Chiral Helical Polymer Nanomaterials with Tunable Morphology

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Chiral Helical Polymer Nanomaterials with Tunable Morphology: Prepared with Chiral Solvent To Induce Helix-Sense-Selective Precipitation Polymerization Yingjie Zhang,†,‡ Jianping Deng,*,†,‡ and Kai Pan*,† †

State Key Laboratory of Chemical Resource Engineering and ‡College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China

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S Supporting Information *

ABSTRACT: We report an unprecedented strategy for using achiral monomer to construct chiral helical polymer nanomaterials with tunable morphologyhelix-sense-selective precipitation polymerization induced by a chiral solvent (HSSPPCS). CHCl3, chiral α-pinene, and an alkanol were employed to constitute a solvent mixture for performing HSSPP-CS. (R)- or (S)-α-pinene worked as chiral source and transferred its chirality to the resulting helical polyacetylenes derived from achiral monomer in the course of polymerization, thereby forming chiral helical polymers and endowing the nanomaterials thereof with optical activity. Simply changing solvent composition provided chiral polymer products varying in morphology from fibrous to rod-like and finally to spherical morphology. This is the first report dealing with chirality transfer from chiral solvent to induce helix-sense-selective precipitation polymerizations. The study establishes a straightforward and effective alternative for making use of achiral monomers to construct chiral helical polymer nanomaterials with diverse morphology.

1. INTRODUCTION Chiral polymer nanomaterials (CPNMs) have gained increasing attention and have constituted an active research area due to the reasonable combination of chiral polymers and specific nanoarchitectures.1−5 Up to now, CPNMs with various morphologies have been developed with examples like tubes,6,7 rods,8 fibers,9,10 sheets,11 films,12 and particles.13 To prepare CPNMs with specific morphologies, a variety of preparation methods have been established for instance selfassembly,8,9,14 templating,5,7 and various polymerization processes (emulsion polymerization,15 suspension polymerization,16 and precipitation polymerization17). Typical CPNMs are spherical particles constructed by amino acid-based polymers,18 helical polyacetylenes,19 and helical polyisocyanides.20 Among the interesting CPNMs, those constructed by chiral helical polymers21−25 are especially attractive because of the well-known chiral amplification effect.4,21,23,26 Consequently, the distinctive properties of helical polymers together with specific morphologies facilitate CPNMs to be used as catalysts for asymmetric catalysis,27 as chiral seeds for enantioselective crystallization,28 and as carriers for enantiodifferentiating release.29 The most straightforward way to prepare chiral helical polymer nanomaterials is by directly polymerizing chiral monomers. However, the limited variety and high cost of chiral monomers have hindered the further development in synthetic helical polymers. Accordingly, utilizing achiral monomers to prepare chiral helical polymer nanomaterials is of great importance.30,31 © XXXX American Chemical Society

Helix-sense-selective polymerizations (HSSPs) of achiral monomers have proved to be effective ways to prepare chiral helical polymers with controlled screw sense.32−34 Up to date, HSSPs can be realized majorly through three approaches according to the chiral sources used, namely, chiral catalytic systems,35 chiral structure-directing agents,26 and external asymmetric field effects.36,37 However, most of the HSSPs reported so far were performed in solutions, thereby providing bulk polymers without a specific morphology and architecture.30,31,38 In our preceding studies, we have prepared chiral helical polyacetylene nanomaterials and hybrid nanomaterials39 derived from achiral monomers by combining HSSP respectively with emulsion polymerization (HSSEP),34 precipitation polymerization (HSSPP),10 and dispersion polymerization (HSSDP).40 Besides, recent studies demonstrate that chirality can smoothly transfer from solvent to preformed polymers.41−45 Stimulated by the report and meanwhile based on our earlier studies,10,34,39,40 we in this study develop a new strategy: helix-sense-selective precipitation polymerization using chiral solvent as chiral source (defined as HSSPP-CS) for preparing chiral helical polymer nanomaterials directly starting from achiral monomer. In the HSSPP-CSs, CHCl3, chiral α-pinene, and an alkanol constituted a ter-solvent system. CHCl3 worked as good solvent while chiral α-pinene Received: September 17, 2018 Revised: October 10, 2018

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DOI: 10.1021/acs.macromol.8b02008 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. A Proposed Schematic Strategy for HSSPP-CS

Table 1. Data of Polymers P1 Prepared with Varied Solvent Composition curve Figure Figure Figure Figure Figure Figure Figure Figure Figure

1, 1, 1, 1, 4, 4, 4, 4, 4,

curve curve curve curve curve curve curve curve curve

e a b, or Figure 4, curve a d b b′ c d e

CHCl3 (mL)

1R (mL)

isobutanola (mL)

Mnb

Mw/Mnb

morphologyc

6 5 4 3 3 3 2 1.5 1

0 1 2 3 2 2 (1S) 2 2 2

0 0 0 0 1 1 2 2.5 3

13200 9700 9400 8000 7100 6000 4300 3800 3500

1.36 1.47 1.22 1.89 1.56 1.43 1.30 1.08 1.09

nanofiber nanofiber nanofiber nanofiber nanorod nanorod nanorod nanoparticle nanoparticle

a

For detailed polymerization conditions, refer to Figures 1 and 4. bDetermined by GPC (polystyrene as standard, DMAC as eluent). cReferring to SEM images. 2.3. Preparation of Chiral Helical Polyacetylene with Diverse Morphology. The achiral acetylene monomers were synthesized according to the method reported previously.47 The nanomaterials were prepared through the precipitation polymerization process. Typical precipitation polymerization was performed as follows. Monomer (0.085 g, 0.25 mmol) was weighed and charged in a vessel, in which solvents CHCl3, chiral α-pinene, and isobutanol were added to dissolve the monomer. The volume of chiral α-pinene was constantly 2 mL and the total volume of solvents kept as 6 mL but with different solvent composition. After complete dissolution, the solution was transferred to a tube already loaded with (nbd)Rh+B−(C6H5)4 catalyst, 0.0013 g ([M]/[Rh] = 100/1, mol/mol). The polymerization was performed at 30 °C for 3 h. The whole procedure for precipitation polymerization was conducted under a N 2 atmosphere. After polymerization, the polymer products were isolated through filtration and washed sequentially by CHCl3 and ethanol. For preparing racemic helical polymers, precipitation polymerization was accomplished in similar procedures but using an equal amount of (S)α-pinene and (R)-α-pinene or without using them. The same procedure was also taken for performing precipitation polymerizations in ter-solvent systems.

and alkanol worked as poor solvents. Chirality transferred from chiral α-pinene to the resulting polymers in the course of polymerization, directly providing chiral helical polyacetylenebased nanomaterials. Using chiral solvent as chiral source allows us to open up a new way toward HSSPs. Moreover, various chiral helical polyacetylene nanomaterials were constructed with diverse morphology from fibrous to rod-like and finally to spherical morphology by simply adjusting the solvent composition.

2. EXPERIMENTAL SECTION 2.1. Materials. Catalyst (nbd)Rh+B−(C6H5)4 was synthesized according to the method reported earlier.46 Propargylamine was obtained from Acros Organics. 3,3,3-Triphenylpropionic acid was bought from J&K, and (S)- and (R)-α-pinene were purchased from TCI. All the reagents were directly used without further purification. Solvents, CHCl3, N,N-dimethylacetamide (DMAC), and isobutanol were purified by distillation before use. 2.2. Measurements. Circular dichroism (CD) and UV−vis absorption spectra were measured on a Jasco-810 spectropolarimeter by dissolving the polymers in DMAC by ultrasonic (approximately c = 10−4 mol/L, by monomer unit). The polymer products were observed by a JSM-7001F (JEOL) scanning electron microscope (SEM). Fourier-transform infrared (FTIR) spectra were recorded on a Nicolet NEXUS 670 spectrophotometer with KBr tablet. 1H and 13C NMR spectra were characterized on a Bruker AV 400 spectrometer. Raman spectra were measured by a microscopic confocal Raman spectrometer−spectrophotometer (Renishaw). The number-average molecular weight and molecular weight polydispersities were determined by GPC calibrated with polystyrenes, using DMAC with LiBr (300 mg/L) as eluent.

3. RESULTS AND DISCUSSION 3.1. Establishment of HSSPP-CS. To achieve the goal of HSSPP-CS, an achiral acetylenic monomer containing three phenyl groups (M1, as presented in Scheme 1) was prepared47 and identified by FTIR (Figure S1, Supporting Information, the same below) and NMR (Figure S2) techniques. The characterizations demonstrate the successful preparation of the monomer. To understand the basic features of the polymer (P1) derived from the monomer (M1), the monomer first underwent routine solution polymerization in pure CHCl3. B


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The resulting polymers were subjected to CD and UV−vis measurement (Figure 1), and apparent CD signals at about 360 nm (Figure 1A) can be observed in all the polymers prepared with chiral additives. Chiral α-pinenes, 1R and 1S, provided polymers with mirror symmetric CD effects as anticipated (Figure 1A, curves b and c). As control tests, pure 1R and 1S were also subjected to CD and UV−vis measurement, and no CD effect can be observed from 300 to 500 nm (Figure S4A). This result indicates that the CD signals in Figure 1A are derived from induced one-handed helical structures in the polymers. In other words, HSSPP-CS was accomplished successfully with chiral α-pinenes as chiral solvent, which can induce predominant helicity in the original racemic helical substituted polyacetylene. To better understand the chirality transfer, the polymer prepared in pure CHCl3 (racemic helical polymer, see above) was dissolved in the same solvent system consisting of CHCl3/ 1R = 4/2 (v/v) which was used as mixed solvent for performing HSSPP-CS. Just as expected, the polymer failed to exhibit optical activity in the mixed solvent (Figure S5). The result demonstrates that in the above HSSPP-CSs chirality transfer occurred in the course of polymerization rather than a postinduction process. On the basis of this observation, we propose that chiral α-pinene underwent noncovalent interactions (van der Waals, −CH/π, and π/π) with the pendant phenyl groups of M1 in the process of polymerization, which induced the otherwise racemic helical polymer to form onehanded helicity and endowed it with optical activity. Theoretically speaking, the chiral α-pinene cannot form hydrogen bonds with amide groups due to its intrinsic character, so chirality transfer only can be realized through the noncovalent interactions aforementioned. Accordingly, a possible mechanism for chirality transfer in the HSSPP-CSs is illustratively presented in Scheme 1. To further ascertain the proposition, another acetylenic monomer (defined as M2, as presented in Figures S6 and S7) having similar chemical structure to M1 but bearing only two phenyl groups was prepared. FTIR (Figure S6) and NMR spectra (Figure S7) together demonstrate the successful preparation of M2. In the same manner as for M1, M2 underwent HSSPP-CS in CHCl3/chiral α-pinene = 4/2 (v/v), and the corresponding polymer (P2) was obtained smoothly. FTIR spectra (Figure S6) of P2 prove the polymerization of M2 preliminarily. GPC measurement was further performed to ensure the polymerization. The average molecular weight of P2 was 10300, and the corresponding Mw/Mn was 1.14. CD and UV−vis absorption spectra of P2 (Figure S8) demonstrate that this polymer failed to form one-handed helical structure because of the absence of CD signals. A comparison between the chemical structures of the two monomers (M1 vs M2) reveals that the phenyl groups play crucial roles in the HSSPPCS and the lack of a third phenyl group resulted in insufficient −CH/π and/or π/π interactions between chiral α-pinene and M2. This further proves our hypothesis about chirality transfer as mentioned above. Unfortunately, because of the unsatisfactory solubility of the chiral polymers, the 1H NMR spectrum cannot provide more information for further determining the specific interactions between phenyl groups and chiral α-pinene. 3.2. Preparation of Chiral Nanomaterials with Varied Morphology by HSSPP-CS. The polymers P1 prepared in solvent systems containing CHCl3/chiral α-pinene at varied ratio were subjected to SEM observation to investigate their

The resulting polymer possesses poor solubility in usual organic solvent, which may be caused by the strong inter- and intramolecular π/π interaction. Nonetheless, it is largely soluble in DMAC. FTIR spectra of M1 and the corresponding polymer are shown in Figure S1. Characteristic peaks at 2114 and 3284 cm−1 in the spectrum of monomer are ascribed to CC and CH stretching vibrations. The disappearance of the two characteristic peaks in the spectrum of the resulting polymer preliminarily proves the occurrence of polymerization. Moreover, GPC was performed to further confirm the successful polymerization. The number-average molecule weight (Mn) of the soluble fraction of P1 is 13200, and the corresponding Mw/Mn is 1.36 (Table 1). This evidently verifies the success in polymerizing M1. Then polymer P1 was dissolved in DMAC for CD and UV− vis measurements, which have been proved to be efficient methods to analyze helical polymers and chiral materials thereof.10,13 As shown in Figure 1 (curve e), intense UV−vis

Figure 1. CD (A) and UV−vis absorption spectra (B) of polymer prepared in solvent system consisting of varied ratio of CHCl3 to chiral α-pinene (1R and 1S). For (A) and (B), the polymers were dissolved in DMAC by ultrasonic.

absorption occurred at 350 nm in Figure 1B, but no noticeable CD signal can be observed in Figure 1A. The results demonstrate that the polymer prepared in pure CHCl3 solvent formed helical structures but with equal amounts of right- and left-handed screw sense, referring to our earlier studies. Namely, this polymer formed racemic helical structures, as frequently observed in earlier optically inactive helical polymers.10,13,40 Next, (R)-α-pinene (1R) and (S)-α-pinene (1S) were separately employed as chiral solvent to conduct HSSP (HSSPP-CS in this study) for preparing helical substituted polyacetylene with predominantly one-handed helices. It is reported that chirality sufficiently transferred from chiral terpenes (e.g., pinenes and limonenes) to achiral π-conjugated polymers via weak noncovalent forces including van der Waals, π/π, and −CH/π interactions, resulting in polymers with chirality.41−45 So herein solvent systems consisting of CHCl3 and chiral α-pinene at varied ratio (CHCl3/chiral α-pinene = 5/1, 4/2, and 3/3, v/v, the same below) were taken for conducting HSSPP-CS of M1 in subsequent studies. GPC data (Table 1) and FTIR spectra measurement (Figure S3) prove the occurrence of polymerization. In addition, in the FTIR spectrum of the polymer prepared in CHCl3/1R = 4:2 (Figure S3, curve a), the newly emerging characteristic peak at 2969 cm−1 can be ascribed to the asymmetric stretching vibration of −CH3, demonstrating the existence of chiral α-pinene in the resulting polymers. C


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Figure 2. Photos and SEM images of chiral helical substituted polyacetylene nanofibers prepared in CHCl3/chiral α-pinene solvent system with different solvent composition: (a) CHCl3/1R = 5/1; (b) CHCl3/1R = 4/2; (c) CHCl3/1R = 3/3; (d) CHCl3/ 1S = 4/2; all in (v/v).

Scheme 2. (A) Illustrative Strategy for Preparing Chiral Helical Polyacetylene Nanomaterials with Tunable Morphology in Ter-Solvent Systems; (B) Proposed Mechanism for the Adjustment of Polymeric Nanomaterials’ Morphology

The results above encourage us to further hypothesize that adding another poor solvent in the precipitation polymerizations may regulate the morphology of polymer products. To justify the hypothesis, isobutanol was employed as a second poor solvent to form a ter-solvent system and from which interesting results have been achieved, as schematically summarized in Scheme 2A and as to be reported in detail below. As shown in Scheme 2A, chiral helical polyacetylene nanomaterials (P1) with tunable morphology were obtained by simply changing the solvent composition. The volume of chiral α-pinene was kept constant, and morphology adjustment was achieved by simply increasing the ratio of isobutanol. Specific digital photos and SEM images of polymers prepared with different solvent compositions are shown in Figure 3. A comparison between the digital photos of precipitation polymerization sample without (Figure 3A) and with (Figure 3B) isobutanol as poor solvent reveals that the addition of isobutanol facilitated the resulting polymer to precipitate out. Moreover, ter-solvent systems with varied compositions resulted in different precipitation phenomena. As shown in the photos (Figure 3A−C), the precipitated polymers existed throughout the whole sample. As a contrast, the polymers in

micromorphologies. The SEM images and corresponding digital photos are exhibited in Figure 2. All the chiral nanomaterials show yellowish color, which is frequently observed in helical polyacetylenes and originated in the helical polymer structures.10,13,15 Our earlier work10 shows that the P1 prepared in pure CHCl3 solvent formed visible polymeric gels constructed by nanofibers. Nonetheless, the present polymers prepared in the CHCl3/chiral α-pinene solvent system precipitated out from the solvent system rather than forming visible gels. This can be explained by the roles of αpinene, which simultaneously works as a chiral source and poor solvent for conducting precipitation polymerizations. In other words, α-pinene promotes polymer chains to precipitate out from solvent, preventing the polymer from forming regular gels. Nevertheless, the SEM images in Figure 2 also reveal that at the microscopic level the addition of chiral α-pinene did not affect the polymer in forming nanofibers. Interestingly, the formed nanofibers exhibited helical architectures, which may arise from the helical twining of polymer chains. The twining of polymer nanofibers keeps similar upon changing solvent composition, which may be caused by the quick precipitation process of polymers. D


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Figure 3. Digital photos and SEM images of polymers prepared in ter-solvent systems with varied solvent composition of CHCl3/1R/isobutanol: (A) 4/2/0; (B) 3/2/1; (C) 2/2/2; (D) 1.5/2/2.5; (E) 1/2/3, in v/v/v.

Figures 3D and 3E precipitated and mainly deposited at the bottom of the reaction vessel. Comparing the SEM images in Figures 3A to 3E, we can find a gradual change in the polymers morphology from fibrous to rod-like morphology and eventually to spherical morphology. Moreover, with increasing the ratio of poor solvent (isobutanol), the polymers tended to transform from continuous polymeric bundles (60 nm in diameter) to individual polymeric aggregates (250 nm in diameter and 1 μm in length) and further broke into nanoparticles (300 nm in diameter). On the basis of the above observation and discussion, we put forward a mechanism addressing the formation of chiral helical polyacetylene nanomaterials with varied morphologies (Scheme 2B). As has been well demonstrated earlier, polymers precipitate out from solvent once growing to a certain length in precipitation polymerizations.17 Solvent systems composed of more good solvent are favorable to form polymers with relatively higher molecular weight (see Table 1). As a result, such longer polymer chains tend to align parallel to form fibrous morphology (Figure 3A); certainly, the tendency of the pendent phenyl groups along the polymer chains to form π−π interactions also makes some contribution to this result. This has been reported in our earlier study.10 When the ratio of poor solvent (isobutanol) is increased, the polymers tend to precipitate out earlier. This does not benefit polymer chains to grow longer and to combine with each other; therefore, the polymers formed rod-like or spherical morphologies instead of continuous fibers. Detailed data from GPC measurement are presented in Table 1. Even though the GPC data only reflect the soluble part of the obtained polymers, they support our analysis to a quite large degree. Apart from molecular weight affecting the resulting polymer morphology, polymer chains stretched or not also should contribute to it. Generally, more good solvent facilitates the polymer chains to be stretched freely, while more poor solvent promotes polymer chains to aggregate. Reasonably, the former case leads to products in fibers while the latter case provides rod-like and even particles.

The above analyses are illustratively presented in Scheme 2B, proposing a possible mechanism for forming helical polymer nanomaterials with varied morphology. 3.3. Helical Polymer Chains Constituting Nanomaterials with Varied Morphologies. Besides the effects on polymers’ morphology, isobutanol was also found to largely influence helical structures of the obtained polymers. As mentioned above (Figure 1), chirality successfully transferred from chiral α-pinene to the resulting polymers in the course of polymerization. Chirality transfer is also expected in the polymers obtained from the ter-solvent mixture systems. To justify this consideration, the products from the ter-solvent systems were subjected to CD (Figure 4) and UV−vis

Figure 4. CD spectra of polymers prepared in ter-solvent system with different solvent composition: (A) CHCl3/1R/isobutanol = 4/2/0 (a); 3/2/1 (b); 3/2(1S)/1 (b′); 2/2/2 (c); (B) CHCl3/1R/ isobutanol = 1.5/2/2.5 (d); 1.5/2(1S)/2.5 (d′); 1/2/3 (e); 1/ 2(1S)/3 (e′) (all in v/v/v). The polymers were dissolved in DMAC by ultrasonic (c = 10−4 M, by monomer units).

absorption spectra (Figure S9) measurement. As shown in Figure 4 and Figure S9, remarkable enhancement in intensity of CD effects can be easily distinguished with the addition of isobutanol, that is, [CHCl3/chiral α-pinene/isobutanol = 3:2:1 and 2:2:2 (v:v:v)] (Figure 4A). The CD and UV−vis measurement was repeated for three times, and the spectra E


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Macromolecules kept nearly constant (all the spectra were measured under nearly the same conditions). Namely, chirality transfer also occurred in the ter-solvent mixture systems, and chirality enhancement can be observed in the polymer products. Furthermore, the solvent system with 1R resulted in polymers showing negative CD signals, while the solvent systems with 1S resulted in polymers showing positive CD signals. The findings keep the same as in the solvent systems without adding isobutanol, which have been reported above. As discussed above and outlined in Scheme 2B, polymer chains tend to aggregate into individual nanorods or nanoparticles upon increasing the ratio of isobutanol. Changes in diameter of the obtained polymers from 60 nm (Figure 3A) to about 250 nm (Figures 3B and 3C) demonstrate the intensified aggregation of polymer chains. The aggregation of polymer chains helps to strengthen the inter- and intramolecular π/π conjugation among the phenyl groups and thus reinforce the chiral interactions between chiral α-pinene and phenyl groups. Accordingly, CD signals with enhanced intense were observed, as stated above. Similar phenomena have been observed previously in the literature.48−50 The stereoregularity of the polymer chains can be determined by NMR and Raman spectra measurements.51 NMR spectra failed to provide useful information because of the poor solubility of the obtained polymers. Raman spectra was carried out, as illustrated in Figure S10 (polymers prepared in CHCl3/1R/isobutanol = 3/2/1 (v/v/v) and in pure CHCl3). The peaks appeared at 1328 and 1644 cm−1 are assigned to cis C−C and CC. Intensity increase in the two peaks signifies the increase of cis content in the polymer chains, and higher cis content is favorable for the polymers to form regular helical structures. The result is in accordance with CD and UV−vis characterization. A further increase in isobutanol amount led to unexpected results. When the amount of isobutanol was further increased to CHCl3/1R/isobutanol = 1.5/2/2.5 (v/v/v), chirality inversion occurred to the polymer. Normally, 1R resulted in polymers with negative CD signals, but polymers with positive CD signals were prepared in the specific solvent composition (curve d, Figure 4B). This phenomenon shows the helical structure of polymer chains transformed to the opposite helicity. Similar phenomena dealing with chirality inversion have been reported,52,53 which may be induced by π−π stacking;54 however, the exact reason is not clear at present yet. When the ratio of isobutanol was increased to CHCl3/1R/ isobutanol = 1/2/3 (v/v/v), chirality inversion still can be observed but the intensity of CD signals weakened for a certain degree (curve e, inset of Figure 4B). To examine the fancy phenomena, 1S was used as chiral solvent instead of 1R for performing HSSPP-CS of monomer M1. We found chirality inversion also occurred, irrespective of the chiral source (curves d′ and e′, Figure 4B). The corresponding UV−vis absorption spectra are presented in Figure S9B. In consideration of the occurrence of chirality inversion, we further tested the stability of the helical structures of polymers prepared in CHCl3/1R/isobutanol = 3/2/1 (v/v/v) and CHCl3/1S/isobutanol = 3/2/1 (v/v/v) by measuring CD and UV−vis spectra at varied temperature (polymer dissolved in DMAC). As illustrated in Figure 5 (CD spectra) and Figure S11 (UV−vis spectra), the CD signals and UV−vis absorptions decreased remarkably when temperature increased from 20 to 90 °C (Figure 5A). Nonetheless, even at 90 °C, considerable CD signals and UV−vis absorptions still remained. These

Figure 5. CD spectra of polymers prepared in CHCl3/1R/isobutanol = 3/2/1 (v/v/v) and CHCl3/1S/isobutanol = 3/2/1 (v/v/v) upon increasing temperature (A) and decreasing temperature (B). Measured by dissolving the polymers in DMAC.

phenomena reveal that elevated temperature can destroy the preferably induced one-handed helices, and the polymer chains underwent conformational transition from one-handed helical conformations to nonhelical conformations. But delightfully, a considerable amount of one-handed helical structures still retained in the polymer chains even at a high temperature up to 90 °C. As demonstrated in Figure 5B and Figure S12, the CD signals and UV−vis absorptions cannot recover completely upon cooling. The phenomena can be explained by the “chiral lock” effects proposed by us in early work.55 As discussed above and illustrated in Scheme 2, 1R/1S interacted with polymers’ phenyl groups and precipitated out together with the polymer chains during polymerization. The chiral molecules fled out from their original positions while heating, which is not favorable for the polymers to keep the preferentially induced one-handed helicity. When decreasing temperature again, most of the chiral molecules failed to return to their original positions due to the huge steric hindrance of polymers’ pendant groups, and thus only part of CD effects recovered. To further examine the chiral induction effects of chiral αpinenes, polymerizations were performed in solvent systems consisting of CHCl3/1R/1S/isobutanol = 3/1/1/1 (v/v/v/v) and CHCl3/isobutanol = 3/3 (v/v). The as-obtained polymers were characterized by CD and UV−vis spectra measurement, as shown in Figure S13. As anticipated, no CD effect can be observed, but strong UV−vis absorptions demonstrate the formation of equal amount of left- and right-handed helices in the polymers. The phenomena clearly verify the crucial effects of 1R or 1S as chiral source for inducing the polymers to take predominant one-helicity. 3.4. Effects of Different Alkanols in HSSPP-CS. As discovered above, isobutanol plays important roles for the polymer to form nanoarchitectures with varied morphology. In the subsequent experiments, alkanols with different polarity; that is, methanol, ethanol, isopropanol, and isobutanol were employed to perform HSSPP-CS in mixed solvents with different composition. Detailed results will be discussed next. First, solvent systems consisting of CHCl3/1R/alkanol = 3/ 2/1 (v/v/v) were taken to accomplish HSSPP-CS. The CD and UV−vis spectra of the obtained polymers are presented in Figure 6. As shown in Figure 6, all the polymers synthesized with different alkanols exhibit intense CD effects (Figure 6A) and UV−vis absorption (Figure 6B) at around 360 nm. The phenomena demonstrate the successful preparation of chiral helical polymers using different types of alkanols. Moreover, the achiral alkanols could not affect the helicity of the polymers. F


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alkanols benefit the transfer of chirality from solvent to polymers.

4. CONCLUSIONS We prepared chiral helical polyacetylene nanomaterials with tunable morphology by directly polymerizing achiral acetylenic monomer via a new strategyhelix-sense-selective precipitation polymerization induced by chiral solvent (HSSPP-CS). Chirality transferred from chiral α-pinenes to polymers in the course of polymerization, thus endowing the polymer products with optical activity. Chirality enhancement and chirality inversion were observed under certain solvent compositions. Moreover, adjustment of polymers’ morphology was achieved, accompanied by the solvent chirality transfer effects. By changing solvent composition, chiral polyacetylene nanomaterials with adjustable morphology from fibrous to rod-like and finally to spherical morphology can be prepared. Poor solvents with appropriate amount and lower polarity help improve the efficiency of chirality transfer. This contribution provides a versatile and straightforward platform for utilizing achiral monomers to construct chiral helical polymer nanomaterials with tunable morphologies.

Figure 6. CD (A) and UV−vis (B) spectra of polymers synthesized in CHCl3/1R/alkanol = 3/2/1 (v/v/v) with different achiral poor solvents. Measured by dissolving the polymers in DMAC.

On the basis of the fact that all polymers obtained in CHCl3/1R/alkanol = 3/2/1 (v/v/v) exhibited strong CD effects, we further increased the ratio of alkanol to CHCl3/1R/ alkanol = 2/2/2 (v/v/v) to further investigate the effects of alkanols. To our surprise, CD effects only can be observed in the polymers with isopropanol and isobutanol as poor solvents (Figure 7). Keeping the amount of alkanols constant, the

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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.8b02008. FTIR spectra of M1 (M2) and corresponding polymers, 1 H NMR and 13C NMR spectra of M1 and 1H NMR spectrum of M2, CD and UV−vis absorption spectra of chiral α-pinene, P1 dissolved in CHCl3/1R = 4:2 (v:v), P2 prepared in CHCl3/1R = 4:2 (v:v), P1 prepared in ter-solvent system with different solvent composition, UV−vis absorption spectra of P1 measured at increasing and decreasing temperature. Raman spectra of P1, SEM of P1 prepared with different alkanols (PDF)

Figure 7. CD (A) and UV−vis (B) spectra of polymers synthesized in CHCl3/1R/alkanol = 2/2/2 (v/v/v) with different achiral poor solvents. Measured by dissolving the polymers in DMAC.

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AUTHOR INFORMATION

Corresponding Authors

disappearance of CD effects with methanol and ethanol as poor solvents demonstrates that alkanols with stronger polarity are not favorable for constructing one-handed polymer helical structures. Moreover, through the comparison between Figures 6 and 7, we found that increasing the amount of alkanols resulted in complete destruction (methanol and ethanol) of the one-handed helical structures. The failure for the polymers to form one-handed helical structures should be caused by the excess formation of hydrogen bonds between polar alkanols and pendant amide groups along polymer chains. In conclusion, the polarity and amount of alkanols both affect chirality transfer remarkably. Besides the influence on chirality transfer, different alkanols may also influence on morphology adjustment of the polymer nanomaterials. Corresponding SEM images are presented in Figure S14. The SEM images reveal the morphologies changed a little when using different types of alkanols; namely, all the polymers formed rod-like morphology, but not as satisfying as using isobutanol as achiral poor solvent. Summarily, the alkanols help promote chirality transfer, but higher solvent polarity and overuse of alkanols are disadvantageous for the polymers to form one-handed helical structures; in other words, lower solvent polarity and appropriate amount of

*E-mail: [email protected] (J.D.). *E-mail: [email protected] (K.P.). ORCID

Jianping Deng: 0000-0002-1442-5881 Kai Pan: 0000-0003-4449-9766 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 (21774009 and 21474007). ABBREVIATIONS CPNMs, chiral polymer nanomaterials; HSSP, helix-senseselective polymerization; HSSPP-CS, helix-sense-selective precipitation polymerization induced by chiral solvent; HSSPP, helix-sense-selective precipitation polymerization; HSSEP, helix-sense-selective emulsion polymerization; HSSDP, helix-sense-selective dispersion polymerization; 1R/ 1S, (R)-/(S)-α-pinene; M1 (M2), monomer 1 (2); P1 (P2), polymer 1 (2). G


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