Oxidative Cross-Coupling between Phenolic Polymer and Phenol

(bisphenol A) was 56/44. An iron salen-catalyzed coup- ling of poly(bisphenol A) with 1 was performed in pyridine at room temperature under air. The r...
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Macromolecules 2004, 37, 7901-7905

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Oxidative Cross-Coupling between Phenolic Polymer and Phenol-Containing Cellulose: Synthesis of a New Class of Artificial Wood Polymers Hiroyuki Tonami, Hiroshi Uyama, and Shiro Kobayashi* Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan Received March 24, 2004; Revised Manuscript Received August 3, 2004 ABSTRACT: Oxidative cross-coupling of phenolic polymers, mainly poly(bisphenol A) including poly(mcresol) and poly(p-tert-butylphenol), onto a phenol-containing cellulose has been performed in pyridine at room temperature under air to produce cellulose-phenolic polymer hybrids, cellulose-graft-phenolic polymers, which is a new synthetic method utilizing phenolic polymers as substrates. Iron salen complex and hydrogen peroxide were used as a catalyst and an oxidizing agent, respectively. During the reaction, the cross-coupling between the cellulose derivative and the phenolic polymer as well as the homocoupling of the phenolic polymer took place. The resultant cellulose-phenolic polymer hybrid was isolated by reprecipitation. Characterization of the hybrid was performed by 1H NMR, SEC, UV, and elemental analysis. The coupled amount of the phenolic polymer on the cellulose could be controlled by the feed ratio. The present study provides a new method for hybrid polymer synthesis by oxidative coupling, leading to production of a new type of biopolymers from abundant renewable resources.

Introduction

Scheme 1

Oxidative polymerization of phenols has been intensively studied to develop economically and ecologically superior processes of polymer synthesis because of the low-energy consumption and the environmentally benign reaction conditions.1-15 Most of these studies dealt with the homopolymerization of monomeric phenols in various reaction systems. For example, several enzyme catalysts were used to produce phenolic polymers.14 These enzymatically synthesized polymers are normally composed of a mixture of phenylene (Ph) and oxyphenylene (Ox) units. In contrast, conventional phenolic resins have been produced using toxic formaldehyde, which causes problems for practical use in some cases. Therefore, it is generally demanded to develop a good way for preparation of phenolic materials free from formaldehyde. A complex of iron with N,N′-ethylenebis(salicylideneamine) (Fe-salen) as well as a horseradish peroxidase enzyme catalyze the oxidative coupling of phenols using hydrogen peroxide as an oxidizing agent.15 While the enzyme is required to use an aqueous solvent for maintaining the catalytic activity, the iron salen complex catalyzes the oxidative polymerization in various organic solvents in the absence of water.12,13 Hence, this complex possesses an advantage over the enzymes for substrates showing low solubility toward aqueous solvents. Cellulose is the most abundant biopolymer of renewable resources; thus, modification of cellulose has been eagerly investigated for scientific and practical interest. From the commercial and environmental viewpoints, covalent modification of cellulose should be one of the most promising routes to afford a new class of highperformance green polymers. Grafting of polymers onto cellulose has been studied to improve the properties and/ or provide novel functions.16-26 So far, vinyl polymers and aliphatic polyesters have been often grafted on the cellulose backbone. Main components of woods are cellulose, hemicellulose, and lignin. In this study, we have synthesized a

hybrid polymer consisting of two components among them, cellulose and a lignin-model polymer, a phenolic polymer synthesized by peroxidase or its model complex (Scheme 1). This hybrid may be regarded as an artificial wood polymer. The hybrid polymers were synthesized by Fe-salen-catalyzed oxidative cross-coupling of the phenolic polymers with phenol-containing cellulose derivatives. To our best knowledge, this is the first example of grafting phenolic polymers onto cellulose polymers. Experimental Section Materials. Tosylcellulose27 and the iron salen complex28 were synthesized according to the literature. Phenolic polymers were synthesized by an oxidative coupling using peroxidase or the iron salen complex as catalyst.4,13,29 The other reagents and solvents were available from Wako Pure Chemical Industries (Japan) and Tokyo Kasei Co. (Japan) and used as received. Their purity was higher than 98%. Preparation of Phenol-Containing Cellulose (1). Tosylcellulose (3.5 g, degree of substitution (DS) ) 0.92) was swelled in 350 mL of N,N-dimethylformamide (DMF) at room temperature. After adding 3.5 g (20 mmol) of sodium 4-hydroxyphenyl acetate, the mixture was stirred at 100 °C under argon to obtain a clear solution. After 2 h, DMF was removed by evaporation under reduced pressure, and then the residue

10.1021/ma049420p CCC: $27.50 © 2004 American Chemical Society Published on Web 09/25/2004

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Macromolecules, Vol. 37, No. 21, 2004 Scheme 2

Table 1. Oxidative Grafting of Poly(bisphenol A) onto 1a entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14

poly(bisphenol A) (mg) 45.6 45.6 45.6 45.6 0 11.0 91.2 91.2 91.2 91.2 34.2 34.2 + 3.8d 34.2 + 3.8 × 2d 34.2 + 3.8 × 3d

H2O2 (µmol)

yield (mg)b

20 40 60 80 20 20 40 80 120 160 12 24 36 48

16 33 42 gel gel gel 20 21 20 17 18 23 35 39

unit ratioc 0.15 1.22 1.50

0.09 0.34 0.92 1.61

a Grafting of poly(bisphenol A) onto 1 (17 mg) using the iron salen complex (0.66 mg) in pyridine (2 mL) at room temperature for 24 h under air. b Methanol-insoluble part. c Unit molar ratio of bisphenol A per glucose, determined by elemental analysis. d Every 20 min, 3.8 mg of poly(bisphenol A) was added to the reaction mixture.

was washed with water and subsequently with methanol repeatedly, followed by drying in vacuo to give polymer 1 (yield 2.5 g) (Scheme 2). 1H NMR (DMSO-d6): δ 3.0-5.6 (br, cellulose backbone and CH2Ar), 6.5-6.8 (br, Ar), 6.9-7.2 (br, Ar), 9.19.4 (br, ArOH); Anal. Found: C, 51.89; H, 5.36; S, 1.76. Other phenol-containing celluloses 2 and 3 were synthesized via similar procedures. 1H NMR of 2 (DMSO-d6): δ 2.4-2.6 (br, CH2Ar), 2.6-2.8 (br, CH2CO), 3.0-5.6 (br, cellulose backbone), 6.5-6.8 (br, Ar), 6.9-7.2 (br, Ar), 9.1-9.4 (br, ArOH). 1H NMR of 3 (DMSO-d6): δ 3.0-5.6 (br, cellulose backbone), 6.6-6.9 (br, Ar), 7.6-7.9 (br, Ar), 10.1-10.4 (br, ArOH). Coupling of Poly(bisphenol A) with Phenol-Containing Cellulose. The following is a typical procedure (entry 14 in Table 1). Phenol-containing cellulose 1 (17 mg), the iron salen complex (0.66 mg, 1.0 µmol), and poly(bisphenol A) (34.2 mg) were dissolved in pyridine (2 mL) at room temperature under air. Hydrogen peroxide (12 µmol) was added with stirring. Every 20 min, 3.8 mg of poly(bisphenol A) and 12 µmol of hydrogen peroxide were added to the reaction mixture three times. After 23 h of further stirring, the reaction mixture was poured into an excess amount of methanol to remove homocoupled poly(bisphenol A) and the catalyst. The precipitate was collected by centrifugation and washed with methanol, followed by drying in vacuo (yield 39 mg). 1H NMR (DMSO-d6): δ 1.0-1.7 (br, CH3) 3.0-5.6 (br, cellulose backbone), 6.3-7.2 (br, Ar), 9.0-9.4 (br, ArOH); Anal. Found: C, 66.79; H, 5.73; S, 0.64. Hydrolysis of the Hybrid. The isolated hybrid (15 mg, entry 12 in Table 1) was dissolved in DMF (4.0 mL). To the solution, added was 50 µL of sodium methoxide solution in methanol (28 w/v %) at room temperature under a nitrogen atmosphere. After 24 h of stirring, the solution was poured

Figure 1. SEC traces of the reaction mixtures obtained by the oxidative coupling of poly(bisphenol A) in the presence of 1 (17 mg), monitored by UV detector (340 nm). (a) Before the reaction (poly(bisphenol A): 34.2 mg), (b) H2O2: 12 µmol, poly(bisphenol A): 34.2 mg, (c) H2O2: 24 µmol, poly(bisphenol A): 38.0 mg, (d) H2O2: 36 µmol, poly(bisphenol A): 41.8 mg, (e) H2O2: 48 µmol, poly(bisphenol A): 45.6 mg, using iron salen catalyst (0.66 mg) in pyridine (2 mL) at room temperature under air. into an excess amount of water and acidified by dilute HCl solution. The precipitate was collected by centrifugation and washed with water, followed by drying in vacuo (yield 6 mg). Measurements. 1H NMR spectra were recorded on a 400 MHz Bruker DPX-400 instrument. SEC analyses were carried out using a TOSOH SC8020 apparatus with a UV detector at 60 °C under the following conditions: TSKgel R-M column (TOSOH, linear-type column covered in all ranges of molecular weight) and 0.1 M LiCl/DMF eluent at a flow rate of 1.0 mL/ min. The calibration curves for SEC analysis were obtained using polystyrene standards. UV spectra were measured on a Hitachi U-2001 spectrometer.

Results and Discussion Oxidative Coupling of Poly(bisphenol A) with a Phenol-Containing Cellulose. A cellulose-phenolic polymer hybrid was synthesized via oxidative coupling of the phenol moiety between cellulose and the phenolic polymer. Cellulose has no phenolic group; thus, a phenol-containing cellulose derivative was designed and prepared as shown in Scheme 2. At first, tosylation of cellulose was performed in N,N-dimethylacetamide (DMA)/LiCl,23 and subsequently a phenol group was introduced into cellulose by reaction of tosyl cellulose with sodium 4-hydroxyphenyl acetate to produce a phenol-containing cellulose (1). DS of the phenol group was determined by elemental analysis and 1H NMR as 0.75. The phenol-containing cellulose was soluble in highly polar organic solvents like DMF, dimethyl sulfoxide (DMSO), and pyridine. Here, poly(bisphenol A)13 with number-average molecular weight of 7.2 × 103 was mainly used as a phenolic polymer, which shows high solubility toward polar organic solvents suitable for the analysis of the oxidative coupling. The unit ratio (Ph/Ox) of poly(bisphenol A) was 56/44. An iron salen-catalyzed coupling of poly(bisphenol A) with 1 was performed in pyridine at room temperature under air. The reaction proceeded by adding aqueous hydrogen peroxide dropwise. After each drop, a small portion of the reaction mixture was analyzed by size exclusion chromatography (SEC) with UV detector at 340 nm (Figure 1). This wavelength was chosen to detect only coupling products containing poly(bisphenol A) chains because 1 has no absorption at 340 nm. Before the reaction, one unimodal

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Oxidative Cross-Coupling of Phenolic Polymers 7903 Scheme 3

peak due to poly(bisphenol A) was observed at a retention time of ca. 17 min (Figure 1a). In the coupling, both the phenolic polymer and 1 have a reactive phenol group. Therefore, the cross-coupling between both polymers as well as homocoupling of the phenolic polymer or 1 simultaneously takes place. For the efficient production of the hybrid polymer, it is necessary to examine the reaction conditions for the selective crosscoupling. When a small amount of hydrogen peroxide was added, the peak due to poly(bisphenol A) was shifted in a shorter elution time (ca. 16 min), and at the same time, a small peak newly appeared at a retention time of ca. 13.5 min (Figure 1b). The former suggests the occurrence of the homocoupling of poly(bisphenol A), and the latter is due to the hybrid formed by cross-coupling between poly(bisphenol A) and the phenol-containing cellulose. In the course of the present oxidative coupling with the addition of hydrogen peroxide, the amount of the hybrid increased gradually (Figure 1c-e). This result indicates that poly(bisphenol A) was coupled with 1, accompanying homocoupling of poly(bisphenol A). The homocoupling of 1 might be suppressed under the present reaction conditions, considering the fact that addition of only one drop hydrogen peroxide to 1 caused gelation in the absence of poly(bisphenol A). The hybrid was isolated by pouring the reaction mixture into an excess amount of methanol; after the reprecipitation, only the peak of the higher molecular weight product was detected in the SEC chart. The structure of the hybrid was confirmed by 1H NMR, UV, and elemental analysis. The proposed structures of the linkage between the phenol group formed by the oxidative grafting are depicted in Scheme 3. When tosyl cellulose was used instead of 1, the hybrid was not obtained. Furthermore, the control experiment showed that the iron salen complex and hydrogen peroxide were absolutely necessary for producing the hybrid. These data clearly indicate that the phenolic group of the backbone is critical to produce the hybrid and that noncovalent bonds such as van der Waals interaction and/or hydrogen bond between cellulose and phenolic polymer are not essential for the hybrid formation. The hybrid synthesis was summarized in Table 1. The appropriate feed amount (entries 1-4) achieved the

sufficient cross-coupling to produce the hybrid containing a large amount of the poly(bisphenol A) moiety. When the feed amount of poly(bisphenol A) was none (entry 5) or small (entry 6), the gel formation was immediately observed by the addition of hydrogen peroxide, indicating the homocoupling of 1 preferentially took place. In using a large amount of poly(bisphenol A) (entries 7-10), the gelation did not take place; however, the SEC analysis of the product showed that the homocoupling of poly(bisphenol A) mainly proceeded (data not shown), and the cross-coupling was not favored. Thus, the hybrid formation was much affected by the feed ratio. The appropriate reaction conditions enabled the efficient hybrid synthesis and control of the hybrid composition. To control the concentration of poly(bisphenol A) more favorable for the cross-coupling, poly(bisphenol A) was added gradually during the oxidative coupling (entries 12-14). In this method, the soluble hybrid bearing 1.61 bisphenol A unit per 1 glucose unit was obtained. Effect of Spacer Structure of Phenol-Containing Cellulose on Oxidative Coupling with Poly(bisphenol A). Effect of the spacer structure between cellulose and phenol group was investigated (Figure 2). Two cellulose derivatives (2 and 3), having nearly the same DS as 1, were synthesized for the oxidative coupling with poly(bisphenol A) (Scheme 4). The coupling was carried out under similar reaction conditions of entries 1-3 in Table 1. In the oxidative coupling in the presence of 2, gelation took place by the addition of 40 µmol of hydrogen peroxide, suggesting higher reactivity of 2 in the oxidative coupling, resulting in the formation of the insoluble gel (Figure 2A). In using 3 as a cellulose backbone, on the other hand, the corresponding hybrid was not formed; only the homocoupling of poly(bisphenol A) took place (Figure 2B). Two factors are considered for no formation of the hybrid: the shorter spacer length of 3 between the cellulose backbone and reactive phenol group and the lower oxidative reactivity of the phenol group due to the substituent of electron-withdrawing carbonyl group. These data suggest that the reactivity of the phenol-containing cellulose should be controlled by the spacer structure for the production of the soluble cellulose-phenolic polymer hybrid.

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Figure 3. SEC traces of (a) the solution before the reaction, (b) reaction mixture, (c) isolated hybrid, and (d) hydrolyzed product of the isolated hybrid, monitored by UV detector (340 nm).

Figure 2. SEC traces of the reaction mixtures obtained by the oxidative coupling of poly(bisphenol A) in the presence of (A) 2 (18 mg) and (B) 3 (17 mg), monitored by UV detector (340 nm). (a) Before the reaction (poly(bisphenol A): 45.6 mg), (b) H2O2: 20 µmol, poly(bisphenol A): 45.6 mg (c) H2O2: 40 µmol, poly(bisphenol A): 45.6 mg. Scheme 4

Figure 4. SEC traces of hydrolyzed products monitored by UV detector (340 nm). (a) Poly(bisphenol A), (b) H2O2: 12 µmol, poly(bisphenol A): 34.2 mg, (c) H2O2: 24 µmol, poly(bisphenol A): 38.0 mg, (d) H2O2: 36 µmol, poly(bisphenol A): 41.8 mg, using iron salen catalyst (0.66 mg) in pyridine (2 mL) at room temperature under air.

Hydrolysis of Hybrid. To confirm the hybrid structure, the isolated hybrid was subjected to the alkaline hydrolysis, and the hydrolyzed product was analyzed by SEC with UV detector at 340 nm. SEC traces of the reaction mixture (entry 12 in Table 1), the isolated hybrid, and the hydrolyzed product are shown in Figure 3. The detected product of the hydrolysis mixture by SEC was the poly(bisphenol A) branch. The hydrolyzed product showed nearly the same retention time as homocoupled poly(bisphenol A) in the reaction mixture. The SEC analysis revealed that the molecular weight of the poly(bisphenol A) branch increased with the addition of hydrogen peroxide (Figure 4). A similar behavior was observed in the homocoupling of poly(bisphenol A). This means that poly(bisphenol A) coupled with the cellulose backbone at the early stage of the reaction would be further coupled with other poly(bisphenol A) molecules, leading to the increase in the molecular weight of the poly(bisphenol A) branch. Coupling of Enzymatically Polymerized Phenolic Polymers with 1. Coupling of other phenolic polymers, poly(m-cresol)4 and poly(p-tert-butylphenol),29 with 1 has been examined. These polymers were synthesized by an enzymatic oxidative polymerization. The

number-average molecular weight and unit ratio (Ph/ OX) of poly(m-cresol) were 2.5 × 103 and 63/37, respectively, and those of poly(p-tert-butylphenol) were 1.6 × 103 and 57/43. The coupling behavior was analyzed by SEC (Figure 5). The formation of the hybrid at the retention time of ca. 13 min was seen as in the case with poly(bisphenol A). The outlines of the reactions were similar regardless of the phenolic polymer structure, although the reactivity was considerably different. These data strongly suggest that the present method can be applied to synthesis of hybrid materials consisting of natural and synthetic phenolic polymers. Conclusion Phenolic polymers were oxidatively cross-coupled with a phenol-containing cellulose by using an iron salen complex as a catalyst, producing cellulose-phenolic polymer hybrids, which may be regarded as artificial wood polymers. This is a new synthetic method of hybrid materials from phenol-containing polymers via oxidative coupling utilizing phenolic polymers as substrates. By selecting appropriate reaction conditions, the crosscoupling between the two polymers preferentially took place to produce the soluble hybrids without gelation. The hybrid composition could be controlled by the amount of hydrogen peroxide and of the feed ratio.

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References and Notes

Figure 5. SEC traces of the reaction mixtures obtained by the oxidative coupling of (A) poly(p-tert-butylphenol) (44 mg) and (B) poly(m-cresol) (21 mg) with 1 (17 mg), monitored by UV detector (340 nm). (a) Before reaction, (b) H2O2: 40 µmol, (c) H2O2: 80 µmol, (d) H2O2: 120 µmol, (e) H2O2: 160 µmol, (f) H2O2: 8 µmol, (g) H2O2: 16 µmol, (h) H2O2: 24 µmol.

Furthermore, the structure of the spacer between cellulose and phenol group in the cellulose backbone and also the structure of phenolic polymers greatly affected the coupling behaviors. Further investigations on syntheses of biopolymer-based hybrids by oxidative couplings are under progress in our laboratory. Acknowledgment. This work was partly supported by the Program for Promotion of Basic Research Activities for Innovative Bioscience, by the 21st century COE program, COE for a United Approach to New Materials Science, and by Japan-U.S. Cooperative Science Joint Program-Japan Society for the Promotion of Science (2003-2005).

(1) Reihmann, M. H.; Ritter, H. Macromol. Chem. Phys. 2000, 201, 798. (2) Fukuoka, T.; Tonami, H.; Maruichi, N.; Uyama, H.; Kobayashi, S.; Higashimura, H. Macromolules 2000, 33, 9152. (3) Liu, W.; Cholli, A. L.; Kumar, J.; Tripathy, S.; Samuelson, L. Macromolecules 2001, 34, 3522. (4) Tonami, H.; Uyama, H.; Kobayashi, S.; Kubota, M. Macromol. Chem. Phys. 1999, 200, 2365. (5) Uyama, H.; Lohavisavapanich, C.; Ikeda, R.; Kobayashi, S. Macromolecules 1998, 31, 554. (6) Dordick, J. S.; Marletta, M. A.; Klibanov, A. M. Biotechnol. Bioeng. 1987, 30, 31. (7) Rao, A. M.; John, V. T.; Gonzalez, R. D.; Akkara, J. A.; Kaplan, D. L. Biotechnol. Bioeng. 1993, 41, 531. (8) Uyama, H.; Kurioka, H.; Kaneko, I.; Kobayashi, S. Chem. Lett. 1994, 423. (9) Hay, A. S.; Blanchard, H. S.; Endres, G. F.; Eustance, J. W. J. Am. Chem. Soc. 1959, 81, 6335. (10) Hay, A. S. J. Polym. Sci., Polym. Chem. Ed. 1998, 36, 505. (11) Gross, R. A., Kaplan, D. L., Swift, G., Eds.; ACS Symp. Ser. 1998, 684. (12) Tonami, H.; Uyama, H.; Kobayashi, S.; Higashimura, H.; Oguchi, T. J. Macromol. Sci., Pure Appl. Chem. 1999, A36, 719. (13) Tonami, H.; Uyama, H.; Oguchi, T.; Higashimura, H.; Kobayashi, S. Polym. Bull. (Berlin) 1999, 42, 125. (14) Kobayashi, S.; Uyama, H.; Kimura, S. Chem. Rev. 2001, 101, 3793. (15) Kobayashi, S.; Higashimura, H. Prog. Polym. Sci., 2003, 28, 1015. (16) Halab-Kessira, L.; Ricard, A. Eur. Polym. J. 1999, 35, 1065. (17) Mais, U.; Binder, W. H.; Knaus, S.; Gruber, H. Macromol. Chem. Phys. 2000, 201, 2115. (18) Carlmark, A.; Malmstro¨m, E. J. Am. Chem. Soc. 2002, 124, 900. (19) Shukla, S. R.; Athalye, A. R. J. Appl. Polym. Sci. 1993, 48, 1877. (20) Yoshinobu, M.; Morita, M.; Sakata, I. J. Appl. Polym. Sci. 1992, 45, 805. (21) Zhang, Z. B.; McCormick, C. L. J. Appl. Polym. Sci. 1997, 66, 307. (22) Berlin, A. A.; Kislenko, V. N. Prog. Polym. Sci. 1992, 17, 765. (23) Kra¨ssig, H. Sven. Papperstidn. 1971, 74, 417. (24) Klemm, D.; Schumann. D.; Udhardt, U.; Marsch, S. Prog. Polym. Sci. 2001, 26, 1561. (25) Heinze, T.; Liebert, T. Prog. Polym. Sci. 2001, 26, 1689. (26) Kobayashi, S.; Uyama, H. Macromol. Chem. Phys. 2003, 204, 235. (27) Rahn, K.; Diamantoglou, M.; Klemm, D.; Berghmans, H.; Heinze, Th. Angew. Makromol. Chem. 1996, 238, 143. (28) Pfeiffer, P.; Breith, E.; Lu¨bbe, E.; Tumaki, T. Ann. 1933, 503, 84. (29) Mita, N.; Tawaki, S.; Uyama, H.; Kobayashi, S. Chem. Lett. 2002, 402.

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