Effect of Polycondensation Conditions on Structure and Thermal

Chapter 17

Downloaded by MONASH UNIV on September 20, 2015 | http://pubs.acs.org Publication Date (Web): November 22, 2013 | doi: 10.1021/bk-2013-1144.ch017

Effect of Polycondensation Conditions on Structure and Thermal Properties of Poly(caffeic acid) Daisuke Ishii,*,1,3 Hiroki Maeda,1 Hisao Hayashi,1 Tomohiko Mitani,2 Naoki Shinohara,2 Koichi Yoshioka,2 and Takashi Watanabe2 1Department

of Materials Chemistry, Graduate School of Science and Technology, Ryukoku University, Otsu, Shiga 520-2194, Japan 2Research Institute for Sustainable Humanosphere (RISH), Kyoto University, Uji, Kyoto 611-0011, Japan 3Current address: Department of Biomaterials Sciences, Graduate School of Agricultural and Life Science, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan *E-mail: [email protected]

Effect of polycondensation conditions of caffeic acid (CA) on solubility and thermal properties of the resultant poly(caffeic acid) (PCA), a homopolyester of CA, was investigated. Polycondensation of CA was performed by transesterification after acetylation using sodium acetate and acetic anhydride. Acetylation and oligomerization of CA was performed by refluxing a mixture of CA, sodium acetate and acetic anhydride at 160 °C for 18 h or at 100 °C for 2 h under a nitrogen atmosphere. Polycondensation of the acetylated-oligomerated CAs was performed at 160 °C or 200 °C and under reduced pressure below 3 kPa. Characterization of PCAs was performed by FTIR, MALDI-TOF-MS, 1H NMR, GPC, DSC, TG-DTA and hotstage-equipped polarized optical microscopy. Both the solubility and thermal properties of PCAs significantly depended on polycondensation conditions. In particular, PCAs polycondensed at 200 °C lost melt fusibility. However, the infusible PCAs showed glass transition at about 110 °C and were thermally processed by hotpress. The thermally processed PCA film retained its original form even after heated up to 300 °C.

© 2013 American Chemical Society In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Introduction Recently, needs for ‘biomass plastics’ that is produced from natural biomasses (woods, agricultural wastes, and many other resources from plants, animals, and micrroorganisms) as starting materials or monomeric components have extensively been increased. Biopolyesters of microbial or synthetic origins, such as poly(hydroxyalkanoate)s and poly(lactide)s, are expected as most promising biomass plastics for commodity or specialty applications (1, 2). These biological polyesters have improved mechanical properties, high processability into films (3, 4), fibers (5–10), and other various forms. Furthermore, the polyesters show biodegradability under in vitro enzymatic (11, 12), alkaline (13), physiological (14, 15), and microbial (16) conditions under the hydrated state. Apart from these advantages, improvement of thermal stability at the elevated temperatures still remains as a major problem. While stereocomplex formation of enantiomeric poly(lactide)s has been proposed for the increment of melting point (17, 18), improvement of thermal degradability should also be attained. One of the expected solutions to improve the thermal stability is the introduction of aromatic moieties as rigid component to polymer backbone. In this context, we intend to utilize lignin-related materials, that can be obtained from wooden or plant biomasses, as starting materials for biomass polymers. Lignin is a natural phenylpropanoid polymer that is biosynthesized in wooden plants by dehydrogenative polymerization of p-hydroxycinnamyl alcohol precursors (19). These cinnamic alcohols originate from aromatic amino acids, i.e., phenylalanine and tyrosine, via shikimic acid pathway. The chemical structure is highly complex because many reaction intermediates are formed during the dehydrogenative polymerization of the cinnamyl alcohols. Although the lignin itself can be abundantly obtained as byproduct in papermaking industry (black liquor from pulping waste solutions) or as remnant of bioethanol production from agricultural resources (20–27), the utilization of lignin as material for functional polymers has been interfered by the inherent structural complexity. Therefore, utilization of low-molecular-weight substances produced by degradation of the lignins followed by selective extraction or separation has been performed to develop structurally-controlled functional materials from the lignins. Plant-derived cinnamic acid derivatives, such as p-coumaric, caffeic and ferulic acids attract attention as precursors for novel heat-resistant bioplastics (28–33). These cinnamic acid derivatives, produced as a precursor of lignins, can be extracted from the plant/wooden biomasses by alkaline extraction or microbial degradation (34, 35). These cinnamic acid derivatives possess rigid phenylpropanoid structure similar to conventional monomers of liquid crystalline polymers (LCPs). Therefore, development of bio-based LCP is expected by the polycondensation. In particular, because CA has two phenolic hydroxyl groups and one carboxyl group, polyesters containing CA as monomeric unit can form multibranched main-chain structure. Previously, several research groups have shown that copolyester of CA with p-coumaric acid possesses various functionalities such as thermotropic liquid crystallity, high heat resistance, adherence, and cell growth properties (30–32). On the other hand, poly(caffeic 238 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

acid) (PCA), a homopolyester of CA, has not been investigated in detail because of the limitation of molecular weight of the homopolyester obtained. In this paper, we report the preparation of high molecular weight PCA by the detailed investigation of polycondensation conditions of CA. Furthermore, thermal, mechanical, and optical properties as well as molecular characteristics of the PCAs are reported.


Experimental Materials Caffeic acid (98%), sodium acetate (97%) and acetic anhydride (93%) were purchased from Wako Pure Chemical Co., Japan. All reagents were used without further purification.

Synthesis of PCA Polycondensation of CA was performed by transesterification after acetylation using sodium acetate and acetic anhydride (Figures 1 and 2). A mixture of CA, sodium acetate and acetic anhydride was refluxed at 160 °C for 18 h or at 100 °C for 2 h under a nitrogen atmosphere. Polycondensation under reduced pressure below 3 kPa was performed at 160 °C for 2 h or at 200 °C for 4 h or 22 h.

Figure 1. Schematic representation of polycondensation reaction of caffeic acid via acetylation followed by transesterification.

239 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 2. Scheme for synthesis of poly (caffeic acid).

Instrumentation FT-IR spectra of CA and PCAs were obtained by KBr method on a HORIBA FT-720 spectrometer. Gel permeation chromatography (GPC) measurements were performed on a Tosoh HLC-8020 system in which TSKGel GMHHR-M column was attached. Chloroform was used as the mobile phase. Injection port, column oven and RI detector were kept at 40 °C. Molecular weights of polymer samples were calibrated by using monodisperse polystyrene standards (Tosoh Corporation, Japan). 1H NMR spectra of the PCAs were obtained on a JEOL Lambda 400 spectrometer at 50 °C using 5 mm o.d. tubes. Sample concentrations were about 10 % (w/v) in DMSO-d6 (Cambridge Isotope Laboratories) containing 0.05 % (v/v) TMS. The chemical shifts of 1H spectra were calibrated by signals at 2.50 ppm assigned to methyl proton of DMSO. Matrix-assisted laser desorption ionization / time-of-flight mass spectrometry (MALDI-TOF-MS) was also performed on a Bruker autoflex III spectrometer under positive ion detection mode. Matrix used was 2,5-dihydroxybenzoic acid. Mass numbers were calibrated by PEG standards. Differential scanning calorimetry (DSC) and thermogravimetry / differential thermal analysis (TG/DTA) were performed on a Rigaku DSC8230 calorimeter and TG8120 thermogravimeter, respectively. These thermal analyses were performed under air atmosphere with heating / cooling runs at 10 K/min. Polarized optical microscopy was performed on a Nikon ECLIPSE 50iPOL microscope equipped with Linkam 10013L hotstage. Optical transmittance of hot-pressed PCA film was measured on a Shimadzu UV3100 spectrophotometer. Dynamic mechanical analysis (DMA) was performed on a DVA-200 rheometer (itk Co. Ltd., Osaka, Japan). 240 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

Results and Discussion Molecular Structure, Solubility, and Molecular Weight Distribution of PCAs


Figure 3 shows FT-IR spectra of CA and PCAs. The intense OH absorption at 3400 cm-1 in CA was rather broadened and diminished in PCAs. In addition, ester C=O (1725-1730 cm-1) and C-O (1095 cm-1) stretching bands were observed in the PCAs. These show that the esterification of CA, whether acetylation or polycondensation, proceeded by the reactions.

Figure 3. FT-IR spectra of CA and PCAs obtained by KBr method.

In order to perform molecular structural analysis of PCAs in the dissolved state, solubility of the PCAs to various solvents were investigated. The results are summarized in Table 1. While CA dissolved in methanol and ethanol, no PCAs were dissolved in these alcohols. Alternatively, the PCAs showed variant solubility dependent on the polymerization conditions. Namely, PCA1 and PCA2 showed similar solubility to chloroform and N,N-dimethylacetamide (DMAc) but showed different solubility to acetone. The PCAs polymerized at 200 °C were insoluble to the solvents investigated, except for PCA3 that only partially dissolved in DMAc. The following analyses were performed on the basis of these results. Figure 4 shows 1H NMR spectra and structural assignments of ACA, PCA1 and PCA2. Signals at 2.28, 6.53, 7.30, 7.63, and 7.65 ppm in ACA and PCA1 were assigned to methyl and caffeoyl protons in 3,4-diacetoxycaffeic acid. Apart from these signals, downfield signals of caffeoyl protons were observed at 6.93, 7.40, 7.84 and 7.88 ppm in all the spectra. These broadened signals are assigned to polymerized portion of CA. These results suggests that, while ACA and PCA1 are mainly composed of 3,4-diacetoxycaffeic acid (DCA) containing small amount of oligomeric CA, PCA2 is mainly composed of the polymerized CA. 241 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

Table 1. Solubility of CA and PCAs Methanol ○

CA


a

Ethanol

Acetone

○

○

a

DMAc

DMSO

○

○

○

○

○

○

○

○

Chloroform ×

PCA1

×

×

○

PCA2

×

×

4

PCA3

×

×

×

×

PCA4

×

×

×

×

PCA5

×

×

×

×

Soluble.

b

Insoluble.

c

Swelling.

d

c

b

Partially soluble.

−

e

×

−

e

×

−

e

d

e

Not investigated.

Figure 4. 1H NMR spectra of ACA, PCA1 and PCA2 in DMSO-d6. The composition and molecular weight distribution of ACA, PCA1, and PCA2 were further analyzed by GPC. Figure 5(a) shows elution profiles of the three samples. ACA and PCA1 showed almost identical elution behavior, in which small amount of polymerized CA eluted first and DCA eluted later. The delayed elution of DCA is possibly caused by the strong interaction between DCA and column bed composed of crosslinked polystyrene gel. Molecular weight distribution (MWD) of these PCAs were estimated from the elution curves assigned to polymerized portion of CA, as shown in Figure 5(b). These profiles also indicate that ACA and PCA1 have almost the same MWD, while PCA2 has larger molecular weight than the two. The weight-averaged MW and polydispersity index of ACA, PCA1 and PCA2 are shown in Table 2. 242 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 5. (a) GPC elution profiles and (b) molecular weight distribution profiles of ACA, PCA1, and PCA2.

Table 2. Weight-averaged molecular weight, Mw, and polydispersity index, PDI, of ACA, PCA1, and PCA2 Mw ×103

PDI

ACA

a

3.8

1.1

PCA1

a

4.2

1.2

25

2.9

PCA2 a

Estimated for polymerized portions of each samples.

The molecular structure of polymerized portion of ACA and PCA1 was further investigated by MALDI-TOF-MS, as shown in Figure 6. Arrays of ionization peaks were observed from m/z = 287 and 449 in PCA1 and 491 and 653 in ACA, with the regular period of 204.

Figure 6. MALDI-TOF-MS spectra of (a)PCA1 and (b) ACA. 243 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


As shown in Figure 7, m/z = 287 and 449 corresponds to the sodium ion-added DCA and monohydroxy DCA dimer, respectively. The value of periodic peak distance corresponds the sum of the mass numbers of CA monomeric unit (m/z = 161) and acetyl group (m/z = 43). Therefore, the appearance of these periodic peaks suggests that the linear sequence of CA units is contained in the backbone of polymerized portions in ACA and PCA1. These linear CA oligomers may be formed by endwise ester exchange reaction of DCA.

Figure 7. Molecular structure of sodium ion-added DCA (m/z = 287) and monohydroxy DCA dimer (m/z = 449).

Thermal and Mechanical Properties of PCA Thermal properties of PCAs significantly depended on the polycondensation conditions. Firstly, thermal fusibility of PCAs was investigated by optical microscopy. Figure 8 shows the optical micrographs of PCAs at 25 °C and the elevated temperatures (135 °C for PCA1 and 250 °C for the other samples). While PCA1 and PCA2 showed melting transition at the elevated temperatures, PCA3, PCA4, and PCA5 did not show melting transition even when heated up to 250 °C. 244 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 8. Optical micrographs of PCAs at 25, 135 (PCA1), and 250 °C.

The difference in fusibility is well correlated to the solubility as mentioned in the former section. Namely, PCA1 with the relatively low molecular weight and the high solubility in various solvents showed melting transition at relatively low temperature. In contrast to PCA1, PCA2 showed complex thermal behavior. Figure 9(a) shows DSC thermogram and polarized optical micrographs at different temperatures of PCA2. PCA2 showed glass transition at 62 °C and melting transition at 164 °C. Furthermore, PCA2 formed liquid crystalline mesophase at 172 °C under shear. Formation of mesophase was also observed under shear at the rubbery state above 200 °C (data not shown). This shows that the highly condensed PCAs act as thermotropic liquid crystalline polymer responsive to external field. The shear-induced mesophase formation of polycondensed PCAs makes marked contrast to other cinnamic acid detivative polyesters that spontaneously form mesophase at the elevated temperatures. The three PCAs 245 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


polycondensed at 200 °C with the poor or no solubility showed infusibility. The insolubility and infusibility imply that the permanent crosslinks in the molecular backbones are formed in these PCAs. However, as seen in DSC thermograms shown in Figure 9(b), these infusible PCAs shows glass transition and thus thermoplasticity.

Figure 9. (a) DSC thermograms of PCA2 taken at first heating and cooling runs. Inset: polarized optical micrographs of PCA2 at 20 °C (glassy solid), 164 °C (melt), 172 °C (liquid crystalline mesophase formed under shear) and 196 °C (isotropic melt). Exothermic peak observed at 196 °C originates from the polycondensation reaction of residual functional groups (phenolic hydroxyl or acetyl and carboxyl groups) and vanishes in second heating. (b) DSC thermograms of PCA3, PCA4, and PCA5. Arrows indicate the glass transition points. PCAs were processed into solid film by hot-press above their glass transition temperatures. The films were colored in reddish brown but showed moderate translucency, as shown in Figure 10(a). However, the films of PCAs polycondensed at relatively low temperatures, such as 160 °C, were too brittle to investigate the mechanical properties. On the other hand, the film of PCA5 hot-pressed at 250 °C showed sufficient mechanical strength that endures rubbing by sandpaper. Dynamic mechanical analysis was performed for the hot-pressed PCA5 film. PCA5 film retained the dynamic storage modulus above 106 Pa even at 300 °C. Furthermore, the sample almost retained its original shape, as shown in Figure 10(b). This indicates the high thermal durability of PCA5 possibly resulting from the chemical crosslinking. The thermal durability of PCAs was semi-quantitatively evaluated by TG/ DTA. For each PCAs, 5 % weight decrease temperature, T5%d, was estimated from TG chart shown in Figure 11. Similarly to the solubility and fusibility, T5%d was correlated with the polycondensation conditions. Namely, PCA2 with the superior solubility and fusibility showed the lowest T5%d of 232 °C. On the other hand, PCA4 and PCA5, that are infusible and insoluble, showed the highest T5%d of 344 °C. All these results indicate that the properties of PCA are affected by the molecular mobility of main chain, as determined by the polycondensation conditions. 246 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 10. (a) Optical transmittance spectrum and appearance (inset) of hot-pressed film of PCA polycondensed at 160 °C for 22 h after acetylated at 160 °C for 18 h. The hot-press was performed at 140 °C. (b) Temperature dependence of dynamic storage, E′, and loss, E′′, moduli and loss tangent, tanδ, of PCA5. Inset: PCA5 film before (left) and after (right) the dynamic mechanical analysis measurement. Sample size is 5 mm×40 mm.

Figure 11. TG curves of PCA2, PCA3, PCA4, and PCA5.

Conclusions PCA, a homopolyester of CA, was prepared by polycondensation via acetylation of CA followed by transesterification. The product of acetylation reaction was mainly diacetoxycaffeic acid (DCA), containing small amount of linearly polymerized DCA. Transesterification of the acetylated product yielded a thermoplastic PCA showing variant thermal behaviors depending on the reaction conditions. In particular, PCA2 retaining solubility and fusibility formed liquid crystalline mesophase under shear at the elevated temperatures. This suggests the thermotropic liquid crystalline nature of PCA. Thermally processed PCA5 247 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

showed dynamic viscoelastic behavior as a glassy polymer retaining its original shape even after heated up to 300 °C. Furthermore, PCA5 was thermally stable that showed 5% weight decrease at 344 °C. These results offer the potential of PCA as biomass-derived novel engineering plastics.


Acknowledgments The present study was supported by Grant-in-Aid for Young Scientists (B) from Japan Society for the Promotion of Science (JSPS) (No. 22710084), Analysis and Development System for Advanced Materials (ADAM) from RISH, Kyoto University, and Innovative Materials and Processing Research Center, Ryukoku University.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12. 13. 14. 15.

16. 17.

Doi, Y. Microbial Polyesters; VCH Publishers: New York, 1990. Ikada, Y.; Tsuji, H. Macromol. Rapid Commun. 2000, 21, 117–132. Aoyagi, Y.; Doi, Y.; Iwata, T. Polym. Degrad. Stab. 2003, 79, 209–216. Iwata, T.; Tsunoda, K.; Aoyagi, Y.; Kusaka, S.; Yonezawa, N.; Doi, Y. Polym. Degrad. Stab. 2003, 79, 217–224. Iwata, T.; Aoyagi, Y.; Fujita, M.; Yamane, H.; Doi, Y.; Suzuki, Y.; Takeuchi, A.; Uesugi, K. Macromol. Rapid Commun. 2004, 25, 1100–1104. Iwata, T. Macromol. Biosci. 2005, 5, 689–701. Tanaka, T.; Fujita, M.; Takeuchi, A.; Suzuki, Y.; Uesugi, K.; Ito, K.; Fujisawa, T.; Doi, Y.; Iwata, T. Macromolecules 2006, 39, 2940–2946. Tsuji, H.; Ikada, Y.; Hyon, S. H.; Kimura, Y.; Kitao, T. J. Appl. Polym. Sci. 1994, 51, 337–344. Takasaki, M.; Ito, H.; Kikutani, T. J. Macromol. Sci., Part B: Phys. 2003, B42, 57–73. Furuhashi, Y.; Kimura, Y.; Yoshie, N.; Yamane, H. Polymer 2006, 47, 5965–5972. Shirakura, Y.; Fukui, T.; Saito, T.; Okamoto, Y.; Narikawa, T.; Koide, K.; Tomita, K.; Takemasa, T.; Masamune, S. Biochim. Biophys. Acta 1986, 880, 46–53. Iwata, T.; Aoyagi, Y.; Tanaka, T.; Fujita, M.; Takeuchi, A.; Suzuki, Y.; Uesugi, K. Macromolecules 2006, 39, 5789–5795. Tsuji, H.; Miyauchi, S. Biomacromolecules 2001, 2, 597–604. Hasirci, V.; Lewandrowski, K.; Gresser, J. D.; Wise, D. L.; Trantolo, D. J. J. Biotechnol. 2001, 86, 135–150. Williams, S. F.; Martin, D. P. In Biopolymers, Volume 4: Polyesters III - Applications and Commercial Products; Doi, Y., Steinbüchel, A., Eds.; Wiley-VCH: Weinheim, Germany, 2002; pp 91−128. Kasuya, K.; Mitomo, H.; Nakahara, M.; Akiba, A.; Kudo, T.; Doi, Y. Biomacromolecules 2000, 1, 194–201. Ikada, Y.; Jamshidi, K.; Tsuji, H.; Hyon, S. H. Macromolecules 1987, 20, 904–906. 248 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


18. Tsuji, H. Macromol. Biosci. 2005, 5, 569–597. 19. Sakakibara, A.; Sano, Y. In Wood and Cellulose Chemistry; Hon, D. N. S, Shiraishi, N., Eds.; Mercel Dekker: New York, 2001; Chapter 4, pp 109−174. 20. Billa, E.; Koukios, E. G.; Monties, B. Polym. Degrad. Stab. 1998, 59, 71–75. 21. Sun, R. C.; Tomkinson, J.; Bolton, J. Polym. Degrad. Stab. 1999, 63, 195–200. 22. Sun, R. C.; Tomkinson, J.; Lloyd Jones, G. Polym. Degrad. Stab. 2000, 68, 111–119. 23. Sun, R. C.; Tomkinson, J.; Wang, S. Q.; Zhu, W. Polym. Degrad. Stab. 2000, 67, 101–109. 24. Xiao, B.; Sun, X. F.; Sun, R. C. Polym. Degrad. Stab. 2001, 74, 307–319. 25. Sun, R. C.; Lu, Q.; Sun, X. F. Polym. Degrad. Stab. 2001, 72, 229–238. 26. El Hage, R.; Brosse, N.; Chrusciel, L.; Sanchez, C.; Sannigrahi, P.; Ragauskas, A. Polym. Degrad. Stab. 2009, 94, 1632–1638. 27. Watanabe, T.; Mitani, T. In The Role of Green Chemistry in Biomass Processing and Conversion; Xie, H., Gathergood, N., Eds.; John Wiley & Sons, Inc.: New York, 2012; pp 281−291. 28. Du, J.; Fang, Y.; Zheng, Y. Polym. Degrad. Stab. 2008, 93, 838–845. 29. Fang, Y.; Zheng, Y.; Hu, F. Polym. Degrad. Stab. 2012, 97, 185–191. 30. Tran, H. T.; Matsusaki, M.; Shi, D. J.; Kaneko, T.; Akashi, M. J. Biomater. Sci. Polym. Ed. 2008, 19, 75–85. 31. Kaneko, T.; Kaneko, D.; Wang, S. Plant Biotechnol. 2010, 27, 243–250. 32. Thi, T. H.; Matsusaki, M.; Hirano, H.; Kawano, H.; Agari, Y.; Akashi, M. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 3152. 33. Xu, S.; Uyama, H.; Whitten, J. E.; Kobayashi, S.; Kaplan, D. L. J. Am. Chem. Soc. 2005, 127, 11745. 34. Matsushita, K.; Adachi, O. Patent Abst. Japan JPA2009-201473, 2009. 35. Tsukada, S.; Ikeda, K.; Yoshimoto, M.; Kurata, R.; Fujii, M.; Ko, N. Patent Abst. Japan JPA2009-100760, 2009.

249 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

Effect of Polycondensation Conditions on Structure and Thermal

Recommend Documents