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Ind. Eng. Chem. Res. 2009, 48, 1706–1711
Synthesis and Properties of Poly(Ester Urethane)s Consisting of Poly(L-Lactic Acid) and Poly(Ethylene Succinate) Segments Jian-Bing Zeng,† Yi-Dong Li,† Wen-Da Li,† Ke-Ke Yang,† Xiu-Li Wang,† and Yu-Zhong Wang*,†,‡ Center for Degradable and Flame-Retardant Polymeric Materials (MoE), College of Chemistry, Sichuan UniVersity, Chengdu 610064, China, and State Key Laboratory of Polymer Materials Engineering, Chengdu 610065, China
An aliphatic polyester based poly(ester urethane) (PEU) consisting of poly(L-lactic acid) and poly(ethylene succinate) was successfully prepared via chain-extension reaction of poly(L-lactic acid)-diol (PLLA-OH) and poly(ethylene succinate)-diol (PES-OH) using 1,6-hexamethlyene diisocyanate (HDI) as a chain extender. PLLA-OH was obtained by direct polycondensation of L-lactic acid in the presence of 1,4-butanediol. PESOH was synthesized by condensation polymerization of succinic acid with excessive ethylene glycol. The structures and molecular weights of PLLA-OH, PES-OH, and PEUs were characterized by proton nuclear magnetic resonance (1H NMR), Fourier transform infrared (FTIR), and gel permeation chromatography (GPC). The PEUs were further studied by the means of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and tensile testing. The data of GPC analysis indicated that high molecular weights for example more than 200 000 g · mol-1 were easily synthesized through chain-extension reaction. The PEUs synthesized with high molecular weight and excellent tensile properties could find some applications in biomaterials and environmental friendly materials. 1. Introduction Although polymeric materials have been making our life very convenient and comfortable, they have encountered two huge challenges recently. The gradual reduction of petroleum resource has been leading to continuously increased cost of traditional polymer. Furthermore, the environmental pollution caused by nondegradation of traditional polymeric materials has accumulated and endangered our lives. A good way to conquer the two issues is the application of renewable biobased resources to prepare biodegradable polymeric materials. As one category of biobased and biodegradable materials, biobased aliphatic polyesters such as poly(lactic acid) (PLA),1-3 poly(butylenes succinate) (PBS),4-6 and poly(hydroxyalkanoic acids) (PHA)3,7,8 and their blends9-12 have recently obtained more and more attention with their excellent properties. The properties of independence from petroleum and complete biodegradation are their two major fetching advantages in comparison with the traditional petroleum based polymeric materials. However, disadvantages such as relatively poorer properties and high cost owing to immature techniques have largely limited the application of these materials. Taking PLA for example, the easily obtainable PLA from condensation polymerization13,14 is difficult to apply to general materials because of its poor physical properties accompanied by low molecular weight. However, high-molecular-weight PLA is usually synthesized through the complicated procedure of ring-opening polymerization of lactide,1 which causes PLA to have high cost and restricts its wide application. In addition, its brittleness is another limitation of the application of PLA. To overcome these limitations and broaden the applications of PLA, many actions have been taken into account to enhance the molecular weight, decrease the cost, and improve the * To whom correspondence should be addressed. Tel. and Fax: +8628-85410259. E-mail address:
[email protected]. † Center for Degradable and Flame-Retardant Polymeric Materials (MoE), College of Chemistry. ‡ State Key Laboratory of Polymer Materials Engineering.
propertiesofPLAbasedmaterials.Twomethods,chain-extension15,16 and melt/solid state polycondensation,17,18 have been successfully applied to prepare high-molecular-weight PLA, but the brittleness could not be changed. Copolymerization of lactide with some other monomers such as ε-caprolactone19 is an effective way to improve the flexibility of PLA based materials, but it can not help to reduce the cost of the polymers as the procedure of ring-opening polymerization was not avoided. Another convenient and useful method, which can improve the flexibility and reduce the cost of PLA based materials, is the coupling reaction of PLA prepolymer with another flexible prepolymer such as PEG20,21 and PCL;22,23 these polymers were mainly designed for biomedical applications. In this study, we prepared a novel poly(ester urethane) by using another biodegradable aliphatic polyester, poly(ethylene succinate) (PES)24 as a flexible component to copolymerize with poly(L-lactic acid) (PLLA) in the presence of 1,6-hexamethlyene diisocyanate (HDI). We expect that the novel copolymer, poly(ester urethane) has improved comprehensive properties. To our best knowledge, no literature has been published on this topic. 2. Materials and Methods 2.1. Materials. Succinic acid (AR grade) and ethylene glycol (AR grade) were bought from Kelong Chemical Corporation (Chengdu, China) and used without further purification. L-Lactic acid with 85 wt % aqueous solution was bought from Guangshui Chemical Plant (Guangshui, China), and the free water was removed by reduced pressure distillation at 80 °C for three hours. Tetrabutyl titanate (Kelong Chemical Corporation, Chengdu, China) with concentration of 0.2 g/mL was prepared by dissolving in anhydrous toluene. Dihydrate tin(II) chloride (SnCl2 · 2H2O, AR grade) obtained from Jinshan Chemical Plant (Chengdu, China) and 1,6-hexamethylene diisocyanate (HDI, AR grade) from Sigma-Aldrich were used without further purification. All other chemicals with AR grade were used as received.
10.1021/ie801391m CCC: $40.75 2009 American Chemical Society Published on Web 01/21/2009
Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 1707 Scheme 1. Preparation of PLLA-OH, PES-OH, and PLEU.
2.2. Synthesis of Poly(Ester Urethane) Containing PLLA and PES Blocks. Dihydroxyl terminated PLLA (PLLAOH) with regulated molecular weight was synthesized via direct condensation polymerization of L-lactic acid in the present of 1,4-butanediol using SnCl2 · 2H2O as a catalyst. The procedure was as follow: 180.0 g (2 mol) L-lactic acid, 2.7 g (0.03 mol) 1,4-butanediol, and 0.91 g (0.5 wt %) SnCl2 · 2H2O were put into a 500 mL three-neck round-bottom flask equipped with mechanical stirrer. The polymerization was first carried out at 160 °C and 4000 Pa for 5 h and then continued at 180 °C and 150 Pa for another 5 h. The resulting polymer was purified by dissolving in chloroform and precipitating in excessive methanol. The white powder product was dried to constant weight in vacuum oven at 40 °C. Poly(ethylene succinate)-diol (PES-OH) with determined molecular weight was prepared by a two-step procedure including esterification and following polycondensation. 1.2 mol ethylene glycol and 1 mol succinic acid were added into a 500 mL three-necked round-bottom flask equipped with mechanical stirrer, water separator, and nitrogen inlet pipe. The esterification was implemented at 180 °C for 4 h, then the catalyst tetrabutyl titanate with 0.1 wt % of the total amount of reactants was introduced into the flask and the polycondensation was continued at 220 °C with vacuum of 30 Pa for 30 min. The resultant polymer was purified by the same procedure with PLLA-OH. The chain-extension reaction was performed in bulk using a glass reactor filled with N2. PLLA-OH and PES-OH were put into the reactor which was vacuumed and purged with nitrogen three times. Then the reactor was immersed in a 160 °C silicone oil bath. The reactants were stirred with a mechanical stirrer when they were completely molten, then the predetermined amount of HDI was injected into the reactor. The chainextension reaction was finished in 1 h. The resulting polymer was also purified using the same procedures as PLLA-OH to remove any potential cross-linked product. 2.3. Fourier Transform Infrared (FTIR) Spectroscopy. FTIR spectra for the polymer samples were recorded on a Fourier transform infrared spectrometer in a range of wave numbers from 4000 to 400 cm-1. The specimens were milled into powders and then mixed and laminated with KBr. The resolution and scanning time were 4 cm-1 and 32 times, respectively.
2.4. Nuclear Magnetic Resonance (NMR) Spectroscopy. Chemical structures and components of polymers were characterized by means of 1H NMR implemented on a Varian Inova 400 spectrometer at ambient temperature, using CDCl3 and tetramethylsilane (TMS), as the corresponding solvent and internal chemical shift standard. 2.5. Gel Permeation Chromatography (GPC). The molecular weights and polydispersity of polymers were determined by GPC at 35 °C with Waters instrument equipped with a model 1515 pump, a 2414 refractive index detector, and a Waters model 717 autosampler. Chloroform was used as the eluant at a flowing rate of 1.0 mL/min, and the sample concentration was 2.5 mg/mL. 2.6. Thermogravimetric Analysis (TGA). TGA was used to determine the thermal stabilities of PLLA-OH, PES-OH, and PEUs. The thermograms were recorded from room temperature to 550 °C at a heating rate of 10 °C/min under N2 atmosphere on a TA Instruments TGA-Q500. 2.7. Differential Scanning Calorimetry (DSC). DSC analysis was carried out on a TA Instrument DSC-Q100. Samples were quickly heated to 160 °C and kept for 5 min to remove thermal history, then were cooled to -50 °C at a rate of 10 °C/min, and finally were reheated to 160 °C at the same rate. 2.8. Tensile Testing. The tensile properties of polymers were measured on an Instron Universal Testing Machine (Model 4302, Instron Engineering Corporation, Canton, MA) at a crosshead speed of 50 mm/min and room temperature. The specimens were prepared by hot pressing and cutting with a dumbbell-shaped cutter. The thickness and width of the specimens were 0.3 mm and 5 mm, respectively, and the length of the sample between the two pneumatic grips of the testing machine was 20 mm. Six time measurements were tested for each sample, and the results were shown by mean values. 3. Results and Discussion 3.1. Synthesis and Characterization of Poly(Ester Urethane)s. Dihydroxyl terminated PLLA (Mn 6120 g · mol-1) was prepared by the condensation polymerization of L-lactic acid in the presence of 1,4-butanediol as shown in Scheme 1a. The chemical structure of PLLA-OH was characterized by 1H NMR (Figure 1). The signals of protons belonging to the inner and outer methylene groups of -OCH2CH2CH2CH2O- were
1708 Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 Table 1. Molecular Weights of Synthesized PLLA and PES Prepolymers prepolymer
Mn,NMRa (g · mol-1)
Mnb (g · mol-1)
Mwb (g · mol-1)
PDIb
PLLA-OH PES-OH
3810 4290
6120 6010
10700 12700
1.75 2.11
a Calculated by NMR analysis. 35 °C.
b
Obtained by GPC in chloroform at
Figure 1. 1H NMR spectrum of PLLA-OH.
found at 1.68 (δHd) and 4.11 (δHc) ppm, respectively. The shifts occurring at 1.58 (δHb) and 5.18 (δHa) ppm can be reasonably assigned to the methyl and methine proton in the repeating unit of PLLA, while the shifts observed at 1.48 (δHb’) and 4.36 (δHa’) ppm were attributed to the corresponding protons at the terminus of PLLA-OH molecules. The number average molecular weight (Mn) of PLLA-OH could be determined through NMR calculating by the following equation: Mn,PLLA-OH ) 72 × 2 ×
I5.18 + I4.36 + 88 + 2 I4.36
(1)
where I5.10 and I4.36 were the peak intensities of the related resonance signals, the numerical values 72, 2, 88, and 2 represented molecular weight of repeating unit, two chain segments at the two sides of -OCH2CH2CH2CH2O-, molecular weight of -OCH2CH2CH2CH2O-, and two hydrogen protons at the ends of the molecular chain, respectively. The Mn,NMR of PLLA-OH obtained by NMR is listed in Table 1. Mn,NMR which was proved by some literature work24,25 was very effective to determine the amount of chain extender, and it was also applied in this study. However, the molecular weights of prepolymer determined by GPC (shown in Table 1) were also important to help us study the results of chain-extension. It could be found that the Mn obtained by GPC was much higher than Mn,NMR. Dihydroxyl terminated poly(ethylene succinate) (Mn 6010 g · mol-1) was synthesized by condensation polymerization of succinic acid and ethylene glycol with the feed molar ratio of 1/1.2 (Scheme 1b). The molecular weight of PES could be readily controlled by constantly removal of excessive diol based on the polycondensation time. Meanwhile, the end groups of PES can be regulated by the adjustment of initial feed molar ratio of ethylene glycol to succinic acid. With the excessive amount of ethylene glycol, it can be expected that PES prepolymer would predominantly be terminated with hydroxyl groups. Figure 2 shows the 1H NMR spectrum of PES-OH as well as the corresponding proton resonance signals assignment. The peaks appearing at 2.66 (δHe) and 4.30 (δHf) ppm were assigned to the methylene protons in the repeating units of PES. The shifts of methylene protons connecting with hydroxyl groups and ester bonds at the terminus of PES chain were seen at 3.82 (δHh) and 4.23 (δHg) ppm. The number average molecular weight of PES-OH can be determined by NMR analysis via the following equation: Mn,PES-OH ) 144 ×
I2.66 + 61 + 1 I3.82
(2)
where I2.66 and I3.82 were the peak intensities of the corresponding methylene protons, the values 144, 61, and 1 were the molecular weights of the repeating unit of PES, OCH2CH2OH, and H, respectively. The value of Mn,NMR of PES-OH is shown in Table
Figure 2. 1H NMR spectrum of PES-OH.
Figure 3. 1H NMR spectra of poly(ester urethane)s.
1. Together with Mn,NMR of PLLA-OH, the amount of chain extender was determined. The molecular weight and distribution of PES-OH were characterized by GPC as shown in Table 1. Similar with PLLA-OH, the number average molecular weight of PES-OH obtained by GPC is higher than that obtained via NMR calculating. Poly(ester urethane)s with different compositions were synthesized via melting chain-extension reaction of PLLA-OH with PES-OH using HDI as a chain extender at 160 °C for 1 h. For comparison, PEU only containing PLLA or PES, named as PLLAU and PESU, was also prepared by utilizing PLLA-OH or PES-OH as the only prepolymer to react with HDI. The polymer containing both PLLA and PES was recorded as PLEU. To get high-molecular-weight polymer, the molar ratio of HDI to the total amount of prepolymers was fixed at 1:1, which was also proved by some other systems.25,26 The structures and compositions of the polymers were characterized by 1H NMR (Figure 3). It was found that the characteristic shifts belonging to PLLA (δHa, δHb) and PES (δHe, δHf) still existed in spectra of PLEU, while the signals belonged to end groups of PLLAOH and PES-OH could not be found, indicating that the chainextension reaction was sufficiently achieved. Furthermore, we could find that the shift of outer methylene protons of -OCH2CH2CH2CH2O- from PLLA-OH was also present in the 1H NMR spectrum of PEU, and its relative intensity became weaker from PEU1 and PEU3 to PEU5 as the decreasing amount
Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 1709 Table 2. Composition and Molecular Weight of Poly(Ester Urethane)s fa (wt %) sample
fPLLA
PLLAU PLEU1 PLEU2 PLEU3 PLEU4 PLEU5 PESU
95.7 86.2 67.1 48.0 28.9 9.6
Fb (wt %)
fPES
fHDI
FPLLA
9.6 28.8 48.0 67.2 86.6 96.3
4.3 4.2 4.1 4.0 3.9 3.8 3.7
95.1 84.1 63.7 47.2 25.1 8.2
FPES
FHDI
Mn × 10-4 c (g · mol-1)
Mw × 10-4 c (g · mol-1)
PDIc
11.2 31.7 48.4 70.8 87.7 96.2
4.9 4.7 4.6 4.4 4.1 4.1 3.8
8.59 9.53 8.29 8.10 8.19 13.87 10.24
18.36 21.24 22.02 27.82 31.84 43.85 19.68
2.14 2.23 2.66 3.43 3.89 3.16 1.92
a Weight fraction in the feed. b Weight fraction in the resulting polymer calculated by NMR analysis. average molecular weight, and polydispersity obtained by GPC in chloroform at 35 °C.
Figure 4. GPC diagrams of PLLA-OH, PES-OH, and PEUs.
of PLLA segments. The shifts of three kinds of methylene protons of HDI in PEUs molecular chain were observed at 3.16 (δHh), 1.50 (δHi), and 1.34 (δHj) ppm. The weight fraction of each component of PEU could be determined by the following equations deduced from the relationship between peak intensity and proton number: 4 × 72 × I5.18 144 × I2.66 + 4 × 72 × I5.10 + 170
(3)
144 × I2.66 144 × I2.66 + 4 × 72 × I5.18 + 170
(4)
FHDI ) 1 - FPLLA - FPES
(5)
FPLLA ) FPES )
where FPLLA, FPES, and FHDI denoted the weight fraction of the related components in the poly(ester urethane), I5.18 and I2.66 represented peak intensities of the corresponding protons, and the numerical values 72, 144, and 170 were the molecular weight of the repeating unit of PLLA, PES molecule, and HDI segment, respectively. The number 4 was a coefficient indicating the number ratio of methylene protons of PES appearing at 2.66 ppm to methine proton of PLLA occurring at 5.18 ppm. The compositions of the polymers are listed in Table 2. f is the weight fraction of the components involved in the feed, while F indicates that in resulting polymer obtained through NMR analysis. From the results, we can see that although the value of F was somewhat different from that of f for each component, they were very close to each other. GPC was implemented to determine the molecular weights of poly(ester urethane)s, and the results are shown in Table 2 and Figure 4. The GPC traces of the PLLAU, PESU, and a typical PLEU (PLEU3) show a unimodal peak in GPC chromatographs, indicating that a complete reaction took place without unreacted prepolymer remaining.27 In addition, the molecular weights of the resulting PEUs, which were more than ten times higher than the prepolymers, can prove that the chainextension reaction is very effective and can also prove that using Mn,NMR of prepolymers to determine the amount of chain
c
Number average molecular weight, weight
extender needed is very useful. Furthermore, we can find that PLLAU and PESU have a narrower molecular weight distribution than PLEUs, and these results can be reasonably attributed to the differences of the prepolymers. For PLEUs, both PLLAOH and PES-OH were used as the combined prepolymers, and their differences in structures and reactivity resulted in wide molecular weight distributions of the polymers. However, in the synthesis of PLLAU or PESU, only one kind of prepolymer PLLA-OH or PES-OH was used. Thus PLLAU and PESU were obtained with relatively narrower molecular weight distributions in comparison with PLEU. FTIR is a useful way to characterize the functional groups present in polymers. Figure 5 shows the FTIR spectra of PLEU3, PLLAU, PESU, and their PLLA-OH and PES-OH precursors. It can be seen from the profiles of PLLAU, PESU, and PLEU that the peak attributed to the -NCO- stretching was not found in the range of around 2200 cm-1.27 This proves that the isocyanate groups of the chain extender have been reacted completely and are absent in the resulting polymers. After chainextension reaction, the peaks appearing around 3500 cm-1 (O-H vibration) in the spectra of prepolymers shifted to about 3350 (N-H vibration) in the spectra of PLLAU, PLEU, and PESU, which indicates that the hydroxyl groups were completely consumed and meanwhile N-H belonging to urethane group was formed. In addition, another characteristic peak of N-H vibration was also observed at about 1530 cm-1. FTIR, NMR, and GPC results provide strong evidence for the successful synthesis of poly(ester urethane)s.27,28 3.2. Thermal and Crystalline Behaviors. Thermogravimetric analysis (TGA) can be used to determine the composition and to study the thermal stabilities of copolymers. Figure 6 shows the TGA curves of PLLAU, PESU, and PLEU3. It can be seen that there was no residue after decomposition of PLLAU, while in the cases of PESU and PLEU, 4 wt % residue was generated at 550 °C. The residue should be caused by the carbonization of PES segments at high temperature. The degradation of PLLAU begins at 225 °C and ends at 293 °C while PESU begins to degrade at 270 °C, at which temperature PLLA has not degraded completely. PLEU undergoes a twostep thermal degradation with the first step happening between 225 and 270 °C and the second step starting at 270 °C. It is obvious that the first step of weight loss of PLEU3 was caused by the thermal degradation of the PLLA segment and the second weight loss step was due to the PES segment. There is an overlapping range of weight loss between the PLLA and PES segments during the thermal decomposition of PLEU3. Therefore, the TGA analysis can not be applied to calculate the accurate contents of PLLA and PES segments in PLEU. A differential scanning calorimetry (DSC) was used to study the thermal transition and crystallization characteristics of the polymers. Figure 7 shows the DSC heating scan of PLLA-OH, PES-OH, and the PEUs. It could be found that the curves of
1710 Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009
Figure 5. FTIR spectra of PLLA-OH, PES-OH, and PEUs.
Figure 6. TGA curves of PLLAU, PESU, and PLEU3. Figure 8. Tg of poly(ester urethane)s versus FPLLA.
Figure 7. DSC thermograms in heating scan of PLLA-OH, PES-OH, and PEUs.
PLEUs show only one glass transition, between the Tg values of PES-OH (-12.2 °C) and PLLA-OH (51.3 °C), which indicates that the PLLA and PES segments are miscible in PLEUs. For the miscible system, the glass transition temperature (Tg) can be evaluated through the Fox equation:29 1/Tg(fox) ) F1/Tg1 + F2/Tg2, where Tg1 and Tg2 are the Tg values of individual segments and F1 and F2 are the weight fraction of corresponding components. The Tg(fox) and Tg(exp) (obtained through DSC) versus FPLLA curves are graphically presented in Figure 8. It can be seen that Tg(exp) is very close to Tg(fox) regardless of composition. This also proves that PLLA and PES segments
are miscible in PLEUs. By comparison of DSC thermograms of PES-OH, PLLA-OH, and PEUs, we can find that no cold crystallization and melting peaks could be detected when the weight fraction of PLLA is more than 25% (PLEU4). Although PLLA-OH can crystallize at the heating rate of 10 °C/min, after chain-extension PLLAU is unable to crystallize, which should result from the disturbance of chain regularity of PLLA molecules owing to the introduction of chain extender. PESU is less sensitive to the chain extender and still able to crystallize, and this is the main reason that PLEU5 with high content of PES segments can crystallize, and the crystallization ability of PLEU decreases and even loses with the increase of FPLLA. In addition, the melting point of PLEU is less than that of PESOH (95 °C) and decreases with the increase of FPLLA, which should be caused by the increased disturbance of crystallite when FPLLA increases. 3.3. Tensile Properties. Mechanical properties are more important than any other properties of polymers when it comes to the real application. Tensile property, as one aspect of the most important mechanical properties, was used to evaluate the tensile strength and strain at break of poly(ester urethane)s synthesized in the study. The results are summarized in Table 3. We can see that the tensile strength of the polymers decreased first and then increased with the decreasing content of PLLA segments, while the strain at break almost always increased. The intrinsic brittle properties of PLLA segments and crystalline properties of PLEU are the two major factors to the alteration.
Ind. Eng. Chem. Res., Vol. 48, No. 4, 2009 1711 Table 3. Mechanical Properties of PEUs polymer
tensile strength (MPa)
strain at break (%)
PLLAU PLEU1 PLEU2 PLEU3 PLEU4 PLEU5 PESU
59.3 ( 2.3 58.6 ( 0.8 33.6 ( 1.3 29.6 ( 2.7 35.2 ( 1.6 42.6 ( 1.5 45.5 ( 1.2
5.2 ( 0.8 8.8 ( 1.0 223 ( 9 525 ( 23 641 ( 15 650 ( 8 653 ( 12
For PLEU1, with FPES less than 10% it exhibits high stress and low strain just like PLLAU. As FPES increased to 48.4%, that is PLEU3, the minimum tensile strength was resulted from amorphous state of PLEU3. Then with the further increase of FPES, the increased crystallization endowed PLEU with increased tensile strength. The crescent strain at break of PLEU should be ascribed to the gradually increased content of PES flexible segment. The results suggest that the tensile properties are adjustable by controlling the composition of PLEU. Consequently, the PLEU can be used as a versatile material via adjusting the tensile properties. 4. Conclusion A series of novel biobased poly(ester urethane)s have been successfully synthesized by chain-extension reaction of two dihydroxyl terminated prepolymers PLLA-OH and PES-OH using HDI as a chain extender. The structure of the poly(ester urethane) was confirmed by the means of 1H NMR, FTIR, and GPC analysis. The GPC results proved that the chain-extension reaction was a very effective method to synthesizing PLLA based copolymers with high molecular weights. The DSC results for the PLEUs showed only one Tg which is in good agreement with the value obtained by Fox equation, which proved that the PLLA and PES segments are completely miscible in PLEU. The results of tensile properties suggested that we can prepare PLLA based materials with controllable physical properties from brittle to flexible materials. Acknowledgment This work was supported by the National Science Fund for Distinguished Young Scholars (50525309). The Analytical and Testing Center of Sichuan University provided NMR analysis. Literature Cited (1) Garlotta, D. A literature review of poly(lactic acid). J. Polym. EnViron. 2001, 9, 63. (2) Duda, A.; Penczek, S. Polylactide [poly(lactic acid)]: Synthesis, properties and applications. Eur. Polym. J. 2006, 42, 468. (3) Sudesh, K.; Iwata, T. Sustainability of biobased and biodegradable plastics. Clean Soil, Air, Water 2008, 36, 433. (4) Lim, S. K.; Jang, S. G.; Lee, S. I.; Lee, K. H.; Chin, I. J. Preparation and characterization of biodegradable poly(butylene succinate)(PBS) foams. Macromol. Res. 2008, 16, 218. (5) Liu, X. Q.; Li, C. C.; Zhang, D.; Xiao, Y. N. Synthesis, characterization and properties of poly(butylene succinate) reinforced by trimellitic imide units. Macrmol. Chem. Phys. 2006, 207, 694. (6) Shih, Y. F.; Wang, T. Y.; Jeng, R. J.; Wu, J. Y.; Teng, C. C. Biodegradable nanocomposites based on poly(butylene succinate)/organoclay. Eur. Polym. J. 2008, 44, 677. (7) van Beilen, J. B.; Poirier, Y. Production of renewable polymers from crop plants. Plant J. 2008, 54, 684.
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ReceiVed for reView September 16, 2008 ReVised manuscript receiVed December 9, 2008 Accepted December 13, 2008 IE801391M
