Article pubs.acs.org/IECR
Optically Active, Magnetic Microparticles: Constructed by Chiral Helical Substituted Polyacetylene/Fe3O4 Nanoparticles and Recycled for Uses in Enantioselective Crystallization Haiyang Zhang, Guangyue Qian, Jiexuan Song, and Jianping Deng* State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *
ABSTRACT: This paper reports a novel category of optically active magnetic microparticles (OAMMPs), which are exclusively constructed by helical polymer chains and Fe3O4 nanoparticles. The resulting microparticles exhibit remarkable optical activity and magnetic property, according to circular dichroism (CD) and UV−vis spectroscopy and vibrating sample magnetometry characterizations. Alkynyl-modified Fe3O4 nanoparticles are employed as a comonomer to fabricate the microparticles. The OAMMPs are further used as a chiral additive to induce enantioselective crystallization of racemic alanine. L-Alanine is preferentially induced to crystallize, forming rod-like crystals, according to CD and scanning electron microscopy characterizations. Also remarkably, the particles possess rapid magnetic responsivity, making it possible to be recycled and reused. The initially induced crystals are further enantioselectively crystallized twice and thrice, and the enantiomeric excess can be increased to 90%. Recyclability of the particles is also clearly verified. The present study not only provides a novel type of advanced functional materials but also creates a “green” strategy for chiral resolution and purification.
1. INTRODUCTION Polymer-based microparticles1−5 (denoted as MPs) have attracted a great deal of interest because they allow a fine control over the morphology and property of polymers. Thanks to the great development of helical polymers,6−21 chiral MPs consisting of optically active helical polymers have constructed a newly emerging active research area over the last few years. They are anticipated to exhibit some exciting properties and significant practical applications,22−24 taking into account the unique chiral amplification effects25,26 in optically active helical polymers. However, the one-component chiral MPs frequently show disadvantages in a lot of applications, in particular, the inconvenience to be recycled, the difficulty to play multiple roles in a cooperative process, and the tendency to dissolve in solvents. Therefore, combining several materials into one entity is an effective way to develop systems combining optical activity with other desirable physical and chemical properties. These versatile properties are usually not performed by the individual components. Among the various chiral composite MPs, magnetic MPs are of wide interest because of their responsivity to magnetic fields. Nevertheless, apart from our earlier reported magnetic composite MPs constructed by polystyrene and Fe3O4 nanoparticles, grafted with helical substituted polyacetylene chains,27 few analogous chiral MPs have been reported yet in the literature. To further simplify the composition and the preparation process, in this article, we will report a novel type of chiral, magnetic MPs exclusively consisting of helical substituted polyacetylene and Fe3O4 nanoparticles. Chirality is a universal phenomenon in biological (macro)molecules. Nonetheless, the enantiomers of a chiral compound may have totally different physiological effects on living organisms. Therefore, chiral resolution is an extremely © 2014 American Chemical Society
significant research topic, especially in the areas of agrochemicals and pharmaceuticals. In recent years, the techniques for chiral resolution have developed rapidly,28,29 among which the crystallization method is a classical process for its easyoperation and capability of separating enantiomers on large scales. So far, microspheres,30 miniemulsions31 and monolayers32 derived from chiral molecules or polymers can be used as unique chiral additives in enantioselective crystallization, in which only one enantiomer is preferentially induced to crystallize.30,33 In addition, optically active helical substituted polyacetylenes have been investigated by our group34−38 as chiral seeding for inducing enantioselective crystallization. Nevertheless, no study has been devoted toward recycling and reuse of the chiral additives after enantioselective crystallization yet. Accordingly, the recycling of chiral additives is currently still a challenging task. The studies along this direction are important in particular from the viewpoint of green chemistry. In this paper, we report an unprecedented category of MPs showing both remarkable optical activity and magnetic property. Different from our earlier Fe3O4−PS−polyacetylene composite microspheres,27 the present MPs were exclusively constructed by helical substituted polyacetylene chains covalently bonded with Fe3O4 nanoparticles. Helical substituted polyacetylene chains and Fe3O4 nanoparticles rendered the MPs with optical activity and magnetic responsivity, respectively. More remarkably, the MPs efficiently induced enantioselective crystallization of racemic alanine. The Received: Revised: Accepted: Published: 17394
August 3, 2014 October 16, 2014 October 17, 2014 October 17, 2014 dx.doi.org/10.1021/ie503114z | Ind. Eng. Chem. Res. 2014, 53, 17394−17402
Industrial & Engineering Chemistry Research
Article
the product was dried in a vacuum overnight, the alkynylMNPs were obtained. The preparation of OAMMPs followed a method reported earlier,44 as briefly stated below. 0.01 g of alkynyl-MNPs was dispersed homogeneously under ultrasonification in 2 mL of chloroform, in which M1 (0.1 g, 0.43 mmol) and (nbd)Rh+B−(C6H5)4 (0.0022 g, 0.0043 mmol) were subsequentially charged. The mixture was added into a predetermined amount of PVA aqueous solution (2 wt %, 50 mL). When the polymerization ended, the microparticles suspended in PVA aqueous solution were separated by the help of a magnet. After the solution was washed and dried, the magnetic microparticles in black were collected. When a certain amount of cross-linking agent (0.0047 g, 0.02 mmol) was added into the chloroform dispersion containing alkynyl-MNPs and monomer, crosslinked magnetic microparticles were obtained. 2.4. Enantioselective Crystallization Experiments. All the enantioselective crystallization experiments were conducted from supersaturated solutions of alanine in deionized water. A typical procedure36 is as follows: D- and L-alanine (each 900 mg) were added in deionized water (10 mL), and the solution was heated to 35 °C under stirring until complete dissolution. A predetermined amount (∼5 mg) of OAMMPs was then added in the above racemic D,L-alanine supersaturated solution and stirred for 15 min. Afterward, the solution was left for ambient cooling to room temperature (approximately 25 °C). After 168 h, the induced crystals were filtrated, dried and subjected to characterization. The residual filtrate was also subjected to measurements. During filtration, OAMMPs dispersed in the residual solution were separated by a magnet, washed with water and dried. The collected OAMMPs were reused for conducting another cycle of enantioselective crystallization, or for further purifying the alanine induced earlier. For comparison, crystallization of alanines was performed by directly using alkynyl-MNPs as an additive in a similar way. Under the same conditions, crystallization was also conducted in the absence of any additive.
magnetic property ensured the MPs to be easily isolated from the crystallization systems just by magnetic separation, by which to have realized the recycling and reuse of the MPs.
2. EXPERIMENTAL SECTION 2.1. Materials. Solvents were purified by distillation. Deionized water was used for preparing oleic acid (OA)-coated magnetic nanoparticles (MNPs) and optically active magnetic microparticles (OAMMPs) via suspension polymerizations. Monomer 1 (M1) was synthesized according to a method previously reported.39 Rhodium catalyst, (nbd)Rh+B−(C6H5)4 (nbd = 2,5-norbornadiene) was prepared by a method in the literature.40 Dipropargyl adipate was synthesized following a procedure41 and used as a cross-linking agent for preparing cross-linked OAMMPs. Propargylamine, (1S)-(−)-camphanic acid and poly(vinyl alcohol) (PVA, polymerization degree 1750 ± 50) were purchased from Aldrich and used as received. OPropargyloxy-N-triethoxysilylpropyl urethane (denoted as OPNTU silane) was purchased from Gelest Inc. and directly used. D- and L-alanine, purchased from Aladdin, were used without further purification. All the other reagents were used as received. 2.2. Measurements. Circular dichroism (CD) and UV−vis absorption spectra were recorded on a Jasco-810 spectropolarimeter. Specific rotations were measured on a JASCO P-1020 digital polarimeter with a sodium lamp as the light source at room temperature. Fourier transform infrared (FT-IR) spectra were recorded with a Nicolet NEXUS 670 spectrophotometer (in KBr tablet). Images of magnetic nanoparticles were observed by using transmission electron microscopy (TEM, JEM-2100 (JEOL)) at an accelerating voltage of 200 kV. The morphology of OAMMPs was observed with scanning electron microscopy (SEM, JSM-7001F (JEOL)). The magnetic property was measured by a vibrating sample magnetometer (VSM, Lake Shore 7410 VSM) at room temperature. Thermogravimetric analyses (TGA) were carried out with a Mettler Toledo TGA instrument at a scanning rate of 10 °C/ min in nitrogen. X-ray diffraction (XRD) analyses were performed with a Shimadzu XRD-6000 instrument. 2.3. Preparation of Optically Active Magnetic Microparticles (OAMMPs). First, oleic acid (OA)-coated magnetic nanoparticles (MNPs) were synthesized by a slight modification of a coprecipitation method in the literature,42 and then OPNTU silane was used to conduct a ligand exchange reaction, yielding alkynyl-MNPs.43 The major processes are described as follows. FeCl3·6H2O (2.05 g) and FeSO4·7H2O (1.18 g) were dissolved in 50 mL of deionized water in a three-necked flask. The solution was intensely stirred under N2 atmosphere at room temperature, followed by adding 15 mL of NH3·H2O (25%, w/w). Afterward, 1 mL of OA was dropped into the solution at 80 °C within 1 h. After 3 h, 0.5 g of sodium chloride and 50 mL of toluene were added in the water dispersion, and then the OA-coated MNPs were extracted from water into the toluene phase. After the aqueous layer was separated and distillation, the organic layer was collected and dried. The assynthesized OA-coated MNPs dispersion in toluene and a small amount of triethylamine (2.5 mL) were charged in a vessel, and then OPNTU silane (1.0 mL) was added dropwise. The reaction lasted for 48 h under N2 at room temperature. Finally, petroleum ether (30 mL) was added to the mixture, followed by magnetic separation. The product was redissolved in toluene. This procedure was repeated for three times. After
3. RESULTS AND DISCUSSION Magnetic particles demonstrate fascinating features,45−47 particularly their easy recycling ability. Meanwhile, chiral microparticles have aroused ever-growing interest on account of their noticeable optical activity. We reason that when optically active MPs integrate with magnetic particles, a new category of chiral materials will be acquired. Such attractive composite MPs will hopefully have significant applications in chiral resolution, asymmetric catalysis, etc. According to the strategy for manufacturing optically active MPs44 and based on our earlier study dealing with chiral magnetic composite particles,27 in the present, we study an originally prepared unique type of optically active, magnetic microparticles (defined as OAMMPs), for which the preparing strategy is presented in Scheme 1. The OAMMPs were synthesized via a suspension polymerization approach. To afford the MPs with a magnetic property, alkynyl-modified Fe3O4 nanoparticles (alkynyl-MNPs) were introduced during the course of polymerization. The synthetic procedure of alkynyl-MNPs includes two major steps. In the first step, coprecipitated Fe3O4 NPs were coated by a layer of oleic acid (OA) molecules. Next, OA molecules were replaced by the alkynyl-containing silane coupling agent (OPNTU silane), making the MNPs coated with alkynyl groups (alkynylMNPs). The alkynyl moieties on the MNPs are polymerizable 17395
dx.doi.org/10.1021/ie503114z | Ind. Eng. Chem. Res. 2014, 53, 17394−17402
Industrial & Engineering Chemistry Research
Article
when an external magnetic field was applied, alkynyl-MNPs could be isolated while OA-coated MNPs could not. This phenomenon can be explained in terms of the different dispersibility of the MNPs. As OA molecules possessed more lipophilic alkyl groups than OPNTU silane, OA-coated MNPs were dispersed so well in toluene that they cannot be separated from the toluene phase. We also found that if toluene was replaced with other more polar solvents (like ethanol), the OAcoated MNPs could be isolated. Moreover, the TEM images of OA-coated MNPs and alkynyl-MNPs (Figure 2A,B) also clearly demonstrate that the former possess better dispersibility than the latter.
Scheme 1. Schematic Strategy for Preparing Optically Active Magnetic Microparticles (OAMMPs, Cross-Linked and NonCross-Linked) by Suspension Polymerization
and thus alkynyl-MNPs can act as a comonomer to fabricate the designed OAMMPs. In addition, we found that alkynylMNPs by themselves cannot play effective roles as a crosslinking agent to form cross-linked MPs, so dipropargyl adipate was employed for preparing cross-linked OAMMPs. The typical photographs in Figure 1 (A1 and B1) show that both OA-coated MNPs and alkynyl-MNPs can be dispersed
Figure 2. Typical TEM images for (A) OA-coated MNPs and (B) alkynyl-MNPs; SEM images for (C) non-cross-linked and (D) crosslinked OAMMPs (for panels A and B, scale bar = 20 nm; for C, scale bar = 100 μm; for D, scale bar = 10 μm).
Following the strategy presented in Scheme 1, we successfully prepared OAMMPs, as shown in Figure 1 (C1). The black color of the MPs is due to the presence of Fe3O4 MNPs, which is evidenced by the characteristic peak of Fe−O in FT-IR spectrum (Figure S2, Supporting Information). Compared with the non-cross-linked OAMMPs, a new weaker peak of CO (1740 cm−1) appeared in the FT-IR spectrum of the cross-linked ones, which is ascribed to the ester group in the cross-linking agent. From the photographs in Figure 1 (C1, C2; cross-linked OAMMPs gave similar photographs), we know that the OAMMPs can be easily separated under an external magnetic field, and the whole process was completed within 10 s. The morphology of the non-cross-linked and cross-linked OAMMPs was further observed with SEM (Figure 2C,D). The MPs surface was quite smooth. This indicates that the OAMMPs were uniformly constructed by MNPs and polymer chains, rather than by a simple physical aggregation of substituted polyacetylene chains and MNPs. The non-crosslinked OAMMPs were 300−400 μm (Figure 2C), whereas the sizes of the cross-linked ones were about 100 μm (Figure 2D), smaller than the former ones. The reduction in dimension is
Figure 1. Typical photographs for (A1/A2) OA-coated MNPs, (B1/B2) alkynyl-MNPs, (C1/C2) non-cross-linked OAMMPs before/after being exposed to a magnetic field; (D) non-cross-linked and (E) cross-linked OAMMPs in tetrahydrofuran (THF).
uniformly in toluene. This is due to the lipophilic property of oleic acid molecules and OPNTU silane molecules attached on the surface of MNPs. The MNPs were further characterized by FT-IR spectroscopy, as illustrated in Figure S1 (Supporting Information). OA-coated MNPs show much stronger peaks of CH and CO stretching vibrations, while the absorption peaks corresponding to SiO and CC can be found in the spectrum of alkynyl-MNPs, indicating that the MNPs were successfully functionalized by oleic acid and alkynyl moieties. From the photographs in Figure 1 (A2 and B2), we observe that 17396
dx.doi.org/10.1021/ie503114z | Ind. Eng. Chem. Res. 2014, 53, 17394−17402
Industrial & Engineering Chemistry Research
Article
flat peak was observed in the identical wavelength range. This phenomenon was also observed in our earlier study.44 The magnetic properties of OA-coated MNPs, alkynylMNPs, and non-cross-linked and cross-linked OAMMPs were investigated by using a VSM, as shown in Figure 4. The
due to the occurrence of cross-linking among the polymer chains forming the OAMMPs, similar to the phenomenon observed earlier.44 The non-cross-linked OAMMPs were soluble in organic solvents, e.g., THF, chloroform and DMF. The color of the solutions was brownish yellow (Figure 1D), darker than that of homo-poly1 (which showed a light yellow color), resulting from the presence of MNPs. Notably, even under an external magnetic field, no free MNPs were observed. This offers further evidence for the chemical connection between MNPs and substituted polyacetylene chains inside the OAMMPs. As a consequence, their molecular weight cannot be measured. Herein, it should be pointed out that the alkynyl-functionalized Fe3O4 NPs can serve as a nano-cross-linker theoretically, but we found their efficiency in cross-linking was not satisfactory. This may be possibly due to the steric hindrance in coordination polymerization. Accordingly, dipropargyl adipate was added to improve the cross-linking degree of OAMMPs. Owing to the effective cross-linking agent, the cross-linked OAMMPs could not dissolve in any solvents (Figure 1E). To explore whether the introduction of MNPs affected the helical structures of the polymer chains constituting the OAMMPs, and to elucidate whether the OAMMPs performed optical activity, we qualitatively measured CD and UV−vis absorption spectra of the OAMMPs in THF solution. The results are shown in Figure 3, in which the spectra of homo-
Figure 4. Hysteresis loops of (A) OA-coated MNPs, (B) alkynylMNPs, (C) non-cross-linked and (D) cross-linked OAMMPs.
maximum saturation magnetization (MSM) of OA-coated MNPs and alkynyl-MNPs was 48.8 and 67.8 emu/g, respectively. This difference in MSM is mainly related to the amount of nonmagnetic materials in the MPs. For OA-coated MNPs, OA molecules have longer molecular chains, resulting in a decrease of MSM compared to MNPs modified by shorter alkynyl chains.43 For the non-cross-linked and cross-linked OAMMPs, the MSM reduced to a level below 10 emu/g (Figure 4C,D). This large reduction is attributed to the plentiful polymers constructing the OAMMPs. With the same proportion of polymers, the two kinds of OAMMPs (crosslinked and non-cross-linked) have nearly the same MSM value. However, such a low MSM is enough to ensure the OAMMPs to be separated with an external magnetic field (Figure 1C2). Thermal properties of the above-obtained MNPs and OAMMPs were measured by TGA technique (Figure 5).
Figure 3. Typical (A) CD and (B) UV−vis spectra of (a) non-crosslinked OAMMPs and (b) homo-poly1 measured in THF solution at room temperature.
poly1/THF solution (homo-poly1, the polymer of M1) are also included. For both homo-poly1 and poly1-OAMMPs, intense CD signals and UV−vis absorption peaks appeared at nearly the same wavelength (∼350 nm), illustrating that the OAMMPs formed by helical structures possessed optical activity, by referring to our earlier intensive studies concerning helical polyacetylenes.36,48,49 That is to say, the helical structures of poly1 chains were not influenced by the presence of MNPs. The cross-linked poly1-OAMMPs were further subjected to CD and UV−vis spectroscopy measurements directly by a compression process.34 Non-cross-linked OAMMPs were also measured by the same method, as to ensure both of them in solid state. The relevant qualitative spectra are displayed in Figure S3 (Supporting Information), from which we find that in the non-cross-linked OAMMPs, both a negative CD signal and UV−vis absorption peak appeared at about 350 nm, almost the same as that in the non-cross-linked OAMMPs solution (Figure 3). But for the cross-linked OAMMPs, a uniform thin film was not obtained because of their rigidity. Accordingly, an obvious
Figure 5. TGA curves of (A) OA-coated MNPs, (B) alkynyl-MNPs, (C) non-cross-linked OAMMPs, (D) cross-linked OAMMPs and (E) poly1-MPs. Heating at 10 °C/min, in nitrogen atmosphere.
Compared to a 38.9% weight loss ratio of the original OAcoated MNPs, the TGA curve of alkynyl-MNPs shows a weight loss of 20.6% (Figure 5A,B). The weight loss values of noncross-linked and cross-linked OAMMPs are very close, i.e., about 79% (Figure 5C,D). As analyzed above, alkynyl-MNPs had less nonmagnetic organic materials than OA-coated MNPs did, while there was a large proportion of polymer in both the 17397
dx.doi.org/10.1021/ie503114z | Ind. Eng. Chem. Res. 2014, 53, 17394−17402
Industrial & Engineering Chemistry Research
Article
Scheme 2. Schematic Representation for Enantioselective Crystallization of L-Alanine by Using OAMMPs
Figure 6. SEM images of alanine crystals induced through enantioselective crystallization: (A) without additive; (B) by using alkynyl-MNPs; (C) by using OAMMPs.
OAMMPs stayed outside the resulting crystals, the OAMMPs can be easily separated with a magnet in the process of filtration. After the solution was washed and dried, the OAMMPs were collected for subsequent reuses. The recycled OAMMPs were reused for another cycle of crystallization, or for further purifying the alanine obtained in the first cycle of enantioselective crystallization. The strategy is illustratively shown in Scheme 2. The detailed experimentation will be discussed below. To acquire deeper insight into the enantioselective crystallization, crystallization of racemic alanine solution was also performed without any additive. Additionally, alkynylMNPs were also used as an additive to conduct the enantioselective crystallization experiment. Both the crystallization processes were performed in the same way as described in Experimental Section. The morphology of the induced alanine crystals is presented in Figure 6. For the alanine crystals obtained without any additive (Figure 6A), and by using alkynyl-MNPs as an additive (Figure 6B), bulk crystals without regular morphology and size were observed. These crystals were found to consist of racemic alanine (by CD spectra and specific rotation measurements). For alanine crystals induced by using OAMMPs, rod-like crystals with regular morphology were observed (Figure 6C). This crystal morphology is consistent with our earlier observation.37 Moreover, these rod-like crystals were confirmed to be predominantly constructed by L-alanine, as reported below. The rod-like crystals obtained via enantioselective crystallization by using OAMMPs were subjected to XRD analyses, together with pure L-alanine and racemic alanine. The XRD patterns are illustrated in Figure 7. The patterns for original pure L-alanine and the L-alanine obtained by enantioselective
two types of OAMMPs. Therefore, the residual mass in the OAMMPs decreased correspondingly. Additionally, the residual amount of poly1-MPs was about 13.7% (Figure 5E), lower than that of OAMMPs, whether cross-linked or non-cross-linked (ca. 20%). This also demonstrates the presence of Fe3O4 nanoparticles inside the OAMMPs. Based on the data above, the content of Fe3O4 in the non-cross-linked and cross-linked OAMMPs can be calculated (respectively 8.8% and 8.5%), which accorded with the feed ratio. In our earlier study, we prepared MPs purely constructed from helical substituted polyacetylenes34,44 and hybrid hollow chiral particles.36 Both of them demonstrated remarkable ability for inducing enantioselective crystallization. As mentioned above, the macromolecular helical structures in OAMMPs were not affected by MNPs. Therefore, the OAMMPs are also expected to have chiral recognition and chiral resolution abilities. Taking racemic alanine as a typical model, OAMMPs were employed for inducing enantioselective crystallization from racemic alanine solution. It should be pointed out that in order to improve the efficiency of crystallization, we selected the cross-linked OAMMPs as an additive, for they have a larger specific surface area than the non-cross-linked ones (the latter ones are larger in size). Scheme 2 depicts the major procedure for enantioselective crystallization of alanine by using OAMMPs in aqueous solution. A typical enantioselective crystallization experiment was performed as briefly stated below. At first, a predetermined amount of OAMMPs was added into a racemic D,L-alanine supersaturated aqueous solution. After the solution stayed for about 144 h at room temperature (ca. 25 °C), crystals began to appear in the solution. After another 24 h, crystallization ended and the crystals were filtrated from the residual solution. As 17398
dx.doi.org/10.1021/ie503114z | Ind. Eng. Chem. Res. 2014, 53, 17394−17402
Industrial & Engineering Chemistry Research
Article
were racemic, whereas crystals induced by using OAMMPs were rod-like crystals predominantly consisting of L-alanine. When alkynyl-MNPs were used as an additive, the induced crystals were still racemic. Thus, the effect of MNPs was excluded; meanwhile, the crucial role played by the helical polymers constituting OAMMPs can be ascertained. The process of enantioselective crystallization was achieved by the enantioselective interaction of L-alanine and helical substituted polyacetylenes constructing the OAMMPs. L-Alanine molecules were preferentially adsorbed on the surface of OAMMPs, resulting from the enantioselective affinity of poly1 toward Lalanine. As temperature decreased, more and more L-alanine molecules were adsorbed on the OAMMPs, which further acted as chiral nucleation sites, finally leading to the formation of macroscopic crystals. Possessing optical activity and magnetic response capability, the as-prepared OAMMPs are expected to be realized in recycling toward enantioselective crystallization. For this purpose, we conducted the following enantioselective crystallization experiments, as illustrated in Scheme 2. After collecting the crystals induced in the first cycle, we dissolved them again to prepare a supersaturated solution of alanine. Then OAMMPs were added in the solution, and the procedure for enantioselective crystallization was the same as in the former cycle, as described in the Experimental Section. To acquire crystals with high purity, we carried out enantioselective crystallization experiment twice more. SEM images of the purified crystals are shown in Figure S5 (Supporting Information). As predicted, the obtained crystals demonstrated a regular rod-like morphology, similar to that of the crystals obtained in the first cycle. Additionally, CD spectra of the L-alanine crystals and the residual solution were quantitively measured, as displayed in Figure 9. Both the
Figure 7. Typical XRD patterns of (a) pure L-alanine crystals, (b) Lalanine crystals obtained by enantioselective crystallization and (c) racemic alanine crystals.
crystallization are almost entirely the same, while noticeably different from racemic alanine crystals. This encourages us to conclude that the crystals induced via enantioselective crystallization by using OAMMPs are primarily constructed by L-alanine. To further confirm our consideration, the obtained crystals and residual solution were investigated by CD and UV−vis spectra measurements (Figure S4, Supporting Information). Then CD spectra were normalized by concentrations, as shown in Figure 8. Like pure L-alanine, the induced L-alanine crystals
Figure 8. CD spectra of (a) pure L-alanine solution, (b) L-alanine crystals induced by OAMMPs, (c) residual solution after removing the L-alanine crystals induced by OAMMPs, (d) alanine crystals induced by alkynyl-MNPs and (e) alanine crystals obtained without any additive. All of the CD measurements were conducted in aqueous solution at room temperature (c = 0.1 mM; crystallization for 168 h).
Figure 9. CD spectra of (a) L-alanine crystals induced by OAMMPs after the 2nd enantioselective crystallization and (b) the residual solution; (c) L-alanine crystals induced by OAMMPs after the 3rd enantioselective crystallization and (d) the residual solution. All of the CD measurements were conducted in aqueous solution at room temperature, c = 0.1 mM.
exhibited a positive CD signal at the identical wavelength (λmax = 205 nm), whereas the residual solution showed a negative signal at a quite similar wavelength. Besides, CD spectra of the alanine crystals obtained without any additive and by using alkynyl-MNPs as additive were also measured, and both of them showed no pronounced CD signals as observed in the induced crystals before. CD signals of OAMMPs or alkynylMNPs did not appear because they were removed from the crystals by magnet. These results provide further evidence for the conclusion that the rod-like alanine crystals obtained by using OAMMPs were mainly formed by L-alanine. The results above demonstrate that for the racemic D,Lalanine solution, alanine crystals obtained without any additive
alanine crystals obtained in the 2nd and 3rd crystallization cycle showed intense positive CD signals at the same wavelength (λmax = 205 nm). Furthermore, with purification, CD signal of the corresponding crystals became stronger, illustrating enhanced purity of L-alanine. However, the CD effects of the residual solutions were also positive, as opposed to Figure 8. The reason for this phenomenon relies on the fact that Lalanine in the residual solution still accounted for the majority, even after crystallization. In all the enantioselective crystal17399
dx.doi.org/10.1021/ie503114z | Ind. Eng. Chem. Res. 2014, 53, 17394−17402
Industrial & Engineering Chemistry Research
Article
strategy, a series of novel chiral magnetic particles will be designed and prepared next. Apart from inducing enantioselective crystallization, other potential applications of the novel chiral microparticles are currently under investigation for instance in asymmetric catalysis, chiral adsorption, etc.
lization processes above, the chiral separation efficiency was investigated, as shown in Table 1. The enantiomeric excess (e.e.) of the crystallized L-alanine increased as crystallization time increased. This trend is in accord with the CD spectra illustrated in Figure 9.
■
Table 1. Data of Enantioselective Crystallization for Further Purification of L-Alanine Crystals entry
e.e. (%)a
time (h)b
crystal yield (%)c
1st crystallization 2nd crystallization 3rd crystallization
70 83 89
144 128 136
35 47 50
Typical FT-IR spectra of OA-coated MNPs and alkynyl-MNPs (Figure S1), typical FT-IR spectra of non-cross-linked and cross-linked OAMMPs (Figure S2), typical CD and UV−vis spectra of non-cross-linked and cross-linked OAMMPs measured on pressed samples (Figure S3), typical CD and UV−vis spectra of pure L-alanine solution, L-alanine crystals induced by OAMMPs, residual solution after removing the Lalanine crystals induced by OAMMPs, alanine crystals induced by alkynyl-MNPs and alanine crystals obtained without any additive (Figure S4), SEM images of L-alanine crystals induced by OAMMPs in the 2nd and 3rd enantioselective crystallization (Figure S5), typical CD and UV−vis spectra of L-alanine crystals induced by OAMMPs after the 2nd enantioselective crystallization and the residual solution, L-alanine crystals induced by OAMMPs after the 3rd enantioselective crystallization and the residual solution (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org.
a Maximum e.e. value, determined by specific rotation. bThe time for crystals appearance, observed by eyes. cDefined as the ratio of precipitated crystals to total racemic mixture.
Because the OAMMPs can be easily recycled, namely, just by using a magnetite, we attempted to achieve the recycling and reuse of the OAMMPs. Accordingly, we subsequently performed enantioselective crystallization of alanine by repeatedly using the same OAMMPs. Excitingly, we found that the OAMMPs could be used for at least trice without noticeably losing their functions. The relevant data are listed in Table 2. In the three cycles of enantioselective crystallization,
■
Table 2. Data of Recycle Utilization of OAMMPs for Inducing Enantioselective Crystallization cyclea
e.e. (%)b
time (h)c
crystal yield (%)d
1 2 3
66 64 67
168 160 192
41 43 39
ASSOCIATED CONTENT
S Supporting Information *
AUTHOR INFORMATION
Corresponding Author
*J. Deng. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21274008, 21174010, 20974007), the Funds for Creative Research Groups of China (51221002), and the “Specialized Research Fund for the Doctoral Program of Higher Education” (SRFDP 20120010130002).
a
Under the same crystallization condition. bMaximum e.e. value, determined by specific rotation. cThe time for crystals appearance, observed by eyes. dDefined as the ratio of precipitated crystals to total racemic mixture.
e.e. nearly kept the same (64%−67%). Nonetheless, we found in the first two cycles, the time for clearly viewing the crystals by eyes was similar (168 and 160 h), but for the 3rd cycle, the crystals appeared much later (192 h). The underlying reason for this occurrence is not clear yet. However, the recycling and reusability of the OAMMPs was definitely demonstrated.
■
REFERENCES
(1) Kelly, T. L.; Wolf, M. O. Template Approaches to Conjugated Polymer Micro- and Nanoparticles. Chem. Soc. Rev. 2010, 39, 1526. (2) Wan, W.; Biyikal, M.; Wagner, R.; Sellergren, B.; Rurack, K. Fluorescent Sensory Microparticles that “Light-up” Consisting of a Silica Core and a Molecularly Imprinted Polymer (MIP) Shell. Angew. Chem., Int. Ed. 2013, 52, 7023. (3) Zhang, M.; Lan, Y.; Wang, D.; Yan, R.; Wang, S.; Yang, L.; Zhang, W. Synthesis of Polymeric Yolk−Shell Microspheres by Seed Emulsion Polymerization. Macromolecules 2011, 44, 842. (4) Liu, S. Q.; Deng, R. H.; Li, W. K.; Zhu, J. T. Polymer Microparticles with Controllable Surface Textures Generated Through Interfacial Instabilities of Emulsion Droplets. Adv. Funct. Mater. 2012, 22, 1692. (5) Di Benedetto, F.; Fasano, V.; Persano, L.; Maruccio, C.; Mele, E.; Potente, G.; Weitz, D. A.; De Lorenzis, L.; Pisignano, D. Rolling Particle Lithography by Soft Polymer Microparticles. Soft Matter 2013, 9, 2206. (6) Cui, J. X.; Zhang, J.; Wan, X. H. Unexpected Stereomutation Dependence on the Chemical Structure of Helical Vinyl Glycopolymers. Chem. Commun. 2012, 48, 4341. (7) Budhathoki-Uprety, J.; Novak, B. M. Synthesis of AlkyneFunctionalized Helical Polycarbodiimides and Their Ligation to Small Molecules Using “Click” and Sonogashira Reactions. Macromolecules 2011, 44, 5947.
4. CONCLUSIONS We successfully prepared a novel type of chiral magnetic composite microparticles via a suspension polymerization method. As a perfect combination of magnetic materials and optically active helical polymers, such microparticles were composed of Fe3O4 magnetic nanoparticles and helical substituted polyacetylenes, showing pronounced optical activity and rapid magnetic responsivity. Furthermore, a kind of bifunctional dipropargyl adipate was introduced as a crosslinking agent, affording microparticles with smaller size and insolubility. Due to the presence of the helical polymer chains of one predominant screw sense, the prepared microparticles excitingly demonstrated the capacity for inducing enantioselective crystallization of L-alanine from racemic alanine. More importantly, they can be easily separated after crystallization, thus realizing the aim for recycle and reuse purposes. Also remarkably, L-alanine with up to 89% e.e. was achieved by enantioselective crystallization for three times. Following this 17400
dx.doi.org/10.1021/ie503114z | Ind. Eng. Chem. Res. 2014, 53, 17394−17402
Industrial & Engineering Chemistry Research
Article
(8) Zhang, X. A.; Chen, M. R.; Zhao, H.; Gao, Y.; Wei, Q.; Zhang, S.; Qin, A. J.; Sun, J. Z.; Tang, B. Z. A Facile Synthetic Route to Functional Poly(phenylacetylene)s with Tunable Structures and Properties. Macromolecules 2011, 44, 6724. (9) Maeda, K.; Mochizuki, H.; Osato, K.; Yashima, E. StimuliResponsive Helical Poly(phenylacetylene)s Bearing Cyclodextrin Pendants that Exhibit Enantioselective Gelation in Response to Chirality of a Chiral Amine and Hierarchical Super-Structured Helix Formation. Macromolecules 2011, 44, 3217. (10) Makiguchi, W.; Kobayashi, S.; Furusho, Y.; Meijer, E. W. Formation of a Homo Double Helix of a Conjugated Polymer with Carboxy Groups and Amplification of the Macromolecular Helicity by Chiral Amines Sandwiched between the Strands. Angew. Chem., Int. Ed. 2013, 52, 5275. (11) Jia, H.; Teraguchi, M.; Aoki, T.; Abe, Y.; kaneko, T.; Hadano, S.; Namikoshi, T.; Ohishi, T. Three Mechanisms of Asymmetric Polymerization of Phenylacetylenes Having an L-Amino Ether Residue and Two Hydroxy Groups. Macromolecules 2010, 43, 8353. (12) Pietropaolo, A.; Nakano, T. Molecular Mechanism of Polyacrylate Helix Sense Switching across Its Free Energy Landscape. J. Am. Chem. Soc. 2013, 135, 5509. (13) Zhang, C. H.; Wang, H. L.; Geng, Q. Q.; Yang, T. T.; Liu, L. J.; Sakai, R.; Satoh, T.; Kakuchi, T.; Okamoto, Y. Synthesis of Helical Poly(phenylacetylene)s with Amide Linkage Bearing L-Phenylalanine and L-Phenylglycine Ethyl Ester Pendants and Their Applications as Chiral Stationary Phases for HPLC. Macromolecules 2013, 46, 8406. (14) Nakano, T.; Okamoto, Y. Synthetic Helical Polymers: Conformation and Function. Chem. Rev. 2001, 101, 4013. (15) Liu, J. Z.; Lam, J. W. Y.; Tang, B. Z. Acetylenic Polymers: Syntheses, Structures, and Functions. Chem. Rev. 2009, 109, 5799. (16) Jain, V.; Cheon, K. S.; Tang, K.; Jha, S.; Green, M. M. Chiral Cooperativity in Helical Polymers. Isr. J. Chem. 2011, 51, 1067. (17) Yashima, E.; Maeda, K.; Iika, H.; Furusho, Y.; Nagai, K. Helical Polymers: Synthesis, Structures, and Functions. Chem. Rev. 2009, 109, 6102. (18) Rudick, J. G.; Percec, V. Induced Helical Backbone Conformations of Self-Organizable Dendronized Polymers. Acc. Chem. Res. 2008, 41, 1641. (19) Fujiki, M. Helix Generation, Amplification, Switching, and Memory of Chromophoric Polymers. Top. Curr. Chem. 2008, 284, 119. (20) Akagi, K.; Mori, T. Helical Polyacetylene−Origins and Synthesis. Chem. Rec. 2008, 8, 395. (21) Shiotsuki, M.; Sanda, F.; Masuda, T. Polymerization of Substituted Acetylenes and Features of the Formed Polymers. Polym. Chem. 2011, 2, 1044. (22) Paik, P.; Gedanken, A.; Mastai, Y. Chiral-MesoporousPolypyrrole Nanoparticles: Its Chiral Recognition Abilities and Use in Enantioselective Separation. J. Mater. Chem. 2010, 20, 4085. (23) Haraguchi, N.; Nishiyama, A.; Itsuno, S. Synthesis of Polymer Microspheres Functionalized with Chiral Ligand by Precipitation Polymerization and Their Application to Asymmetric Transfer Hydrogenation. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 3340. (24) Thvedt, T. H. K.; Kristensen, T. E.; Sundby, E.; Hansen, T.; Hoff, B. H. Enantioselectivity, Swelling and Stability of 4Hydroxyprolinol Containing Acrylic Polymer Beads in the Asymmetric Reduction of Ketones. Tetrahedron: Asymmetry 2011, 22, 2172. (25) Green, M. M.; Park, J. W.; Sato, T.; Teramoto, A.; Lifson, S.; Selinger, R. L. B.; Selinger, J. V. The Macromolecular Route to Chiral Amplification. Angew. Chem., Int. Ed. 1999, 38, 3138. (26) Palmans, A. R. A.; Meijer, E. W. Amplification of Chirality in Dynamic Supramolecular Aggregates. Angew. Chem., Int. Ed. 2007, 46, 8948. (27) Liu, D.; Zhang, L.; Li, M. K.; Yang, W. T.; Deng, J. P. Magnetic Fe3O4-PS-Polyacetylene Composite Microspheres Showing Chirality Derived From Helical Substituted Polyacetylene. Macromol. Rapid Commun. 2012, 33, 672. (28) Xie, R.; Chu, L. Y.; Deng, J. G. Membranes and Membrane Processes for Chiral Resolution. Chem. Soc. Rev. 2008, 37, 1243.
(29) Okamoto, Y.; Ikai, T. Chiral HPLC for Efficient Resolution of Enantiomers. Chem. Soc. Rev. 2008, 37, 2593. (30) Medina, D. D.; Goldshtein, J.; Margel, S.; Mastai, Y. Enantioselective Crystallization on Chiral Polymeric Microspheres. Adv. Funct. Mater. 2007, 17, 944. (31) Menahem, T.; Mastai, Y. Enantioselective Crystallization in Miniemulsions Based on Chiral Surfactants. New J. Chem. 2008, 32, 925. (32) Ejgenberg, M.; Mastai, Y. Conglomerate Crystallization on SelfAssembled Monolayers. Chem. Commun. 2011, 47, 12161. (33) Mastai, Y. Enantioselective Crystallization on Nanochiral Surfaces. Chem. Soc. Rev. 2009, 38, 772. (34) Zhang, D. Y.; Song, C.; Deng, J. P.; Yang, W. T. Chiral Microspheres Consisting Purely of Optically Active Helical Substituted Polyacetylene: The First Preparation via Precipitation Polymerization and Application in Enantioselective Crystallization. Macromolecules 2012, 45, 7329. (35) Chen, B.; Deng, J. P.; Cui, X.; Yang, W. T. Optically Active Helical Substituted Polyacetylenes as Chiral Seeding for Inducing Enantioselective Crystallization of Racemic N-(tert-Butoxycarbonyl)alanine. Macromolecules 2011, 44, 7109. (36) Chen, B.; Deng, J. P.; Yang, W. T. Hollow Two-Layered Chiral Nanoparticles Consisting of Optically Active Helical Polymer/Silica: Preparation and Application for Enantioselective Crystallization. Adv. Funct. Mater. 2011, 21, 2345. (37) Li, W. F.; Liu, X.; Qian, G. Y.; Deng, J. P. Immobilization of Optically Active Helical Polyacetylene-Derived Nanoparticles on Graphene Oxide by Chemical Bonds and Their Use in Enantioselective Crystallization. Chem. Mater. 2014, 26, 1948. (38) Chen, B.; Deng, J. P.; Tong, L. Y.; Yang, W. T. Optically Active Helical Polyacetylene@silica Hybrid Organic-inorganic Core/Shell Nanoparticles: Preparation and Application for Enantioselective Crystallization. Macromolecules 2010, 43, 9613. (39) Tabei, J.; Nomura, R.; Masuda, T. Conformational Study of Poly(N-propargylamides) Having Bulky Pendant Groups. Macromolecules 2002, 35, 5405. (40) Schrock, R. R.; Osborn, J. A. π-Bonded Complexes of the Tetraphenylborate Ion with Rhodium(I) and Iridium(I). Inorg. Chem. 1970, 9, 2339. (41) Nomura, R.; Yamada, K.; Tabei, J.; Takakura, Y.; Takigawa, T.; Masuda, T. Stumuli-Responsive Organogel Based on Poly(Npropargylamide). Macromolecules 2003, 36, 6939. (42) El-Boubbou, K.; Gruden, C.; Huang, X. F. Magnetic GlycoNanoparticles: A Unique Tool for Rapid Pathogen Detection, Decontamination, and Strain Differentiation. J. Am. Chem. Soc. 2007, 129, 13392. (43) Liu, D.; Chen, H. Y.; Deng, J. P.; Yang, W. T. Optically Active, Magnetic Gels Consisting of Helical Substituted Polyacetylene and Fe3O4 Nanoparticles: Preparation and Chiral Recognition Ability. J. Mater. Chem. C 2013, 1, 8066. (44) Zhang, H. Y.; Song, J. X.; Deng, J. P. The First Suspension Polymerization for Preparing Optically Active Microparticles Purely Constructed from Chirally Helical Substituted Polyacetylenes. Macromol. Rapid Commun. 2014, 35, 1216. (45) Gijs, M. A.; Lacharme, F.; Lehmann, U. Microfluidic Applications of Magnetic Particles for Biological Analysis and Catalysis. Chem. Rev. 2009, 110, 1518. (46) Agrigento, P.; Beier, M. J.; Knijnenburg, J. T. N.; Baiker, A.; Gruttadauria, M. Highly Cross-Linked Imidazolium Salt Entrapped Magnetic ParticlesPreparation and Applications. J. Mater. Chem. 2012, 22, 20728. (47) Okada, T.; Takeda, Y.; Watanabe, N.; Haeiwa, T.; Sakai, T.; Mishima, S. Chemically Stable Magnetic Nanoparticles for Metal Adsorption and Solid Acid Catalysis in Aqueous Media. J. Mater. Chem. A 2014, 2, 5751. (48) Luo, X. F.; Deng, J. P.; Yang, W. T. Helix-Sense-Selective Polymerization of Achiral Substituted Acetylenes in Chiral Micelles. Angew. Chem., Int. Ed. 2011, 50, 4909. 17401
dx.doi.org/10.1021/ie503114z | Ind. Eng. Chem. Res. 2014, 53, 17394−17402
Industrial & Engineering Chemistry Research
Article
(49) Deng, J. P.; Chen, B.; Luo, X. F.; Yang, W. T. Synthesis of Nano-Latex Particles of Optically Active Helical Substituted Polyacetylenes via Catalytic Microemulsion Polymerization in Aqueous Systems. Macromolecules 2009, 42, 933.
17402
dx.doi.org/10.1021/ie503114z | Ind. Eng. Chem. Res. 2014, 53, 17394−17402
