Enzymatic Synthesis and Properties of Novel Biobased Elastomers

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Chapter 17

Enzymatic Synthesis and Properties of Novel Biobased Elastomers Consisting of 12-Hydroxystearate, Itaconate and Butane-1,4-diol Mayumi Yasuda, Hiroki Ebata, and Shuichi Matsumura* Faculty of Science and Technology, Keio University, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan *[email protected]

A biobased elastomer was prepared by the lipase-catalyzed polymerization of methyl 12-hydroxystearate (12HS-Me), dimethyl itaconate and butane-1,4-diol using immobilized lipase from Burkholderia cepacia and subsequent thermal crosslinking. The molecular weight of the polyester was significantly increased by the ring-opening copolymerization of a cyclic butylene itaconate oligomer and 12HS-Me. The produced polyester with a Mw of 160000 g/mol consisted of randomly distributed 12HS units and butylene itaconate units. Thermal crosslinking of the polyester was carried out at 180 °C using a hot-press to obtain the transparent elastomer. The hardness of the elastomer increased with decreasing 12HS content. In contrast, Young’s modulus of the elastomer decreased with increasing 12HS content.

Introduction Biobased polymers produced from renewable biomass resources such as plant oil are attractive with respect to saving fossil carbon resources, reduction of CO2 emission and energy minimization for sustainable development and the establishment of a sustainable society. Such green polymers should be produced by environmentally benign processes, e.g., enzymatic and solvent-free processes, and by avoiding the use and generation of hazardous materials. Unlike fossil © 2010 American Chemical Society In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


resources, plant oil is regarded as an abundant renewable feedstock for next generation polymers (1–6). About one hundred million tons of plant oil are produced annually. In recent years, soybean oil has been used as the feedstock for the production of paint, printing ink and biofuel. Furthermore, the application of plant oil as the chain extender for polyurethanes and crosslinkers for biodegradable polymers has been reported (7–14). However, as in the case of soybean oil, the price of plant oil is dependent on both its production and its demand in food supplies. Non-edible oils may not cause this conflict. Among plant oils, castor oil has attracted attention as a non-edible feedstock for the production of functional materials because of its relative abundance. Castor oil is obtained from the bean of the castor plant and 530,000 tons of it are produced annually. Approximately 85-90% of the triglyceride-derived fatty acid in castor oil is ricinoleic acid, 12-hydroxy-cis-9-octadecenoic acid. 12-Hydroxystearic acid (12HS) is produced industrially by the hydrogenation of ricinoleic acid. Thus, 12HS is attractive as a renewable biobased starting material for the production of various polymeric materials. Elastomers show rubber elasticity at room temperature and are widely used in industrial applications. Elastomers are produced mainly from petroleum feedstocks and their consumption is increasing annually. The polybutadiene series and polybutadiene-acrylonitrile series elastomers are both widely used. A thermosetting poly(ricinoleic acid)-type elastomer was prepared by the enzyme-catalyzed polymerization of ricinoleic acid and subsequent crosslinking (15, 16). A thermoplastic elastomer composed of 12HS and 12-hydroxydodecanoate (12HD) was synthesized by lipase-catalyzed polymerization (17). As biodegradable plastics, poly(butylene succinate) (PBS) is produced by the polymerization of succinic acid and butane-1,4-diol (BD). The former can be produced by the fermentation method and the latter can be produced by the reduction of succinic acid. Thus, PBS is regarded as a potentially biobased plastic. Itaconic acid is also produced by the fermentation method. Itaconic acid has a polymerizable C=C double bond in its structure (18, 19), thus it is an attractive monomer for the production of functional polyesters. However, the synthesis of polyesters involving itaconic acid has not been reported to date. In this study, a novel biobased elastomer was prepared by the lipase-catalyzed copolymerization of 12HS, itaconic acid and BD with subsequent thermal crosslinking. Some physicochemical properties of the elastomer were also reported.

Experimental Part Materials and Measurements Methyl 12-hydroxystearate (12HS-Me) was purchased from Sigma (St. Louis, MO, USA). Dimethyl itaconate (IA-Me) and BD were purchased from Wako Pure Chemical Industries, Ltd. (Tokyo, Japan). 12HD was purchased from Aldrich (Milwaukee, WI, USA). 1-Hexanol was purchased from Junsei Chemical 238 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Co. Itd. (Tokyo, Japan). 4-Decanol was purchased from Tokyo Kasei Kogyo Co. Inc. (Tokyo, Japan). Molecular sieves 4A (MS4A) were purchased from Junsei Chemical Co., Ltd. (Tokyo, Japan), and were dried at 150 °C for 2 h before use. Immobilized lipase from Candida antarctica (lipase CA: Novozym 435; 10000 PLU·g-1 propyl laurate units; lipase activity based on ester synthesis) was kindly supplied by Novozymes Japan, Ltd. (Chiba, Japan). Lipase from Burkholderia cepacia immobilized on Diatomaceous Earth with 0.5 units·mg-1 (lipase PS-D) was purchased from Wako Pure Chemical Industries, Ltd. (Tokyo, Japan). The immobilized enzymes were dried in a vacuum (3 mmHg) over P2O5 at 25 °C for 2 h. The weight-average molecular weight (Mw), number-average molecular weight (Mn), polydispersity index (Mw/Mn) and monomer conversion were measured using size exclusion chromatography (SEC) with SEC column (Shodex K-805L, Showa Denko Co., Ltd., Tokyo, Japan) at 37 °C with a refractive index detector. Chloroform was used as the eluent at 1.0 mL·min-1. The SEC system was calibrated with polystyrene standards with narrow molecular weight distribution. 1H NMR spectra were recorded on a Varian MERCURY plus 300 spectrometer operating at 300 MHz. The 13C NMR and HMBC spectra were recorded with a Lambda 300 Fourier Transform Spectrometer (JEOL, Ltd., Tokyo, Japan) operating at 75 MHz. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) was performed with a Bruker Ultraflex mass spectrometer equipped with a nitrogen laser. The detection was performed in the reflectron mode, 2,5-dihydroxybenzoic acid was used as the matrix, sodium bromide was used as the cation source, and positive ionization was used.

Scheme 1. Direct polycondensation of 12HS-Me, BD and IA-Me by lipase CA and PS-D.

239 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


The glass transition temperature (Tg), crystallization temperature (Tc), and melting temperature (Tm) of the polymer were determined by differential scanning calorimetry (DSC-60, Shimadzu Co., Kyoto, Japan). The heating rate was 10 °C·min-1 within a temperature range of -150 to 60 °C. The polymer samples were heated in a nitrogen flow at a rate of 10 °C·min-1 from 30 to 60 °C, cooled to -150 °C at a rate of -20 °C·min-1, and then scanned at the same heating rate and over the same temperature range. The crosslinking behavior of the polymer was evaluated with a Scanning Vibrating Needle Curemeter (SVNC, RAPRA, Heisen Yoko Co., Ltd., Japan). The Young’s modulus of elastomers was measured using an Autograph instrument (Shimadzu Co., Kyoto, Japan). The hardness of the elastomers was measured using a Haze meter (Nippon Denshoku Industries Co., Ltd., Japan).

General Enzymatic Polymerization Procedure Direct Polycondensation of 12HS-Me, BD and IA-Me The general procedure for the enzymatic polymerization of 12HS-Me, BD and IA-Me was carried out in a screw-capped vial with MS4A placed at the top of the vial to absorb the condensation byproducts, such as water or methanol. The preparation of poly(76.5% 12HS/BD/IA) with a Mw of 40300 is described as a typical example. A mixture of 12HS-Me (182 mg, 0.58 mmol), BD (6.5 mg, 0.072 mmol), IA-Me (11.4 mg, 0.072 mmol) and immobilized lipase PS-D (240 mg, 120 wt% relative to substrate) was stirred under a nitrogen atmosphere in an oil bath at 80 °C for 5 d. After the polymerization, the reaction mixture was dissolved in chloroform (20 mL), and the insoluble enzyme was removed by filtration. The solvent was then evaporated under reduced pressure to obtain the polymer. The crude polymer was reprecipitated from chloroform using methanol to remove any unreacted monomers. The molecular weight was determined using SEC. The molecular structure and monomer composition were determined by 1H NMR, 13C NMR and HMBC experiments. The spectral data of poly(76.5 mol% 12HS/BD/IA) is shown as being representative. Poly(76.5 mol% 12HS/BD /IA) 1H NMR (300 MHz, CDCl3): δ 0.88 (t, 3H, -CH3, J = 6.6 Hz), 1.25 (m, 22H, -CH2-, 12HS), 1.49 [m, 4H, -CO(CH2)9CH2-, -CH2(CH2)4CH3, 12HS], 1.60 (m, 2H, -COCH2CH2-, 12HS), 1.70 (m, 4H, -CH2CH2CH2CH2-, BD), 2.27 (t, 2H, -COCH2-, J = 7.5 Hz), 3.33 [m, 2H, -COC(CH2)CH2 COO-, IA], 4.09, 4.19 (m, 4H, -CH2CH2CH2CH2-, BD), 4.89 (tt, 1H, HC-O, J1 = J2 = 6.3 Hz, 12HS), 5.72, 6.32 (m, 2H, -C=CH2). 13C NMR (75 MHz, CDCl3): δ 14.0 (CH3), 22.5-31.7 (-CH2-, 12HS), 25.1-25.3 (-CH2CH2CH2CH2-, BD), 34.7 (-COCH2-, 12HS), 37.6 [-COC(CH2)CH2COO-, IA], 63.6 (-CH2CH2CH2CH2-, BD), 64.3, 64.4 (-CH2CH2CH2CH2-, BD), 74.0 (CH-O, 12HS), 127.7, 128.3 (-C=CH2), 133.8, 134.3 (-C=CH2), 165.8, 166.0 (-COCCH2-, IA), 170.4, 170.6 (-CCH2CH2COO-, IA), 173.6, 173.8 (-COCH2-, 12HS). 240 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 1. Effects of temperature on Mw, Mw/Mn and polymer yield using lipase CA (a) and lipase PS-D (b). Reaction conditions: 12HS-Me (0.58 mmol), BD (0.072 mmol) and IA-Me (0.072 mmol) were polymerized by lipase CA (50 wt%) or lipase PS-D (100 wt%) in toluene (100 µL) with MS4A placed at the top of the polymerization vessel for 5 d. Mw and Mw/Mn (●); polymer yield (○).

Enzymatic Preparation of Cyclic BD/IA Oligomer The cyclic BD/IA oligomer was prepared by the reaction of BD and IA-Me using lipase in a dilute toluene solution with MS4A placed at the top of the flask in the vapor phase above the reaction mixture. BD (72 mg, 0.8 mmol) and IAMe (79 mg, 0.5 mmol) were dissolved in toluene (30.2 mL) in a round-bottomed flask, after which immobilized lipase CA (151 mg, 100 wt% relative to substrate) was added and the reaction mixture was stirred in an oil bath at 80 °C for 48 h. After the reaction, the reaction mixture was diluted with chloroform (20 mL), and insoluble enzyme was removed by filtration. The solvent was then evaporated under reduced pressure to obtain cyclic oligomers consisting mainly of cyclic BD/ IA dimer in almost quantitative yield. Purification was carried out by silica gel column chromatography using ethyl acetate-hexane (1:1 v/v, Rf = 0.40) as an eluent to obtain the cyclic BD/IA oligomer as white crystals in 40% yield. The molecular structure of the cyclic BD/IA oligmer was confirmed by 1H NMR spectroscopy, MALDI-TOF MS and elemental analysis. Cyclic BD/IA oligomer 1H NMR (300 MHz, CDCl3): δ 1.70 (m, 4H, -CH2CH2CH2CH2-), 3.35 [m, 2H, -COC(CH2)CH2COO-], 4.12, 4.19 (m, 4H, -CH2CH2CH2CH2-), 5.72, 6.32 (m, 2H –C=CH2).



Enzymatic Ring-Opening Polymerization of Cyclic BD/IA Oligomer and 12HS-Me The general procedure for the enzymatic ring-opening polymerization of cyclic BD/IA oligomer and 12HS-Me was carried out in a screw-capped vial with MS4A placed at the top of the vial to absorb the condensation byproduct. The preparation of poly(58.8 mol% 12HS/BD/IA) with a Mw of 102000 is described as a typical example. A mixture of cyclic BD/IA oligomer (29.5 mg, 0.16 mmol), 12HS-Me (151 mg, 0.48 mmol) and lipase PS-D (252.7 mg, 140 wt% relative to substrate) was stirred under a nitrogen atmosphere in an oil bath at 80 °C for 4 d. After the polymerization, the reaction mixture was dissolved in chloroform (20 mL), and the insoluble enzyme was removed by filtration. The solvent was then evaporated under reduced pressure to obtain the polymer. The polymer was reprecipitated from chloroform using methanol to remove any unreacted monomers. The molecular weight was determined by SEC. The molecular structure and monomer composition were determined by 1H NMR, 13C NMR and HMBC experiments. The spectral data of poly(58.8 mol% 12HS/BD/IA) obtained by the ring-opening polymerization of cyclic oligomer and 12HS-Me using lipase PS-D was in complete agreement with those obtained by the direct polycondensation of 12HS-Me, BD and IA-Me using lipase PS-D.

Results and Discussion Synthesis and Characterization Direct Polycondensation of 12HS-Me, BD and IA-Me 12HS-Me, BD and IA-Me were copolymerized by immobilized lipase CA or lipase PS-D in toluene to produce poly(12HS/BD/IA) as confirmed by 1H and 13C NMR spectroscopy (Scheme 1). It was found that the monomer ratio of the copolymer agreed with the initial monomer feed ratio when lipase PS-D was used. However, when lipase CA was used, 12HS was less reactive compared to BD and IA-Me. Thus, the sequences of the monomers in the copolymer chain differed depending on the lipase origin. Details are discussed in a later section. Figure 1 shows the effects of temperature on Mw, Mw/Mn and polymer yield. Similar tendencies were observed when using lipase CA and lipase PS-D, i.e., Mw and polymer yield increased with increasing temperature from 30 °C to 80 °C. Both the highest Mw and polymer yield were obtained at 80 °C, then both gradually decreased due to thermal crosslinking at the IA moiety and deactivation of the enzyme. Based on these results, further studies were carried out at 80 °C. Figure 2 shows the time course of the lipase-catalyzed polymerization of 12HS-Me, BD and IA-Me. The Mw of poly(12HS/BD/IA) gradually increased with time and reached a Mw of about 30000 after a 5 d reaction at 80 °C using either lipase CA and lipase PS-D as shown in Figure 2. Figure 3 shows the effects of enzyme concentration on Mw, Mw/Mn, and polymer yield. When lipase CA was used, the highest Mw of poly(12HS/BD/IA)-1 was produced at an immobilized lipase concentration of 80 wt% as shown 242 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

in Figure 3a. The molecular weight decreased with a lipase concentration higher than 80 wt%. On the other hand, when using lipase PS-D, the Mw of poly(12HS/BD/IA)-2 increased with increasing lipase concentration up to 120 wt% and then remained almost constant as shown in Figure 3b.


Lipase-Catalyzed Ring-Opening Polymerization of Cyclic BD/IA Oligomer and 12HS-Me In order to further increase the Mw of the polymer for better mechanical properties, lipase-catalyzed ring-opening polymerization of the cyclic oligomer may be effective as reported previously (20–22). Therefore, cyclic BD/IA oligomer was first prepared, followed by ring-opening copolymerization with 12HS as shown in Scheme 2. It was found that significantly higher molecular-weight poly(12HS/BD/IA) was produced by the ring-opening copolymerization of cyclic BD/IA oligomer and 12HS-Me using lipase PS-D when compared to direct polycondensation. Figure 4a shows the effects of enzyme concentration on Mw and polymer yield by the copolymerization of cyclic BD/IA oligomer and 12HS-Me. The greatest Mw of poly(12HS/BD/IA)-2 was produced when 140 wt% lipase PS-D was used after 2 d at 80 °C. Figure 4b shows the time course of the copolymerization of cyclic BD/IA oligomer and 12HS-Me using lipase PS-D. The Mw of poly(12HS/BD/IA)-2 gradually increased with time and reached the highest Mw of 160000 after a 4 day reaction at 80 °C using 140 wt% immobilized lipase PS-D. On the other hand, the Mw of poly(12HS/BD/IA)-1 was only 39000 using 70 wt% lipase CA after a 4 d reaction at 80 °C (not shown). Characterization of Poly(12HS/BD/IA) Figure 5 shows HMBC spectra of poly(12HS/BD/IA)s produced by the ringopening polymerization using lipase CA and lipase PS-D. No correlation of 12HS and IA was observed in Figure 5a. This indicates that when the polymerization was carried out using lipase CA, no ester bond formed between the hydroxy group of 12HS and the carboxy group of IA. On the other hand, an ester bond was formed between 12HS and IA by lipase PS-D as confirmed by the HMBC spectrum in Figure 5b (marked x). Based on these results, the proposed molecular structure of poly(12HS/BD/IA) is shown in Scheme 1 and Scheme 2. In order to further analyze the difference of the two polymer structures produced by lipase CA and lipase PS-D, the reactivities of the two carboxy groups of IA were compared. At first, 12HS-Me having a secondary hydroxy group was reacted with cyclic BD/IA oligomer using lipase CA to produce poly(12HS/BD/IA)-1 with a Mw of 26000. However, the molar ratio of 12HS and IA of the polymer did not agree with the feed molar ratio. Contrary to this, 12HD having a primary hydroxy group was reacted with cyclic BD/IA oligomer using lipase CA to produce a higher molecular weight poly(12HD/BD/IA) with a Mw of 86000, and the molar ratio of 12HD and IA of the polymer agreed with the feed monomer ratio. This difference might be due to the reactivity of the hydroxy group of 12HD and 12HS towards IA in the enzyme-activated intermediate 243 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 2. Effects of reaction time on Mw, Mw/Mn and polymer yield using lipase CA (a) and lipase PS-D (b) at 80 °C. Reaction conditions are the same with Figure 1 except reaction time. Mw and Mw/Mn (●); polymer yield (○).

Figure 3. Effects of enzyme concentration on Mw, Mw/Mn and polymer yield using lipase CA (a) and lipase PS-D (b) at 80 °C for 5 d. Reaction conditions are the same with Figure 1 except enzyme concentration. Mw and Mw/Mn (●); polymer yield (○).



Scheme 2. Ring-opening polymerization of cyclic BD/IA oligomer and 12HS-Me by lipase CA and PS-D.

Figure 4. (a) Effects of enzyme concentration on Mw, Mw/Mn and polymer yield. Reaction conditions: Cyclic BD-IA oligomer (0.025 mmol) and 12HS-Me (0.2 mmol) were polymerized by lipase PS-D in the presence of MS4A placed at the top of the polymerization vessel at 80 °C for 2 d. Mw and Mw/Mn (●); polymer yield (○). (b) Time course of the ring-opening copolymerization of cyclic BD/IA oligomer and 12HS-Me. Reaction conditions: Cyclic BD/IA oligomer (0.025 mmol) and 12HS-Me (0.2 mmol) were polymerized by lipase PS-D (140 wt%) at 80 °C. Mw and Mw/Mn (●); polymer yield (○). 245 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 5. HMBC spectra of poly(12HS/BD/IA) prepared by lipase CA (a) and lipase PS-D (b) (CDCl3). x indicates the ester bond of 12HS and IA.



(23–25). The secondary hydroxy group of 12HS-Me was less reactive with the carboxy group of IA by lipase CA when compared to the primary hydroxy group of 12HD. Also, the reactivity of the two carboxy groups of IA might differ. This difference would be more pronounced in the enzyme-activated intermediate of IA. In order to compare the reactivity of the two carboxy groups of IA, IA-Me was reacted with 1-hexanol (primary alcohol) and 4-decanol (secondary alcohol) using lipase CA and lipase PS-D in toluene. It was found that both carboxy groups of IA-Me (A and B in Table 1) reacted equally with 1-hexanol by both lipases. Also, both carboxy groups of IA-Me reacted equally with 4-decanol by lipase PS-D. However, the carboxy group adjacent to the C=C double bond (A) of IA-Me barely reacted with 4-decanol by lipase CA (Table 1). Based on these results, it was concluded that the carboxy group of IA-Me adjacent to the C=C double bond and the secondary hydroxy group of 12HS-Me were less reactive by lipase CA. Thus, no correlation between 12HS and IA was observed in Figure 5a, because 12HS was exclusively bound to IA via BD when using lipase CA. Thermal Properties The thermal properties of poly(12HS/BD/IA)-2, poly(12HS) and poly(BD/IA) were measured using DSC. The results are shown in Table 2. A single crystallization peak at Tc = -40 °C was observed at a cooling rate of -20 °C·min-1. The Tm of the copolymer was measured at a heating rate of 10°C·min-1 and a single melting peak at around Tm = -25 °C was observed. Poly(12HS/BD/IA)-2 showed a Tg of around -77 °C, and was a viscous liquid at room temperature. Tm and Tc of the poly(12HS/BD/IA)-2 were similar to those of poly(12HS). On the other hand, Tg values of the poly(12HS/BD/IA)-2 were slightly lower than that of poly(12HS). Crosslinking Behavior of Poly(12HS/BD/IA) Poly(12HS/BD/IA)-2 was thermally crosslinked at 180 °C using a hot-press machine and crosslinking was evaluated by a SVNC. The results are summarized in Figure 6. It was found that the viscous poly(12HS/BD/IA)-2 prepared by lipase PS-D was readily crosslinked at 180 °C to produce a crosslinked polymer sheet as shown in Figure 6 (lines 1 - 3). The crosslinking of the polymer occurred between the C=C double bonds of IA. Therefore, the crosslinking was facilitated with increasing IA content. On the other hand, no significant crosslinking of poly(12HS/BD/IA)-1 prepared by lipase CA was observed by heating at 180 °C as shown in Figure 6 (lines 4 and 5). It seems that this difference is caused by the differences in polymer structures. No significant crosslinking was observed for poly(12HS/BD/IA)-1 prepared by lipase CA probably due to the crosslinking site of the polymer, i.e., the C=C double bond of the IA unit might be distributed on the center part of the polymer chain as shown in Scheme 1, with relatively long 12HS chains on both sides. Thus, IA might exist as a core moiety and the 12HS chain might be covering the core moiety like a shell. Therefore, intramolecular crosslinking preferentially occurred rather than intermolecular crosslinking. On the other hand, when poly(12HS/BD/IA)-2 was prepared by lipase PS-D, 247 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Table 1. Reactivity of IA-Me and alcohol by lipasea Substrate

Reaction rate (mol%) Lipase PS-D

Lipase CA

A

B

A

B

1-Hexanol

94.2

91.5

97.5

98.0

4-Decanol

50.9

52.6

3.4

21.2

a


Reaction conditions: IA-Me and alcohol (molar ratio of 1/2) were stirred with 100 wt% lipase in toluene (1.8 g substrate/mL toluene) at 80 °C for 2 d.

Table 2. Thermal properties of poly(12HS/BD/IA)

a

12HS in polymer (mol%)

Tm (°C)

Tc (°C)

Tg (°C)

100a

-24.6

-37.0

-55.7

88.2

-24.2

-39.0

-84.8

82.4

-25.4

-39.7

-76.2

79.7

-25.8

-40.5

-76.6

71.9

-25.7

-41.0

-77.6

67.2

-25.6

-40.0

-78.7

0b

-

-

-42.0

poly(12HS),

b

poly(BD/IA)

the IA unit was randomly distributed in the polymer chain and intermolecular crosslinking occurred. The crosslinked poly(12HS/BD/IA)-2 sheet with a thickness of 1 mm was soaked in chloroform in order to remove any uncrosslinked soluble fractions. As determined by the weight loss of the sample, the insoluble gel fraction of the crosslinked polymer was 95.8%. The FT-IR absorption peak at 1640 cm-1, corresponding to the C=C of the polymer, became weak after crosslinking. This indicated that the crosslinking reaction occurred at the C=C group by a free radical crosslinking mechanism (26, 27). The crosslinked polymer sheet was a soft and transparent elastomer. The light transmission rate of the crosslinked polymer sheet as measured by Haze meter was 92.3%, indicating high transparency similar to acrylate resin and glass. The haze of the crosslinked polymer sheet was 20.3%. 248 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 6. Crosslinking behavior of poly(12HS/BD/IA) prepared by lipase PS-D (lines 1-3) and lipase CA (lines 4 and 5) using a hot-press machine at 180 °C. Crosslinking was evaluated by SVNC. 1: Mw = 25000, IA 19.7 mol%; 2: Mw = 37000, IA 15.9 mol%; 3: Mw = 9000, IA 14.9 mol%; 4: Mw = 33000, IA 7.5 mol%; 5: Mw = 6000, IA 23.7 mol%.

Figure 7. Effects of the 12HS content on the hardness by durometer C (●) and Young’s modulus (○) of thermally crosslinked poly(12HS/BD/IA)-2 film. Hardness and Mechanical Properties of the Crosslinked Poly(12HS/BD/IA)-2 The crosslinked poly(12HS/BD/IA)-2 sheet showed a hardness of 30 - 66 based on the durometer C as shown in Figure 7. This indicated that the crosslinked poly(12HS/BD/IA) is softer than conventional natural and synthetic rubbers. The hardness of the poly(12HS/BD/IA) sheet was dependent on the 12HS content, decreasing with increasing 12HS content of the copolymer as shown in Figure 7. 249 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


The hexyl side group of 12HS may endow a flexibility to the elastomer, thus, with the increasing 12HS content, the hardness of the polymer sheet became softer. A poly(78.4 mol% 12HS/BD/IA)-2 film with a Mw of 131000 showed a tensile strength at break of 310 KPa and an elongation at break of 130%. On the other hand, poly(47.4 mol% 12HS/BD/IA) film with a Mw of 105000 showed a tensile strength at break of 630 KPa and an elongation at break of 66%. The Young’s modulus of poly(12HS/BD/IA) films with varying monomer compositions were measured and the results are shown in Figure 7. The Young’s modulus of the copolymer gradually decreased with increasing 12HS content. The mechanical properties of the elastomers were highly dependent on the 12HS content.

Conclusion Poly(12HS/BD/IA) having a Mw of 30000 was produced by the direct polycondensation of 12-Me, BD and IA-Me using 120 wt% immobilized lipase PS-D and 80 wt% immobilized CA in toluene at 80 °C for 5 d. Significantly higher molecular weight poly(12HS/BD/IA)-2 having a Mw of 160000 was produced by the ring-opening polymerization of cyclic BD/IA and 12HS-Me using 140 wt% immobilized lipase PS-D in toluene at 80 °C for 4 d. Poly(12HS/BD/IA) was a viscous liquid having low Tc of – 40 °C, Tg of –77 °C and Tm of –25 °C. Poly(12HS/BD/IA)-2 produced by lipase PS-D was readily crosslinked by hot-press at 180 °C to form a soft and transparent elastomer. Crosslinking was facilitated by increasing the IA content in the copolymer.

Acknowledgments Immobilized lipase from Candida antarctica (Novozym 435) was kindly supplied by Novozymes Japan Ltd. (Chiba, Japan). This work was supported by High-Tech Research Center Project for Private Universities: matching fund subsidy from MEXT, 2006-2011.

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