Alkynylated Cellulose Nanocrystals ... - ACS Publications

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Alkynylated Cellulose Nanocrystals Simultaneously Serving as Chiral Source and Stabilizing Agent for Constructing Optically Active Helical Polymer Particles Huli Yu and Jianping Deng* State Key Laboratory of Chemical Resource Engineering and College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: To take advantage of the chirality of cellulose nanocrystals (CNCs) and to develop novel chiral polymer materials, alkynylated CNCs (alkynyl-CNCs) were prepared and copolymerized with an achiral acetylenic monomer through suspension polymerization in aqueous media. The chirality of the alkynyl-CNCs was efficiently transferred to racemic helical polymer chains, by which inducing predominantly onehanded polymer helicity. Moreover, alkynyl-CNCs simultaneously acted as a comonomer and stabilizing agent, directly providing optically active microparticles (400−800 μm) constructed by chirally helical polymer chains. So the alkynylated CNCs played triple roles in our strategy: chiral source, comonomer, and stabilizing agent for performing suspension polymerizations. SEM images showed the successful formation of microparticles with regular spherical morphology, while circular dichroism spectra demonstrated the formation of one-handed helical polymer chains and optical activity of the microparticles thereof. The present study opens new opportunities for using CNCs and for preparing novel optically active helical polymer materials.

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INTRODUCTION Natural biomacromolecules provide abundant renewable building blocks for constructing green materials. The studies along this direction are gathering rapidly increasing attention in diverse disciplines. In fact, however, natural biomacromolecules also offer rich chiral resources for establishing chiral materials. Delightfully, this has been well recognized recently. Typically impressive examples include biomacromolecule-based chiral bioconjugates1 and hybrids,2−4 chiralized graphene5 and silica,6,7 etc. The resulting functional materials were investigated in potential applications like chiral resolution8 and chiral adsorption.9 More recently, cellulose nanocrystals (CNCs) have stimulated much more interest due to the rigid, highly crystallized, 1-D nanoscaled structures and the ability to form liquid crystals.10 So far remarkable progress has been achieved in using CNCs for fabricating chiral architectures. For instance, MacLachlan’s group has made significant achievements in preparing chiral silica materials by using CNCs.11 Besides, chiral films12 and chiral hybrids13 were also prepared by different groups. Reportedly, the materials derived from CNCs incorporating metals14 can work as chiral catalysts for performing asymmetric reactions. In addition, both pristine CNCs15 and CNCs containing grafts16 can be used as stabilizers for constructing polymer particles through suspension polymerization approaches. In the present study we for the first time prepared optically active helical polymer microparticles using alkynylated CNCs as the only chiral source. Our strategy (Scheme 1B) shows the following highlights: (1) Biomacromolecular CNCs were alkynylized to form the monomer (alkynyl-CNCs), which copolymerized with an achiral monomer to form the designed © XXXX American Chemical Society

composite particles. (2) The chirality of alkynyl-CNCs was efficiently transferred to the resulting microparticles through the “sergeants and soldiers rule”.17,18 (3) To fabricate polymer particles, alkynyl-CNCs served as stabilizing agent without adding other additives for performing suspension polymerization. Accordingly, alkynyl-CNCs in the present study simultaneously played triple roles: chiral source, stabilizing agent, and comonomer. Therefore, our strategy opens a new route for using CNCs to establish advanced functional materials. Another feature that also deserves being highlighted is the asymmetric polymerization19 involved in the study. As far as asymmetric polymerization is concerned, the strategy in the present work is also obviously different from the usual processes for preparing chirally helical polymers derived from achiral monomers. Routinely, helix-sense-selective polymerization (HSSP20,21) and postinduction (chiral induction22,23) processes are taken for controlling the helicity of racemic helical polymers. Nonetheless, in both of the two routes, small chiral molecules are used as chiral source, which probably limits the further progress of chiral materials due to the extremely high cost and particularly limits the types and numbers of such chiral compounds. In the present study, we successfully used biomacromolecules, i.e., CNCs derivative, as chiral inducer for controlling the handedness of helical polymers. Herein it is also important to point out that optically active particles,24,25 especially those constructed by chirally helical polymers,26−31 have constituted an emerging research area due to their Received: September 22, 2016

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DOI: 10.1021/acs.macromol.6b02070 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Strategy for Preparing Optically Active Microparticles (MPs) by Suspension Polymerization with Different Stabilizing Agents: (A) Pristine CNCs, (B) Alkynyl-CNCs, and (C) PVP

Briefly, the CMCs/water suspension (8.23 g/36.5 mL) was put in an ice bath and stirred at room temperature, in which 95−98 wt % sulfuric acid (35.5 mL) was added dropwise. Within 5 min, the temperature was heated to 45 °C, at which the suspension was stirred vigorously for 2 h. Subsequently, the suspension was diluted 10-fold by water to stop the reaction, neutralized with sodium hydroxide aqueous solution, and purified by dialyzing for 2 weeks. The product was lyophilized to provide white powder with a yield of 28% (of initial weight). Preparation of Alkynylated CNCs (Alkynyl-CNCs). According to the literature,39,40 the above-prepared CNCs (0.5 g) were dispersed in DMF (100 mL) containing triethylamine (5 mL) and charged in a vessel, in which OPNTU silane (2.5 mL) was added dropwise. The reaction system was stirred with a magnetic stirrer for 48 h under N2 at room temperature. After that the suspension was successively washed with DMF and THF each for three times by centrifuge (10 min at 12 000 rpm). Finally, the product was redispersed in a small amount of deionized water and freeze-dried into a white powder (0.41 g, a yield of 82%). Preparation of Microparticles (MPs). The microparticles (MPs) were prepared through suspension polymerization established by us earlier.41 Taking the preparation of MPs with 0.14 wt % CNCs as example, the polymerization was carried out as follows. CNCs (0.035 g) were dispersed in deionized water (25 mL) under ultrasonification for 10 min. To avoid overheating, the sample was sonicated at 0 °C. Under the protection of nitrogen atmosphere, the suspension was stirred at a rate of 350 rpm in an ice bath for 10 min. Then the mixture of monomer M (0.05 g, 0.25 mmol) and CA (0.0186 g, 0.075 mmol) which were previously dissolved in CHCl3 (0.8 mL) was added to the CNCs/water suspension. After 30 min, a solution of Rh catalyst (0.002 g, 0.004 mmol, 0.2 mL CHCl3) was dropped into the above reaction system. Maintained at 0 °C for 3 h, the suspension system was heated at a rate of 8 °C/h until the temperature reached 30 °C and then kept for 4 h to complete polymerization with evaporation of CHCl3 by bubbling N2. After filtration and drying, C-MPs were obtained. The preparation conditions and the data about the resulting MPs are summarized in Table 1.

significant potentials as chiral materials in asymmetric catalysis,32 chiral resolution,8 and enantioselective release.33 Therefore, the investigations in the study are expected to inspire more interest in HSSPs, chiral polymer particles, biomacromolecules, etc.

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EXPERIMENTAL METHODS

Materials. Solvents were purified by distillation. Rhodium catalyst, (nbd)Rh+B−(C6H5)4 (nbd = 2,5-norbornadiene), was prepared in a reported method.34 Achiral monomer (defined as M, as molecularly presented in Scheme 1) was synthesized according to a procedure reported earlier.35 A bifunctional butynyl ester (dibutynyl adipate) was prepared referring to the method in the literature36 and was employed as cross-linking agent (CA, Scheme 1) to prepare cross-linked microparticles. Alkynylglucose was synthesized by a reported method.37 O-Propargyloxy-N-triethoxysilylpropylurethane (OPNTU silane) was purchased from Gelest Inc. and used directly. Cellulose microcrystals (CMCs) with average length about 25 μm were purchased from Aldrich and used as received. Polyvinylpyrrolidone (PVP K30) was purchased from Beijing Chemical Reagent Co. and used without further purification. Deionized water was used in all the experiments. The other reagents were used as received. Measurements. Circular dichroism (CD) and UV−vis absorption spectra were obtained on a Jasco-810 spectropolarimeter. The morphology of the MPs was observed with a JSM-7001F (JEOL) scanning electron microscope (SEM). The dimensions of CNCs or alkynyl-CNCs were determined by TEM images, which were obtained from a Hitachi H-800 electron microscope. Solid state 13C crosspolarization-magic angle spinning (CP-MAS) NMR spectra of CNC and alkynyl-CNC were recorded at room temperature on a Bruker ACSEND 400WB NMR spectrometer. FT-IR spectra were recorded on a Nicolet NEXUS 670 spectrophotometer (in KBr tablets). XPS measurements were performed with an ESCALAB 250Xi electron spectrometer. The optical microscopic images of suspension samples were obtained with Axioskop 40A Pol optical microscope (POM, Carl Zeiss). Preparation of Cellulose Nanocrystals (CNCs). CNCs were prepared from acid hydrolysis of CMCs, as described previously.38 B

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Macromolecules Table 1. MPs Synthesized with Different Stabilizing Agents (SA)a sample

SA

C-MPs

CNCs

A-CMPs

alkynylCNCs

P-MPs

PVP

SA mass concnb (wt %)

yield of MPsc (wt %)

CD effect

0.14 0.28 0.33 0.14

55.8 80.0 74.5 59.8

yes

0.16 0.18 0.8

77.3 80.2 75.5

no

Figure 1. TEM images of CNCs (a) and alkynyl-CNCs (b) water suspension. Phosphotungstic acid was used as dyeing agent for (b), since alkynyl-CNCs were not obviously observed in our testing process.

no

a

Polymerization conditions: monomer M (0.05 g, 0.25 mmol), CA (0.0186 g, 0.075 mmol), (nbd)Rh+B−(C6H5) (0.002 g, 0.004 mmol), CHCl3 (1 mL), deionized water (25 mL), stirring rate of 350 rpm. bSA means stabilizing agent; mass concentration was determined by the ratio of SA (g) to 25 g of water. cThe yield of MPs was the ratio of dried MPs to the initial total amount of monomer and SA.

subjected to XPS measurement (Figure S4). A peak corresponding to Si 2p appeared, demonstrating the presence of silicon in the sample. Therefore, CNCs were successfully alkyne-functionalized. Moreover, the amount of Si represents the amount of CC groups. Thus, the degree of surface substitution (the number of CC groups per glucose unit, DSS) value, about 1.1, was determined from the surface Si mass concentration (Table S1). To use CNCs and alkynyl-CNCs as stabilizing agent for performing suspension polymerizations, their dispersion stability in water is an important factor. As demonstrated in Figure 2a, alkynyl-CNCs were more stable than CNCs in water

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RESULTS AND DISCUSSION Taking an achirally substituted acetylene as monomer (M) model, we totally prepared three groups of MPs (Scheme 1, CMPs, P-MPs, and A-C-MPs). Among the three groups, C-MPs and P-MPs were prepared respectively with pristine CNCs and PVP (polyvinylpyrrolidone) as stabilizing agent and were taken as reference microparticles (Table 1). A-C-MPs were prepared using alkynyl-CNCs as stabilizing agent. The investigations clearly proved our success in preparing the anticipated MPs constructed by chirally helical substituted polyacetylene, as will be reported below. Rodlike CNCs can stabilize oils in water.42 First, we directly used CNCs as stabilizing agent for performing suspension polymerization of monomer M (Scheme 1A). The process led to polymer MPs (C-MPs) as expected. Unfortunately, C-MPs failed to show optical activity due to CNCs just serving as “inert” stabilizing agent. However, alkynyl-CNCs copolymerized with monomer M in solution (Figure S1, Supporting Information), providing chirally helical copolymers. This encouraged us to design alkynyl-CNCs as chiral inducer meanwhile as stabilizing agent in the suspension polymerizations. In principle, alkynyl-CNCs may play the role of “sergeants” in copolymerzing with achiral monomer (Scheme 1B; M, serves as “soldiers”), in which the sergeants may promote the soldiers to arrange uniformly along the polymer chains (“sergeants and soldiers rule”17,18). So the practice is expected to result in chirally helical polymers and optically active MPs thereof. TEM images of CNCs and alkynyl-CNCs are shown in Figure 1, from which we know that there was little change in the size between CNCs and alkynyl-CNCs, about 200−400 nm in length and 20−30 nm in width. The dimensions of CNCs and alkynyl-CNCs are in good agreement with the earlier reports.10 After modification with OPNTU silane, the new chemical shift at δ 71 ppm in solid 13C NMR spectra demonstrated the appearance of CC groups (Figure S2). Additionally, the chemical shift for the alkynyl-CNCs at δ 77 ppm almost disappeared fully, while chemical shifts at 84 and 65 ppm were obviously enhanced. It was due to the hydroxyl groups at C2, C3, or C5 reacted with OPNTU silane. Besides, in FT-IR spectra (Figure S3), weak peaks of Si−C stretching43 (1300−1200 cm−1) and Si−O (Si−C) stretching44 (850−750 cm−1) also showed that a certain amount of hydroxyl groups reacted with OPNTU silane. The products were further

Figure 2. Typical photographs of (a) cellulose/water suspensions and (b−d) MPs. The mass concentration of (a-1) CMCs, (a-2) CNCs, and (a-3) alkynyl-CNCs was 0.5 wt % (25 mg/5 mL). Samples of (b) PMPs, (c) C-MPs, and (d) A-C-MPs were prepared with 0.8 wt % PVP, 0.28 wt % CNCs, and 0.16 wt % alkynyl-CNCs as stabilizing agent, respectively. Photographs of other C-MPs and A-C-MPs are illustrated in Figure S5.

suspension. The reason is that some original surface hydroxyl groups of alkynyl-CNCs were substituted by hydrophobic C C groups, which weakened the hydrogen-bonding interaction among CNCs. Taking CNCs and alkynyl-CNCs as stabilizing agent, we successfully prepared MPs, for which the photographs are presented in Figure 2c,d. The size of the C-MPs and A-C-MPs varied from 400 to 700 μm, similar to P-MPs (Figure 2b, 400− 600 μm). All the MPs prepared with PVP, CNCs, and alkynylCNCs showed a yellow color, which is in accordance with C

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Figure 3. SEM images of MPs: (a) P-MPs, (b) C-MPs (0.28 wt % CNCs), and (c) A-C-MPs (0.16 wt % alkynyl-CNCs). The MPs were directly observed by SEM.

in the course of forming MPs, irrespective of the used stabilizing agents. However, alkynyl-CNCs were found inside the A-C-MPs (Figure 3c-2, lower), and some were even completely coated by polymer chains. It showed that alkynylCNCs took part in polymerization, just as anticipated. The resulting MPs were also characterized by FT-IR (Figure S7), in which no apparent difference was observed due to the used low content of CNCs and alkynyl-CNCs. According to our strategy as illustrated in Scheme 1 and the discussion above, alkynyl-CNCs took part in suspension copolymerization with monomer M. If this is true, the chirality of the alkynyl-CNCs may be efficiently transferred to the asprepared A-C-MPs. To elucidate this issue, all the three sets of MPs (P-MPs, C-MPs, A-C-MPs) were subjected to CD and UV−vis absorption spectra measurements, since the two techniques in particular the former have been proved effective for exploring chiral helical macromolecules and optical activity of the nanoarchitectures thereof.33,39,41,45The recorded spectra are presented in Figure 4. For comparison, we first characterized CNCs and alkynyl-CNCs by CD and UV−vis absorption spectroscopies (Figure 4a,b). In Figure 4a, the CD spectra of CNCs and alkynyl-CNCs were quite similar in the detected wavelength region (300−600 nm), and no CD peak was observed. However, the CD signals of the MPs differed remarkably. As shown in Figure 4c, no CD effect was observed (350−500 nm) in neither P-MPs nor CMPs because their helical structures were integrally racemic.46 In contrast, a significant positive Cotton effect was observed at 425 nm in A-C-MPs (Figure 4c-3). Therefore, A-C-MPs were optically active. Different from alkynyl-CNCs, the CD spectrum

earlier reports and originated from the helical substituted polyacetylenes.41 Among the three concentrations, 0.14, 0.28, and 0.33 wt % (Table 1), CNCs at a concentration of 0.28 wt % provided C-MPs showing the narrowest size distribution. When alkynyl-CNCs were used, a concentration of 0.16 wt % led to the best A-C-MPs in comparison with the concentration of 0.14 and 0.18 wt %. Therefore, we can take 0.28 wt % CNCs or 0.16 wt % alkynyl-CNCs separately as stabilizing agent for preparing MPs. To further explore the morphology of the MPs, SEM images were observed, as presented in Figure 3. More SEM images for C-MPs and A-C-MPs can be found in Figure S6. In accordance with the observations above, the dimensions of P-MPs, C-MPs, and A-C-MPs are about 500−600 μm. In Figure 3a-1, the outer surface of P-MPs looked smooth. Nonetheless, CNCs were noticeably aggregated on the surface of C-MPs (Figure 3b-1). The aggregated CNCs reached 5 μm and above in length and 1 μm in width, much larger than the initially used CNCs (about 200−400 nm in length and 20−30 nm in width). This observation is mainly ascribed to the hydrogen bond formed through hydroxyl groups in CNCs when water was evaporated when drying MPs.43 In contrast, alkynyl-CNCs tended to be individually distributed, as observed in Figure 3c-1. Most of them kept the same length as the originally added alkynylCNCs. This phenomenon is because the CC bonds facilitated the alkynyl-CNCs to take part in copolymerization, making them distributed individually rather than aggregated. Inside the MPs, there were some wrinkled structures more than 2 μm in length (Figure 3a-2, Figure 3b-2, and Figure 3c-2, upper). The unique wrinkled structures seem to be generated D

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Figure 4. CD (a, c) and UV−vis (b, d) spectra of CNCs, alkynyl-CNCs, and MPs. (c-1) P-MPs, (c-2) C-MPs (0.28 wt % CNCs), and (c-3) A-CMPs (0.16 wt % alkynyl-CNCs). The spectra were measured with samples fixed between two glass slides.

Figure 5. POM images of the suspension systems stabilized by CNCs (a) and alkynyl-CNCs (b−d). The mass concentration of CNCs or alkynylCNCs was 0.16 wt %. (b) Sample before polymerization, (c) Sample in polymerization. (d) Sample after polymerization.

and the alkynyl-CNCs were defficient in solubility, so it is difficult to further quantitatively analyze the MPs, e.g., to determine the enantiomeric excess (ee) value. In addition, currently we cannot control the polymers’ moleculer weight due to lacking convenient and effective way to achieve “living” polymerization of acetylenics especially in aqueous media. To understand more deeply the origin of A-C-MPs’ optical activity, we next investigated whether alkynyl-CNCs assembled into liquid crystal, since CNCs can form liquid crystal structures under certain conditions.10 If this is the case, achiral monomer M may have underwent asymmetric polymerization in the liquid crystals.47 To make this question clear, we first explored the ability of CNCs and alkynyl-CNCs to form chiral liquid crystals. POM images (Figure S10) of both CNCs and alkynyl-CNCs suspensions demonstrated a fingerprint texture characteristic when the mass concentration was higher than 9 wt %, showing the formation of chiral nematic order of the liquid crystals. However, when the mass concentration was lower than 9 wt %, no fingerprint texture was observed. Then

of A-C-MPs assumed an intense peak, showing that the optical activity of A-C-MPs originated in the polymer. We thus conclude that the substituted polyacetylene chains took onehanded helical conformation in A-C-MPs, referering to our earlier intensive studies dealing with helically substituted polyacetylenes and the particles thereof.33,41,45,46 The modification of CNCs with alkyne groups made a big difference on the optical activity of MPs. The conclusion is further evidenced by the spectra in Figures S8 and S9. In Figure S8, the sample of A-C-MPs (0.16 wt % alkynyl-CNCs) was measured after twisting for 0°, 15°, 30°, 45°, 60°, and then 90°; however, the CD peaks kept unchanged. In Figure S9, all the A-C-MPs demonstrated remarkable CD effect, while no CD effect was observed in CMPs, as summarized in Table 1. The corresponding UV−vis adsorptions are presented in Figure 4b,d. Accordingly, alkynylCNCs copolymerized with monomer M, by which the chirality of CNCs was successfully transferred to the as-formed A-CMPs. It should be noted that the A-C-MPs were cross-linked E

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Scheme 2. (A) Solution Polymerization of Monomer M To Form Racemic Helical Polymer (PM), Which Was Treated with Alkynyl-CNCs under Conditions for the Corresponding Suspension Polymerization, but No Polymer Particles Were Formed (B); (C) Monomer M Underwent Solution Polymerization with Chiral Catalyst System (for More Details, See the Supporting Information)

Figure 6. CD (a) and UV−vis absorption (b) spectra of polymer (PM, Mn: 6300, Mw/Mn: 1.90; obtained by solution polymerization in CHCl3).

Figure 7. CD (a) and UV−vis (b) spectra of polymer (PM) prepared with chiral catalyst systems. In the spectra, R-PM (Mn: 5800, Mw/Mn: 1.81) and S-PM (Mn: 5700, Mw/Mn: 1.68) represented the polymer obtained by {[Rh(nbd)Cl]2 + (R)-(+)-1-phenylethylamine} and {[Rh(nbd)Cl]2 + (S)-(−)-1-phenylethylamine}, respectively. Samples were measured in CHCl3.

F

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investigations demonstrated that the molecular chirality of the glucose units in the alkynyl-CNCs served as the chiral origin, while the rodlike structure of alkynyl-CNCs played the role of stabilizing agent for forming the optically active microparticles. Essentially, we take each rodlike alkynyl-CNC as an entity and make a full use of alkynylated CNCs which play triple roles: chiral source, copolymer, and stabilizing agent. Indeed, the chirality of the CNCs at the three levels may be judiciously used for creating novel chiral materials. We will continue our studies along the attractive research directions.

we characterized three samples, namely, the polymerization system before polymerization, in polymerization, and after polymerization by POM (Figure 5b−d). The POM image of pristine CNCs suspension (0.16 wt %) was taken for comparison (Figure 5a). However, all the samples did not produce liquid crystals because of the low mass concentration of CNCs and alkynyl-CNCs, showing that the achiral monomer polymerized in the suspension system without the participation of liquid crystal field. We thus rule out the effects of liquid crystal field on the polymerization of monomer M. We accomplished additional experiments to further justify our considerations. Monomer M was first polymerized by solution polymerization (Scheme 2A), and then the thusobtained polymer (PM) was examined in the aqueous suspension system containing 0.16 wt % alkynyl-CNCs under the conditions identical to suspension polymerization (Scheme 2B). Nonetheless, we found no polymer MPs were formed even in the presence of alkynyl-CNCs. The as-treated polymer was further tested by CD and UV−vis spectra measurements (Figure 6). The CD spectrum maintained the same as that of the polymer prepared by solution polymerization. That is no CD effect was observed at the range of 300−600 nm, at which UV−vis adsorption clearly appeared. It further demonstrates that the racemic helical polymer (PM)41 remained even in the presence of akynyl-CNCs. Therefore, we further conclude that the aforementioned A-C-MPs’ optical activity could not be postinduced by alkynyl-CNCs. To acquire more insights into the effects of alkynyl-CNCs in forming chiral helical polymer microparticles, we further conducted the subsequent investigations. Monomer M was polymerized using chiral catalyst system (Scheme 2C) according to the literature.48 However, no CD peak was observed in the resulting polymer (Figure 7). The reason may be that the helix inversion barrier was not sufficient enough for the helices to retain the preferentially induced single-handed screw sense.46 On the contrary, our investigations demonstrate that monomer M copolymerized with alkynyl-CNCs to form AC-MPs, in which the chirality of alkynyl-CNCs transferred to the polymer through the “sergeants and soldiers rule”.17,18 In particular, importantly, the induced preferential helicity was efficiently maintained due to the presence of alkynyl-CNCs. With the optically active A-C-MPs in hand, we began to consider another interesting question: CNCs contain three levels of chirality (molecular chirality of the glucose units, helical chirality of the screw-shaped nanocrystal structure, and chiral-nematic phase of the lyotropic liquid crystal), so what is the exact chiral source in alkynyl-CNCs, by which to induce preferential helicity in the helical polymers and to construct the optically active MPs? In the three-level chirality, the last one, i.e., chiral liquid crystal, has been ruled out, as demonstrated in Figure 5. Moreover, as discussed above (Figure 4), when pristine CNCs were used as stabilizing agent for performing suspension polymerization of monomer M, no CD effect was observed in the resulting microparticles, showing that the helical chirality of the pristine CNCs did not induce any chirality during the polymerization. Accordingly, molecular chirality of the glucose units in the alkynyl-CNCs seems to serve as the chiral origin. For this assumption, specific investigations provided support. Both our additional experiments (Scheme S1 and Figure S11) and early report49 proved that alkynylated glucose played the role of “sergeants” in driving the soldiers (achiral monomer M units) to arrange in the identical helicity along the copolymer chains. Thus, our

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CONCLUSIONS Using alkynyl-CNCs as stabilizing agent, we successfully fabricated microparticles (400−800 μm) through the suspension polymerization approach. Alkynyl-CNCs copolymerized with the achiral monomer and served as sergeants, directing the soldiers (achiral monomer units) to arrange in the same manner along the helical copolymer chains. Meanwhile, the stabilizing effect of the alkynyl-CNCs was demonstrated and contributed to the formation of regular, optically active microparticles. The study opens new routes for preparing optically active helical polymer-based particles. The established strategy is expected to provide various polymers particles possessing potentials as novel, chiral materials. It also provides new strategies for taking advantage of the chirality of biomacromolecules.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02070. Data for CNCs, alkynyl-CNCs, and MPs; CD and UV− vis spectra of copolymers; POM images, solid 13C CPMAS NMR, FT-IR, and XPS survey spectra of CNCs and alkynyl-CNCs; typical photographs, SEM images, FT-IR, CD, and UV−vis spectra of MPs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax +86-10-6443-5128; Tel +86-10-6443-5128; e-mail [email protected] (J.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21274008, 21174010, 20974007) and the Funds for Creative Research Groups of China (51521062).

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REFERENCES

(1) Shao, H.; Lockman, J. W.; Parquette, J. R. Coupled Conformational Equilibria in β-Sheet Peptide-Dendron Conjugates. J. Am. Chem. Soc. 2007, 129, 1884−1885. (2) George, J.; Thomas, G. Surface Plasmon Coupled Circular Dichroism of Au Nanoparticles on Peptide Nanotubes. J. Am. Chem. Soc. 2010, 132, 2502−2503. (3) Graf, P.; Mantion, A.; Haase, A.; Thünemann, A. F.; Mašić, A.; Meier, W.; Luch, A.; Taubert, A. Silicification of Peptide-Coated Silver NanoparticlesA Biomimetic Soft Chemistry Approach toward Chiral Hybrid Core-Shell Materials. ACS Nano 2011, 5, 820−833. (4) Wang, Y.; Xu, J.; Wang, Y. W.; Chen, H. Y. Emerging Chirality in Nanoscience. Chem. Soc. Rev. 2013, 42, 2930−2962. G

DOI: 10.1021/acs.macromol.6b02070 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules (5) Qing, G. Y.; Zhao, S. L.; Xiong, Y. T.; Lv, Z. Y.; Jiang, F. L.; Liu, Y.; Chen, H.; Zhang, M. X.; Sun, T. Chiral Effect at Protein/Graphene Interface: A Bioinspired Perspective to Understand Amyloid Formation. J. Am. Chem. Soc. 2014, 136, 10736−10742. (6) Liu, B.; Cao, Y. Y.; Huang, Z. H.; Duan, Y. Y.; Che, S. Silica Biomineralization via the Self-Assembly of Helical Biomolecules. Adv. Mater. 2015, 27, 479−497. (7) Liu, B.; Han, L.; Che, S. Formation of Enantiomeric ImpellerLike Helical Architectures by DNA Self-Assembly and Silica Mineralization. Angew. Chem., Int. Ed. 2012, 51, 923−927. (8) Shen, J.; Okamoto, Y. Efficient Separation of Enantiomers Using Stereoregular Chiral Polymers. Chem. Rev. 2016, 116, 1094−1138. (9) Wei, W. L.; Qu, K. G.; Ren, J. S.; Qu, X. G. Chiral Detection Using Reusable Fluorescent Amylose-Functionalized Graphene. Chem. Sci. 2011, 2, 2050−2056. (10) Habibi, Y.; Lucia, L. A.; Rojas, O. J. Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications. Chem. Rev. 2010, 110, 3479−3500. (11) Shopsowitz, K. E.; Qi, H.; Hamad, W. Y.; MacLachlan, M. J. Free-Standing Mesoporous Silica Films with Tunable Chiral Nematic Structures. Nature 2010, 468, 422−425. (12) Querejeta-Fernández, A.; Chauve, G.; Methot, M.; Bouchard, J.; Kumacheva, E. Chiral Plasmonic Films Formed by Gold Nanorods and Cellulose Nanocrystals. J. Am. Chem. Soc. 2014, 136, 4788−4793. (13) Weng, X. L.; Bao, Z. B.; Zhang, Z. G.; Su, B. G.; Xing, H. B.; Yang, Q. W.; Yang, Y. W.; Ren, Q. L. Preparation of Porous Cellulose 3,5-Dimethylphenylcarbamate Hybrid Organosilica Particles for Chromatographic Applications. J. Mater. Chem. B 2015, 3, 620−628. (14) Kaushik, M.; Basu, K.; Benoit, C.; Cirtiu, C. M.; Vali, H.; Moores, A. Cellulose Nanocrystals as Chiral Inducers: Enantioselective Catalysis and Transmission Electron Microscopy 3D Characterization. J. Am. Chem. Soc. 2015, 137, 6124−6127. (15) Nypelö, T.; Rodriguez-Abreu, C.; Kolen’ko, Y. V.; Rivas, J.; Rojas, O. J. Microbeads and Hollow Microcapsules Obtained by SelfAssembly of Pickering Magneto-Responsive Cellulose Nanocrystals. ACS Appl. Mater. Interfaces 2014, 6, 16851−16858. (16) Tang, J. T.; Lee, M. F. X.; Zhang, W.; Zhao, B. X.; Berry, R. M.; Tam, K. C. Dual Responsive Pickering Emulsion Stabilized by Poly[2(dimethylamino)ethyl methacrylate] Grafted Cellulose Nanocrystals. Biomacromolecules 2014, 15, 3052−3060. (17) 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, 3138−3154. (18) Palmans, A. R. A.; Vekemans, J. A. J. M.; Havinga, E. E.; Meijer, E. W. Sergeants-and-Soldiers Principle in Chiral Columnar Stacks of Disc-Shaped Molecules with C3 Symmetry. Angew. Chem., Int. Ed. Engl. 1997, 36, 2648−2651. (19) Okamoto, Y.; Nakano, T. Asymmetric Polymerization. Chem. Rev. 1994, 94, 349−372. (20) Akagi, K. Helical Polyacetylene: Asymmetric Polymerization in a Chiral Liquid-Crystal Field. Chem. Rev. 2009, 109, 5354−5401. (21) Chen, J. L.; Yang, L.; Wang, Q.; Jiang, Z. Q.; Liu, N.; Yin, J.; Ding, Y. S.; Wu, Z. Q. Helix-Sense-Selective and Enantiomer-Selective Living Polymerization of Phenyl Isocyanide Induced by Reusable Chiral Lactide Using Achiral Palladium Initiator. Macromolecules 2015, 48, 7737−7746. (22) Yashima, E.; Maeda, K.; Okamoto, Y. Memory of Macromolecular Helicity Assisted by Interaction with Achiral Small Molecules. Nature 1999, 399, 449−451. (23) Shimomura, K.; Ikai, T.; Kanoh, S.; Yashima, E.; Maeda, K. Switchable Enantioseparation Based on Macromolecular Memory of a Helical Polyacetylene in the Solid State. Nat. Chem. 2014, 6, 429−434. (24) Arias, S.; Freire, F.; Quiñoá, E.; Riguera, R. Nanospheres, Nanotubes, Toroids, and Gels with Controlled Macroscopic Chirality. Angew. Chem., Int. Ed. 2014, 53, 13720−13724. (25) Huang, H. J.; Yuan, Y. B.; Deng, J. P. Helix-Sense-Selective Precipitation Polymerization of Achiral Monomer for Preparing Optically Active Helical Polymer Particles. Macromolecules 2015, 48, 3406−3413.

(26) Liu, J. Z.; Lam, J. W. Y.; Tang, B. Z. Acetylenic Polymers: Syntheses, Structures, and Functions. Chem. Rev. 2009, 109, 5799− 5867. (27) Reuther, J. F.; Siriwardane, D. A.; Campos, R.; Novak, B. M. Solvent Tunable Self-Assembly of Amphiphilic Rod−Coil Block Copolymers with Chiral, Helical Polycarbodiimide Segments: Polymeric Nanostructures with Variable Shapes and Sizes. Macromolecules 2015, 48, 6890−6899. (28) Kulikov, O. V.; Siriwardane, D. A.; Reuther, J. F.; McCandless, G. T.; Sun, H. J.; Li, Y.; Mahmood, S. F.; Sheiko, S. S.; Percec, V.; Novak, B. M. Characterization of Fibrous Aggregated Morphologies and Other Complex Architectures Self-Assembled from Helical Alkyne and Triazole Polycarbodiimides (R)- and (S) Families in the Bulk and Thin Film. Macromolecules 2015, 48, 4088−4103. (29) Li, S.; Liu, K.; Kuang, G.; Masuda, T.; Zhang, A. Thermoresponsive Helical Poly(phenylacetylene)s. Macromolecules 2014, 47, 3288−3296. (30) Miyagi, Y.; Sogawa, H.; Shiotsuki, M.; Sanda, F. Synthesis of Optically Active Conjugated Polymers Bearing m-Terphenylene Moieties by Acetylenic Coupling Polymerization: Chiral Aggregation and Optical Properties of the Product Polymers. Macromolecules 2014, 47, 1594−1603. (31) Wang, R.; Zheng, Y. J.; Li, X. F.; Chen, J. X.; Cui, J. X.; Zhang, J.; Wan, X. H. Optically Active Helical Vinylbiphenyl Polymers with Reversible Thermally Induced Stereomutation. Polym. Chem. 2016, 7, 3134−3144. (32) Thvedt, T. H. K.; Kristense, T. E.; Sundby, E.; Hansen, T.; Hoff, B. H. Enantioselectivity, Swelling and Stability of 4-Hydroxyprolinol Containing Acrylic Polymer Beads in the Asymmetric Reduction of Ketones. Tetrahedron: Asymmetry 2011, 22, 2172−2178. (33) Liang, J. Y.; Wu, Y.; Deng, J. P. Construction of Molecularly Imprinted Polymer Microspheres by Using Helical Substituted Polyacetylene and Application in Enantio-Differentiating Release and Adsorption. ACS Appl. Mater. Interfaces 2016, 8, 12494−12503. (34) Schrock, R. R.; Osborn, J. A. π-Bonded Complexes of the Tetraphenylborate Ion with Rhodium(I) and Iridium(I). Inorg. Chem. 1970, 9, 2339−2343. (35) Tabei, J.; Nomura, R.; Masuda, T. Conformational Study of Poly(N-propargylamides) Having Bulky Pendant Groups. Macromolecules 2002, 35, 5405−5409. (36) Nagao, Y.; Takasu, A. Click Polyester’’: Synthesis of Polyesters Containing Triazole Units in the Main Chain via Safe and Rapid ‘‘Click’’ Chemistry and Their Properties. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 4207−4218. (37) Ryu, E.-H.; Zhao, Y. Efficient Synthesis of Water-Soluble Calixarenes Using Click Chemistry. Org. Lett. 2005, 7, 1035−1037. (38) Bondeson, D.; Mathew, A.; Oksman, K. Optimization of the Isolation of Nanocrystals from Microcrystalline Cellulose by Acid Hydrolysis. Cellulose 2006, 13, 171−180. (39) Zhang, H. Y.; Qian, G. Y.; Song, J. X.; Deng, J. P. Optically Active, Magnetic Microparticles: Constructed by Chiral Helical Substituted Polyacetylene/Fe3O4 Nanoparticles and Recycled for Uses in Enantioselective Crystallization. Ind. Eng. Chem. Res. 2014, 53, 17394−17402. (40) Sai, H.; Fu, R.; Xing, L.; Xiang, J.; Li, Z.; Li, F.; Zhang, T. Surface Modification of Bacterial Cellulose Aerogels’ Web-like Skeleton for Oil/Water Separation. ACS Appl. Mater. Interfaces 2015, 7, 7373−7381. (41) 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, 1216−1223. (42) Cherhal, F.; Cousin, F.; Capron, I. Structural Description of the Interface of Pickering Emulsions Stabilized by Cellulose Nanocrystals. Biomacromolecules 2016, 17, 496−502. (43) Andresen, M.; Johansson, L. S.; Tanem, B. S.; Stenius, P. Properties and Characterization of Hydrophobized Microfibrillated Cellulose. Cellulose 2006, 13, 665−677. H

DOI: 10.1021/acs.macromol.6b02070 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (44) Zhang, Z.; Sèbe, G.; Rentsch, D.; Zimmermann, T.; Tingaut, P. Ultralightweight and Flexible Silylated Nanocellulose Sponges for the Selective Removal of Oil from Water. Chem. Mater. 2014, 26, 2659− 2668. (45) Huang, H. J.; Deng, J. P.; Shi, Y. Optically Active Physical Gels with Chiral Memory Ability: Directly Prepared by Helix-SenseSelective Polymerization. Macromolecules 2016, 49, 2948−2956. (46) 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, 4909−4912. (47) Akagi, K.; Piao, G.; Kaneko, S.; Sakamaki, K.; Shirakawa, H.; Kyotani, M. Helical Polyacetylene Synthesized with a Chiral Nematic Reaction Field. Science 1998, 282, 1683−1686. (48) Teraguchi, M.; Tanioka, D.; Kaneko, T.; Aoki, T. Helix-SenseSelective Polymerization of Achiral Phenylacetylenes with Two NAlkylamide Groups to Generate the One-Handed Helical Polymers Stabilized by Intramolecular Hydrogen Bonds. ACS Macro Lett. 2012, 1, 1258−1261. (49) Suenaga, M.; Kaneko, Y.; Kadokawa, J.; Nishikawa, T.; Mori, H.; Tabata, M. Amphiphilic Poly(N-propargylamide) with Galactose and Lauryloyl Groups: Synthesis and Properties. Macromol. Biosci. 2006, 6, 1009−1018.

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DOI: 10.1021/acs.macromol.6b02070 Macromolecules XXXX, XXX, XXX−XXX