Facile Synthesis of Optically Active and Thermoresponsive Star Block

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Letter Cite This: ACS Macro Lett. 2018, 7, 127−131

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Facile Synthesis of Optically Active and Thermoresponsive Star Block Copolymers Carrying Helical Polyisocyanide Arms and Their ThermoTriggered Chiral Resolution Ability Qian Wang, Ben-Fa Chu, Jia-Hong Chu, Na Liu, and Zong-Quan Wu* Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Anhui Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei, 230009 Anhui Province, China S Supporting Information *

ABSTRACT: A left-handed helical poly(phenyl isocyanide) bearing a norbornene unit and a Pd(II) complex on each terminus was prepared. The norbornene terminus was core cross-linked with a bisnorbornene linker via ring-opening metathesis polymerization (ROMP), yielding a star polymer carrying left-handed helical arms decorated with Pd(II) units at the exterior. The optical activities of the helical arms were maintained after the cross-linking reaction. The Pd(II) units on the surface of the star polymer were chain extended with a new phenyl isocyanide bearing three hydrophilic triethylene glycol monomethyl chains, which afforded an amphiphilic star block copolymer carrying helical arms. Such a star block copolymer showed excellent thermoresponsiveness with the lower critical solution temperature (LCST) around 55 °C. This optically active and thermoresponsive star polymer can enantioselectively capture the S-enantiomer of racemic methyl benzyl alcohol solution at a temperature lower than the LCST and precipitated when the temperature was higher than the LCST, leaving the R-enantiomer in the solution. The enantiomeric excess (ee) of the isolated enantiomer is up to 75%.

S

In this contribution, we report on the synthesis of an optically active and thermoresponsive star block copolymer carrying amphiphilic block helical poly(phenyl isocyanide) (PPI) arms through the combination of the ring-opening metathesis polymerization (ROMP) and the Pd(II)-initiated isocyanide polymerization. The interior blocks of the arms are left-handed PPI segments and are hydrophobic, while the exterior blocks are hydrophilic PPI blocks bearing a triethylene glycol monomethyl ether chain. Moreover, it also showed thermoresponsiveness in water. The lower critical solution temperature (LCST) is around 55 °C and depends on the concentration. Taking advantage of the optical activity and the thermoresponsiveness, this star block copolymer showed good performance on chiral resolution. The enantiomeric excess (ee) of the separated enantiomer is up to 75% by using racemic D/Lmethyl benzyl alcohol as the model compound. As displayed in Scheme 1, a NB-functionalized Pd(II) (NBPd(II)) catalyst was designed and synthesized following the reported literature with modifications.23 Polymerization of enantiopure phenyl isocyanide monomer L-1 carrying L-alanine with a decyl ester linker was performed in THF at 55 °C ([L1]0 = 0.2 M, [L-1]0/[Pd]0 = 100). The afforded helical PPI was fractionated by acetone, which yielded a norbornene-terminated, left-handed helical polymer NB-poly-L-1100.24 The

timulated by the helical structures in biomacromolecules, many studies have been done on optically active helical polymers recently.1−5 These investigations can not only deepen our understanding of living systems but also promote the development of novel functional materials for chiral recognition,6,7 asymmetric catalysis,8 and enantiomer separation,9,10 and so on. Polyisocyanide is one of the most studied helical polymers due to its unique rigid rod helical structure and a range of appealing applications.11,12 To fabricate novel chiral functional materials with excellent performance, a variety of block and brush polymers containing optically active helical polyisocyanide segments have been reported. However, core cross-linked star polymers carrying helical polyisocyanide arms with controlled handedness have rarely been explored. Core cross-linked star polymers with multiple arms connected to a central core have gained increasing attention in recent years.13,14 The three-dimensional globular compact structure results in a unique set of physical properties that cannot be accessed by the linear analogues. The physical and chemical properties of a star polymer are mainly dependent on the grafted polymers at the exterior. Thus, a variety of star polymers with different structure and compositions have been reported.15,16 However, to the best of our knowledge, few of them are optically active.17−19 Synthesis of an optically active star polymer carrying unique helical arms may provide new functional materials with novel properties and great potentials in many fields.20−22 © XXXX American Chemical Society

Received: November 6, 2017 Accepted: January 2, 2018

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DOI: 10.1021/acsmacrolett.7b00875 ACS Macro Lett. 2018, 7, 127−131

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three triethylene glycol monomethyl ether chains (Scheme 1). The chain extension reaction was performed in THF at 55 °C ([3]0/[Pd]0 = 50, [3]0 = 0.2 M). SEC analysis indicated the chain extension occurred because the elution peak caused the resulting star block copolymer 4L to be shifted to the shorter retention-time region (Figure 1a). The Mn of 4L was estimated to be 95.1 kDa, larger than that of the 2L precursor (Mn = 63.9 kDa, Mw/Mn = 1.20), while the dispersity was still narrow (Mw/ Mn = 1.21). Based on the values of SEC, the average number of the arms of the star polymer was estimated to be around 10, similar to those reported in the literature.12,26 The structure of the star block copolymer 4L was further characterized by 1H NMR and FT-IR spectroscopies (Figure S5 and S6, SI). Because the polymerization of phenyl isocyanide with the NBPd(II) as initiator and the ROMP both proceed in living/ controlled manner,25,27−29 a variety of well-defined core crosslinked star polymers bearing helical arms with different Mn and narrow Mw/Mn were facilely prepared and isolated in high yields (Table 1 in SI). The normalized CD and UV−vis spectra of 4L were also displayed in Figure 1b. Due to the chain extension reaction of monomer 3 bearing more phenyl rings, the absorption at the phenyl region was significantly increased. The chain-extended star polymer 4L also exhibited negative CD on the region of the backbone with the Δε364 of −9.85. The decreased Δε364 is ascribed to the poly-3m segment containing both left- and righthanded helices because monomer 3 did not contain any chiral units and amide pendents and could not maintain the onehanded helicity during the chain extension reaction. The structures of the core cross-linked star polymer 2L and amphiphilic star block copolymer 4L were further investigated by atomic force microscopy (AFM) observations. As shown in Figure 2a and b, both 2L and 4L exhibited spherical

Scheme 1. Synthesis of Thermoresponsive Optically Active Star Block Copolymer

number-average molecular weight (Mn) and its distribution (Mw/Mn) were, respectively, estimated to be 31.8 kDa and 1.15, by size exclusion chromatography (SEC) with equivalence to polystyrene standards (Figure 1a). The single left-handed

Figure 1. SEC curves (a) and CD and absorption spectra (b) of NBpoly-L-1100 and the corresponding star polymer 2L and the star block copolymer 4L.

helicity was confirmed by CD and UV−vis spectroscopies, which showed an intense negative CD at 364 nm, corresponding to the absorption of the CN backbone. The molar CD intensity at 364 nm (Δε364) was determined to be −20.0 (Figure 1b), which was consistent with the value of optical rotation (−1856, c = 0.1, CHCl3, 25 °C). Copolymerization of NB-poly-L-1100 with a bisnorbornene linker via ROMP using Grubbs’ second-generation catalyst in THF at 25 °C ([1]0/[linker]0/[G2]0 = 30/30/1) afforded an anticipated star polymer 2L carrying left-handed helical arms. The recorded SEC curve was shifted to the higher-Mn region and remains symmetric and single model. The Mn of star polymer 2L increased to 63.9 kDa, while the dispersity kept narrow (Mw/ Mn = 1.20). This result confirmed that the copolymerization of NB-terminated poly-L-1100 with the bisnorbornene linker via ROMP did take place. It has been reported that the Pd(II) units can tolerate the ROMP condition, and the Pd(II) terminus of helical PPI arms was active enough to reinitiate a new polymerization of isocyanide monomers.25 Thus, the isolated star polymer 2L was treated with a new phenyl isocyanide monomer (3) containing

Figure 2. AFM height images of 2L (a) and 4L (b). (c) TEM image of 4L. (d) DLS curves NB-poly-L-1100 and the resulting star polymer 2L and 4L.

nanoparticle morphologies with good homogeneity on AFM height images. The average diameter of 2L is ca. 50 nm, which increased to ca. 70 nm in 4L, further confirming the chain extension reaction. The TEM image of 4L also showed spherical nanoparticles with ca. 72 nm in diameter. The formation of the star block copolymer was further studied by dynamic light scattering (DLS) analyses. Both 2L and 4L exhibited symmetric and single-model DLS curves (Figure 2d). 128

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suggested the thermoresponsive star block copolymer may be recycled in chiral resolution. Due to the chirality of the left-handed helical poly-L-1100 segment, the synthetic star polymers possess plenty of chiral microenvironments and have high optical activity, which may be utilized in chiral recognition.32,33 To verify this, the enantioselective absorption capability of the star polymers 2L and 4L were then investigated by taking racemic methyl benzyl alcohol (R- and S-5) as model compounds. First, 2L was added to the aqueous solution of R- and S-5 at room temperature (see Supporting Information for details). Due to the insolubility of 2L in water, the enantioselective absorption of R- and S-5 can be followed by measuring the CD and UV−vis spectra. As shown in Figure S8 in SI, R- and S-5 showed negative and positive CD at 260 nm, respectively, with mirror images to each other. The CD intensities of both R- and S-5 were monitored upon the addition of 2L. As expected, the CD changes of the two enantiomers are different. For R-5, there were no substantial changes on CD and UV−vis spectra upon the addition of 2L, and even the incubation solution stood for 1 week. However, for S-5, remarkable changes on CD and UV− vis spectra were clearly observed. It takes about 45 h to reach a balance. On the basis of CD and UV changes, about 32% of S-5 was absorbed, while only 8% was absorbed for R-5 under the same conditions. Thus, the ratio of the absorbed S-5 to R-5 was about 4/1 (Figure S8, SI). When racemic solution of R/S-5 in water was incubated with 2L, CD silent solution gradually showed negative CD at the absorption region of 5 and increased with the incubation time. After 45 h, the CD became constant, implying the enantioselective absorption reached a balance. The ee of the solution was determined to be 30% by CD and UV−vis spectra. These results indicated that the optically active star polymer carrying left-handed helical PPI arms has good chiral recognition ability and can be used in enantiomer separation. However, the efficiency is relatively low and requires a long time, probably because of the heterogeneous interaction of the insoluble star polymer 2L with the R/S-5 in solution. Inspired by the excellent chiral recognition ability of the star polymer 2L, the chiral recognition and resolution properties of 4L were then investigated. Due to the good water solubility of 4L, the homogeneous chiral recognition of 4L with R/S-5 may be more efficient and has higher enantioselectivity. Moreover, because of the excellent thermoresponsiveness in water, the amphiphilic star polymer 4L can be precipitated at a temperature higher than its LCST and separated from the solution by centrifugation. To verify this hypothesis, racemic R/ S-5 was added to the aqueous solution of 4L at room temperature. The process of the enantioselective absorption was monitored by measuring the enantiomeric excess (ee) of 5 in solution by HPLC (see SI for details). To estimate the ee value of the solution, the incubation solution was heated to its LCST at appropriate time intervals and centrifuged. The star block copolymer 4L with the absorbed enantiomer was precipitated, while the unabsorbed enantiomer remained in solution. Thus, a small aliquot of the upper clear solution was subjected to HPLC analysis. The ee value determined by HPLC was plotted against the absorption time which was displayed in Figure 4c. It was found that the S-5 was enantioselectively absorbed by the star polymer and left R-5 in solution. The ee value increased very quickly and eventually reached a constant of 33% after 4 h. It should be noted that the enantioselective absorption of R/S-5 by 2L requires 45 h to

The hydrodynamic size of NB-poly-L-1100, 2L, and 4L was, respectively, estimated to be 16, 55, and 75 nm. The increased size of 2L suggested the formation of core cross-linked star polymers. Collectively, these studies confirmed the formation of well-defined optically active star amphiphilic block copolymer. The star block copolymer is not only optically active but also thermoresponsive. Star polymer 2L is only dissolvable in organic solvents such as chloroform and THF, while the amphiphilic star block copolymer 4L has good solubility in most common organic solvents and in water as well because of the amphiphilic character. Interestingly, it showed excellent thermoresponsive behavior in water. As shown in Figure 3a, the

Figure 3. (a) Plots of the transmittance and hydrodynamic size of the 4L in water with temperature (insets: photographs of the aqueous solution of 4L in water at 25 and 55 °C, c = 0.5 g/L). (b) Plots of the transmittance of 4L in water with the temperature in different concentrations.

clear transparent solution of 4L gradually turned to turbid upon heating and turned back to clear solution again when it was recooled to room temperature. A plot of the transmittance of the aqueous solution (0.5 mg/mL) at 700 nm with temperature suggested the LCST is ca. 50 °C. The optical transmittance curve of 4L in water upon the heating and cooling cycle is reversible with small hysteresis, indicating the excellent thermoresponsive behavior of the amphiphilic star block copolymer. The thermoresponsiveness of the star polymer may be attributed to the variation on the hydrophilic/ hydrophobic balance of poly-3m segments. The poly-3m block is hydrophilic at the temperature below LCST, and it became hydrophobic when the temperature was higher than its LCST.30,31 Further studies revealed that the LCST at 1.0 and 2.0 mg/mL concentrations was 47.5 and 43 °C, respectively (Figure 3b). Probably the higher concentration can facilitate polymer collapse and subsequent interchain aggregation of the hydrophilic poly-3m segments. Note that the ester linkages in the pendants of the poly-3m segment give more flexibility than that of the amide linkage, which may facilitate the interchain aggregation during the thermo-induced phase transition. The heating-induced phase transition of the amphiphilic star block copolymer was also confirmed by DLS analysis (Figure 3a). The DLS studies indicated that the hydrodynamic diameter of 4L is ca. 75 nm and kept constant at the temperature below the LCST. When the solution was heated to 55 °C, the hydrodynamic size considerably increased and eventually reached 1.24 μm, likely due to the aggregation of the star polymers. This thermoresponsiveness is because the poly3m segment of the star block copolymer is swollen below the LCST temperature and undergoes entropy-driven phase transition to the collapsed hydrophobic polymer at the LCST. Very interestingly, the turbid solution of 4L in water can be facilely precipitated from the solution via high-speed centrifugation at temperature higher than LCST. This property 129

DOI: 10.1021/acsmacrolett.7b00875 ACS Macro Lett. 2018, 7, 127−131

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Figure 5. Possible mechanism for the enantioselective absorption and release. Experimental conditions: star polymer 4L (2.5 mg) was added to a solution of racemate R/S-5 (0.5 mg, 0.001 mol/L) in H2O. The ee values were determined by HPLC using a Chiralcel OD-H column, eluent: i-PrOH/hexane (v/v = 95/5), flow rate: 0.5 mL/min.

Figure 4. (a) CD and UV−vis spectra of the racemic solution of R/S-5 with the presence of 2L in water at 25 °C. (b) Plots of the ee value of R/S-5 with the absorption time by 2L and 4L. (c) HPLC curves of the racemic R/S-5, absorbed S-5, and the resulting solution of R-5. (d) Plots of the ee value of the enantioselective absorption by 2L and 4L with the recycle time.

enantiomer separation on these two compounds. The results were summarized in Table S2 in SI. In summary, optically active star block polymers carrying helical arms were facilely synthesized and characterized. Such polymers exhibited excellent thermoresponsiveness. Interestingly, this star block copolymer exhibited good performance on chiral recognition. Taking advantage of the thermoresponsiveness, the enantiomers can be facilely separated, and the ee value of the isolated enantiomer is up to 75%. We believe the present study provides not only a novel chiral material with great potential applications but also an approach for facile synthesis of optically active functional materials. Considering the modifications on the structure of monomers and polymers, more interesting chiral materials with multiresponsiveness can be anticipated.

reach constant (Figure 4b). After the absorption reached to balance, the incubation solution of 4L was then heated to 55 °C (>LCST). The precipitated star polymer encapsulated with S-5 was isolated by centrifugation. After removal of the upper clear solution, the absorbed S-5 was isolated by washing the precipitated solid successively with n-hexane. The ee of isolated S-5 was estimated to be 75% by HPLC (Figure 4c). It worthy to note that about 30% of total R/S-5 was absorbed, and others were left in solution. Calculations based on ee and yield found the ratio of S-5 to R-5 is 7/1 in absorbed sample, and it is 1/2 in the free samples in supernate. The fraction of isolated S-5 is 26% with respect to the initial racemate feed and is 52% with respect to the initial S-5 enantiomer (Table S2, SI). To get more details of the chiral resolution, the apparent association constants (Ka) of 4L to R- and S-5 in water at 25 °C were estimated to be 1.20 × 104 and 2.65 × 105, respectively, by UV−vis titration (Figures S10−S11, SI). The Ka for S-5 is about 22 times larger than that of R-5, confirming the excellent enantioselectivity of 4L. For comparison, the Ka values at high temperature were also investigated. The UV−vis titrations were carried out at 40 °C to avoid the precipitate of the polymer. It was found that the Ka’s for 4L with R- and S-5 were, respectively, determined to be 1.14 × 104 and 2.63 × 105 (Figures S10−S11, SI). The Ka for both enantiomers was almost the same as that obtained at 25 °C, and the high enantioselectivity was maintained. Moreover, the star block copolymer 4L can be facilely recovered and reused in the enantiomer separation following the procedure described above. The performance of the chiral resolution was also very high and comparable to those by using the fresh star polymer 4L. As shown in Figure 4d, the star block copolymer can be reused for at least four cycles in the enantiomer separation of R/S-5 through the heating-induced phase transition without significant loss of its enantioselectivity. A possible mechanism for the enantiomer separation was outlined in Figure 5. Lastly, the chiral resolution abilities of star polymer 4L to methyl benzyl amine and 1-phenyl-1,2ethanediol were investigated following the same procedure. To our delight, it also showed good performance on the

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00875. General consideration, synthesis of NB-Pd(II) and bisnorbornene linker, synthesis of NB-poly-L-1100, synthesis of 2L and 4L, typical procedure for chiral resolution, additional references, and additional tables and figures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.W.). ORCID

Zong-Quan Wu: 0000-0001-6657-9316 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is partially supported by the National Natural Scientific Foundation of China (21622402, 51673057, and 21574036). Z.W. thanks the 1000plan Program for Young Scholars of China for Financial Support. 130

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DOI: 10.1021/acsmacrolett.7b00875 ACS Macro Lett. 2018, 7, 127−131