Biomacromolecules 2001, 2, 29-31
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Enzymatic Synthesis of Cross-Linkable Polyesters from Renewable Resources Takashi Tsujimoto, Hiroshi Uyama, and Shiro Kobayashi* Department of Materials Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan Received September 22, 2000; Revised Manuscript Received November 6, 2000
Enzymatic synthesis of a new class of cross-linkable polyesters has been achieved. Polymerization of divinyl sebacate and glycerol using Candida antarctica lipase as catalyst in the presence of unsaturated higher fatty acids produced the polyesters having an unsaturated group in the side chain. The polyester was subjected to hardening by cobalt naphthenate catalyst or thermal treatment, yielding cross-linked transparent film. Polymeric materials production from renewable plantbased substrates has received world-wide attention from both social and environmental viewpoints, since their use contributes to global sustainability without depletion of scarce resources. We have achieved a new cross-linkable polyphenol from cardanol, a main component obtained by thermal treatment of cashew nut shell liquid (CNSL).1 Although CNSL is nearly one-third of the total nut weight and much CNSL is formed as a byproduct from mechanical processes for the edible use of the cashew kernel, only a small part of CNSL has been used in industrial fields. On the other hand, our finding for new applications of CNSL will provide a new insight in development of polymeric materials from renewable resources. Lipase is widely used as a catalyst for modification of plant oils (triglycerides).2 Lipase-catalyzed hydrolysis of triglycerides and esterification of glycerol with fatty acids take place by choosing the reaction conditions, which leads to production of functional oils by the selective hydrolysis (or esterification) via lipase catalysis. Lipase also acted as a catalyst for synthesis of biodegradable polyesters and polycarbonates.3 So far, various monomer combinations have produced the polymers under mild reaction conditions. Furthermore, the specific catalysis of lipase induced the enantio- and regioselective polymerizations, yielding new functional polymers,4 which cannot be obtained by conventional polymerizations. In the lipasecatalyzed (co)polymerization of racemic lactones in four- or six-membered rings, the enantioselection took place to give the optically active polyesters.4d Lipase catalyzed the regioselective polymerization of diacid derivatives and polyols such as glycerol and sugar alcohols to give the reactive polyesters having hydroxy groups in the main chain.4e,f This study deals with synthesis of cross-linkable polyesters having an unsaturated group in the side chain by the lipase-catalyzed polymerization of divinyl sebacate (1) and glycerol (2) in the presence of unsaturated higher fatty acids (3) from plant oils. A desired structure of the repeating units in the polymer is shown in Scheme 1. The resulting polymer was synthesized by a single-step facile procedure and found
Scheme 1
to be an efficient prepolymer for coating materials. In a relevant study, enzymatic synthesis of unsaturated polyesters having fumarate or maleate groups in the main chain was reported, which were synthesized by the polycondensation of the corresponding diacid derivatives with glycols.5 These main-chain type unsaturated polyesters can be used for conventional thermosetting resins with vinyl monomers. In this study, Candida antarctica lipase was used as catalyst, which showed high catalytic activity toward polycondensation of divinyl esters and glycols and ring-opening polymerization of lactones and cyclic carbonates.6 For 3, employed were oleic, linoleic, and linolenic acids (3a, 3b, and 3c, respectively). The polymerization of 1 and 2 in the presence of 3 was carried out in bulk under argon.7 In most cases, the monomers were quantitatively consumed. After the polymerization, the polymeric materials were purified by washing the reaction mixture with a mixed solvent of methanol and water (95:5 vol %). During the purified procedure, the oligomer soluble in the solvent might be lost. Molecular weight was determined by size exclusion chromatography (SEC). Polymerization results are summarized in Table 1. In the polymerization of 1 and 2 in the presence of 3b at 60 °C for 24 h under argon (entry 4), the oily polymer was obtained in 58% yield. The molecular weight determined by SEC was 4700. In the polymerization without the enzyme (control experiment), the polymeric materials were not
10.1021/bm000097h CCC: $20.00 © 2001 American Chemical Society Published on Web 12/19/2000
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Table 1. Lipase-Catalyzed Polymerization of 1 and 2 in the Presence of 3a entry
unsaturated acid
temp (°C)
pressure (Pa, ×10-3)
yieldb (%)
Mnc (×10-3)
1 2 3 4 5 6 7 8
3a 3a 3b 3b 3b 3b 3c 3c
60 60 45 60 75 60 60 60
100 2.7 100 100 100 2.7 100 2.7
54 77 26 58 74 77 56 71
5.5 7.5 3.9 4.7 6.1 8.5 5.6 8.0
polymer Mw/Mnc 1.6 6.1 1.2 1.4 1.9 3.3 1.4 3.4
3/1d
3/2d
0.93 0.81 1.2 1.1 1.1 0.86 1.0 1.1
1.1 0.94 1.2 1.3 1.3 1.1 1.3 1.3
a Polymerization of 1 and 2 in the presence of 3 (each 2.0 mmol) using lipase catalyst (100 mg) in bulk for 24 h. b Methanol/water (95:5 vol %)insoluble part. c Determined by SEC using tetrahydrofuran as eluent, calibrated with polystyrene standards. d Determined by 1H NMR.
Figure 2. Expanded and (B) entry 6.
Figure 1.
1H
NMR spectrum of polymer (entry 4).
formed, indicating that the present polymerization took place through the enzyme catalysis. The polymer structure was analyzed by 1H NMR spectroscopy (Figure 1). There were characteristic peaks (peaks A and B) at δ 5.4 and 4.2 due to protons of unsaturated and glyceride units, respectively. The ratio of integrated areas between the former peak and a triplet peak (peak H) at δ 0.9 ascribed to terminal methyl protons of the unsaturated fatty acid was 4:3, indicating that the unsaturated group was not reacted during the polymerization. Assignment of other peaks is shown in Figure 1 (typically, a unit having the linoleyl group at the 2-position of glycerol). Unit ratios of the polymer, the ratio between the unsaturated group and sebacate unit (3/1) and that between the unsaturated group and glyceride unit (3/2), were determined from the integrated area of peaks B, D, and H. The values of 3/1 and 3/2 were 1.1 and 1.3, respectively, which were relatively close to the feed ratios. The temperature effect on the polymerization has been examined (entries 3-5). The polymer yield and molecular weight increased as a function of temperature, whereas the polymer composition was almost the same. The polymerization results hardly depended on the acid structure (entries 1, 4, and 7). To increase the molecular weight in the enzymatic synthesis of polyesters, the polymerization has been often carried out under reduced pressure.8 The leaving group (water, or acetaldehyde) was removed from the reaction mixture, leading to the shift of the equilibrium to the polymer. When the polymerization was performed under the reduced pressure (2700 Pa) (entries 2, 6, and 8), the polymer yield
13C
NMR spectra of polymers of (A) entry 4
and molecular weight increased in comparison with those of the polymer obtained under ambient conditions. We have reported the microstructural analysis of the glyceride unit in the lipase-catalyzed polymerization of 1 and 2 by 13C NMR.4e Figure 2 shows expanded 13C NMR spectra of polymers obtained under ambient and reduced pressures. For both cases, the trisubstituted unit was mainly formed. The content of the trisubstituted unit in the polymer obtained under reduced pressure was larger than that obtained under ambient pressure, which may be related to the formation of the higher molecular weight polymer under reduced pressure. The curing of the polymers obtained under reduced pressure was examined by two methods: oxidation catalyzed by cobalt naphthenate (3 wt % for 4) in air and thermal treatment (150 °C for 2 h). The sample film was prepared on a glass slide by using an applicator for 50 µm thickness and allowed to stand at 25 °C under 70% humidity. The curing was monitored by pencil scratch hardness.9 In both methods, 4b and 4c were cured within 2 h to give transparent films; however, cross-linking of 4a did not take place. These data indicate that 4b and 4c were good cross-linkable prepolymers and two or three unsaturated groups in the side chain were required for the hardening. The pencil scratch hardness values of the thermally treated film from 4b and 4c after 1 day were 5B and 2B, respectively. Afterward, the hardness was almost constant. FT-IR analysis of the cured film suggests that the cross-linking mechanism of the present polymer was similar to that of the oil autoxidation.1b,c In conclusion, novel cross-linkable polyesters were conveniently synthesized by lipase-catalyzed polymerization of divinyl sebacate and glycerol in the presence of unsaturated fatty acids. The resulting polymer was cured by cobalt naphthenate or thermal treatment. The present prepolymer
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is synthesized from renewable plant-based materials without use of toxic reagents under mild reaction conditions. In the curing stage, the cross-linked polymeric film is obtained in the absence of organic solvents at an ambient temperature under air. Therefore, the present method is expected as an environmentally benign process of polymer coating, giving an example system of green polymer chemistry.1b,c,10 Further investigations on the production of cross-linkable polymers from renewable resources are under way in our laboratory. Acknowledgment. This work was partly supported by a Grant-in-Aid for Specially Promoted Research (No. 08102002) from the Ministry of Education, Science, and Culture, Japan. We acknowledge the gift of divinyl sebacate and lipase from Shin-etsu Chemical Co. and Novo Nordisk Bioindustry, Ltd., respectively. References and Notes (1) (a) Uyama, H.; Kobayashi, S. CHEMTECH 1999, 29 (10), 22. (b) Ikeda, R.; Tanaka, H.; Uyama, H.; Kobayashi, Macromol. Rapid Commun. 2000, 21, 496. (c) Ikeda, R.; Tanaka, H.; Uyama, H.; Kobayashi, Polym. J. 2000, 32, 589. (d) Uyama, H.; Ikeda, R.; Yaguchi, S.; Kobayashi, S. ACS Symp. Ser. 2000, no. 764, 113. (2) (a) Bornscheuer, U. T.; Yamane, T. Enzymol. Microb. Technol. 1994, 16, 864. (b) Partali, V.; Kvittingen, L.; Sliwka, H.-R.; Anthonsen, T. Angew. Chem., Int. Ed. Engl. 1996, 35, 329. (3) (a) Kobayashi, S.; Shoda, S.; Uyama, H. In The Polymeric Materials Encyclopedia; Salamone, J. C., Ed.; CRC Press: Boca Raton, 1996; pp 2102-2107. (b) Kobayashi, S.; Shoda, S.; Uyama, H. In Catalysis in Precision Polymerization; Kobayashi, S., Ed.; John Wiley & Sons: Chichester, 1997; Chapter 8. (c) Ritter, H. In Desk Reference of Functional Polymers, Syntheses and Applications; Arshady, R., Ed.; American Chemical Society: Washington, DC, 1997; pp 103113. (d) Gross, R. A., Kaplan, D. L., Swift, G., Eds. ACS Symp. Ser. 1998, No. 684. (e) Kobayashi S.; Uyama H. In Materials Science and TechnologysSynthesis of Polymers; Schlu¨ter, A.-D., Ed.; Wiley-
(4)
(5)
(6)
(7)
(8) (9) (10)
VCH: Weinheim, 1998; Chapter 16. (f) Kobayashi, S. J. Polym. Sci., Polym. Chem. Ed. 1999, 37, 3041. (a) Wallace, J. S.; Morrow, C. J. J. Polym. Sci., Polym. Chem. Ed. 1989, 27, 2553. (b) Svirkin, Y. Y.; Xu, J.; Gross, R. A.; Kaplan, D. L.; Swift, G. Macromolecules 1996, 29, 4591. (c) Xie, W.; Li, J.; Chen, D.; Wang, P. G. Macromolecules 1997, 30, 6997. (d) Kikuchi, H.; Uyama, H.; Kobayashi, S. Macromolecules 2000, 33, 8971. (e) Uyama, H.; Inada, K.; Kobayashi, S. Macromol. Rapid Commun. 1999, 20, 171. (f) Uyama, H.; Klegraf, E.; Wada, S.; Kobayashi, S. Chem. Lett. 2000, 800. (g) Matsumura, S.; Suzuki, Y.; Tsukuda, K.; Toshima, K.; Doi, Y.; Kasuya, K. Macromolecules 1998, 31, 6444. (h) Bisht, K. S.; Svirkin, Y. Y.; Henderson, L. A.; Gross, R. A.; Kaplan, D. L.; Swift, G. Macromolecules 1997, 30, 7735. (i) Bisht, K. S.; Deng, F.; Gross, R. A.; Kaplan, D.; Swift, G. J. Am. Chem. Soc. 1998, 120, 1363. (j) Co´rdova, A.; Hult A.; Hult, K.; Ihre, H.; Iversen, T.; Malmstro¨m, E. J. Am. Chem. Soc. 1998, 120, 13521. (k) Kumar, A.; Gross, R. A. Biomacromolecules 2000, 1, 133. (l) Co´rdova, A.; Iversen, T.; Hult, K. Polymer 2000, 40, 6709. (a) Geresh, S.; Gilboa, Y. Biotechnol. Bioeng. 1990, 36, 270. (b) Geresh, S.; Gilboa, Y.; Abrahami, S.; Bershadsky, A. Polym. Eng. Sci. 1993, 33, 311. (c) Mezoul, G.; Lalot, T.; Brigodiot, M.; Mare´chal, E. Macromol. Rapid Commun. 1995, 16, 613. (a) Uyama, H.; Yaguchi, S.; Kobayashi, S. J. Polym. Sci., Polym. Chem. Ed. 1999, 37, 2737. (b) Uyama, H.; Suda, S.; Kikuchi, H.; Kobayashi, S. Chem. Lett. 1997, 1109. (c) Deng, F.; Gross, R. A. Int. J. Biol. Macromol. 1999, 25, 153. The following is a typical procedure for the polymerization (entry 4). Under argon, a mixture of divinyl sebacate (0.51 g, 2.0 mmol), glycerol (0.18 g, 2.0 mmol), and linoleic acid (0.56 g, 2.0 mmol) was placed in a dried test tube and sealed. The tube was kept under gentle stirring at 60 °C for 24 h. A small amount of THF was added to the mixture and the part of the organic solution was separated by filtration. The filtrate was poured into a large amount of methanol/ water (95:5 vol %). The resulting oily precipitates were collected by centrifugation, followed by drying in vacuo to give 0.60 g of the polymer (yield 58%). (a) Linko, Y.-Y.; Seppa¨la¨, J. CHEMTECH 1996, 26 (8), 25. (b) Uyama, H.; Inada, K.; Kobayashi, S. Polym. J. 2000, 32, 440. Guevin, P. R., Jr. J. Coat. Technol. 1995, 67, 61. Kobayashi, S. High Polym., Jpn. 1999, 48, 124.
BM000097H
