Micelle-provided microenvironment facilitating the formation of single-handed helical polymer-based nanoparticles

Abstract

Microenvironments have been found to play critical roles that cannot be realized in bulk reaction systems. Optically active particles, particularly those constructed by chiral helical polymers, have attracted much interest. However, most of the optically active polymer particles were derived from chiral monomers. In the present study, achiral substituted acetylene monomer underwent polymerizations in micelles (emulsion polymerizations) and in solution (solution polymerizations) in the presence of rhodium catalyst and chiral additive. The emulsion polymerizations provided optically active polymer nanoparticles consisting of helical polymers forming predominantly one-handed screw sense, while the corresponding solution polymerizations provided helical polymers containing both right- and left-handed helices in identical content (racemic helical polymers). The exciting finding of helix-sense-preferring effect in emulsion polymerizations is due to the specific spatial microenvironment offered by the micelles. To further improve the thermal stability of the induced macromolecular helicity, core/shell structured particles were prepared by successive emulsion polymerization route, in which the shells provided protection for the cores.

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Introduction

Spatial microenvironment confinement makes it possible to realize reactions otherwise impossible in conventional bulk solutions^1,2 and thereby to prepare advanced materials.^3,4 Microenvironments can be provided by nanotubes/tunnels,⁵ molecular sieves,⁶ and gels.⁷ In polymer science, spatial confinement effects have been used to prepare novel materials⁸ and improve properties of polymers.⁹ Despite enormous efforts focused on investigating the microenvironment confinement effects on the primary structures of polymers, no study has been devoted to systemically exploring their effects on the secondary structure of polymers.

Optically active helical polymers have gathered ever-increasing attention due to their intriguing helical structures¹⁰ and significant potential applications in diverse areas such as chiral recognition,¹¹ chiral separation,¹² and asymmetric catalysis.¹³ In the present study, we found that spatial microenvironment confinement promoted racemic helical polymers (helical polymers with both left- and right-handed screw sense in the same mixture) to take on one predominant handedness. Taking advantage of this intriguing effect, we prepared optically active polymer particles derived from achiral monomer. The underlying strategy is outlined in Scheme 1. When achiral acetylenic monomer (M) underwent emulsion polymerization in the presence of rhodium catalyst and chiral additive (CA, Scheme 1A), helical polymers adopting predominantly one-handed screw sense were obtained, thus providing optically active polymer nanoparticles. In sharp contrast, when the polymerization system was transferred to solution (Scheme 1B), helical polymers were formed but containing equal amounts of left- and right-handed helicity; more precisely, no chiral induction occurred in the solution polymerization even in the presence of chiral additive; it was just as if the solution polymerizations occurring without chiral additive added.¹⁴ The intriguing findings demonstrate the significant roles played by the micelles, which provided spatial confinement effects for inducing preferential helicity from the racemic helical polymer chains. Based on these findings, this study further provides a novel strategy for preparing optically active helical polymers and nanoparticles thereof by starting from achiral monomers. To further improve the thermal stability of the induced macromolecular helicity, core/shell nanoparticles were successfully prepared, as illustrated in Scheme 1A.

Scheme 1 A schematic showing polymerization of achiral monomer (M) in the presence of chiral additive (CA): (A) emulsion polymerization (HSSEP); (B) solution polymerization; route (A) also includes the preparation of optically active core/shell nanoparticles.

Herein, it is worth mentioning that elegant methodologies have been established to control the helicity of synthetic helical polymers, i.e., helix-sense-selective polymerizations (HSSPs),¹⁵ which are of great importance for realizing the same target using achiral monomers instead of expensive chiral counterparts. In the preceding studies, we established the first helix-sense-selective polymerization in chiral micelles^16,17 and the first helix-sense-selective precipitation polymerization¹⁸ for constructing optically active polyacetylene particles. The present strategy is essentially different from the earlier studies and worth being particularly highlighted—the spatial microenvironment provided by the micelles plays a vital role in controlling the screw sense of the helical polymers formed in situ inside the micelles. Compared to the counterparts in the literature, the present strategy shows a clear advantage: to directly provide optically active nanoparticles starting from achiral monomers. This cannot be achieved by usual HSSP processes.

Experimental

Materials

Solvents were distilled under reduced pressure under N₂ atmosphere. Deionized water was used for all the emulsion polymerizations. The achiral monomer was synthesized according to a previous report.¹⁴ Triton X-100 (polyethylene glycol tert-octylphenyl ether) (Alfa), N,N-dimethylaniline (DMA) (Alfa), (R)-(+)- or (S)-(−)-phenethylamine (R-PEA and S-PEA) (Aldrich), D- or L-menthol (TCI), [(nbd)RhCl]₂ (Aldrich) and phenylacetylene (Aldrich) were used as received. Benzoylperoxide (BPO) was obtained from Beijing Chemical Reagent Company and purified by recrystallization before use. Methyl methacrylate (MMA) was bought from Aldrich and purified by distillation before use. (nbd)Rh⁺B⁻(C₆H₅)₄ was prepared as reported in the literature.¹⁹

Measurements

FT-IR spectra were obtained on a Nicolet NEXUS 670 spectrophotometer. Circular dichroism (CD) and UV-Vis absorption spectroscopy measurements were conducted on a Jasco 810 spectropolarimeter. Molecular weights and molecular weight polydispersities were determined by gel permeation chromatography (GPC, Shodex KF-850) column calibrated using polystyrenes with THF as eluent. Transmission electron microscopy (TEM) was performed on the polymer emulsions using a Hitachi H-800 electron microscope after a moderate dilution with deionized water.

Helix-sense-selective emulsion polymerizations

The main procedure is as follows. Predetermined amounts of Triton X-100 (2.39 g, 3.7 mmol) were dissolved in deionized water (17 mL) in a dry glass flask equipped with a reflux condenser, stirrer, N₂ inlet, and a dropping funnel. The aqueous solution was stirred at 30 °C until the emulsifier was dissolved completely. Then, the solution of achiral monomer M (0.0744 g, 0.37 mmol) and chiral PEA (1.85 mmol) in dimethylformamide (DMF, 2 mL) was added dropwise to the abovementioned solution. The solution mixture was subsequently stirred for 30 min followed by the addition of (nbd)Rh⁺B⁻(C₆H₅)₄ (0.002 g, 0.0037 mmol) in DMF solution (1 mL). The polymerization proceeded under N₂ at 30 °C for 3 h forming seed emulsion.

For the [(nbd)RhCl]₂/chiral PEA coordination complex, a similar method was followed. The procedure of emulsion polymerization in the presence of D- or L-menthol was the same as the case using PEA. After polymerization, the resulting polymer emulsions were diluted by deionized water for CD and UV-Vis absorption measurements. To acquire pure polymer nanoparticles, the emulsifier Triton X-100 was repeatedly excluded by centrifugation (centrifuge: GL-22MS, max. speed 22 000 rpm). The pure polymer nanoparticles were subsequently used for GPC, CD and UV-Vis absorption spectra measurements.

Synthesis of optically active core/shell nanoparticles

With the polymer emulsion prepared above as seed emulsion, a predetermined amount of MMA was added in the emulsion followed by stirring for 30 min. Then, a predetermined amount of BPO/DMA catalyst (catalyst/MMA = 1 mol%) in DMF (1 mL) was added dropwise to the system. The radical polymerization of the MMA proceeded for 4 h at room temperature, providing optically active core/shell nanoparticles. All the polymerizations were carried out under N₂.

Solution polymerizations

For a typical solution polymerization, predetermined amounts of achiral monomer M (0.0744 g, 0.37 mmol), (nbd)Rh⁺B⁻(C₆H₅)₄ (0.0020 g, 0.0037 mmol) and chiral PEA (1.85 mmol) were dissolved in 3 mL chloroform (CHCl₃). The polymerization was performed under N₂ at 30 °C. The solutions containing the resulting polymer were poured into a large excess of hexane to precipitate out the polymer for GPC, CD and UV-Vis absorption spectra measurements. For the [(nbd)RhCl]₂/chiral PEA coordination complex, a similar operation was followed.

Results and discussion

In the present study, an achiral substituted acetylene monomer (M) underwent aqueous emulsion polymerizations with (nbd)Rh⁺B⁻(C₆H₅)₄ as catalyst using R- and S-PEA separately as chiral additive (CA). The underlying strategy is schematically presented in Scheme 1A. More details are presented in the Experimental section. The emulsion polymerizations proceeded smoothly and the resulting polymer emulsions kept stable for at least one month. The transmission electron microscopy (TEM) image of the polymer emulsion is shown in Fig. 1A and clearly demonstrates the formation of polymer nanoparticles (polyM nanoparticles). The average diameter of the particles was 115 nm. The number-average molecular weight (M_n) of polyM with R- and S-PEA as additive was 8100 and 9000 and the corresponding molecular weight distribution (M_w/M_n) was 1.72 and 1.70, respectively, as shown in Table S1 in the ESI.†

Fig. 1 Typical TEM images of (A) optically active polymer nanoparticles and (B) optically active core/shell nanoparticles.

According to the earlier studies,^16–18 circular dichroism (CD) and UV-Vis absorption spectroscopy techniques are powerful for us to analyze the helical structures of substituted polyacetylene particles. The predominant helicity of substituted polyacetylenes can be truly reflected by CD signals. The polymer emulsions obtained were characterized by CD and UV-Vis spectroscopy techniques and the spectra obtained are presented in Fig. 2. In Fig. 2A, obvious positive and negative CD signals are found at about 350 nm for polyM emulsions prepared in the presence of chiral PEAs (spectra (a) and (g) in Fig. 2A). Moreover, intense UV-Vis absorptions are observed at about 350 nm (Fig. 2B), similar to the CD spectra. Referring to our earlier series of studies on helical substituted polyacetylenes,^14,16–18 the CD and UV-Vis absorption spectra (Fig. 2, spectra (a) and (g)) demonstrate that polyM adopted helical conformations with one predominant helicity. It is further indicated that the polyM emulsions formed possessed optical activity. To acquire more information, we subsequently conducted more experiments by preparing polyM emulsions in the presence of (nbd)Rh⁺B⁻(C₆H₅)₄ but without chiral additive and with equal content of R-PEA and S-PEA, while keeping other conditions unchanged. GPC data are presented in Table S1 in the ESI.† The polymer emulsions obtained were further characterized by CD and UV-Vis absorption spectroscopies and the spectra obtained are also presented in Fig. 2. In sharp contrast, no CD signal is found in the wavelength range from 300 to 450 nm in the control samples (Fig. 2A, spectra (e) and (f)); however, intense UV-Vis absorptions are still observed at the wavelength of 350 nm in the control samples (Fig. 2B, spectra (e) and (f)). The observations demonstrate that the two emulsion processes without chiral PEA and with equal content of R-PEA and S-PEA hardly affected the helical structures of polyM; namely, although polyM still adopted helical conformations, it did not possess optical activity in the two cases. This further indicates that chiral additive is a necessary factor for constructing optically active polymer nanoparticles starting from achiral monomers by the process presented in Scheme 1A. We thus conclude that helix-sense-selective emulsion polymerization (HSSEP) successfully occured in this case.

Fig. 2 (A) CD and (B) UV-Vis spectra of polyM emulsions. Spectra (a) and (g), polyM emulsions with R- and S-PEA, respectively; (b) monomer emulsion; (c) and (d) monomer emulsions with R- and S-PEA, respectively; (e) polyM emulsion without chiral PEA; (f) polyM emulsion with equal content of R-PEA and S-PEA. The spectra were recorded at 25 °C.

Furthermore, we attempted to elucidate the driving force for the HSSEP process as reported above. The CD effects observed in Fig. 2A cannot arise from emulsifier Triton X-100, monomer M and chiral PEA. This assumption was confirmed by the fact that no CD signal was found in the wavelength range from 300 to 450 nm in the control samples (monomer emulsion with or without chiral PEA), as shown in Fig. 2A (spectra (b–d)). The corresponding UV-Vis absorption spectra are presented in Fig. 2B. Accordingly, the CD effects in Fig. 2A (spectra (a) and (g)) are proposed to originate in the one-handed helices in polyM emulsions with chiral additive present. That is, we established an unprecedented strategy for preparing helical polymers taking one predominant helicity through the HSSEP process, directly constructing optically active polymer nanoparticles. The influence of the amount of chiral PEA used in the HSSEP process was then investigated. The CD and UV-Vis spectra of the obtained polyM emulsions are summarized in Fig. 3. This shows that the higher the concentration of the chiral PEA, the larger the intensity of the induced CD signal on the polymer emulsion (Fig. 3A). When the mole ratio of chiral PEA to monomer M is 1, weak CD signals are found around 350 nm (Fig. 3A, spectra (c) and (e)), demonstrating that the as-formed nanoparticles possessed optical activity. Further decrease of PEA led to no CD effect (Fig. 3A, spectrum (d)). Thus, we think the minimum mole ratio of the chiral PEA to monomer M necessary to induce helical sense predominance in the nanoparticles is 1 : 1. We also used chiral menthol instead of PEA to induce the achiral monomer M to undergo the HSSEP process. The GPC data are presented in Table S1† and the relative CD and UV-Vis spectra are shown in Fig. 3C and D. Excitingly, obvious CD effects are observed at about 350 nm in Fig. 3C, just like the spectra of polyM emulsions with chiral PEA present (Fig. 2A). Thus, both chiral PEA and menthol are effective in inducing the achiral monomer to undergo helix-sense-selective emulsion polymerization. Extended investigations are currently ongoing in our lab to acquire more information regarding the effect of micelle dimension on the HSSEP process.

Fig. 3 (A) CD and (B) UV-Vis spectra of polyM emulsions synthesized in the presence of chiral PEA at various concentrations. (C) CD and (D) UV-Vis spectra of polyM emulsions obtained using chiral menthol. The spectra were obtained at 25 °C.

For the achieved HSSEPs in micelles described above, catalyst systems may be considered to exert a large influence on them. To elucidate this issue, [(nbd)RhCl]₂/chiral PEA coordination complex was used instead of the above (nbd)Rh⁺B⁻(C₆H₅)₄/chiral PEA to initiate the HSSEP of monomer M, while keeping the other conditions unchanged. The polymer emulsions obtained were further characterized. The data from GPC measurement are shown in Table S2.† The corresponding CD and UV-Vis spectra are presented in Fig. 4. In this case, intense CD effects are also observed at about 350 nm in Fig. 4A, which is consistent with the observations in Fig. 2A. This demonstrates that monomer M successfully underwent an HSSEP process and thus the polyM obtained showed optical activity. In addition, polymerizations using [(nbd)RhCl]₂/chiral menthol obtained similar results (Table S2† for GPC data, Fig. S1† for CD and UV-Vis spectra).

Fig. 4 (A) CD and (B) UV-Vis spectra of polyM emulsions obtained using [(nbd)RhCl]₂ and chiral PEA. (C) CD and (D) UV-Vis spectra of polyM nanoparticles measured in water dispersion and CHCl₃ solution, respectively. The spectra were obtained at 25 °C.

To understand more deeply the HSSEP process mentioned above, pure helical polymer nanoparticles were acquired from polyM emulsion with S-PEA (taken as representative) by a repeated centrifugal separation process. The pure polymer nanoparticles were separately dispersed in water and dissolved in chloroform (CHCl₃) for measuring CD and UV-Vis absorption spectra. The results are presented in Fig. 4C and D. (Note: pure helical polymer nanoparticles obtained from polyM emulsion in the presence of R-PEA supported the same conclusion.) In Fig. 4C (spectrum (a)), an obvious CD signal is observed at around 350 nm, similar to the corresponding polyM emulsion (Fig. 2A), demonstrating that polyM chains retained the preferred helicity when dispersed in water. However, when polyM nanoparticles were dissolved in CHCl₃, no CD signal appeared in the wavelength range 300–500 nm (Fig. 4C, spectrum (b)), indicating that the polymer chains lost optical activity once dissolved in this solvent. This further means that racemization occurred to the chirally helical polymer in solution state. In addition, for the polymer solution, when compared to the polymer nanoparticles discussed above (λ_max, 350 nm, Fig. 2B and 4D), pronounced red shifts occurred in the UV-Vis absorption of the polymer solution (λ_max, 390 nm, Fig. 4D, spectrum (b)). The red shifts resulted from the prolonged effective conjugation length in helical polymer chains in the solution state, as discussed in detail earlier.²⁰

To explore the mechanism of the HSSEP process established above, we subsequently conducted more experiments in CHCl₃ solution under conditions comparable to the emulsion polymerizations in the presence of (nbd)Rh⁺B⁻(C₆H₅)₄ and chiral PEA, as schematically shown in Scheme 1B. The solution polymerizations also proceeded smoothly and provided corresponding polymers in quantitative yields. The M_n of polyM prepared using R- and S-PEA was 10 300 and 9500, and the M_w/M_n was 1.83 and 1.87, respectively. The polymers obtained were further subjected to CD and UV-Vis absorption spectroscopy measurement. The obtained spectra are presented in Fig. 5. No CD signal is found in the polymer (Fig. 5A), however, pronounced UV-Vis absorption peaks are observed at around 390 nm in the corresponding spectra (Fig. 5B). These observations demonstrate that the achiral monomer failed to undergo helix-sense-selective polymerization in solution, even in the presence of chiral additive. We further increased the concentration of chiral PEA in the solution system, but regretably, optically inactive polymer was still obtained. Solution polymerizations using [(nbd)RhCl]₂/chiral PEA obtained similar results (Fig. S2† for CD and UV-Vis spectra). We further conducted another experiment by adding chiral PEA into the pre-formed polyM emulsion, which was obtained in the absence of chiral PEA (i.e. sample (e) in Fig. 2A). However, no CD effect (300–450 nm) was found in this case either.

Fig. 5 (A) CD and (B) UV-Vis spectra of polyM solutions after solution polymerization in the presence of (nbd)Rh⁺B⁻(C₆H₅)₄ and chiral PEA. The solution concentration was approximately 1 mmol L⁻¹ by monomer units. (C) CD and (D) UV-Vis spectra of polyphenylacetylene emulsions in the presence of chiral PEA. The spectra were obtained at 25 °C.

A combination of the investigations above clearly shows us that polyM can form predominantly one-handed helical structures only by the HSSEP approach and only in polymer emulsions. Therefore, we propose that the micelles play a critical role in controlling the helical structures of polymer chains. Definitely, both the chiral additive and the micelles play essential roles in the HSSEP process. The chiral additives provide the chiral origin and form hydrogen bonds with the monomer, changing the initial achiral monomer into a pseudo-chiral monomer. The microenvironment confinement effects offered by the micelles lend a hand by assisting the pseudo-chiral monomer in forming predominantly one-handed helical polymer, leading to optically active polymer nanoparticles. To verify the hypothesis, we subsequently used phenylacetylene instead of the achiral monomer M to perform HSSEP in the same way, and the obtained CD and UV-Vis absorption spectra are shown in Fig. 5C and D. Just as expected, no CD signal was found in the samples examined (Fig. 5C), because of the absence of hydrogen bonds between phenylacetylene and chiral PEA.

With the prepared polymer emulsions in hand, we subsequently explored the temperature effects on the predominantly one-handed helical structures of the polymer in the nanoparticles. The corresponding CD and UV-Vis spectra are presented in Fig. 6 and S3 in the ESI.† Only little change in the CD and UV-Vis absorption spectra is found in Fig. 6A and B when the temperature varied from 30 to 50 °C, demonstrating the induced favorable helical structures in polyM in the nanoparticles possessed relatively high thermal stability under the conditions examined. However, when the temperature was increased to 80 °C, a remarkable decrease occurred in the intensity of the CD and UV-Vis signals, as observed in Fig. 6 and S3.† This observation indicates that the induced chiral helical structures were probably destroyed at high temperature, which is not favorable for practical applications of these optically active polymer nanoparticles.

Fig. 6 Effects of temperature on (A) CD and (B) UV-Vis spectra of polyM emulsion obtained by HSSEP (in (B) R-PEA gave similar results, so the related spectra are presented in Fig. S3 in the ESI†).

We previously reported that the stability of substituted polyacetylene can be improved by forming core/shell nanoparticles in which the shell provided the desirable protection for the polymeric cores inside.²¹ Herein, we further designed and prepared core/shell nanoparticles with the purpose of improving the stability of the induced helicity by using the protection provided by the outside shells. The underlying strategy for preparing core/shell nanoparticles is illustrated in Scheme 1A. With the polyM emulsion as seed emulsion, methyl methacrylate (MMA) was directly added to it for the subsequent free radical polymerization. The hydrophobicity of MMA made it quickly disperse inside the micelles and finally distribute around the core particles constituted from polyM chains. Conventional radical initiators like 2,2-azobisisobutyronitrile (AIBN) should be triggered at a moderately high temperature, which may lead to disruption of the induced, predominantly one-handed helical structures of polyM. Thus, in this study, we tactfully solved this problem using a redox initiation system, that is, the BPO/DMA system.²² The free radical polymerization proceeded at room temperature, providing the desired core/shell nanoparticles.

The practice mentioned above led to the anticipated core/shell nanoparticles, as revealed by the TEM image (Fig. 1B). The diameter of the core/shell nanoparticles is approximately 200 nm, much bigger than the initial pure polymer cores (115 nm, Fig. 1A). FT-IR spectra analysis further supported this result, as shown in Fig S4 in the ESI.† The vibrational absorption peaks at 1516 and 1652 cm⁻¹ are assigned to the amide groups in polyM chains, whereas the absorption peak at 1732 cm⁻¹ is attributed to the ester group in poly(methyl methacrylate) (PMMA) chains. This result indicates the successful preparation of core/shell nanoparticles. Herein, we point out that the diameter of the core/shell nanoparticles in theory can be tuned by the dosage of MMA. In more detail, appropriately more MMA loading may result in larger particles.²¹ To investigate whether the formation of shells could affect the helical conformations of the polyM chains inside, we examined the optical activity of the core/shell nanoparticles by CD and UV-Vis techniques. The obtained spectra are shown in Fig. 7. Remarkable CD and UV-Vis absorptions can be observed around 350 nm in Fig. 7A and B, just like the pure polymer nanoparticles discussed above (Fig. 2A and B). Therefore, the formation of PMMA shells surrounding the cores did not affect the helical structures of the polyacetylene cores inside formed by HSSEP. We subsequently explored whether the stability of the induced helicity could be remarkably enhanced by the outside shells. CD and UV-Vis spectra of the core/shell nanoparticles measured at varied temperatures are presented in Fig. 7C, D and S5 in the ESI.† These show that the CD and UV-Vis intensity of the core/shell nanoparticles could be maintained without change even up to 80 °C, demonstrating the enhanced stability of the helical structures against heating. Accordingly, compared with the results in Fig. 6, it can be concluded that the stability of the induced, predominately one-handed helical structures was remarkably improved after forming core/shell nanoparticles.

Fig. 7 (A) CD and (B) UV-Vis spectra of core/shell nanoparticles in the presence of chiral PEA. Effects of temperature on (C) CD and (D) UV-Vis spectra of core/shell nanoparticles (in (D) R-PEA gave similar results, so the related spectra are presented in Fig. S5 in the ESI†).

Conclusions

Achiral monomer successfully underwent helix-sense-selective emulsion polymerizations (HSSEPs) in micelles, directly providing optically active helical polymer nanoparticles. The stability of the induced helical structures with predominant one-handed screw sense was remarkably improved through forming core/shell nanoparticles. The present investigations largely extend our understanding of microenvironment confinement effects on the secondary structure of synthetic helical polymers. Based on these findings, we established an unprecedented strategy for preparing chiral helical polymer nanoparticles derived from achiral monomer. More importantly, the spatial microenvironment effects may also exist in other situations possessing special geometries. The related investigations will bring significant implications to bear, not only on polymer science, but also on related disciplines, including chemistry, materials, pharmaceutics, and bio-engineering.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (21474007, 21274008, 21174010) and the Funds for Creative Research Groups of China (51221002).

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedures; GPC, FT-IR spectra, CD and UV-Vis spectra of polymers. See DOI: 10.1039/c6ra10610b

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