Lei
Ding
ab,
Yi
Li
ab,
Jianping
Deng
*ab and
Wantai
Yang
*ab
aState Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, China. E-mail: dengjp@mail.buct.edu.cn; Fax: +86-10-6443-5128; Tel: +86-10-6443-5128yangwt@mail.buct.edu.cn
bCollege of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, China
First published on 11th November 2010
We present a facile approach for the preparation of hydrophobic helical poly(N-propargylamide)s in aqueous medium instead of toxic and volatile organic solvent by using a monomer/cyclodextrin inclusion complex. Four hydrophobic substituted acetylene monomers were investigated in this study. Their inclusion complexes with hydroxypropyl-β-cyclodextrin (HP-β-CD) and hydroxypropyl-γ-cyclodextrin (HP-γ-CD) were prepared in water and identified with FT-IR and NMR spectroscopy. Polymerizations of the complexes were successfully carried out in aqueous solution in the presence of a water-soluble Rh-based catalyst [Rh(cod)2BF4; cod = 1,5-cyclooctadiene]. The as-prepared poly(N-propargylamide)s exhibited little difference in composition from the counterparts obtained in organic solvent according to FT-IR analysis. Circular dichroism and UV-Vis absorption spectra demonstrated that the as-prepared poly(N-propargylamide)s could take ordered helical conformations. The reusability of cyclodextrins in the polymerization was investigated quantitatively, which should be one of cyclodextrins' advantages but has never been highlighted before in literature. The versatile method for preparing hydrophobic helical poly(N-propargylamide)s can efficiently reduce the use of noxious organic solvents and can be applicable to the preparation of other helical polymers.
The importance of using water as medium for chemical reactions is ever-increasing as a way to environmentally benign events.22,23Water is thus considered as the favored solvent in chemistry and also in polymer research field.24–27Polymerizations in aqueous medium are recently drawing growing attention owing to the advantages of using water as solvent. Many reports have been found to deal with the aqueous radical polymerization of vinyl monomers.26–29 In particular, aqueous catalytic polymerizations, which combine the advantages of relatively easy control of the microstructure of the polymersviacatalytic polymerization and the benefits of using water as polymerization solvent, are gathering continuously increasing interest.25
In order to prepare hydrophobic helical poly(N-propargylamide)s in aqueous medium, we created a facile approach via polymerizing the monomer/cyclodextrin inclusion complex in water.30Cyclodextrins (CDs), as cyclic oligosaccharides consisting of 6 (α), 7 (β), or 8 (γ) glucopyranose units linked by 1,4-α-glucosidic bonds,31,32 have been widely applied in polymer chemistry such as for the preparation of supramolecular polyrotaxanes,33 chemical sensors,34 supramolecular self-assemblies,35 hydrogels,36 and nanogels.37 CDs have also been extensively used in click chemistry and the related reactions,38,39 and attracted much attention as aqueous-based hosts for chiral recognition40 and chiral separations.41 CDs' unique structures of a polar hydrophilic outer shell and relatively hydrophobic cavity are in particular interesting to polymer chemists because CDs can include a suitable guest to form a host/guest inclusion complex, by which hydrophobic monomers can undergo polymerizations in aqueous medium instead of noxious organic solvents. The group of Ritter has reported the first study using CD during the formation of different polymers in aqueous solution and made significant contributions in this interesting research field.42–46
In our previous research, CDs were used to prepare CD/GMA47 and CD/MMA inclusion complexes,48 with which novel hydrogels were successfully obtained and UV-induced polymerizations were smoothly carried out, respectively. We also reported the first catalytic polymerizations in aqueous medium by using monomer/cyclodextrin inclusion complex.30 However, some monomers with large phenyl pendent groups cannot be prepared viaHP-β-CD/monomer inclusion complex. In the present article, we successfully circumvent this problem by employing HP-γ-CD. It should be stressed that this is the first attempt in the literature to use γ-CD/monomer inclusion complex in catalytic polymerizations, although Uyar et al. reported the radical polymerization of styrene in γ-CD channels.49 In addition, HP-β-CD and HP-γ-CD were used in this work because their solubility in water was much better than that of β-CD and γ-CD. We further found that the poly(N-propargylamide)s separately obtained via two different routes (polymerization in aqueous media by using monomer inclusion complex vs.polymerization in organic solvent by directly using monomer) exhibited no pronounced difference in terms of polymers' yield, composition, and the ability to form helical conformations. Also of particular importance is that cyclodextrin used in the polymerization system can be reused several times, further demonstrating that cyclodextrins can be used as an environment-friendly material.
Monomer | HP-β-CD | HP-γ-CD | |||
---|---|---|---|---|---|
Entry |
|
Appearance of the complex in watera | Inclusion complex or not | Appearance of the complex in watera | Inclusion complex or not |
a “Clear” means that no free monomer was left and “turbid” means that only part of the monomer was included by CD under the investigation conditions. | |||||
1 |
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Clear | Yes | Clear | Yes |
2 |
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Clear | Yes | Clear | Yes |
3 |
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Clear | Yes | Clear | Yes |
4 |
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Turbid | No | Clear | Yes |
We define three parameters as the following to investigate the reusability of HP-β-CD quantitatively. The total conversion (X) of monomer 2 is defined as
![]() | (1) |
![]() | (2) |
![]() | (3) |
The formation of an inclusion complex was preliminarily proved by observing the change in the appearance of the involved aqueous solution. When the solution of monomer 4/THF was added into the HP-γ-CD aqueous solution, the monomer immediately precipitated and dispersed in the system. Nevertheless after stirring at room temperature for about 30 min, the monomer precipitate disappeared and a clear, homogenous solution was formed. For a further confirmation of the occurrence of inclusion, NMR and FTIR spectroscopy techniques were employed, both of which have been proved effective for the confirmation of inclusion complex.47,48,51,52 The relevant results are presented in Fig. 1 (NMR) and Fig. 2 (FTIR), and will be discussed in detail later.
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Fig. 1 (A) 1H NMR spectra of a: HP-γ-CD; b: monomer 4/HP-γ-CD inclusion complex; and c: monomer 4. (B) 13C NMR spectra of d: HP-γ-CD; e: monomer 4/HP-γ-CD inclusion complex; and f: monomer 4. The spectra of HP-γ-CD and monomer 4/HP-γ-CD were recorded in D2O while the spectrum of monomer 4 was acquired in CHCl3. |
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Fig. 2 FTIR spectra of a: HP-γ-CD; b: monomer 4/HP-γ-CD inclusion complex; and c: monomer 4 (KBr tablet). |
A comparison of the 1H and 13C NMR spectra of the monomer 4/HP-γ-CD inclusion complex and those of pure monomer 4 and pure HP-γ-CD is presented in Fig. 1. Table 2 and Table 3 illustrate the corresponding chemical shifts of monomer 4 observed in Fig. 1. The data in Table 2 show that all the corresponding signals of the protons in monomer 4 shifted downfield after the inclusion with the exception of the proton in the –NH– group. Furthermore, no new peaks were observed after the inclusion and only chemical shifts occurred to the protons, indicating that the complexation was a physical process rather than a chemical reaction. In order to investigate the inclusion mode of monomer 4/HP-γ-CD inclusion complex, we further measured 13C NMR spectra of the pure HP-γ-CD, the pure monomer 4, and their inclusion complex. The results are presented in Fig. 1B and the detailed data are listed in Table 3. For the guest monomer 4, C (1–3) and C (6–11) showed downfield shifts, while only C–5 (carbon in the carbonyl group) shifted in the opposite direction. This is in good agreement with the observations in the 1H NMR spectra above, namely, all the protons shifted downfield except the proton in the amide group. A combination of the above results clearly demonstrates that in monomer 4, atoms C–1 to C–11, except C–5, are in the same environment after the formation of inclusion complex. Therefore, the inclusion complex was formed in a mode as schematically presented in Scheme 1. This inclusion complex mode is similar to that already found in the glycidyl methacrylate (GMA/HP-β-CD) system.47
Proton | δ free | δ complex | Δδ |
---|---|---|---|
a The data were based on Fig. 1A in the text. The spectrum of monomer 4/HP-γ-CD was recorded in D2O while the spectrum of monomer 4 was acquired in CHCl3. | |||
H-1 | 2.134 | 1.960 | −0.174 |
H-3 | 3.944 | 3.818 | −0.126 |
H-4 | 5.347 | 5.549 | +0.202 |
H-7 | 1.567 | 1.427 | −0.140 |
H-9 | 7.285 | 7.257 | −0.028 |
H-10 | 7.369 | 7.330 | −0.039 |
H-11 | 7.255 | 7.167 | −0.088 |
Carbon | δ free | δ complex | Δδ |
---|---|---|---|
a The data were based on Fig. 1B in the text. The spectrum of monomer 4/HP-γ-CD was recorded in D2O while the spectrum of monomer 4 was acquired in CHCl3. | |||
C-1 | 71.30 | 69.03 | −2.27 |
C-2 | 79.55 | 78.03 | −1.52 |
C-3 | 29.44 | 29.19 | −0.25 |
C-5 | 177.03 | 179.21 | +2.18 |
C-6 | 46.90 | 46.64 | −0.26 |
C-7 | 26.96 | 26.72 | −0.24 |
C-8 | 144.88 | 144.67 | −0.21 |
C-9 | 128.75 | 128.70 | −0.05 |
C-10 | 127.11 | 127.10 | −0.01 |
C-11 | 126.38 | 125.72 | −0.66 |
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Scheme 1 A schematic illustration of the monomer 4/HP-γ-CD inclusion complex. |
A further comparison between the 1H NMR spectrum of the monomer 4/HP-γ-CD inclusion complex (spectrum b, Fig. 1) and that of pure HP-γ-CD (spectrum a) demonstrates that all the corresponding signals of HP-γ-CD slightly shifted downfield in the presence of monomer 4. In theory, the signals at δ = 5.27, 5.07 and 3.97 ppm, which correspond to protons outside the cavity, should shift upfield. The detailed reason for this phenomenon is not yet clear at present. However, in the 13C NMR spectra, the carbons inside the cavity of HP-γ-CD shifted downfield while the carbons outside the cavity (δ = 101– 98, and 73–71 ppm) shifted upfield after inclusion occurrence, which is well in agreement with the result of monomer 4.
Besides NMR spectroscopy, FTIR spectroscopy measurements also provided powerful evidence for the formation of inclusion complex, as illustrated in Fig. 2. When compared to the spectra of the pure HP-γ-CD (spectrum a in Fig. 2) and monomer 4 (spectrum c), the characteristic bands of monomer 4 at 3321, 3286, 1649, and 1524 cm−1 all appeared in the spectrum of the monomer 4/HP-γ-CD inclusion complex (spectrum b) and the characteristic peaks of the cyclodextrin at 3395 and 1148–1027 cm−1 were also observed there. These observations indicated the formation of the expected inclusion complex. It should be pointed out here that before the measurement of FTIR spectroscopy, the free monomer in the inclusion complex was entirely excluded.
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Scheme 2 Preparation and polymerization of monomer 4/HP-γ-CD inclusion complex in water in comparison to polymerization of uncomplexed monomer 4 in chloroform. |
Some earlier investigations demonstrated that,42–46 once the monomer in a monomer/cyclodextrin inclusion complex forms a polymer through radical polymerization, the resulting polymer frequently breaks away from the cyclodextrin and precipitates out from the solution. Hence a pure polymer can be obtained just by a simple filtration. This is also true in our catalytic polymerizations of hydrophobic substituted acetylene monomers. The resultant polymers were isolated by a facile filtration, and washed with water and then ethyl acetate to remove the free cyclodextrins and the residual monomer, respectively. After re-precipitation from THF into n-hexane, the as-prepared polymer was subjected to GPC and the results are listed in Table 4. From Table 4, although the number average molecular weights (Mns) of the polymers prepared in aqueous medium were a bit lower than those of their counterparts obtained in organic solvent, no pronounced difference can be observed in the yields of the polymers. Just as aforementioned, in the present aqueous polymerization systems, the monomer/cyclodextrin inclusion complexes were water-soluble but the as-prepared polymers were water- insoluble. Therefore, in principle the polymers should break away from the cavity of cyclodextrins and then precipitated, resulting in the termination of the polymer chains propagation. As a consequence, the molecular weights of the polymers are relatively low.
Entry | Monomer | Cyclodextrin | Solvent | Yield (%) | M n b | M w/Mnb |
---|---|---|---|---|---|---|
a The concentration of monomer was 0.1 mol L−1; the concentration of catalyst ([Rh(cod)2BF4]) was 1.0 mmol L−1; the polymerization temperature was 30 °C; and the polymerization time was 3 h. b Determined by GPC, using polystyrene as the standard and THF as eluent. c Cyclodextrin was not used, and the catalyst used was [Rh(nbd)B(C6H5)4]. | ||||||
1 | 1 | HP-β-CD | H2O | 97 | 4600 | 1.22 |
2 | 1 | /c | CHCl3 | 98 | 9000 | 1.69 |
3 | 2 | HP-β-CD | H2O | 97 | 6200 | 1.93 |
4 | 2 | /c | CHCl3 | 95 | 9000 | 2.35 |
5 | 3 | HP-β-CD | H2O | 94 | 4500 | 1.18 |
6 | 3 | /c | CHCl3 | 96 | 6500 | 2.41 |
7 | 4 | HP-γ-CD | H2O | 96 | 4000 | 1.13 |
8 | 4 | /c | CHCl3 | 97 | 4200 | 3.71 |
To further clarify the assumption that the polymers prepared via aqueous polymerization fell off the cyclodextrins during polymerization, the polymers prepared by the two different approaches (cf.Scheme 2) were subjected to FTIR measurement. The relevant results were presented in Fig. 3, where two spectra (spectra a and b: for polymer 4 prepared in aqueous medium and in organic solvent, respectively) are shown simultaneously for a visually clear comparison. It can be seen that the two spectra were almost the same in the characteristic bands of the two polymers. The peaks appeared at 3356, 1647, and 1515 cm−1 were assigned to the amine groups and amide groups in the side chains of the polymers. More importantly, hydroxyl group (–OH) was hardly observed in the FTIR spectrum of polymer 4 obtained in aqueous solution, indicating that there was no cyclodextrin molecules threaded on the polymer chains. It is worthwhile to point out that polymer 4 prepared via aqueous polymerization showed little difference in solubility in organic solvent from the polymer 4 prepared in organic solvent. This is also in well accordance with the fact that HP-γ-CD broke away from the polymer chains during polymerization; otherwise the residual HP-γ-CD would adversely affect the solubility of the polymer.
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Fig. 3 FTIR spectra of a: polymer 4 prepared by using monomer 4/HP-γ-CD inclusion complex in water; and b: polymer 4 prepared in chloroform (KBr tablet). |
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Fig. 4 A comparison of the UV-vis spectra of polymer 2 and polymer 4 prepared in water and in chloroform. For the polymerization conditions, cf.Table 4, entry 3 and 4 (polymer 2); entry 7 and 8 (polymer 4). |
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Fig. 5 A comparison of A: UV-vis absorption spectra and B: circular dichroism spectra of polymer 3 prepared in water and polymer 3 prepared in chloroform. For the polymerization conditions, cf.Table 4, entry 5 and 6. |
Fig. 4 shows the UV-Vis spectra of the two polymers from monomer 2 and monomer 4 (for polymer 2, entry 3, in water; entry 4, in chloroform in Table 4. For polymer 4, entry 7, in water; and entry 8, in chloroform in Table 4). All the spectra showed a strong UV-vis absorption at about 390 nm. Although the absorption strength of the polymers prepared in water was not the same as that of the counterparts obtained in chloroform, little difference was observed in the spectra in terms of profile and the maximum wavelength. This indicates that polymerization in aqueous medium can also allow the polymers to form helical structures. The UV-Vis absorption spectra and circular dichroism effects of the two polymers derived from monomer 3 (entry 5, in water; and entry 6, in chloroform in Table 4) were displayed in Fig. 5. Similar to the observations in polymer 2 and 4, the two spectra in Fig. 5 were almost the same in their profiles either in their UV-Vis spectra or circular dichroism spectra. Monomer 3 has a chiral center and thus a circular dichroism signal can be seen in the circular dichroism spectra of polymer 3. Polymer 1 prepared by using a monomer 1/HP-β-CD inclusion complex did not show any absorption peaks in a wavelength range from 300 to 400 nm in both UV-Vis and circular dichroism spectra because polymer 1 cannot form a helical conformation under the examined conditions, as reported elsewhere.10 According to the above discussions, polymerizations by using monomer/cyclodextrin inclusion complexes can provide the expected polymers and did not affect the secondary structures of the thus-prepared polymers, irrespective of their ability or inability to adopt helical structure.
HP-β-CD mmol | M2 mmol | Complex b mmol | X c % | X e c % | R CD c % | M n d | M w/Mnd |
---|---|---|---|---|---|---|---|
a The concentration of the monomer was 0.1 mol L−1; the concentration of the catalyst ([Rh(cod)2BF4]) was 1.0 mmol L−1; the polymerization temperature was 30 °C; and the polymerization time was 3 h. b Theoretical quantity. c The total conversion, extra conversion, and HPCD surplus ratio were defined in the experimental part. d Determined by GPC, using polystyrene as the standard and THF as eluent. | |||||||
0.4 | 0.8 | 0.2 | 95.3 | 281.2 | 96.4 | 4700 | 1.18 |
0.8 | 0.8 | 0.4 | 97.7 | 95.4 | 96.0 | 3700 | 1.57 |
0.8 | 0.4 | 0.4 | 96.7 | 0 | 94.5 | 4200 | 1.28 |
This phenomenon can be easily explained by the extra conversion Xe. For example, when 0.4 mmol HP-β-CD and 0.8 mmol monomer 2 were used, 0.7624 mmol polymer 2 (based on the molecular weight of monomer 2) was obtained. Based on the mode of the monomer/HP-β-CD inclusion complex discussed above, only 0.2 mmol polymer 2 (based on the molecular weight of monomer 2; theoretical quantity) should be obtained. However, the extra conversion was 281.2% according to eqn (2). It thus can be further inferred that HP-β-CD was used repeatedly in the system based on the fact that the extra conversion was larger than 0; and in particular, the extra conversion was larger than 100% also further clearly demonstrates that HP-β-CD was used repeatedly more than once.
The concentration of the residual HP-β-CD after one batch of polymerization in the solution was also investigated. We found that in all the entries in Table 5, the HPCD surplus ratios were 90% and above, demonstrating that a majority of HP-β-CD was unthreaded from the polymer chain and remained in solution. This was also consistent with the results of the FTIR measurements in Fig. 2. Therefore, we conclude that most of the cyclodextrin used in the system can be reused in polymerization, and we believe that the present technique will find some practical applications in the synthesis of hydrophobic helical polymers.
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