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Alternating Copolymerization of Carbon Dioxide and Epoxide [2]. The First Example of Polycarbonate Synthesis from 1-atm Carbon Dioxide by Manganese Porphyrin.
Studies in Surface Science and Catalysis 153 S.-E. Park, J.-S. Chang and K.-W. Lee (Editors) ©2004 Published by ElsevierB.V.
247
Alternating Copolymerization of Carbon Dioxide and Epoxide [2]. The First Example of Polycarbonate Synthesis from 1-atm Carbon Dioxide by Manganese Porphyrin. Hiroshi Sugimoto, Hiromistu Ohshima, and Shohei Inoue Faculty of Engineering, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan The first successful example of the formation of polycarbonate from 1-atm carbon dioxide and epoxide was achieved by the alternating copolymerization of carbon dioxide and cyclohexene oxide with (porphinato)manganese acetate under the 1-atm pressure of carbon dioxide to give a copolymer with alternating sequence. 1. INTRODUCTION Chemical fixation of carbon dioxide (CO2) is of much interest and importance from the viewpoints of energy resources as well as biological photosynthesis. Since we discovered that the Zn-complex catalyzed alternating copolymerization of CO2 and epoxide to afford a linear polycarbonate , various catalysts, including Zn, Al, Mg, Cd, Cr, and lanthanoid complexes, have been developed for the controlled alternating copolymerization. However, in many cases, a high CO2 pressure, a high reaction temperature, and a long reaction time were needed to realize the activity of the catalyst and/or the high CO2 content in the obtained copolymer. In
the course
of our studies on the alternating
Q ^ C rf\ s
copolymerization with transition-metal complexes, we have
t/~X'^' Hiin-N^3'
found that manganese porphyrin, [(TPP)Mn(OAc), or 1],
^^y==<^\^f~~\
which has been reported by us to bring about the living
\J
homopolymerization of epoxide, readily brings about the
(TPP)Mn(OAc)(1)
,}
alternating copolymerization of CO2 and epoxide to afford polycarbonate.3 In the present paper, we report the alternating copolymerization of CO2 and cyclohexene oxide (CHO) by using (TPP)Mn(OAc) as the initiator even under the 1-atm CO2 atmosphere (Scheme 1).
248
2. EXPERIMENTAL 2.1. Synthesis of (TPP)Mn(OAc) (1) For the synthesis of (TPP)Mn(OAc) (1), a CHC13 solution (1 L) of tetraphenylporphyrin (10 g) was added to arefluxing acetic acid solution (1.4 L) of Mn(OAc)24H2O (10 g), and the mixture was kept refluxing for 6 h under aerobic conditions. The resulting solution was evaporated to dryness, and the residue was dissolved in CHCI3, followed by filtering off the insoluble fractions. The filtrate was evaporated to dryness to leave a greenish powder, which was dissolved in 1 L of refluxing acetic acid. The solution was concentrated to 300 mL and kept overnight at room temperature to give greenish-purple needles. The resulting crystals were isolated, dried at 100 °C in vacuo for 24 h, and subjected to repeated azeotropic distillations with benzene until the IR absorption due to free acetic acid disappeared. 2.2. Copolymerization of CO2 andCHO A typical procedure for the copolymerization of CO2 and CHO by 1 was as follows: A stainless steel autoclave (150 mL) containing 1 (0.05 mmol), dried in vacuo, was purged with nitrogen. CHO (25 mmol) was added with a syringe. The mixture was pressurized by CO2 up to 50 atm (52 kg/cm2, 735 psi). After the mixture was stirred for 24 h at 80 °C, the autoclave was cooled, and excess CO2 was discharged, and then to which was added methanol to stop the polymerization. A small potion of the products was subjected to H NMR and IR for determining the polymer structure and to GPC for estimating the average molecular weights.
2.3. Copolymerization of CO2 and CHO under 1-atm CO2 Into a 100-mL round-bottomed flask attached to a three-way stopcock containing 1 (0.05 mmol) under dry CO2, CHO (25 mmol) was added with a syringe. With vigorous, CO, was passed through the mixture of 1 and CHO for 5 min, and then a rubber balloon (2 L) filled with 1-atm CO2 was attached to the top of the three-way stopcock in order to provide enough amount of CO2 to the reaction system. After the mixture was stirred for 90 h at 80 °C, the flask was cooled, and then to which was added methanol to stop the polymerization.
249
3. RESULTS AND DISCUSSION
Copolymerization of CO2 and CHO was first attempted by using (TPP)Mn(OAc) (1) as the initiator under 50 atm of CO2-pressure at 80 °C. When CHO (25 mmol; 2.5 g) was added to 1 (0.05 mmol; 0.036 g) in stainless autoclave and pressurized by CO2 at 50 atm, the copolymerization proceeded smoothly to yield a polymeric product (2.8 g) in 24 h. The IR spectrum of the product showed two strong absorptions at 1,750 and 1,280 cm"1 due to C=O and C-0 stretching vibrations, respectively. In the ! H NMR spectrum for the same product, a signal was observed at 8 4.7 ppm which was assignable to the methine proton next to the carbonate linkage in the repeating oxycarbonyloxy(l,2-cyclohexene) units. From these observations, the product is considered the alternating copolymer of CO2 and CHO, poly(cyclohexene carbonate) (78 % yield), where the polycarbonate linkage estimated from ' H NMR exceeded 99 % (Table 1, run 1). The number average molecular weight of the poly(cyclohexene carbonate), as estimated by GPC based on the polystyrene standards, was 6,700. MJMn was calculated from the GPC chromatogram to be 1.3. This value indicated that the molecular weight of the copolymer had a fairly narrow distribution (Table 1, run 1). The copolymerization of CO2 and 1,2-epoxypropane (PO) gave much different results. For the reaction of CO2 and PO, the IR and 1H NMR spectra indicated the concomitant formation of propylene carbonate, the one-to-one cyclic adduct between CO2 and PO (31 % yield). For the alternating copolymerization, the catalyst is most important. When a manganese complex of a Schiff base, (Salophen)MnOAc (2), was used as the initiator under the similar conditions, the alternating copolymer was not obtained, and the homopolymer of CHO was the only product (32 % yield, Mn = 6,000, MJMn= 3.3) (Table 1, run 4). On the contrary, Mn(OAc)2, which is used as one of the source materials in the synthesis of catalyst complexes, could not give rise to the alternating copolymerization of CO2 and CHO or homopolymerization of CHO(Tablel,run5). Different from other reported catalysts, which bring about the copolymerization of CO2 and epoxide only under the high CO2 pressure (> 6.8 atm)4, (TPP)MnOAc (1) afforded polycarbonate even under the CO2 atmosphere of 1 atm, although the reaction proceeded more slowly than that under 50 atm of CO2 pressure ([CHO]0/[l]o = 500). The copolymer obtained after 90-h reaction contains 95 % of carbonate as estimated by *H NMR (59 % yield,
250 Table 1. Copolymerization of CO2 and CHO by Manganese Catalysts.a reaction conditions ran catalyst
pressure
temp.
(atm)
product time
yield
carbonate linkages
MJMnc
(h)
1
1
50
80
24
78
99
6,700
1.3
2
1
1
80
90
59
95
3,000
1.6
3
1
1
25
90
25
86
1,800
1.5
4
2
50
80
24
32
0
6,000
3.3
5
Mn(OAc) 2
50
80
24
0
0
-
-
a
Without solvent, [CHO]0/[catalyst]0 = 500. b Estimated by *H NMR. based on standard polystyrenes.
c
Estimated by GPC
Mn = 3,000, MJMn = 1.6) (Table 1, run 2). To the best of our knowledge, this is the first successful example of the alternating copolymerization of CO2 and CHO under 1-atm CO2. The alternating copolymerization under 1-atm CO2 proceeded very slowly but surely even at 25 °C to generate polycarbonate (yield = 25 %, Mn = 1,800, MJMn = 1.6) (Table 1, run 3). Some kinds of additives, such as triphenylphospine and 4-(dimethylamino)pyridine, may enhance the reactivity of the catalyst complex for the alternating copolymerization of CO2 and CHO, leading to the formation of the alternating copolymer, even when the complex itself does not work efficiently as a catalyst. However, in the presence of additives, such as triphenylphospine, pyridine, or 1-methylimidazole, the copolymerization initiated by 1 was decelerated, and the Mn and the CO2 content of the produced copolymer were lowered. REFERENCES 1. S. Inoue, H. Koinuma, and T. Tsuruta, J. Polym. Sci., Polym. Lett. Ed., 7 (1969) 287. 2. M. Kuroki, T. Aida, and S. Inoue, Makromol. Chem., 189 (1988) 1305. 3. H. Sugimoto, H. Ohshima, and S. Inoue, J. Polym. Sci., Polym. Chem., in press. 4. The CO2 pressure that Coates et al employed for the alternating copolymerization of CO2 and epoxide was the lowest that could be found in the papers; M. Cheng, E.B. Lobkovsky, and G.W. Coates, J. Am. Chem. Soc, 120 (1998) 11018.
247
Alternating Copolymerization of Carbon Dioxide and Epoxide [2]. The First Example of Polycarbonate Synthesis from 1-atm Carbon Dioxide by Manganese Porphyrin. Hiroshi Sugimoto, Hiromistu Ohshima, and Shohei Inoue Faculty of Engineering, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan The first successful example of the formation of polycarbonate from 1-atm carbon dioxide and epoxide was achieved by the alternating copolymerization of carbon dioxide and cyclohexene oxide with (porphinato)manganese acetate under the 1-atm pressure of carbon dioxide to give a copolymer with alternating sequence. 1. INTRODUCTION Chemical fixation of carbon dioxide (CO2) is of much interest and importance from the viewpoints of energy resources as well as biological photosynthesis. Since we discovered that the Zn-complex catalyzed alternating copolymerization of CO2 and epoxide to afford a linear polycarbonate , various catalysts, including Zn, Al, Mg, Cd, Cr, and lanthanoid complexes, have been developed for the controlled alternating copolymerization. However, in many cases, a high CO2 pressure, a high reaction temperature, and a long reaction time were needed to realize the activity of the catalyst and/or the high CO2 content in the obtained copolymer. In
the course
of our studies on the alternating
Q ^ C rf\ s
copolymerization with transition-metal complexes, we have
t/~X'^' Hiin-N^3'
found that manganese porphyrin, [(TPP)Mn(OAc), or 1],
^^y==<^\^f~~\
which has been reported by us to bring about the living
\J
homopolymerization of epoxide, readily brings about the
(TPP)Mn(OAc)(1)
,}
alternating copolymerization of CO2 and epoxide to afford polycarbonate.3 In the present paper, we report the alternating copolymerization of CO2 and cyclohexene oxide (CHO) by using (TPP)Mn(OAc) as the initiator even under the 1-atm CO2 atmosphere (Scheme 1).
248
2. EXPERIMENTAL 2.1. Synthesis of (TPP)Mn(OAc) (1) For the synthesis of (TPP)Mn(OAc) (1), a CHC13 solution (1 L) of tetraphenylporphyrin (10 g) was added to arefluxing acetic acid solution (1.4 L) of Mn(OAc)24H2O (10 g), and the mixture was kept refluxing for 6 h under aerobic conditions. The resulting solution was evaporated to dryness, and the residue was dissolved in CHCI3, followed by filtering off the insoluble fractions. The filtrate was evaporated to dryness to leave a greenish powder, which was dissolved in 1 L of refluxing acetic acid. The solution was concentrated to 300 mL and kept overnight at room temperature to give greenish-purple needles. The resulting crystals were isolated, dried at 100 °C in vacuo for 24 h, and subjected to repeated azeotropic distillations with benzene until the IR absorption due to free acetic acid disappeared. 2.2. Copolymerization of CO2 andCHO A typical procedure for the copolymerization of CO2 and CHO by 1 was as follows: A stainless steel autoclave (150 mL) containing 1 (0.05 mmol), dried in vacuo, was purged with nitrogen. CHO (25 mmol) was added with a syringe. The mixture was pressurized by CO2 up to 50 atm (52 kg/cm2, 735 psi). After the mixture was stirred for 24 h at 80 °C, the autoclave was cooled, and excess CO2 was discharged, and then to which was added methanol to stop the polymerization. A small potion of the products was subjected to H NMR and IR for determining the polymer structure and to GPC for estimating the average molecular weights.
2.3. Copolymerization of CO2 and CHO under 1-atm CO2 Into a 100-mL round-bottomed flask attached to a three-way stopcock containing 1 (0.05 mmol) under dry CO2, CHO (25 mmol) was added with a syringe. With vigorous, CO, was passed through the mixture of 1 and CHO for 5 min, and then a rubber balloon (2 L) filled with 1-atm CO2 was attached to the top of the three-way stopcock in order to provide enough amount of CO2 to the reaction system. After the mixture was stirred for 90 h at 80 °C, the flask was cooled, and then to which was added methanol to stop the polymerization.
249
3. RESULTS AND DISCUSSION
Copolymerization of CO2 and CHO was first attempted by using (TPP)Mn(OAc) (1) as the initiator under 50 atm of CO2-pressure at 80 °C. When CHO (25 mmol; 2.5 g) was added to 1 (0.05 mmol; 0.036 g) in stainless autoclave and pressurized by CO2 at 50 atm, the copolymerization proceeded smoothly to yield a polymeric product (2.8 g) in 24 h. The IR spectrum of the product showed two strong absorptions at 1,750 and 1,280 cm"1 due to C=O and C-0 stretching vibrations, respectively. In the ! H NMR spectrum for the same product, a signal was observed at 8 4.7 ppm which was assignable to the methine proton next to the carbonate linkage in the repeating oxycarbonyloxy(l,2-cyclohexene) units. From these observations, the product is considered the alternating copolymer of CO2 and CHO, poly(cyclohexene carbonate) (78 % yield), where the polycarbonate linkage estimated from ' H NMR exceeded 99 % (Table 1, run 1). The number average molecular weight of the poly(cyclohexene carbonate), as estimated by GPC based on the polystyrene standards, was 6,700. MJMn was calculated from the GPC chromatogram to be 1.3. This value indicated that the molecular weight of the copolymer had a fairly narrow distribution (Table 1, run 1). The copolymerization of CO2 and 1,2-epoxypropane (PO) gave much different results. For the reaction of CO2 and PO, the IR and 1H NMR spectra indicated the concomitant formation of propylene carbonate, the one-to-one cyclic adduct between CO2 and PO (31 % yield). For the alternating copolymerization, the catalyst is most important. When a manganese complex of a Schiff base, (Salophen)MnOAc (2), was used as the initiator under the similar conditions, the alternating copolymer was not obtained, and the homopolymer of CHO was the only product (32 % yield, Mn = 6,000, MJMn= 3.3) (Table 1, run 4). On the contrary, Mn(OAc)2, which is used as one of the source materials in the synthesis of catalyst complexes, could not give rise to the alternating copolymerization of CO2 and CHO or homopolymerization of CHO(Tablel,run5). Different from other reported catalysts, which bring about the copolymerization of CO2 and epoxide only under the high CO2 pressure (> 6.8 atm)4, (TPP)MnOAc (1) afforded polycarbonate even under the CO2 atmosphere of 1 atm, although the reaction proceeded more slowly than that under 50 atm of CO2 pressure ([CHO]0/[l]o = 500). The copolymer obtained after 90-h reaction contains 95 % of carbonate as estimated by *H NMR (59 % yield,
250 Table 1. Copolymerization of CO2 and CHO by Manganese Catalysts.a reaction conditions ran catalyst
pressure
temp.
(atm)
product time
yield
carbonate linkages
MJMnc
(h)
1
1
50
80
24
78
99
6,700
1.3
2
1
1
80
90
59
95
3,000
1.6
3
1
1
25
90
25
86
1,800
1.5
4
2
50
80
24
32
0
6,000
3.3
5
Mn(OAc) 2
50
80
24
0
0
-
-
a
Without solvent, [CHO]0/[catalyst]0 = 500. b Estimated by *H NMR. based on standard polystyrenes.
c
Estimated by GPC
Mn = 3,000, MJMn = 1.6) (Table 1, run 2). To the best of our knowledge, this is the first successful example of the alternating copolymerization of CO2 and CHO under 1-atm CO2. The alternating copolymerization under 1-atm CO2 proceeded very slowly but surely even at 25 °C to generate polycarbonate (yield = 25 %, Mn = 1,800, MJMn = 1.6) (Table 1, run 3). Some kinds of additives, such as triphenylphospine and 4-(dimethylamino)pyridine, may enhance the reactivity of the catalyst complex for the alternating copolymerization of CO2 and CHO, leading to the formation of the alternating copolymer, even when the complex itself does not work efficiently as a catalyst. However, in the presence of additives, such as triphenylphospine, pyridine, or 1-methylimidazole, the copolymerization initiated by 1 was decelerated, and the Mn and the CO2 content of the produced copolymer were lowered. REFERENCES 1. S. Inoue, H. Koinuma, and T. Tsuruta, J. Polym. Sci., Polym. Lett. Ed., 7 (1969) 287. 2. M. Kuroki, T. Aida, and S. Inoue, Makromol. Chem., 189 (1988) 1305. 3. H. Sugimoto, H. Ohshima, and S. Inoue, J. Polym. Sci., Polym. Chem., in press. 4. The CO2 pressure that Coates et al employed for the alternating copolymerization of CO2 and epoxide was the lowest that could be found in the papers; M. Cheng, E.B. Lobkovsky, and G.W. Coates, J. Am. Chem. Soc, 120 (1998) 11018.