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Macromolecules 2005, 38, 2154-2158
Ring Size Effect of Crown Ether on the Fixation of Carbon Dioxide into an Oxirane Polymer Shin-ichi Yamamoto,† Osamu Moriya,† and Takeshi Endo*,‡ Department of Applied Chemistry, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa, 239-8686, Japan, and Department of Polymer Science and Engineering, Faculty of Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata, 992-8510, Japan Received August 9, 2004; Revised Manuscript Received December 15, 2004
ABSTRACT: Several methacrylate derivatives having a crown ether group, the size of which was changed from 12-crown-4 to 16-crown-5, were newly prepared and copolymerized with glycidyl methacrylate under radical conditions. The obtained copolymers (CP1-5) were employed for the fixation of CO2 into the oxirane moieties using lithium salt as a catalyst. The conversion of oxirane to five-membered cyclic carbonate groups by fixation of CO2 was related with the ring size of crown ethers and reflected the association constants for Li+. The most efficient fixation of CO2 in nitromethane solution was observed in the use of CP3, having 14-crown-4 groups, in which oxirane groups were transformed into the corresponding cyclic carbonate groups quantitatively, and further, the reaction proceeded readily even in the solid state.
Introduction The fixation of carbon dioxide (CO2), which is a prominent greenhouse gas, into organic compounds is very important from an economical and an environmental point of view.1 The use of CO2 as a starting material for the preparation of chemicals is a practical strategy to solve the problems of CO2. As a possible and effective candidate, the reaction of oxirane compounds with CO2 has been investigated enthusiastically.2,3 We have also reported on the reaction of CO2 and oxiranes in the presence of catalytic amounts of alkali metal salts such as lithium bromide (LiBr) or sodium iodide (NaI) to afford the corresponding five-membered cyclic carbonates under an atmospheric pressure of CO2.4 In line with such investigations, the fixation of CO2 into poly(glycidyl methacrylate) (PGMA), which shows the advantage of easy separation from the reaction mixture, has been already reported (Scheme 1).5 For this successful fixation, the reaction had to be carried out in a homogeneous reaction system. Consequently, the aprotic polar solvents such as DMF and N-methylpyrrolidone (NMP) were required to dissolve the metal salts as the catalyst. This seems to set a limit on the versatility of our method. As a possible modification to overcome the limitation, the use of the copolymer containing oxirane and crown ether groups was examined. In the copolymer, the fixation of CO2 into oxirane groups proceeded effectively with NaI catalyst even in nitromethane.6 The successful fixation should be indebted to the presence of the crown ether of 15-crown-5 on the copolymer, which promoted the catalyst to be soluble in nitromethane and dissociate to ionic species. In addition, the copolymer seems to provide an effective reaction environment in which the catalyst and oxirane groups are located on the same polymer backbone. Such findings have led us to perform a more detailed investigation on the roles of crown ether groups. In this work, we intend to get the information on the relationship between the ring sizes of crown ether and * Author to whom correspondence should be addressed. † National Defense Academy. ‡ Yamagata University.
Scheme 1
lithium salt catalyst, which enhance the fixation of CO2 into the oxirane groups on the copolymer containing the crown ethers. Experimental Section Measurements. 1H and 13C NMR spectra were recorded on either a Bruker DMX-500 or a JEOL AL-300 spectrometer using tetramethylsilane (TMS) as an internal standard in chloroform-d (CDCl3) and dimethyl sulfoxide (DMSO)-d6. IR spectra were recorded on a Jasco FT/IR-230 spectrometer. Number-average molecular weights (Mn) and polydispersity ratios (Mw/Mn) of polymers were estimated by a Tosoh HLC8220 GPC system equipped with a refractive index detector and three consecutive columns (limitation of size exclusion is 4 × 106, 6 × 104, and 1 × 104, respectively) with tetrahydrofuran (THF) as an eluent at a flow rate of 0.6 mL/min at 40 °C using a calibration curve of polystyrene standards. Materials. Unless stated otherwise, all the chemicals and reagents were obtained commercially and used without further purification. Glycidyl methacrylate (GMA, Tokyo Kasei Kogyo Co., Inc.), N,N-dimethylformamide (DMF, Kanto Chemical Co., Inc.), and nitromethane (Kanto Chemical Co., Inc.) were distilled over CaH2. Diethyl ether (Kanto Chemical Co., Inc.) was distilled over sodium-benzophenone ketyl before use. Methacryloyl chloride, R,R′-azobisisobutyronitrile (AIBN), 12crown-4, lithium triflate (LiOTf) (the above were obtained from Tokyo Kasei Kogyo Co., Inc.), lithium chloride (LiCl), lithium iodide (LiI), lithium perchloride (LiClO4), lithium acetate (LiOAc) (the above were obtained from Wako Pure Chemical Co., Inc.), and LiBr (Kanto Chemical Co., Inc.) were used as received. Methacryloyloxymethyl-12-crown-4 (1),7 methacryloyloxymethyl-14-crown-4 (3),7 methacryloyloxymethyl-15crown-5 (4),8 2-hydoroxymethyl-13-crown-4 (6),9 2-hydroxymethyl-16-crown-5 (7),10 and 14-crown-4-ether11 were prepared according to the reported procedures. Preparation of Methacryloyloxymethyl-13-crown-4 (2). A solution of 6 (0.81 g, 4 mmol) and triethylamine (0.5 g, 5 mmol) in dry diethyl ether (10 mL) was added dropwise to methacryloyl chloride (0.41 g, 4 mmol) at 0 °C for 5 min and
10.1021/ma040131a CCC: $30.25 © 2005 American Chemical Society Published on Web 02/11/2005
Macromolecules, Vol. 38, No. 6, 2005 stirred at room temperature for 1 h. The reaction mixture was extracted with diluted aqueous HCl, washed with water, and dried over MgSO4. After filtration, the ether layer was evaporated under reduced pressure. The residue was purified by chromatography on alumina (ethyl acetate eluent) to obtain 0.71 g (2.5 mmol) of 2 as a colorless oil in 62% yield:1H NMR (CDCl3) δ 1.87 (s, 3 H, -CH3), 2.16-2.21 (m, 1 H, >CH-), 3.54-3.66 (m, 16 H, -CH2-O-C), 4.14 (d, 2 H, -C(dO)-OCH2-, J ) 6.2 Hz), 5.48 (s, 1 H, CH2d), 6.02 (s, 1 H, CH2d) ppm; 13C NMR (CDCl3) δ 18.73 (-CH3), 40.08 (>CH-), 63.85, 69.55, 70.62, 70.91, 71.03, 71.33 (-CH2-O), 125.75 (CH2d), 136.77 (dCCdO) ppm; IR (neat): 2859 (CH3, CH2), 1716 (CdO of ester), 1637 (CdC), 1130 (C-O) cm-1; Anal. Calcd for C14H24O6: C, 58.32; H, 8.39. Found: C, 58.59; H, 8.10. Preparation of Methacryloyloxymethyl-16-crown-5 (5). By the procedure described for 2, this compound was obtained from 7 (0.98 g, 4.0 mmol), triethylamine (0.50 g, 5.0 mmol), and methacryloyl chloride (0.41 g, 4 mmol) as a colorless oil (0.90 g, 2.7 mmol) in 68% yield: 1H NMR (CDCl3) δ 1.87 (s, 3 H, -CH3), 2.19-2.25 (m, 1 H, >CH-), 3.51-3.73 (m, 20 H, -CH2-O-C), 4.10 (d, 2 H, -C(dO)-O-CH2-, J ) 6.9 Hz), 5.49 (s, 1 H, CH2d), 6.03 (s, 1 H, CH2d) ppm; 13C NMR (CDCl3) δ 18.72 (-CH3), 39.99 (>CH-), 64.04, 67.55, 69.99, 70.49, 70.51(-CH2-O), 125.83 (CH2d), 136.74 (dCCdO) ppm; IR (neat): 2865 (CH3, CH2), 1718 (CdO of ester), 1637 (CdC), 1122 (C-O) cm-1; Anal. Calcd for C15H26O7: C, 56.59; H, 8.23. Found: C, 56.58; H, 8.14. Typical Procedure for Polymerization. Total monomer (4 mmol), AIBN (0.02 g, 0.12 mmol, 3 mol%), and DMF (2 mL) were fed into a glass tube. After three freeze-pump-thaw cycles, the glass tube was sealed under vacuum and the reaction mixture was heated at 60 °C for 20 h. The mixture was diluted with DMF and poured into diethyl ether (200 mL) to precipitate a resulting polymer. The polymer was collected by filtration and dried in vacuo: 1H NMR (DMSO-d6) δ 0.791.50 (m, -CH3), 1.78-2.10 (b, -CH2-C, >CH-), 2.64 (b, -CH2-O-C), 2.85 (b, -CH2-O-C), 3.24 (b, >CH-O), 3.563.95 (b, -CH2-O-C), 4.48 (b, -C(dO)-O-CH2-) ppm; 13C NMR (DMSO-d6) δ 16.28, 18.17 (-CH3), 41.22 (>C-), 43.50 (-CH2-C), 44.00 (>CCH-O), 52.58, 53.45, 64.90, 66.11, 69.75, 70.08 (-CH2-O), 175.81, 176.60 (>CdO) ppm; IR (KBr): 1728 (CdO of ester), 1135 (CO), 906 (C-O of oxirane) cm-1. Typical Procedure for Fixation of CO2 in Nitromethane Solution. The polymer containing 1 mmol equiv of oxirane groups, catalyst (1.5 mol%), and nitromethane (1 mL) was added to a glass tube. After the mixture was degassed by a freeze-pump-thaw cycle, CO2 gas was flowed into the glass tube under atmospheric pressure and heated at 100 °C. The reaction mixture was dissolved into DMF and poured into diethyl ether (100 mL) to precipitate a polymer. The polymer was collected by filtration and dried in vacuo: Yield ) 93100%; 1H NMR (DMSO-d6) δ 0.81-0.99 (m, -CH3), 1.56-2.07 (m, -CH2-C, >CH-), 3.50-3.91 (m, -CH2-O-C), 4.27 (br, -C(dO)-O-CH2-), 4.61 (s, -CH2-O-C(dO)-O), 5.06 (s, >CH-O-C(dO)-O) ppm; 13C NMR (DMSO-d6) δ 16.72, 18.30 (-CH3), 39.92 (-CH2-C), 44.40 (>CCH-O), 154.61 (-O-C(dO)O-), 175.43, 176.25 (C-C(dO)-O-) ppm; IR (KBr): 1800 (CdO of carbonate), 1730 (CdO of ester), 1166 (C-O) cm-1. Typical Procedure for Fixation of CO2 under SolidState Conditions. The polymer containing 1 mmol equiv of oxirane groups, catalyst (1.5 mol%), and acetonitrile (1 mL) was added to a glass tube. The reaction mixture was stirred until it changed to a homogeneous solution, and then the solvent was evaporated under reduced pressure. After the sample was dried in a vacuum, CO2 gas was flowed into the glass tube under atmospheric pressure and heated at 80 or 100 °C. After the reaction, CO2 in the vessel was purged and the obtained residue was employed for analysis without further purification: 1H NMR (DMSO-d6) δ 0.79-0.96 (m, -CH3), 2.48 (br, >CH-), 3.53-3.81 (m, -CH2-O-C), 4.20-4.48 (m, -C(d O)-O-CH2-), 4.64 (s, -CH2-O-C(dO)-O), 5.09 (s, >CHO-C(dO)-O) ppm; 13C NMR (DMSO-d6) δ 16.65, 18.23
Fixation of Carbon Dioxide into an Oxirane Polymer 2155 (-CH3), 44.09 (-CH2-C), 44.47 (>CCH-O), 154.73 (-O-C(dO)O-), 175.59, 176.25 (C-C(dO)-O-) ppm; IR (KBr): 1802 (Cd O of carbonate), 1729 (CdO of ester), 1167 (C-O) cm-1.
Results and Discussion Copolymerization. The methacrylates containing crown ether (CMA1-5) were prepared according to the reaction procedure illustrated in Scheme 2. The radical copolymerizations of GMA with CMA1-5 using AIBN as an initiator afforded the corresponding copolymers (CP1-5, Mn ) 9400-13 800) in good yields. The Mn’s and Mw/Mn’s of the resulting polymers are summarized in Table 1. The ratio of crown ether unit in CPs was in accord with the feed ratio of the corresponding materials. The obtained CPs were soluble in common solvents such as chloroform, ethyl acetate, and THF. Fixation of CO2. In our previous report, the fixation of CO2 into the oxirane polymer having a crown ether group was carried out in nitromethane with NaI as a catalyst.6 This reaction system showed the advantageous effects of the crown ether unit upon the fixation. The reaction could be conducted by the use of nitromethane as a solvent, which dissolved the polymer but not the catalyst NaI, instead of the aprotic polar solvents such as DMF or NMP. In such a reaction, the ring size of the crown ether, which is related with the cation species of the catalyst, should affect the efficiency of the fixation. Therefore, at first, more detailed examinations on the reaction by the use of the several crown ethers having different ring sizes and LiBr, which was a good catalyst for this fixation, as well as NaI, were made.4 The results are summarized in Table 2.The fixation of CO2 into PGMA without a crown ether hardly proceeded (entry 1 in Table 2), while the fixation in the presence of monomeric crown ether (12-crown-4) proceeded to give the corresponding polymer having a fivemembered cyclic carbonate group in 35% yield (entry 2 in Table 2). Although the yield was low, the results demonstrated that the crown ether (12-crown-4) obviously contributed to the fixation of CO2 by activating the catalyst. The introduction of CMA units into the oxirane polymer resulted in the more efficient fixation of CO2. In the case of the copolymers of GMA and CMA1 (CP1), the yield of the cyclic carbonate groups increased moderately to 42% (entry 3 in Table 2). Next, the fixation of CO2 was examined on the copolymers (CPs) having the different sizes of crown ether moieties. The results indicated that the order of activating the catalyst, reflected in the yields, was 14-crown-4 > 15crown-5 > 16-crown-5 > 12-crown-4 > 13-crown-4 (entries 3-7 in Table 2). This tendency was in accord with the association constants of the crown ethers for Li+, which were reported by Czech et al.9 In that report, the association constants have assessed by solvent extraction of aqueous lithium picrate in the presence of various crown ethers. Therefore, it was reasonable to speculate that the crown ether moieties in the copolymer coordinated with LiBr dissolve and activate it by generating the corresponding ionic species. The reason that the crown ethers 12-crown-4 and 13-crown-4 were not effective in comparison with others might be due to their smaller ring sizes. In general, the complex of Li+ and the crown ether 12-crown-4 is said to form a 1:2 sandwiched structure in nitromethane solution.12 Consequently, the oxirane moieties cannot come into contact with the catalyst held by the crown ethers
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Macromolecules, Vol. 38, No. 6, 2005 Scheme 2
Table 1. Radical Copolymerization of GMA with CMAa feed composition (mol %) entry
GMA
1 2 3 4 5 6
100 90 90 90 90 90
CMA (ring size) 1 (12C4) 10 2 (13C4) 10 3 (14C4) 10 4 (15C5) 10 5 (16C5) 10
yield (%)b 92 90 88 85 89 91
compositionc x:y
Mn (Mw/Mn)d
polymer
92:8 89:11 91:9 92:8 91:9
11 400 (2.38) 10 100 (2.40) 13 800 (2.12) 10 100 (3.30) 10 500 (2.46) 9 400 (2.44)
PGMA CP1 CP2 CP3 CP4 CP5
a Conditions: total monomer 4 mmol, 3 mol% AIBN, DMF 2 mL, 60 °C for 20 h. b Ether-insoluble parts. c Estimated by comparing the integrated values of the 1H NMR signal of the methine proton of the oxirane unit (3.24 ppm) with that of methylene protons of the crown ether unit (3.56-3.95 ppm). d Estimated by GPC (THF, polystyrene standards).
Scheme 3
efficiently, especially with the cation species Li+, due to steric hindrance. Various lithium salts were employed for the reaction in which the copolymer (CP3) having the crown ether 14-crown-4 was employed to get information about the catalytic activities of anionic species (Table 3). In three of the lithium halides, the order of the catalytic activity for the fixation was Br- > I- > Cl-. It is known that the salt consisting of a more nucleophilic anion and a more Lewis acidic cation is generally more active for the ring-opening of oxiranes.13,14 However, in this reaction, bromide anion, which had a moderate nucleophilicity, gave the best result on the fixation of CO2 and iodide anion showed a slightly low catalytic activity. On the other hand, the use of chloride anion, a powerful nucleophile, was not effective for the transformation into
cyclic carbonate. This might be due to the low ability of chloride anion as a leaving group. Consequently, the carboxylate anion formed through the introduction of CO2 hardly bonded in place of chloride to the intramolecular methine carbon to afford the cyclic product. This seemed to lead the intermolecular attack of the carboxylate anion to other oxirane group, which resulted in gelation. Such speculation was supported by the results of the fixation using other anions. The nonnucleophilic anions such as perchlorate and triflate showed no catalytic activity for the formation of the cyclic carbonate, whereas the use of a higher nucleophilic acetate anion was ineffective for the cyclization but led to the gelation of CP3. We examined the fixation of CO2 into CP3 under solvent-free conditions (Table 4). Reaction systems that
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Fixation of Carbon Dioxide into an Oxirane Polymer 2157
Table 2. Fixation of CO2 into the Coplymer of GMA with CMAa
entry
polymer
1 2d 3 4 5 6 7
PGMA PGMA CP1 CP2 CP3 CP4 CP5
ring size
12C4 13C4 14C4 15C5 16C5
y unit (%)
fixation of CO2b (%)
log Ka for Li+ c
8 11 9 8 9
2 35 42 18 100 71 60
4.93 4.93 3.80 5.15 4.94 NDe
a Conditions: [oxirane] ) 1 M solution in nitromethane; LiBr 0 1.5 mol%; under CO2 atmosphere (1 atm) at 100 °C for 24 h. b Estimated by comparing the integrated values of the 1H NMR signal of the methine proton of the oxirane unit (3.24 ppm) with that of the cyclic carbonate unit (5.09 ppm). c Deuteriochloroformwater system at room temperature (22-23 °C). d Added 10 mol% of 12-crown-4. e Not determined.
Table 3. Fixation of CO2 into CP3 Catalyzed by Several Lithium Saltsa entry
catalyst X-
fixation of CO2b (%)
gelation
1 2 3 4 5 6
Cl Br I ClO4 TfO AcO
32 100 82 trace 0 2
O X X O X O
Figure 1. Relationship between time and the conversion of the oxirane group to carbonate groups in CP3 (a) in nitromethane ([oxirane]0 ) 1 M, 1.5 mol% LiBr), (b) in the solid state (5 mol% LiBr). The reaction temperature was 100 °C. Scheme 4
a Conditions: [oxirane] 0 ) 1 M solution in nitromethane; catalyst ) 1.5 mol%; under CO2 atmosphere (1 atm) at 100 °C for 24 h. b Estimated by comparing the integrated values of the 1H NMR signal of the methine proton of the oxirane unit (3.24 ppm) with that of the cyclic carbonate unit (5.09 ppm).
Table 4. Fixation of CO2 into CP3 Catalyzed by LiBr in the Solid Statea entry
LiBr mol%
temp °C
time h
fixation of CO2b %
1 2 3 4 5 6 7
1.5 1.5 1.5 1.5c 5.0 5.0 10.0
80 80 100 100 100 100 100
24 48 24 24 18 24 24
25 18 57 25 80 74 87
a Conditions: 1 mmol of oxirane unit under CO atmosphere (1 2 atm). b Estimated by comparing the integrated values of the 1H NMR signal of the methine proton of the oxirane unit (3.24 ppm) with that of the cyclic carbonate unit (5.09 ppm). c Added 10 mol% of 14-crown-4.
avoid the use of organic solvents are thought to be practical and agreeable procedures in the view of environmental problems. At 100 °C, the conversion to the cyclic carbonate groups increased with the amount of the catalyst LiBr and reached 87% in the use of 10 mol% of the catalyst (entry 6 in Table 4), while the lower conversion such as 25% was recorded in the fixation conducted at 80 °C with 1.5 mol% of LiBr (entry 1 in Table 4). Under analogous conditions, a yield of 57% was observed in the reaction conducted at 100 °C (entry 3 in Table 4). However, the presence of the gelled products, insoluble in DMSO and DMF, was observed in the all cases. Meanwhile, the fixation of the mixture of PGMA and 14-crown-4 was examined (entry 4 in Table 4). However, the conversion to carbonate was 25%, which was lower than the result of the corresponding copolymer (entry 3 in Table 4). It was assumed that the decrease of mobility of the activated LiBr coordinated by 14-crown-4 due to the solid-state condition.
Figure 1 depicts the relationships between time and the conversion to the carbonate groups by the fixation of CO2 in nitromethane solution and in the solid state, respectively. The reaction proceeded rapidly at the initial stage of the reaction in both cases. In the solid state, the fixation rate was depressed after ca. 4 h and the conversion was retained around 70%. On the other hand, in solution, the fixation proceeded almost quantitatively. The results indicated that the catalyst was activated through forming a complex with the crown ether moieties. However, in the solid state, the complex of the catalyst could react with only neighboring oxiranes due to less mobility. The finding that the conversion depended on an amount of the catalyst in the solid state may support this interpretation (Table 4). Further, the decrease of conversions of 79% at 18 h to 75% at 24 h was observed. The explanation may be accepted for the phenomena that the elimination of CO2 occurred from the formed cyclic carbonates, which led to the cross-linking reaction through an attack of the generated alcoholic anion to other oxiranes to give the insoluble products. This gelation may be another reason for the saturation of the conversion, as stated in the former solid-state reaction because the conversion was estimated on the basis of the protons of the cyclic carbonate group, but not the ether group. However, about the gelation itself, the elimination of CO2 was thought to be one of the reasons for the gelation since the obtained polymers at the early stage in the solid state contained a small amount of insoluble matter, which should be formed by intermolecular fixation of CO2. More detailed experiments are required to present a clear explanation for these phenomena. Scheme 4 showed the plausible mechanism of the fixation of CO2 based on the results mentioned in this
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work. At first, LiBr was activated by the formation of the complex with crown ether moieties bonded to the polymer backbone. Then, the neighboring oxiranes assembled to the immobilized catalyst and constructed the effective reaction environment for the fixation of CO2. In this environment, the activated catalyst coordinated with neighboring oxiranes, and then the fixation of CO2 through the nucleophilic attack of the oxygen of oxirane to the center carbon of CO2 proceeded. After the formation of the cyclic carbonate groups, the anion species of the catalyst may be caught again by the countercation on the crown ether moieties, although the presences of the free anion and the cation on the crown ether ring through the reaction have not been confirmed. Summary In this article, the fixation of carbon dioxide into the oxirane polymers containing crown ether moieties (CPs) catalyzed by lithium salts was demonstrated. The fixation of CO2 into the polymers proceeded more effectively in comparison with the polymer having no crown ether moiety, PGMA. In the fixation, the crown ether moiety was thought to contribute to the solvation and activation of lithium salts. Among crown ethers with several ring sizes, the combination of LiBr and 14crown-4 showed the most effective catalytic activity. This seemed to be reasonable in consideration of the association constant of the crown ether for lithium cation. In addition, the fixation of CO2 under solventfree conditions was possible, although the formation of a small amount of gelled product was observed. Such a solid-state reaction should be a more practical and
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convenient procedure in which no organic solvent is required. Further investigations are in progress in line with such a reaction system, and the results will be presented soon. References and Notes (1) (a) Organic and Bioorganic Chemistry of Carbon Dioxide; Inoue, S., Yamazaki, N., Eds.; Kodansha Ltd.: Tokyo, 1982. (b) Halmann, M. M. Chemical Fixation of Carbon Dioxide Methods for Recycling CO2 into Useful Products; CRC Press: Boca Raton, FL, 1993. (c) Keim, W. Carbon Dioxide as a Source of Carbon; Aresta, M., Forti, G., Eds.; NATO ASI series: Dordrecht, 1986; Vol. 206. (2) Behr, A. Carbon Dioxide Activation by Metal Complexes, VCH: Weinheim, 1988; p 91. (3) (a) Rokicki, G. Makromol. Chem. 1985, 186, 331. (b) Nomura, R.; Kori, M.; Matsuda, H. Makromol. Chem., Rapid Commun. 1988, 9, 739. (c) Kihara, N.; Kushida, Y.; Endo, T. J. Polym. Sci., Part A: Polym. Chem. 1996, 34, 2173. (4) Kihara, N.; Hara, N.; Endo, T. J. Org. Chem. 1993, 58, 6198. (5) Kihara, N.; Endo, T. Macromolecules 1992, 25, 4824. (6) Yamamoto, S.; Moriya, O.; Endo, T. Macromolecules 2003, 36, 1514. (7) Collie, L.; Denness, J. E.; Parker, D.; O’Carroll, F.; Tachon, C. J. Chem. Soc., Parkin Trans. 2 1993, 1747. (8) Higashiyama, N.; Nakamura, H.; Mishima, T.; Shiokawa, J.; Adachi, G. J. Electrochem. Soc. 1991, 138, 594. (9) Czech, B. P.; Babb, D. A.; Son, B.; Bartsch, R. A. J. Org. Chem. 1984, 49, 4805. (10) Tomoi, M.; Abe, O.; Ikeda, M.; Kihara, K.; Kakiuchi, H. Tetrahedron Lett. 1978, 33, 3031. (11) Liu, Y.; Inoue, Y.; Hakushi, T. Bull. Chem. Soc. Jpn. 1990, 63, 3044. (12) Smetana, A. J.; Popov, A. I. J. Solution Chem. 1980, 9, 183. (13) Rao, A. S.; Paknikar, S. K.; Kirtane, J. G. Tetrahedron 1983, 39, 2323. (14) Parker, R. E.; Isaacs, N. S. Chem. Rev. 1959, 59, 737.
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