Demonstrating the 81-Helicity and Nanomechanical Function of Self

Article pubs.acs.org/Macromolecules

Demonstrating the 81‑Helicity and Nanomechanical Function of SelfOrganizable Dendronized Polymethacrylates and Polyacrylates Katerina A. Andreopoulou,† Mihai Peterca,†,‡ Daniela A. Wilson,† Benjamin E. Partridge,† Paul A. Heiney,‡ and Virgil Percec*,† †

Roy & Diana Vagelos Laboratories, Department of Chemistry and ‡Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States S Supporting Information *

ABSTRACT: A library of six dendronized polymers synthesized via single-electron transfer living radical polymerization (SET-LRP) of self-assembling dendrons with a methacrylate or acrylate apex is reported. Structural and retrostructural analysis by X-ray diffraction, molecular modeling, reconstructed electron density maps, and circular dichroism demonstrated that dendronized polymethacrylates and polyacrylates adopt a chiral, 81-helix induced by self-organization of the dendritic side groups. The helical handedness is selected by the stereocenter of the ethylene glycol linker between the dendron and polymer. Upon heating, oriented fibers of polymers with this linker undergo a phase transition between two columnar hexagonal phases, accompanied by unidirectional macroscopic thermal expansion. Replacement of the flexible linker with a shorter, rigid linker eliminates this expansion. Polyacrylates exhibit substantially quicker phase transition dynamics than polymethacrylates. The relationship between the primary structure of the dendritic monomer and the macroscopic thermal response elucidated here is expected to guide the design of additional macromolecules with nanomechanical function.

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chromophoric backbone itself.32,40,44 Unlike pristine PPA, which upon heating undergoes a helix−coil transition accompanied by intramolecular electrocyclization and chain cleavage,47 dendronized PPAs exhibited an unprecedented helix−helix transition mediated by the jacketing dendrons via cis-cisoidal to cis-transoidal isomerization and no intramolecular electrocyclization. This thermoreversible cis-cisoidal to cistransoidal isomerization at the molecular level resulted in the macroscopic elongation of an oriented fiber of the PPAs.45,46 The anisotropic thermal expansion was exploited to lift an object with a mass 250 times greater than the fiber.45 Such macroscopic motions depend on a nanoscale amount of work being conducted by billions of assemblies in a cooperative and reproducible fashion and may be exploited in molecular nanomachines.48−53 In contrast to semirigid PPAs,54−59 atactic pristine polymethacrylates and polyacrylates produce nonhelical backbones with no well-defined conformation. Functionalization of polymethacrylates with chiral lysine- or 4-aminoproline-based dendrimers induces an undetermined helical conformation in solution, which is unaffected by the tacticity of the polymer backbone.22,60−63 A polymethacrylate dendronized with a tapered self-assembling dendron linked to the backbone via a flexible ethylene glycol linker,64−70 12-4EO-PMA, was shown to

INTRODUCTION Dendronized polymers have been advanced as precise synthetic mimics of complex biomacromolecules,1 such as tobacco mosaic virus (TMV).2−4 TMV consists of 2130 identical proteins which self-organize into a porous helical column. The pore of this column can be occupied by viral DNA, which adopts a helical conformation templated by the protein coat.3 Analogous helical dendronized polymers with dendritic side chains attached by noncovalent and covalent interactions have been developed.1,5−13 The elaboration of function in self-organizing materials requires control over the assembly of individual molecules over different hierarchical levels.1,14,15 The helicity of chiral dendronized polymers has been investigated extensively in solution using circular dichroism (CD) spectroscopy,12,16−19 and their behavior has been subject to theoretical treatment.20−22 However, there are comparatively few investigations into bulk assemblies of chiral dendronized polymers with rigorous analysis of the structure of the polymer.23−28 In most cases, rigid or semirigid backbones have been used to induce helical assembly, with the appended dendrons simply selecting the handedness of the already helical polymer.29−32 Our laboratory has previously investigated the supramolecular selforganization of cis-poly(phenylacetylene)s (PPAs) 33−38 dendronized with a diversity of self-assembling first- and second-generation dendrons.28,32,39−46 Structural analysis demonstrated that the polymer backbones in such polymers adopt a helical conformation, which could be monitored via the © XXXX American Chemical Society

Received: June 8, 2017 Revised: June 30, 2017

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DOI: 10.1021/acs.macromol.7b01216 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Differential Scanning Calorimetry (DSC). Thermal transitions were determined on a TA Instruments Q100 differential scanning calorimeter equipped with a refrigerated cooling system with 10 °C/ min heating and cooling rates. Indium was used as calibration standard. The transition temperatures were reported as the maxima and minima of their endothermic and exothermic peaks, respectively. An Olympus BX51 optical microscope (100× magnification) equipped with a Mettler FP82HT hot stage and a Mettler Toledo FP90 central processor was used to identify and verify the thermal transitions and to characterize the anisotropic textures. X-ray Diffraction (XRD). X-ray diffraction (XRD) measurements were performed using Cu Kα1 radiation (λ = 1.542 Å) from a BrukerNonius FR-591 rotating anode X-ray source equipped with a 0.2 × 0.2 mm2 filament operated at 3.4 kW. Osmic Max-Flux optics and triple pinhole collimation were used to obtain a highly collimated beam with a 0.3 × 0.3 mm2 spot on a Bruker-AXS Hi-Star multiwire area detector. To minimize attenuation and background scattering, an integral vacuum was maintained along the length of the flight tube and within the sample chamber. Samples were held in quartz capillaries (0.7−1.0 mm in diameter), mounted in a temperature-controlled oven (temperature precision: ±0.1 °C; temperature range from −10 to 210 °C). The distance between the sample and the detector was 12.0 cm for wide-angle diffraction experiments and 54.0 cm for intermediate-angle diffraction experiments. Aligned samples for fiber XRD experiments were prepared using a custom-made extrusion device.71 The powdered sample (∼10 mg) was heated inside the extrusion device above the isotropization temperature. After slow cooling from the isotropic phase, the fiber was extruded in the liquid crystal phase and cooled to 23 °C. Typically, the aligned samples have a thickness of 0.3−0.7 mm and a length of 3−7 mm. All XRD measurements were done with the aligned sample axis perpendicular to the beam direction. Primary XRD analysis was performed using Datasqueeze.72 Circular Dichroism and UV Spectroscopy. Circular dichroism (CD) and UV spectroscopy measurements were performed in both solution and thin film on a Jasco J-720 spectropolarimeter. The temperature was controlled for solution experiments by an integrated Peltier temperature controller (Jasco PTC-423) and for thin film experiments by a Thermo Neslab RTE-111 refrigerator circulator with digital temperature controller. Trifluoroethane and dodecane (spectrophotometric grade, 99%) were used as solvents for the solution experiments. Thin films were cast on round quartz plates by spincoating with a Chemat Technology spin-coater (model KW-4A). Samples weighing from 1.3 to 11.5 mg were dissolved in 100 μL of CHCl3 or hexane and added in one portion to the quartz plate. The quartz slides were spin-coated in two stages: stage I (3000 rpm for 6 s) and stage II (7000 rpm for 30 s), allowing a good dispersal of the liquid at low speeds and homogenizing the coating at high speed. Synthesis of Dendritic Monomers. (4-3,4,5)12G1-CO2EO4H (1),64 (S)-(4-3,4,5)12G1-CO2EO*-(EO)3H (2),73 (4-3,4,5)12G1CH2OH (3),74 (4-3,4,5)16G1-CO2EO4H (4),64 12-4EO-MA (5),64 12-CH2O-MA (7),74 and 16-4EO-MA (8)64 were synthesized according to literature procedures. 12-4EO*-MA (6). (S)-(4-3,4,5)12G1-CO2EO*-(EO)3H (2)73 (2.00 g, 1.69 mmol), dry Et3N (1.03 g, 10.14 mmol), and dry CH2Cl2 (30 mL) were added to a round-bottomed flask, which was sealed with a rubber septum and flushed with argon. The solution was cooled to 0 °C with an ice bath, and methacryloyl chloride (0.70 g, 6.76 mmol) was added via syringe. The reaction mixture was then stirred at 23 °C until TLC (2:1 hexane/ethyl acetate) showed completion (2 h). The mixture was filtered, and the solvent was removed by rotary evaporation. The crude product was redissolved in CH2Cl2 and precipitated in MeOH, filtered, and purified by column chromatography on silica gel (eluent: 2:1 hexane/ethyl acetate). The product was precipitated in MeOH from a minimum amount of CH2Cl2, filtered, and dried under vacuum, affording 6 as a white solid (1.67 g, 79%). Purity (HPLC): 99.9%. 1H NMR (CDCl3) δ, ppm: 7.39 (s, 2H), 7.35 (d, 4H), 7.27 (d, 2H), 6.91 (d, 4H), 6.78 (d, 2H), 6.15 (s, 1H), 5.58 (s, 1H), 5.07 (s, 4H), 5.04 (s, 2H), 4.30 (m, 4H), 3.96 (m, 6H), 3.83 (m, 1H), 3.75−3.66 (m, 13H), 1.96 (s, 3H), 1.79 (m, 6H), 1.48 (m,

generate cylindrical supramolecular columns capable of selforganizing into columnar hexagonal arrays.64−68 It was not possible to determine by oriented fiber XRD whether the polymer backbone adopted a helical conformation or whether dendrons stacked in a nonhelical disklike conformation.67,68 Reversible anisotropic thermal expansion similar to that of dendronized PPAs was exhibited by oriented fibers of 12-4EOPMA.68 Here, a library containing 12-4EO-PMA and five related achiral and chiral dendronized polymethacrylates and polyacrylates is reported. A combination of X-ray diffraction, circular dichroism spectroscopy, electron density reconstruction, and molecular modeling provides definitive demonstration that the polymers adopt a helical conformation in solution and in bulk, induced by the dendrons jacketing the polymer backbone. Anisotropic thermal expansion of elongated fibers was observed in dendronized polymethacrylates and polyacrylates. The dependence of the thermoreversible actuation of these polymethacrylates on the primary structure of the monomer was investigated, revealing that the linker between the dendron and polymer backbone plays a crucial role in enabling the nanomechanical function.

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EXPERIMENTAL SECTION

Materials. Methacryloyl chloride (97%, Lancaster), acryloyl chloride (98%, Aldrich), methyl 2-bromopropionate (98%, Aldrich), and ethyl 2-bromoisobutyrate (98%, Aldrich) were used as received. K2CO3 (from Acros) was dried at 80 °C. Triethylamine (from Fisher) was distilled from CaH2. CH2Cl2 (from Fisher, ACS reagent) was dried over CaH2 and freshly distilled before use. THF (from Fisher, ACS reagent) was dried over sodium ketyl until the solution turned purple and freshly distilled before use. Methods. Assessment of Purity. The purity of the intermediary and final products was assessed by a combination of techniques that includes thin-layer chromatography (TLC), high pressure liquid chromatography (HPLC), 1H and 13C NMR, and matrix-assisted laser desorption−ionization time-of-flight (MALDI-TOF) mass spectrometry. TLC was carried out on precoated glass plates (silica gel with F254 indicator; layer thickness, 200 μm; particle size, 2−25 μm; pore size, 60 Å, from Sigma-Aldrich). HPLC was carried out using Shimadzu LC-20AD high-performance liquid chromatograph pump, a PE Nelson Analytical 900 Series integration data station, a Shimadzu RID-10A refractive index (RI) detector, and three AM gel columns (a guard column, 500 Å, 10 μm, and 104 Å, 10 μm). THF was used as solvent at an oven temperature of 40 °C. UV absorbance at 254 nm was used as detector. Relative weight-average (Mw) and numberaverage (Mn) molecular weights were determined with the same instrument in gel permeation chromatography (GPC) mode from a calibration plot constructed from polystyrene standards. 1H NMR (500 MHz) and 13C NMR (126 MHz) spectra were recorded on a Bruker DRX 500 instrument using the solvent indicated and TMS as internal standard. Matrix-Assisted Laser Desorption/Ionization Time-of-Flight (MALDI-TOF) Mass Spectrometry. MALDI-TOF mass spectrometry was performed on a PerSeptive Biosystems-Voyager-DE (Framingham, MA) mass spectrometer equipped with a nitrogen laser (337 μm) and operating in linear mode. Internal calibration was performed using Angiotensin II and Bombesin as standards. The analytical sample was obtained by mixing the THF solution of the sample (5−10 mg/ mL) and THF solution of the matrix (3,5-dimethoxy-4-hydroxy-transcinnamic acid or 4-hydroxybenzylidenemalononitrile, 10 mg/mL) in a 1:5 v/v ratio. The prepared solution of the sample and the matrix (0.5 μL) was loaded on the MALDI plate and allowed to dry at 23 °C before the plate was inserted into the vacuum chamber of the MALDI instrument. The laser steps and voltages applied were adjusted depending on both the molecular weight and the nature of each analyzed compound. B


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Macromolecules Scheme 1. Synthesis of First-Generation Dendritic Monomers and Dendronized Polymersa

Reagents and conditions: (i) Et3N, CH2Cl2, 0−23 °C, 2 h; (ii) Cu0, ligand, DMSO−THF or DMSO−cyclohexane or DMSO−DME, 23 °C, 21−24 h; full details in Table ST1.

a

6H), 1.29 (m, 51H), 0.91 (t, 9H). 13C NMR (CDCl3) δ, ppm: 166.0, 159.0, 152.6, 142.7, 130.2, 129.4, 129.3, 128.6, 125.7, 125.0, 114.5, 114.1, 109.4, 74.7, 73.9, 71.1, 70.7, 70.6, 69.1, 68.8, 68.1, 68.0, 67.8, 63.9, 31.9, 29.7−29.3, 26.1, 22.7, 18.3, 17.3, 14.1. 12-4EO-Ac (9). (4-3,4,5)12G1-CO2EO4H (1)64 (4.00 g, 3.43 mmol), dry Et3N (2.08 g, 20.58 mmol), and dry CH2Cl2 (50 mL) were added in a round-bottom flask, which was sealed with a rubber septum and flushed with argon. The solution was cooled to 0 °C with an ice bath, and acryloyl chloride (1.24 g, 13.72 mmol) was added via syringe. The reaction mixture was then stirred at 23 °C until TLC (2:1 hexane/ethyl acetate) showed completion (2 h). The mixture was filtered, and the solvent was removed by rotary evaporation. The crude product was redissolved in CH2Cl2 and precipitated in MeOH, filtered, and purified by column chromatography on silica gel (eluent: 2:1 hexane/ethyl acetate). The product was precipitated in MeOH from a minimum amount of CH2Cl2, filtered, and dried under vacuum, affording 9 as a white solid (3.17 g, 76%). Purity (HPLC): 99.9%. 1H NMR (CDCl3) δ, ppm: 7.37 (s, 2H), 7.34 (d, 4H), 7.24 (d, 2H), 6.89 (d, 4H), 6.75 (d, 2H), 6.42 (d, 1H), 6.13 (dd, 1H), 5.80 (d, 1H), 5.03 (s, 2H), 5.00 (s, 4H), 4.45 (t, 2H), 4.29 (t, 2H), 3.94 (m, 6H), 3.82 (t, 2H), 3.68 (m, 10H), 1.77 (m, 6H), 1.45 (m, 6H), 1.26 (m, 48H), 0.88 (t, 9H). 13C NMR (CDCl3) δ, ppm: 166.2, 159.1, 152.6, 142.7, 130.9, 130.2, 129.5, 129.3, 128.6, 128.3, 124.9, 114.5, 114.1, 109.4, 74.7, 71.2, 70.7−70.6, 69.3, 69.1, 68.1, 68.0, 64.1, 63.7, 31.9, 29.7−29.3, 26.1, 22.7, 14.1. 12-4EO*-Ac (10). (S)-(4-3,4,5)12G1-CO2EO*-(EO)3H (2)73 (2.00 g, 1.69 mmol), dry Et3N (1.03 g, 10.14 mmol), and dry CH2Cl2 (30 mL) were added to a round-bottomed flask, which was sealed with a rubber septum and flushed with argon. The solution was cooled to 0 °C with an ice bath, and acryloyl chloride (0.61 g, 6.76 mmol) was added via syringe. The reaction mixture was then stirred at 23 °C until TLC (2:1 hexane/ethyl acetate) showed completion (2 h). The

mixture was filtered, and the solvent was removed by rotary evaporation. The crude product was redissolved in CH2Cl2 and precipitated in MeOH, filtered, and purified by column chromatography on silica gel (eluent: 2:1 hexane/ethyl acetate). The product was precipitated in MeOH from minimum amount of CH2Cl2, filtered, and dried under vacuum, affording 10 as a white solid (1.70 g, 81%). Purity (HPLC): 99.9%. 1H NMR (CDCl3) δ, ppm: 7.36 (s, 2H), 7.32 (d, 4H), 7.24 (d, 2H), 6.88 (d, 4H), 6.75 (d, 2H), 6.40 (d, 1H), 6.13 (dd, 1H), 5.80 (d, 1H), 5.04 (s, 4H), 5.00 (s, 2H), 4.28 (m, 4H), 3.93 (m, 6H), 3.81 (m, 1H), 3.71−3.63 (m, 10H), 1.78 (m, 6H), 1.44 (m, 6H), 1.26 (m, 51H), 0.88 (t, 9H). 13C NMR (CDCl3) δ, ppm: 166.0, 159.1, 152.6, 142.7, 131.0, 130.3, 129.4, 129.3, 128.6, 128.3, 124.9, 114.5, 114.1, 109.4, 74.7, 73.9, 71.1, 70.9, 70.6 × 2, 69.1, 68.8, 68.1, 68.0, 67.8, 63.7, 31.9, 29.7−29.3, 26.1, 22.7, 17.3, 14.1. Cu0-Catalyzed SET-LRP of Dendritic Monomers. Polymerizations of dendritic monomers 5−10 to synthesize dendronized polymers 11−16 were carried out using the following general procedure for the polymerization of 12-4EO-MA (5) to prepare 124EO-PMA (11). Experimental parameters and characterization by GPC are provided in Table ST1. 12-4EO-PMA (11). Monomer 12-4EO-MA (5)64 (0.20 g, 0.16 mmol), ethyl-2-bromoisobutyrate (1.6 mg, 0.0082 mmol), Cu0 (0.5 mg, 0.0079 mmol), bipyridine (3.7 mg, 0.024 mmol), freshly distilled THF (0.5 mL), and DMSO (0.3 mL) were placed in a dried Schlenk tube. The reaction mixture was degassed by freeze−pump−thaw cycles, filled with argon, and stirred at 25 °C for 18−24 h. The final polymerization mixture was diluted with THF and passed through SiO2 to remove copper species. Traces of unreacted monomer were removed by passing a petroleum ether solution of the crude polymer through neutral alumina. The monomer-free polymer was redissolved in a minimum volume of THF and precipitated in a 10-fold amount of MeOH affording 11 as a white powder (0.13 g, 65%). C


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Figure 1. DSC traces of (a) 12-4EO-PMA, (b) 12-4EO-PAc, and (c) 12-4EO*-X, where X = PMA and PAc, recorded with heating and cooling rates of 10 °C/min. Phases determined by XRD, heating cycle, annealing temperature and periods, transition temperatures (in °C), and associated enthalpy changes (in parentheses in kcal/mol) are indicated.

Figure 2. Idealized schematic representations of four previously proposed67 models for the supramolecular columns of 12-4EO-PMA: (a) achiral nonhelical 80 column of stacked disks; (b) chiral 81-helical column; (c) achiral 84-helical column with a single polymer chain; (d) 84-helical column with achiral arrangement of dendrons and a chiral backbone multiplex of four polymer chains. In each schematic representation (bottom), the polymer backbone is shown with and without its jacketing dendrons.

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CO2EO4*-MA] (12, 12-4EO*-PMA); poly[(4-3,4,5)12G1CH 2 O-MA] (13, 12-CH 2 O-PMA), poly[(4-3,4,5)16G1CO2 EO 4-MA] (14, 16-4EO-PMA); poly[(4-3,4,5)12G1CO2EO4-Ac] (15, 12-4EO-PAc); and poly[(4-3,4,5)12G1CO2EO*4-Ac] (16, 12-4EO*-PAc). Thermal Analysis by DSC. Differential scanning calorimetry (DSC) experiments have shown that 12-4EO-PMA undergoes a phase transition at approximately 46 °C.64 Oriented fiber X-ray diffraction analysis identified that 124EO-PMA forms a 2D columnar hexagonal phase with intracolumnar order (Φhio) at room temperature and generates upon heating a liquid crystalline 2D columnar hexagonal without intracolumnar order (Φh) phase.67,68 DSC scans of 124EO-PAc, 12-4EO*-PMA, and 12-4EO*-PAc (Figure 1) show a similar endotherm upon first heating at 10 °C/min at 45, 28, and 35 °C, respectively, corresponding to a phase transition between the Φhio and Φh phases, as determined by X-ray diffraction (XRD) to be discussed later. These first heating scans were recorded for samples which had been stored at 23 °C (ambient temperature) for several days, and therefore they can be considered as having been annealed. In polymethacrylates 12-4EO-PMA and 12-4EO*-PMA, this phase transition is not observed or weakly observed upon subsequent cooling and

RESULTS AND DISCUSSION Synthesis of Dendronized Polymethacrylates and Polyacrylates. A library of six dendritic monomers was synthesized from first-generation minidendrons previously reported by our laboratory (Scheme 1). Dendritic alcohols (4-3,4,5)12G1-CO 2 EO 4 H (1), 6 4 (S)-(4-3,4,5)12G1CO2EO4*H (2),73 (4-3,4,5)12G1-CH2OH (3),74 and (43,4,5)16G1-CO2EO4H (4)64 were reacted with methacryloyl chloride in the presence of triethylamine in CH2Cl2 to give dendritic methacrylates (4-3,4,5)12G1-CO2EO4-MA (5), (S)(4-3,4,5)12G1-CO2EO4*-MA (6), (4-3,4,5)12G1-CH2O-MA (7), and (4-3,4,5)16G1-CO2EO4-MA (8). Dendritic alcohols 1 and 2 were also reacted with acryloyl chloride under the same reaction conditions to give dendritic acrylates (4-3,4,5)12G1CO2EO4-Ac (9) and (4-3,4,5)12G1-CO2EO4*-Ac (10) in 76% and 81% yield, respectively. The six dendritic monomers (5− 10) were polymerized via single-electron transfer living radical polymerization75−78 using Cu(0) and Me6-TREN in DMSO (Table ST1), with methyl 2-bromopropionate as initiator for dendritic methacrylates and ethyl 2-bromoisobutyrate as initiator for dendritic acrylates, to give six dendronized polymers: poly[(4-3,4,5)12G1-CO2EO4-MA] (11), hereafter abbreviated as 12-4EO-PMA; poly[(S)-(4-3,4,5)12G1D


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Macromolecules

Figure 3. Wide-angle XRD patterns of oriented fibers of (a) 12-4EO-PMA and (b) 12-4EO-PAc measured at (left to right) 26, 60, and 30 °C. In all patterns e represents diffuse equatorial features, and h1 and h2 represent helical features. (c, d) Azimuthal angle (χ) plots integrating the regions containing (c) diffuse equatorial features e and helical features h1 and (d) helical features h2.

reheating (Figure 1a,c). Annealing 12-4EO-PMA at 20 °C for 240 min followed by cooling to −40 °C and reheating regenerates the Φhio-to-Φh peak (Figure 1a), while annealing 12-4EO*-PMA at 20 °C for 120 min followed by cooling and reheating increases the enthalpy of the Φhio-to-Φh peak by a factor of 8 (Figure 1c). In contrast, the transition between the Φhio and Φh phases is readily observed in the DSC traces of polyacrylates 12-4EO-PAc and 12-4EO*-PAc upon first cooling and second heating (Figure 1b,c), and annealing 12-4EO-PAc for 120 min at 30 °C increases the enthalpy of the Φhio-to-Φh transition by only 24% (Figure 1b). The presence of a peak corresponding to the Φhio-to-Φh transition for the polyacrylates at a heating and cooling rate of 10 °C/min, and the substantial increase in the enthalpy of that transition upon annealing the polymethacrylates, indicates that the dynamics of the Φhio-toΦh phase transition are much quicker in the polyacrylates than in the polymethacrylates. This Φhio-to-Φh phase transition denotes the onset of macroscopic expansion, to be discussed later.

Structural and Retrostructural Analysis by XRD. X-ray diffraction (XRD) of oriented fibers of 12-4EO-PMA had previously elucidated that supramolecular columns of 12-4EOPMA comprise repeating units containing eight monomeric units of the polymer backbone, stacked at a distance of 4.9 Å.67 Furthermore, off-meridional features suggested that the aromatic dendrons were tilted by ∼50° with respect to the column axis.67 At that time, three models consistent with these observations were proposed for the structure of the supramolecular column (Figure 2): a nonhelical 80 column, an 81helical column, and an 84-helical column which can be constructed from one or four polymer chains and therefore provides two distinct models for self-assembly. The nonhelical 80 column (Figure 2a) is generated by stacking disks containing eight dendrons atop each other with no rotation between adjacent disks. The dendrons are connected by the polymer chain within a single disk, and the chain then “drops down” to an adjacent disk and thus forms the backbone of the column. Adjacent dendrons in the polymer backbone can adopt a conformation in which they are separated by ∼5 Å without E


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forms the basis for a nanomechanical function which will be described later. Diffuse equatorial and helical features in the XRD diffraction patterns of chiral 12-4EO*-PMA and 12-4EO*-PAc at 25 °C (Figure 4) indicate that a similar ordered Φhio phase is

disrupting the bond angles and bond lengths of the tetrahedral polymethacrylate carbons (Figure S1). This distance is consistent with the drop-down distance required between stacked disks of 12-4EO-PMA.67 The lack of rotation between disks means that the backbone and dendrons of this column are positioned in an achiral arrangement (Figure 2a). In contrast, the 81-helical column (Figure 2b) comprises a helix formed from dendrons rotated by 45° and translated along the column axis by a small distance (one-eighth of the helical pitch) with respect to each other. The 81-helical backbone adopts a chiral conformation, and the jacketing dendrons adopt a chiral arrangement. These 81-helices are chiral and may exist in leftor right-handed forms (Figure 2b, bottom) or as a racemic mixture of left- and right-handed helical columns. The 84-helical column presents an achiral arrangement of dendrons (Figure 2c,d).79 These dendrons can be connected by polymer chains in two ways: either all dendrons are attached to a single backbone (Figure 2c), or the dendrons are attached to four separate, intertwined backbones (Figure 2d). In the former case, the backbone conformation is similar to that in the nonhelical 80-column, in which the polymer chain connects eight dendrons within one layer and then “drops down” to the next layer (Figure 2c, bottom). In this case, the polymer backbone and the arrangement of the dendrons are achiral. Alternatively, the 84-column can be generated from four intertwined helical polymer chains (Figure 2d). The resultant multiplex79 of polymer chains has a helical sense and can be left- or right-handed. Therefore, a multiplex of four polymer chains provides a chiral backbone but maintains the achiral arrangement of dendrons. These four models (80-column with achiral backbone and achiral dendrons; 81-helix with chiral backbone and chiral dendrons; single chain 84-column with achiral backbone and achiral dendrons; and multiplexed 84-helix with chiral backbone but achiral dendrons) could not previously be discriminated.67,68 Diffraction patterns recorded upon first heating of oriented fibers of 12-4EO-PMA and 12-4EO-PAc demonstrate the presence of a more highly intracolumnar ordered Φhio phase followed by a transition to a lower order Φh phase with no intracolumnar order (Figure 3, left and center). Equatorial diffuse features (denoted e in Figure 3a−c) and off-meridional features on the first layer line (denoted h1 in Figure 3) in the XRD patterns of the Φhio phase indicate the presence of high intracolumnar order in the Φhio phase compared to no intracolumnar order in the Φh phase. The helical features h2 represent tilting of the aromatic portion of the dendrons with respect to the column axis of approximately 37°. This is in good agreement with previous reports of a 40°−50° tilt in 12-4EOPMA.68 Upon first cooling, 12-4EO-PAc regenerates the Φhio phase (Figure 3b), whereas the diffraction pattern of 12-4EOPMA has weak equatorial diffuse features from the Φhio phase and instead is more similar to the Φh phase (Figure 3a). This suggests coexistence of the Φhio and Φh phases in the cooled sample of 12-4EO-PMA at 30 °C and supports the observation by DSC that the dynamics of the Φhio-to-Φh phase transition are much quicker in 12-4EO-PAc than in 12-4EO-PMA. The diameter of the supramolecular columns generated from 12 to 4EO-PMA and 12-4EO-PAc are almost identical, with the polyacrylate diameter slightly smaller: 60.7 vs 59.6 Å, respectively, in the Φhio phase at 30 °C, and 55.4 vs 54.4 Å in the Φh phase at 90 °C. The substantial decrease (8.7%) in the column diameters of both polymers at elevated temperature

Figure 4. Wide-angle XRD patterns of oriented fibers of (a) 12-4EO*PMA and 12-4EO*-PAc measured at 25 °C. (b, c) Azimuthal angle (χ) plots integrating the regions containing (b) diffuse equatorial features e and helical features h1 and (c) helical features h2.

generated from these chiral analogues as from achiral 12-4EOPMA and 12-4EO-PAc. The diameters of the supramolecular columns in the Φhio phase of chiral 12-4EO*-PMA and 124EO*-PAc (Dcol = 62.0 and 60.0 Å, respectively) are slightly larger than those of achiral 12-4EO-PMA and 12-4EO-PAc (Dcol = 60.7 and 59.6 Å) and may arise from less compact packing of dendrons due to the methyl group attached to the stereocenter in the ethylene glycol linker. Upon heating into the Φh phase, the column diameter of the chiral polymers decreases to 58.1 Å. This decrease in Dcol is substantially less than that exhibited by the achiral polymers. XRD data and structural analysis of the polymers discussed here are summarized in Table 1. The number of dendritic monomer units per stratum of the column, μ, can be determined using μ = (NAρAt)(Mwt)−1, where NA = 6.022 × 1023 mol−1, ρ is the experimental density of the polymer, A is the unit cell area of the ab-plane, t is the average stratum thickness calculated from the meridional pattern, and Mwt is the molecular weight of one dendritic F


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Macromolecules Table 1. Powder XRD Data and Structural Analysis of Dendronized Polymers compound 12-4EO-PMA 12-4EO-PAc 12-4EO*-PMA 12-4EO*-PAc 12-CH2O-PMA 16-4EO-PMA

T (°C)

phasea

a (Å)b

30 90 28 90 20 60 30 50 20 90 35 70

Φh Φh Φhio Φh Φhio Φh Φhio Φh Φh Φh Φhio Φh

60.7 55.3 59.6 54.4 62.0 58.1 60.0 58.1 43.6 44.2 66.1 62.9

io

d10, d11, d20, d21 (Å)c 52.6, 48.0, 51.7, 47.2, 53.8, 50.4, 52.1, 50.4, 37.7, 38.0, 56.9, 54.7,

30.2, 26.4, 19.9 27.6, 24.0, 18.1 29.8,25.9, 19.4 27.2, 23.6, 17.9 30.8, 26.9, 20.2 28.9, 25.2, 19.0 29.8, 26.1, 19.7 28.9, 25.2, 19.0 21.8, 18.9, − 22.1, 19.1, − 32.9, 28.9, 21.6 31.2, 27.4, 20.7

A10, A11, A20, A21 (au)d 51.6, 61.5, 49.8, 63.1, 52.4, 57.9, 50.7, 57.6, 86.4, 85.6, 67.1, 66.9,

23.5, 18.2. 6.6 17.4, 15.7, 5.4 24.8, 17.9, 7.4 18.1, 15.1, 3.7 24.4, 18.7, 4.5 20.9, 16.8, 4.4 26.4, 19.6, 3.4 21.6, 18.6, 2.1 6.6, 6.9, − 6.8, 7.6, − 18.1, 13.5, 1.3 15.2, 16.3, 1.6

Phase notation: Φhio = 2D columnar hexagonal phase with intracolumnar order; Φh = 2D liquid crystalline columnar hexagonal phase without intracolumnar order. bLattice parameter a calculated using a = [(2/√3)d10 + 2d11 + (4/√3)d20]/3. cExperimental d-spacings. dScaled amplitude of the diffraction peaks calculated from the diffraction peak area after applying appropriate Lorentzian and multiplicity corrections.

a

Figure 5. Spectra of spin-coated thin films of chiral dendronized polymers measured by (top) CD and (bottom) UV spectroscopy. (a, b) Variable temperature CD and UV spectra of (a) 12-4EO*-PMA cast from hexane (3.2% w/v) and (b) 12-4EO*-PAc cast from hexane (1.1% w/v). (c) Comparison of CD and UV spectra of 12-4EO*-PMA (red) and 12-4EO*-PAc (blue) cast from hexane (3.2% w/v and 1.1% w/v, respectively) at 0 °C. (d) Comparison of CD and UV spectra of 12-4EO*-PMA cast from hexane (3.2% w/v) immediately after spin-coating (green) and after annealing at 23 °C for 2 days (purple) and 5 days (red).

monomer unit. Taking experimental values of ρ, t, and Mwt for 12-4EO-PMA (1.03 g/cm3, 4.9 Å, and 1238.8 g/mol, respectively) indicates that there are 7.8, i.e., ∼8, dendrons per column stratum. Four alternate models have been proposed for the stacking of these dendrons (Figure 2): a nonhelical 80 column composed

of stacked disks (Figure 2a), an 81-helical column (Figure 2b), and an 84-helical column with either a single achiral polymer chain (Figure 2c) or a chiral multiplex of polymer chains (Figure 2d).67,68 Discriminating between a helical and nonhelical model for the supramolecular structure of 12-4EO-PMA by XRD was not possible previously due to the broadness of G


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Macromolecules the diffraction peaks on the first layer line and the consequent inability to differentiate between meridional and near-axis off meridional diffraction features.67 An absence of meridional features supports a helical model of self-assembly. As for previous work on 12-4EO-PMA, it is not clear in the XRD patterns of 12-4EO-PMA (Figure 3a), 12-4EO-PAc (Figure 3b), and 12-4EO*-PMA (Figure 4a) whether there is a meridional diffraction feature on the first layer line. However, the absence of a meridional diffraction feature on the first layer line can be discerned in the XRD pattern of 12-4EO*-PAc (Figure 4a,b). Hence, XRD indicates that 12-4EO*-PAc selfassembles into a helical structure rather than a nonhelical stack of supramolecular disks. This can likely be extended to 12-4EOPMA, 12-4EO-PAc, and 12-4EO*-PMA. The presence of helical features (h1) in the diffraction patterns of the Φhio phase of 12-4EO-PMA, 12-4EO-PAc, and their chiral analogues further supports a helical model of self-assembly. However, discrimination by XRD between the 81- and 84-helical models is not possible with our data. Demonstration of Helicity in the Columns of 124EO*-PMA and 12-4EO*-PAc by CD. Elimination of the nonhelical model of self-assembly (Figure 2a) results in two competing helical models: an 81-helix with a chiral arrangement of dendrons and a chiral helical backbone (Figure 2b) and an 84-helix with a chiral multiplex of four helical polymer chains with an achiral arrangement of dendrons (Figure 2d). Hence, circular dichroism spectroscopy (CD), which is commonly used to investigate the formation of supramolecular helices in solution and thin film, was applied to discriminate between these two alternative helical models. Of these two competing models, the dendrons jacketing the polymer backbone adopt a chiral arrangement only in the 81-helical column (Figure 2). Therefore, the observation of CD signals arising from the aromatic dendrons is indicative of an 81-helical column. Introduction of a stereocenter into the ethylene glycol linker of a series of dendronized poly(phenylacetylene)s induced a CD signal arising both from the chromophoric polymer backbone and the dendrons.32,40,44 This demonstrated that the helical sense of both the polymer backbone and of the dendrons jacketing the backbone could be selected by the stereocenter on the ethylene glycol linker. In the current polymethacrylates and polyacrylates, a chiral stereocenter has also been incorporated in the ethylene glycol linker in 124EO*-PMA and 12-4EO*-PAc. However, the lack of a chromophore in the polymer backbone means that helicity can only be monitored in the appended dendrons. Therefore, CD spectroscopy can discriminate between an 81-helix, in which both the dendrons and polymer backbone adopt a chiral arrangement and hence a CD signal should be observed, and a multiplexed 84-helix, in which the dendrons adopt an achiral arrangement and only the nonchromophoric polymer backbone adopts a chiral conformation; hence, no CD signal is expected. CD and UV spectra of thin films of 12-4EO*-PMA and 124EO*-PAc at 0 °C demonstrate that these polymers form chiral helices (Figure 5a). The peaks in the CD and UV spectra, at 235, 275, and 310 nm, arise from the self-assembling dendrons attached to the polymer backbone and are identical in both 124EO*-PMA and 12-4EO*-PAc. The wavelengths of these features correlate well with other examples of similar selfassembling dendrons in which chiral induction occurs.80 Heating films of 12-4EO*-PMA and 12-4EO*-PAc from the Φhio phase at 0 °C to 55 and 50 °C in the Φh phase, respectively, eliminates the induced CD signal (Figure 5b,c).

The intensity of the CD signal remains constant on heating from 0 to ∼35 °C, at which point there is a sharp reduction in CD intensity, corresponding to the temperature of the Φhio-toΦh phase transition (Figure 1c). This suggests that the helical arrangement of the dendrons and helical conformation of the polymer backbone are disrupted during heating and are less well-correlated in the Φh phase without intracolumnar order. Furthermore, CD provides further support that the dynamics of the formation of the Φhio phase are slower for 12-4EO*-PMA than for 12-4EO*-PAc. No CD signal is observed for a freshly cast film of 12-4EO*-PMA and instead evolves during annealing at 23 °C for 2 days (Figure 5d). Longer annealing times do not provide any further enhancement of the CD signal. Thin films and solutions of the monomeric dendron (S)-(43,4,5)12G1-CO2EO*-(EO)3H (2) also exhibit CD signals demonstrating that the dendrons adopt a chiral helical arrangement (Figure 6). The similarity of the CD patterns

Figure 6. Temperature dependence of the (top) CD and (bottom) UV spectra of chiral dendron (S)-(4-3,4,5)12G1-CO2EO*-(EO)3H (2) (a) as a spin-coated film cast from CHCl3 (1.3% w/v) and (b) in trifluoroethanol solution (2.5 × 10−4 M).

obtained from chiral dendron 2, 12-4EO*-PAc, and 12-4EO*PMA implies that the tacticity of 12-4EO*-PMA has negligible impact on its conformation. The helical arrangement of dendrons, both attached to (Figure 5) and independent of (Figure 6) a polymer backbone, demonstrates that the PMA and PAc polymers adopt a helical conformation, which is induced by the self-assembly of the jacketing dendrons. Hence, CD has refuted the multiplexed 84-helix with an achiral arrangement of dendrons and shown that 12-4EO*-PMA, 124EO*-PAc, and, by extension, their achiral analogues selfassemble into a chiral 81-helix. H


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Figure 7. Comparison of (a) wide-angle XRD pattern collected from an oriented fiber of 12-4EO*-PMA in the Φhio phase at 26 °C with (b) XRD pattern simulated from atomistic model and (c) XRD pattern simulated from molecular model. Broken blue ellipses in (a) denote tilt features. (d) Atomistic model used for simulation in (b). (e) Comparison of modeled column stratum with tilt angle of (left) 0° and (right) 37° according to an 81-helical model (Figure 2b). (f) Molecular models of polymer backbone in PMA and PAc showing helical conformation. (g) Molecular model used for simulation in (c).

Molecular Modeling and Simulation of XRD. The combination of XRD and CD has discriminated between the four previously proposed67,68 models (Figure 2) for the selforganization of 12-4EO-PMA, 12-4EO-PAc, and their chiral analogues. The experimental XRD pattern of 12-4EO*-PMA in the Φhio phase (Figure 7a) shows tilt features displaced by 37° from the meridian (Figure 7a, broken blue ellipses). Simulation (Figure 7b) of XRD patterns from atomistic models applicable to all four polymers with tilt angles of 0° and 37° (Figure 7d) reproduces the experimental pattern only when the dendrons are tilted down toward the column axis (compare Figure 7b, left and right). Further evidence for the tilted dendrons in 12-4EOPMA, 12-4EO-PAc, and their chiral analogues is provided from molecular models (Figure 7f). The experimental column diameter of 12-4EO*-PMA (Dcol,XRD ≈ 60 Å) is consistent with the column diameter of a modeled column with 37° tilt (Dcol,model ≈ 60 Å; Figure 7e). Dendrons with 0° tilt would generate substantially larger columns (Dcol,model ≈ 70 Å). Dendronized polymers were modeled by dendrons tilted by 37° appended to a helical polymer backbone, such that the dendrons in the ith and (i + 8)th position are identical (Figure 7f). The XRD pattern simulated for this full molecular model (Figure 7g) compares well with the experimental XRD patterns of all four polymers, and hence the proposed model provides a good approximation of the supramolecular structure of these dendronized polymers. Simulations of the XRD expected from both atomistic and molecular models (Figures 7b and 7c, respectively) demonstrate the importance of this tilt angle in accurately reproducing the experimental XRD pattern. The similarity of the XRD patterns obtained from achiral 124EO-PMA and 12-4EO-PMA compared to their chiral analogues 12-4EO*-PMA and 12-4EO*-PAc suggests that all four polymers assemble to give approximately identical

structures. Coupled with CD experiments on the chiral dendronized polymers, this indicates that all four polymers assemble into near-identical helical structures. Nolte and coworkers have previously suggested that helicity in macromolecules arises from the incorporation of a stereocenter into an otherwise nonhelical molecule.81 However, the achiral and chiral dendronized polymers reported here instead demonstrate that the achiral polymers can also be helical and that the stereocenter simply selects the handedness of the already helical structure. This model invoking helix-sense selection of an already helical structure has been observed previously in dendronized covalent28,31,44,46 and supramolecular45,82−85 polymers. Correlating Concerted Molecular Motion with Anisotropic Macroscopic Expansion. A series of dendronized poly(phenylacetylene)s, in which a decrease in column diameter on the microscopic scale was accompanied by an increase in the length of a macroscopic extruded fiber, were utilized to generate a nanomechanical functional material.45,46 The anisotropic thermal expansion of a fiber of dendronized poly(phenylacetylene) was exploited to lift an object with a mass 250 times greater than the fiber. The mechanism of this expansion was proposed to require unwinding of the helical polymer backbone via cisoidal-to-transoidal isomerization upon heating. Monitoring fibers of 12-4EO-PMA and 12-4EO-PAc by transmission optical microscopy during heating revealed reversible68 anisotropic thermal expansion (Figure 8), similar to that observed in poly(phenylacetylene)s.45 The length of a fiber of 12-4EO-PMA increased by 28% upon heating from 25 to 95 °C (Figure 8a), while the length of a fiber of 12-4EO-PAc increased by 21% upon heating from 25 to 90 °C (Figure 8b). Even at high temperature, the fiber maintains its form due to I


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Figure 8. Macroscopic thermal expansion of an oriented fiber of (top) 12-4EO-PMA and (bottom) 12-4EO-PAc. Temperature and length, L, relative to length at 25 °C are indicated.

the high viscosity of its nonfluid liquid crystalline Φh phase.45 To demonstrate that the increase in the macroscopic length of the fiber is correlated to the molecular motion of individual polymer chains, the length of a column defined by an individual molecule (Lcol) can be compared with the length of the macroscopic fiber (Lfiber). The parameter Lcol can be defined in terms of the volume and diameter of the microscopic column, Vcol and Dcol, such that Lcol = 4Vcol/(πDcol2) and hence Lcol ∝ Dcol−2. If the macroscopic expansion of the column results from a change in the conformation of individual polymer chains, a linear correlation between Lfiber and Dcol−2 is expected. The expansion of the fiber length, Lfiber, and column diameter, Dcol−2, for 12-4EO-PMA and 12-4EO-PAc is well correlated as a function of temperature, as predicted (Figure 9). Hence, the macroscopic extension of the fiber must be a consequence of the elongation of individual polymer chains with concomitant decrease of the column diameter. This molecular motion likely involves unwinding of the helical polymer backbone with concerted movement of the dendrons along the column axis (to increase the fiber length) and toward the center of the column (to reduce the column diameter). This inference agrees with CD studies which showed that the helicity of the dendrons and polymer backbone is disrupted upon heating at ∼35 °C for 12-4EO*-PMA and 12-4EO*-PAc (Figure 5a,b). The onset of macroscopic expansion (Figure 9) correlates well with the disruption of helicity observed by CD (Figure 5a,b) and the phase transition between the Φhio and Φh phases observed by DSC (Figure 1c). Tuning Macroscopic Function via Changes to Molecular Structure. Changes to the molecular structure of dendronized poly(phenylacetylene)s were manifested as changes in the nanomechanical function of these polymers, indicating allosteric regulation between the primary structure of the dendron and the thermal behavior of the polymer backbone.45 To elucidate whether movement of the dendrons closer to the polymer backbone requires a flexible linker between the dendron and polymer backbone, a dendronized polymer without a flexible linker, 12-CH2O-PMA (13, Scheme 1) obtained from monomer 12-CH2O-MA (7),74 was studied by XRD and optical microscopy. 12-CH2O-PMA has a rigid

Figure 9. Comparison of the thermal expansion of the fiber length (macroscopic scale) and column diameter (microscopic scale) along the axis of supramolecular columns self-assembled from (a) 12-4EOPMA and (b) 12-4EO-PAc.

CH2O spacer between the dendron and polymer backbone rather than the ethylene glycol linker present in 12-4EO-PMA and 12-4EO-PAc. XRD of oriented fibers of 12-CH2O-PMA shows formation of only a Φh phase without intracolumnar order, with column diameters of 43.6 and 44.0 Å at 20 and 90 °C, respectively (Table 1). Formation of the more highly intracolumnar ordered Φhio phase is disfavored due to the lack of a flexible spacer to offset the steric crowding between dendrons caused by the short length of the repeating unit along the polymer backbone (two carbon atoms). No significant change in the length of a macroscopic fiber of 12-CH2O-PMA was observed by optical microscopy (Figure 10 and Figure S2). Hence, the rigid CH2O linker prevents the dendrons from undergoing the conformational change necessary to exhibit macroscopic expansion by eliminating the more highly ordered Φhio phase. The first-order transition between the Φhio and Φh phases was previously used as a diagnostic for nanomechanical function in dendronized poly(phenylacetylene)s.45 In that work, the phenyl group of the monomer provided the necessary relief of unfavorable steric crowding at the backbone.45 This provides further evidence that the macroscopic thermal J


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Figure 10. Comparison of the thermal expansion of the fiber length (macroscopic scale) along the axis of supramolecular columns selfassembled from 12 to 4EO-PMA (blue squares), 16-4EO-PMA (green triangles), and 12-CH2O-PMA (red circles).

expansion is directly correlated to the motion of individual polymer chains. Previous work on dendronized poly(phenylacetylene)s showed that increasing the length of the alkyl chain at the periphery of the dendron increased the extension of the macroscopic fiber.39,45 XRD of oriented fibers of 16-4EO-PMA (13, Scheme 1) reveals a low-temperature Φhio phase (Dcol,35 °C = 66.1 Å) and a high-temperature Φh phase (Dcol,70 °C = 62.9 Å). Increasing the length of the peripheral aliphatic chains of m4EO-PMA from m = 12 to 16 facilitates greater thermal expansion (Figure 10 and Figure S3), as observed for poly(phenylacetylene)s.45 Reconstruction of Electron Density Maps. There are two modes via which dendrons could move to reduce the diameter of the supramolecular column: increased tilting of dendrons with no lateral movement or movement of dendrons with a fixed tilt closer to each other in the plane perpendicular to the column axis. Electron density maps were reconstructed from experimental XRD data of 12-4EO-PMA in the Φhio phase at 25 °C and in the Φh phase at 95 °C (Figure 11). The relatively high electron density of the aromatic dendrons defines the supramolecular column, the center of which contains the relatively low electron density polymethacrylate backbone. Comparing the relative electron density profiles at 25 and 95 °C (Figure 11c) shows an increased electron density at the center of the columns at 95 °C. Furthermore, the decrease in the distance between the peaks of the electron density profile (4.6 Å) almost fully accounts for the decrease in Dcol (5.3 Å), implying that the decrease in the column diameter arises almost entirely due to movement of the aromatic dendrons, rather than a change in their tilt relative to the column axis.

Figure 11. Reconstructed relative electron density maps for (a, b) 124EO-PMA at (a) 25 °C and (b) 95 °C and (c) 12-CH2O-PMA at 25 and 110 °C. (d) Cross-sectional plots of relative electron density maps in (a) and (b) at y = 0.

via the transfer of chirality from the chiral linker to the aromatic dendrons jacketing the polymer backbone. A combination of CD spectroscopy with powder and oriented fiber XRD, molecular modeling, and reconstructed electron density maps elucidated that the these polymers assemble into an 81-helix in which the polymethacrylate and polyacrylate backbones are chiral and helical, thus answering a question about the supramolecular structure of these polymers first raised over 20 years ago.67 Dendronized polymers with flexible ethylene glycol linkers between the dendron and polymer backbone exhibit two 2D columnar hexagonal phases: one with intracolumnar order (Φhio) and one without intracolumnar order (Φh). Transition between these phases is accompanied by an unwinding of the polymer backbone with concomitant movement of the dendrons toward the center of the supramolecular column and disruption to the helicity of the polymer backbone (Figure 12). DSC and XRD experiments showed that the dynamics of polymethacrylates is markedly slower than those of polyacrylates, most likely due to increased steric interactions in the polymer backbone. This conformational change was correlated with a macroscopic thermal expansion, manifested as elongation of an oriented fiber along the axis of the supramolecular columns of up to 37%. The flexible ethylene glycol linker was shown to be critical for this expansion (Figure 12). The polymethacrylates and polyacrylates reported here reveal the major role played by the primary structure of the monomer in obtaining thermally controlled nanomechanical function and are expected to facilitate the design of ever more powerful molecular actuators and functional materials.

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CONCLUSIONS The synthesis as well as structural and retrostructural analysis of a library of dendronized polymethacrylates and polyacrylates with flexible ethylene glycol linkers, and rigid CH2O linkers, is reported. Incorporation of a chiral ethylene glycol linker allowed the investigation of the supramolecular self-organization of these polymers by CD spectroscopy for the first time K


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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.7b01216. Experimental parameters for polymerization of 5−10, modeling of “drop-downs” in polymer backbone in 80and 84-columns, optical microscopy images of 12-CH2OPMA and 16-4EO-PMA, and additional CD spectra of 12-4EO*-PMA and 12-4EO*-PAc in solution and thin film (PDF)

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REFERENCES

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Figure 12. Schematic representation of the influence of dendritic primary structure on thermal expansion.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; tel +1-215-573-5527; fax +1215-573-7888 (V.P.). ORCID

Mihai Peterca: 0000-0002-7247-4008 Daniela A. Wilson: 0000-0002-8796-2274 Benjamin E. Partridge: 0000-0003-2359-1280 Virgil Percec: 0000-0001-5926-0489 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the National Science Foundation (DMR1066116 (V.P.) and DMR-1120901 (V.P. and P.A.H.)), the Humboldt Foundation (V.P.), and the P. Roy Vagelos Chair at Penn (V.P.) is gratefully acknowledged. B.E.P. thanks the Howard Hughes Medical Institute for an International Student Research Fellowship. L


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