Article Cite This: Macromolecules XXXX, XXX, XXX-XXX
pubs.acs.org/Macromolecules
Stereocomplexation of Helical Polycarbodiimides Synthesized from Achiral Monomers Bearing Isopropyl Pendants Dumindika A. Siriwardane,† Oleg Kulikov,† Yekaterina Rokhlenko,‡ Sahila Perananthan,† and Bruce M. Novak*,† †
Department of Chemistry and Biochemistry, University of Texas Dallas, Richardson, Texas 75080, United States Department of Chemical and Environmental Engineering, Yale University, 9f Hillhouse Avenue, New Haven, Connecticut 06511, United States
‡
S Supporting Information *
ABSTRACT: A high level of the permanent asymmetry was built into the poly(Nmethyl-N′-(2-isopropyl-6-methylphenyl)carbodiimide) system by introducing a bulky, substituted phenyl group which revealed a very interesting phenomenological behavior upon heating. This polymer undergoes P/M racemization upon thermal annealing, thus leading to the formation of a stereocomplexed structure. Predominantly P and M helices have been obtained through helix sense selective polymerization by using chiral BINOL-Ti(IV) diisopropoxide initiator with achiral N-methyl-N′-(2-isopropyl-6-methylphenyl)carbodiimide monomer. Upon thermal annealing, the specific optical rotation (SOR) of the single-handed polymer begins to decrease but never reaches zero. The SOR plateaus at a large value (−286° for M helices or +283° for P helices), and shortly thereafter the polymer forms a precipitate. The process that polymer undergoes is attributed to stereocomplexation between two complementary strands via racemization. Inspired by the phenomena analogous to classical leucine zippers with isobutyl termini (interlocking motifs), a unique polycarbodiimide scaffold bearing isopropyl pendant groups was designed to play a vital role in the aggregation process with a calculated energy barrier of around 19 ± 0.4 kcal/mol. To investigate the effect of regioregularity in isopropyl groups, a series of isomeric polymers bearing isopropyl segments at the ortho, meta, and para positions have been synthesized, and their self-assembly behavior has been studied by using AFM, SEM, p-XRD, and TEM analytical techniques. To take advantage of both isopropyl zipping motif and increased solubility in organic solvents imparted by octadecyl lateral chains, a new block copolymer, poly(N-methyl-N′-(2-isopropyl-6-methylphenyl)carbodiimide)-b-poly(N-phenyl-N′-octadecylcarbodiimide) (P-1,2), was designed. The first block, containing the substituted aryl functional group, contributes to the stereocomplexation phenomena, while the second block copolymer, composed of the octadecyl group, imparts solubility and morphological attributes. This unique polymeric scaffold exhibits interesting morphologies such as spherical particles, capsules, wrinkled surface patterns, and fiber-like motifs, which may be associated with supramolecular aggregation. Detailed stereocomplex formation studies will bestow new possibilities in diverse areas, including drug delivery applications, catalysis, and chiral separations.
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catalysts, and for chiral separation. The presence of “ridges-ingrooves” or “knobs-in-to-holes” model of the propagating chains in these macromolecules enhances the formation of the heterochiral associations which are energetically more favorable than respective homochiral species.14−16 The assemblies formed between two complementary strands are denoted as stereocomplexes.17−21 These kinds of complexes can give rise to intriguing properties which are not seen in the parent, enantiomerically pure strands.8,22,23 Stereocomplexation provides an unprecedented opportunity to impart some valuable biological, chemical, and physical properties in polymeric platforms which can be used in various applications such as
INTRODUCTION The helix is well-known motif which can be found in many biological macromolecules. At the molecular level, DNA, polypeptides, enzymes, and proteins acquire preferred handedness to confine into the specific helical structure, and further they form supramolecular assemblies through various hydrophobic and electrostatic interactions.1−3 Inspired by helical nature, these structural scaffolds regulate essential and specific functions in various biological systems. Synthetically, we are intrigued to create diverse helical polymer architectures because of their miscellaneous applications like molecular recognition,4−6 enantioseparation,7 optoelectronic properties,8 external stimuli responsive behavior,9 and formation of various tunable self-assemblies.10−13 The screw sense nature of polycarbodiimides offers a fascinating way to self-assemble into supramolecular constructs that can be used as drug carriers, as © XXXX American Chemical Society
Received: July 30, 2017 Revised: November 2, 2017
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DOI: 10.1021/acs.macromol.7b01633 Macromolecules XXXX, XXX, XXX−XXX
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profiles on powder samples. Tapping mode atomic force microscopy (TM-AFM) was done by using a Nanoscope IV Multimode Veeco instrument on silicon wafers (diameter, r = 2.5 cm, Wafer World). All imaging was performed at room temperature with silicon cantilever with a nominal constant of 42 N/m and 320 kHz OTESPA tip. Scanning electron microscopy imaging was done by using a Zeiss Supra 40 instrument at the UTD nano characterization facility. The samples were mounted on silicon wafers and coated with conductive Pd/Au film. The potential applied to the inspected sample was 10 kV. Transmission electron micrographs were acquired on a Tecnai Spirit electron microscope under 200 kV at UT Southwestern Medical School. The samples were mounted on the carbon-coated copper grid, and negative staining with 2% aqueous uranyl acetate was applied to the specimens. Small-angle X-ray scattering (SAXS) was performed in a vacuum with a pinhole-collimated instrument (Rigaku SMAX3000) using Cu Kα radiation (λ = 1.542 Å). The beam has a 1 mm diameter at the sample plane and the accessible range of scattering vectors from 0.02 to 0.22 Å−1. SAXS data were calibrated with a silver behenate standard. Polymer Synthesis. Synthesis of Poly(N-methyl-N′-(2-isopropyl6-methylphenyl)carbodiimide), P-1. The monomer:initiator ratio used here was 250:1, and (R)-, (S)-, and achiral (RAC)-polymers were synthesized by using (R)-, (S)-, and (RAC)-BINOL-Ti(IV) diisopropoxide initiators, respectively. All polymerizations have been carried out inside the glovebox. First, 2.000 g (250 equiv, 10.6 mmol) of N-methyl-N′-(2-isopropyl6-methylphenyl)carbodiimide monomer was placed in an oven-dried sample vial along with a stirring bar. Then, 19.1 mg (1 equiv, 42.5 μmol) of (R)-BINOL-Ti (IV) isopropoxide initiator was added into the vial along with 0.2 mL of anhydrous chloroform. It was stirred overnight until a very viscous sample of polymer was obtained. The same procedures and the same amounts were used to obtain (S)- and (RAC)-polymers. After 15 h, the polymers were dissolved in toluene and precipitated into methanol. This step was repeated three times to obtain pure fiber-like, off-white polymers. Then all polymers were dried under vacuum for 2 days. (R)-P-1, yield = 1.88 g (94%). 1H NMR (500 MHz, toluene-d8, δ ppm): 6.90,6−6.82 (b, Ar−H), 3.43−3.23 (b, N−CH3), 2.71, 2.62, 2.32 (N−CH3), 1.47 (b, methine H), 1.35 (b, Ar−CH3), 1.07, 0.81, 0.62 (b, isopropyl CH3). 13C NMR (500 MHz, toluene-d8, δ ppm): 146.20 (CN, imine carbon), 142.88, 141.97, 124.56, 123.20, 122.49 (Ar−CC), 34.14 (amine N−CH3), 27.89 (Ar−CH3), 21.33 (methine secondary C), 18.81 (isopropyl CH3); 25 °C[α]435 nm = +688° (toluene), C = 2 mg/mL, Mn = 22 776 Da. (S)-P-1, yield = 1.78 g (90%). 1H NMR (500 MHz, toluene-d8, δ ppm): 6.97−6.74 (b, Ar−H), 3.44−3.22 (b, N−CH3), 2.69, 2.63 (b, N−CH3), 1.47−1.33 (Ar−CH3), 1.12, 1.06 (b, methine H), 0.81(b, isopropyl CH3). 13C NMR (500 MHz, toluene-d8, δ ppm): 148.08− 143.05 (CN, imine carbon and Ar−CC overlapped), 127.17− 123.48 (Ar−CC), 57.03 (b, N−CH3) 34.58 (Ar−CH3), 28.24− 22.08 (b, methine secondary C), 19.05 (isopropyl CH3); 25 °C[α]435 nm = −714° (toluene), C = 2 mg/mL, Mn = 23 106 Da. (RAC)-P-1, yield = 1.73 g (87%). 1H NMR (500 MHz, toluene-d8, δ ppm): 6.89, 6.81 (b, Ar−H), 3.43, 3.32, 3.22 (b, N−CH3), 1.34 (Ar− CH3), 1.16 (C−H, methine H), 1.08, 0.81 (isopropyl CH3). 13C NMR solid state, δ ppm: 143.15 (b, CN), 126.45−123.24 (b, Ar−CC), 55.63 (b, N−CH3), 34.24 (b, Ar−CH3), 28.25−22.85 (b, methine C), 18.99 (b, isopropyl CH3), Mn = 18 123 Da. Synthesis of Poly(N-methyl-N′-(2-isopropyl-6-methylphenyl)carbodiimide)-b-poly(N-methyl-N′-octadecylcarbodiimide), P-1,2. Inside the glovebox, 0.250 g (40 equiv, 1.3 mmol) of N-methyl-N′(2-isopropyl-6-methylphenyl)carbodiimide and 14.9 mg of (S)BINOL-Ti(IV) diisopropoxide initiator were placed along with 3.0 mL of chloroform, and it was allowed to stir for 2 h. While stirring, every 30 min an aliquot of 20 μL was taken and mixed with 4.0 mL of ether. Then it was injected into GC-MS to determine the percentage of monomer consumption. After 2 h (90% of monomer consumed), the second monomer, N-phenyl-N′-octadecylcarbodiimide, M-2 (2.000 g, 162 equiv, 5.4 mmol), was added, and the mixture was allowed to stir overnight until gel-like product formed. The following
catalysis, drug delivery, and improving thermal and mechanical properties.24−27 For example, Coates and co-workers have synthesized poly(propylene succinate)-based stereocomplexes which possess improved thermal stability and semicrystalline properties.28 The synthetic route reported by Qiao and coworkers to prepare syndiotactic and isotactic PPMA (poly(methyl methacrylate))-based triple-helix stereocomplexes is important for the creation of new functional nanomaterials.29 Notably, Nanda and co-workers created triple-helix stereocomplex based on L- and D-proline composed collagen peptides.15 Its complexation behavior has been characterized by using various techniques, such as p-XRD, SAXS, AFM, and TEM techniques. Here we report the formation of stereocomplex between complementary strands of poly(N-methyl-N′-(2-isopropyl-6methylphenyl)carbodiimide), P-1, under thermal annealing conditions. The first observation of such polymer system was made by Novak and co-workers in 2004.30 Predominantly, P and M helices have been synthesized by using achiral N-methylN′-(2-isopropyl-6-methylphenyl)carbodiimide (M-1) monomer along with chiral (R)/(S) BINOL-Ti(IV) diisopropoxide initiator through helix-sense-selective polymerization. Loss of specific optical rotation (SOR) without reaching zero value (plateau out around +286° for P helices) has been observed upon thermal annealing, and interestingly, shortly thereafter the precipitate was formed. This unusual observation clearly distinguishes the P-1 polymer system from more than 100 polycarbodiimides reported so far. For instance, the helical polycarbodiimides synthesized from achiral monomers of Nhexyl-N′-(R)carbodiimides (where R = isopropyl, hexyl, or phenyl) were found to be optically active, and the SOR is temperature-dependent. Upon prolonged heating, the SOR is diminished and reached zero, which is common behavior for almost all polycarbodiimide systems.30 Investigated by different techniques including polarimetry, vibrational circular dichroism (VCD), AFM, TEM, SEM, p-XRD, and SAXS, parent singlehanded helices undergo partial racemization to form stereocomplex upon thermal annealing. In this complexation, the isopropyl scaffold plays a pivotal role in self-organizing molecules as evident by VCD, SAXS, and p-XRD data. To take advantage of this complexation technique, various aggregated morphologies, i.e., spheroids and capsules, have been produced from polycarbodiimide block copolymers and further successfully visualized by using different imaging techniques, such as AFM, TEM, and SEM.
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EXPERIMENTAL SECTION
Instrumentation. 1H NMR and 13C NMR were recorded by using a Bruker Advance III 500 MHz NMR spectrometer at room temperature. Specific optical rotation data were recorded on a JASCO P-1010 polarimeter at λ = 435 nm and at a sample concentration 2.0 mg/mL by using 100 mm path length cell. Solution state vibrational circular dichroism spectra were obtained by using a Bio Tool Chiral-2X VCD spectrometer by dissolving samples in deuterated toluene (C = 25 mg/mL and l = 50 μm). A Malvern Zetasizer particle sizer Nano ZC model equipped with a He−Ne laser source at 633 nm (max 4 mW) was used for dynamic light scattering (DLS) measurement. Size exclusion chromatography (SEC) on a Viscotek VE 3580 system equipped with ViscoGel columns (GMHHR-M) connected to a refractive index detector was used to determine molar mass of all polymers by using chloroform as an eluent. Transmittance measurements were done by using a UV−vis spectrometer (UV-1601PC SHIMADZU) at 600 nm wavelength. A Raguku Ultima III X-ray diffractometer was used to record all p-XRD B
DOI: 10.1021/acs.macromol.7b01633 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 1. Synthesis of P-1: Poly(N-methyl-N′-(2-isopropyl-6-methylphenyl)carbodiimide)
Figure 1. Charts showing a change of SOR upon thermal annealing for P-1, P-2, and block copolymer P-1,2 synthesized from (S)-BINOL-Ti(IV) initiator. day, the polymer was dissolved in chloroform and precipitated from methanol two times. The polymer was dried under vacuum for 2 days. Yield = 1.93 g (86%). 1H NMR (500 MHz, CDCl3, δ ppm): 7.13− 7.01 (b, Ar−H), 3.48 (N−CH2 from octadecyl chain), 3.13, 2.89 (b, N−CH3), 1.28−0.90 (overlapped peaks from octadecyl chain, Ar− CH3, methine H, and isopropyl groups). 13C NMR (126 MHz, CDCl3, δ ppm): 148.69, 148.32 (CN of both P-1 and P-2 blocks), 144.26, 143.17, 141.28, 132.66, 132.58, 130.29, 128.90, 128.24, 124.70, 123.85, 122.93 (Ar−CC), 54.10 (Ar−CH3), 34.24 (N−CH3 from P-1), 32.31 (N−CH2 octadecyl chain), 30.13, 30.06, 29.75, 28.48, 28.30, 27.32, 25.13, 24.18, 23.35, 23.06, 21.71, 19.38, 18.96, 18.81, 18.40, 14.47 (CH2, isopropyl CH3 and terminal CH3 from octadecyl chain); 25 C ° [α]435 nm = −498° (toluene), C = 2 mg/mL, Mn = 13 405 Da. Determination of the Activation Energy. 2 mg/mL of Poly-1 of (R), (S) configurations and Poly-1,2 were dissolved in toluene separately. These polymer solutions were incubated at 40, 50, 60, and 70 °C, and the specific optical rotation was measured independently. Then the graphs of SOR vs time, ln[α/α0] vs time, and Arrhenius plots were drawn. Determination of Chirality. VCD spectra of the abovementioned samples were recorded upon thermal annealing. For those experiments, a 1.0 mL aliquot of each sample was taken before and after the thermal annealing process. Dynamic Light Scattering Experiment. Hydrodynamic radius of polymer samples was measured before and after the thermal annealing process. Transmittance. 2 mg/mL solutions of (R)-, (S)-, (RAC)-Poly-1 and Poly-1,2 were prepared, and transmittance of polymer solutions was measured at λ = 600 nm upon thermal annealing at 70 °C. Then the transmittance % was plotted against time. p-XRD (Powder X-ray Diffraction). The powder sample of each polymer was placed on quartz holder, and the p-XRD profile was recorded. Morphological Studies. Tapping-Mode Atomic Force Microscopy (TP-AFM). 2 mg/mL polymer solutions were prepared, and thermal annealing was done until the SOR reached the plateau value. 1.0 mL of the polymer solution was spun-cast on silicon wafer for AFM imaging. Samples were prepared before and after thermal annealing. Scanning Electron Microscopy (SEM). For thin film preparations, the solid polymer samples were used before and after thermal
annealing. SEM imaging was performed by using bulk materials and electrosprayed samples. For this, the samples of 0.43% (w/w) concentration were used in toluene, and electrospraying was carried out by applying 12 kV between the needle tip and grounded collector drum. Transmission Electron Microscopy (TEM). For TEM imaging, the samples of all polymers were inspected before and after thermal annealing.
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DISCUSSION Use of chiral initiators with the achiral monomer is one of the most interesting ways to obtain single-handed polymers. During the polymerization reaction, the initial chiral stimulant (i.e., either chiral end group or continuous chiral influence from chiral monomers) solely biases the formation of specific handedness, which is called helix sense selective polymerization.31,30,32 Through this polymerization either P (righthanded) helix or M (left-handed) helix can be formed and the interconversion between two helical senses is also possible, but it does not readily occur. However, the transformation of kinetically controlled excess helical sense into energetically favored thermodynamically stable conformation takes place via helical inversion barrier, and the energy associated with this process is called helical inversion energy (or activation energy, Ea). By installation of substituted bulky functional groups into two tunable pendant motifs of carbodiimide monomer, static helices with higher helical inversion energy can be obtained. Bearing in mind that 2,6-disubstitution pattern may be used to introduce persistence asymmetry to the monomer, we have synthesized N-methyl-N′-(2-isopropyl-6-methylphenyl)carbodiimide monomer, M-1. Helix-sense selective polymerization was employed by using both chiral (R)- and (S)-BINOL Ti(IV) diispropoxide initiators with achiral M-1. Being fully regioregular polymer, 2,6-disubstituted aryl scaffold appended to sp2 nitrogen at imine position provides polycarbodiimide with permanent asymmetry. C
DOI: 10.1021/acs.macromol.7b01633 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. Schemes for different molecular motions and racemization pathways.
Figure 3. VCD spectra of P-1 polymers before and after thermal annealing.
(ω), and N−aryl bond rotation (θ) (Figure 2). Helical racemization takes place via the rotation of the helical backbone, and it requires the highest energy. Imine inversion is the second molecular motion which can cause the change of the chirality of the polymer. The lowest energy barrier is attributed to N−aryl bond rotation, and this can be facilitated by bulky, asymmetric substituent at imine position of the polymer system. N−aryl bond rotation contributes to different positioning of isopropyl scaffold, and this bias is to change helicity of the backbone. It was concluded that a permanent asymmetry gained by the polymer system is due to the presence of bulky, substituted aryl pendant groups which partially locked rotation around the N−aryl bond, thus preventing complete racemization. Their supramolecular assemblies are influenced by isopropyl groups which are involved in interlocking interaction to fuse together adjacent polymer chains in the complex bundles. This aggregation causes the polymer to precipitate by losing its chirality which refers to the stereocomplexation phenomenon. By analogy with the wellknown for biological macromolecules leucine zipper arrangement, one can assume that the presence of isopropyl groups appended to polycarbodiimide scaffolds is essential to form a stereocomplex.33,34 This interdigitation phenomenon is very common in regulatory proteins which have leucine residue at every seventh position along their α-helical backbone (Figure S31-1).35,36 The presence of isobutyl segment in leucine residue facilitates interdigitation of adjacent helices (Figure S31-2); thus, it forms dimeric bundled structure due to “knobs-intoholes” packing (see animated P and M helices bundle in the Supporting Information). We postulate the formation of stereocomplex as a result of interdigitation of the isopropyl groups belonging to adjacent polymer chains. Change of
P-1 polymer shows very unusual behavior upon thermal annealing as reported previously by our group.30 Before thermal annealing, this polymer exhibited high specific optical rotation (SOR) that tends to decrease without hitting zero (Figure 1). Shortly after plateauing at around 286° for (R)-P-1, the polymer solution formed a precipitate when heated for several hours. The hypothesis behind this phenomenon is the formation of stereocomplex via racemization process between the parent helical sense and “in situ” generated opposite helical sense. The formation of enantiomeric helices is entropically and thermodynamically favorable process that involves various types of noncovalent interactions, such as hydrophobic and electrostatic forces. The racemization process is confirmed by a decrease of SOR values upon prolonged heating and disappearance of bisignate peak in VCD spectra. Even though SOR tends to descend over time, it comes to the plateau value at −286° for (S)-P-1, implying that the polymer complex retains residual chirality. For this particular polymer, there are potentially two types of chirality present in the molecule, and one of them is due to the helical amidine backbone, whereas another contributor is assigned to the specific arrangement of asymmetric, aryl groups (i.e., helical twist is induced during polymer chain growth and locked in the position mainly by steric hindrance among aromatic pendant groups). For a complete racemization, all isopropyl and methyl groups should be aligned in the same registry to allow helical inversion. The presence of residual chirality (also confirmed by VCD data) implies the incomplete racemization due to inability to remove a high level of cooperativity of bulky, substituted aryl groups. Indeed, three molecular motions were hypothesized to be a cause of this interesting behavior of P-1. The proposed mechanism involves helical racemization (φ), imine inversion D
DOI: 10.1021/acs.macromol.7b01633 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. Graphs for activation energy barriers for P-1, P-2, and P-1,2.
Figure 5. (A) Cartoon showing the formation of stereocomplex, (B) p-XRD profile for annealed (S)-P-1, and (C) SAXS profiles for P-1 polymers.
equation (Figure S32). Then, using the Arrhenius equation, the energy of activation (Ea) can be calculated (Figure 4). From this method, Ea for P-1 was calculated to be 19 ± 0.4 kcal/mol. For the P-1,2 and P-2, it was found to be 20 ± 0.3 and 22 ± 0.4 kcal/mol, respectively. This is in accordance with the previous studies when the energy of activation was found to be 22.8 and 25.6 kcal/mol for poly(N,N′-dihexylcarbodiimide) and poly(Nphenyl-N′-hexylcarbodiimide), respectively. On the basis of these results, it can be inferred that the presence of aryl pendant groups influenced formation of static helices with high helix inversion energy barrier.30 In P-1, 2,6-disubstituted aryl group at the imine position contributes to the stability of helical sense, unlike a small methyl group at the amine position which lowers the activation barrier for this polymer to favor racemization process. Powder X-ray Profile Analysis. p-XRD profiles of all polymers before (initial) and after thermal annealing were analyzed. In all polymer samples, the sharp, intense peak is observed at 12 Å. This first-order peak may appear due to intertwined super helices formation shown in the model (Figure S33). Peaks at 7.12 and 4.47 Å could be assigned to periodic arrangement of the helical turn. Interestingly, the new peak appeared at 4 Å after thermal annealing at 70 °C (which is undetectable in initial (R)- and (S)-P-1) may be associated with complexation of complementary strands (Figure 5B). Hamilton
chirality upon thermal annealing was further investigated by using VCD. Determination of the Helicity of Polymer. VCD is one of the most powerful tools for determining the absolute configuration of the chiral analyte. It combines capabilities of IR spectroscopy with vibration dichroism to show bisignate peak indicative of sample chirality. For the initial (S) polymer, it shows +/− bisignate peak at 1640 cm−1, corresponding to the imine stretch. Upon thermal annealing, the bisignate couplet is diminished once the polymer solution became turbid. After disappearance of bisignate couplets, new peaks arose at 1606, 1494, and 1462 cm−1. The peaks at 1606 and 1494 cm−1 (asymmetric C−H bending or scissoring) can be assigned to chirality changes in aromatic CC stretches, whereas the band at 1462 cm−1 (C−H bending or scissoring) may be attributed (Figure 3) to the zipping of adjacent isopropyl groups projected toward each other. The energy associated with this process can be determined by observing loss of the optical activity. When two helical senses are in dynamic equilibrium, the first-order rate equation can be used, and all concentration terms should be replaced with those of SOR obtained since the concentration of excess helical sense cannot be measured directly. The plots of ln(α0/ α) vs time were constructed to find the observed rate constant kobs at different temperatures by using first-order kinetics E
DOI: 10.1021/acs.macromol.7b01633 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules and co-workers reported the range of 3.8−4.50 Å for hydrophobic side chain interactions of terminal isopropyl groups in dimeric amide foldamers (as determined by single crystal X-ray analysis).37 Again, the appearance of the 4 Å peak should be indicative of interactions of isopropyl groups in this complexation. These data are consistent with SAXS experiment results (Figure 5C). Notably, all peak intensities have been increased that is conclusive of crystallinity changes as a result of macromolecules reorganization/reordering when specimen was thermally annealed (Figure 6).
Figure 7. Cartoon showing a block copolymer (P-1,2) composed of a short block of P-1with isopropyl substituent and a long block of P-2 with octadecyl chains.
studies on (S)-P-1,2 were performed to measure the particle size (Figure 8). At room temperature, hydrodynamic radius (Dh) was found to be 1778 nm, implying that block copolymer occupies a random volume when the main chain is fully extended. After 4 h, Dh became 342 nm which is suggestive of sphere formation (more compact arrangement), and further annealing at 70 °C causes the polymer to form a larger aggregate with Dh around 3380 nm (potentially, clustering of individual spheres). It seems likely that driving force of the sphere formation is interlocking of isopropyl scaffolds at the core of the assembly. DLS experiment was also conducted for homopolymer P-1 having (S) helical sense. It suggested the formation of spheres when samples were thermally annealed for 4 h while further heating caused the appearance of large aggregates. DLS findings in solution are successfully confirmed by TEM and SEM techniques (Figure 9). We hypothesize that P-1,2 scaffold may be organized in such a way that individual macromolecules fuse together to form a spherical aggregate having a core composed of a short block with zipped isopropyl groups and an outer shell (or corona) which is formed by long block with interdigitating octadecyl side chains. According to the 1H NMR data, poly(N-methyl-N′-(2isopropyl-6-methylphenyl)carbodiimide)-b-poly(N-methyl-N′octadecylcarbodiimide), P-1,2, incorporates 10% of poly(Nmethyl-N′-(2-isopropyl-6-methylphenyl)carbodiimide) block. Based on the p-XRD profile, it mainly exhibits peaks associated with poly(N-methyl-N′-octadecylcarbodiimide) block (i.e., sharp peak at 26 Å) while overlapping peaks from the short
Figure 6. p-XRD profiles for P-1 polymers before and after annealing.
Toward the Block Copolymer (P-1,2). Detailed studies using SOR, VCD, p-XRD, and SAXS data provided unambiguous evidence for changes in an excess helical sense during the course of racemization via different molecular motions which led to stereocomplex formation. Zipping or side-chain interactions among isopropyl groups play a vital role in this process as evident by VCD data. To take advantage of this complexation, block copolymer of P-1,2 (composed of a short block of P-1 and a long block of P-2) was synthesized (Scheme 2 and Figure 7). Because of the presence of octadecyl chains in a long segment of P-2, the solubility of a block copolymer, P-1,2 has been enhanced. The thermal annealing experiment for this block copolymer P-1,2 was performed to show gradual decrease of SOR to zero over time at elevated temperature. This behavior is in turn with VCD studies data. Observed phenomenon can be portrayed as the racemization upon thermal annealing, and it never causes a solid to form due to the presence of the octadecyl chains that greatly enhanced the solubility in toluene. By employing DLS, TEM, and SEM techniques, aggregation behavior of (S)-P-1 and (S)-P-1,2 has been investigated. DLS Scheme 2. Synthesis of Block Copolymer P-1,2
F
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Figure 8. DLS profiles for (S)-P-1 and (S)-P-1,2.
Figure 9. Formation of spherical aggregates from P-1,2 upon thermal annealing.
Figure 10. Chart showing change transmittance percentage for P-1 type polymers and P-1,2 upon thermal annealing.
Figure 11. Cartoon representation for stereocomplexation upon thermal annealing.
Transmittance Experiment To Prove Racemization/ Stereocomplexation. It appears that P-1 polymer solution turned turbid as the complexation progressed over time upon thermal annealing. Therefore, to validate the proposed mechanism, the change of transmittance was measured while annealing the sample at 70 °C. For this experiment, the (S)-,
isopropyl block cannot be observed (Figures S34 and S35). The major peak at 26 Å may arise due to the specific packing of individual helices. Further, we hypothesize that the presence of long interdigitating octadecyl chains may facilitate packing of macromolecules. G
DOI: 10.1021/acs.macromol.7b01633 Macromolecules XXXX, XXX, XXX−XXX
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polymers tend to assemble into nanofibers with average diameter around 15−30 nm. When annealed, P-1 formed both irregular and nearly spherical aggregates which are in the range of 180−200 nm and, also, very small 20−50 nm motifs. In addition to that (RAC)-P-1 polymer was inspected prior to annealing, and in this case, the “maggot-like” crystalline domains were observed (Figure S38) as predominant motif which converted into very large aggregations upon thermal annealing. Conversely, the block copolymer of P-1,2 was observed as elongated helical domains that after annealing transformed into large aggregates stacked together (Figure 12C,D). Also, for inspected polymer samples P-2, there are no significant changes in morphology before and after annealing process (Figure S39). SEM of Bulk and Electrosprayed Morphologies. For the P-1 polymer prior to thermal annealing, fiber-like morphology was observed while macroporous interior structure was obtained upon thermal annealing at 70 °C (Figure 13A). Interestingly, both fiber-like and macroporous morphologies could be seen in the initial (RAC)-P-1 (Figure S41). We believe that “knobs-in-to-holes” packing of enantiomeric helices causes the polymer to form porous network. SEM imaging of the block copolymer of P-1,2 showed the mixed fiber-like morphology accompanied by a few spherical aggregates (before annealing). After thermal annealing at 70 °C, this polymer generated well-defined spheres (Figure 13E,F) with a diameter of 4−6 μm. For the homopolymer P-2, fiber-like morphology was shown as a predominant motif in bulk. Insoluble poly(Nmethyl-N′-(2-isopropylphenyl)carbodiimide), P-3, and poly(Nmethyl-N′-(3-isopropylphenyl)carbodiimide), P-4, were confined into unique, homogeneous morphology at room temperature. When the isopropyl group is at the ortho position (P-3), the polymer samples displayed clusters of the fused grainy-like motifs (Figure S42). In contrast, the corresponding metaisomer (P-4) tends to form very crystalline, hard material (Figure S43), and the para-isomer (P-5) forms crystalline solid material upon thermal annealing (Figure S44). It was noted that morphologies might look different depending on the way a polymer specimen was prepared (processed). In order to understand how electrospraying affects morphologies in the newly synthesized polymeric platforms before and after annealing, we examined P-1 and P-1,2 samples. The solvent of interest was toluene with a sample concentration of 0.4% (w/w). The morphologies we obtained are quite interesting and homogeneous. For initial P-1 before annealing, the large, hollow hemispheres were obtained whereas an annealed sample revealed the formation of microspheres which seemed to be porous in appearance (Figure S45). Another typical morphology which resulted from precipitate/turbid solution upon thermal annealing was chip-like motif (Figure 14 and Figure S46). For the block copolymer P-1,2, plate-like morphologies in the 10−17 μm range with dimpled surfaces were obtained. Noteworthy, some fibers were found for annealed sample of P-1,2 polymer which further was electrosprayed (Figure 14, lower panel, and Figures S47 and S48). Importantly, upon thermal annealing this polymer did not form any turbid solution due to the presence of peripheral octadecyl groups as they drastically enhanced the solubility of macromolecule in organic solvents. We believe that presence of this octadecyl scaffold also contributes to increase of polymer viscosity which is a prerequisite for successful fiber extrusion. As anticipated, TEM studies of P-1 polymer revealed the formation of aggregates upon thermal annealing. Nanofibrillar
(R)-, (RAC)-types of P-1 and P-1,2 (block copolymer) were used to compare the transmittance change rate as expressed by equations outlined in Figure 10. Based on the results for (R)and (S)-configurations, polymers undergo around 100 min induction period before dropping down the value of transmittance while analysis of (RAC) implied instant changes upon annealing. It is believed that racemization caused the formation of helices of opposite handedness. The behavior of racemic polymer vs excess helical sense polymer is interesting according to this transmittance experiment. As the racemic polymer (RAC) is composed of a mixed array of 50:50 complementary strands, the induction period is absent whereas the singlehanded polymer requires this period of time for macromolecules to rearrange. Once the opposite handedness is achieved in situ, it tends to undergo “knobs-in-to-holes” packing which is energetically favorable process (Figure 11). However, the transmittance change rate is nearly the same for all P-1 type polymers. For P-1,2, for instance, which has short block composed of isopropyl scaffold and a long block of octadecyl chains around the polycarbodiimide backbone, transmittance changes very slowly, and it does not drop down significantly over time. Also, the presence of octadecyl chains on the periphery of the polymer backbone enhances its solubility. Investigation of Complexation of Different Polymer Derivatives. The synthesis of different polymer derivatives (i.e., P-3, P-4, and P-5) having pendant isopropyl group at the ortho, meta, and para position, respectively, was performed to investigate the complexation behavior (Scheme 3). UnfortuScheme 3. Synthesis of a Polymer Bearing Isopropyl Groups at the Different Positions
nately, both ortho and meta derivatives formed insoluble, gelled polymer material during the polymerization that is suggestive of aggregation. Even though the para derivative forms a soluble polymer, it does not retain excess helical sense as evident by VCD and SOR data. Further, morphological and p-XRD studies of these polymers were performed in order to clarify the mechanism of aggregation (Figure S36). Morphological Studies of Helical Homopolymers and Block Copolymers. For morphological studies of newly synthesized polymers, a number of analytical techniques, including AFM, SEM, and TEM determinations, have been used before and after thermal annealing. It was found that morphology is greatly dependent on sample preparation. Therefore, it was of interest to study electrospun samples along with thin film morphologies and bulk material of the same polymers. AFM Imaging of P-1, P-2, and P-1,2 Polymers. The initial P-1 polymers were inspected by using AFM. These H
DOI: 10.1021/acs.macromol.7b01633 Macromolecules XXXX, XXX, XXX−XXX
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Figure 12. AFM images for P-1 (A, B) and P-1,2 (C, D), 3 × 3 μm.
Figure 13. SEM images for P-1 (A, B), P-2 (C, D), and P-1,2 (E, F) before and after annealing.
morphology was observed for P-1 and P-1,2 by the TEM technique, thus providing unambiguous evidence for the presence of elongated motifs (Figures S49 and S50). Moreover, upon thermal annealing, P-1 forms large aggregates as a result of supramolecular bundling.
annealing takes place due to racemization of single-handed molecular screws causing different molecular motions and zipping of isopropyl scaffold in aryl pendant groups as demonstrated by SOR, VCD, and transmittance data. Zipping of isopropyl scaffold causes the polymer to aggregate by forming a complex of enantiomeric helices, thus decreasing overall chirality. Stereocomplex aggregated morphologies were studied by AFM, SEM, and TEM techniques to reveal the formation of fibers, spheres, and maggot-like architectures. These results strongly suggested that short block segments may crystallize together to form a core of the round-shaped particles. To take advantage of macromolecular zipping, a block
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CONCLUSIONS By using 2,6-disubstituted aryl scaffold in imine position as a bias, permanent asymmetry was successfully introduced to polycarbodiimide chain through helix sense selective polymerization of achiral N-methyl-N′-(2-isopropyl-6-methylphenyl)carbodiimide monomer. An unusual behavior upon thermal I
DOI: 10.1021/acs.macromol.7b01633 Macromolecules XXXX, XXX, XXX−XXX
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Figure 14. SEM images detailing observed morphologies for electrosprayed P-1 (top panel) and P-1,2 (bottom panel) before and after thermal annealing.
Wijayantha Asanga Perera for p-XRD analysis at The University of Texas at Dallas, the Department of Chemistry and Biochemistry, and Prof. Jeffery L. White at Oklahoma State University, Department of Chemistry, for 13C NMR-solid state analysis. We gratefully acknowledge the NSF-MRI grant (CHE1126177) used to purchase the Bruker Advance III 500 NMR instrument.
copolymer with two chemically distinct segments was synthesized and examined in solution and in solid state. Also, the electrospraying technique was successfully applied to this polymeric platform to generate unique architectures, such as capsules, caps, and toroids. Aforementioned aggregations are insightful toward tuning and improvement of polymer thermal and physical properties like glass transition and melting temperatures and crystallinity of the material which is important for various applications. Overall, understanding the mechanism of the complex formation based on stereoregularity changes may advance an area of molecular separation, chiral catalysis, and drug delivery.
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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.7b01633. Further experimental details for the synthesis of ureas, monomers, polymers, and their characterization using NMR, AFM, SEM, TEM, p-XRD, and DLS techniques as well as additional graphs for activation energy calculation (PDF) Animated video featuring “knob-in-to-hole” packing of complementary helical strands (MPG)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*(B.M.N.) E-mail
[email protected]. ORCID
Dumindika A. Siriwardane: 0000-0001-9978-9651 Funding
Funding for this work was provided by Faculty-startup fund from The University of Texas at Dallas (UTD). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are thankful to Prof. Chinedum Osuji at Yale University for providing SAXS analysis, Prof. Kenneth J. Balkus Jr. and J
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