Optically Active Physical Gels with Chiral Memory Ability: Directly

Apr 11, 2016 - Helix-sense-selective polymerizations (HSSPs) have been attracting much attention for controlling the chirality of synthetic helical po...
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Optically Active Physical Gels with Chiral Memory Ability: Directly Prepared by Helix-Sense-Selective Polymerization Huajun Huang, Jianping Deng,* and Yan Shi 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: Helix-sense-selective polymerizations (HSSPs) have been attracting much attention for controlling the chirality of synthetic helical polymers. However, up to date HSSPs have provided only optically active helical polymer solutions and emulsions. The present contribution reports on the first HSSPs for directly constructing optically active physical gels (OAPGs) by using achiral substituted acetylene monomer with (R)- or (S)-1phenylethylamine as chiral additive. The one-handed helical conformations of the resulting polymers and the optical activity of the gels thereof were verified by CD and UV−vis absorption spectra. SEM images show that the OAPGs were constructed by helical nanofibers with remarkable one-handed screw sense. After complete removal of the chiral additive, the purified OAPGs and the polymers forming the gels still showed intense CD signals, demonstrating their excellent chiral memory ability. This significant advantage and the simple preparative methodology endow the OAPGs with great potentials as chiral functional materials.

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INTRODUCTION Inspired by the overwhelming importance of natural helical architectures in organisms, scientists have been paying special attention toward synthetic chirally helical polymers and accordingly have achieved significant progress.1−3 Nowadays, synthetic helical polymers have found important applications as chiral seeds in enantioselective crystallization,4−6 as catalysts for asymmetric catalysis,7−11 as probes for chiral recognition,12 etc. However, an intractable task in synthesizing chirally helical polymers is the need of expensive chiral monomers. Another challenge is the very limited types and number of available chiral monomers. To solve the problems, helix-sense-selective polymerizations (HSSPs) using achiral monomers have been proved to be one of the most important strategies for preparing helical polymers with controlled helicity. Nevertheless, to our knowledge, the HSSPs reported so far were majorly limited to resulting in optically active helical polymer solutions.13−17 We recently developed helix-sense-selective emulsion polymerization (HSSEP)18,19 and helix-sense-selective precipitation polymerization (HSSPP)20 strategies, by which to directly fabricate optically active helical polymer nanoparticles. Based on the investigations, this article will report the first HSSPs for directly establishing optically active helical polymer-derived physical gels formed through π−π interaction. In sharp contrast with common optically active physical gels which are usually constructed by small chiral molecules21−24 or by nonhelical chiral macromolecules,25,26 we in the present work designed and successfully prepared a novel kind of optically active physical gel (OAPG) constructed by chirally helical substituted polyacetylene through helix-sense-selective © XXXX American Chemical Society

polymerization (HSSP) strategy. HSSPs provided helical polymer chains with predominant one helicity, which further formed OAPGs by intra- and intermolecular π−π stacking interaction.27,28 In the course of polymerization and gel formation, chirality transfer occurred from the chiral additive, and then chirality amplification29,30 was demonstrated by the resulting OAPGs. More excitingly, the resulting gels remained optically active even after the chiral additive was excluded thoroughly. It means that the chirality can be remembered in the gels, as reported in other chiral polymers.31−33 The asprepared OAPGs show superiority since the optical activity arouse solely from the predominant one-handed helical conformation of the polymer chains. This “clean” structure makes the OAPGs advantageous in practical uses.

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RESULTS AND DISCUSSION To construct the target polymer physical gels, we especially designed a monomer containing three phenyl groups, as shown in Figure 1A. The monomer was obtained in a high yield (75 wt %) and characterized by FT-IR (Figure S1, Supporting Information, the same below), NMR (Figure S2), and elemental analysis (see Experimental Methods section) techniques. These characterizations together confirmed the successful synthesis of the monomer. Then, the monomer underwent solution polymerization in organic solvents such as N,N-dimethylformamide (DMF), tetrahydrofuran (THF), Received: March 24, 2016 Revised: April 6, 2016

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butanone, toluene, xylene, CH2Cl2, CHCl3, and (R)- and (S)-1phenylethylamine ((R)- and (S)-PEA, used as chiral additives later in this study). The polymerization was catalyzed by Rh catalyst (Figure 1A). After polymerization for 3 h, the reaction system turned into white opaque gel only when CH2Cl2 or CHCl3 was used as solvent. The other solvents hindered the aggregation of polymer chains, namely disrupting the gelation process, due to either forming hydrogen bonds (DMF, THF, butanone) or π−π interactions (toluene, xylene, (R)- and (S)PEA) with the side groups in polymer. Considering the too low boiling point of CH2Cl2, we finally chose CHCl3 as the favorable solvent for gel preparation in all the experiments below. The gelation process is irreversible because the gels were formed through polymerization. In addition, the gelation is also irreversible to temperature. The as-prepared gels remained the shape below 60 °C (the boiling point of CHCl3 is about 61 °C), and the freeze-dired gels could even retain the shape at a temperature up to 150 °C. Moreover, the OAPGs (both asprepared gels and freeze-dried gels) were insoluble in usual solvents, like DMF, THF, acetone, butanone, toluene, xylene, CH2Cl2, and CHCl3 (only slightly soluble in DMAC). The above observations show the high stability of the OAPGs in diverse conditions. The polymerization system established above shows a desirable simplicity. We kept constant the ratio of monomer to catalyst ([M]/[Rh] = 1/100, in mol) and adjusted the monomer concentration ([M]) to determine the suitable conditions for forming OAPG. Therefore, we polymerized the monomer in a range of concentration and found that the critical

Figure 1. (A) Schematic illustration of the helix-sense-selective polymerization (HSSP) for preparing optically active physical gel (OAPG). (B) Illustration for the formation of OAPG by helical nanofibers through π−π interaction between phenyl groups, in which chiral additive molecules (blue five-pointed stars) are physically embedded in the as-prepared OAPG. (C) After elimination of all the chiral additive, the OAPG still remembered the chirality because the one-handed helical structures of the corresponding polymers remained well.

Figure 2. Photos (in the reaction tubes) and SEM images (freeze-dried samples) of optically active physical gels (OAPGs) prepared in CHCl3 at varied monomer concentrations ([M]): (A) [M] = 0.05 mol/L, without chiral additive; (B) [M] = 0.1 mol/L, (R)-PEA as chiral additive; (C) [M] = 0.05 mol/L, (R)-PEA as chiral additive; (D) [M] = 0.025 mol/L, (R)-PEA as chiral additive; (E) [M] = 0.017 mol/L, (R)-PEA as chiral additive; (F) [M] = 0.05 mol/L, (S)-PEA as chiral additive. The ratio of chiral PEAs to CHCl3 was kept at 1/100 (v/v). Scale bar: 2 μm (in the insets, 500 nm). B

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Figure 3. CD (A, C, E) and UV−vis (B, D, F) spectra of optically active physical gels prepared in CHCl3 at different monomer concentrations: (a) [M] = 0.05 mol/L, without chiral additive; (b) [M] = 0.1 mol/L, (R)-PEA as chiral additive; (c) [M] = 0.05 mol/L, (R)-PEA as chiral additive; (d) [M] = 0.025 mol/L, (R)-PEA as chiral additive; (e) [M] = 0.017 mol/L, (R)-PEA as chiral additive; (f) [M] = 0.05 mol/L, (S)-PEA as chiral additive. For (A) and (B), the polymer (10−4 mol/L by monomer units) was dissolved in DMAC. For (C) and (D), the polymer (10−4 mol/L by monomer units) was homogeneously dispersed in CHCl3 by ultrasonic. For (E) and (F), the gels were directly pressed between two pieces of quartz glass for measurement.

smaller ones. This well answers the question why the fiber diameters in the OAPGs are larger than that of the achiral gel, as mentioned earlier. In particular, a careful comparison between Figures 2C and 2F discloses that the use of (R)- or (S)-PEA in the polymerizations led to fibers with opposite helix sense. The opposite helicity will be further supported by CD spectra (see below). The driving forces for the formation of helical nanofibers include two aspects: (1) the π−π interaction between the phenyl groups along the polymer chains, as illustrated in Figure 1B,C (for more discussion, see below); (2) the chiral transfer and amplification effect29,30 occurring in the course of polymerization. Namely, chiral information transferred from small chiral additives to macromolecules and then from macromolecules to nanofibers. The OAPGs were prepared at different monomer concentration, as shown in Figure 2B−E (the SEM images are arranged in the order of decreasing monomer concentration). As monomer concentration decreased, the resulting gels

concentration to form an integral gel was approximately 0.017 mol/L. This critical concentration varied little when a little amount of chiral PEAs (chiral PEAs/CHCl3 = 1/100, v/v) was added as additive, as to be reported later on. The obtained gels seem undistinguishable in visual appearance when prepared with or without chiral PEAs as additive (Figure 2A,C,F, the photographs). Surprisingly, the SEM images demonstrate that the inner structures of the gels (freeze-dried) differ largely. Although all these gels are constructed by polymer nanofibers, the thickness and morphology of the nanofibers are remarkably different. For the gel obtained from polymerization without chiral additive (achiral gel, Figure 2A), the fiber diameters are only about 50 nm, while for the gels (OAPGs) obtained with (R)- or (S)-PEA as chiral additive, the corresponding fiber diameters increased to over 100 nm (Figure 2C,F). More excitingly, the fibers in the achiral gel are continuous and smooth; in sharp contrast, the fibers in OAPGs seem to be formed by helically twining of C

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Macromolecules became softer and weaker, but they would not flow out of the reaction vessel by gravity until reaching the aforementioned critical concentration (Figure 2E). On the other hand, by comparing SEM images, we can find that the helical nanofibers formed at a monomer concentration of 0.05 mol/L (Figure 2C,F) possessed the most distinguishable helices among the gels. It means that a highest chiral transfer efficiency was achieved in the cases (Figure 2C,F). Furthermore, this observation is in well agreement with CD spectra as to be presented below. A careful comparison between the present monomer and similar monomers (with fewer or without benzene rings; the corresponding polymers could not form gels) polymerized by the same Rh catalyst in our previous investigations20,34−37 enables us to elucidate the gelation mechanism of the polymers, as illustrated in Figure 1B. Different from the racemic physical gels constructed by stereocomplexes of poly(phenylacetylene)s,38 in which gelation was induced and stabilized by intermolecular hydrogen bonds and complementary helices, the OAPGs synthesized in the present study were formed by intra- and intermacromolecular π−π stacking interaction. During the formation of OAPGs, including both the chiral and achiral gels, π−π interaction occurred among the phenyl groups and then made the polymer chains concentrated and oriented. The π−π interaction extended through the propagation of the polymer chains. Eventually, bundles of polymers formed fiber-like architectures at nanoscale. Noticeably, the nanofibers in OAPGs took chiral helical structures due to the chiral transfer and amplification effect29,30 originated from the chiral additives. At last, the nanofibers constituted the gel skeleton by stacking process. Since the gels were constructed by physical rather than chemical means, the polymers can be investigated further in solution. The corresponding data are summarized in Table S1. Number-average molecular weights of the polymers were found to be around 85 000 g/mol, which convincingly confirm the successful polymerization of the monomer. Typical Raman spectra (Figure S3) show that the cis contents of the polymer chains constructing achiral gel and OAPGs are both higher than 90%. Moreover, the cis content in the OAPG (>93%) is even greater than that in the achiral gel (90.6%), which further indicates that the helix-sense-selective polymerization could produce polymers with higher cis content. In order to acquire more insights into the helical structure of the polymers and the optical activity of the OAPGs thereof, we next characterized them by CD and UV−vis absorption spectroscopy measurements, since these two techniques, especially the former, have been proved efficient to analyze macromolecular helical structures and thus have been utilized in nearly all the related studies in the literature. Herein, the asprepared gels (in the presence of chiral PEAs) underwent CD and UV−vis absorption characterizations in three situations: (situation 1) polymers (derived from freeze-dried gels, the same below) were dissolved in N,N-dimethylacetamide (DMAC) (Figure 3A,B); (situation 2) polymers were dispersed in CHCl3 (Figure 3C,D); and (situation 3) gels directly pressed between two pieces of quartz glass (Figure 3E,F). In addition, situations 2 and 3 only provided qualitative test results. Nonetheless, the spectra data obtained from them were also important because they could provide information much closer to the actual gel state of the products. The CD and UV−vis absorption characterizations of free polymers were conducted quantitatively in solution state

(Figure 3A,B). Achiral gel shows no CD signal over 300 nm in all the three testing situations as mentioned above due to the lack of any chiral factor or asymmetric stimulus during the whole preparation procedure. In contrast, OAPGs obtained from HSSP by using R-PEA (negative CD signals, Figure 3A(a−e)) and S-PEA (positive CD signal, Figure 3A(f)) as chiral additive show intense symmetrical CD signals at about 360 nm. Corresponding UV−vis absorptions were observed at the identical wavelength in both chiral and achiral gels. As a comparison, no CD signal and UV−vis absorption of chiral PEAs could be found above 300 nm (for more discussion, see below). Therefore, we can definitely conclude that the polymer chains (from OAPGs) in solution adopted predominant onehanded helical conformation, which enabled the polymer solutions to show significant CD effect. However, this conclusion cannot be simply extended to the OAPGs because polymer chains are strongly restricted by each other in the gels. Therefore, the OAPGs were further dispersed in CHCl3 by strong ultrasonic for quantitative CD and UV−vis absorption characterizations, as presented in Figure 3C,D. In this measuring situation, the aforementioned nanofibers were broken into smaller polymer aggregates which could be homodispersed in the solvent and kept for a long time. The recorded CD and UV−vis absorption spectra seem quite similar to the spectra recorded in situation 1 (Figure 3A,B), except for a slight red-shift of about 10 nm and the relatively stronger intensity in spectra. These changes can be explained by the difference in the two states of solution and dispersion. Similar phenomena have already been observed and discussed in detail in our earlier articles.35−37 The OAPGs also underwent CD (Figure 3E) and UV−vis (Figure 3F) measurements in gel state by directly pressing between two pieces of quartz glass (situation 3, stated above). The resulting CD spectra were much complicated than the two cases discussed above (Figure 3A,C). Compared with the observations in situation 2 (Figure 3C), the CD signals around 370 nm could be attributed to the one-handed helical conformation of polymers. However, their intensity seemed weaker than the corresponding data obtained in situation 2 due to the different measurement methods. Noticeably, the variation trend among the CD intensities of the samples remained nearly unchanged when compared to Figure 3A,C. This suggests that the chiral helical conformation of polymers constituting the OAPGs changed little, no matter whether they were dissolved or dispersed for CD measurement. Especially interestingly, unexpected strong and wide CD signals can be observed ranging from 400 nm and above (Figure 3E). The CD signals also depended on using (R)-PEA (positive CD signals) or (S)-PEA (negative CD signals) as chiral additive. The observations further indicate the existence of chirality on a larger scale. Taking the SEM images in Figure 2 into consideration, the large-scale chirality could be attributed to the helical nanofibers in OAPGs. It is indicated that chiral transfer and chiral amplification (from small molecular level to nanoscale level) efficiently occurred in the course of HSSPs and in the formed OAPGs. No matter in which situation the OAPGs were measured, the intensity of the CD signals which originated in the one-handed helical structure of polymers (360 nm, in DMAC solutions; 370 nm, in CHCl3 dispersions and the original state of gels) enhanced with the reduction of monomer concentration at the beginning and then reached the maximum value at 0.05 mol/L (Figure 3A,C,E). Afterward, it remained D

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Figure 4. CD spectra of optically active physical gels synthesized in CHCl3 ([M] = 0.05 mol/L) with (R)- or (S)-PEA and dissolved in DMAC with increasing (A) and decreasing (B) temperature. Concentration of the solutions: 10−4 mol/L by monomer units.

Nevertheless, a considerable content of one-handed helical structures still retained well along the polymer chains even at 90 °C. After lowering the temperature to 30 °C, the departed chiral PEA molecules could not all return to the original positions due to large steric hindrance. As a consequence, the CD spectra would not recover to the original intensity. Similar phenomena were also observed in UV−vis absorption spectra (Figure S5). The above observations demonstrate the relatively high thermal stability of the induced preferable helices and meanwhile promote us to consider the question: If all chiral small molecules are washed out of the OAPGs, will the onehanded helical conformations of the polymers still remain? Accordingly, we subsequently made special effort to explore this question, as to be reported later on. As discussed above, we propose that the OAPGs were constructed by one-handed helical substituted polyacetylene in the course of HSSP, with the help of chiral PEAs as helicitydirecting agent. However, there is another possibility; i.e., the preferential helicity may be induced in achiral helical polymerbased gels by chiral additive, as reported in an earlier study.39 We next tried to examine such a chiral induction by immersing the as-prepared achiral physical gel for over 12 h in the same chiral PEA solutions used for preparing OAPGs. The product underwent CD and UV−vis measurements in all the three situations as mentioned above. The relevant spectra are presented in Figure S6. The CD spectra obtained from situation 3 show the lack of CD signal (Figure S6A), which indicates that the polymers in gel state could not been induced to form one-handed helices. In sharp contrast, obvious CD signals were observed in the CD spectra obtained from situations 1 and 2 (Figures S6C and S6E). The results tell us that chiral induction took place only after the gels were dissolved or dispersed. In the latter case, the dispersed gel could be swollen by chiral PEAs. By comparing the observations with Figure 3, the occurrence of HSSP and the formation of onehanded helical polymers and chiral nanofibers in OAPGs are clarified further. In the gel state, polymer chains were strongly restricted by the π−π stacking interaction among themselves due to the large content of phenyl groups (as illustrated in Figure 1B). The helical structures of the polymer chains were locked so firmly that they could not be changed even by a long-time chiral induction. Accordingly, we further hypothesize that the onehanded helical structures of the polymers in OAPGs could possibly remain even after removing all the chiral additives. On the basis of this hypothesis, we further washed out fully the chiral PEAs from the OAPGs by repeatedly immersing the

unchanged from 0.05 to 0.025 mol/L; however, the CD signals weakened by a further reduction in monomer concentration. Combining with the SEM observations, a monomer concentration at 0.05 mol/L seemed to be the most favorable one for preparing the OAPGs. The induced chirality of the macromolecular helices in the HSSPs is mainly determined by two factors: [chiral PEAs]/[M] and [M]. The [chiral PEAs] was kept constant, so the former factor increased with the decrease of the latter one. When we decreased [M], the intensity of CD signals increased at the beginning because [chiral PEAs]/[M] increased. The increase of [chiral PEAs]/[M] means that more chiral PEA molecules can simultaneously interact with monomer molecules and thus transfer “chirality” more efficiently. But if [M] decreases to below a certain degree (0.025 mol/L), the concentration of and the interaction force among the resulting polymer chains will decrease. As a consequence, a weakened CD signal was observed. In order to fully understand the relationship between optical activity of OAPGs and the chiral PEAs, we additionally synthesized a gel also by HSSP process but using racemic PEA ((R)- and (S)-PEA in identical amount) as additive (the gel is named as rac-gel). Just as expected, the CD spectra (Figure S4A) of this gel and the corresponding polymer solution showed no CD signal in its UV−vis absorption range (Figure S4B) as otherwise observed in Figure 3A,E. This result convincingly confirms the helix sense controlling role played by chiral PEAs. It is worth mentioning that strong UV−vis absorption still occurred in the rac-gel, as illustrated in Figure S4B. Referring to our earlier studies dealing with helical polyacetylenes,34,36 this observation indicates that the polymer chains in the rac-gel formed racemic helical structures, i.e., with both L- and R-handed helicity in identical content. Thermal stability of the induced preferential macromolecular helices was also explored by measuring CD and UV−vis absorption spectra at varied temperature (polymer solution in DMAC), as illustrated in Figure 4 (CD spectra) and Figure S5 (UV−vis absorption spectra). The CD intensity decreased with temperature increasing (Figure 4A) and could not completely recover again by decreasing temperature (Figure 4B). But to our delight, we found that even at a temperature up to 90 °C, intense CD effect and UV−vis absorption still occurred. This phenomenon can be explained by the “chiral lock” effect put forward earlier by us.20 Chiral PEA molecules “fled” from the polymer chains upon heating, and the thus-formed bare helices were much easier to undergo conformational transition to a certain degree from one-handed helices to nonhelical structures or even to helical structures with the opposite screw sense. E

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Figure 5. CD (A) and UV−vis absorption (B) spectra of the eluates derived from immersing the optically active physical gels in CHCl3. For (A), (R)- or (S)-PEA was used as chiral additive in gel preparation; for (B), (R)-PEA was used as chiral additive in gel preparation. For the gel prepared by using (S)-PEA as chiral additive, the relevant UV−vis absorption spectra gave the same result and thus presented in Figure S7.

Figure 6. CD (A, C) and UV−vis absorption (B, D) spectra of optically active physical gels before (a, b) and after (c, d) the elimination of (R)-PEA (a, c) or (S)-PEA (b, d) residual. For (A) and (B), the polymer (10−4 mol/L by monomer units) was dissolved in DMAC. For (C) and (D), the gels were directly pressed between two pieces of quartz glass for measurement. All the gels were prepared in a CHCl3 solution of chiral PEAs (chiral PEAs/CHCl3 = 1/100 (v/v)) by the same monomer concentration of 0.05 mol/L.

results clearly show that the peaks of chiral PEAs disappeared in the HPLC spectra of the thoroughly purified OAPGs. Additionally, the dry gels were subjected to a thermal treatment at 200 °C (higher than the boiling point of chiral PEAs) for 30 min in a vacuum. Residual weights of the gels were recorded again, so their mass loss rates can be calculated (Table S2). The mass loss rates (20 ± 1%) of purified OAPGs and achiral gel are quite close. This weight loss is caused by the slow degradation of polymers at such a high temperature. Meanwhile, the mass loss rates (≥40%) of the original OAPGs are much larger (the initial content of chiral PEAs was about 36 wt %, and part of chiral PEAs had leaked from the gel in the drying process). Therefore, the extra weight loss in the original OAPGs should be due to the evaporation of chiral PEAs. The above observations, i.e., the HPLC and the thermogravimetric analyses, directly confirm that all the chiral PEA molecules were removed from OAPGs through the washing process.

OAPGs in a large amount of CHCl3. Moreover, CHCl3 was renewed every day. The content of PEA molecules in the eluate was traced by CD and UV−vis measurements, as shown in Figure 5 and Figure S7. Comparing the spectra with the CD and UV−vis absorption spectra of pure chiral PEAs and the monomer (Figure S8), we can find that chiral PEAs (possibly including a little amount of monomer or oligomer) appeared in the initial eluate. However, all the small molecules were excluded from OAPGs after washing for 5 days, according to the corresponding spectra in Figure 5 and Figure S7. To further confirm the successful elimination of all chiral PEAs, the original OAPGs (without removing chiral PEAs), purified OAPGs (after washing for 5 days in CHCl3), and achiral gel (also washed in the same way) were dried up at 50 °C in vacuum until constant weight. The weights of the dry gels were recorded. Then all the dry gels, together with pure chiral PEAs, underwent HPLC analyses, as shown in Figure S9. The F

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liquid chromatography) analyses were performed on FL2200-2 (FL2200-2 pump and UV detector). Monomer Preparation. The monomer was synthesized according to a method reported earlier,34,35 as briefly depicted below. 3,3,3Triphenylpropionic acid (3.02 g, 10 mmol), isobutyl chloroformate (1.3 mL, 10 mmol), and 4-methylmorpholine (1.1 mL, 10 mmol) were added in THF (100 mL) sequentially. After magnetic stirring for 15 min at 30 °C, propargylamine (0.65 mL, 10 mmol) was dropwise added in the solution. The reaction system was kept under stirring at 30 °C for 4 h, and then the white precipitate was filtered off. The filtrate was diluted with ethyl acetate, washed with 2 M HCl aqueous solution thrice, and then washed with saturated aqueous NaHCO3 to neutralize the solution. Afterward, the solution was dried over anhydrous MgSO4, filtered, and concentrated by a rotary evaporator. The coarse monomer was purified further by recrystallization thrice from THF/n-hexane. The resulting white powder-like crystals were dried in vacuum. Elemental analysis of monomer: Anal. Calcd for C24H21NO: C, 84.92; H, 6.24; N, 4.13. Found: C, 84.87; H, 6.24; N, 4.15. Preparation of Optically Active Physical Gels by HelixSense-Selective Polymerization. In order to synthesize the OAPGs with homogeneous optical activity, a solution of R- or S-PEA (chiral PEAs/CHCl3 = 1/100, v/v) was prepared ahead and kept as the reaction solvent. Then, the solution was equally added into two vessels already loaded with monomer and catalyst, (nbd)Rh+B−(C6H5)4 ([Rh]/[M] = 1/100, mol/mol). After complete dissolution of the chemicals, the Rh catalyst solution was immediately transferred into the tube containing the monomer solution. The polymerization was carried out under N2 at 30 °C for 3 h. Gels were formed gradually along with polymerization procedure. Before freeze-drying, these gels should undergo solvent exchange sequentially by ethanol and distilled water. For the preparation of rac-gel, the reaction solvent was changed to the racemic PEA solution, but the polymerization operation remained exactly the same. The racemic PEA solution was prepared by directly mixing identical volume of aforementioned (R)- and (S)-PEA solutions. Chiral Induction of Achiral Gel. Purified CHCl3 was added into one vessel already loaded with monomer ([M] = 0.05 mol/L) and catalyst ((nbd)Rh+B−(C6H5)4, [Rh]/[M] = 1/100 (mol/mol)). The polymerization progress was carried out under N2 at 30 °C for 3 h providing the achiral gel. Then the gel was immersed in a solution of (R)- or (S)-PEA (chiral PEAs/CHCl3 = 1/100 (v/v), about 80 mL in total) for over 12 h at room temperature. Chiral Memory Experiment. The OAPGs ([M] = 0.05 mol/L) prepared with (R)- and (S)-PEA were separately immersed in 100 mL of pure CHCl3. The washing process was conducted for 5 days by replacing the eluate with pure solvent every day.

Next we measured CD and UV−vis absorption spectra of the OAPGs before and after eliminating chiral PEA molecules, as displayed in Figure 6. Excitingly, the spectra of the polymer solutions (Figure 6A,B) and the corresponding original OAPGs (Figure 6C,D) both strongly demonstrated the chiral memory ability of OAPGs. Moreover, the memory efficiency is about 78% (the average value of OAPGs fabricated by R-PEA and SPEA as chiral additive), which can be estimated according to the CD signal intensity at 360 nm (in polymer solution). This result definitely demonstrates that the chirality of OAPGs can be sufficiently maintained: even without replacing the chiral PEA molecules with achiral reagent. Furthermore, the thermal stability of the memorized one-handed helices was still high, according to the CD and UV−vis absorption spectra shown in Figures S10 and S11. To our knowledge, this kind of chiral memory in optically active gels is extremely rare in the literature and thus quite interesting in polymer chemistry. The OAPGs as a novel kind of material hold bright prospect in practical applications especially in chiral-related fields.

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CONCLUSIONS We established an unprecedented kind of optically active physical gels (OAPGs) constructed solely by one-handed helical substituted polyacetylene through helix-sense-selective polymerization approach. CD and UV−vis absorption spectra demonstrated the one-handed helical conformation of the polymers in OAPGs as well as the helix-sense controlling role of chiral 1-phenylethylamine. SEM images showed that the OAPGs were constructed by helical nanofibers. Combining these two characterizations, chiral transfer and chiral amplification effects during the gel formation process were revealed. The CD and UV−vis absorption spectra measurements conducted at varied temperatures demonstrated that the one-handed helical structures still remained even at a temperature up to 90 °C. After excluding all the chiral additives, the gels retained the optical activity due to the strong intra- and intermolecular π−π interaction between the phenyl groups along the polymer chains. The as-prepared OAPGs are expected to be conveniently applied in diverse domains associated with chirality, especially as novel chiral monolithic columns for HPLC, distinctive chiral reactors toward asymmetric catalysis and helix-sense-selective polymerization of other achiral monomers, etc.

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EXPERIMENTAL METHODS

ASSOCIATED CONTENT

S Supporting Information *

Materials. The Rh catalyst (nbd)Rh+B−(C6H5)4 was prepared according to a procedure reported previously.40 Propargylamine was obtained from Acros Organics. 3,3,3-Triphenylpropionic acid and (R)and (S)-1-phenylethylamine (PEA) were bought from TCI. The other reagents were purchased from Aldrich. All the reagents were directly used without further purification. CHCl3, N,N-dimethylacetamide (DMAC), and ethanol were distilled under reduced pressure before use. Measurements. FT-IR spectra were recorded using a Nicolet NEXUS 870 infrared spectrometer (KBr tablet). 1H and 13C NMR spectra were recorded on a Bruker AV 400 spectrometer. Elemental analysis was performed on an Elementar vario EL cube element analyzer. Circular dichroism (CD) and UV−vis absorption spectra were measured using a Jasco-810 spectropolarimeter. The internal structure of polymer gels was observed with a Hitachi S-4800 scanning electron microscope (SEM). The molecular weights and molecular weight polydispersities were determined by GPC calibrated with polystyrenes, with DMAC as eluent. Raman spectra were measured using a Renishaw inVia Raman microscope. HPLC (high performance

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00612. Data for the gels and corresponding polymers; NMR spectra of the monomer used in this article; FT-IR spectra of monomer and polymer; Raman spectra of freeze-dried gels synthesized with and without chiral additive; CD and UV−vis absorption spectra of the racgel synthesized with racemic PEA solution; UV−vis absorption spectra of OAPGs dissolved in DMAC at varied temperature, etc. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-10-6443-5128; Tel: +86-10-6443-5128; e-mail: [email protected]. G

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Macromolecules Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21474007, 21274008, and 21174010), the Funds for Creative Research Groups of China (51221002), and the “Specialized Research Fund for the Doctoral Program of Higher Education” (SRFDP 20120010130002).

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DOI: 10.1021/acs.macromol.6b00612 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b00612 Macromolecules XXXX, XXX, XXX−XXX