Combination of RAFT and Pd(II)-Initiated Isocyanide Polymerizations

Feb 2, 2018 - We report a new method for facile synthesis of block and star copolymers composed of helical poly(phenyl isocyanide) and polyacrylate ...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Combination of RAFT and Pd(II)-Initiated Isocyanide Polymerizations: A Versatile Method for Facile Synthesis of Helical Poly(phenyl isocyanide) Block and Star Copolymers Zhi-Qiang Jiang, Song-Qing Zhao, Yi-Xu Su, Na Liu, and Zong-Quan Wu* Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, and 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: We report a new method for facile synthesis of block and star copolymers composed of helical poly(phenyl isocyanide) and polyacrylate segments via combination of Pd(II)initiated isocyanide polymerization and reversible addition− fragmentation chain transfer (RAFT) controlled radical polymerization. First, an alkyne−Pd(II) complex containing benzyl trithiocarbonate substituent was designed and synthesized. The Pd(II) moiety of the complex can catalyzed the living polymerization of phenyl isocyanides, while the benzyl trithiocarbonate unit can be used as a chain transfer agent for RAFT polymerization. Upon combination of the two living polymerizations, a series of block copolymers containing poly(phenyl isocyanide) and polyacrylate, poly(aryl vinyl) blocks with expected molecular weights (Mns) and narrow molecular weight distributions (Mw/Mns) can be facilely prepared under controlled manner. Some of these block copolymers exhibited interesting self-assembly and stimuli-responsive properties. Taking advantage of this synthetic approach, core cross-linked star block copolymer carrying polyacrylate-block-polyisocyanide arms can be readily prepared through the copolymerization with a cross-linker, ethylene glycol dimethacrylate. Moreover, miktoarm star copolymers carrying helical polyisocyanides and polyacrylates arms can also be prepared under controlled manner.

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INTRODUCTION The helix is one of the most interesting structures in the nature. It is ubiquitous both in macroscale and microscale such as helix in the galaxy and double helix in DNA.1,2 Stimulated by the perfect helical structures in nature and their close relations to the sophisticated functions in living system, chemists have been challenged to develop artificial helical polymers and oligomers.3−9 Through these studies, a variety of new chiral materials with novel functions and properties were fabricated.10−14 Synthetic helical polymers with controlled helix sense have exhibited a range of applications including chiral recognition,10,11 enantiomer separation,12,13 and liquid crystallization.14 In this context, helical polyisocyanide is of particular interest. It is a kind of conjugated polymer and can adopt a rigid-rod helical conformation.15−18 Optically active helical polyisocyanide has been widely used in supramolecular selfassembly, enantiomer separation, and asymmetric catalysis,19,20 among others. Incorporation of helical polyisocyanide onto conjugated polymers forming the block copolymers can control the self-assembly morphology and consequently tuning the optoelectronic properties.21,22 Hybrid silica nanoparticles grafted with optically active helical polyisocyanides turned out to be an effective material for enantiomer separation.12 Polyisocyanides bearing proline pendants showed significantly enhanced enantioselectivity and diastereoselectivity in aldol reaction comparing with the corresponding small molecule.23 © XXXX American Chemical Society

Reversible addition−fragmentation chain transfer (RAFT) polymerization is one of the most versatile controlled radical polymerizations because of its mild polymerization condition and applicable to most common vinyl monomers such as styrene, (meth)acrylates, (meth)acrylamides, and others.24−30 Moreover, it can tolerance to various functional groups and compatibility with a wide range of conditions (bulk, solution, emulsion, and suspension).31−36 Recently, we reported a family of alkyne−Pd(II) complexes that can initiate living polymerizations of various isocyanide monomers, forming well-defined stereoregular helical polyisocyanides under controlled manner.37,38 We envision that combination of the Pd(II)-initiated isocyanide polymerization with RAFT polymerization may bring out a synthetic approach for synthesis of new chiral materials with novel properties and functions. However, to the best of our knowledge, the compatibility of RAFT with Pd(II)initiated isocyanide polymerization has never been investigated to date. Compared to the linear polymer, core cross-linked star (CCS) polymers composed of cross-linked core and densely arms have exhibited unique properties and broad applications that cannot be accessed by the linear analogues.39−44 Generally, Received: December 15, 2017 Revised: January 21, 2018

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DOI: 10.1021/acs.macromol.7b02663 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of BSTP−Pd(II) Complex and the Block Copolymers

Figure 1. (a) SEC curves of poly-1a50 and poly-1L50 (eluent = THF, temperature = 40 °C). (b) CD UV−vis spectra of poly-1L50 measured in THF at 25 °C.

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RESULTS AND DISCUSSION As shown in Scheme 1, the BSTP−Pd(II) was synthesized in two steps. First, 3-benzylsulfanylthiocarbonylsulfanylpropionic acid reacted with 2-propynylamine in CH2Cl2 using 1-ethyl-3(3-(dimethylamino)propyl)carbodiimide hydrochloride (EDCI) as catalyst to afford BSTP−alkyne in 86% yield, which was treated with trans-dichlorobis(triethylphosphine)palladium in CH2Cl2 and Et3N with the presence of cuprous iodide as catalyst. The desired BSTP−Pd(II) was isolated in 82% yield as a yellow solid. The structure of BSTP−Pd(II) was fully characterized by 1H and 13C NMR, mass spectrum, FT-IR, and elemental analysis (Figures S1−S6). 1H NMR of BSTP− Pd(II) showed characteristic resonances of PCH2CH3 at 1.92 (CH2) and 1.16 ppm (CH3). Besides, the integration area ratios of CH3 at 1.13−1.21 ppm and the phenyl protons at 7.27−7.33 ppm was roughly estimated to be 18:5, which agreed with the proposed structure. The FT-IR spectrum also confirmed the structure of BSTP−Pd(II) because diagnostic vibrations appeared at 2132 and 1643 cm−1 respectively ascribed to the CC and CO groups were clearly discerned. With the catalyst in hands, our efforts were directed to its polymerization ability. The catalytic activity of BSTP−Pd(II) on isocyanide polymerization was first studied by treating it with a phenyl isocyanide bearing a decyl carbon chain (1a) in THF at 55 °C ([1a]0/[BSTP−Pd(II)]0 = 50, [1a]0 = 0.2 M). As anticipated, the polymerization afforded poly-1a50 (the subscript indicates the initial feed ratio of monomer to catalyst)

the functions and the responsiveness of a star polymer were mainly controlled by the density and the properties of the arms. In this context, star block copolymer and miktoarm star polymers containing two or more different blocks on the arms have attracted great attention for their structural diversity and variety applications.45−49 However, to the best of our knowledge, reports on such kinds of star polymers are very rare, probably owing to the limited synthetic approaches.50 In this work, we report a new method for facilely synthesis of block and star polymers carrying helical polyisocyanide, polyacrylate, and poly(aryl vinyl) segments. A new Pd(II) complex containing a RAFT chain transfer agent 3benzylsulfanylthiocarbonylsulfanylpropionic amide (BSTP) and an alkyne−Pd(II) unit was designed and synthesized (Scheme 1). Such BSTP−Pd(II) complex can not only catalyze the living polymerization of isocyanide but also promote a RAFT polymerization of acrylate and aryl vinyl monomers. Upon combination of the two living polymerizations, a variety of block copolymers, core cross-linked star block copolymers, and miktoarm star polymers composed of helical polyisocyanide and polyacrylate segments with controlled molecular weights (Mns), narrow molecular weight distributions (Mw/ Mns), and tunable compositions can be facilely obtained. The block copolymers showed interesting self-assembly and stimuliresponsive properties, while the star polymers carrying excess of one-handed helical arms exhibited excellent performance on enantioselective crystallization. B

DOI: 10.1021/acs.macromol.7b02663 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

polymers showed narrow Mw/Mns (Table S2 and Figure S15). These studies clearly demonstrated that the BSTP−Pd(II) complex is a good chain transfer agent for RAFT polymerization, and the Pd(II) unit has no negative effect on the controlled RAFT polymerizations. Since the BSTP−Pd(II) catalyst can promote both the living polymerizations of phenyl isocyanide and acrylate monomers, block copolymer and core cross-linked star polymers composed of helical polyisocyanide and polyacrylate segments may be facilely produced through the combination of the two living polymerizations. Thus, the compatibility of RAFT with Pd(II)initiating isocyanide polymerization was investigated. First, the isolated poly-1L100 bearing BSTP terminus was employed as a macromolecular chain transfer agent for RAFT polymerization. As shown in Scheme 1, the isolated poly-1L100 was treated with 2a and AIBN in THF at 55 °C to perform a block copolymer containing helical poly-1L100 and poly-2an segments. However, the polymerization did not take place; even it was carried out at high temperature (100 °C in toluene) and for a long time (24 h). Probably, the CN double bonds of the polyisocyanide segment interfered with the RAFT polymerization. To verify this, we carried out RAFT homopolymerization of 2a with the presence of poly-1L100 under the same conditions (see Supporting Information for details). No RAFT polymerization took place even after a long reaction time. Therefore, the block copolymerization was switched to a reversed chain extension sequence. That is, the Pd(II)-terminated poly-2a100 was used as a macroinitiator to copolymerize a isocyanide monomer. The block polymerization of 1a with Pd(II)-terminated poly-2a100 was carried out in THF at 55 °C ([1a]0 = 0.2 M, [1a]0/ [Pd(II)]0 = 50), following the procedure described above. SEC analysis of the resulting polymer indicated chain extension occurred (Figure 2). Comparing to the poly-2a100 homopolymer precursor, the SEC trace of the isolated poly(2a100-b-1a50) was shifted to shorter retention time region. The Mn of the isolated block copolymer poly(2a100-b-1a50) was determined to be 23.8 kDa, larger than that of the poly-2a100 precursor (Mn = 11.3 kDa, Mw/Mn = 1.14), while the Mw/Mn remains narrow, which is 1.22. The FT-IR spectrum of the block copolymer showed more signals in addition to those of poly-2a100. For example, an intense absorption located at 1602 cm−1 was attributed to the characteristic of CN vibration of poly-1a50 segment. 1H NMR analyses also support the formation of expected block copolymer because the resonances attributed to both poly-2a100 and poly-1a50 segments can be clearly observed (Figure 3). For example, the peaks at 7.15−7.53 and 5.47−6.03 ppm were assigned to the phenyl resonances of the poly-1a50 segment, while the resonance at 2.16−2.41 ppm came from the CH protons of the poly-2a100 block. Moreover, the block ratio of poly-1a50 to poly-2a100 deduced from the chemical shifts at 5.47−6.03 (phenyl protons of poly-1a50) and 2.16−2.41 ppm (CH of poly-2a100 backbone) is ca. 0.50, which agree well with the initial feed ratio of the two monomers used in the copolymerization ([1a]0/[2a]0 = 0.5). To get more details on the block copolymerization, 31P NMR spectra of BSTP−Pd(II) complex, poly-1a50, poly-2a100, and poly(2a100-b-1a50) were recorded and are displayed in Figure 4. As anticipated, poly2a100 exhibited the same 31P chemical shift as that of BSTP− Pd(II) and was located at 17.7 ppm on 31P NMR spectra, suggesting the Pd(II) unit was maintained during the RAFT polymerization of 2a. However, after the copolymerization of 1a with poly-2a100, the 31P resonance of poly(2a100-b-1a50) was shifted to 14.5 ppm, similar to that of the poly-1a 50

in almost quantitative yield. The Mn and Mw/Mn were determined to be 12.4 kDa and 1.19, respectively, by size exclusion chromatography (SEC) analysis with equivalent to polystyrene standards. Further studies revealed that the Mn of the isolated poly-1ams was linearly correlated to the initial feed ratio of monomer to catalyst, and all the isolated polymers showed narrow dispersity with Mw/Mn < 1.23 (Table S1 and Figure S12). The polymerization of an enantiopure phenyl isocyanide (1L) bearing L-alanine ester with a long decyl carbon chain with BSTP−Pd(II) was conducted under the same procedure. The isolated poly-1L50 (Mn = 15.2 kDa, Mw/Mn = 1.22) showed intense CD at the absorption region of the polyisocyanide backbone (Figure 1b). The molar CD intensity at 364 nm of poly-1L50 was estimated to be −15.4, suggesting the formation of an excess of left-handed helix. These results indicated the BSTP−Pd(II) is a good catalyst for the living polymerization of phenyl isocyanide, affording well-defined helical polyisocyanide in high yields with controlled Mn and narrow Mw/Mn. Moreover, it can be concluded that the introduction of BSTP group onto the Pd(II) catalyst has no negative effect on the Pd(II)-initiated living polymerization of isocyanide. After the polymerization of isocyanide using BSTP−Pd(II) as catalyst was established, RAFT polymerization using BSTP− Pd(II) as chain transfer agent was then investigated. As outlined in Scheme 1, the polymerization was carried out in THF at 55 °C using n-butyl acrylate (2a) as a representative monomer with azoisobutyronitrile (AIBN) as initiator ([2a]0 = 2.0 M, [2a]0/[BSTP−Pd(II)]0/[AIBN]0 = 100/1/1). SEC trace of the produced poly-2a100 shows a single model elution peak at high molecular weight region (Figure 2). The Mn and Mw/Mn of

Figure 2. (a) SEC curves of poly-2a100, poly(2a100-b-1a50), and poly(2a100-b-1L50). SEC conditions: eluent = THF; temperature = 40 °C.

poly-2a100 were estimated to be 11.3 kDa and 1.14, respectively. The structure of poly-2a100 was further confirmed by FT-IR and 1 H NMR spectra. An absorption band at 1722 cm −1 corresponding to νC=O vibration was observed in the FT-IR spectrum. 1H NMR showed resonances of CO2CH2 and CH of the main chain located at 3.91−4.16 and 2.24−2.34 ppm, respectively. Interestingly, the resonances of the CH3 on the terminal Pd(PEt3)2Cl can be clearly observed at 1.09−1.20 ppm, suggesting the structure of the Pd(II) unit was maintained during the RAFT polymerization. To verify the living nature of the polymerization, a series of polymerizations in different initial feed ratios of 2a to BSTP−Pd(II) were performed under the same experimental conditions. It was found that the Mn of the afforded poly-2ams increased linearly and in proportion to the initial feed ratio of 2a to BSTP−Pd(II), and all the isolated C

DOI: 10.1021/acs.macromol.7b02663 Macromolecules XXXX, XXX, XXX−XXX

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copolymer and further supported that the Pd(II) units of the poly-2a 100 terminus were active enough and were all participated in the block copolymerization. Since the block copolymerization can be conducted in one-pot by sequential addition of the two monomers, poly(2a200-b-1a20) was then attempted to be prepared in one-pot through the copolymerization of 2a and 1a monomers at the same time by using the BSTP-Pd(II) catalyst. However, 1H NMR and SEC analyses suggested the generated polymer was almost a poly1am homopolymer (Figures S16 and S17). This result is reasonable because only a small amount of 2a was copolymerized at the beginning stage of the polymerization. After that the CN double bonds of the formed polyisocyanide terminated the RAFT polymerization of 2a. The low polydispersity index of poly(2a100-b-1a50) block copolymer suggested the block copolymerization of 1a using poly-2a100 as macroinitiator may proceed in a living/controlled manner. To verify this, the block copolymerization was performed with the presence of dimethyl terephthalate as the internal standard and was followed by 1H NMR and SEC to estimate the monomer conversion and Mn and Mw/Mn values of the generated block copolymers. As anticipated, the recorded SEC of the poly(2a100-b-1am)s isolated at different polymerization stages exhibited symmetric and single model elution peaks. The Mn increased linearly and in proportion to the conversion of monomer 1a. All the isolated block copolymers showed narrow molecular weight distributions with Mw/Mn < 1.34 (Figure 5). Note that the block copolymerization was relatively fast; more than 83% of monomer 1a was consumed within 6 h. Kinetic investigation indicated the block copolymerization obeyed the first-order rate law. The appearance rate constant was estimated to be 2.0 × 10−4 s−1, which is almost the same as that of the homopolymerization under the same experimental conditions. These analyses further confirmed the living nature of the block copolymerization. Taking advantage of this synthetic method, a family of block copolymers with expected Mns, narrow Mw/Mn, and tunable compositions were facilely prepared and isolated in high yields (Table S3). The block copolymer poly(2a100-b-1L50) (Mn = 25.3, Mw/Mn = 1.23) obtained through the copolymerization of 1L with Pd(II)-terminated poly-2a100 was optically active. As shown in Figure S22, the CD spectrum of poly(2a100-b-1L50) measured in THF at 25 °C showed intense Cotton effect at the absorption region of the polyisocyanide backbone, which was very similar to that of the poly-1L50 homopolymer. The Δε364 of the poly-1L50 segment of the block copolymer was estimated

Figure 3. 1H NMR (400 MHz) spectrum of poly-2a100 (a), poly-1a50 (b), and poly(2a100-b-1a50) (c) recorded in CDCl3 at 25 °C.

Figure 4. 31P NMR (162 MHz) spectra of BSTP−Pd(II) complex, poly-1a50, poly-2a100, and poly(2a100-b-1a50) recorded in CDCl3 at 25 °C.

homopolymer, and no peak at 17.7 ppm can be discerned. These results confirmed the formation of expected block

Figure 5. (a) Time-dependent size exclusion chromatograms of poly(2a100-b-1am)s prepared via block copolymerization of 1a using Pd(II)terminated poly-2a100 as a macroinitiator in THF at 55 °C. (b) Plots of the Mn and Mw/Mn values as a function of the conversion of monomer 1a initiated by poly-2a100 in THF at 55 °C ([1a]0/[Pd(II)]0 = 50, [1a]0 = 0.20 M). D

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Figure 6. (a) SEC curves of poly-2b200 and the resulting poly(2b200-b-1L50) block copolymer. (b) Transmittance (900 nm) vs temperature curves and photographs of the phase transition of poly(2b200-b-1L50) in H2O/THF (v/v = 9/1, c = 2 mg/mL).

to be −15.5, which suggested an excess of left-handed helix was produced. After the block copolymerization method was established, other block copolymers such as poly(2b200-b-1L50) and poly(2c100-b-1L50) can also be facilely prepared by using Nisopropylacrylamide (2b) and 4-vinylpyridine (2c) as the first monomer instead of 2a. The structures of poly(2b200-b-1L50) and poly(2c100-b-1L50) were verified by FT-IR, 1H NMR, CD, and UV−vis spectroscopies (Figures S23−S32). The stimuliresponsive and self-assembly properties of these block copolymers were then investigated. Because of the amphiphilic character, poly(2b200-b-1L50) has good solubility in most common organic solvents and limited solvable in water. Thus, the thermoresponsiveness of poly(2b200-b-1L50) was studied in water with the 10% of THF (2.0 mg/mL). As displayed in Figure 6b, the clear and transparent solution turned to turbid upon heating from 20 to 38 °C, and it turned back to transparent again when cooling back to room temperature. Plot of the transmission at 900 nm with temperature suggested the lower critical solution temperature (LCST) is 27 °C (Figure 6b). When cooled to room temperature, the transmittance was recovered with slight hysteresis. Further studies revealed that the LCST is dependent on concentration; decrease the concentration, LCST will increase. For instance, when the concentration decreased to 0.5 and 0.2 mg/mL, LCST of poly(2b200-b-1L50) increased to 30 and 35 °C, respectively. The heating-induced phase transition was also supported by dynamic light scattering (DLS) analysis. The average diameter of poly(2b200-b-1L50) in THF and water (v/v = 10/90, c = 0.5 g/L) was ca. 260 nm at 25 °C, which increased to ca. 900 nm when heated up to 35 °C (Figure S27). According to the DLS analyses, the LCST was determined to be 31 °C, in agreement with the transmittance studies. Poly(2c100-b-1L50) also has good solubility in most common organic solvents, but it has limited solubility in water. The solubility of poly(2c100-b-1L50) in water is ≤0.01 mg/mL. The self-assembly behavior of poly(2c100-b-1L50) was then investigated. First, poly(2c100-b-1L50) was dissolved in chloroform (0.10 mg/mL) and spin-coated to the mica wafer. It was subjected to AFM observations after the solvent was evaporated. Spherical nanoparticles with ca. 75 nm in diameter and ca. 4.1 nm in height were observed on the AFM image (Figure 7a), which was ascribed to the self-assembly of poly(2c100-b-1L50) due to the immiscibility of the two blocks. DLS and static light scattering (SLS) studies of poly(2c100-b1L50) in CHCl3 were then carried out to investigate the selfassembled supramolecular structure (Figures S34 and S35).

Figure 7. AFM height images of thin film spin-coated from a solution of poly(2c100-b-1L50) in CHCl3 (c = 0.1 mg/mL) (a), and after being annealed at 160 °C for 10 h (b) and 28 h (c).

According to the DLS and SLS analyses, the radius of gyration (Rg) and hydrodynamic radius (Rh) of poly(2c100-b-1L50) in CHCl3 was ca. 21 and 28 nm (Rg/Rh = 0.75), respectively, which suggested the self-assembled structure is a spherical micelle.51 The large difference in diameter and height on the AFM image of poly(2c100-b-1L50) was probably caused by the collapse of the self-assembled structure on the substrate. When the sample on mica wafer was annealed at 160 °C for 10 h, the spherical nanoparticles were transferred to wormlike morphology with ca. 30 nm in width and 2.5 nm in height (Figure 7b). Further annealing at 160 °C for 28 h led to the formation of tinier wormlike structure (Figure 7c). The reason for the thermal annealing induced fiber formation is not quite clear at the current stage, probably the thermal annealing allows the aggregates to reach thermodynamic equilibrium.52−54 Synthesis of Core Cross-Linked Star Polymers. The combination of RAFT and Pd(II)-initiated isocyanide polymerizations for the synthesis of hybrid block copolymers using BSTP−Pd(II) as catalyst may be applied to the synthesis of star polymers. Generally, core cross-linked star polymer can be prepared via “core-first” or “arm-first” strategies.50 Herein, the star polymer was first attempted to be prepared via “arm-first” strategy, followed a reported procedure with modifications.24 As shown in Scheme 2, poly-2a80 (Mn = 8.7 kDa, Mw/Mn = 1.19) was first prepared and then copolymerized with a crosslinker ethylene glycol dimethacrylate (3) ([3]0/[BSTP]0/ [AIBN]0 = 10/1/0.1) via RAFT polymerization. The structure of the afforded star polymer CCS-1 was confirmed by SEC, 1H NMR, and FT-IR. The elution peak of CCS-1 was shifted to shorter retention region as comparing to that of the linear poly2a80 precursor. The Mn and Mw/Mn were determined to be 13.2 kDa and 1.26, respectively (Figure 8a). The increased Mn supports the core cross-linked reaction was occurred. The Pd(II) units of the star polymer may initiate a polymerization of isocyanide, which would yield a star polymer carrying hybrid E

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Macromolecules Scheme 2. Synthesis of Star Polymer

which indicated all the Pd(II) unites were involved in the polymerization of 1L. All these studies confirmed the formation of the expected star polymer CCS-1 and CCS-2. The morphology of star polymers CCS-1 and CCS-2 was investigated by AFM and TEM. As expected CCS-1 exhibited well-defined spherical nanoparticles on AFM image with ca. 40 nm in diameter and ca. 4 nm in height (Figure 9b). TEM study

Figure 8. (a) SEC of poly-2a80, CCS-1, and CCS-2 (eluent = THF, temperature = 40 °C). (b) FT-IR spectra of poly-2a80, CCS-1, and CCS-2 measured at 25 °C using KBr pellets.

arms. To verify this hypothesis, the isolated CCS-1 was allowed to initiate the polymerization of 1L in chloroform at 55 °C ([1L]0 = 0.2 M, [1L]0/[Pd(II)]0 = 100/1) to graft helical polyisocyanide arms onto the star polymer. As anticipated, the Mn and Mw/Mn of the resulting polymer determined by SEC was 46.7 kDa and 1.30 (Figure 8a), which suggested the helical arms were linked to the star polymer CCS-1 and afford the miktoarm star polymer CCS-2. The relatively high Mn of the miktoarm star polymer was probably due to the rigid rod helical conformation of the poly-1Lm segments. The structures of the star polymers carrying homopolymer poly-2am arms and hybrid arms were characterized by 1H, 31P NMR (Figures S36 and S37), and FT-IR (Figure 8b). Because of the similar chemical composition of monomer 2a and cross-linker 3, FT-IR and of the linear poly-2a80 precursor and core cross-linked star polymer CCS-1 were almost the same (Figure 8b). However, a weak resonances at 6.10 and 5.56 ppm were observed on the 1 H NMR resulting from the residue of the unreacted double bond of 3 in CCS-1 (Figure S36). Comparing to CCS-1, new vibrations were observed in the FT-IR of CCS-2 such as vibrations at 1600 and 1633 cm−1 corresponding to the CN and CO (amide) of polyisocyanide arms (Figure 8b). The 1H NMR of CCS-2 exhibited new characteristic resonance at 6.28−7.20 and 4.20− 7.20 ppm, corresponding to the phenyl and OCH2 resonances of the polyisocyanide segment (Figure S36). The structures of CCS-1 and CCS-2 were further investigated by 31P NMR (Figure S37); the signal at 17.7 ppm of CCS-1 was completely shifted to 14.5 ppm on the 31P NMR spectrum of CCS-2,

Figure 9. TEM (a, c) and AFM height (b, d) image of CCS-1 (a, b) and CCS-2 (c, d). All samples were prepared from the THF solutions at room temperature (c = 0.1 mg/mL).

also confirmed the spherical nanoparticle of CCS-1 (Figure 9a). It should noted that after the Pd(II)-initiated 1L polymerization the resulting star polymer CCS-2 contained two different kinds of arms. The poly-2am segment is random coil while the poly-1Lm segment is rigid rod; thus, the CCS-2 may have a unique morphology that differs from that of CCS-1. As shown in Figure 9c, compared to CCS-1, larger size spherical nanoparticles with diameter of ca. 70 nm were clearly observed. Moreover, there is a dark loop surrounding the spheres on the F

DOI: 10.1021/acs.macromol.7b02663 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

after the chain extension with 1L. The Mn of the resulted CCS5 was of 43.6 kDa, while the Mw/Mn remains narrow and was 1.38. 1H NMR and FT-IR of these star polymers were almost the same with the miktoarm star polymer prepared by the “armfirst” strategy because of the same chemical compositions (Figures S39 and S40). The average hydrodynamic diameters of CCS-3, CCS-4, and CCS-5 were measured by DLS and are shown in Figure 10b. It confirmed that the star block copolymer were obtained successfully because the average diameter increased from 40 to 60 nm after the chain-extension reaction of 1L. The diameter of CCS-5 was determined to be ca. 250 nm. TEM studies also revealed the formation of expected star polymers. Well-defined spherical nanoparticles were observed on the TEM images of CCS-4 and CCS-5 (Figure S41). The diameters of CCS-4 and CCS-5 were estimated to be ca. 43 and 230 nm, respectively, which are agreed with the DLS analyses. Chiral Resolution Ability. The circular dichroism (CD) and UV−vis spectra of the CCS-2 are displayed in Figure S42; the intense negative Cotton effect at 364 nm (Δε364 = −16.5) corresponding to the helical poly(phenyl isocyanide) backbone suggested the excess of left-handed helical arms of CCS-2. Thus, this chiral material may be used in chiral recognition and enantiomer separations.10,12,13 Enantioselective crystallization was performed by using racemic threonine as model compound because of its biological important.56,57 A small amount of CCS-2 (9.0 mg) was added into a supersaturated acetonitrile solution (3 mL) of racemic threonine (250 mg). After the solution was standing at room temperature in an open vial for several hours, needle crystals were gradually formed. The CD spectrum of the induced crystal showed a positive Cotton effect around 210 nm, suggesting L-threonine was enantioselectively crystallized (Figure 11a). Based on CD and UV−vis analyses of

TEM images (Figure 9d) which can be well explained by the structure of CCS-2. For the CCS-1, the Pd(II) units were located at the surrounding of the core of the star polymer, so core−shell structures were observed with dark center and lighter color on shell. After polymeization with 1L, the Pd(II) units were extended to the exterior of the star polymer. Thus, there is a dark loop surrounding the nanoparticles. Interestingly, well-defined unique “poached egg” morphology was oberved on the AFM height image of CCS-2 (Figure 9d). The average diameter of the “yolk” core is ca. 30 nm, which is similar to CCS-1, indicating the central structure of the star polymer was maintained after the growth of the second arms of poly-1Lm. The thickness of the “albumen” was ca. 35 nm, larger than that of the “yolk” because of the rigid structure of poly1Lm and the longer polymer chain. The size of the star polymers was also investigated by DLS. As shown in Figure S38, the average hydrodynamic diameter of poly-2a80 was determined to be ca. 5 nm because of its linear structure and was molecularly dissolved in THF. After crosslinking reaction, the diameter of CCS-1 increased to ca. 50 nm and was in agreement with the size determined by TEM and AFM. In contrast, average hydrodynamic diameter of CCS-2 increased to ca. 76 nm because of the rigid polyisocyanides arms (Figure S38), which is a little bit larger than the average diameter measured in TEM and AFM. By using this synthetic method, star block copolymer can also be prepared followed a procedure similar to the reported in the literature with modifications.55 As illustrated in Scheme 2, cross-linker 3 were first polymerized with the BSTP−Pd(II) in THF ([3]0 = 0.02 M, [3]0/[BSTP−Pd(II)]0/[AIBN]0 = 10/1/ 0.1) to form a cross-linked macroinitiator CCS-3 bearing Pd(II) units and the RAFT chain transfer agent BSTP. Monomer 2a was first polymerized with CCS-3 via RAFT polymerization ([2a]0/[BSTP]0/[AIBN]0 = 100/1/1), following the procedure described above. The generated cross-linked star polymer CCS-4 carrying poly-2a100 arms. The Pd(II) units was located at the terminus of the poly-2a100 arms and were located at the exterior, which was then chain extended with monomer 1L in CHCl3 at 55 °C ([1L]0/[Pd(II)]0 = 100/1). The block copolymerization afforded star block polymers which arms were composed of poly(2a100-b-1L100) blocks. The formation of cross-linked catalyst CCS-3, star polymer CCS4, and the chain-extended star block copolymer CCS-5 was first confirmed by SEC analyses. As outlined in Figure 10a, the SEC trace of CCS-3 was shifted to higher Mn weight region after the RAFT polymerization of 2a. The Mn and Mw/Mn of CCS-3 were determined to be 4.4 kDa and 1.32, respectively. After the polymerization of 2a, the Mn of CCS-4 was increased to 18.7 kDa and the Mw/Mn was 1.36. The Mn was further increased

Figure 11. (a) CD and UV−vis spectra for the enantioselective crystallization of racemic threonine induced by CCS-2 in acetonitrile at room temperature. (b) SEM of the L-threonine crystal induced by CCS-2.

the induced crystal and enantiopure L-threonine, the ee value of the crystal was determined to be 95%. The morphology of the induced crystals was further investigated by SEM. As shown in Figure 11b, well-defined needlelike morphology with good homogeneity was clearly observed on the induced L-threonine crystal by CCS-2 (Figure 11b). In contrast, the racemic crystals of D/L-threonine obtained without any addition of CCS-2 showed irregular bulk morphology on the SEM image (Figure S43). All these results undoubtedly indicated the synthetic CCS-2 was an excellent chiral material with great potentials in chiral resolution.

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CONCLUSION In conclusion, a new catalyst BSTP−Pd(II) containing a RAFT chain transfer agent and a Pd(II) units was designed and

Figure 10. SEC (a) and DLS curves (b) of CCS-3, CCS-4, and CCS-5. SEC condition: eluent = THF; temperature = 40 °C. DLS were recorded in THF at 25 °C with c = 0.1 mg/mL. G

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Macromolecules

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synthesized. The Pd(II) unit of the catalyst can initiated a living polymerization of isocyanide, while the BSTP moiety can promote a RAFT polymerization. Upon combination of these two polymerizations, a variety of hybrid block copolymers containing helical polyisocyanide and polyacrylate segments can be facilely prepared under living/controlled manner. Moreover, miktoarm star polymer and star block copolymer containing optically active polyisocyanide and polyacrylate block can be readily prepared. Because of the chirality, the star polymer can be used in the chiral resolution; the enantiomeric excess (ee) value of the induced crystals of model a compound, racemic threonine, is up to 95%. We believe the present study provides not only a method for synthesis of chiral materials with control structures but also a clue for designing novel catalysts for functional materials.

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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.7b02663. Synthesis and characterization of RAFT-Pd(II) catalyst and the related mediates; polymerization procedure and additional spectroscopic data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.-Q.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 supported by the National Natural Scientific Foundation of China (21622402, 51673057, and 21574036). Z.-Q. Wu thanks the 1000plan Program for Young Talents of China. N. Liu thanks Anhui Provincial Natural Science Foundation (1608085MB41).

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DOI: 10.1021/acs.macromol.7b02663 Macromolecules XXXX, XXX, XXX−XXX