J Polym Res (2010) 17:847–851 DOI 10.1007/s10965-009-9376-6
SHORT COMMUNICATION
Ring-opening polymerization of ε-caprolactone initiated by heteropolyacid Guangwen Cheng & Xiaodong Fan & Wei Pan & Yuyang Liu
Received: 14 July 2009 / Accepted: 6 December 2009 / Published online: 16 December 2009 # Springer Science+Business Media B.V. 2009
Abstract In this work, phosphotungstic acid (PTA) as a novel initiator was reported for the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). It was found that PTA was an efficient initiator. ROP of ε-CL can be readily initiated by PTA at room temperature and form poly(ε-caprolactone) with narrow molecular weight distribution. Polymerization mechanism study indicates that the polymerization proceeds via acyl-oxygen bond cleavage.
Heteropolyacid is an eco-friendly and commercially available solid acid. As a “green” catalyst, it is widely used in various chemical reactions such as alkylation [13], cycloaddition [14], hydroformylation [15], oxidation [16], and polymerization [17, 18]. In this work, a heteropolyacid, phosphotungstic acid (PTA) was first used as an initiator for the ROP of ε-CL.
Keywords Phosphotungstic acid . ε-caprolactone . Ring-opening polymerization
Experimental Materials
Introduction In recent years, poly(ε-caprolactone) (PCL) has attracted much attention due to its biodegradable, biocompatible, and permeable properties [1, 2]. PCL is usually synthesized by the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). Many metal complexes have been used as initiators for ROP of ε-CL, such as Al [3–5], Sn[6, 7], Zn [8, 9] and Ln [10–12] complexes. Despite the success of these initiators in the synthesis of PCL, some problems are still encountered such as the utilization of high reaction temperature, and the requirement of oxygen-free environment. Therefore, it is crucial significance to develop a simple, convenient and efficient metal complex initiator for the ROP of ε-CL from both practical and fundamental viewpoints. G. Cheng (*) : X. Fan : W. Pan : Y. Liu Applied Chemistry Department, School of Science, Northwestern Polytechnical University, Xi’an 710072, China e-mail:
[email protected]
ε-CL was purchased from Alfa, dried over CaH2 and distilled at reduced pressure. Phosphotungstic acid (H3PW12O40•xH2O, PTA) was purchased from Tianjin Kermel Chemical Company, and dried at 150°C for 2 h before use. Methylene dichloride (CH2Cl2) and anhydrous methanol (CH3OH) were both purchased from Tianjin Kermel Chemical Company, and used as received. Characterizations Infrared (IR) spectrum was acquired on a Nicolet IS10 spectrometer, KBr film. 1 H-NMR spectrum was recorded by INOVA-400 spectrometer (Varian), CDCl3 as the solvent, and tetramethylsilane (TMS) as the internal standard. UV–vis spectrometer analysis was performed on a UV-2550 (Shimadzu). H2O as solvent. Polymer’s molecular weight was measured by sizeexclusion chromatography with multi-angle laser light scattering detection (SEC-MALLS). SEC was performed using a HPLC pump (Waters 515) and a column (300 mm× 0.8 mm, MZ-Gel SDplus 500Å 5μm). Column effluent was
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precipitation under vacuum at room temperature for 24 hr. The monomer conversion was determined by gravimetric method. PCL-1: IR (cm−1) 3440(νOH, s), 2945(νas (C-H), s), 2865 (νs (C-H), s), 1724(νs (C=O), s), 1470(δ(C-H), m), 1365 (νas(C-O), s), 1294(ω(C-H), m), 1243 (βOH, s), 1190(νC-O-C, s). 1H-NMR(CDCl3 as solvent)δ(ppm) 1.38(cH), 1.63(bH, dH), 2.30(eH), 3.65(fH), 4.05(aH). SEC-MALLS: Mw = 5060, Mn =4700, Mw/Mn =1.077. DSC (melting point) 60.14°C.
0.9 0.8 Conversion (%)
0.7 0.6
0.024 g 0.036 g 0.072 g
0.5 0.4 0.3 0.2 0.1
Results and discussion
0.0 0
2
4
6
8
10
Time (h)
Fig. 1 Conversion versus time curves of ε-CL polymerization initiated by PTA (ε-CL=1.25 g, reaction temperature=25°C, PTA=0.024 g, 0.036 g, 0.072 g, respectively )
monitored by a miniDawn light-scattering detector (Wyatt technology, Santa, Barbara, CA, USA) and an Optilab rEX differential refractometer (Wyatt Technology). Two 25 mm high-pressure filters with 0.22 and 0.1μm pore (Millipore) were used for on-line filtration. The mobile phase was THF (chromatographic grade) with a flow rate of 0.5 ml/min. The end group of polymer was characterized by acidbase titration according to GB12008.3–89. Thermal behavior of polymer was investigated by differential scanning calorimetry (DSC) on a TA Q50 instrument (TA, UAS) under nitrogen gas with a heating rate of 10°C/min. ROP of ε-CL ε-CL (1.25 g) and PTA ( 0.036 g) were charged into a round-bottom flask equipped with a magnetic stirrer. The reaction mixture was stirred at 25°C for a desired reaction time (1 hr). The reaction was terminated by 0.5 M NaOH solution. The crude product was dissolved in CH2Cl2 (5 ml) and then precipitated in CH3OH (20 ml) for three times. The PCL-1 (0.5 g) was finally obtained through drying the Table 1 The relationship between ε-CL conversion and molecular weight of PCLa
a Reaction conditions: ε-CL= 1.25 g, PTA=0.036 g, T=25°C
Figure 1 shows the experimental results for the ε-CL polymerization initiated by PTA in different concentrations of initiator. As can be seen, the monomer conversion increases rapidly with time indicating a fast polymerization rate at the early stage of polymerization. Moreover, the polymerization rate increases with the concentration of initiator. This indicates that PTA possesses high activity as initiator for ε-CL polymerization. After a high monomer conversion, the conversion versus time plot presents a strong curvature indicating a decreased polymerization rate, which may be attributed to the increased viscosity of polymerization system. Interestingly, however, the molecular weight of resulting polymer does not increase with the monomer conversion, and its molecular weight distribution always keeps narrow during the whole polymerization. As shown in Table 1, the molecular weight of PCL increases first, and then decreases with ε-CL conversion. This may be related to the side reaction of chain degradation [19, 20]. At the late stage of polymerization, compared with the chain propagation, the chain degradation will dominate, which may cause a decrease of the molecular weight. Table.1 also shows the molecular weight distribution of PCL is always less than 1.5, which implies only one kind of active species is present in the ε-CL polymerization A sample of PCL (PCL-1) has been prepared and subjected to SEC-MALLS, 1H-NMR, UV-vis, and end group analysis. As shown in the 1H-NMR of PCL-1(Fig. 2), the peak at 3.65 ppm can be assigned to methylene protons
Conversion (%)
Molecular weight (Mn)
Molecular weight distribution (Mw/Mn)
10.6 25.1 38.4 45.0 53.2 72.8 80.1 92.3
2142 3082 3574 5315 5963 5802 4330 3991
1.064 1.115 1.076 1.12 1.219 1.251 1.317 1.445
Ring-opening polymerization of ε-caprolactone initiated by heteropolyacid
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5
4
c
e
a
Absorption
b+d
CDCl3
ε–CL
PTA ε-CL/PTA
3
2
TMS 1
f
0 200
10
Fig. 2
9
8
7
6
5 ppm
4
3
2
1
0
400
300
500
λ (nm)
Fig. 4 UV-spectra of ε-CL, PTA, and their mixture (H2O as solvent)
1
H-NMR spectrum of PCL-1
from –CH2OH group, which is further verified by the molecular weight analysis. Because if integrating the area of peak e and peak f in Fig. 2, the polymerization degree and number average molecular weight of PCL-1 can be calculated to be 40 and 4692 respectively, which is very close to the value of 4700 obtained by SEC-MALLS analysis as shown in Fig. 3. In other words, the end group of PCL-1 must contain hydroxyl group. The content of hydroxyl group (namely hydroxyl value) is 23.68 mg KOH/g, which was measured by acid-base titration. The functionality of hydroxyl group for PCL-1 can be calculated to be 1.98 through combing the molecular weight and the hydroxyl value (Eq. 1). In addition, there is the coordination between PTA and ε-CL, which was verified by the UV-vis spectrometer analysis. Fig. 4 shows the UV-spectra of ε-CL, PTA and their mixture. As can be seen, the spectrum of ε-CL and PTA mixture blueshifted compared with that of PTA. This is because that the Fig. 3 SEC-MALLS chromatogram of PCL-1. (Mw =5060, Mn =4700, Mw/Mn =1.077, The red curve shows the relationship between differential refractive index and elution time, and the black one shows the relationship between light scattering intensity and elution time)
interaction between ε-CL and PTA makes the density of electron cloud increase in PTA molecule [21]. Mn x f ¼ ð1Þ 56:1 1000 Where f represents functionality of hydroxyl group, Mn represents number average molecular weight, and x represents hydroxyl value, which is defined by the equivalent amount of potassium hydroxide corresponding to the number of hydroxyl groups in 1 g of polymer. In addition, the number average molecular weight of PCL-1 (4700) was obtained by SEC-MALLS. Base on these experiment results and the literature analysis [22, 23], we speculated the mechanism of ε-CL polymerization initiated by PTA. As shown in Scheme 1, PTA exerts a significant activating effect on the monomer through coordination. The ROP of ε-CL is initiated by the proton ionized from PTA, and the ε-CL monomer ring is opened via acyl-oxygen bond cleavage since PTA is a strong proton acid. There may be a degradation reaction of
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Scheme 1 The mechanism of ε-CL polymerization initiated by PTA
O O
O O
O O
3
O
O W O
+ H3PW12O40
+
+ 3H
O
3+ [PW12O40]
3 H+
H3PW12O40
O W O
O + HOCH2CH2CH2CH2CH2C [PW12O40]3-
+ [PW12O40]3-
3
O O
O W O
O
3n O
O
H OCH2CH2CH2CH2CH2C OCH2CH2CH2CH2CH2C+ [PW12O40]3n
3
3n OH O O H OCH2CH2CH2CH2CH2C OCH2CH2CH2CH2CH2C OH n
O O W O O O O H OCH2CH2CH2CH2CH2C OCH2CH2CH2CH2CH2C OCH2CH2CH2CH2CH2C O CH2CH2 (back-biting)
degradation O
O O HOCH2CH2CH2CH2CH2COCH2CH2CH2CH2CH2C O CH2CH2
+
O
O O W O O O O H OCH2CH2CH2CH2CH2C OCH2CH2CH2CH2CH2C OCH2CH2CH2CH2CH2C O CH2CH2 O H OCH2CH2CH2CH2CH2C OCH2CH2CH2
(interchange esterification) O HOCH2CH2CH2CH2CH2C
degradation
O OCH2CH2CH2CH2CH2C
2
OCH2CH2CH2
+
O HOCH2CH2CH2CH2CH2C OCH2CH2 O O W O represents the microstructure of H3PW12O40
polymer chain during the polymerization, which results in a decrease of product’s molecular weight [20]. In summary, the results obtained in the present study indicate that phosphotungstic acid can serve as an efficient initiator for the ε-CL polymerization. This work extends the
application of heteropolyacid as a “green” initiator in polymer synthesis. Acknowledgments This study was financially supported by a Meterage Foundation Project (contract grant number: J142006A201).
Ring-opening polymerization of ε-caprolactone initiated by heteropolyacid
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