http://www.e-polymers.org e-Polymers 2011, no. 081 ISSN 1618-7229

Ring-opening polymerization of using ionic liquids as catalysts

Jiancheng Zhou, Ling Cheng, Dongfang Wu*

*School of Chemistry and Chemical Engineering, Southeast University, Jiangning District, Nanjing 211189, China; e-mail: [email protected].

(Received: 17 April, 2010; published: 15 August, 2011)

Abstract: The ring-opening polymerization of ethylene carbonate was examined using ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4) and 1-butyl-3-methylimidazolium chlorozincate ([bmim]Cl-(ZnCl2)x), as polymerization catalysts. It was shown that the polymerization was accompanied with decarboxylation and chain cleavage reaction. As the reaction time increased, the monomer conversion increased and the content of ethylene carbonate units in the resultant polymer decreased, while the polymer molecular weight increased at first, reached a maximum and then decreased. It was also found that not only the polymerizing activity of the [bmim]Cl-(ZnCl2)x but also its performance for suppressing the decarboxylation and chain cleavage increased with the molar fraction of ZnCl2. It was deduced that the catalytic performance of ionic liquids mainly depended on their inorganic anions and that the larger the amount of these anions in the reaction system, the better the catalytic performance. The polymerizing activity of ionic liquids was much higher than conventional catalysts often used for the polymerization of ethylene carbonate. Keywords: polymer synthesis; ionic liquid; polycarbonate; ring-opening polymerization

Introduction Aliphatic polycarbonates have received considerable attention due to their outstanding biocompatibility and biodegradability leading to a number of their applications in biomedical materials [1, 2] as well as in polyurethane and surfactant industry [3, 4]. A popular way to synthesize these polymers is the ring-opening polymerization of cyclic , usually carried out in the presence of different kinds of initiators and/or catalysts [5, 6]. In industry, five-membered cyclic carbonates such as ethylene carbonate and can be easily obtained through an environmentally friendly process, i.e. alcoholysis, which does not rely on the petrochemical (epoxide) and poisonous [7]. Nevertheless, the ring-opening polymerization of these cyclic carbonates of the smallest ring size is rather difficult to take place and involves partial decarboxylation and loss of , irrespective of the polymerization conditions [8-14]. For example, the polymerization of ethylene carbonate results in the formation of a copolymer consisting of units and ethylene carbonate units, i.e. poly(ethylene ether-carbonate)s, as represented by the following chemical equation [9]: O O O CH2CH2O nmCH2CH2OCO + CO2 ↑ O (1)

1 It has been reported that ethylene carbonate can be polymerized using Lewis acids, transesterification catalysts and bases as initiators or as polymerization catalysts [5, 9-14]. When Lewis acids or transesterification catalysts were used, the resultant polymers normally contained about 40-50 mol% of ethylene carbonate units for the reaction temperature of 150-170 oC and the reaction time of 70-100 h [10-13]. When bases were used, the resultant polymers contained 10-20 mol% of ethylene carbonate links for the temperature of 150 oC and the time of 72-98 h [9, 14]. One of main disadvantages of these initiators or catalysts is that they all have a low activity of polymerization, which leads to a high reaction temperature and a long reaction time, needed to reach 100% conversion of ethylene carbonate. For instance, the reaction temperature of 180-200 oC was found to be suitable for the polymerization of ethylene carbonate in the presence of potassium hydroxide [9]. Ionic liquids, which are noncorrosive and nonflammable organic salts comprised entirely of ions, are liquids below 100 oC and, as consequence of their nature, show some intriguing features such as extremely low vapor pressure, high thermal stability, the ability to retain the liquid state over a wide temperature range and the ability to solvate many organic, inorganic and polymeric materials [15]. In recent years, ionic liquids have received much attention as green or catalysts used for numerous chemical reactions [16, 17]. As for polymerizations in ionic liquids, olefin polymerization [17, 18], oxidative polymerization of benzene to poly(p-phenylene) [19], galvanostatic polymerization of 3-methylthiophene to poly(3-methylthiophene) [20], enzymatic polymerization [21] and ring-opening polymerizations of ε- caprolactone [21], 2-ethyl-2-oxazoline [22], γ-benzyl-L-glutamate-N-carboxyanhydride [23], etc. were reported. In this work, an investigation of ionic liquids for the ring-opening polymerization of ethylene carbonate was conducted. Ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4, hereinafter referred to as IL A) and 1-butyl-3- methylimidazolium chlorozincate ([bmim]Cl-(ZnCl2)x, referred to as IL Bx, where x is the ZnCl2 molar fraction in the mixture, i.e. x=nZnCl2/(nZnCl2+n[bmim]Cl)), were used as polymerization catalysts. The monomer conversion, the content of ethylene carbonate units in the resultant polymer and the polymer molecular weight were all examined as the function of reaction time.

Results and discussion

Polymer product The FT-IR spectra of ethylene carbonate and its polymerizing reaction mixtures are illustrated in Fig. 1, where the polymerizations catalyzed with IL A for 6 h and with IL B0.67 for 15 h give the monomer conversions of 23.5% and 100%, respectively (see Tab. 1). It can be seen that the C=O stretching vibrational frequency of the polymer product (1745-1747 cm-1) is lower than that of ethylene carbonate (1809 cm-1), indicating that the ring-opening polymerization of ethylene carbonate has effectively proceeded in the presence of the ionic liquids examined. However, a noticeable feature of the polymerization catalyzed with ionic liquids is that vigorous gas evolution took place from the reaction mixture at the beginning of the reaction and continued until the reaction termination, which represents the occurrence of frequent decarboxylation and loss of carbon dioxide during polymerization.

2

C

791 1745 1265 1029

B

791

Transmittance 1809 1747 1029 1264 A

1809

3600 2800 2000 1200 400 Wave number (cm-1)

Fig. 1. IR spectra of ethylene carbonate (A) and its polymerizing reaction mixtures o o obtained with IL A at 150 C for 6 h (B) and with IL B0.67 at 150 C for 15 h (C).

Fig. 2. 1H-NMR spectra of the reaction mixtures obtained by the polymerization of ethylene carbonate catalyzed with IL A at 150 oC for 0, 6, 10 and 15 h.

The strong characteristic absorption peaks at 1745-1747 and 1264-1265 cm-1 in the FT-IR spectra of the polymerizing mixtures are attributed to the carbonyl (C=O) and carbon-oxygen (C-O) stretching vibrations of ethylene carbonate units in the polymer, respectively; the peaks at 1029 and 791 cm-1 are, however, ascribed to the carbon-

3 oxygen (C-O) stretching vibration of the ethylene oxide units. These confirm that the repeating units of the resultant polymer are a mixture of the monomeric units (ethylene carbonate units) and the decarboxylated units (ethylene oxide units). The polymer chemical structure is also revealed by the 1H-NMR spectra of the polymerizing reaction mixtures, illustrated in Fig. 2 for the ring-opening polymerization catalyzed with IL A. There are four kinds of spectrum signals corresponding to different protons in these spectra. The singlet at 4.54 ppm is ascribed to the methylene protons of unreacted monomer (referred to as proton a); the two triplets at 4.26 and 3.70 ppm are attributed to the methylene protons of the ethylene carbonate units in the polymer, i.e. methylene protons adjacent to the oxygen atom (proton b) and distant from the oxygen atom (proton c), respectively; the multiplet at 3.50-3.66 ppm is assigned to the methylene protons of the ethylene oxide units in the polymer (proton d). Thus it can be seen that the polymer product contains ethylene oxide units and ethylene carbonate units. It should be further mentioned that the obtained polymer has a linear structure with ethylene carbonate-ethylene oxide repeating units (see Eq. 1). For the polymerization of ethylene carbonate, the smallest ring size formed by the intramolecular cyclization is an eight-membered ring [9]. However, the formation of these rings by intramolecular cyclization reactions is rare. As is well known, five- and six-membered rings form easily by intramolecular cyclization. As the ring size increases from seven- membered ring, intramolecular cyclization becomes harder, and cyclization is very unlikely if it produces nine- to twelve-membered rings. The detailed studies of intracyclization vs linear polymerization can be found elsewhere [24, 25].

Catalytic performance of ionic liquids By peak integration of the 1H-NMR spectra of the polymerizing mixtures, the monomer conversion and the polymer composition can be calculated as follows: A + A + A X ()% = b c d ×100 (2) Aa + Ab + Ac + Ad A + A Y ()mol% = b c ×100 (3) Ab + Ac + Ad where Aa, Ab, Ac and Ad are, respectively, the peak intensities of protons a, b, c and d in the 1H-NMR spectra, X the conversion of ethylene carbonate, and Y the content of carbonate units in the polymer product. The computational results are listed as a function of the reaction time in Tab. 1. The polymer molecular weights determined with the GPC measurements are also given in this table. Fig. 3 illustrates the molecular weight change of the reaction mixtures with the reaction time for the ring- opening polymerization catalyzed with IL A. To fully justify the importance of the ionic liquids, a blank test was previously carried out without using any ionic liquid in the reaction medium and resulted in only 15.1% conversion of ethylene carbonate after 15 h reaction at 150 oC. It shows that all the ionic liquids examined are active in the ring-opening polymerization of ethylene carbonate. As expected, for any ionic liquid, as the reaction time increases the monomer conversion increases. However, for any ionic liquid, the content of carbonate units always has its maximum at the early stage of the polymerization, and then decreases with increasing the reaction time. These results are also obvious from Fig. 2, which

4 shows that at the early stage of reaction, the proton peaks due to the unreacted monomer (proton a) and due to the polymer (protons b, c and d) are both visible.

Tab. 1. Experimental results of the ring-opening polymerization of ethylene carbonate catalyzed with various ionic liquids at 150 oC.

Ionic liquid Time (h) Conversion (%) Carbonate unit Number-average PDI (a) content (mol%) molecular weight IL A 6 23.5 50.0 387 1.24 9 48.6 40.5 590 1.33 12 71.0 30.0 1332 1.38 15 100 18.0 922 1.60

IL B0.67 6 25.5 48.0 998 1.33 9 52.1 34.7 1690 1.36 12 79.6 28.9 2273 1.24 15 100 14.3 2809 1.39

IL B0.50 6 26.3 34.2 602 1.28 9 43.6 27.1 899 1.33 12 69.9 23.5 1022 1.23 15 97.0 16.8 956 1.40

IL B0.33 6 11.2 19.8 231 1.21 9 20.4 11.5 332 1.30 12 31.0 9.9 444 1.41 15 42.0 5.6 602 1.43 a) Polydispersity index, equal to the ratio of weight-average molecular weight to number-average molecular weight.

Fig. 3. GPC spectra of the reaction mixtures obtained by the polymerization of ethylene carbonate catalyzed with IL A at 150 oC for 6, 9, 12 and 15 h.

5 As the reaction time increases, the proton peak intensity of the monomer decreases and the proton peak intensity of ethylene oxide units in the polymer increases. However, the two proton peak intensities of ethylene carbonate units in the polymer reach their maxima at the early stage of reaction, and then seem to somewhat decrease with increasing the reaction time. It can also be seen from Tab. 1 that for the IL A and IL B0.50, the molecular weight of the polymer always increases at first, reaches a maximum, and then decreases with increasing the reaction time, indicating the polymerization may be accompanied not only with the decarboxylation but also with the polymer chain cleavage reaction. For the IL B0.67 and IL B0.33, the polymer molecular weight always increases in the examined range of the reaction time; however, it can be foreseen that if the reaction will continue for some more time, the polymer molecular weight affirmatively decreases after it reaches a maximum. The changing rule of the polymer molecular weight can also be concluded from Fig. 3. The peaks at about 9.4 min are due to the monomer and the peaks at earlier elution times are due to the polymer. In the early stage of reaction (<12 h), both monomer and polymer peaks are visible, with the monomer peak intensity decreasing while the polymer peak intensity increases. After 15 h, only the polymer peak is visible. The molecular weight of the polymer increases (the elution time decreases) as the reaction time increases up to 12 h and then starts to decrease.

As shown in Tab. 1, the IL B0.33 gives a fairly low monomer conversion, a low content of carbonate units and a low polymer molecular weight, indicating a very poor catalytic performance of the neutral ionic liquid. Compared with the IL B0.50, the IL B0.67 needs a shorter time to reach 100% monomer conversion and results in a higher content of carbonate units and a higher polymer molecular weight. Especially, the IL B0.67 gives 48.0 mol% of the content of carbonate units at the early stage of the polymerization and molecular weight of maximum 2809 after 15 h reaction. These results show that the larger the molar fraction of ZnCl2 in the IL Bx, the higher the catalytic activity of polymerization and the more effectively the decarboxylation and polymer chain cleavage are both suppressed.

In addition, it is found from Tab. 1 that the IL A and IL B0.67 need approximatively the same time to reach 100% monomer conversion, showing that their catalytic activities of polymerization are very close each other. Nevertheless, for any reaction time, the IL B0.67 gives a little lower content of carbonate units and a much higher molecular weight of the polymer than the IL A does. Therefore, the IL A has a much worse performance for suppressing the polymer chain cleavage and a bit better performance for suppressing the decarboxylation.

On the polymerization and catalytic site in ionic liquids The ring-opening polymerization in the presence of ionic liquids seems to divide into two stages. In stage 1, there exist only two main reactions, the polymerization and decarboxylation, resulting in the fact that the monomer conversion and the polymer molecular weight increase and the content of carbonate units decreases with increasing the reaction time. However, not only the polymerization and decarboxylation but also the polymer chain cleavage occur in stage 2, where the polymer molecular weight and the content of carbonate units decrease and the monomer conversion increases with increasing the reaction time if the 100% conversion is still not reached. All these reactions may be strongly dependent on the nature of the catalytic site in ionic liquids.

6 The IL A and IL B0.33 are neutral, while the IL B0.50 and IL B0.67 are acidic. The acidity of the IL Bx increases with increasing the molar fraction of ZnCl2. As mentioned above, the larger the molar fraction of ZnCl2, the higher the catalytic activity of the IL Bx for the polymerization and the more effectively the decarboxylation and polymer chain cleavage are both suppressed. However, there is no evidence that the catalytic performance of ionic liquids is related to their acidities, since the neutral IL A also shows a good polymerizing performance. It is known that ionic liquids typically consist of organic nitrogen-containing heterocylic cations and inorganic anions [16, 17]. In this study, the cation is [bmim]+ - - and the anion is BF4 or [Cl-(ZnCl2)x] . For the IL B0.67, IL B0.50 and IL B0.33, there is difference only in the amount of ZnCl2; therefore, it can be deduced that the catalytic performance of ionic liquids mainly depends on their inorganic anions and that the larger the amount of these anions in the reaction system, the better the catalytic performance. Moreover, the zinc or boron atom in the anions of ionic liquids may act as the main catalytic site for the ring-opening polymerization of ethylene carbonate, even for neutral ionic liquids.

Conclusions

The catalytic performances of the IL A and IL Bx ionic liquids for the ring-opening polymerization of ethylene carbonate were examined. It is affirmed that the polymerization takes place accompanied with decarboxylation and chain cleavage reaction to give a copolymer consisting of ethylene oxide units and ethylene carbonate units. It is found that for any ionic liquid examined, as the reaction time increases the monomer conversion increases and the content of carbonate units in the resultant polymer, always having its maximum at the early stage of the polymerization, decreases; however, the polymer molecular weight increases at first, reaches a maximum, and then decreases with increasing the reaction time. Experimental results also show that with the increase of the molar fraction of ZnCl2, not only the polymerizing activity of the IL Bx but also its performance for suppressing the decarboxylation and chain cleavage increase. Compared with the IL B0.67, the IL A has the same catalytic activity of polymerization, a much worse performance for suppressing the chain cleavage and a bit better performance for suppressing the decarboxylation. It is concluded that the catalytic performance of ionic liquids mainly depended on their inorganic anions and that the larger the amount of these anions in the reaction system, the better the catalytic performance.

Experimental part

Materials The chemical reagents used were of AR grade and purchased from China market. Ethylene carbonate was dried over P2O5 and then fractionally distilled under vacuum. Other chemicals were used without further purification. Ionic liquids were prepared according to the procedures reported in the literature [26-28].

Polymerization of ethylene carbonate The 4.0 mol (352.0 g) of ethylene carbonate and the 0.04 mol of one ionic liquid were mixed in a 500-ml four-necked round-bottom flask equipped with a mechanical stirrer, thermometer, nitrogen inlet and reflux condenser. After the mixture was heated to

7 150 oC in an oil bath, the ring-opening polymerization proceeded under stirring for at least 15 h. Nitrogen gas was continuously passed through the flask during the reaction. About 2 ml of the reaction mixture was sampled at given times using a syringe to prevent air and moisture contamination of the reaction.

Analysis of polymerizing reaction mixture Infrared spectra of the polymerizing reaction mixtures were recorded on a Nicolet 5700 Fourier transform infrared (FT-IR) spectrometer. 1H-NMR (300 MHz) spectra of the reaction mixtures were obtained by a Bruker 300 spectrometer with CDCl3 as and TMS as internal standard. After being dissolved in THF and then neutralized to pH = 7 with acetic acid, the reaction mixtures were also measured for the determination of the polymer molecular weight by a PerkinElmer Series 200 gel permeation chromatography (GPC) instrument equipped with a refractive index detector. THF was used as eluent at a flow rate of 2.0 ml/min at 40 oC. The calibration plot was constructed using polystyrene standards.

Acknowledgements This work was supported by the National Natural Science Foundation of China under Grant No. 31070517, and the Program for New Century Excellent Talents in University under Grant No. NCET-07-0185.

References [1] Dadsetan, M.; Christenson, E. M.; Unger, F.; Ausborn, M.; Kissel, T.; Hiltner, A.; Anderson, J. M. J. Control. Release. 2003, 93, 259. [2] Liu, Z. L.; Zhou, Y.; Zhuo, R. X. J. Polym. Sci. Part A: Polym. Chem. 2003, 41, 4001. [3] Cuscurida, M.; Gipson, R. M. US. Pat. 4, 488, 982, 1984. [4] Harris, R. F. J. Appl. Polym. Sci. 1992, 44, 605. [5] Rokicki, G. Prog. Polym. Sci. 2000, 25, 259. [6] Darensbourg, D. J.; Choi, W.; Karroonnirun, O.; Bhuvanesh, N. Macromolecules. 2008, 41, 3493. [7] Wang, D. P.; Yang, B. L.; Zhai, X. W.; Zhou, L. G. Fuel Process. Technol. 2007, 88, 807. [8] Keki, S.; Torok, J.; Deak, G.; Zsuga, M. Macromolecules. 2001, 34, 6850. [9] Lee, J. C.; Litt, M. H. Macromolecules. 2000, 33, 1618. [10] Harris, R. F.; McDonald, L. A. J. Appl. Polym. Sci. 1989, 37, 1491. [11] Soga, K.; Hosada, S.; Tazuke, Y.; Ikeda, S. J. Polym. Sci.: Polym. Lett. Ed. 1976, 14, 161. [12] Storey, R. F.; Hoffman, D. C. Macromolecules. 1992, 25, 5369. [13] Vogdanis, L.; Heitz, W. Macromol. Rapid Commun. 1986, 7, 543. [14] Vogdanis, L.; Martens, B.; Uchtmann, H.; Hensel, F.; Heitz, W. Macromol. Chem Phys. 1990, 191, 465. [15] Rogers, R. D.; Seddon, K. R. Science. 2003, 302, 792. [16] Zhao, D. B.; Wu, M.; Kou, Y.; Min, E. Z. Catal. Today. 2002, 74, 157. [17] Welton, T. Chem. Rev. 1999, 99, 2071. [18] Stenzel, O.; Brüll, R.; Wahner, U. M.; Sanderson, R. D.; Raubenheimer, H. G. J. Mol. Catal. A: Chem. 2003, 192, 217. [19] Kobryanskii, V. M.; Arnautov, S. A. Chem. Commun. 1992, (9), 727. [20] Arbizzani, C.; Soavi, F.; Mastragostino, M. J. Power Sources. 2006, 162, 735.

8 [21] Marcilla, R.; de Geus, M.; Mecerreyes, D.; Duxbury, C. J.; Koning, C. E.; Heise A. Eur. Polym. J. 2006, 2, 1215. [22] Guerrero-Sanchez, C.; Hoogenboom, R.; Schubert, U. S. Chem. Commun. 2006, (36), 3797. [23] Mori, H.; Iwata, M.; Ito, S.; Endo, T. Polymer. 2007, 48, 5867. [24] Odian, G. Principles of Polymerization, John Wiley & Sons, New York, 1991. [25] Flory, P. J. Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, 1953. [26] Wilkes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, C. L. Inorg. Chem. 1982, 21, 1263. [27] Zhang, Q. G.; Wei, Y. J. Chem. Thermodynamics. 2008, 40, 640. [28] Lecocq, V.; Graille, A.; Santini, C. C.; Baudouin, A.; Chauvin, Y.; Basset, J. M.; Arzel, L.; Bouchu, D.; Fenet, B. New J. Chem. 2005, 29, 700.

9