Supramolecular Helix–Helix Block Copolymers - Macromolecules

Sep 28, 2016 - Two chemically distinct monotelechelic helical polymers were synthesized using anionic and reversible addition–fragmentation chain-tr...
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Supramolecular Helix−Helix Block Copolymers Anna Croom, Kylie B. Manning, and Marcus Weck* Molecular Design Institute and Department of Chemistry, New York University, New York, New York 10003, United States S Supporting Information *

ABSTRACT: Two chemically distinct monotelechelic helical polymers were synthesized using anionic and reversible addition−fragmentation chain-transfer (RAFT) polymerizations. A chiral poly(isocyanide) block was obtained using a palladium−ethynyl complex modified with the N1,N3-bis(6-butyramidopyridin-2yl)-5-hydroxyisophthalamide (Hamilton wedge) moiety as a catalyst employing anionic polymerization. A complementary barbiturate-functionalized chain-transfer agent was used to polymerize chiral N-(1-(naphthalen-2-yl)ethyl)methacrylamides by RAFT polymerization. The assembly into helix−helix supramolecular block copolymers in chloroform via hydrogen bonding was analyzed by 1H NMR spectroscopy, resulting in an average measured association constant of (9.5 ± 0.5) × 103 M−1. After block copolymer formation, the secondary structures of both helical polymers were maintained within the block copolymer, as evidenced by circular dichroism and infrared spectroscopies. Films were prepared from a 1:1 mixture of polymers in solution and were analyzed by WAXS and DSC to evaluate organization in the solid state. While diblock formation in the solution phase is readily obtainable, there was little evidence supporting a self-assembly assisted microstructure in the solid state. This work demonstrates a synthetic methodology for obtaining two telechelic helical polymers capable of supramolecular assembly in solution toward the goals of developing multifunctional polymeric ensembles.

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via block extension16 and postpolymerization ligation strategies, which often prove to be nonquantitative.17 Covalently linked helix-containing main-chain BCPs have relied heavily on the incorporation of polypeptides as the helical block.18−26 The use of synthetic helical blocks has only recently been realized by the installation of poly(isocyanate)s, poly(isocyanide)s, poly(guanidine)s, and poly(acetylene)s to obtain rod−coil27−35 and rod−rod block copolymers.36−39 To generate chemically distinct, main-chain block copolymers when an iterative strategy cannot be applied, postpolymerization ligation strategies are the preferred method employed by polymer chemists.17 Despite the concerns for ligation compatibility and BCP fidelity, the pursuit of diverse polymeric materials containing a helical domain is an attractive synthetic target for advanced materials and nanoarchitectures. Utilizing high-yielding, postpolymerization click ligation strategies, Novak and co-workers studied the synthesis of amphiphilic di- and triblock copolymers containing a helical poly(carbodiimide) segment31,40 as well as their resulting tunable nanostructural30 and bulk behaviors.41 Similar strategies have been applied by other research groups to generate rod− rod BCPs displaying hierarchical self-assembly and phase separations26 as well as diverse mesophases.38 These examples demonstrate that the incorporation of a helical segment into main-chain BCPs expands the scope of applications of the

INTRODUCTION Main-chain supramolecular block copolymers (BCP)s rely on modular, directional, and reversible noncovalent interactions to fabricate complex macromolecular architectures through rational design.1−5 With tunable properties such as microphase separation,6 self-healing,7 and as thermally sensitive substrates,8,9 supramolecularly functionalized polymers are attractive targets for materials chemists. To date, most main-chain supramolecular BCPs are composed of random coil polymers such as poly(methacrylate)s, poly(acrylate)s, poly(styrene)s, poly(norbornene)s, and poly(ether)s.1,2,10 There are very few examples of the use of a controlled polymerization method to afford main-chain supramolecular BCPs where a target polymer block possesses a higher order structure, such as a helix or a sheet.11 In biological contexts, a helix imparts structural rigidity, chirality, and spatial organization to its environment. By virtue, helices facilitate complex biological functions such as chiral catalysis, genetic encoding, and the hierarchical assembly of cellular components.12−14 Harnessing the importance of the helix in biology, polymer chemists have sought to include helical structures in synthetic systems to perform molecular recognition, and nanoscaffolding, and to impart long-range molecular alignment.15 Exploitation of the characteristics of helices in synthetic systems is especially advantageous toward the realization of complex block copolymer blends. Polymer chemists are fairly limited in the methods available to obtain diverse BCP blends, typically relying on the compatibility of polymerization methods with macroinitiators © XXXX American Chemical Society

Received: June 30, 2016 Revised: September 13, 2016

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Figure 1. Supramolecular assembly of two chemically distinct helical polymers via the Hamilton wedge and barbiturate recognition pairs.

Scheme 1. Synthetic Route toward the HW-Pd Initiator for the Polymerization of Helical Poly(isocyanide)s

material by introducing properties such as chirality, rigidity, and microphase alignment. A supramolecular BCP strategy can be applied to address the synthetic limitations plaguing the development of diverse BCP blends, and specifically it can be applied toward the installation of helical blocks into polymeric assembles.42 To overcome such synthetic challenges, functionalized initiators and chain-transfer agents (CTAs) containing supramolecular recognition motifs have been developed to generate telechelic polymers with quantitative end-group fidelity. Hydrogen-bonding supramolecular recognition allows for strong, yet reversible and stimuliresponsive assembly of complementarily functionalized components.1,10,43 Ring-opening metathesis polymerization (ROMP)1 initiators, reversible addition−fragmentation chain-transfer (RAFT),44−51 and atom-transfer radical polymerization (ATRP)52 CTAs have been used obtain polymers with quantitative incorporation of the functional end-group in combination with control over polymer molecular weight and dispersity. These functional initiators and CTAs, however, have not been adapted to polymerize monomers that afford helical polymers. In this contribution, we close this gap and report the synthesis of two monotelechelic helical polymers obtained using controlled polymerization techniques with terminal hydrogen-bonding recognition units that drive the assembly toward an A/B supramolecular helix−helix BCP. In particular, we investigated functionalizable initiators and CTAs capable of generating helical poly(isocyanide)s and poly(methylmethacrylamide)s, respectively. By modifying the

initiators and CTAs to contain supramolecular recognition units, complementarily end-functionalized helical polymers are obtainable (Figure 1). Poly(isocyanide)s are known to form stable helices with high helix-inversion barriers53,54 (>85 kJ/mol)55 and have been αfunctionalized when polymerized with modified nickel (II)56 or palladium (II)34,57 initiators by anionic polymerization. Enantiomerically enriched chiral side chains can promote a preferred helical handedness.58,59 Poly(isocyanide)s containing supramolecular recognition units have not been reported using either Ni(II) or Pd(II) initiators. In this contribution, we report the synthesis of a poly(isocyanide) α-functionalized with a N1,N3-bis(6-butyramidopyridin-2-yl)-5-hydroxyisophthalamide (Hamilton wedge, HW) by palladium-mediated anionic polymerization. Poly(N-(1-(naphthalen-2-yl)ethyl)methacrylamide) (poly(NEMAM)s) have been shown to adopt a helical structure based on the chirality of the naphthylethyl side chains.60−62 The secondary helical structure of this polymer is different in comparison to poly(isocyanide)s as it features dynamic hydrogen-bonding events along the polymer backbone that affect the overall secondary structure. A commercially available RAFT CTA was modified to contain a terminal barbiturate (Ba), the complementary unit to the HW receptor. Polymerization of chiral NEMAMs assisted by the modified RAFT CTA will generate a ω-Ba-functionalized helical polymer. We rationalize that the assembly of two helical polymers into supramolecular BCPs will give rise to materials with distinct architectural domains. Ripe is the possibility to develop B

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Table 1. Polymer Data and Molar Circular Dichroism Observed at Various Wavelengths polymer

Mna (g/mol)

Đa

monomer

Δε236 (M−1 cm−1)

Δε283 (M−1 cm−1)

Δε249 (M−1 cm−1)

Δε364 (M−1 cm−1)

HW-Poly-1 HW-Poly-2 Ba-Poly-3 Ba-Poly-4 Ba-Poly-5

13100 9000 13100 14900 14700

1.36 1.38 1.25 1.49 1.38

(+)-menthol (−)-menthol (S)-NEMAM (R)-NEMAM (S)-NEMAM

+2.30 −3.21 −2.90 +2.51 −2.84

−0.89 +1.18 −1.23 +1.55 −1.06

+8.48 −9.16 −0.13 +0.18 −0.12

−4.04 +4.57 −0.04 +0.06 +0.03

a

The Mn and Mw/Mn (Đ) were determined by SEC in THF using poly(styrene) standards. Samples were dissolved in distilled chloroform (c = 3.75 μM, l = 0.2 cm path length).

disappearance of the alkyne C−H proton at δ 3.0 ppm in the H NMR spectrum. When the palladium is coordinated to the alkyne of HW-Pd, a downfield shift of the alkyne carbon from δ 83.5 to 95.8 ppm was observed in the 13C NMR spectrum. A single signal at δ 18.0 ppm in the 31P NMR spectrum suggested one phosphorus species was present, and this shift is consistent with the resonances observed for similar reported initiators.34 Poly(isocyanide)s with an excess helical sense are commonly formed from the polymerization of chiral monomers.53 Aryl isocyanide monomers bearing (+)- and (−)-menthol pendant groups were synthesized according to the literature and polymerized using HW-Pd to afford polymers of preferential helical handedness.58 The polymerizations were performed in a Schlenk tube under an argon atmosphere. The reactions were heated to 55 °C and stirred for 12−16 h. The crude polymer was purified by precipitation and filtration and rinsed with acetone. Polymer dispersities range between 1.36 and 1.38 (Table 1 and Figures S17, S18). The presence of the HW moiety on the polymer chain-end is confirmed by visualizing the N−H amide protons with chemical shifts of δ 8.4 and 8.1 ppm in the 1H NMR spectra (Figures S10 and S11). End-group analysis by 1H NMR spectroscopy to determine polymer molecular weights could not be performed due to lack of resolution of comparable main-chain and end-group signals. For all samples, the Mn values obtained by SEC are generally higher than the theoretical feed ratio as has been observed for rigid-helical systems when compared to poly(styrene) standards.65 Synthesis of Ba-Functionalized Polymers. A barbiturate containing RAFT CTA was targeted for the polymerization of NEMAM. The carboxylic acid functional group of 4-cyano-4((phenylcarbonothioyl)thio)pentanoic acid (CPADB) allows for the facile modification of the RAFT CTA as shown in

multicomponent assemblies featuring modular, organizable, and chiral polymer blocks, affording “plug-and-play” building blocks with unique 3D architectures. Herein we contribute a description of the synthesis and assembly of telechelic HWfunctionalized poly(isocyanide)s with Ba-functionalized poly(NEMAM)s into diblock copolymers affording helix/helix supramolecular BCPs. The persistence of the secondary structure of each block upon assembly was studied by circular dichroism (CD) and infrared (IR) spectroscopies. Bulk material properties in the solid state were explored by wide-angle X-ray scattering (WAXS). These results display the potential to utilize supramolecular assembly to incorporate well-defined, persistent helical structures into advanced polymeric ensembles.

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RESULTS AND DISCUSSION Synthesis of HW-Functionalized Polymers. Polymerization initiators can be modified with molecular recognition units that are transferred, quantitatively, to the polymer chainend during the initiation step. Recent reports show the controlled polymerization of isocyanides via phenylethynyl palladium initiators, which resulted in monotelechelic polymers.34,63,64 By devising a strategy to attach the Hamilton wedge recognition unit in the para-position of the phenylethynyl−palladium complex, quantitative end-group functionalization can be achieved. The HW-modified phenylethynylene derivative, 2, was synthesized in two steps using Schotten−Baumann and Williamson ether coupling conditions, starting from 4-ethynylaniline and 6-bromohexanoyl chloride to form a phenylethynylene linker and subsequently attaching the HW moiety (Scheme 1). To generate HW-Pd, 2 was treated with Pd(PEt3)2(Cl)2 in the presence of CuI and diethylamine. Successful formation of HW-Pd was confirmed by the C

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observed at 364 nm to indicate an excess of the M-helical sense (HW-Poly-1, Figure 2). Polymerization of methylmethacrylamides bearing (S)- and (R)-ethylnaphthyl moieties in the side chains gives rise to the negative and positive Cotton effects at 283 nm for Ba-Poly-3 and Ba-Poly-4, respectively (Figure 2). Therefore, selectively right- and left-handed helices can be generated by both polymer systems in an enantiomeric excess. Detailed configurational assignment for poly(NEMAM)s has not been reported. Therefore, all polymer samples will be addressed with respect to the sign of the Cotton effect visible in their CD spectra. IR Spectroscopy Studies. IR spectroscopy is a useful method for monitoring subtle changes within a helical polymer, especially those that possess intramolecular hydrogen bonds.68 For polypeptides, it is well established that the wavenumbers corresponding to intramolecularly hydrogen-bonded amide stretches can divulge information about conformation.68 This diagnostic technique can also be used to probe the conformation of poly(NEMAM)s, which adopt a helical structure via amide hydrogen bonds along the polymer backbone. Previous reports show that the absorption of poly(NEMAM) at 3450 cm−1 corresponds to the stretching vibration of the amide N−H bond and ranges from approximately 3450 to 3320 cm−1, with lower wavenumbers indicative of greater intramolecular hydrogen bonding. Similarly the amide carbonyl CO stretch ranges from 1660 to 1620 cm−1 with lower wavenumbers denoting a more hydrogenbonded structure.61,62 Homopolymer Ba-Poly-3 shows an amide N−H stretch at 3448 cm−1 with a broad shoulder around 3370 cm−1 and a carbonyl CO stretch at 1652 cm−1 suggestive of a partially intrachain hydrogen-bonded structure (Figure S41). Poly(isocyanide), HW-Poly-1, contains side chains with ester linkages and therefore does not utilize intrachain hydrogen bonding for helix stabilization. The IR spectrum for HW-Poly-1 displays a sharp stretch at 1716 cm−1 that correlates to the ester carbonyl of the menthoxy side chain along with a small broad stretch at 1656 cm−1 assigned to the imino CN. IR spectral analysis of the supramolecularly assembled helix−helix (Ba-Poly-3/HW-Poly-1) will highlight if there has been any disruption of the helical poly(NEMAM) with respect to the amide stretches. Wide-Angle X-ray Scattering Studies. A preliminary study of the helical homopolymers in the solid state was performed using wide-angle X-ray scattering (WAXS) to monitor the solid-state helical pitch and diameter of each polymer.53,69 The X-ray diffraction pattern of HW-Poly-1 shows a relatively intense Bragg reflection in the small-angle region corresponding to an interplanar distance of 19.63 Å, most likely corresponding to the diameter of the polymer stem (Figure 3). This distance correlates well with other reported poly(aryl isocyanide)s with interplanar distances of approximately 20 Å obtained using atomic force microscopy.70 Two weaker, broad maxima are also observed in the small-angle region corresponding to distances of 7.51 and 5.01 Å (Figure 3). Despite periodicity observed in the small-angle region of the diffraction pattern of HW-Poly-1, the absence of reflections in the wide-angle region suggests no long-range ordering of polymer chains. The X-ray diffraction pattern for Ba-Poly-3 shows a very weak maximum with an interplanar distance of 12.56 Å corresponding to the polymer stem and indicative of amorphous film formation (Figure 3). This agrees well with previously reported X-ray diffraction data of a film prepared

Scheme 2. Barbiturate moiety 4 was coupled to the CTA utilizing Mitsunobu conditions yielding the final Ba-CTA. Formation of the new ester moiety was confirmed by visualizing a triplet at δ 4.1 ppm (ester−CH2) in the 1H NMR spectrum. The 13C NMR spectrum displayed conversion of the acid to an ester with signals at δ 172.8 and 65.5 ppm (ester−CH2). The functional CTA was used in the RAFT polymerization of (R)or (S)-N-(1-(naphthalen-2-yl)ethyl)methacrylamide (NEMAM), synthesized by known protocols, installing the Ba moiety at the polymer chain-end.62 Polymerizations of chiral NEMAMs with Ba-CTA were performed under argon in THF at 60 °C for 12−18 h. Crude polymers were purified via precipitation in cold methanol. The poly(NEMAM)s displayed monomodal distributions and low dispersities ranging between 1.25 and 1.49, as determined by SEC in THF (Figures S19−S21). 1H NMR spectra of the purified polymers contained all respective polymer resonances. The imide protons of the Ba end-group were not immediately visible in the 1H NMR polymer spectra but could be resolved upon association with the HW moiety (vide inf ra). Analysis of a low molecular weight polymer by MALDI-TOF displayed periodic molecular ion peaks for the mass of the polymer and Ba end-group with fragmentation patterns attributable to the mass of the repeat unit. A minor series indicative of fragmentation of the labile thioester at the polymer chain-end were also visible and have been observed for other RAFT polymers (Figures S22 and S23).48 Higher-molecular-weight Ba-polymers and all HW-polymers could not be analyzed by MALDI-TOF. Therefore, the Mn values obtained via SEC are used as representative molecular weights. Circular Dichroism Studies of Homopolymers. Both HW-Poly and Ba-Poly display an excess helical handedness resulting from their respective chiral monomers which are observed as positive and negative Cotton effects by CD spectroscopy.66 The differential absorbance of circularly polarized light by a helical polymer with a predominant helix sense gives rise to the spectral features seen in Figure 2. The

Figure 2. CD spectra displaying positive and negative Cotton effects for HW-Poly-1 and HW-Poly-2 (left) and Ba-Poly-3 and Ba-Poly-4 (right) in CHCl3 (c = 3.75 μM).

imino group of the poly(isocyanide) backbone of HW-Poly has a n−π* transition at 364 nm. A π−π* transition at 249 nm is observed for the aromatic chromophore of the aryl side chains.66 Ba-Poly experiences electronic transitions associated with absorbance at 283 and 236 nm for naphthyl π−π* transitions due to intramolecular hydrogen bonding.60 For both polymer systems, the helical sense of the polymers is dependent on the chiral side-chains. When (−)-menthoxycarbonylphenyl isocyanide is polymerized, a strong positive Cotton effect is observed at 364 nm and is indicative of an excess P-helical sense (HW-Poly-2, Figure 2) as determined by experimental and computational methods.67 When (+)-menthoxycarbonylphenyl isocyanide is polymerized, the opposite Cotton effect is D

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moiety shifts downfield from δ 8.4 ppm in the unbound state to δ 9.9 ppm for the bound host−guest complex (Figures S24 and S25). Conversely, as sBa is added to excess, the Ba N−Hc proton shifts upfield from δ 12.8 to 11.1 ppm at 1.0 mol equiv. The association constant was calculated to be Ka = 2.0 × 104 M−1. ITC experiments were also performed by titrating sBa into HW-Poly to obtain a Ka = 2.2 × 104 M−1, which is in good agreement with the 1H NMR experimental data as well as previously published reports (Figure S34).1 Similarly, Ba-Poly was titrated with sHW and the proton resonances were monitored by 1H NMR spectroscopy. As sHW was added to Ba-Poly, the N−Ha protons of the HW moiety shifted upfield from δ 9.8 to 8.9 ppm as excess sHW was added. The N−Hc of Ba-Poly shifted downfield to δ 12.5 ppm at 1.0 mol equiv, which is representative of the bound state (Figures S27 and S28). The association constant was calculated as Ka = 2.0 × 104 M−1. ITC experiments were performed to determine a Ka = 1.8 × 104 M−1 (Figure S34). These experiments support the availability of each functionalized polymer chain-end as a supramolecular recognition site. Characterization of Supramolecularly Assembled A/B Helical Polymers. The hydrogen-bonding interaction between the Hamilton wedge and barbiturate recognition units has been used in a variety of reports to assemble polymer blocks and other macromolecular structures.1,44,73 By relying on the strong HW/Ba hydrogen-bonding interaction, a supramolecular A/B helix−helix block copolymer was formed from an equimolar mixture of homopolymers in chloroform. Determination of the Association Constant (Ka). The supramolecular assembly of telechelic polymers in solution can be affected by a variety of factors such increased entropy for higher molecular weight polymers and steric congestion leading to a decrease in association constants.74−76 Despite the entropic and steric considerations, we demonstrate the reliable solutionstate assembly of HW-Poly with Ba-Poly as the first supramolecular helix−helix block copolymer. To confirm the supramolecular assembly of HW-Poly with the Ba-Poly, and to assess the strength of the binding, titration studies were performed and visualized by 1H NMR spectroscopy and ITC. The assembly process of HW-Poly and Ba-Poly can be followed in situ by the characteristic changes in the chemical shifts of the N−H protons of the respective recognition pairs. An average association constant (Ka) of (9.5 ± 0.5) × 103 M−1 was determined by monitoring the chemical shifts of the hydrogenbonding imide and amide proton resonances of the Ba or HW

Figure 3. Wide-angle X-ray diffraction pattern of homopolymer films HW-Poly-1 (left axis) and Ba-Poly-3 (right axis) in reflection mode.

from isotactic poly((−)-NEMAM) (DP ≈ 700) which displayed a relatively weak maximum around 12.6 Å.60,71 Supramolecular Assembly with Small Molecule Analogues. To investigate the supramolecular interaction at the polymer chain-end for HW-Poly and Ba-Poly, small molecule analogues of the complementary recognition units were synthesized according to previously published procedures: tert-butyldimethylsiloxy (TBS) protected Hamilton wedge (sHW) and 5-ethyl-5-(hex-5-yn-1-yl)pyrimidine-2,4,6(1H,3H,5H)-trione (sBa) (Figures S26 and S29).46,72 The assembly of each polymer with the respective complementary small molecule can be visualized by 1H NMR spectroscopic techniques as well as isothermal titration calorimetry (ITC) (Figure 4). When HW is assembled with Ba, the N−Ha and N− Hb groups shift downfield from δ 8.4 and 7.7 ppm, respectively, in the unbound molecule to δ 9.8 and 9.5 ppm. The N−Hc proton resonance for the Ba moiety shifts from δ 8.8 ppm for the unbound species to resonances above δ 12 ppm for the bound molecule. Monitoring the HW-Ba assembly by 1H NMR spectroscopy confirmed the responsiveness of polymer endgroups to small molecule derivatives and is suggestive of high end-group fidelity for both systems given the dependence on concentration for the technique. To study the supramolecular assembly with small molecule analogues, HW-Poly was titrated by adding incremental molar aliquots of sBa and monitoring the proton resonances via 1H NMR spectroscopy. As sBa is added, the N−Ha of the HW

Figure 4. ITC binding isotherms for (a) HW-Poly with sBa and (b) Ba-Poly with sHW in CHCl3 at 25 °C. E

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Figure 5. Chemical shift of the amide protons of Ba-Poly upon hydrogen bonding as a function of equivalents of HW-Poly (CDCl3, 500 MHz, 25 °C). The concentration of Ba-Poly was kept constant at 1.11 mM (left). Average ITC binding isotherm for the titration of Ba-Poly with HW-Poly in CHCl3 at 25 °C (right).

an intense Δε whereas Ba-Poly-5 showed no differential absorbance (Figure 6). Because there was negligible contribu-

moieties, respectively (Figures S30−S33). A representative binding curve of the titration of HW-Poly-2 into Ba-Poly-4 is shown in Figure 5: upon assembly the imide N−Hc proton resonance of the Ba-Poly-4 moves downfield to δ 12.2 ppm, which is indicative of hydrogen bonding with the Hamilton wedge. A gradual upfield shift is visible for the amide N−Ha protons of HW-Poly-2 from δ 9.4 to 8.8 ppm upon addition of excess polymer. Previously, our group demonstrated the supramolecular assembly between HW- and Ba-functionalized poly(norbornene)s with an observed Ka of (1.4−1.5) × 104 M−1.77 The average association constant obtained for our system correlated well with this data, confirming the assembly of the two structurally distinct, helical polymer blocks by hydrogen bonding. ITC studies were performed by titrating Ba-Poly with HWPoly. A subtle heat change was visible from the average binding isotherm (Figure 5 and Figure S35) and corresponded to an average Ka = 210 M−1. We experienced challenges performing the ITC experiments, and we noticed a propensity for the injection needle tip to bead with polymer solution during loading. It is possible that the weight of the 5 mM polymer solution in chloroform caused the titrant to be leached from the injection needle into the sample cell during the pretitration equilibration period, resulting in undocumented binding events and therefore a lower Ka. Circular Dichroism Studies. The helical conformation of some polymers can be influenced by the presence of other optically active species in solution as a result of the dynamic nature of helical stabilization.78−80 Polymers with restricted backbone rotation or sterically bulky pendant groups tend to be less influenced by optical activity in the environment.81,82 Therefore, CD spectroscopy was used to observe the extent to which HW-Poly-1 and Ba-Poly-5 were able to maintain their respective optical activity as an A/B supramolecular block copolymer in solution. The CD spectrum for an equimolar mixture of HW-Poly-1 and Ba-Poly-5 was compared to the spectra of the homopolymers. The CD spectrum of the mixture should approximate the sum of the contribution of the differential absorbances represented as molar circular dichroism (Δε) of both polymer blocks. At 249 and 364 nm, HW-Poly-1 displayed

Figure 6. CD spectra for homopolymers HW-Poly-1 (dashed) and BaPoly-5 (gray) at 3.75 μM in CDCl3 and a 1:1 mixture of the polymers (solid).

tion in differential absorbance from Ba-Poly-5 at these wavelengths, the consistency in the molar circular dichroism of the homopolymer and the mixture suggests that the helical structure of the poly(isocyanide) block is not affected by the assembly with the helical Ba-Poly-5 block. At 283 nm, the CD signal observed from the mixture was approximately the sum of the contributions of Δε of each homopolymer, indicating a retention in the helical conformation of the poly(NEMAM) block in the presence of the poly(isocyanide) block (Table S3). These results suggested that in an assembled mixture of helical HW-Poly-1 and Ba-Poly-5 the helical structures of each block coexist in solution and remain stable. In an effort to explore the assembly of helical polymers with opposite signs for their Cotton effects, additional CD experiments were conducted. Because the strength of the CD signal for Ba-Poly is less than HW-Poly and because some of the chromophore absorbances overlap, we observed the CD of Ba-Poly-4 displaying a positive Cotton effect at 283 nm in a pseudo-racemic mixture. To generate the pseudo-racemic mixture, M-helical HW-Poly-1 with a negative Cotton effect at 364 nm was mixed with a P-helical unfunctionalized poly(isocyanide) with the opposite Cotton effect (Figure 7). First, unfunctionalized P-helical poly(isocyanide) was added to a 3.75 μM solution of M-helical HW-Poly-1, and a pseudoracemic solution was formed (dotted line, Figure 7). One molar equivalent of Ba-Poly-4 was added, and the relatively flat F

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polymer blends. Meanwhile, aggregation and cluster formation as well as the degree of amorphous or crystalline character of the components can yield a complex and not well-understood interplay of forces responsible for the overall material.1,76,88−95 Chen et al. describe how segregation effects between polar head groups (e.g., HW/Ba) versus the polymer chain can yield significant changes in the film state, with not only formation of phase separated diblock structures but also formation of clusters composed of singularly hydrogen-bonding moieties.76 With the aforementioned knowledge related to supramolecular assembly and its impact on phase separation, we hypothesized that based on the given molecular weights and block ratios (approximately 1:1), a fairly amorphous film would be observed with low probability for microphase separation assisted by supramolecular assembly. In order to preliminarily investigate how the assembly and mixing behavior affect the solid-state character, the WAXS profiles of the homopolymers were compared to the 1:1 assembled mixture prepared in chloroform. The X-ray diffraction pattern from the film of HWPoly-1 and Ba-Poly-3 displays a very weak maximum at 19.74 Å, assigned to the polymer stem of HW-Poly-1. The broad nature is indicative of a relatively amorphous solid (Figure 8). The

Figure 7. CD spectrum of a pseudo-racemic mixture of HW-Poly-1 and an unfunctionalized poly(isocyanide) in CHCl3 (dashed). One molar equivalent of Ba-Poly-4 (gray) was added, and the CD spectrum of the polymer mixture was obtained (solid).

baseline of the pseudo-racemic host mixture allowed for the less obstructed visualization of the CD spectra of Ba-Poly-4 (solid line, Figure 7). From the spectra it is evident that the excess helical conformation of Ba-Poly-4 is retained in the mixture and is not affected by the presence of either poly(isocyanide) species. The same experiment was performed utilizing 1H NMR spectroscopy. As seen in the Supporting Information (Figure S36), an equivalent of Ba-Poly-4 was added to the pseudoracemic mixture. The signal for the Ba proton at δ 12.6 ppm indicated assembly between Ba-Poly and HW-Poly despite the presence of excess unfunctionalized polymer.74 From these experiments, we can conclude that a mixture of HW- and Bapolymer forms an assembled supramolecular block copolymer, which contains helical domains that are persistent and not diminished by the presence of another chiral species in solution. IR Spectroscopy of Assembled Polymers. As previously stated, IR spectroscopy is a valuable means to elucidate the conformation of poly(NEMAM) due to the reliance of intramolecular hydrogen bonds to stabilize the secondary structure. Poly(isocyanide)s show no stretching frequencies above 3000 cm−1, making IR spectroscopy a good diagnostic technique to monitor changes in the intramolecularly hydrogen-bonded network of Ba-Poly-3 within the assembled mixture. An equimolar mixture of Ba-Poly-3 and HW-Poly-1 displays negligible changes in the wavenumbers of the amide N−H (3447 cm−1) and carbonyl CO (1652 cm−1) stretches of Ba-Poly-3, indicative of no significant change in the intramolecular amide hydrogen-bonded network responsible for stabilizing the helical structure in poly(NEMAM)s.60−62 All of the vibrational stretches characteristic of HW-Poly-1 remain constant in the assembled mixture, further supporting the spectral evidence from CD experiments that there is no significant change in the secondary structure of each polymer block upon supramolecular block copolymer formation in solution (Figure S41). Wide-Angle X-ray Scattering Studies of Polymer Mixtures. Ideally, incorporation of helical blocks into covalent BCPs can assist in hierarchical self-assembly, by which microphase separation of the block copolymer competes with the tendency for each helical block to form ordered domains.27,83 The addition of supramolecular groups at the chain-ends adds a degree of complexity toward the rational design of phase-separated copolymers in the solid state with polymers (especially high molecular weight) typically displaying regions of macrophase separation, microphase separation, and disordered phases.84−87 An interplay of forces is responsible for the final organization of BCPs in the film state. Hydrogen bonding can compete with the chemical incompatibility of

Figure 8. Wide-angle X-ray diffraction pattern of films of HW-Poly-1 (left axis) and Ba-Poly-3 (right axis) versus the 1:1 solution-assembled polymer film (right axis) prepared from chloroform in reflection mode.

WAXS profile of the 1:1 assembled film contained features more similar to HW-Poly-1, with the diameter of the polymer stem displaying miniscule changes from 19.63 to 19.74 Å, thus indicating no significant change in the side-chain extension (stem diameter) of HW-Poly-1. No peaks or features related to Ba-Poly-3 were observable. The lack of new Bragg reflections in the wide-angle region suggested there is little to no mixed (eutectic) crystals. This suggested two phase separation or disordered regions may dominate in the solid state, but it is not conclusive as to whether small regions of diblocks exist. X-ray diffraction patterns from a 1:1 mixture of unfunctionalized polymers96 (no hydrogen-bonding groups) displayed that the diffraction pattern of the mixture mirrored the diffraction patterns of the homopolymers, especially that of the poly(isocyanide) which is relatively more crystalline (Figures 9 and S42). The d-spacing of each Bragg diffraction respective to each homopolymer was retained in the mixture, and these values matched those observed in the functionalized polymers. When comparing the 1:1 mixture of functionalized polymers to the 1:1 physical mixture, the functionalized polymers displayed a diffraction pattern that is notably more amorphous. It is clear that the presence of the hydrogen G

DOI: 10.1021/acs.macromol.6b01410 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

merization of chiral arylisocyanide monomers using a Hamilton wedge-functionalized ethynylphenyl−palladium (II) derivative yielded α-functionalized helical poly(isocyanide)s. The use of a barbiturate-functionalized RAFT CTA for the radical polymerization of chiral NEMAMs generated ω-functionalized helical poly(NEMAM). 1H NMR specctroscopic studies confirmed the directional supramolecular assembly of the polymer blocks by complementary recognition motifs. These results showed that the strength of the hydrogen-bonded pairs prevail against the solution dynamics of these sterically cumbersome materials at the given molecular weights. CD and IR spectroscopies confirmed retention of each secondary structure in the assembled BCP. It is clear that the supramolecular forces that led to directionality in solution do not always translate to molecular ordering in the solid state, with WAXS, DSC, and optical microscopy suggestive of a low degree of diblock formation and largely disordered or two-phase structures. The synthesis of functional initiators and CTAs for well-defined helical polymers and successful supramolecular assembly in solution state is a first step toward the fabrication of advanced polymeric ensembles.

Figure 9. Wide-angle X-ray diffraction pattern from a 1:1 solutionassembled polymer film (right axis) prepared from HW-Poly-1 and BaPoly-3 in chloroform and a 1:1 mixture of unfunctionalized poly(isocyanide) and poly(NEMAM) (left axis) prepared from chloroform in reflection mode.

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bonding end-groups affect the film formation to some degree, but it is inconclusive as to if this is because of some degree of supramolecular assembly. We have shown the HW/Ba moieties was bound in solution, but it is conceivable that as the solution was concentrated, the dynamic HW/Ba bond and polymer incompatibility caused supramolecular diblocks to break with prevailing large-scale phase separation or disorder of the homopolymers. The lack of microstructure observed in this supramolecular system was supported by differential scanning calorimetry (DSC). No thermal transitions were obtained for the homopolymers and 1:1 solution-assembled mixtures in a range of −80 to 200 °C, which is suggestive of disorder at the given molecular weights and sample preparation. In order to establish an appropriate temperature window with no degradation, thermogravimetric analysis (TGA) was performed on Ba-Poly-3 and HW-Poly-1. The TGA trace of Ba-Poly-3 shows 10% degradation between 300 and 310 °C, with 100% degradation reached at 430 °C. HW-Poly-1 exhibited 10% degradation between 280 and 325 °C, gradually reaching 100% at 375 °C. Although DSC analysis did not reveal thermal transitions for the homopolymers and the solution assembled mixtures, optical microscopy revealed that upon heating BaPoly-3 gradually melted over a large temperature window (80− 280 °C) (Figure S43). The large melting-point range supports relatively weak Bragg diffractions observed by WAXS and the lack of defined thermal transitions by DSC. Previous studies report thermal transitions at 171 °C for isotactic and 151 °C poly(NEMAM) with a DP of 700. The polymers used for NMR spectroscopic and ITC assembly studies are small enough to characterize the assembly strength and fidelity; however, they are too small to obtain insights into long-range molecular packing. Complex studies comparing diblocks of varying molecular weights, ratios, covalent analogues, and comparison to theoretically predicted phase behavior will yield a better understanding of the phase behavior of these materials in the future.

EXPERIMENTAL SECTION

All reagents were purchased from Acros Organics, Sigma-Aldrich, or Alfa Aesar and used without further purification unless otherwise noted. Methacryloyl chloride was distilled prior to use. 4,4′-Azobis(4cyanovaleric acid) (ACVA) was recrystallized from MeOH. NMR spectra were recorded using either a Bruker AV-400 (1H: 400.1 MHz; 13 C: 100.6 MHz; 31P: 162 MHz) or an AV-600 (1H: 600 MHz, 13C: 150 MHz) spectrometer. Chemical shifts are reported in ppm and referenced to TMS. Mass spectra of samples in acetonitrile were acquired with an Agilent 6224 Accurate-Mass TOF/LC/MS spectrometer. Gel-permeation chromatograms (GPC) were obtained from a Shimadzu pump coupled to a Shimadzu UV detector with THF as the mobile phase. The flow rate was 1 mL/min on an American Polymer Standards column set (100, 1000, and 100 000 Å, linear mixed bed). The GPC was calibrated using poly(styrene) standards and carried out at 25 °C. Mw, Mn, and Đ represent weight-average molecular weight, number-average molecular weight, and dispersity, respectively. UV−vis spectra were recorded with a PerkinElmer Lamda 950 UV−vis spectrometer using distilled CHCl3 and 1 cm quartz cuvettes. Thermogravimetric analysis was performed with a PerkinElmer Pyrus I TGA by scanning from 25 to 450 °C with a rate of 10 °C/min. X-ray diffraction data were collected on a Bruker D8 DISCOVER GADDS microdiffractometer equipped with a VÅNTEC-2000 area detector. X-rays generated with sealed Cu tube were monochromated by a graphite crystal and collimated by a 0.5 mm MONOCAP (λ = 1.541 78 Å). Samples were prepared by dissolving a 1:1 mixture of HW-Poly and Ba-Poly (approximately 10 mg) polymer in 1 mL of chloroform. Differential scanning calorimetry (DSC) was performed using a PerkinElmer Pyris1 DSC. Samples were heated at a rate of 10 °C/min. Optical micrographs were performed using a Zeiss Axioskop 40 polarized optical microscopy coupled to a Linkam LTS 350 heating stage and Infinity 1 camera with heating at a rate of 10 °C/min. Matrix-assisted light desorption ionization time-of-flight (MALDI-ToF) data were recorded on a Bruker UtrafleXtreme using trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malonitrile (DCTB) as the matrix. The HW-Pd sample was prepared using a layer-by-layer approach,97 with DCTB dissolved in a 1:1 MeOH/ DCM solution. A low-molecular-weight Ba-Poly sample was prepared using 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB) as matrix and AgTFA and/or LiTFA as a cationization agent. The polymer solution and the salt were premixed in a ratio of 100:10:1 (dry-droplet method). IR spectra were obtained on a Nicolet 6700 FTIR using attenuated total reflectance (ATR) mode. Circular dichroism spectra were collected using an Aviv stopped flow CD spectropolarimeter Model 202SF (Lakewood, NJ). The

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CONCLUSIONS We describe the synthesis of telechelic, helical poly(isocyanide)s and poly(NEMAM)s containing terminal complementary supramolecular hydrogen-bonding groups. PolyH

DOI: 10.1021/acs.macromol.6b01410 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules spectra were obtained using a 2 mm path length quartz cell in CHCl3. Isothermal titration calorimetry was performed on a Nano isothermal titration calorimeter low volume (Nano ITC LV) from TA Instruments. Measurements were obtained at 25 °C in distilled, degassed chloroform. The buret was stirred at 300 rpm with 20 injection volumes of 2.5 μL. Compound 1. A flask containing 4-ethynylaniline (0.663 g, 5.66 mmol) was evacuated and purged with N2, charged with 50 mL of dry, distilled chloroform, and submerged in an ice bath to cool to 0 °C. Triethylamine (1.18 mL, 8.49 mmol) was added dropwise followed by 6-bromohexanoyl chloride (1.04 mL, 6.79 mmol). After 14 h, the reaction was diluted with dichloromethane (100 mL) and washed with diluted HCl (3 × 100 mL) followed by brine (2 × 80 mL). The organic layer was dried over MgSO4, and the solvent was removed under reduced pressure. The purified compound was obtained after column chromatography gradient elution on SiO2 with DCM/Et3N (99.5/0.5) increasing to 10% MeOH/DCM. Yellow solid, 1.48 g (89%). 1H NMR (400 MHz; CDCl3, δ ppm): 7.46 (q, J = 9.8, 4H), 7.26 (d, J = 6.3, 1H), 3.42 (t, J = 6.7, 2H), 3.04 (s, 1H), 2.38 (t, J = 7.4, 2H), 1.90 (t, J = 7.5, 2H), 1.76 (s, 2H), 1.53 (s, 2H). 13C NMR (600 MHz; CDCl3, δ ppm): 24.7 27.8, 32.5, 33.7, 37.5, 117.8, 119.5, 133.02, 138.4, 171.3. FT-IR (ATR, cm−1): 3278 (alkyne νC−H), 3302 (νN−H), 2104 (νC−C, alkyne), 1660 (νCO, amide). Compound 2. A flask and reflux condenser were evacuated and purged with N2 and charged with HO-wedge (0.350 g, 0.694 mmol), Cs2CO3 (0.452 g, 1.39 mmol), and catalytic NaI. The solids were dissolved in 12 mL of dry, distilled THF. In a separate flask, 6-bromoN-(4-ethynyl)hexamide (0.220 g, 0.75 mmol) was dissolved in 6 mL of dry, distilled THF and added dropwise to the stirred solution of HOwedge and base. The reaction was heated to reflux and stirred for 42 h. The reaction was removed from heat and filtered to remove solids. The filtrate was diluted with EtOAC (100 mL) and washed with brine (3 × 50 mL). The organic portion was dried over Na2SO4, and the solvent was removed by reduced pressure. The product was purified by column chromatography with 7% MeOH/DCM as the eluent. The product stains red with vanillin, Rf = 0.55 (MeOH/DCM, 7/93). Pale yellow solid, 0.449 g (90%). 1H NMR (400 MHz; CDCl3, δ ppm): 8.70 (s, 2H), 8.40 (s, 2H), 8.27 (s, 1H), 7.92−7.83 (m, 6H), 7.64 (t, J = 8.1, 2H), 7.57 (d, J = 8.6, 2H), 7.39 (t, J = 8.6, 4H), 3.79 (t, J = 5.9, 2H), 3.02 (s, 1H), 2.36 (t, J = 7.4, 7H), 1.73 (dd, J = 14.9, 7.4, 7H), 1.62 (s, 2H), 1.38 (s, 2H), 1.25 (s, 2H), 0.98 (t, J = 7.4, 7H). 13C NMR (125 MHz; CDCl3, δ ppm): 13.9, 19.0, 25.2, 25.7, 28.6, 37.6, 39.6, 68.3, 83.5, 109.8, 110.5, 117.2, 117.7, 119.5, 133.0, 135.9, 138.8, 140.8, 149.4, 150.1, 159.7, 164.4, 172.0, 172.4. FT-IR (ATR, cm−1): 2106 (νC−C, alkyne), 1667 (νCO, amide), 3293 (νN−H). HRMS-ESI: Mtheoretical = 717.327 482, Msample = 717.327 952, ΔM = 0.47 mmass units (0.65 ppm), C40H43N7O6 (M+). HW-Pd. Hamilton wedge functionalized 4-ethynylphenyl (0.200 g, 0.279 mmol), dichlorobis(triethylphosphine)palladium (II) (0.120 g, 0.289 mmol), and CuI (3.6 mg, 0.019 mmol) were added to a flask purged with Ar. The solids were dissolved in 20 mL of distilled DCM, and 5 mL of diethylamine was added. After stirring for 15 h, the solvent was removed, and the residue was purified by column chromatography with MeOH/DCM (5/95). Pale yellow solid, 0.187 g (61%). 1H NMR (600 MHz; CDCl3, δ ppm): 8.84 (s, 1H), 8.48 (s, 1H), 8.21 (s, 1H), 7.87 (s, 5H), 7.67 (d, J = 8.1, 2H), 7.47 (d, J = 10, 2H), 7.42 (s, 2H), 7.16 (d, J = 8.5, 2H), 3.80 (s, 2H), 2.37 (d, J = 7.6, 7H), 2.21 (m, 2H), 1.95 (tt, J = 8.2,14H), 1.73 (dd, J = 14.8, 7.4, 7H), 1.64 (s, 3H), 1.40 (s, 2H), 1.30−1.25 (m, 2H), 1.20−1.15 (m, 23H), 0.99 (t, J = 7.2, 8H). 13C NMR (150 MHz; CDCl3, δ ppm): 8.5, 13.9, 15.5, 18.9, 25.3, 25.7, 28.6, 29.4, 37.7, 39.5, 68.3, 94.8, 109.7, 110.4, 117.3, 117.7, 119.7, 123.8, 131.3, 135.8, 136.0, 140.8 149.4, 150.9, 159.6, 164.8 171.7, 172.3. 31P NMR (161 MHz; CDCl3, δ ppm): 17.98. FT-IR (ATR, cm−1): 2117 (νC−C, alkyne), 1668 (νCO, amide), 3294 (νNH). Typical Polymerization for HW-Poly. In a Schlenk tube purged with argon, the monomer (157.48 mg, 37 mmol) was dissolved in 2.25 mL of dry, distilled THF and degassed via freeze−pump−thaw. The initiator (16.74 mg, 0.015 mmol) was dissolved in 0.4 mL and added to the monomer mixture. The reaction was heated to 55 °C for 12 h,

whereupon the polymerization mixture was precipitated into 50 mL of MeOH/H2O (3:1). The brown solids were further purified via dialysis in DCM using 3500 MWCO dialysis tubing. After 24 h, the content of the dialysis tubing was precipitated into cold MeOH, and the polymer was obtained as a solid after rinsing with acetone. Brown solid, HWPoly-1:0.130 g, Mn = 13 100, Đ = 1.36 (SEC, THF); HW-Poly-2: 0.82g, Mn = 9000, Đ = 1.38 (SEC, THF). Ba-CTA. PPh3 (0.289 g, 1.1 mmol) was dissolved in freshly distilled THF (40 mL) in a three-neck flask under argon and placed over an ice bath. Diisopropyl azodicarboxylate (DIAD) (0.224 g, 1.1 mmol) was added dropwise and stirred until the solution became cloudy, indicating formation of the betaine complex. Commercial CPADB (0.243 g, 0.871 mmol) was dissolved in THF (10 mL) and added to the cloudy solution dropwise. The solution became clear upon deprotonation of the carboxylic acid. Compound 646 (0.316 g, 0.968 mmol) in THF (10 mL) was added to the mixture dropwise. The solution was stirred for 12 h at room temperature and then diluted with DCM (100 mL). The organic phase was washed with water (2 × 150 mL). The organic layer was dried over magnesium sulfate and concentrated. The crude product was purified on neutral alumina using 5% MeOH:DCM to afford a bright pink viscous oil. Yield 202 mg, (40%). 1H NMR (400 MHz; CDCl3, δ ppm): 8.44 (s, 2H), 7.92− 7.89 (m, 2H), 7.58−7.54 (m, 1H), 7.41−7.37 (m, 2H), 4.10 (t, J = 6.78, 2H), 2.73−2.60 (m, 3H), 2.47−2.40 (m, 1H), 2.03 (q, J = 7.41, 2H), 1.99−1.94 (m, 4H), 1.66−1.61 (m, 3H), 1.25 (d, 18H), 0.88 (t, J = 7.43, 3H). 13C NMR (150 MHz, CDCl3): 222.6, 172.8, 172.0, 148.9, 144.7, 133.3, 128.9, 126.9, 118.7, 65.5, 57.6, 45.9, 38.9, 33.7, 32.7, 30.0, 29.5, 29.4, 29.3, 29.2, 28.7, 26.0, 25.3, 24.3, 9.7. ESI-MS (M + Na)+ C30H41N3O5S2, theoretical: 587.25; experimental: 587.26. Typical RAFT Polymerization for Ba-CTA. The desired amounts of monomer, Ba-CTA, ACVA, and anhydrous THF were placed in a Schlenk flask. The typical molar ratio was 100:1:0.1 [M]:[CTA]:[I], respectively. The freeze−pump−thaw method was employed over five cycles, and the flask was backfilled with argon. The polymerization mixture was heated to 60 °C and quenched in liquid nitrogen after approximately 12−18 h. Crude polymers were purified via precipitation in cold MeOH three times.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01410. Additional characterization data for all homopolymers and copolymers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(M.W.) E-mail: [email protected]. Author Contributions

A.C. and K.M. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support has been provided by the National Science Foundation (CHE-1506890). We also acknowledge the NSF CRIF Program (CHE-0840277) for the purchase of a Bruker GADDS microdiffractometer. The NMR spectrometers were acquired through the National Science Foundation (CHE01162222). The MALDI-TOF MS was acquired through the National Science Foundation under Award CHE-0958457. We graciously thank Dr. Elizabeth Elacqua for obtaining MALDITOF data and for her insightful contributions to this manuscript. I

DOI: 10.1021/acs.macromol.6b01410 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b01410 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b01410 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b01410 Macromolecules XXXX, XXX, XXX−XXX