Facile Synthesis of Hybrid Silica Nanoparticles Grafted with Helical

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Facile Synthesis of Hybrid Silica Nanoparticles Grafted with Helical Poly(phenyl isocyanide)s and Their Enantioselective Crystallization Ability Li Yang, Yang Tang, Na Liu, Chun-Hua Liu, Yunsheng Ding, and Zong-Quan Wu* Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology and Anhui Key Laboratory of Advanced Functional Materials and Devices, Hefei 230009, China S Supporting Information *

ABSTRACT: In this contribution, we report on the facile synthesis of hybrid silica nanoparticles grafted with helical poly(phenyl isocyanide)s via both “grafting from” and “grafting to” strategies. First, triethoxysilanyl functionalized alkyne− Pd(II) initiator was anchored onto the surface of bare silica nanoparticles through silanization coupling reaction. Polymerization of phenyl isocyanide using the Pd(II)−anchored silica nanoparticles lead to the formation of hybrid nanoparticles grafted with helical poly(phenyl isocyanide)s. The surfaceinitiated polymerization was revealed to proceed in a living/controlled chain-growth manner, afforded the hybrid nanoparticles with controlled thickness. 31P NMR analysis indicated the initiation efficiency of the surface-anchored Pd(II) initiators is very high, and almost quantitative. The grafting density was determined to be ∼0.89 nm2/chain based on the thermal gravity analysis (TGA). Polymerization of optically active phenyl isocyanide bearing an L-alanine with a long decyl chain using the Pd(II)anchored silica nanoparticles formed chiral hybrid nanoparticles grafted with helical poly(phenyl isocyanide) arms in preferred handedness. Second, the hybrid silica nanoparticles were prepared via “grafting to” strategy. Well-defined triethoxysilanyl terminated poly(phenyl isocyanide) was prepared in controlled manners. The polymer was grafted to the surface of bare silica nanoparticles via the silanization coupling reaction, afforded hybrid silica nanoparticles grafted with helical poly(phenyl isocyanide). TGA indicates the grafting density is ∼0.76 nm2/chain. Taking advantage of this synthetic method, left-handed helical poly(phenyl isocyanide) was grafted to the surface of silica nanoparticles, generated chiral hybrid silica nanoparticles with high optical activity. Such chiral nanoparticle exhibited good performance in enantioselective crystallization of racemic Bocalanine. The enantiomeric excess (ee) of the induced crystal is up to 95%.

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INTRODUCTION Helix is one of the most important and fundamental secondary structure in living system and is closely related to the sophisticated functions of biomacromolecules, such as α-helix in protein and double helix in DNA.1 Since the discovery of the helical structure in biomacromolecules, chemists have been challenged to develop artificial polymers and oligomers with stable helical conformation.2 However, the number and type of artificial helical polymers are still very limited to date due to the difficult in synthesis, albeit they possessing great applications in many areas.3 Among the reported helical polymers, polyisocyanides are of particular interest,4 not only because of their interesting rigid rod helical conformation but also for the wide applications in chiral recognition, enantiomer separation and liquid crystallization.5 Helical polyisocyanides have been fabricated into block copolymers,6 grafted polymer brushes,7 and star polymers5b,8 to give birth of new chiral materials with novel functions. However, to the best of our knowledge, welldefined organic/inorganic hybrid nanoparticles grafted with helical polyisocyanides have rarely been reported. The surface modifications of silica nanoparticles by chemically bound polymers have attracted ever increasing attention in © XXXX American Chemical Society

the past few decades due to their fascinating optical, electronic, magnetic and catalytic properties.9 Polymer-grafted silica nanoparticles typically consist of organic polymer shells with incorporated inorganic cores.10 Although the physical properties of nanoparticles are governed by both the size and shape of inorganic cores and surrounding organic layer, the chemical properties of nanoparticles were mainly determined by the organic polymer shell. Thus, synthesis of hybrid silica nanoparticles grafted with distinct helical polymers may provide new materials with novel functions and properties. Covalently bonded hybrid nanoparticles are usually prepared via either “grafting to”11 or “grafting from”12 methods. Taking advantage of these strategies, a variety of hybrid silica nanoparticles grafted with different polymer arms such as polystyrene,13 polyacrylamide, 14 poly(N-isopropylacrylamide), 15 poly(phenyleneethynylene),16 and polypeptide17 have been reported, which exhibited distinct properties that cannot be obtained from the respective homopolymers. Therefore, Received: August 26, 2016 Revised: September 23, 2016

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DOI: 10.1021/acs.macromol.6b01870 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of Hybrid Silica Nanoparticles Grafted with Helical Poly(phenyl isocyanide)s via “Grafting from” Strategy

Figure 1. TEM images of (a) bare silica nanoparticles, (b) Pd(II)-anchored silica nanoparticles (SiO2−Pd(II)), and (c) helical poly(phenyl isocyanide)−grafted hybrid silica nanoparticles (SiO2−poly-1100).

grafted with helical poly(phenyl isocyanide)s via both the “grafting from” and “grafting to” strategies. The Pd(II) complex was first anchored onto the surface of silica nanoparticles, which initiate the living polymerization of phenyl isocyanide afforded well-defined core/shell nanoparticles grafted with stereoregular helical polyisocyanides with controlled thickness. Polymerization of chiral phenyl isocyanide bearing an L- or Dalanine pendant with a long decyl chain formed optically active organic/inorganic nanoparticles grafted with helical poly(phenyl isocyanide)s with preferred handedness. Moreover, the hybrid silica nanoparticles were prepared via the “grafting to” strategy through the silanation coupling reaction of triethoxysilanyl-terminated poly(phenyl isocyanide)s with bare silica nanoparticles. Grafted a left-handed helical polyisocyanide onto the silica nanoparticles afforded chiral nanoparticles with high optical activity, which were evidenced by circular dichroism (CD) spectroscopy and optical rotation. Strong Cotton effects were observed on CD spectra of the chiral nanoparticles due to the chirality of the single handed helical poly(phenyl isocyanide) arms (see below for details). Such chiral materials can be applied in the enantioselective crystallization of racemic compounds. The enantiomeric excess (ee) of the induced crystal of Boc-alanine can be up to 95%.

grafting well-defined helical polyisocyanide to the surface of silica nanoparticles may provide new organic/inorganic hybrid materials with interesting properties and functions. Resolution of a chiral isomer from its enantiomeric antipode has attracted considerably research attention in recent years. Despite of the great developments in asymmetric synthesis and HPLC (high-performance liquid chromatography), chiral resolution by enantioselective crystallization continues to be one of the main method to obtain pure enantiomers because of its simplicity, wide range of applications, and cost efficiency.18 In this context, development of novel chiral materials to induce crystallization of a chiral isomer from the racemate with high enantiomeric selectivity is of great required. Deng and coworkers reported that chiral nanoparticles containing helical polyacetylenes can induced enantioselective crystallization of some amino acids.19 Although helical polyisocyanides have been widely used in many fields, theirs application in enantioselective crystallization have never been explored. Because of the rigid-rod helical conformation the main chain, these materials may have excellent performance in enantioselective crystallization. Recently, we reported a family of alkyne−palladium(II) complexes which can initiate living polymerizations of various isocyanide monomers, formed well-defined stereoregular helical polyisocyanides in high yields.20 Taking advantage of this method, various polyisocyanides with different topologies can be facilely synthesized, including hybrid block copolymers, star polymers and polymer brushes.7b,8,21 In this contribution, we report the controlled synthesis of hybrid silica nanoparticles

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RESULTS AND DISCUSSION Synthesis of Hybrid Silica Nanoparticles via “Grafting from” Strategy. First, Pd(II) complex for the polymerization of phenyl isocyanide containing a triethoxysilane headgroup was prepared according to Scheme 1. Triethoxy(3B


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Figure 2. FT-IR spectra (a) and TGA curves (b) of bare silica nanoparticle, Pd(II)-anchored silica nanoparticle SiO2−Pd(II), and the resulting helical poly(phenyl isocyanide)-grafted hybrid silica nanoparticle SiO2−poly-1100. FT-IR spectra were measured at 25 °C using KBr pellets. TGA was performed in air at a heating rate of 10 °C/min.

Figure 3. (a) Size exclusion chromatograms of poly(phenyl isocyanide)-grafted silica nanoparticles SiO2−poly-1ms prepared via the surface-initiated polymerization of monomer 1 using SiO2−Pd(II) in different initial feed ratio of monomer to initiator. (b) Plots of Mn and Mw/Mn values of SiO2− poly-1ms with the initial feed ratio of monomer to initiator.

cm−1 can be also observed, which were assignable to the bending of Si−OH and Si−O−Si bonds, respectively.15a After the silanization coupling reaction, the resulting SiO2−Pd(II) showed additional vibration absorptions at 1538 cm−1 due to the N−H vibrations:at 2130 and 1720 cm−1, respectively,ascribed to the vibrations of CC and CO groups. These results support the anchoring of Pd(II) initiators on the silica surface. TGA analysis further supports the formation of Pd(II)anchored silica nanoparticle. As displayed in Figure 2b, there is about ∼3.0 wt % difference in the weight retention at 800 °C between the bare and Pd(II)-anchored silica nanoparticles. Using the weight retention values obtained from the TGA data, grafting density of the Pd(II) initiators at the surface of the silica nanoparticle (∼70 nm in diameter) was roughly estimated to be ∼0.91 nm2/initiator. With the SiO2−Pd(II) in hand, subsequent efforts were directed toward the surface-initiated polymerization of phenyl isocyanide. Polymerization decyl 4-isocyanobenzoate (1) with SiO2−Pd(II) was performed in THF at 55 °C ([1]0 = 0.2 M, [1]0/[Pd]0 = 100), followed the procedure reported by our group previously.21 After 16 h, the reaction solution was precipitated into a large amount of methanol, and the precipitated solid was isolated by filtration. As shown in Figure 3a, the size exclusion chromatography (SEC) of the afforded SiO2−poly-1100 (the subscript indicates the initial feed ratio of monomer to Pd(II) unit, the same below) exhibited a symmetric and single model elution peak. The number-average molecular weight (Mn) and its distribution (Mw/Mn) of the

isocyanatopropyl)silane was treated with prop-2-yn-1-ol in dichloromethane at room temperature with the presence of triethylamine as base. The alkyne group was then installed with a Pd(II) unit by reacted with trans-bis(triethylphosphine)palladium(II) dichloride in dichloromethane using copper(I) chloride as catalyst.20,21 The afforded triethoxysilane-terminated alkyne−Pd(II) complex (TEOS−Pd(II)) was isolated in 86% yield. Through silanization coupling reaction, the TEOS-Pd(II) was anchored onto the surface of the spherical bare silica nanoparticles (∼70 nm in diameter) prepared using the Stöber process,22 which has been well-known to produce spherical silica nanoparticles with relatively narrow size distributions. The diameters of the bare and Pd(II)-anchored silica nanoparticles SiO2−Pd(II) were initially determined by transmission electron microscope (TEM). As shown in parts a and b of Figure 1, the average diameter of SiO2−Pd(II) was estimated to be ∼78 nm, which is almost identical to that of the bare silica nanoparticle (∼70 nm). Dynamic light scatting (DLS) analyses suggested the hydrodynamic diameters of bare and Pd(II)anchored silica nanoparticles are 72 and 80 nm, respectively, which agree well with the TEM observations (Figure S4, Supporting Information). The evidence for the Pd(II)anchored silica nanoparticles were also come from FT-IR and TGA. Figure 2a showed the FT-IR spectra of bare and Pd(II)anchored silica nanoparticles. For bare silica nanoparticle, vibration absorption peaks characteristic of tetrahedron silica structures were located at 1100 cm−1 (Si−O stretching) and 465 cm−1 (Si−O bending). The absorptions at 945 and 800 C


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Macromolecules hybrid nanoparticle were estimated to be 2.96 × 104 and 1.14, respectively, by SEC with equivalent to polystyrene standards. The narrow molecular weight distribution suggested the surface-initiated polymerization may proceeded in a living/ controlled chain-growth manner. To verify this, a series of polymerizations of monomer 1 with SiO2−Pd(II) was carried out in different initial feed ratio of monomer to initiator. As outlined in Figure 3a, all the isolated core/shell polymers showed single model elution peaks on the SEC curves. Moreover, the Mns of the hybrid nanoparticles were linearly correlated to the initial feed ratio of monomer to initiator, while the Mw/Mns were narrow with Mw/Mn < 1.20 (Figure 3b). These results revealed that the surface-initiated polymerization of phenyl isocyanide did proceed in a controlled chain-growth manner. To further verify the living nature, poly-1m arms were etched from the isolated hybrid silica nanoparticles using hydrofluoric acid (see Supporting Information for details). The structures of poly-1m arms were confirmed by 1H NMR and FT-IR (Figure S1 and S2, Supporting Information). The Mn and Mw/Mn values of the etched poly-1m were determined by SEC with equivalent to polystyrene standard. As expected, all the etched poly-1m arms exhibited single model and symmetric elution peaks on the SEC traces (Figure S3a, Supporting Information). Moreover, the Mns of the poly-1ms showed a linear correlation to the initial feed ratios of monomer to Pd(II) initiator (Figure S3b, Supporting Information). Although the Mn values of the etched poly-1m arms were much lower than that of the precursor SiO2−poly-1m hybrid nanoparticles, the molecular weight distributions of poly-1ms were still narrow with Mw/Mn < 1.20 (Table S1 in Supporting Information). The structure of the poly(phenyl isocyanide)-grafted silica nanoparticle was further verified by TEM observation. As shown in Figure 1c, well-defined spherical nanoparticles with ∼120 nm diameter was clearly observed on the TEM image of SiO2−poly-1100. The diameter was about 50 nm larger than that of the bare silica nanoparticle, suggesting the formation of organic/inorganic hybrid silica nanoparticle. DLS analyses indicated the average diameter of the resulting hybrid silica nanoparticle SiO2−poly-1100 is about 128 nm, which agree well with the TEM observation (Figure S4, Supporting Information). DLS studies also suggested that the average diameter of the hybrid silica nanoparticles was depending on the length of the poly(phenyl isocyanide) arms. The diameter increased with the increase of the chain lengths of the poly-1ms. For example, the diameters of SiO2−poly-1120 and SiO2−poly-1100 were estimated to be 140 and 128 nm, respectively, larger than that of the SiO2−poly-180 which is ca. 90 nm (Figure S4, Supporting Information). Note that the three hybrid silica nanoparticles have the same silica cores. Thus, the shell thickness of the hybrid poly(phenyl isocyanide)-grafted silica nanoparticles can be facilely tuned through the variation on the chain length of poly-1m arms. FT-IR spectrum of SiO2−poly-1100 was shown in Figure 2a. In addition to the vibration coming from the silica core, the vibration absorptions attributable to the poly-1100 arms can be clearly observed. For example, the sharp absorption at 1740 cm−1 was ascribed to the vibration of ester CO; the absorption at 1630 cm−1 is the characteristic absorption of C N of the poly(phenyl isocyanide) backbone. From TGA curve, the weight retention at 800 °C for SiO2−poly-1100 was determined to be ∼65.6 wt % (Figure 2b). Thus, the grafting density of poly-1100 chains at the surface of silica nanoparticles can be roughly estimated to be ∼0.89 nm2/chain, which is relatively high. Since the grafting density of the Pd(II) initiator

at the surface of the silica nanoparticle is 0.91 nm2/initiator, the initiation efficiency for the surface-initiated polymerization of monomer 1 is almost quantitative (∼98%), which was further confirmed by 31P NMR analysis (see below). 1H and 13C NMR spectra of SiO2−poly-1100 was almost the same to that of the poly-1100 prepared via the polymerization of 1 using a phenylacetylene Pd(II) complex as catalyst,21 further support the formation of the hybrid silica nanoparticles grafted with poly-1100 arms (Figure S5 and S6, Supporting Information). Moreover, SiO2−poly-1100 showed a sharp singlet at 162.5 ppm on the 13C NMR assigned the imino carbons of the main chain of poly-1100 arms. The half-bandwidth was estimated to be ca. 30 Hz, which indicates the poly-1100 arms of the nanoparticle have high stereoregularity. 31P NMR spectra of TEOS−Pd(II), SiO2−Pd(II) and the resulting SiO2−poly-1100 were displayed in Figure 4. The TEOS−Pd(II) and SiO2−Pd(II) exhibited

Figure 4. 31P NMR (121.5 MHz) spectra of TEOS−Pd(II), Pd(II)anchored silica nanoparticle SiO2−Pd(II), and the resulting hybrid silica nanoparticle SiO2−poly-1100 measured in CDCl3 at 25 °C.

almost the same chemical shift at 17.6 ppm on 31P NMR spectra, suggested the structure of the Pd(II) units was maintained after the silanization coupling reaction of TEOS− Pd(II) with the bare silica nanoparticle. However, after the surface-initiated polymerization of monomer 1, the resulting hybrid SiO2−poly-1100 showed an resonance at 14.3 ppm on 31 P NMR, which can be assigned to the PEt3 of the Pd(II) complex resided at the poly(phenyl isocyanide) chain end.20,21 Remarkably, no signals at 17.6 ppm attributable to the unreacted Pd(II) initiator could be observed, indicated all the anchored Pd(II) initiators were participated in the polymerization. That is, the initiation efficiency of the surface-anchored Pd(II) complex is very high (almost quantitative), which is consistent to the TGA results. To get more details of the surface-initiated polymerization, the polymerization of 1 with SiO2−Pd(II) was performed with the presence of polystyrene (Mn = 2620, Mw/Mn = 1.06) as internal standard in THF at 55 °C, and the polymerization was followed by SEC. The time-dependent SEC curves were displayed in Figure 5. Comparing to the polystyrene standard, monomer 1 was continually consumed with the progress of the polymerization. Accompanied by the conversion of the monomer 1, a new elution peak appeared at high molecularweight region corresponding to the poly-1m-grafted silica nanoparticle was observed, which gradually shifted to the shorter retention time region with the progress of the D


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chain-growth manner. Therefore, a variety of organic/inorganic hybrid silica nanoparticle grafted with stereoregular helical poly(phenyl isocyanide)s with controlled Mn, narrow Mw/Mn were facilely prepared and isolated in high yields (Table 1). Table 1. Results for the Surface-Initiated Polymerizations of Monomer 1 by SiO2−Pd(II) in THF at 55 °Ca run 1 2 3 4 5 6

[1]0/[Pd]0b

SiO2−poly-1m

Mn (Da)c

Mw/Mnc

yieldd

20 40 60 80 100 120

SiO2−poly-120 SiO2−poly-140 SiO2−poly-160 SiO2−poly-180 SiO2−poly-1100 SiO2−poly-1120

× × × × × ×

1.13 1.17 1.19 1.18 1.19 1.20

91% 92% 95% 92% 95% 91%

7.70 1.16 1.65 2.38 2.89 3.65

3

10 104 104 104 104 104

a The polymerizations were carried out according to Scheme 1. bThe initial feed ratio of monomer 1 to Pd(II) unit. cThe Mn and Mw/Mn data of poly-1m-grafted nanoparticles were determined by SEC analyses with equivalent to polystyrene standards. dIsolated yields obtained by gravimetric method.

Figure 5. Time-dependent SEC curves for the surface-initiated polymerization of monomer 1 using SiO2−Pd(II) in THF at 55 °C with the presence of polystyrene (Mn = 2620, Mw/Mn = 1.06) as internal standard ([1]0 = 0.2 M, [1]0/[Pd]0 = 100). SEC conditions: eluent = THF; temperature = 40 °C. The Mn and Mw/Mn of the hybrid silica nanoparticles were determined by SEC with equivalent to polystyrene standards.

To further confirm the helical conformation of the grafted poly(phenyl isocyanide) arms on the organic/inorganic hybrid silica nanoparticles, optically active phenyl isocyanide monomers 2L and 2D were prepared and polymerized by SiO2− Pd(II) in THF at 55 °C ([2L or 2D]0 = 0.2 M, [2L or 2D]0/ [Pd]0 = 100), followed the same procedure described above (Scheme 1). SEC analyses of the isolated hybrid silica nanoparticles, SiO2−poly-2L100 and SiO2−poly-2D100 indicated the polymerizations were succeeded (Figure S8, Supporting Information). The optical activity of hybrid silica nanoparticles grafted with helical poly-2L100 and poly-2D100 arms were investigated by CD and UV−vis absorption spectra. As shown in Figure 7, both SiO2−poly-2L100 and SiO2−poly-2D100 showed intense absorption at 364 nm owing to the n−π* transition of CN of the main chain. Strong negative CD at 364 nm was observed on the CD spectrum of SiO2−poly-2L100, suggesting the poly-2L100 arms of the nanoparticle possessing a predominated left-handed helical conformation. As expected, due to the predominated right-handed helical conformation of poly-2D100, SiO2−poly-2D100 showed an intense positive CD at 364 nm, which is almost a mirror image to that of the SiO2− poly-2L100. The molar CD intensity at 364 nm (Δε364) for SiO2−poly-2L100 and SiO2−poly-2D100 were estimated to be −16.5 and +15.8, respectively. Because of the stable helical

polymerization. It was found that more than 86% of 1 was consumed within 16 h (Figure 6a). Kinetic studies revealed that the polymerization obey the first-order rate law, and the apparent rate constant was determined to be 7.6 × 10−5 s−1. The Mn value of the generated hybrid silica nanoparticle kept growth and eventually reached to 2.38 × 104 as determined by SEC. Moreover, the Mn increased linearly and in proportion to the conversion of monomer 1 (Figure 6b). All the hybrid nanoparticles isolated at different polymerization stages showed narrow molecular weight distribution with the Mw/Mn < 1.20. To get more details of the grafted poly-1m arms, the isolated hybrid silica nanoparticles were etched by hydrofluoric acid. The Mns of the etched poly-1m arms of the hybrid silica nanoparticles isolated at different polymerization stages were also linearly correlated to the conversion of monomer 1 (Figure S7, Supporting Information). All the poly-1m arms exhibited narrow dispersity as determined by SEC (Mw/Mn < 1.20), although the Mn values were lower than that of the SiO2−poly1m particles. Collectively, these results support the surfaceinitiated polymerization of phenyl isocyanide using the Pd(II)anchored silica nanoparticle was proceed in a living/controlled

Figure 6. (a) Plots of the conversion of monomer 1 with the polymerization time of surface-initiated polymerization using SiO2−Pd(II) in THF at 55 °C and the first-order kinetic plot for the polymerization. (b) Plots of Mn and Mw/Mn values of hybrid silica nanoparticle SiO2−poly-1m as a function of the SiO2−Pd(II) initiated conversion of 1 in THF at 55 °C ([1]0 = 0.2 M, [1]0/[Pd]0 = 100). E


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(Scheme 2). Hybrid silica nanoparticles grafted with poly-1100 was obtained through the silanization coupling reaction of the bare silica nanoparticle (∼70 nm diameter) with poly-1100TEOS in ethanol with the presence of ammonium hydroxide at room temperature for 12 h. The structure of the afforded hybrid silica nanoparticle poly-1100−SiO2 was confirmed by SEC, 1H NMR, TGA, FT-IR, DLS, and TEM observation. The recorded SEC curves of poly-1100−OH and poly-1100− SiO2 were displayed in Figure 8a. The elution peak of poly1100−SiO2 shifted to the shorter retention time region as compared to that of poly-1100−OH, suggested the molecular weight was considerably increased. The Mn of poly-1100−SiO2 was estimated to be 3.95 × 104, larger than that of the poly1100−OH (Mn = 2.80 × 104) precursor, while the Mw/Mn was still narrow (Mw/Mn = 1.21). 1H NMR spectrum of poly-1100− SiO2 measured in CDCl3 at 25 °C was almost the same to that of the poly-1100−OH, revealed the structure of the poly(phenyl isocyanide) was maintained after the silanization coupling reaction (Figure S9, Supporting Information). The structure of poly-1100−SiO2 prepared via the “grafting to” strategy was also confirmed by TGA analysis. As shown in Figure 8b, the weight retention of poly-1100−SiO2 at 800 °C was estimated to be 67.0 wt %, which is about 27.4 wt % different to that of the bare silica nanoparticle (∼94.4 wt %). On the basis of this analysis, the grafting density of poly-1100−SiO2 was roughly estimated to be ∼0.76 nm2/chain, which was lower than that prepared via “grafting from” strategy. FT-IR analyses of the poly-1100−OH and the resulting poly-1100−SiO2 also confirmed the formation of poly-1100-grafted hybrid silica nanoparticle, because the vibration absorptions assigned to the silica core and the poly1100 arms can be clearly observed. For example, the FT-IR spectrum of poly-1100−SiO2 showed absorptions at 1630 and 1740 cm−1 come from the CN and CO groups of the poly-1100 arms; and at 1100 and 465 cm−1 ascribed to the Si−O stretching and bending signals of the silica core (Figure S10, Supporting Information). DLS curves of the poly-1100−OH and poly-1100−SiO2 were shown in Figure 8c, together with the bare silica nanoparticle for comparison. Both poly-1100−OH and poly-1100−SiO2 showed symmetric and narrow DLS curves due to the narrow molecular weight distributions. The hydrodynamic diameter for poly-1100−OH was ca. 8.0 nm, because

Figure 7. CD and UV−vis absorption spectra of the chiral hybrid silica nanoparticles SiO2−poly-2L100 and SiO2−poly-2D100 grafted with helical poly-2L100 and poly-2D100 arms with preferred handedness (c = 0.2 g/L, THF, 25 °C).

conformation, there was no obviously change could be observed on the CD and UV−vis spectra of the hybrid silica nanoparticles at the temperature range from −10 to 50 °C measured in THF. The optical rotations of the SiO2−poly2L100 and SiO2−poly-2D100 were determined to be −1235 and +1168 (c = 0.1, CHCl3, 25 °C), which were consistent to the CD and UV−vis analyses. Collectively, these studies revealed that the poly(phenyl isocyanide)s arms of the hybrid silica nanoparticles possess stable helical conformation with a preferred handedness. Synthesis of Hybrid Silica Nanoparticles via “Grafting to” Strategy. The silica nanoparticle grafted with stereoregular helical poly(phenyl isocyanide) was then attempted to synthesized via “grafting to” strategy. As shown in Scheme 2, hydroxyl terminated stereoregular poly(phenyl isocyanide), poly-1100−OH was prepared through the living polymerization of monomer 1 using the HO-functionalized alkyne-Pd(II) complex as catalyst in THF at 55 °C ([1]0 = 0.20 M, [1]0/ [Pd]0 = 100), followed the procedure reported by our group previously.21b The poly-1100−OH (SEC: Mn = 2.80 × 104, Mw/ Mn = 1.16, Figure 8a) was then reacted with triethoxy(3isocyanatopropyl)silane in dichloromethane with triethylamine as catalyst, which lead to the formation of poly-1100-TEOS

Scheme 2. Synthesis of Hybrid Silica Nanoparticles Grafted with Stereoregular Helical Poly(phenyl isocyanide)s via “Grafting to” Strategy

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Figure 8. (a) Size exclusion chromatograms of poly-1100−OH and the resulting poly-1100−SiO2 hybrid silica nanoparticle. (b) TGA curves for bare silica nanoparticle and the resulting hybrid silica nanoparticle poly-1100−SiO2 grafted with poly-100 arms (TGA was performed in air at a heating rate of 10 °C/min). (c) DLS curves for poly-1100−OH, bare silica nanoparticle, and hybrid silica nanoparticle poly-1100−SiO2. (d) TEM image of the poly-1100−SiO2 hybrid silica nanoparticle. SEC conditions: eluent = THF; temperature =40 °C.

the polymer was well molecularly dissolved. The hydrodynamic diameter of poly-1100−SiO2 was increased to 120 nm, larger than that of the bare silica nanoparticle (∼70 nm) and the poly1100−OH precursor, indicated the poly-1100 was grafted onto the surface of silica nanoparticle. The morphology of poly1100−SiO2 was further investigated by TEM. As shown in Figure 8d, well-defined spherical nanoparticles were clearly observed on the TEM image. The average diameter of the nanoparticles was estimated to be 110 nm, about 40 nm larger than that of the bare silica nanoparticles (∼70 nm). These studies confirmed the succeeded synthesis of hybrid silica nanoparticle grafted with stereoregular helical poly(phenyl isocyanide) arms via “grafting to” strategy. Taking advantage of this method, a series of hybrid silica nanoparticles grafted with helical poly(phenyl isocyanide) can be facilely prepared (Table S2, Supporting Information). After “grafting to” strategy for the synthesis of poly(phenyl isocyanide)-grafted hybrid silica nanoparticles was established, the effort was directed to the preparation of chiral hybrid silica nanoparticles grafted with optically active, single-handed helical poly(phenyl isocyanide) arms. As shown in Scheme 2, HOterminated left-handed helical poly-2L100−OH (Mn = 3.82 × 104, Mw/Mn = 1.16) bearing an L-alanine pendant with a decyl chain was prepared according to the reported procedure.21b The helicity of poly-2L100−OH was studied by CD and UV−vis spectra. As displayed in Figure 9, poly-2L100−OH showed intense negative CD at 364 nm and the Δε364 was estimated to be −20.5, confirmed the single left-handed helical conformation of the main chain.21 After poly-2L100−OH reacted with triethoxy(3-isocyanatopropyl)silane, it was grafted to the surface of silica nanoparticles via silanization coupling reaction, afforded the expected hybrid nanoparticle poly-2L100−SiO2 grafted with left-handed helical poly-2L100 arms. SEC analysis

Figure 9. CD and UV−vis spectra of poly-2L100−OH and the resulting poly-2L100−SiO2 hybrid silica nanoparticle measured in THF at 25 °C (c = 0.20 g/L).

indicated the resulting poly-2L100−SiO2 has Mn of 4.92 × 104 Da (Figure S11, Supporting Information), larger than that of the poly-2L100−OH precursor, while the Mw/Mn was narrow (Mw/Mn = 1.16). CD and UV−vis spectra of poly-2L100−SiO2 were similar to those of the poly-2L100−OH precursor (Figure 9). The Δε364 was estimated to be −19.5, almost the same to that of the poly-2L100−OH, suggesting the single left-handed helical conformation of poly-2L100 arms was maintained after grafted to the surface of the silica nanoparticles. The structure of poly-2L100−SiO2 was further confirmed by FT-IR, DLS, TEM, and TGA analyses (Figure S12−S15, Supporting Information). DLS and TEM studies indicated poly-2L100− SiO2 have a diameter of ca. 110 nm, about 40 nm larger than that of the bare silica nanoparticles (Figures S13 and S14, Supporting Information). The graft density of poly-2L100−SiO2 G


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Macromolecules

Figure 10. (a) CD and UV−vis spectra of crystal and the rest solution of the enantioselective crystallization of Boc-alanine induced by poly-2L100− SiO2 in methanol at room temperature. CD and UV−vis spectra of commercial enantiomeric pure Boc-D- and Boc-L-alanine were shown for comparison. (b) Plot of ee (%) value of the solution as a function of crystallization time of the enantioselective crystallization of Boc-alanine in methanol induced by poly-2L100−SiO2. SEM images of the crystals obtained via crystallization of Boc-alanine without (c) and with (d) the presence of poly-2L100−SiO2.

was estimated to be ∼0.78 nm2/chain, based on the TGA analysis (Figure S15, Supporting Information). Enantioselective Crystallization. Since the hybrid silica nanoparticles SiO2−poly-2L100 and SiO2−poly-2D100 and poly2L100−SiO2 bear dense and optically active helical poly(phenyl isocyanide) arms, these materials may be used in chiral recognition and enantiomer separation. Each optically active hybrid silica nanoparticle has a large amount of chiral microenvironment and can be considered as a bulk “chiral” entity. Thus, it may act as nucleation sites for induce preferential crystallization of one enantiomer from the racemic solution. To verify this hypothesis, the enantioselective crystallization of racemic compounds using the chiral hybrid silica nanoparticles was then performed. In this work, Bocalanine were used as model system for enantioselective crystallization, mainly because of its biological importance and extensive used as chiral source and building block in organic synthesis. Because of the high optical activity, poly2L100−SiO2 was first used in the enantioselective crystallization. A small amount of poly-2L100−SiO2 was added into a supersaturated methanol solution of racemic Boc-alanine, a typical model compound for enantioselective crystallization.19 After the solution was standing at room temperature in an open vial for several hours, needle crystals were gradually formed. CD spectrum of the induced crystal showed negative Cotton effect around 210 nm, suggesting Boc-D-alanine was enantioselective crystallized (Figure 10a). Comparing the molar CD intensity at 210 nm of the induced crystal with the commercial enantiopure Boc-D-alanine, the ee value of the crystal was determined to be 95%, which was also confirmed by optical rotation ([α]25D = −26.4, c = 0.15, methanol).19 The process of the crystallization was then followed by measuring CD and UV−vis spectra of the rest solution. As expected, increasing positive CD at 210 nm was observed with the process of crystallization, which indicated Boc-D-alanine was gradually

induced to crystallize by the chiral poly-2L100−SiO2, left the antipode Boc-L-alanine in the solution (Figure S16, Supporting Information). A plot of the ee value of the solution with the crystallization time was displayed in Figure 10b. It was found that the ee was gradually increased, and eventually reached ca. 95%. Therefore, it can be concluded that the chiral hybrid silica nanoparticle poly-2L100−SiO2 grafted with optically active helical poly(phenyl isocyanide) arms are good chiral materials for enantioselective crystallization. For comparison, crystallization of racemic Boc-alanine was performed under the same experimental condition but without addition of chiral poly-2100−SiO2 nanoparticle. Although crystals were obtained, however, no CD was observed on either the crystals or the rest solution. The crystals obtained with and without the presence of chiral poly-2L100−SiO2 nanoparticle were investigated by scanning electron microscope (SEM). As displayed in Figure 10c, without the addition of chiral poly-2L100−SiO2, the racemic crystals showed octahedral structure. In sharp contrast, crystals induced by the chiral poly2L100−SiO2 nanoparticle showed needle-like morphology with high regularity. These results demonstrated that the facilely synthesized organic/inorganic hybrid silica nanoparticles grafted with single handed helical poly(phenyl isocyanide) are good chiral materials for enantioselective crystallization. The enantioselective crystallization was further performed using hybrid silica nanoparticle SiO2−poly-2L100 prepared via “grafting from” strategy. The induced crystals also showed negative CD at 210 nm, similar to that induced by poly-2L100− SiO2 (Figure S17, Supporting Information). However, because of lower optical activity of SiO2−poly-2L100 (Δε364 = −16.5), the ee value of the induced crystals was estimated to be 82%, lower than that obtained by using poly-2L100−SiO2. Accordingly, SiO2−poly-2D100 nanoparticle enantioselectively induced crystallization of Boc-L-alanine, and ee value was estimated to be ca. 80%. Lastly, optically active phenyl isocyanide H


Article

Macromolecules

Single- and Double-Stranded Helical Polymers: Synthesis, Structures, and Functions. Acc. Chem. Res. 2008, 41, 1166. (c) Yashima, E.; Maeda, K.; Iida, H.; Furusho, Y.; Nagai, K. Helical Polymers: Synthesis, Structures, and Functions. Chem. Rev. 2009, 109, 6102. (d) Zhang, D.-W.; Zhao, X.; Hou, J.-L.; Li, Z.-T. Aromatic Amide Foldamers: Structures, Properties, and Functions. Chem. Rev. 2012, 112, 5271. (e) Reuther, J. F.; Novak, B. M. Evidence of EntropyDriven Bistability through 15N NMR Analysis of a Temperature- and Solvent-Induced, Chiroptical Switching Polycarbodiimide. J. Am. Chem. Soc. 2013, 135, 19292. (f) Reuther, J. F.; Bhatt, M. P.; Tian, G.; Batchelor, B. L.; Campos, R.; Novak, B. M. Controlled Living Polymerization of Carbodiimides Using Versatile, Air-Stable Nickel(II) Initiators: Facile Incorporation of Helical, Rod-like Materials. Macromolecules 2014, 47, 4587. (g) Shen, J.; Okamoto, Y. Efficient Separation of Enantiomers Using Stereoregular Chiral Polymers. Chem. Rev. 2016, 116, 1094. (h) Freire, F.; Quiñoá, E.; Riguera, R. Supramolecular Assemblies from Poly(phenylacetylene)s. Chem. Rev. 2016, 116, 1242. (3) (a) Maeda, K.; Wakasone, S.; Shimomura, K.; Ikai, T.; Kanoh, S. Helical polymer brushes with a preferred-handed helix-sense triggered by a terminal optically active group in the pendant. Chem. Commun. 2012, 48, 3342. (b) Wang, R.; Li, X.; Bai, J.; Zhang, J.; Liu, A.; Wan, X. Chiroptical and Thermotropic Properties of Helical Styrenic Polymers: Effect of Achiral Group. Macromolecules 2014, 47, 1553. (c) Zhu, Y.-Y.; Yin, T.-T.; Li, X.-L.; Su, M.; Xue, Y.-X.; Yu, Z.-P.; Liu, N.; Yin, J.; Wu, Z.-Q. Synthesis and Chiroptical Properties of Helical Polyallenes Bearing Chiral Amide Pendants. Macromolecules 2014, 47, 7021. (d) Chae, C.-G.; Shah, P. N.; Seo, H. B.; Lee, J.-S.; Min, J. Synthesis of Novel Amphiphilic Polyisocyanate Block Copolymer with Hydroxyl Side Group. Macromolecules 2014, 47, 1563. (e) Sakai, N.; Satoh, T.; Kakuchi, T. Rod-Like Amphiphile of Diblock Polyisocyanate Leading to Cylindrical Micelle and Spherical Vesicle in Water. Macromolecules 2014, 47, 1699. (f) Jiang, S.; Zhao, Y.; Wang, L.; Yin, L.; Zhang, Z.; Zhu, J.; Zhang, W.; Zhu, X. Photocontrollable induction of supramolecular chirality in achiral side chain Azo-containing polymers through preferential chiral solvation. Polym. Chem. 2015, 6, 4230. (g) Reuther, J. F.; Siriwardane, D. A.; Kulikov, O. V.; Batchelor, B. L.; Campos, R.; Novak, B. M. Facile Synthesis of Rod−Coil Block Copolymers with Chiral, Helical Polycarbodiimide Segments via Postpolymerization CuAAC “Click” Coupling of Functional End Groups. Macromolecules 2015, 48, 3207. (h) Huang, H.; Yuan, Y.; Deng, J. Helix-Sense-Selective Precipitation Polymerization of Achiral Monomer for Preparing Optically Active Helical Polymer Particles. Macromolecules 2015, 48, 3406. (i) Rodríguez, R.; Quiñoá, E.; Riguera, R.; Freire, F. Architecture of Chiral Poly(phenylacetylene)s: From Compressed/Highly Dynamic to Stretched/Quasi-Static Helices. J. Am. Chem. Soc. 2016, 138, 9620. (4) (a) Schwartz, E.; Koepf, M.; Kitto, H. J.; Nolte, R. J. M.; Rowan, A. E. Helical poly(isocyanides): past, present and future. Polym. Chem. 2011, 2, 33. (b) Kouwer, P. H. J.; Koepf, M.; Le Sage, V. A. A.; Jaspers, M.; Van Buul, A. M.; Eksteen-Akeroyd, Z. H.; Woltinge, T.; Schwartz, E.; Kitto, H. J.; Hoogenboom, R.; Picken, S. J.; Nolte, R. J. M.; Mendes, E.; Rowan, A. E. Responsive biomimetic networks from polyisocyanopeptide hydrogels. Nature 2013, 493, 651. (c) Hu, G.; Li, W.; Hu, Y.; Xu, A.; Yan, J.; Liu, L.; Zhang, X.; Liu, K.; Zhang, A.-F. Water-Soluble Chiral Polyisocyanides Showing Thermoresponsive Behavior. Macromolecules 2013, 46, 1124. (5) (a) Wu, Z.-Q.; Nagai, K.; Banno, M.; Okoshi, K.; Onitsuka, K.; Yashima, E. Enantiomer-Selective and Helix-Sense-Selective Living Block Copolymerization of Isocyanide Enantiomers Initiated by Single-Handed Helical Poly(phenyl isocyanide)s. J. Am. Chem. Soc. 2009, 131, 6708. (b) Miyabe, T.; Iida, H.; Banno, M.; Yamaguchi, T.; Yashima, E. Synthesis and Visualization of a Core Cross-Linked Star Polymer Carrying Optically Active Rigid-Rod Helical Polyisocyanide Arms and Its Chiral Recognition Ability. Macromolecules 2011, 44, 8687. (c) Miyabe, T.; Iida, H.; Ohnishi, A.; Yashima, E. Enantioseparation on poly(phenyl isocyanide)s with macromolecular helicity memory as chiral stationary phases for HPLC. Chem. Sci. 2012, 3, 863.

monomers, 2L and 2D were employed in the crystallization of racemic Boc-alanine under the same conditions to those of SiO2−poly-2L100 and poly-2L100−SiO2, however, no enantiomeric selectivity was observed on the crystallizations. Thus, it can be concluded that the enantiomeric selectivity of the crystallization induced by the chiral hybrid silica nanoparticles was come from the helical sense of the poly(phenyl isocyanide) arms.

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CONCLUSIONS In summary, well-defined organic/inorganic hybrid silica nanoparticles grafted with stereoregular helical poly(phenyl isocyanide) with high grafting density were facilely synthesized via both “grafting from” and “grafting to” strategies. The surface-initiated polymerization of the phenyl isocyanides by Pd(II)-anchored silica nanoparticle was revealed to proceed in living/controlled chain-growth manner, afforded the hybrid silica nanoparticles in high yields with controlled thickness. Moreover, the hybrid silica nanoparticles can also be prepared via “grafting to” strategy through the silanization coupling reaction of TEOS-terminated stereoregular poly(phenyl isocyanide) with bare silica nanoparticle. Chiral hybrid silica nanoparticles grafted with optically active helical poly(phenyl isocyanide)s can be obtained via these two synthetic methods by using chiral phenyl isocyanide monomers. The chiral hybrid nanoparticles have been demonstrated the exciting utility for enantioselective crystallization of racemic Boc-alanine. The ee value of the induced enantiomer can be up to 95%. Thus, we believe the present study provides not only synthetic methods for controlled preparation of well-defined organic/inorganic hybrid silica nanoparticles grafted with stereoregular helical poly(phenyl isocyanide) but also new chiral materials for enantiomer separation.

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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/acs.macromol.6b01870. Additional experimental details and spectral data (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21371043, 21574036, and 21622402). Z.-Q.W. thanks the Thousand Young Talents Program of China for Financial Support.

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REFERENCES

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