic acid. Deprotonation gives anions with 1–4 negative charges, which can act as multidentate ligands. This catalyst system is effective for methylates but not for acrylates.
added to the sample to dissolve the polymer. PMMA was isolated by precipitation in hexane and dried in vacuum at 25 C for 48 h.
3. Results and discussion
Methyl methacrylate, N-butyl methacrylate, N-butyl acrylate, Octyl acrylate (chemically pure, Shanghai Chemical Reagent Co., Ltd.) were purified by extracting with 5% sodium hydroxide aqueous solution, followed by washing with water and dried with sodium sulfate anhydrous for overnight, finally distillated under vacuum. N,N-dimethylformamide (DMF) (analytical reagent, Shanghai No. 1 Chemical Reagent Factory) was distillated at reduced pressure. AIBN (chemically pure, Shanghai No. 4 Reagent Factory) was recrystallized from ethanol. FeCl3 (Jinshan Chemical Co.; analytical-reagent-grade, anhydrous), pyromellitic acid (Chang shu Lianbang Chemical Co.; 99.5%), tetrahydrofuran (THF) (analytical reagent, Shanghai Chemical Reagent Co., Ltd.) were used without further purification. 2.2. Characterization Conversion of monomer was determined by gravimetry. Molecular weights and molecular weight distributions were measured by Waters 1515 GPC, equipped with microstyragel columns (HR1, HR3, HR4) with THF as a mobile phase at a flow rate of 1 cm3 /min operated at 30 C. Polystyrene standards were used. Thus all molecular weights should be considered as polystyrene equivalent molecular weights. Calibration of the instrument was performed with standard samples of poly(styrene) between 5 · 105 and 1 · 102 g mol1 . 1 H NMR spectra were recorded in CDCl3 with an INOVA 400 MHz spectrometer at ambient temperature.
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
1
2
3
4
5
6
7
8
Time(h)
Fig. 1. Kinetic plot for the reverse ATRP of MMA. [MMA]0 ¼ 6.31 mol/l, [MMA]0 :[AIBN]0 :[FeCl3 ]0 :[pyromellitic acid]0 ¼ 500:1:3:6, T ¼ 100 C, VðMMAÞ =VðDMFÞ ¼ 2=1 ðV=VÞ.
2.2 25000
20000
The general procedure of the polymerization was as follows: FeCl3 (0.0378 mol/l), pyromellitic acid (0.0756 mol/l) and N,N-dimethylformamide were added to an ampule tube under stirring, three cycles of vacuumnitrogen were applied in order to remove the oxygen. After the catalyst was dissolved (about 10 min), MMA (6.31 mol/l) with AIBN (0.0126 mol/l) dissolved in advance was added via an argon-washed syringe. Then the ampule tube was sealed under nitrogen and placed in an oil bath thermostated at the desired temperature. After a given time, the tube was broken open, and THF was
15000
2.0
Mw /Mn ----Mn,th
1.8
Mn
2.3. Polymerization
Mn(GPC)
1.6 10000
Mw /Mn
2.1. Materials
MMA was polymerized in N,N-dimethylformamide using FeCl3 /pyromellitic acid as catalyst and AIBN as the initiator at 100 C ([MMA]0 /[AIBN]0 /[FeCl3 ]0 / [pyromellitic acid]0 ¼ 500/1/3/6). The kinetic plot of ln½M0 =½M versus time is shown in Fig. 1. The straight line through original point indicates that the concentration of propagating radicals remained constant during the polymerization. The polymerization showed that the molecular weights increased linearly with conversion and they were close to the predicted values (Fig. 2). The polydispersity index of the resulting polymer decreased from 1.63 to 1.28 in the
ln[M] 0 /[M]
2. Experimental section
1.4
5000
1.2
0 0
10
20
30
40
50
60
70
80
90
1.0 100
conversion(%)
Fig. 2. Dependence of molecular weights and polydispersities on monomer conversion for the reverse ATRP of MMA. Experimental conditions are the same as in Fig. 1.
G. Wang et al. / European Polymer Journal 39 (2003) 2161–2165
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Table 1 Polymerization of MMA by reverse ATRP using AIBN as the initiator Entry
[I]0 /[FeCl3 ]0 / [pyromellitic acid]0
Time (h)
Conversion (%)
Mn;GPC
Mn;th
Mw =Mn
Initiation efficiency
1 2 3 4 5 6 7 8 9
1:1:0.5 1:0.5:1 1:1:1 1:1:2 1:2:1 1:2:2 1:2:4 1:3:6 1:4:8
2 1.5 2 2.5 3 3.5 4 6 15
75.0 70.0 70.0 71.5 72.6 72.5 74.3 71.0 71.0
26 600 26 500 26 900 34 600 23 500 23 600 26 200 20 050 20 500
18 750 17 500 17 500 17 900 18 150 18 150 18 600 17 750 17 750
1.53 1.71 1.55 1.44 1.33 1.34 1.33 1.29 1.25
0.7 0.66 0.65 0.52 0.77 0.77 0.71 0.89 0.87
Conditions: [I]0 /[MMA]0 ¼ 1/500, [MMA]0 /[DMF] ¼ 2/1 (V/V), Mn;th ¼ ð½MMA0 =2½AIBN0 MWMMA ConversionÞ, Initiation efficiency ¼ Mn;th =Mn;GPC .
o
T=110 C 3.0
o
T=90 C o
T=100 C
2.5
Ln([M]0 /([M])
2.0 o
T=80 C 1.5
1.0
0.5
0.0 0
10
20
30
40
50
60
Time(h)
Fig. 3. Kinetic plots of MMA polymerization at different temperatures. [MMA]0 ¼ 6.31 mol/l, [MMA]0 :[AIBN]0 :[FeCl3 ]0 : [pyromellitic acid]0 ¼ 500:1:3:6.
-9.0
-9.5
-10.0 app
Ln(Kp )
course of conversion from 10% to 80%. The linear line of Mn versus conversion indicates the absence or an insignificant amount of transfer reaction. In comparison with the reverse ATRP of MMA using isophthalic acid as ligand [14], the initiation efficiency was higher. This may due to the strong coordination ability of pyromellitic acid, which would decrease the possibility of irreversible termination. The ratio of [FeCl3 ]0 /[I]0 played an important role in the polymerization rate and the molecular weight and molecular weight distribution. From Table 1, we can see that, as more FeCl3 /pyromellitic acid was added (entries 2, 4, 7, 8, 9), slower polymerization rates and narrower molecular weight distributions were observed, which suggests that the concentration of FeCl3 has a favorable influence on the activation as well as equilibrium of reverse ATRP. The initiator efficiency also increased from 0.66 to 0.87 with an increase in [FeCl3 ]0 /[pyromellitic acid]0 . With increasing the amount of pyromellitic acid (entries 1, 3, 4 and entries 5, 6, 7), the rate of polymerization and initiator efficiency varied little while the molecular weight distribution of the products remained narrow. When the ratio of [FeCl3 ]0 /[pyromellitic acid]0 was changed from 0.5:1 to 2:1 (entries 2, 3, 5), the molecular weight distribution of the products decreased from 1.71 to 1.31 and the initiation efficiency increased from 0.66 to 0.77. This implies that FeCl3 itself is an effective catalyst in reverse ATRP. The effect of reaction temperature on the rate of polymerization for the reverse ATRP of MMA was studied over a temperature range of 80–110 C. Kinetic plots are shown in Fig. 3. The first-order time-conversion plots have a good linearity at all temperature indicating that the radical concentrations in all these systems are low enough to prevent significant radical termination. On the basis of the Arrhenius plot in Fig. 4, the apparent 6¼ activation energy was calculated Eapp ¼ 24 kcal/mol for the reverse ATRP of MMA initiated by AIBN/FeCl3 / pyromellitic acid. The activation energy for MMA
-10.5
Y = 22.74 + 12135.03∗ X R=0.97
-11.0
-11.5
-12.0 -0.00285
-0.00280
-0.00275
-0.00270
-0.00265
-0.00260
-1/T
Fig. 4. Effect of reaction temperature on Kpapp for the AIBN/ FeCl3 /pyromellitic acid-mediated reverse ATRP of MMA in DMF. Experimental conditions are the same as in Fig. 3. 6¼ propagation is known to be Eprop ¼ 5:3 kcal/mol [15], Eq. (1) shows how the enthalpy of the equilibrium and
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the activation energy of propagation are related to the apparent activation energy, so we could calculated the 0 enthalpy of the equilibrium DHeq ¼ 18:7 kcal/mol for the reverse ATRP of MMA catalyzed by FeCl3 /pyromellitic acid. This value is higher than the corresponding values for MMA catalyzed by the Cl-mediated system 0 (DHeq ¼ 9:7 kcal/mol) [16] or by FeCl3 /isophthalic acid 0 system (DHeq ¼ 10:47) [14]. This difference seems to
originate in the variation of the catalyst structure with solvent [17]. 0 6¼ 6¼ DHeq ¼ Eapp Eprop
ð1Þ
A 1 H NMR spectrum of the resulting polymer is shown in Fig. 5. The signals at 0.83–1.21 ppm are assigned to the protons of methyl groups of
Fig. 5. 1 H-NMR spectrum of PMMA initiated with AIBN/FeCl3 /pyromellitic acid.
Fig. 6. GPC curves of PMMA before (b) and after (a) chain extension T ¼ 100 C, [MMA]0 ¼ 6.38 mol/l, [MMA]0 :[toluene]0 ¼ 1.7:1, [MMA]0 :[PMMA-Cl]0 :[FeCl2 ]0 :[PPh3 ]0 ¼ 500:1:2:4, time ¼ 24 h.
G. Wang et al. / European Polymer Journal 39 (2003) 2161–2165
2165
Table 2 Polymerization of various monomers by reverse ATRP Monomer
Time (h)
Conversion (%)
Mn;GPC
Mn;th
Mw =Mn
Methyl methacrylate N-butyl methacrylate Methyl acrylate N-butyl acrylate Octyl acrylate
6 115 139 139 139
88.6 70 0 0 0
27 800 37 900 – – –
22 150 25 025 – – –
1.28 1.24 – – –
Conditions: [monomer]0 :[AIBN]0 :[FeCl3 ]0 :[pyromellitic acid]0 ¼ 500:1:2:4, [monomer]0 /[DMF] ¼ 2/1 (V/V), T ¼ 100 C.
–C(CH3 )(COOCH3 ), the signals from 1.44 to 2.07 ppm are attributed to the methylene group of –CH2 –, and the peaks at 3.41–3.60 ppm correspond to methoxy groups in the main chain. The single signal at 3.78 ppm is for the methoxy group next to the halogen chain end and is consistent with that reported by Sawamoto and coworkers [18]. An additional method toward verifying the functionality of a polymer prepared by ATRP is as a macroinitiator for the same or other monomers. The chain extension polymerization of MMA with PMMA (Mn ðGPCÞ ¼ 23 000, Mw =Mn ¼ 1:33) as the macroinitiator was successfully performed. As shown in Fig. 6, the Mn ðGPCÞ for 93% conversion of chain-extended PMMA increased from 23 100 to 54 000, however, the polydispersity index (1.49) was a little higher than that of the macroinitiator, which indicates that a small amount of the macroinitiator probably did not participate the reaction. Polymerizations of various polar monomers were examined using FeCl3 /pyrimillitic acid catalyst system. As shown in Table 2, this catalyst system can catalyze the polymerization of methacrylates effectively, however, this is not the case for acrylates. This may be explained by the fact that acrylates generate highly reactive radical species but give less reactive carbon–halogen bonds than those of methacrylates [19].
4. Conclusion In conclusion, pyromellitic acid has been successfully employed as a new ligand in the iron-mediated reverse ATRP of methyl methacrylate. The polymerizations showed a linear increase of molecular weights with conversion and low polydispersities throughout the reactions. Increasing the dosage of catalyst would decrease the rate of polymerization and enhanced the degree of controlled polymerization. This catalyst system is effective for methylates but not for acrylates.
Acknowledgements We gratefully acknowledge National Nature Science Foundation of China (no. 20176033) and Nature Science Foundation of Jiangsu Province (no. BK2001141) for support in this work. References [1] Wang JS, Matyjaszewski K. J Am Chem Soc 1995; 117: 5614–5. [2] Kato M, Kamigaito M, Sawamoto M, Higashimura T. Macromolecules 1995;28:1721–3. [3] Percec V, Barboiu B. Macromolecules 1995;28:7970–2. [4] TPT L, Moad G, Rizzardo E, Thang SH. WO patent 1998; 9801478. [5] Moad G, Rizzardo E, Solomon DH. Macromolecules 1982;15:909–14. [6] Kukulj D, Shooter AJ. Macromolecules 1999;32:2110–9. [7] Xia JH, Matyjaszewski K. Macromolecules 1997;30:7697– 700. [8] Ando T, Kamigaito M, Sawamoto M. Macromolecules 1997;30:4507–10. [9] Wang JS, Matyjaszewski K. Macromolecules 1995; 28: 7572–3. [10] Xia JH, Matyjaszewski K. Macromolecules 1997;30:7692–6. [11] Moineau G, Dubois Ph, Jer^ ome R, Senninger T, Teyssie Ph. Macromolecules 1998;28:7572–3. [12] Saikia PJ, Goswami A, Baruah SD. J Appl Polym Sci 2002;85:1236–45. [13] Teodorescu M, Gaynor SG, Matyjaszewski K. Macromolecules 2000;33:2335–9. [14] Zhu SM, Yan DY, Zhang GS. J Polym Sci, Part A: Polym Chem 2001;39:765–74. [15] Gilbert RG. Pure Appl Chem 1996;68:1491–4. [16] Wang JL, Grimaud T, Matyjaszwski K. Macromolecules 1997;30:6507–12. [17] Davis KA, Paik HJ, Matyjaszewski K. Macromolecules 1999;32:1767–76. [18] Ando T, Kamigaito M, Sawamoto M. Macromolecules 1998;31:6708–11. [19] Kamigaito M, Ando T, Sawamoto M. Chem Rev 2001; 101:3689–745.
Reverse atom transfer radical polymerization of methyl methacrylate with FeCl3/pyromellitic acid Gang Wang, Xiulin Zhu *, Zhenping Cheng, Jian Zhu School of Chemistry and Chemical Engineering, Suzhou University, 1 Shizi Street, Suzhou 215006, PR China Received 3 April 2003; received in revised form 29 May 2003; accepted 30 May 2003
Abstract IronIII chloride coordinated by pyromellitic acid was successfully used as the catalytic system in reverse atom transfer radical polymerization of MMA. Well-defined poly(methyl methacrylate) with narrow molecular weight distribution was synthesized in N,N-dimethylformamide at 80–110 C. Chain extension was performed to confirm the living nature of the polymer. The presence of the end chloride atom on the resulting PMMA was demonstrated by 1 HNMR spectroscopy. This catalyst system is effective for reverse ATRP of methacrylates but not for acrylates. 2003 Elsevier Ltd. All rights reserved. Keywords: Reverse atom transfer radical polymerization; Methyl methacrylate; Pyromellitic acid
1. Introduction Interest in living/controlled free radical polymerization has been stimulated to a great extent by the impressive progress made in several methods such as atom transfer radical polymerization (ATRP), nitroxyl radical mediated polymerization (NMP), reversible addition fragmentation transfer polymerization (RAFT), and in the field of synthetic well-defined polymers. Research groups such as those of Matyjaszewski [1], Sawamoto [2], Percec [3], Rizzardo [4,5] have successfully introduced these approachs into polymerization chemistry as a novel ÔlivingÕ/controlled free radical polymerization. By far the most successful catalysts have been those based upon electron-donating compounds such as bipyridine (bipy) [1], N-alkyl-2-pyridylmethanimines (NPMI) [6], N,N,N0 ,N0 -tetramethylethylenediamine (TMEDA) [7] and triphenylphosphine (PPh3 ) [8] as metal complex ligands. However, the catalyst system of ATRP is generally toxic and expensive and the catalyst such as CuCl is unstable under air and moisture. To overcome these
*
Corresponding author. Tel./fax: +86-512-65112796. E-mail address: [email protected] (X. Zhu).
shortcomings, a so-called reverse ATRP with 2,20 -azobis-isobutyronitrile (AIBN)/CuBr2 /bipy [9] and AIBN/ CuBr2 /dNbipy [10] as the initiating systems have been developed. In this process, a higher oxidation state transition-metal species Mtnþ1 X =Lx and a conventional radical initiator were used instead of a lower oxidation state Mtn =Lx and halide species RX, respectively. Teyssie and coworkers [11] reported the reverse ATRP of MMA using FeCl3 and AIBN in the presence of PPh3 . The ligands in iron-mediated reverse ATRP are usually bipy [12] and PPh3 [11]. Recently, Matyjaszewski and coworkers [13] reported that the reverse ATRP initiated by AIBN/FeBr3 /onium salts, which led to a controlled polymerization of both MMA and MA, while for styrene uncontrolled molecular weights and high polydispersities were obtained. Yan and coworkers [14] reported the reverse ATRP of MMA initiated by AIBN/FeCl3 / isophthalic acid, the polymerization was controlled up to a molecular weight of 50,000, and the polydisperity index is 1.4, the observed molecular weight was higher than the theoretical value, which indicates the relatively low initiator efficiency. In this work we investigated a new catalytic system based on iron complexes with pyromellitic acid for reverse ATRP of MMA in the presence of a conventional
0014-3057/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0014-3057(03)00137-X
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G. Wang et al. / European Polymer Journal 39 (2003) 2161–2165
radical initiator, AIBN. Pyromellitic acid is a quaternary carboxylic acid. Deprotonation gives anions with 1–4 negative charges, which can act as multidentate ligands. This catalyst system is effective for methylates but not for acrylates.
added to the sample to dissolve the polymer. PMMA was isolated by precipitation in hexane and dried in vacuum at 25 C for 48 h.
3. Results and discussion
Methyl methacrylate, N-butyl methacrylate, N-butyl acrylate, Octyl acrylate (chemically pure, Shanghai Chemical Reagent Co., Ltd.) were purified by extracting with 5% sodium hydroxide aqueous solution, followed by washing with water and dried with sodium sulfate anhydrous for overnight, finally distillated under vacuum. N,N-dimethylformamide (DMF) (analytical reagent, Shanghai No. 1 Chemical Reagent Factory) was distillated at reduced pressure. AIBN (chemically pure, Shanghai No. 4 Reagent Factory) was recrystallized from ethanol. FeCl3 (Jinshan Chemical Co.; analytical-reagent-grade, anhydrous), pyromellitic acid (Chang shu Lianbang Chemical Co.; 99.5%), tetrahydrofuran (THF) (analytical reagent, Shanghai Chemical Reagent Co., Ltd.) were used without further purification. 2.2. Characterization Conversion of monomer was determined by gravimetry. Molecular weights and molecular weight distributions were measured by Waters 1515 GPC, equipped with microstyragel columns (HR1, HR3, HR4) with THF as a mobile phase at a flow rate of 1 cm3 /min operated at 30 C. Polystyrene standards were used. Thus all molecular weights should be considered as polystyrene equivalent molecular weights. Calibration of the instrument was performed with standard samples of poly(styrene) between 5 · 105 and 1 · 102 g mol1 . 1 H NMR spectra were recorded in CDCl3 with an INOVA 400 MHz spectrometer at ambient temperature.
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0
1
2
3
4
5
6
7
8
Time(h)
Fig. 1. Kinetic plot for the reverse ATRP of MMA. [MMA]0 ¼ 6.31 mol/l, [MMA]0 :[AIBN]0 :[FeCl3 ]0 :[pyromellitic acid]0 ¼ 500:1:3:6, T ¼ 100 C, VðMMAÞ =VðDMFÞ ¼ 2=1 ðV=VÞ.
2.2 25000
20000
The general procedure of the polymerization was as follows: FeCl3 (0.0378 mol/l), pyromellitic acid (0.0756 mol/l) and N,N-dimethylformamide were added to an ampule tube under stirring, three cycles of vacuumnitrogen were applied in order to remove the oxygen. After the catalyst was dissolved (about 10 min), MMA (6.31 mol/l) with AIBN (0.0126 mol/l) dissolved in advance was added via an argon-washed syringe. Then the ampule tube was sealed under nitrogen and placed in an oil bath thermostated at the desired temperature. After a given time, the tube was broken open, and THF was
15000
2.0
Mw /Mn ----Mn,th
1.8
Mn
2.3. Polymerization
Mn(GPC)
1.6 10000
Mw /Mn
2.1. Materials
MMA was polymerized in N,N-dimethylformamide using FeCl3 /pyromellitic acid as catalyst and AIBN as the initiator at 100 C ([MMA]0 /[AIBN]0 /[FeCl3 ]0 / [pyromellitic acid]0 ¼ 500/1/3/6). The kinetic plot of ln½M0 =½M versus time is shown in Fig. 1. The straight line through original point indicates that the concentration of propagating radicals remained constant during the polymerization. The polymerization showed that the molecular weights increased linearly with conversion and they were close to the predicted values (Fig. 2). The polydispersity index of the resulting polymer decreased from 1.63 to 1.28 in the
ln[M] 0 /[M]
2. Experimental section
1.4
5000
1.2
0 0
10
20
30
40
50
60
70
80
90
1.0 100
conversion(%)
Fig. 2. Dependence of molecular weights and polydispersities on monomer conversion for the reverse ATRP of MMA. Experimental conditions are the same as in Fig. 1.
G. Wang et al. / European Polymer Journal 39 (2003) 2161–2165
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Table 1 Polymerization of MMA by reverse ATRP using AIBN as the initiator Entry
[I]0 /[FeCl3 ]0 / [pyromellitic acid]0
Time (h)
Conversion (%)
Mn;GPC
Mn;th
Mw =Mn
Initiation efficiency
1 2 3 4 5 6 7 8 9
1:1:0.5 1:0.5:1 1:1:1 1:1:2 1:2:1 1:2:2 1:2:4 1:3:6 1:4:8
2 1.5 2 2.5 3 3.5 4 6 15
75.0 70.0 70.0 71.5 72.6 72.5 74.3 71.0 71.0
26 600 26 500 26 900 34 600 23 500 23 600 26 200 20 050 20 500
18 750 17 500 17 500 17 900 18 150 18 150 18 600 17 750 17 750
1.53 1.71 1.55 1.44 1.33 1.34 1.33 1.29 1.25
0.7 0.66 0.65 0.52 0.77 0.77 0.71 0.89 0.87
Conditions: [I]0 /[MMA]0 ¼ 1/500, [MMA]0 /[DMF] ¼ 2/1 (V/V), Mn;th ¼ ð½MMA0 =2½AIBN0 MWMMA ConversionÞ, Initiation efficiency ¼ Mn;th =Mn;GPC .
o
T=110 C 3.0
o
T=90 C o
T=100 C
2.5
Ln([M]0 /([M])
2.0 o
T=80 C 1.5
1.0
0.5
0.0 0
10
20
30
40
50
60
Time(h)
Fig. 3. Kinetic plots of MMA polymerization at different temperatures. [MMA]0 ¼ 6.31 mol/l, [MMA]0 :[AIBN]0 :[FeCl3 ]0 : [pyromellitic acid]0 ¼ 500:1:3:6.
-9.0
-9.5
-10.0 app
Ln(Kp )
course of conversion from 10% to 80%. The linear line of Mn versus conversion indicates the absence or an insignificant amount of transfer reaction. In comparison with the reverse ATRP of MMA using isophthalic acid as ligand [14], the initiation efficiency was higher. This may due to the strong coordination ability of pyromellitic acid, which would decrease the possibility of irreversible termination. The ratio of [FeCl3 ]0 /[I]0 played an important role in the polymerization rate and the molecular weight and molecular weight distribution. From Table 1, we can see that, as more FeCl3 /pyromellitic acid was added (entries 2, 4, 7, 8, 9), slower polymerization rates and narrower molecular weight distributions were observed, which suggests that the concentration of FeCl3 has a favorable influence on the activation as well as equilibrium of reverse ATRP. The initiator efficiency also increased from 0.66 to 0.87 with an increase in [FeCl3 ]0 /[pyromellitic acid]0 . With increasing the amount of pyromellitic acid (entries 1, 3, 4 and entries 5, 6, 7), the rate of polymerization and initiator efficiency varied little while the molecular weight distribution of the products remained narrow. When the ratio of [FeCl3 ]0 /[pyromellitic acid]0 was changed from 0.5:1 to 2:1 (entries 2, 3, 5), the molecular weight distribution of the products decreased from 1.71 to 1.31 and the initiation efficiency increased from 0.66 to 0.77. This implies that FeCl3 itself is an effective catalyst in reverse ATRP. The effect of reaction temperature on the rate of polymerization for the reverse ATRP of MMA was studied over a temperature range of 80–110 C. Kinetic plots are shown in Fig. 3. The first-order time-conversion plots have a good linearity at all temperature indicating that the radical concentrations in all these systems are low enough to prevent significant radical termination. On the basis of the Arrhenius plot in Fig. 4, the apparent 6¼ activation energy was calculated Eapp ¼ 24 kcal/mol for the reverse ATRP of MMA initiated by AIBN/FeCl3 / pyromellitic acid. The activation energy for MMA
-10.5
Y = 22.74 + 12135.03∗ X R=0.97
-11.0
-11.5
-12.0 -0.00285
-0.00280
-0.00275
-0.00270
-0.00265
-0.00260
-1/T
Fig. 4. Effect of reaction temperature on Kpapp for the AIBN/ FeCl3 /pyromellitic acid-mediated reverse ATRP of MMA in DMF. Experimental conditions are the same as in Fig. 3. 6¼ propagation is known to be Eprop ¼ 5:3 kcal/mol [15], Eq. (1) shows how the enthalpy of the equilibrium and
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the activation energy of propagation are related to the apparent activation energy, so we could calculated the 0 enthalpy of the equilibrium DHeq ¼ 18:7 kcal/mol for the reverse ATRP of MMA catalyzed by FeCl3 /pyromellitic acid. This value is higher than the corresponding values for MMA catalyzed by the Cl-mediated system 0 (DHeq ¼ 9:7 kcal/mol) [16] or by FeCl3 /isophthalic acid 0 system (DHeq ¼ 10:47) [14]. This difference seems to
originate in the variation of the catalyst structure with solvent [17]. 0 6¼ 6¼ DHeq ¼ Eapp Eprop
ð1Þ
A 1 H NMR spectrum of the resulting polymer is shown in Fig. 5. The signals at 0.83–1.21 ppm are assigned to the protons of methyl groups of
Fig. 5. 1 H-NMR spectrum of PMMA initiated with AIBN/FeCl3 /pyromellitic acid.
Fig. 6. GPC curves of PMMA before (b) and after (a) chain extension T ¼ 100 C, [MMA]0 ¼ 6.38 mol/l, [MMA]0 :[toluene]0 ¼ 1.7:1, [MMA]0 :[PMMA-Cl]0 :[FeCl2 ]0 :[PPh3 ]0 ¼ 500:1:2:4, time ¼ 24 h.
G. Wang et al. / European Polymer Journal 39 (2003) 2161–2165
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Table 2 Polymerization of various monomers by reverse ATRP Monomer
Time (h)
Conversion (%)
Mn;GPC
Mn;th
Mw =Mn
Methyl methacrylate N-butyl methacrylate Methyl acrylate N-butyl acrylate Octyl acrylate
6 115 139 139 139
88.6 70 0 0 0
27 800 37 900 – – –
22 150 25 025 – – –
1.28 1.24 – – –
Conditions: [monomer]0 :[AIBN]0 :[FeCl3 ]0 :[pyromellitic acid]0 ¼ 500:1:2:4, [monomer]0 /[DMF] ¼ 2/1 (V/V), T ¼ 100 C.
–C(CH3 )(COOCH3 ), the signals from 1.44 to 2.07 ppm are attributed to the methylene group of –CH2 –, and the peaks at 3.41–3.60 ppm correspond to methoxy groups in the main chain. The single signal at 3.78 ppm is for the methoxy group next to the halogen chain end and is consistent with that reported by Sawamoto and coworkers [18]. An additional method toward verifying the functionality of a polymer prepared by ATRP is as a macroinitiator for the same or other monomers. The chain extension polymerization of MMA with PMMA (Mn ðGPCÞ ¼ 23 000, Mw =Mn ¼ 1:33) as the macroinitiator was successfully performed. As shown in Fig. 6, the Mn ðGPCÞ for 93% conversion of chain-extended PMMA increased from 23 100 to 54 000, however, the polydispersity index (1.49) was a little higher than that of the macroinitiator, which indicates that a small amount of the macroinitiator probably did not participate the reaction. Polymerizations of various polar monomers were examined using FeCl3 /pyrimillitic acid catalyst system. As shown in Table 2, this catalyst system can catalyze the polymerization of methacrylates effectively, however, this is not the case for acrylates. This may be explained by the fact that acrylates generate highly reactive radical species but give less reactive carbon–halogen bonds than those of methacrylates [19].
4. Conclusion In conclusion, pyromellitic acid has been successfully employed as a new ligand in the iron-mediated reverse ATRP of methyl methacrylate. The polymerizations showed a linear increase of molecular weights with conversion and low polydispersities throughout the reactions. Increasing the dosage of catalyst would decrease the rate of polymerization and enhanced the degree of controlled polymerization. This catalyst system is effective for methylates but not for acrylates.
Acknowledgements We gratefully acknowledge National Nature Science Foundation of China (no. 20176033) and Nature Science Foundation of Jiangsu Province (no. BK2001141) for support in this work. References [1] Wang JS, Matyjaszewski K. J Am Chem Soc 1995; 117: 5614–5. [2] Kato M, Kamigaito M, Sawamoto M, Higashimura T. Macromolecules 1995;28:1721–3. [3] Percec V, Barboiu B. Macromolecules 1995;28:7970–2. [4] TPT L, Moad G, Rizzardo E, Thang SH. WO patent 1998; 9801478. [5] Moad G, Rizzardo E, Solomon DH. Macromolecules 1982;15:909–14. [6] Kukulj D, Shooter AJ. Macromolecules 1999;32:2110–9. [7] Xia JH, Matyjaszewski K. Macromolecules 1997;30:7697– 700. [8] Ando T, Kamigaito M, Sawamoto M. Macromolecules 1997;30:4507–10. [9] Wang JS, Matyjaszewski K. Macromolecules 1995; 28: 7572–3. [10] Xia JH, Matyjaszewski K. Macromolecules 1997;30:7692–6. [11] Moineau G, Dubois Ph, Jer^ ome R, Senninger T, Teyssie Ph. Macromolecules 1998;28:7572–3. [12] Saikia PJ, Goswami A, Baruah SD. J Appl Polym Sci 2002;85:1236–45. [13] Teodorescu M, Gaynor SG, Matyjaszewski K. Macromolecules 2000;33:2335–9. [14] Zhu SM, Yan DY, Zhang GS. J Polym Sci, Part A: Polym Chem 2001;39:765–74. [15] Gilbert RG. Pure Appl Chem 1996;68:1491–4. [16] Wang JL, Grimaud T, Matyjaszwski K. Macromolecules 1997;30:6507–12. [17] Davis KA, Paik HJ, Matyjaszewski K. Macromolecules 1999;32:1767–76. [18] Ando T, Kamigaito M, Sawamoto M. Macromolecules 1998;31:6708–11. [19] Kamigaito M, Ando T, Sawamoto M. Chem Rev 2001; 101:3689–745.