Colloid Polym Sci (2013) 291:1999–2005 DOI 10.1007/s00396-013-2941-x

ORIGINAL CONTRIBUTION

Acidic ionic liquids catalyst in homo and graft polymerization of ε-caprolactone Amir Abdolmaleki & Zahra Mohamadi

Received: 29 September 2012 / Revised: 29 December 2012 / Accepted: 11 March 2013 / Published online: 20 March 2013 # Springer-Verlag Berlin Heidelberg 2013

Abstract Biodegradable polycaprolactone was prepared by ring-opening polymerization in presence of ionic liquids as efficient, inexpensive, nontoxic, and easily handled acid catalysts. The resulting polymer exhibited good yield and inherent viscosity between 0.10 and 0.18 dL/g. The chemical structure of obtained polymer was verified by the 1HNMR and Fourier transform infrared spectroscopy (FT-IR) spectra. In continuation, the obtained polymer was applied to improve quality level and mechanical properties and also to reduce the hydrophilic properties of the starch, so the ring-opening polymerization of ε-caprolactone was investigated in the presence of starch hydroxyl groups as initiator and ionic liquid as catalyst. The obtained starch-graftedpolycaprolactone was verified by 1H-NMR, FT-IR spectra, and field emission scanning electron microscopy analysis. Keyword Ionic liquid . Biodegradable . Polycaprolactone . Starch-grafted-polycaprolactone

Introduction In recent years, there has been an increasing concern about landfilling with nondegradable materials such as plastics. This concern led to the development of alternative biodegradable materials. Aliphatic polyesters are a group of remarkably biocompatible and biodegradable polymers widely used in biomedical applications. For example, they have been employed as resorbable implant materials for tissue engineering and drug delivery [1]. Among the numerous polyesters A. Abdolmaleki (*) : Z. Mohamadi Department of Chemistry, Isfahan University of Technology, Isfahan 84156/83111, Iran e-mail: [email protected] A. Abdolmaleki e-mail: [email protected]

studied so far, polycaprolactone (PCL) has proven to be the most attractive and useful class of biodegradable polyesters because of its proven biocompatibility, permeability, and good material properties. Polycaprolactone was usually synthesized by ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). Metal complexes, e.g., tin [2, 3], aluminum [4–7], iron [8, 9], yttrium [10, 11], zinc [12, 13], bismuth [14], ruthenium [15], scandium [16, 17], and zirconocene [18], were the most commonly explored catalysts for production of polycaprolactone. Despite the success of these initiators in the synthesis of PCL, some problems are still encountered such as utilization of high-reaction temperature and the toxicity of the heavy metal catalysts. Also, because of the potentially harmful effect of trace metallic residues, there are some concerns about using them in biomedical fields. Ring-opening polymerization of ε-caprolactone catalyzed by enzymes had also been investigated [19, 20]. Although the toxic, metal-based catalytic systems can be replaced by enzymatic polymerization, it remains in research and development stage because of the high cost of enzymes production and immobilization. Also, the polymerization of caprolactone can be catalyzed by organic species, e.g., different tertiary phosphines [21], (dimethylamino)pyridine [22], and N-heterocyclic carbene complexes [23, 24]. In spite of not contaminating the polymer with metal residue, it is not being easily recoverable from the reaction mixture, so it leads to the problematic catalyst recovery and recycling. From green chemistry points of view, further development of a recoverable catalyst for green and sustainable synthesis of polycaprolactone in an industrial scale is desirable. Ionic liquids were recently recognized as green solvents because of their recyclability and low volatility. Some ionic liquids had proved to be not only as solvents but also as effective catalysts for polymerization reaction [25–33]. The low cost, availability, easy procedure, and

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workup of the ionic liquids made them attractive for largescale operations. Therefore, in this project, they were successfully applied in the synthesis of polycaprolactone. Poly(ε-caprolactone) is one of the most hydrophobic biodegradable polymers with good mechanical properties currently available [34, 35]. However, the high cost of PCL and its low melting point (∼67 °C) have prevented its widespread industrial use. These facts motivated researchers to synthesize completely biodegradable polymers based on starch, a totally biodegradable, cheap, and natural biopolymer obtained from renewable sources. However, hydrophilicity of pure starch-based materials limits their applications, for example, their mechanical properties decrease with the water intake [36]. Several studies had been performed to combine the properties of starch and PCL by the grafting reaction between hydroxyl groups of the starch and ε-caprolactone monomers. This led to fully biodegradable starch-grafted-polycaprolactone (starch-gPCL) product with improved properties. Although graft polymerization was commonly done by catalysts such as (Sn(Oct)2 [37–39], Ti(OnBu)4, Al(O-iPr)3, or AlEt3) [39], their applications were limited because of their operation in anhydrous conditions, high costs, and toxicity. Furthermore, triethylaluminium as catalyst was extremely air and water sensitive; therefore, it was difficult to handle. Also, it released ethane, a very flammable by-product, during the reaction. So due to above problems, finding a new and simple technology was very important. Therefore, starch-graftedpolycaprolactone was synthesized in the presence of acidic ionic liquids in good yield.

Experimental section Materials Chemicals and materials: benzyl alcohol (Hopkin & Williams, 99 %), ε-caprolactone (Merck, 98 %), starch (Merck), sulfuric Scheme 1 ROP of ε-caprolactone using ionic liquids catalyst

Colloid Polym Sci (2013) 291:1999–2005

acid (98 %, commercial), hydrochloric acid (37 %, commercial), N-methyl-2-pyrrolidone (NMP, Merck), morpholine (>98 %, Merck), toluene (commercial) dried over barium oxide, tetrahydrofuran (THF, Aldrich >99 %), n-hexane (commercial), ethanol (commercial), methanol (commercial), and dichloromethane (>99 %, Merck) were used for the synthesis of the polymers and ionic liquids. Instrumentation The 1H-NMR spectra were recorded on Bruker Avance 400 spectrometer operating on polymers solution in DMSO-d6. Chemical shifts were given in the δ scale in parts per million. Fourier transform infrared spectroscopy (FT-IR) spectra were recorded using a Jasco-680 FT-IR spectrophotometer with KBr pellet. Vibration bands were reported as the wavenumber (in per centimeter). Inherent viscosities of polymer solution (0. 5 % w/v) in DMF were determined at 30 °C by a standard procedure using a Cannon Fenske routine viscometer. The field emission scanning electron microscopy (FE-SEM) micrographs of polymer were taken on a Hitachi (S-4160). Melting points were taken with a Gallenkamp melting point apparatus. Hammett acidity function was calculate based on UV spectroscopy [40, 41]. Ionic liquids synthesis Pyrrolidinium bisulfate [H-NMP]HSO4 In a round-bottomed flask, to a cold mixture of 1methyl-2-pyrrolidone (0.96 mL, 10 mmol) in 15 mL of dichloromethane, 0.53 mL of concentrated sulfuric acid (98 %, d=1.84) was added dropwise. The reaction mixture was stirred at room temperature for 3 h, then the solvent was evaporated under reduced pressure to produce [H-NMP]HSO4 (100 %) [40, 41]. Spectral data obtained through FT-IR spectroscopy were as follows (KBr, in per centimeter): υ=3,600–2,700 (OH, NH, br), 1,693 (C=O, sh), 1,671 (NH, w), 1,483 (C–N, sh), 1,287 (s),

Colloid Polym Sci (2013) 291:1999–2005 Table 1 Effect of reaction time and catalyst on the εcaprolactone polymerization a

Measured at a concentration of 0.5 g/dL in DMF at 30 °C

b Viscosity average molecular weight

Catalyst

[Mor-H]HSO4 [NMP-H]HSO4 [NMP-H]Cl

2001

36 h

48 h

Yield (%)

ηa (dL/g)

Yield (%)

ηa (dL/g)

Mvb ×10-4

84 76 67

0.181 0.175 0.108

93 84 74

0.162 0.158 0.098

1.36 1.30 0.78

1,229 (s), 1,175 (S=O, s), 1,064 (s), 1,016 (s), 881 (s), 854 (s), and 591 (S–O, s); 1H-NMR (400 MHz, DMSO-d6, in parts per million): δ=9.15 (4H, s), 3.27 (t, 2H, CH2), 2.65 (s, 3H, CH3), 2.16 (t, 2H, CH2), and 1.86 (m, 2H, CH2); H0 =0.58. Other ionic liquids had been synthesized by the same procedure.

(C=O, s), 1,471 (C–H, m), 1,368 (C–O, s), 1,295 (C–H, m), 1,244 (OH, s), 1,191 (C–O–C, s), 1,106, 1,046, 961, 731; 1H-NMR (400 MHz, DMSO-d6, in parts per million): δ=1.3 (2H, CH2), 1.55 (4H, CH2), 2.3 (2H, −CO–CH2), 3.98 (2H, –CH2–O–CO–), 5.11 (2H, ph–CH2–), and 7.36 (5H, ph).

Pyrrolidinium chloride [H-NMP]Cl

Preparation of starch-grafted-polycaprolactone

Regarding oily, yellow liquid, spectral data obtained through FT-IR spectroscopy were as follows (KBr, in per centimeter): υ=3,400–2,930 (OH, NH, br), 1,660 (C=O, sh), 1,507 (C–N, sh), 1,400, 1,300, 1,260, 1,110, 1,010, 950, 920, 750, 650, and 610; 1H-NMR (400 MHz, DMSO-d6, in parts per million): δ= 6.3 (4H, s), 3.3 (t, 2H, CH2), 2.69 (s, 3H, CH3), 2.18 (t, 2H, CH2), and 1.89 (m, 2H, CH2); H0 =2.15.

Starch-g-PCL was synthesized via the ROP of ε-CL with starch as an initiator and pyrrolidinium chloride as catalyst. After azeotropic dehydration of 0.6 g starch with toluene for 6 h, ionic liquid as a catalyst was added to the solution and stirred for 2 h at 110 °C. Then, 5 mL of ε-caprolactone was added into the reaction flask, and it was stirred under nitrogen atmosphere for 36 h at 110 °C. At last, the toluene in reaction flask was removed by rotovap. The polymer was precipitated with cold ethanol and washed with hot water to remove unreacted ε-caprolactone, starch, and residual catalyst. The resulted precipitate was a mixture of the grafted copolymer and homo-PCL (starch-gPCL/PCL). Spectral data obtained through FT-IR spectroscopy were as follows (KBr, in per centimeter): υ=3,433, 2,943, 2,866, 1,726, 1,637, 1,459, 1,420, 1,397, 1,367, 1,294, 1,241, 1,187, 1,045, and 731; 1H-NMR (400 MHz, DMSO-d6, in parts per million): δ=1.31 (2H, m, CH2), 1.54 (4H, m, CH2), 2.27 (2H, t, –CO–CH2), 3.98 (2H, t, –CH2– O–CO–), 3.5–3.75, 4.3–4.5, and 5.0–5.5 (m, broad peak, starch).

Morpholinium bisulfate [H-Mor]HSO4 With regard to white solid, spectral data obtained through FTIR spectroscopy were as follows (KBr, in per centimeter): υ=3,700–2,015 (OH, NH, br), 1,563 (NH, s), 1,455 (C–N, s), 1,422 (s), 1,300 (s), 1,224 (s), 1,189 (S=O, s), 1,100 (C–O, s), 1,060 (s), 893 (s), 870 (s), and 590 (S–O, s); 1H-NMR (400 MHz, DMSO-d6, in parts per million): δ=8.8 (s, 1H, OH), 7.2 (s, 2H, NH), 3.8 (m, 4H CH2), and 3.0 (m, 4H, CH2); pKa1 =2.26, H0 =1.31. Polymers synthesis Preparation of polycaprolactone In a typical reaction, 8 mL of dry toluene, 30 μL of benzyl alcohol, and 2.2 mL of ε-caprolactone were mixed in a three-neck flask. Then, the ionic liquid, as catalyst, was added to the solution. The reaction solution was then stirred under a nitrogen atmosphere at 52 °C. The reaction was quenched by adding excess MeOH, followed by THF. Excess solvent was removed from this solution by rotovap led to an oily, yellow liquid product precipitated in 30 mL of cold hexane. Then, it was filtered and dried in vacuum and stored under nitrogen atmosphere. Spectral data obtained through FT-IR spectroscopy were as follows (KBr, in per centimeter): υ=2,944 (C–H, s), 2,856 (C–H, s), 1,724

Fig. 1 FT-IR spectrum of poly (ε-caprolactone) synthesized in ionic liquids

2002

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Fig. 3 FT-IR spectrum of starch-g-polycaprolactone synthesized in ionic liquid Fig. 2 1H-NMR (400 MHz) spectrum of poly(ε-caprolactone) in DMSO-d6

Results and discussion Characterization of polycaprolactone Polycaprolactone was synthesized by the ring-opening polymerization of the ε-caprolactone and benzyl alcohol as an initiator in the presence of acidic ionic liquids. These ionic liquids offered significant advantages due to their high catalytic efficiency, low cost of production, excellent operational stability, and recoverability. Moreover, the problem of the polymer contamination with metal residue can be removed by ionic liquids use. A plausible mechanism of the ROP of ε-caprolactone catalyzed by ionic liquids via an activated monomer mechanism was presented in Scheme 1. Because of the polymer thermal degradation occurrence at the reaction temperature above 100 °C, the reaction was carried out at 52 °C [42]. The experimental results of the ε-caprolactone polymerization by using different ionic liquids at various times were shown in Table 1. According to the obtained results, as the reaction time increased, the monomer conversion increased too, but the inherent viscosity decreased. The inherent viscosity decreasing was probably because of increasing in intramolecular transesterification which led to degradation and cyclic oligomers formation and consequently decreasing the molecular weight [42–45]. Scheme 2 Synthesis of starchgrafted-polycaprolactone

The intrinsic viscosity of poly(ε-caprolactone) was measured with a Cannon Fenske routine viscometer in N,Ndimethylformamide at 30 °C. The viscosity average molecular weight of PCL was calculated according to the following equation [17]: ½ηðdL=gÞ ¼ 1:94  104 Mv0:73 The polymers’ structure was investigated by combination of melting point analysis, FT-IR, and 1H-NMR spectra. The melting point was 58 °C, in good agreement with listed values of ∼60 °C [46, 47]. The IR spectrum of polymer was in a good agreement with poly(ε-caprolactone) structure (Fig. 1). The characteristic vibrations of linear ester carbonyl groups (C=O) and C–O bonds were observed, respectively, at 1,724 and 1,191 cm−1. A typical 1H-NMR spectrum of the resulting polymer was shown in Fig. 2; the protons on the benzyl alcohol group were presented at 7.36 and 5.11 ppm. The peaks between 1.30 and 1.55 ppm are assigned to proton resonance of methylene in repetitive unit, and the peaks around 2.3 and 3.98 ppm are assigned to the methylene protons from (–CO–CH2) and (–CH2–O–CO–), respectively. Characterization of starch-grafted-polycaprolactone Starch-grafted-PCL copolymer was prepared in toluene by reacting starch and ε-caprolactone using pyrrolidinium chloride as catalyst. Since pyrrolidinium bisulfate and morpholinium

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Fig. 4 1H-NMR (400 MHz) spectrum of starch-g-polycaprolactone in DMSO-d6

bisulfate led to starch hydrolysis, they could not be used successfully in copolymer synthesis. The general reaction was briefly shown in Scheme 2. During graft polymerization, because of the formation of non-grafted polyester chains to PCL homopolymer by residual water of starch and other impurities in the reaction system, starch-g-PCL was purified via toluene extraction. Grafting efficiency (GE) was calculated according to the following equation [48], and it was equal to 11 %: GE ¼ Mðgrafted PCLÞ=½M ðgrafted PCLÞ þ M ðfree PCLÞ Where Mðgrafted PCLÞ ¼ MðstarchgPCLÞ  MðstarchÞ The structure of starch-g-PCL was characterized by FTIR and 1H-NMR, and the good adhesion of two components was proved by FE-SEM observations. Fig. 5 FE-SEM images of native starch (a1, a2) and starchg-PCL (b1, b2) prepared by in situ polymerization of εcaprolactone using [NMP-H]Cl catalyst after toluene extraction

2003

Notable difference between FT-IR spectra of starch and PCL was the strongest peak that appeared at 3,433 cm−1 (s, OH) for starch and at 1,726 cm−1 for PCL, assigned to the carbonyl group in PCL. However, both peaks were recorded in the spectrum of purified starch-g-PCL (Fig. 3). The product was characterized using 1H-NMR analysis in DMSO-d6 as a solvent. A typical spectrum was shown in Fig. 4. The peaks related to the polycaprolactone units were clearly presented in the range of 1.2–4.0 ppm; the peaks at 3.65 and 2. 2 ppm could be assigned to (–CO–CH2) and (CH2–O–CO), respectively, and the peaks at 1.3–1.5 ppm could be assigned to polyester methylene in repetitive unit (H3, H4). The proton resonances of the starch can be observed as the small and broad peaks in the region 3.4–3.8 and 5.0–5.4 ppm. The FE-SEM images of the starch-g-PCL composite and native starch were compared. It was found that the surface of starch granules were smooth (Fig. 5a), whereas the surface of starch-g-PCL (Fig. 5b) were rough and polygonal. Also, in the latter, there were some connections between granules which could be clearly seen. Therefore, a very good interfacial adhesion between the starch and the PCL chains (Fig. 5b) was obviously confirmed which would be helpful for improvement of mechanical properties of the modified composites. All these analyses, FT-IR, 1H-NMR, and FE-SEM, confirmed that ε-CL was polymerized from starch hydroxyl group with [H-NMP]Cl as catalyst.

Conclusion Polycaprolactone was synthesized by the ring-opening polymerization of ε-caprolactone and benzyl alcohol as an initiator in the presence of acidic ionic liquids. These ionic liquids offered significant advantages due to their high catalytic efficiency, low cost of production, and excellent operational

2004

stability. These catalysts were easily recovered from the polymerization solution. The polymer had no contamination. The resulting polymer exhibited good yield, and its inherent viscosity ranged between 0.10 and 0.18 dL/g. The chemical structure of obtained polymer was verified by the 1H-NMR and FT-IR spectra. ROP was initiated from the hydroxyl group in the presence of ionic liquids led to chemically modification of starch. The obtained starch-g-polycaprolactone was verified by 1H-NMR, FTIR spectra, and FE-SEM analysis. This copolymer had efficiently proven the interfacial adhesion improvement and presumably the mechanical properties of the compatible composites. Acknowledgments We gratefully acknowledge the partial financial support from the Research Affairs Division Isfahan University of Technology (IUT), Isfahan. Further partial financial support of Iran Nanotechnology Initiative Council, National Elite Foundation, and Center of Excellency in Sensors and Green Chemistry (IUT) are also gratefully acknowledged.

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