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ABC Supramolecular Triblock Copolymer by ROMP and ATRP Diane S. Lye,† Yan Xia,‡,§,⊥ Madeleine Z. Wong,† Yufeng Wang,‡,§,⊥ Mu-Ping Nieh,*,‡,§,⊥ and Marcus Weck*,† †

Molecular Design Institute and Department of Chemistry, New York University, New York, New York 10003, United States Department of Chemical and Biomolecular Engineering, University of Connecticut, Storrs, Connecticut 06269, United States § Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, United States ⊥ Department of Biomedical Engineering, University of Connecticut, Storrs, Connecticut 06269, United States ‡

S Supporting Information *

ABSTRACT: We report the controlled polymerization of three heterotelechelic polymers from supramolecular motifbearing initiators, their assembly into supramolecular A/B/C triblock copolymers, the morphology of the resulting block copolymers, and their disassembly. The individual blocks are synthesized in a controlled or living fashion via ring-opening metathesis polymerization (ROMP) of norbornenes (block B) or atom-transfer radical polymerization (ATRP) of styrenes (block A) and methacrylates (block C). The building blocks are assembled via metal-coordination between blocks A and B using a palladated SCS pincer complex and pyridine, and hydrogen bonding between blocks B and C using Hamilton Wedge and barbituric acid derivatives. Scanning-transmission electron microscopy and small-angle X-ray scattering reveal lamellar domains of approximately 23, 15, and 21 nm for bulk samples of the A/B and B/C diblock copolymers and A/B/C triblock copolymers, with 1:1 and 1:1:1 block feed ratios, respectively. Nanoscale features are destroyed when the materials are exposed to dimethylformamide and/or triphenylphospine, removing noncovalent linkages between individual blocks.

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tion,24,25 materials with shape-memory from backbone stereocomplementarity,26 columnar liquid-crystalline phase formation with the addition of hydrogen-bonding dendronized sidechains,27−31 and light-responsive morphology-controlled drug release via a supramolecular branched BCP.32 Most studies for the formation of nanoscale features from triblock copolymers are based on covalent systems with functionality and application preprogrammed in the polymer backbone33−36or side-chains37−39 by design. The only example of a triblock BCP responsive to two orthogonal stimuli (temperature and gas) utilizes a single supramolecular moiety.40 Indeed, few examples exist combining supramolecular directionality, phase-separation behavior, and stimuli-responsivity. Noncovalent block copolymer systems that study selfassembly behavior utilizing interactions such as metal coordination41,42 or hydrogen bonding41,43,44 have been reported. An attractive feature of these materials is their degree of modularity via the assembly of two or more telechelic polymer chains to yield any desired BCP sequence and composition. Multiple polymerization strategies (e.g., reversible addition−fragmentation transfer polymerization (RAFT),45−50

INTRODUCTION Self-assembled materials with ordered periodicities on the nanometer scale have become a cornerstone of nanotechnology. Supramolecular block copolymers, a subclass of selfassembled materials that have the ability to microphase-separate into three-dimensional architectures on the nanoscale, have found applications as self-healing1,2 and stimuli-responsive materials,3−5 as well as in optics,6−8 nanolithography,9 electronic devices,10 porous membranes,11 and the fabrication of nanostructured thin-films.12 Block copolymers (BCPs) with immiscible polymer backbones microphase separate due to mutual repulsion of dissimilar blocks and packing constraints, imposed by the connectivity of each block.13 A variety of ordered structures can be constructed from BCPs, depending on the number of blocks, volume fraction, chain flexibility, architecture, and the extent of repulsion between chemically connected blocks.14 The majority of research on supramolecular BCPs containing nanoscale features have centered around diblock copolymers, several of which employed nonspecific and nondirectional interactions. Examples include nanostructured films of BCPs with varying block copolymer segregating strength, sustained by hydrogen-bonding unit pairs,15−19 nanoporous films based on metal-coordinating diblocks,20−23 the addition of hydrogen-bonding small molecules to BCPs to undergo morphological transforma© 2017 American Chemical Society

Received: January 23, 2017 Revised: April 26, 2017 Published: May 18, 2017 4244

DOI: 10.1021/acs.macromol.7b00169 Macromolecules 2017, 50, 4244−4255

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Scheme 1. Schematic Representation of the Synthesis of Supramolecular A/B/C Triblock Copolymers via Self-Assembly (m ≈ n ≈ p ≈ 100)

Scheme 2. Synthesis of Pin ATRP Initiator and block A: α-Pd-pin-PS,a m ≈ 100

a Key: m ≈ 100. (a) 2-Bromo-2-methyl-propionic acid, EDC, DMAP, CH2Cl2, 12 h, room temperature, 68%; (b) styrene/PMDETA/CuBr = 500/2/ 1 ratio, 2.5 h, 110 °C; (c) Pd(PhCN)2Cl2, CH2Cl2/ CH3CN (v/v:1/1), AgBF4, NaCl, 12 h, room temperature, 99%.

nitroxide-mediated polymerization (NMP),51 ring-opening polymerization (ROP),52 ROMP,53−58 and ATRP59,60 have enabled access to telechelic hydrogen-bonding or metalcoordinating supramolecular blocks. In this contribution, we report the synthesis of a supramolecular main-chain A/B/C triblock copolymer, its selfassembly, resultant morphological properties, and controlled disassembly. Controlled or living polymerization of three endfunctionalized polymer blocks is achieved via ATRP and ROMP from initiators containing supramolecular units, using styrene, methacrylate, and norbornene octyl ester as monomers. Self-assembly of these blocks yields the desired A/B/C triblock copolymers, where “/” denotes a supramolecular interaction between the two blocks: poly(styrene)/poly-

(norbornene octyl ester)/poly(methyl methacrylate) (PS/ PNB/PMMA) (Scheme 1). Our synthetic methodologies avoid postpolymerization functionalization instead using initiators, catalysts, or chain-terminating groups containing recognition motifs to yield telechelic end-functionalized polymers. Two orthogonal supramolecular recognition pairs are employed: blocks A and B are sustained by the coordination of Pd(II) SCS-pincer complex (pin) with pyridine (pyr), while hydrogen-bonding Hamilton Wedge (HW)/barbiturate (Ba) motifs join blocks B and C. We characterize the microphase separation of our A/B/C main-chain supramolecular BCPs via scanning-transmission electron microscopy (STEM) and smallangle X-ray scattering (SAXS). Investigations into unfunctionalized poly(norbornene)-based morphology are scarce, focusing 4245


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Macromolecules Scheme 3. Synthesis of Heterotelechelic Block B: α-HW-PNB-ω-pyr PNB, n ≈ 100a

a

Key: (a) Grubbs’ modified 1st generation initiator 6, DCE, room temperature, 35 min, followed by addition of CT 7, 12 h.

Scheme 4. Synthesis of Ba-ATRP Initiator and Block C: α-Ba-PMMA, p ≈ 100a

a

Key: (a) 2-Bromo-2-methylpropanoyl bromide, DMF, CH2Cl2, 12 h, room temperature, 47%; (b) MMA/10/PMDETA/CuBr2 = 300/1/0.5/0.5 in 50% (v/v) anisole, Cu(0), 35 °C, 5 h.

units had to be incorporated into the chain-ends. Monotelechelic terminal blocks A and C were accessed via ATRP initiators bearing supramolecular recognition units, ensuring low molecular weight distributions and a high degree of chainend functionalization. A pyr-based ATRP initiator was not a viable target due to the high likelihood of coordination of the pyridine to the copper catalyst during the polymerization. Therefore, we incorporated the pincer ligand and barbiturate into the ATRP initiators. Styrene and MMA were the monomers selected for the ATRP to fabricate polymer blocks A and C.

on norbornenes with order-inducing pendant groups that dictate self-assembly and lead to highly defined nanoscale features,61,62 or ROMP-based comb copolymers.63,64 Our report closes this gap by introducing a nonfunctional norbornene monomer as building block for a microphase separating block copolymer system. Finally, we show that the addition of solvents and/or additives selectively displaces the metal-coordinating and hydrogen-bonded terminal blocks.

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RESEARCH DESIGN The successful assembly of our main-chain supramolecular A/ B/C BCP hinged upon the presence of the heterotelechelic middle block B (Scheme 3) containing two orthogonal supramolecular termini. We decided to use a ROMP-based polymer block with a hydrogen-bonding motif and a metalcoordinating unit on the α- and ω-termini via methodology previously developed by us.65 For polymer blocks A and C, complementary metal-coordinating and hydrogen-bonding

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RESULTS AND DISCUSSION

The supramolecular recognition unit-containing initiators were synthesized by esterification of 2-bromo-2-methylpropionate with either pin−OH or Ba−OH to yield compounds 2 and 10 respectively (Schemes 2 and 4). Initiators 2 and 10 were used to conduct the ATRP in the presence of Cu(0) and CuBr2, or 4246


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Figure 1. Semilogarithmic kinetic plots and the dependence of Mn vs monomer conversion for the ATRP of functionalized initiators: (A) pin-2bromo-2-methylpropanoate (2) with styrene and (B) Ba-2-bromo-2-methylpropanoate (10) with MMA. From left to right, ⧫ tracks the consumption of monomer over time, and likewise from left to right; • tracks the corresponding progression of Đ values over time.

CuBr catalysts and PMDETA as ligand, at elevated temperatures under inert conditions, resulting in α-pin-PS and α-BaPMMA polymers of controlled molecular weights and low dispersities as determined by gel-permeation chromatography (GPC) (Mn = 13 800 g/mol, Mw = 16 700 g/mol, and Đ = 1.21 for α-pin-PS and Mn = 25 000 g/mol, Mw = 29 000 g/mol, and Đ = 1.16 for α-Ba-PMMA). GPC analyses showed a monomodal dispersity for blocks A and C (Figure 2). α-PinPS was subsequently palladated with Pd(PhCN)2Cl2 and v/v 1:1 CH2Cl2/CH3CN for 12 h at ambient conditions to obtain α-Pd-pin-PS. The kinetics of both ATRP reactions were monitored by 1H NMR spectroscopy and GPC (Figures 1 and 2). For both polymerizations, the plots of ln([M]0/[M]) versus time and Mn versus conversion are linear, confirming a high degree of control. When 2 was palladated prior to polymerization and used as initiator in the ATRP, GPC analysis of the resulting polymers consistently showed a monomodal but broad peak with a dispersity of Đ = 1.9 suggesting a loss of control over the polymerization due to the presence of the palladium complex. Glass transition temperatures of Tg = 98 °C for α-Pd-pin-PS and Tg = 98 °C for α-Ba-PMMA were obtained via differential scanning calorimetry (DSC). We synthesized the heterotelechelic α-HW-PNB-ω-pyr PNB block using methodologies described by us previously.65 The ROMP of norbornene was initiated with a Grubbs first generation initiator, modified with a HW-styrene via crossmetathesis, and terminated with a pyr-CTA. GPC analyses indicated a monomodal distribution (Figure 2) with low dispersities (Mn = 35 700 g/mol, Mw = 40 700 g/mol, Đ = 1.14). The initiation was complete as evidenced by the disappearance of the benzylidene peak at 19.43 ppm in the NMR spectrum during the polymerization. No Tg was observed for the α-HW-PNB-ω-pyr PNB by DSC. Self-Assembly of A/B, B/C, and A/B/C Block Copolymers. The hydrogen bonding HW-Ba interaction was previously shown to be orthogonal to the pin-pyr metalcoordination,66,68 as was likewise observed in our self-assembly

Figure 2. GPC traces: block A, α-pin-PS, Mn = 13 800 g/mol, Đ = 1.21; block B, α-HW-PNB-ω-pyr PNB, Mn = 35 700 g/mol, Đ = 1.14; block C, α-Ba-PMMA, Mn = 25 000 g/mol, Đ = 1.16.

experiments (Figure 3). This allowed for the mixing of all three supramolecular blocks in solution in one pot in a 1:1:1 molar ratio based on polymer molecular weight calculations, as determined by end group analysis versus polymer backbone via 1 H NMR spetroscopy, to yield the desired A/B/C BCP. The fully assembled metal-coordinated A/B diblocks, hydrogenbonded B/C diblocks, and A/B/C supramolecular triblocks were studied by 1H NMR spectroscopy (Figures 3 and S4). For the A/B diblock formation, a 1:1 molar ratio of α-Pdpin-PS (block A) and α-HW-PNB-ω-pyr PNB (block B) were dissolved as physical mixtures in chloroform. No changes in the 1 H NMR spectrum were observed with respect to the SPh 4247


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Figure 4. Titration curve of Blocks B and C in CHCl3: to a solution of α-HW-PNB-ω-pyr PNB, 20 equiv of α-Ba-PMMA was added.

recognition pairs for the formation of main and side-chain supramolecular polymers have been well established and our results are on par with what has been reported.58,73 Both pairs display high association constants (Ka ≈ 103 - 105 M−1 for HWBa and Ka ≈ 1015 M−1 for pin-pyr)73 in halogenated solvents. To obtain the A/B/C triblock copolymer, α-Pd-pin-PS, αHW-PNB-ω-pyr PNB, and α-Ba-PMMA were assembled in a one pot fashion in chloroform with AgBF4. After the mixture was stirred for 30 min at room temperature, the AgCl was removed via syringe filter. The mixture was then concentrated in vacuo, and the polymer assembly obtained annealed in a likewise fashion as to its B/C counterpart: a degassed chamber at 60 °C in the presence of chloroform for 48 h. The 1:1:1 ratio of α-Pd-pin-PS, α-HW-PNB-ω-pyr PNB, and α-Ba-PMMA in CHCl3 was analyzed by 1H NMR spectroscopy showing the same SPh shifts from 7.41 and 7.85 ppm to 7.57 and 8.09 ppm respectively, as described above for the diblock, indicating the coordination between pyridine and the palladium pincer complex had taken place (Figure 3, trace 5). HW signals shifted upon addition of the Ba-containing polymer block from 8.05 and 8.81 ppm to 9.1 and 9.55 ppm (Figure 3, trace 6). With the assembled A/B/C triblock copolymer in hand, we chose to investigate preliminary evidence of microphase separation behavior at an easily targetable data point. We chose the ratio of a 1:1:1 assembly in the solid state as a straightforward assessment of our system, to see if supramolecular triblock copolymers of this class show any microphase separation behavior. We also studied our supramolecular triblock copolymer’s subsequent disassembly in this composition. To observe the microphase behavior of our self-assembled BCPs in the bulk, ultrathinly sectioned samples had to be obtained for examination under scanning transmission electron microscopy (STEM). The PNB block at room temperature tends to be soft and viscous, properties that were imparted to all of our BCP samples (A/B, B/C, A/B/C). Sectioning via cryotome at subzero temperatures enabled the soft BCP samples to be sliced. Samples were also examined under SAXS to corroborate images observed under STEM. Self-assembled hydrogen-bonded B/C block copolymers exhibited lamellar phases of 15 nm in the STEM (Figure 5). The lamellar features, which show long persistence lengths across hundreds of nanometers, display turns and junctions. To confirm our STEM findings, we carried out SAXS studies on the B/C block copolymers. A broad shoulder was observed in the SAXS data (Figure 7A) from 0.02 to 0.1 Å−1, suggesting an irregular d-spacing of between 6 and 30 nm consistent with the STEM micrographs.

Figure 3. 1H NMR of homopolymers α-Pd-pin-PS, α-HW-PNB-ωpyr PNB, and α-Ba-PMMA, and their self-assembly in CHCl3. Solid boxes indicate shifts of proton peaks from metal-coordination while dotted boxes indicate shifts from hydrogen-bonding.

(7.41, 7.85 ppm) and HW (7.88, 8.81 ppm) proton peaks. (Figure 3, traces 1, 2, and 4). An excess of AgBF4 was then added to the mixture and the reaction stirred at room temperature for 10 min at which point a cloudy white precipitate, presumably AgCl, formed in solution. The mixture was stirred for an additional 10 min to ensure complete metalcoordination, syringe-filtered to remove AgCl, and the solvent was removed in vacuo to provide the self-assembled A/B diblock copolymer. The addition of a multifold excess of AgBF4 during the metal-coordination event pushes the equilibrium forward toward quantitative assembly of a metal−ligand pin-pyr complex as AgCl salt crashes out. The mixture was analyzed by 1 H NMR spectroscopy, whereby the SPh signals shifted from 7.41 and 7.85 ppm to 7.57 and 8.09 ppm, respectively, indicating the coordination between pyridine and the palladium pincer complex had taken place (Figure 3, trace 5). Hydrogen-bonded B/C polymers were obtained by mixing a 1:1 molar ratio of α-HW-PNB-ω-pyr PNB and α-Ba-PMMA polymer blocks in chloroform, drying the mixture in vacuo and annealing it at 60 °C in a degassed chamber, in the presence of chloroform vapors for 48 h. A 1:1 mixture of α-HW-PNB-ωpyr PNB (block B) and α-Ba-PMMA (block C) in CHCl3 was analyzed by 1H NMR spectroscopy. The HW signals shifted from 8.05 and 8.81 ppm to 9.1 and 9.55 ppm, respectively (Figure 3, trace 6). Hydrogen-bonding was also monitored via 1 H NMR spectroscopy by titrating the α-Ba-PMMA into a solution of α-HW-PNB-ω-pyr PNB in CHCl3. An association constant (Ka) of 2.87 × 103 M−1 was determined (Figure 4). The synthetic and supramolecular chemistries of these two 4248


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Because of the difficulty of background subtraction, we were not able to accurately determine the decay constant at high q. Microphase separation as observed in our A/B and B/C polymer samples corroborates the ability for PS/PNB and PNB/PMMA to undergo phase separation due to chemical incompatibility. Studies have observed phase separation between PS/PMMA on surfaces ranging from 1:1 to 9:1 ratios and 35 to 1450 × 103 g/mol17,80−82 but not at lower molecular weights.83,84 Introduction of a middle block B (such as a polymer like PNB) with a higher interaction parameter χ with either PS or PMMA, would lead to a higher likelihood of observing phase separation after assembly of the triblock copolymer. STEM and SAXS results of the A/B/C assembly showed that the introduction of a PNB block was sufficient to effect phase separation between PS and PMMA as A/B and B/ C assemblies, as well as in the full triblock assembly when PNB was installed as a middle block. The A/B/C triblock copolymers were subsequently disassembled in a similar fashion as shown elsewhere via 1H NMR spectroscopy65−72,85 and studied under SAXS, by (a) breaking the metal-coordinating block A from B/C via the addition of PPh3 and (b) breaking the hydrogen-bonds between blocks C from A/B by increasing the temperature or using a competitive solvent such as DMF. A 4-fold equivalent of PPh3 was added to a solution of the A/B/C triblock copolymer in chloroform. The stronger coordination complex formed between PPh3 and Pdpin results in the release of the pyridine ligand from the Pdpincer complex,85 which goes hand in hand with the disruption of α-Pd-pin-PS from the rest of the assembly. In severing the bonding between α-Pd-pin-PS from the rest of the assembly, a comparison of the original SAXS curves of A/B/C (Figure 8A) to that of the disassembled mixture (Figure 8B), the first slope transition from q−2 to q−4 is smeared in Figure 8B, suggesting possible disruption of the original lamellar phase. The A/B/C triblock copolymer was then disassembled at the hydrogen-bonding junction. Disassembly of block C from the original A/B/C assembly was carried out by exposing the triblock copolymer to hydrogen-bond disrupting DMF solvent vapors at 60 °C for a week. The dissembled polymer was cooled back down by briefly exposing it in a vial to liquid nitrogen to freeze the target disassembled structure, and left in vacuum at ambient temperature for an additional 2 days to remove residual solvent. The SAXS data (Figure 8C) show that slope transitions between q−2 and q−4 compared to that of the triblock copolymer (Figure 8A) were smeared presumably due to the rearrangement of the block copolymer in the solvent environment. The slope transition at q ∼ 0.06 Å−1 observed in Figure 8C is gentler compared to the transition in Figure 8, parts A and B. This suggests that the disassembly of the hydrogen bonding unit might have a greater impact in

Figure 5. STEM image of assembled B/C block copolymer (A). Bar chart of all feature sizes on a 1 μm2 area shows a mode of 15 nm-sized features, and a mean of 13.6 nm-sized features (B).

In the metal-coordinated A/B diblock copolymers, an irregular lamellar pattern was observed of approximately 23 nm (Figure 6A). Under bright-field STEM mode, after staining, the poly(norbornene) blocks were presented as darker regions while the poly(styrene) blocks appeared as lighter regions. A lamellar pattern in a striped fashion was observed with intermittent dark nanometer-sized circular features sitting at various junctions across several lamellar phases, the latter of which are artifacts of OsO2 particles generated from the staining process.74 A similar broad shoulder was observed in the corresponding SAXS data from 0.02 to 0.1 Å−1 (Figure 7B), implying the irregular spacing observed in STEM images. However, the SAXS intensity increases with q at the higher q regime with a possible peak positioned higher than 0.2 Å−1, indicating the presence of other well-defined spacing at a length scale