Facile Synthesis of a High Molecular Weight Amphiphilic Aramid − ROMP Block Copolymer

: Herein we report the facile synthesis of an amphiphilic rod − coil block copolymer obtained by the coupling of an amine-terminated poly(dimethylpropylamine norbornene imide) (PDMAPNI) and a penta ﬂ uorophenol ester-terminated poly(dimethoxybenzyl p -aminobenzoate) (PAram). Postpolymerization amide N-deprotection of the block copolymer yielded a strongly aggregating water-soluble rod − coil copolymer. Transmission electron microscopy revealed the formation of large ribbonlike aggregates with sizes up to 50 nm in thickness and 300 nm in length. b -Polymethylated (2-(3-(Dimethylamino)propyl)-3 a ,4,7,7 a -tetrahydro-1 H -4,7-meth- anoisoindole-1,3(2 H )-dione) (PAram- b -PTMAPNI). methyl iodide was to the block copolymer PAram- b -PDMAPNI in DMF- d , NMR Experimental procedures, 1 H and 13 C NMR spectra, and GPC elugrams (PDF)


■ INTRODUCTION
Over the past decades, many efforts have been directed toward the synthesis of well-defined block copolymers due to their unique and tunable properties compared to homopolymers. A diverse number of the reported polymers show the ability to self-assemble in solution, most often as a consequence of solvophobic effects. 1,2 The range of structures that can be adopted by block copolymers varies from vesicles 3,4 or micelles 5−7 to nanotubes 8−10 depending on parameters such as the nature of the different blocks or their length, for example. Block copolymer self-assemblies open the door to nanostructured materials possessing intriguing properties and have already shown to be useful for applications in optical devices, 11 advanced lithography, 12 drug release, 13 or microelectronics. 14 Ring-opening metathesis polymerization (ROMP) is an ideal choice when it comes to the synthesis of functional polymers. On the one hand, this method tolerates a large number of functional groups and is therefore suitable to prepare functional polymer blocks 15 derived from functional monomers. On the other hand, its living polymerization character enables the direct attachment of a further block via ROMP or an effective end-functionalization, which allows postpolymerization reactions with another block. For this purpose, numerous chain-end functionalizing agents have been reported that lead to amines, 16,17 alcohols, 18 thiols, 19 or aldehydes, 17 providing different possibilities of coupling reactions with distinct polymer blocks. 20 End-functionalized poly(norbornene) derivatives have been extensively used for the direct coupling to a hydrophilic block like polyethylene (PEG) 21,22 or served as a macroinitiator for atom transfer radical polymerization (ATRP). 23−25 Interesting properties of block copolymers can be derived from blocks consisting of poly(aromatic amide)s (polyaramids). These structures are known from very resistant materials such as Kevlar. The reason for the high toughness of these compounds is based on their molecular structure. Intramolecular hydrogen bonds and π−π stacking are responsible for these unique characteristics but also result in drawbacks such as weak solubility and hence processability. One way to overcome this disadvantage is the attachment to a solubilizing polymer, opening the door to new applications. There are only few reports on block copolymers using aramids. So far, they have been used to improve the toughness of epoxy resins, 26 as macroinitiators for ATRP 27,28 or RAFT. 29 They have also been coupled to polyethers 30,31 or aliphatic polyamides. 32 However, aramids are highly interesting due to their ability to aggregate via the above-mentioned interactions even in the absence of solvophobic effects, thus making them promising candidates for the study of self-assembled structures. 33 Rod−coil copolymers between short oligobenzamides and solubilizing poly(ethylene glycol) chains were reported by our group previously. 34−37 ROMP polymers have been combined with ATRP 38 or RAFT 39 as well, but to the best of our knowledge, the synthesis of a block copolymer prepared from a polyaramid and a polynorbornene block prepared by ROMP has never been reported.
Herein we report the facile synthesis of a high molecular weight block copolymer prepared via the direct coupling of poly(dimethylpropylamine norbornene imide) (PDMAPNI) and poly(dimethoxybenzyl p-aminobenzoate) (PAram). The blocks were coupled by substituting the highly electrophilic pentafluorophenol (PFP) ester at the chain end of the aramid block with the amine end-functionalized poly(norbornene imide).

■ EXPERIMENTAL SECTION
Materials. Standard 1 H, 13 C, and 19 F nuclear magnetic resonance spectra were recorded either on a Bruker Avance III 300 at a frequency of 300 MHz ( 1 H) and 75 MHz ( 13 C) or at 400 MHz ( 1 H), 100 MHz ( 13 C), and 376 MHz ( 19 F) on a Bruker DPX 400 spectrometer. All NMR signals were referenced internally to residual solvent signals. Matrix-assisted laser desorption and ionization (MALDI) mass spectra were recorded on a Bruker ultrafleXtreme. For gel permeation chromatography in chloroform, an instrument consisting of a Duratec vacuum degasser, a KNAUER Smartline Pump 1050, and a set of two MZ-Gel SDplus linear columns (300 × 8 mm, 5 μm particle size) was used. Signal detection occurred by use of an Applied Biosystems 759A UV detector (set to 254 nm wavelength) and a Knauer Smartline 2300 RI detector (refractive index). Calibration was done using Malvern PolycalTM UCS-PS polystyrene standards as for gel permeation chromatography in THF. The instrument is an automated Viscotek GPCmax VE-2001 with a set of two Viscotek T6000 M linear columns (300 × 8 mm, 5 μm particle size). Signal detection occurred by use of a Viscotek Smartline 2600 UV detector (set to 254 nm wavelength) and a Viscotek VE 3580 RI detector (refractive index). Transmission electron microscopy images were recorded on a Tecnai Spirit (BioTwin lens, 120 kV LaB6 emitter) transmission electron microscope (FEI, Netherlands). To determine the manner of aggregation, sample solutions were deposited onto carbon-coated copper grids (200 mesh, Plano GmbH, Germany), which were placed on a filter paper. Samples were allowed to dry overnight protected from dust.

■ RESULTS AND DISCUSSION
We recently reported the synthesis of high molecular weight aramids obtained by the controlled anionic polymerization of pentafluorophenyl (PFP) N-benzyl p-aminobenzoates. 40 We showed that those polymer chains were still bearing a PFP ester at the chain end and could, therefore, be useful for the coupling to other polymer blocks carrying a terminal nucleophile for further functionalization. This polymerization method is also interesting as it is possible to obtain very high molecular weight N-benzyl protected and therefore organo-soluble aramids. The cleavage of this N-benzyl group on the amide and thus recovery of the amide proton induces strong intermolecular hydrogen bonding which initiates aggregation. Furthermore, those hydrogen bonds combined with aromatic interactions between the polymer chains render such types of polymers insoluble in most solvents. However, in the case where block copolymers of N-unsubstituted aramids with N-alkyl-substituted aramids have been prepared, very good solubility in organic solvents can be observed. 42 In this report, we exploit this aggregating behavior in order to induce self-assembly between the aramid blocks. The PAram block was obtained by polymerization of perfluorophenyl 4-((2,4-dimethoxybenzyl)amino)benzoate (2) which was synthesized from 4-aminobenzoic acid (Scheme 1). The synthesis pathway of the aramid monomer started with a reductive amination of the aromatic amino acid with 2,4-dimethoxybenzaldehyde using triacetoxyborohydride, which lead to product 1 (91%). 43 This compound was further converted to the monomer 2 via a Steglich esterification in low yield (36%) due to repeated purification cycles. Monomer 2 was then polymerized to give PAram with a molecular weight around 20 kDa (see Scheme 1). Unfortunately, the polymer could not be analyzed by matrix-assisted-laser-desorption−ionization timeof-flight mass spectrometry (MALDI-ToF MS) due to its poor ability to get ionized. However, 19 F and 1 H NMR spectroscopy as well as gel permeation chromatography (GPC) in DMF and in chloroform confirmed the formation of the polymer. 19 F NMR spectroscopy in particular clearly showed the presence of the PFP ester at the chain end of the PAram block (see Supporting Information), which was required for further coupling to the second nucleophilic polymer block.
In parallel to our research on the polymerization of aromatic amino acid derivatives, our group also published several methods for the end functionalization of poly(norbornene imide) using chain transfer agents. 17−22 The monomer 2-(3-(dimethylamino)propyl)-3a,4,7,7a-tetrahydro-1H-4,7-methanoisoindole-1,3(2H)-dione (3, 76%) was obtained by condensing exo-norborn-5-ene−2,3-dicarboxylic anhydride with 3-(dimethylamino)-1-propylamine and was polymerized using Grubbs' first-generation catalyst (G1). The protocol for the chain transfer of the poly(norbornene imide) derivative to a Boc-protected 1,4-but-2-ene diamine was published previously by our group and should lead to the Boc-protected amine endfunctionalized polymer (Pre-PDMAPNI) with a molecular weight of around 20 kDa. 16 Because of the high molecular weight of the polymer, end-group analysis by 1 H NMR spectroscopy or isotopically resolved MALDI-ToF mass spectrometry was unfortunately not successful. The presence of the protected amine end group was, however, indirectly shown by the successful coupling to the aramide block (see below).

Scheme 1. Synthetic Route to Monopentafluorophenyl Ester End-Functional Polyaramid Param
after a simple postpolymerization deprotection with hydrochloric acid solution (see Scheme 2). We believe that the broad polydispersity index (Đ = 2.1) of polymer PDMAPNI, which is unusual for ROMP, is due to the presence of the amine in the monomer structure, which might coordinate to the propagating ruthenium complex and decrease the efficiency of the polymerization. Nonetheless, GPC (DMF) and 1 H NMR spectroscopy confirm the formation of a polymer of around 22 kDa molecular weight. The Boc deprotection reaction of the Pre-PDMAPNI block could unfortunately not be conclusively shown by MALDI-ToF mass spectrometry. Therefore, the Bocdeprotected polymer was analyzed only by 1 H NMR spectroscopy and GPC in DMF (see Supporting Information). Boc deprotection could not be verified by 1 H NMR spectroscopy due to the low signal intensity of the Boc end group compared to the polymer backbone peaks. Successful deprotection was proved via the coupling to the PAram block as clearly indicated in GPC traces (Figure 1).
The two blocks PDMAPNI and PAram were coupled in a simple and straightforward manner upon mixing them together in DMF at room temperature for 5 days. Completion of the reaction was monitored via DMF GPC (see Supporting Information). Purification of the block copolymer PAram-b-PDMAPNI was achieved using recycling GPC in chloroform, giving a copolymer of molecular weight of 27 kDa and a dispersity index of Đ = 1.6 ( Figure 1). Successful coupling was confirmed by diffusion ordered (DOSY) NMR spectroscopy (see Figure 1). Further, it was characterized by means of 1 H and 19 F NMR spectroscopy (see Supporting Information).
In order to induce a change in solubility of one block, and hence generate an amphiphilic character of the whole copolymer, PAram-b-PDMAPNI was treated with methyl iodide in deuterated DMF, leading to the quaternization of the dimethylaminopropyl groups of the PDMAPNI block. At that stage of postpolymerization modification (Scheme 3, step 1, ST1, 30 mg/mL), the two blocks were still soluble and observable by 1 H NMR spectroscopy (DMF-d 7 ). Furthermore, no large aggregates were observed by transmission electron microscopy (TEM), even though a difference in contrast between the images of the block copolymer before and after quaternization could be observed (see Supporting Information).
Significant aggregation was induced by the addition of deuterated water to the DMF solution of PAram-b-PTMAPNI up to a ratio a of 1:1 DMF to D 2 O (ST2, Scheme 3). 1 H NMR spectroscopy showed the disappearance of the hydrophobic aromatic PAram block protons, indicating signal broadening due to strong aggregation. The aromatic polyamide moiety cannot be solvated properly by this solvent mixture. The TEM images showed the formation of large aggregates of the block copolymers up to several hundred nanometers in length (Figure 2).
At this stage, we assume that the block copolymer PAram-b-PTMAPNI (Scheme 3, ST2) adopts the shape of an amphiphilic coil−coil block copolymer. According to TEM microscopy ( Figure 2, left) large aggregates are formed due to the amphiphilic nature of the diblock copolymer. As the hydrophobic effect forces the aromatic moieties to be spatially closer, aromatic interactions might take place, leading to the formation of brush or starlike aggregates (Figure 2, right). In a third step (Scheme 3, ST3), the addition of trifluoroacetic acid (TFA) cleaves the benzylic N-protecting group on the amide nitrogen and forces the aramid block to adopt a linear rodlike structure. The change of configuration of the amide bonds from cis to trans (with respect to the phenyl rings) 44  aromatic interactions. Thus, the aramid moieties rigidify into linear rods and therefore force the block copolymer to adopt a different more well-defined shape.
The large aggregates visible in the TEM images before addition of TFA ( Figure 2) are replaced by "flake"-like aggregates (Figure 3, top, A−C). These "flakes" consist of fibers with lengths from 100 to 300 nm and typical thicknesses of 20 to 40 nm, which we believe to be the ordered PAram moieties. The greater TEM contrast of the fibers compared to the background indicates a larger electron density as it is the case for aggregated aromatic structures. Furthermore, the fibers are surrounded by diffuse areas of lower contrast that we believe to be the less electron-dense PTMAPNI chains. One linear PAram chain of 15−20 kDa would have a length of around 35−50 nm (estimated via Chem3D molecular mechanics calculations), thus matching the width of the observed fibers. It is interesting to see that the fibers do not appear to have the same width along the contour length of the fibers. We believe that the PAram blocks align side by side (like in the case of Kevlar) forming twisted ribbonlike structures. Because of the twisting of the ribbon, we believe that the width of the fibers might not appear constant in the projection images ( Figure 3D).
The aggregation behavior of linear amide-N-unprotected aramid polymers has been reported before. 34−37 However, very high molecular weight polyaramids are typically completely insoluble in common organic solvents. In the presented case, the high solubility of the charged poly(norbornene imide) derivative in a DMF/water mixture enables the aggregates to remain in solution despite the high molecular weight of the aramid block. We believe that our synthetic method allows the synthesis of even higher molecular weight polyaramid block copolymers which should self-assemble into even larger well-defined self-assembled structures. Synthetic efforts toward such interesting nanostructures are currently underway.

■ CONCLUSIONS
In this report, we showed the facile and rapid synthesis of a block copolymer consisting of a high molecular weight aramid and a polynorbornene imide derivative block. We proved that pentafluorophenol esters, due to their high electrophilicity, are convenient end groups to build aramid block copolymers. Finally, we showed the formation of self-assemblies of a polynorbornenimide (PNI)−aramid block copolymer into ribbons with lengths up to 300 nm through postpolymerization reactions. Nanostructures were obtained following N-quaternization of the PNI derivative block, changing the hydrophilic character of the solvent and removal of the N-protecting group of the aramid block. Changing the postpolymerization modifications or their sequence could lead to several other types of arrangements and thus of other highly interesting nanostructures, which we will investigate in future reports. The potential modifications of our ROMP monomers as well as the aromatic amides are highly versatile, and a wide range of chemical functionalities can easily be incorporated into the polymer structures. The synthesis of block copolymers consisting of customizable aromatic amides and norbornene derivatives and their subsequent aggregation following the herein reported methodology provides the potential for a broad spectrum of new nano-organized macromolecules.