Chemo-Enzymatic Syntheses of Polyester-Urethanes - American

Chapter 16

Downloaded by PENNSYLVANIA STATE UNIV on August 20, 2013 | http://pubs.acs.org Publication Date (Web): August 11, 2010 | doi: 10.1021/bk-2010-1043.ch016

Chemo-Enzymatic Syntheses of Polyester-Urethanes Karla A. Barrera-Rivera,a Ángel Marcos-Fernández,b and Antonio Martínez-Richaa,* aDepartamento

de Química, Universidad de Guanajuato, Noria alta s/n. Guanajuato, Gto., 36050, México. bDepartamento de Química y Tecnología de Elastómeros, Instituto de Ciencia y Tecnología de Polímeros (CSIC), Juan de la Cierva No. 3, 28006 Madrid, Spain. *[email protected]

The enzymatic synthesis of α-ω-telechelic polycaprolactone diols (HOPCLOH) and triblock copolymers was studied. Synthesis of α-ω-telechelic PCL diols was achieved by enzymatic ring opening polymerization with Yarrowia lipolytica lipase immobilized on a macroporous resin Lewatit VP OC 1026, and using diethylene glycol and poly(ethylene glycol) as initiators. Biodegradable linear polyester-urethanes were prepared from synthesized PCL diols and hexamethylenediisocyanate (HDI). Depending on the length of PCL in HOPCLOH, the polymers were amorphous or semicrystalline. Measured mechanical properties strongly depend upon the degree of crystallinity of HOPCLOH.

Introduction The development of injectable materials to be used in non-invasive surgical procedures has triggered much attention in recent years. These materials are required to display low viscosity at insertion time, while a gel or solid consistency is developed in situ, later on. Block copolymers comprising poly(ethylene glycol) (PEG) segments and biodegradable polyester blocks such as poly(lactic acid), poly(glycolic acid) and poly(caprolactone) (PCL), have been described by various groups (1–3). The implementation of green chemistry in the field of polymer © 2010 American Chemical Society In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


science includes the design and synthesis of sustainable plastics. Such plastics should be produced from feedstocks derived from renewable or biomass resources by environmentally benign processes, such as enzymatic and solvent free processes, and avoiding the use of hazardous materials. Moreover, they should be chemically recyclable and biodegradable. In addition, a high-performance material that leads to a reduction in its consumption is very important. A chemically recyclable and biodegradable polymer contains enzymatically cleavable linkages, such as ester and carbonate linkages. Such a polymer chain can be further broken down by environmental microbes during biodegradation. Also, it can be cleaved by a specific enzyme into oligomers or monomers that can be repolymerized in the reverse reaction of the enzyme (chemical recycling) (4). Polyurethane polymers are extremely important and versatile materials having numerous applications in foams, surface and textile coatings, adhesives and elastomers. They are used in a wide variety of industries such as furniture, construction, aircraft and automobile manufacture and mining equipment. These materials are manufactured from hydroxyl terminated polyester resins made by the high temperature Lewis acid catalyzed condensation of a diacid and diol, or hydroxyl terminated polyethers derived from propylene oxide, in both cases the subsequent reaction with diisocyanates produces the polyurethane polymer (5). Enzyme-catalyzed polymerization may become a versatile method for the production of sustainable polyurethanes, as lipase, for example, is a renewable catalyst with high catalytic activities (6). The most prominent advantage of using a hydrolysis enzyme for the production of polymers is the reversible polymerization-degradation reaction that allows chemical recycling (7). The focus of this work is the synthesis of α, ω-telechelic poly(ε-caprolactone) diols (HOPCLOH), diblock and triblock copolymers using a one-step enzymatic method, useful for polyurethane synthesis.

Experimental Materials ε-CL (Aldrich) was distilled at 97-98 °C over CaH2 at 10 mm Hg. Diethylene glycol (DEG) and poly(ethylene glycol) with different molecular weights (PEG200), (PEG400) and (PEG1000), Lewatit VP OC 1026 beads, stannous 2-ethylhexanoate, hexamethylenediisocyanate (HDI) and 1,2-dichloroethane anhydrous 99.8 % were purchased from Sigma Aldrich and used as received. Instrumentation Solution 1H and 13C-NMR spectra were recorded at room temperature on a Varian Gemini 2000 instrument. Chloroform-d (CDCl3) was used as solvent. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) spectra were recorded in the linear mode by using a Voyager DE-PRO time-of-flight mass spectrometer (Applied Biosystems) equipped with a nitrogen laser emitting at λ= 337 nm with a 3 ns pulse width and working in positive-ion mode and delayed extraction. A high acceleration voltage of 20 kV was employed. 228 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 1. Synthesis of α, ω-telechelic poly(ε-caprolactone) diols (HOPCLOH). 2,5-dihydroxybenzoic acid (DHB) was used as matrix. Samples were dissolved in acetonitrile and mixed with the matrix at a molar ratio of approximately 1:100. DSC thermograms were obtained in a Mettler-Toledo 820e calorimeter using heating and cooling rates of 10 °C/min. Thermal scans were performed from 0 °C to 100 °C. FT-IR spectra were obtained with the ATR technique on films deposited over a diamond crystal on a Perkin-Elmer 100 spectrometer with an average of 4 scans at 4 cm-1 resolution. Gel permeation chromatography (GPC-MALLS) was used to determine molecular weights and molecular weight distributions, Mw/Mn, of macrodiols samples. The chromatographic set-up used consists of an Alliance HPLC Waters 2695 Separation Module having a vacuum degassing facility online, an auto sampler, a quaternary pump, a columns thermostat, and a Waters 2414 Differential Refractometer for determining the distribution of molecular weight. The temperature of the columns was controlled at 33 °C by the thermostat. Tensile properties were measured in a MTS Synergie 200 testing machine equipped with a 100 N load cell. Type 3 dumbbell test specimens (according to ISO 37) were cut from film. A crosshead speed of 200 mm/min was used. Strain was measured from crosshead separation and referred to 12 mm initial length. Five samples were tested for each polymer composition.

Lipase Isolation and Immobilization Lipase production by Yarrowia lipolytica (YLL) was made as previously reported by Barrera et al (8). Before immobilization, Lewatit 1026 beads were activated with ethanol (1:10 beads: ethanol), washed with distilled water and dried under vacuum for 24 h at room temperature. The beads (1g) were shaken in a rotatory shaker in 15 mL of lipase solution with 0.1568 mg/mL of YLL at 4°C for 229 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 2. Synthesis of PEG-PCL diblock and triblock copolymers. 24 h. After incubation, the carrier was filtered off, washed with distilled water and then dried under vacuum for 24 h at room temperature.

Synthesis of α,ω-Telechelic Poly(ε-caprolactone) Diols (HOPCLOH) Samples DEG1PCL, (10 mmol of ε-CL, 1 mmol of DEG), DEG2PCL (10 mmol of ε-CL, 0.5 mmol of DEG) and DEG3PCL (10 mmol of ε-CL,0.25 mmol of DEG) were placed in a 10 mL vial previously dried, and in all cases 12 mg of immobilized YLL was added. Vials were stoppered with a teflon silicon septum and placed in a thermostated bath at 120 °C for 6 h. No inert atmosphere was used. After the reaction was stopped, the enzyme was filtered off and the final polymer was analyzed by FT-IR, 1H and 13C-NMR, GPC-MALLS, DSC and MALDI-TOF.

Synthesis of the PEG-(CL)n Copolymers The PEG-(CL)n copolymer was synthesized by a ring opening polymerization reaction. PEG200PCL (10 mmol of ε-CL, 1 mmol of PEG200), PEG400PCL (10 mmol of ε-CL, 1 mmol of PEG400) and PEG1000PCL (10 mmol of ε-CL, 0.1 mmol of PEG1000) were placed in a 10 mL vial previously dried, and then 12 mg of immobilized YLL was added. The reaction proceeded at 120 °C for 6 h. By varying the CL/PEG ratios and PEG molecular weight, caprolactone blocks of different length were produced. Depending on the molecular weight of the CL segment, soft waxes to hard solids were obtained. PCL diols were dried at 70 °C in vacuo for 12 h, and stored at ambient temperature in a dessicator at vacuum until used. 230 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Table 1. Molecular Weights of the synthesized poly(ε-caprolactone) diols Mn (GPC)

Mw/Mn (GPC)

Mn (MALDI)

Mn (Da) 1H-NMR

PCLPEG200

3817

1.132

974

1066

PCLPEG400

4083

1.163

1120

1211

PCLPEG1000

4481

1.214

971

2504

PCLDEG1

4321

1.181

1363

836

PCLDEG2

5101

1.272

1978

1305

PCLDEG3

7426

1.531

2429

1780

Table 2. Percent of bisubstitution (% Bi (OH)), monosubstitution (% Mono (OH)) in the synthesized poly(ε-caprolactone) diols % Mono (OH)

% Bi (OH)

HAPCLOH (%)

PCLPEG200

47

53

10

PCLPEG400

44

56

13

PCLPEG1000

52

48

56

PCLDEG1

34

66

8

PCLDEG2

29

71

9

PCLDEG3

23

77

48

Synthesis of PCL Polyurethanes Dry PCL diol (1.5 g) and HDI in the appropriate amount (OH:NCO ratio = 1:1) and 2 mL of 1,2-dichloroethane were charged into a round bottom flask. The catalyst, stannous 2-ethylhexanoate (1% mol by PCL diol moles) was added, and stirred for 4 h at 80 °C. The resulting solution was poured over a leveled glass. The solution was covered by a conical funnel to protect it from dust and to avoid the excessively fast solvent evaporation, and allowed to stand at ambient temperature for 24 h. The film was then released and dried in vacuum. Samples for physical characterization were cut from films; film thickness ranged from 50-80 µm.

Results and Discussion Lipase Isolation and Immobilization Lipase with a protein concentration of 0.1568 mg/mL was obtained. Lewatit VP OC 1026, a crosslinked polystyrene (macroporous) resin with a matrix active group di-2-ethylhexyl phosphate, was used. The saturation time for YLL absorption was 30 min. The enhanced adsorption rates of polystyrenic resins are attributed to stronger hydrophobic interactions between styrenic surfaces, functional groups and YLL. The dependence of adsorption rate on particle size is 231 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 3. DSC second heating curves (20 °C/min) for the synthesized poly(ε-caprolactone) diols. Top: PCLPEG200 (black), PCLPEG400 (red) and PCLPEG1000 (blue). Bottom: PCLDEG1 (black), PCLDEG2 (red) and PCLDEG3 (blue). due to the pore size that is limiting protein transport to the inside of the particles. Final resin with a protein content of 0.136 mg of protein/g of resin and a protein adsorption of 87 % was used in polymer synthesis. Synthesis of α,ω-Telechelic Poly(ε-caprolactone) Diols (HOPCLOH) The synthetic pathway is described in Figure 1. The first step was the formation of the PCL diols by the ring-opening polymerization of ε-CL and using DEG as initiator in the presence of immobilized YLL. Under the same conditions, CL and DEG were allowed to polymerize in the absence of enzyme as control. After precipitation, no corresponding polymers could be obtained, which indicate that the lipase enzymes actually catalyze the polymerization of CL and DEG. Results for the three synthesized PCL diols are shown in Tables 1 and 2. Synthesis of the PEG-(CL)n Copolymers The synthetic pathway is described in Figure 2. Three PEG-PCL copolymers (shown in Tables 1 and 2) with different molecular weights and compositions were easily synthesized by changing the feed molar ratio of ε-CL/PEG in the presence 232 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 4. Stress–strain graph for the synthesized polyurethanes at room temperature. of immobilized lipase. The apparent molecular weights and polydispersities of all samples were also measured by GPC. As shown in Table 1, the diblock copolymers exhibited a narrow molecular weight distribution. In the last column of Table 2 we report the percentage of HAPCLOH present in the polymers. In some cases (PCLPEG1000 and PCLDEG3) in which the PEG1000 and DEG concentrations are lower (0.1 mmol and 0.25 mmol respectively) it reaches more than 50 % of the total polymer chains. These behavior could be associated to an increase in the chain length of PCL and a decrease in the initiator concentration, which causes that the proportional molecules of water present in the reaction medium (9) that initiates the reaction becomes larger. DSC results show crystallization due to caprolactone segments, with an increase in the melting point when polymer length increases. Melting temperatures decrease in the polymers which have a lesser content of HAPCLOH chains. In the second scan a biphasic behavior can be observed, attributed to the existence of a multiphase morphology (Figure 3). In the second stage, OH-terminated PCL was reacted with a stoichiometric amount of HDI (respect to HO hydroxyl groups), to form the PCL polyurethane. Finally, the chemical composition of the polymers was studied by different techniques. The FT-IR spectra of the polymers revealed the complete disappearance of the isocyanate peak 2268 cm-1 and the appearance of urethane bands at 3320 and 2263 cm-1. The presence of urethane groups in the polymer was corroborated by 13C-NMR, where the characteristic peak at 156.3 ppm was clearly observed, while the isocyanate band at 122.1 ppm totally disappeared. 233 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Table 3. Mechanical properties of the synthesized polyurethanes Polymer

Strain at yield (%)

Stress at break (MPa)

Strain at break (%)

Modulus (MPa)

PCLPEG200HDI

42 ± 4

43 ± 5

1678 ± 161

11 ± 2

PCLPEG400HDI

27 ± 1.3

26 ± 1.4

1698 ± 36

4 ± 0.2

PCLPEG1000HDI

16 ± 0.2

14 ± 0.1

20 ± 2

222 ± 7.3

PCLDEG1HDI

67 ± 8

45 ± 6

1434 ± 127

11 ± 2.2

PCLDEG2HDI

39 ± 2

32 ± 2

1779 ± 137.4

122 ± 47

PCLDEG3HDI

23 ± 3

21 ± 2.2

953 ± 373

228 ± 10

The stress-strain curves of the different polyurethanes are shown in Figure 4. Characteristic values derived from these curves are presented in Table 3. Two very differentiated behaviors can be observed. When PCL segments crystallize extensively, the polymers behave as tough plastics and present a high modulus followed by yielding and high extension. A very small increase in the stress value is observed until the narrow part of the specimen is completely extended and reaches the wide part of the dumbbell; after that, the stress increases again until the sample breaks. When the crystallization of the PCL segments is limited, the polymer behaves as an elastomer with low modulus, and a steady increase of stress as a function of strain until rupture is seen. For samples PCLPEG400HDI and PCLPEG200HDI, the stress-strain diagrams are curved for practically the entire range of stress. PCLPEG400HDI shows a lower modulus than PCLPEG200HDI. This behavior can be associated to the PCL chain length, which is shorter for the PCLPEG400 diol. In both cases, PCL chains are amorphous and does not crystallize (as observed by DSC). PEG content in the polyurethane is larger in PCLPEG400HDI, and this reflects in a softer material. In both cases, it can be concluded that final polymers exhibit good mechanical performance.

Conclusion A series of PCL–PEG–PCL block copolymer diols, and α,ω-telechelic poly (ε-caprolactone) diols were successfully synthesized for the first time, using immobilized lipase from Yarrowia lipolytica as catalyst. Results demonstrated that these diols are useful for polyurethane syntheses. The polymers based on them showed good mechanical behavior as tough plastics or rubbers depending on crystallization of the PCL segments.

Acknowledgments Financial support by Consejo Nacional de Ciencia y Tecnología (CONACYT) Grant SEP-2004-C01- 47173E. We are indebted to Rosa Lebrón-Aguilar (CSIC) 234 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

and Ricardo Vera-Graziano (IIM-UNAM) for obtaining MALDI-TOF spectra and GPC-MALLS data.

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

Chemo-Enzymatic Syntheses of Polyester-Urethanes - American

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