Article pubs.acs.org/Macromolecules
Orthogonal Synthesis of Covalent Polydendrimer Frameworks by Fusing Classical and Onion-Peel Phosphorus-Based Dendritic Units Nadia Katir,† Nabil El Brahmi,† Nathalie Marcotte,‡ Jean Pierre Majoral,§ Mosto Bousmina,*,† and Abdelkrim El Kadib*,† †
Euromed Research Center, Engineering Division, Euro-Mediterranean University of Fes (UEMF), Fès-Shore, Route de Sidi Hrazem, 30070 Fès, Morocco ‡ Institut Charles Gerhardt UMR 5253, CNRS/ENSCM/UM, 8 rue de l’Ecole Normale, Montpellier F-34295 Cedex, France § Laboratoire de Chimie de Coordination (LCC), CNRS, 205 route de Narbonne, 31077 Toulouse, France S Supporting Information *
ABSTRACT: We report novel and new giant three-dimensional polymers having dendrimers as repeating units. The approach is illustrated here for macromolecular synthesis by polymeric condensation of well-defined single phosphorus dendrimers units. Specifically, classical and onion-peel phosphorus dendrimers, constructed by a divergent method from a cyclotriphosphazene core, were fused within the same tectonic nanostructure by several polymeric condensation approaches including hydrazine-to-aldehyde Schiff-base formation and amine-tocarboxylic acid peptide-like coupling. These reticular, easy to run metalfree routes afford a new library of hyperbranched macromolecular materials, featuring various phosphorus layers (both alternated and dissymmetrical), well-defined textured nanospheres, and controllable nanometric ordered substructures. The scope of the concept is successfully expanded to the integration of electro-redox viologen units resulting in the synthesis of new photoactive macromolecular materials.
1. INTRODUCTION High molecular weight macromolecules are a class of pervasive materials at the borderline of molecular chemistry and materials science. Polymer synthesis, engineering, and manufacturing stimulate tremendous efforts to provide novel commodities and constitute one of most vibrating field of research in advanced science and technology.1−5 Among these macromolecular architectures, the cross-linked ones are very attractive for various applications in chromatography, gas storage, separation, filtration, sensing, automotive, and aerospace sectors.6−10 A typical example is the so-called covalent organic frameworks (COFs) prepared through chemical condensation of multifunctional reactive monomeric units that allow a threedimensional growth of the resulting scaffold.11−13 This affords a large panel of macromolecular diversity with a tunable reactivity that was advantageously exploited in bifunctional catalysis,14 CO2-capture and release,15,16 gas separation,17 molecularly imprinting for host−guest interactions, and sensing technologies.18,19 Dendrimers are another class of emerging polymeric-like architectures. They are highly branched macromolecules with well-defined and nanosized monodisperse structure that find applications especially in medical and catalysis fields.20,13 The possibility to design such scaffold with controlled multivalency by increasing the dendrimer generation is one of the most important features of this chemistry.21,22 In addition, dendrimer © XXXX American Chemical Society
chemistry allows the modulation of other parameters including their size, shape, topology, surface chemistry, flexibility, and architecture. Consequently, dendrimers stand as prominent contenders to design highly tunable nanoarchitectures. Following our involvement in the chemistry of phosphorus dendrimers and their outstanding properties in medicinal chemistry, biotechnology, and materials science,23−27 we recently reported the preparation of novel onion-peel dendritic structures, a recent synthetic methodology pioneered by Roy’s group.28,29 We showed that each phosphorus layer in the construct has a distinguished reactivity,30 thereby increasing the versatile behavior of phosphorus dendrimers for which, hitherto, the reactivity is mainly developed in the outer shell.30,31 As an example, regioselective incorporation of Au(I) within the onion dendritic structure was performed thanks to the incorporation of PN−PS ligands in a specific internal layer.32 This new directional synthetic strategy parallels the recent finding that underlies the importance of each function within the dendrimer scaffolds with a special emphasis to the hitherto-underestimated pivotal role of the internal core.33 While extensive attempts were made to diversify the chemical skeleton of the dendrimer itself, only partially extended Received: June 27, 2016 Revised: July 26, 2016
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DOI: 10.1021/acs.macromol.6b01365 Macromolecules XXXX, XXX, XXX−XXX
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P2: To a solution of 2-NNH2 (40 mg, 5.94 × 10−6 mol) in 4 mL of THF was added a solution of 1-CHO (20 mg, 2.32 × 10−5 mol) in 5 mL of THF. The mixture was stirred for 48 h at room temperature and then filtered. The solid was washed with THF, then with EtOH, and finally with acetone to afford P2 as a white powder. P3: To a solution of 2-NNH2 (40 mg, 5.94 × 10−6 mol) in 4 mL of THF was added a solution of 2-CHO (34 mg, 1.19 × 10−5 mol) in 6 mL of THF. The mixture was stirred for 48 h at room temperature and then filtered. The solid was washed with THF, then with EtOH, and finally with acetone to afford P3 as a white powder. 13C CP MAS NMR (100 MHz): δ 32.35 (N-Me), 121.32 (C6H4), 128.05 (C6H4, C6H5), 132.89 (CHN, C6H5 and C6H4), 151.17 (C6H4). 31P CP MAS NMR (162 MHz): δ 8.35 (N3P3, PPh2), 56.47 (PS). P4: To a solution of 2-NNH2 (40 mg, 5.94 × 10−6 mol) in 4 mL of THF was added a solution of 3-CHO (40.5 mg, 5.91 × 10−6 mol) in 4 mL of THF. The mixture was stirred for 48 h at room temperature and then filtered. The solid was washed with THF, then with EtOH, and finally with acetone to afford P4 as a white powder. 13C CP MAS NMR (100 MHz): δ 32.13 (N-Me), 121.42 (C6H4), 128.17 (C6H4, C6H5), 132.49 (CHN, C6H5 and C6H4), 151.14 (C6H4). 31P CP MAS NMR (162 MHz): δ 8.52 (N3P3, PPh2), 57.14 (PS). P5: To a solution of 3-NNH2 (30 mg, 2.05 × 10−6 mol) in 4 mL of THF was added a solution of 1-CHO (14 mg, 1.62 × 10−5 mol) in 4 mL of THF. The mixture was stirred for 48 h at room temperature and then filtered. The solid was washed with THF, then with EtOH, and finally with acetone to afford P5 as a white powder. 13C CP MAS NMR (100 MHz): δ 32.28 (N-Me), 121.81 (C6H4), 123.17 (C6H4), 128.59 (C6H5, C6H4), 132.30 (CHN, C6H5 and C6H4), 150.86 (C6H4), 154.57(C6H4). 31P CP MAS NMR (162 MHz): δ 11.42 (N3P3, PPh2) 60.09 (PS). P6: To a solution of 3-NNH2 (30 mg, 2.05 × 10−6 mol) in 4 mL of THF was added a solution of 2-CHO (23.5 mg, 8.23 × 10−6 mol) in 6 mL of THF. The mixture was stirred for 48 h at room temperature and then filtered. The solid was washed with THF, then with EtOH, and finally with acetone to afford P6 as a white powder. 13C CP MAS NMR (100 MHz): δ 32.02 (N-Me), 121.22 (C6H4), 128.15 (C6H4, C6H5), 132.41 (CHN, C6H5 and C6H4), 150.60 (C6H4), 153.83 (C6H4). 31 P CP MAS NMR (162 MHz): δ 9.46 (N3P3, PPh2), 58.21 (PS). P7: To a solution of 3-NNH2 (30 mg, 2.05 × 10−6 mol) in 4 mL of THF was added a solution of 3-CHO (28 mg, 4.09 × 10−6 mol) in 4 mL of THF. The mixture was stirred for 48 h at room temperature and then filtered. The solid was washed with THF, then with EtOH, and finally with acetone to afford P7 as a white powder. 13C CP MAS NMR (100 MHz): δ 32.11 (N-Me), 121.79 (C6H4), 128.28 (C6H5, C6H4), 132.64 (CHN, C6H5 and C6H4), 150.99 (C6H4), 152.57 (C6H4). 31 P CP MAS NMR (162 MHz): δ 9.45 (N3P3, PPh2), 58.32 (PS). P8: A solution of 1-NNH2 (40 mg, 9.87 × 10−5 mol) in 5 mL of acetone was added to a solution of Vio-CHO (20 mg, 2.92 × 10−5 mol) in 6 mL of acetone. The mixture was stirred for 72 h at room temperature and then filtered. The solid was washed with acetone to afford P8 as an orange powder. 13C CP MAS NMR (100 MHz): δ 31.13 (N-Me), 63.90 (N−CH2), 127.35 (C6H4, NC5H4), 136.63 (CHN and C6H4), 144.66 (NC5H4), 149.93 (NC5H4). 31P CP MAS NMR (162 MHz): δ −144.18 (hept, J = 701.0 Hz, PF6), 19.57 (PS). P9: A solution of 2-NNH2 (40 mg, 5.93 × 10−6 mol) in 5 mL of acetone was added to a solution of Vio-CHO (49 mg, 7.16 × 10−5 mol) in 12 mL of acetone. The mixture was stirred for 72 h at room temperature and then filtered. The solid was washed with acetone to afford P9 as an orange powder. 13C CP MAS NMR (100 MHz): δ 31.81 (N-Me), 63.76 (N−CH2), 120.08 (C6H4, NC5H4), 128.62 (C6H5, C6H4, NC5H4), 131.79 (CHN, C6H5 and C6H4), 144.99(NC5H4), 153.28 (C6H4, NC5H4). 31P CP MAS NMR (162 MHz): δ −143.82 (hept, J = 705.2 Hz, PF6), 10.63 (N3P3, PPh2), 57.67 (PS). P10: To a solution of 3-NNH2 (40 mg, 2.74 × 10−6 mol) in 5 mL of acetone was added to a solution of Vio-CHO (45 mg, 6.58 × 10−5 mol) in 6 mL of acetone. The mixture was stirred for 72 h at room temperature and then filtered. The solid was washed with acetone to afford P10 as an orange powder. 13C CP MAS NMR (100 MHz): δ 32.06 (N-Me), 64.43 (N−CH2), 122.38 (C6H4, NC5H4), 129.00 (C6H5, C6H4, NC5H4), 130.11 (C6H4, NC5H4), 132.07 (C6H4,
polymeric structures were designed giving rise to hyperbranched macromolecular architectures.18,19,34−37 Hitherto, giant three-dimensional macromolecular architectures derived from polymerization of dendrimers as repeating units remains elusive and have eluded the realm of polymer chemistry. Conceptually, we describe herein for the first time a new and novel category of macromolecules by polymerizing dendritic units to achieve true three-dimensional giant structures, called here “polydendrimers”. We illustrate this approach using phosphorus dendrimers as monomer-like units. The remarkable multifunctionality of the primarily building motifs used herein, compared to the simpler single units involved in covalent organic frameworks (COFs) and hyperbranched polymers, affords a large panel of macromolecular nanospheres with increased phosphorus diversity including cyclotriphosphazene cores, branching ligands, and bridging terminal groups. The choice of these phosphorus-based building blocks was dictated by several advantages: (i) their well-established and versatile reactivity,38 with 31P NMR constituting an investigating tool of the layer modification;39 (ii) their internal hydrophobicity, somewhat similar to the astonishing backbone of natural proteins; (iii) their improved thermal and chemical stability;40 and (iv) the thermally induced ring-opening ability of the cyclotriphosphazene core affording linear polyphosphazene polymers,41−43 a supplementary key to access more diversified polymeric architectures.44 No other synthetic methodology of this versatility and chemical diversity attractiveness is currently known.
2. EXPERIMENTAL SECTION Materials. All reactions were carried out in the absence of air using standard Schlenk techniques and vacuum-line manipulation. Chemicals were purchased from Sigma-Aldrich or Strem and used without further purification; solvents were dried and distilled by routine procedures. Characterization. NMR spectra were recorded with Bruker AV 400 and HD 400 spectrometers. Cross-polarization magic-anglespinning (CP/MAS) 13C and 31P NMR spectra were acquired on a Bruker Avance 400 WB spectrometer operating at 100 and 162 MHz, respectively. Fourier transformed infrared spectra were obtained with a PerkinElmer Spectrum 100FT-IR spectrometer on neat samples (ATR FT-IR). Nitrogen sorption isotherms at 77 K were obtained with a Micromeritics ASAP 2010 apparatus. Prior to measurement, the samples were degassed for 8 h at 120 °C. The surface area (SBET) was determined from BET treatment in the range 0.04−0.3 p/p0 assuming a surface coverage of the nitrogen molecule estimated to be 13.5 Å. Xray powder diffraction (XRD) patterns were recorded on a D8 Advance Bruker AXS system using Cu Kα radiation with a step size of 0.02° in the 2θ range from 0.3° to 10° for SAXS and from 0.45° to 87° for WAXS (geometry: Bragg−Brentano, θ/2θ mode). DRUV spectra were measured in the 200−800 nm range using Spectralon as the reference on a PerkinElmer Lambda 40 spectrometer equipped with an integrating sphere (Lapshere, North Sutton, NH). Scanning electronic microscopy (SEM) images were obtained using a JEOL JSM 6700F. Transmission electronic microscopy (TEM) images were obtained using JEOL JEM 2010 at an activation voltage of 200 kV. Synthesis of Polymers P1−P12. P1: To a solution of 1-NNH2 (20 mg, 4.94 × 10−5 mol) in 3 mL of THF was added a solution of 1-CHO (42.5 mg, 4.93 × 10−5 mol) in 5 mL of THF. The mixture was stirred for 48 h at room temperature and then filtered. The solid was washed with THF, then with EtOH, and finally with acetone to afford P1 as a white powder. 13C CP MAS NMR (100 MHz): δ 31.18 (N-Me), 121.15 (C6H4), 127.70 (C6H4), 128.63 (C6H4), 133.50 (CHN and (C6H4)), 150.05 (C6H4), 154.71 (C6H4). 31P CP MAS NMR (162 MHz): δ 9.33 (N3P3−O), 17.31 (N3P3−N). B
DOI: 10.1021/acs.macromol.6b01365 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 1. Chemical Structures of Dendrimers D-CHO, D-NNH2, and D-COOH
Scheme 2. Illustration of the Chemical Fusion Occurring between Two Different Elementary Units To Afford Interconnected Building Blocks: (a) Schiff-Base Formation; (b) Peptide Cross-Coupling; (c) Extended Polymerization Allows for Further Growth and Formation of Hyperbranching Units
138.75 (CHN and (C6H4), 140.41 (C6H4), 151.24 (C6H4), 167.10 (C6H4). 31P CP MAS NMR (162 MHz): δ 9.13 (N3P3−O), 27.79 (N3P3−N), 60.73 (PS). P12: To a mixture of 2-COOH (60 mg, 7.63 × 10−6 mol), EDCI (38 mg, 2.10 × 10−4 mol), and DMAP (22 mg, 1.83 × 10−5 mol) in 20 mL of THF was added a solution of 1-NNH2 (12 mg, 2.96 × 10−5 mol). The mixture was stirred for 120 h at room temperature and then filtered. The residue was washed with THF then with CH2Cl2 to afford P12 as a white powder. 13C CP MAS NMR (100 MHz): δ 40.76 (N-Me), 122.39 (C6H4), 131.99 (C6H4), 139.99 (C6H4), 151.19 (C6H4),
NC5H4), 137.75 (CHN, C6H5 and (C6H4), 145.48 (NC5H4), 151.39 (C6H4, NC5H4), 153.69 (C6H4, NC5H4). 31P CP MAS NMR (162 MHz): δ −143.96 (hept, J = 702.6 Hz, PF6), 10.70 (N3P3, PPh2), 57.23 (PS). P11: To a mixture of 1-COOH (20 mg, 5.95 × 10−6 mol), EDCI (15 mg, 7.85 × 10−5 mol), and DMAP (9 mg, 7.4 × 10−5 mol) in 10 mL of THF was added a solution of 1-NNH2 (4.8 mg, 1.19 × 10−5 mol). The mixture was stirred for 120 h at room temperature and then filtered. The residue was washed with THF and then with CH2Cl2 to afford P11 as a white powder. 13C CP MAS NMR (100 MHz): δ 35.52 (N− CH3), 39.73 (N−Me), 121.03 (C6H4), 122.04 (C6H4), 132.03 (C6H4), C
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Macromolecules Table 1. Polymeric Materials Derived from the Fusion of Classical and Onion-Peel Dendrimers polymer
onion-peel dendrimer
classical dendrimer
types of phosphorus units
P1
1-NNH2
1-CHO
2
P2 P3 P4 P5 P6 P7 P8 P9 P10 P11
2-NNH2 2-NNH2 2-NNH2 3-NNH2 3-NNH2 3-NNH2 1-NNH2 2-NNH2 3-NNH2 1-NNH2
1-CHO 2-CHO 3-CHO 1-CHO 2-CHO 3-CHO Vio-CHO Vio-CHO Vio-CHO 1-COOH
5 6 7 6 7 8 2 5 6 3
P12
1-NNH2
2-COOH
4
31
P NMR core
31
P NMR PS
9.33 17.31
SBET (m2 g−1) 181
8.35 8.52 11.42 9.45 9.45 19.57
56.47 57.14 60.09 58.21 58.32
10.63 9.13 27.79 9.14 27.27
57.67 60.73
19 3 7 26
10
61.78
Figure 1. High magnification DRIFT spectra of 1-NNH2, 1-CHO, and P1: left (4000−2000 cm−1) and right (2000−1200 cm−1). 168.22 (CON). 31P CP MAS NMR (162 MHz): δ 9.14 (N3P3−O), 27.27 (N3P3−N), 61.78 (PS).
accessed following the early described procedure46,47 and obtained quantitatively in 88% yield (Scheme 1). The preparation of 2-NNH2 consists in nucleophilic substitution of the P(S)Cl2 end groups of the “classical” phosphorus dendrimer D1 with 4-(diphenylphosphino)phenol affording a dendrimer decorated with 12 diphenylphosphino groups. The Staudinger reaction48,49 between these terminated diphenylphosphines and the azidothio-bis-hydrazinophosphine afforded 2-NNH2 with the simultaneous release of nitrogen as the sole byproduct. Similar approach affords quantitative yield of 3-NNH2. 2-NNH2 and 3-NNH2 feature 12 and 24 methyl hydrazine terminal groups, respectively30 (S1 in Supporting Information). The multistep preparation of 1-COOH and 2-COOH consists in an aldol-type reaction, which proceeds at 95 °C (48−72 h) through the condensation of the surface aldehyde of 1-CHO and 2-CHO, respectively, with an excess of malonic acid in pyridine solution, in the presence of traces of piperidine.50 The products were isolated in quantitative yields (∼89%) with excellent purity (S1 in Supporting Information). It is worth noting that the diverging route used to construct D-CHO and D-COOH results in classical dendrimers built from periodically alternated repetitive units of the branching phosphorus, whereas construction of D-NNH2 allows different layers with distinguished phosphorus units to be introduced in an onion-peel structure. The various surface functional groups introduced in those dendrimers may act as elementary building blocks for organizing a more complex tridimensional structure
3. RESULTS AND DISCUSSION We synthesized up to three generations of phosphorus dendrimers (D = 1, 2, or 3, Scheme 1) using hexacyclotriphosphazene as a central hydrophobic core, each of them bearing various surface functional groups, i.e., methyl hydrazines (D-NNH2), aldehydes (D-CHO), and carboxylic acids (D-COOH). The syntheses were adapted from the well-known methodologies previously reported by one of us.45 Experimental details of the syntheses and full characterization of the dendrimers are given in S1 of the Supporting Information. Briefly, the phosphorus dendrimer of generation 0 1-CHO was prepared by chloride substitution of the starting hexachlorocyclotriphosphazene (PN) 3 Cl 6 core by p-hydroxybenzaldehyde, allowing its isolation in 95% yield (Scheme 1). The first and second generation aldehyde-terminated phosphorus dendrimer (2-CHO and 3-CHO) were prepared by a repetition of two successive reactions: (i) condensation of peripheral aldehydes with dichlorophosphothiohydrazide and (ii) substitution of the attached chlorides by p-hydroxybenzaldehyde45 (S1 in Supporting Information). In parallel, three methyl hydrazine-terminated dendrimers were designed also from the hexachlorocyclotriphosphazene (PN)3Cl6. The smallest building block, the azidohexamethylhydrazinocyclotriphosphazene (denoted as 1-NNH2), was D
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Macromolecules at a micrometric scale (Scheme 2). We then explored the possibility to fuse these elementary building blocks, namely by condensation between D-CHO and D-NNH2, to form Schiff base and peptide coupling reaction between D-COOH and D-NNH2, which will allow access to hybrid mixtures of classical and onion-peel structures, a concept not explored so far for macromolecular nanoarchitecture engineering. The choice of the methylhydrazine as a reactive linker instead of simple amine emanates from the great stability of P−N(CH3)−NCH compared to the more moisture sensitive −CHN− imine bonds. Polymerization by Schiff-Base Formation. Stoichiometric amounts of terminal functional groups in D-NNH2 and D-CHO (D = 1, 2, or 3) were mixed under gentle conditions, without any noble metal or toxic reagents. After 48 h of stirring at room temperature in THF, the mixture was filtered off and washed twice with THF and acetone. The resulting P1 to P7 polymeric materials (Table 1) were insoluble in most organic solvents (CHCl3, CH2Cl2, THF, CH3CN, DMF, CH3SOCH3, and CH3−CH2−OH). This behavior is the first indication of the occurrence of a reticular cross-linking in the three dimensions between the functional units of the starting dendrimers (Scheme 2). The chemical structure of these polymeric materials were elucidated by FTIR analysis (S2 in Supporting Information) and solid-state 13C and 31P NMR spectroscopies (S3 and S4 in Supporting Information). Salient evidence for the consumption of both methylhydrazine (NMe-NH2) and terminal aldehydes comes from changes occurring in the FTIR spectra of the condensed materials compared to the two starting precursors. As an example, Figure 1 shows the IR spectral changes obtained for P1, which are similar for all the polymeric materials. The signal of the NH2 groups at 3288 and 3324 cm−1 almost disappeared, indicating a nearly quantitative consumption of the NH2 group involved in the formation of the CN bond. Meanwhile, the aldehyde-terminated precursor 1-CHO shows a significant reduction of the signal at 1705 cm−1, thereby evidencing the consumption of the CHO units during the multicondensation step. However, few aldehydes are still present within the material network as illustrated by the observation of a small remaining vibration band at 1705 cm−1 (Figure 1). One may think that as the cross-linking occurs randomly in three dimensions, functional groups can be buried in inaccessible porous network or hindered from reactivity because of their entrapment in rigid positions. The remaining aldehydes and methyl hydrazine will serve to further introduce other “smaller” functionalities. Additional insight has been gained from solid-state 13C and 31 P NMR analyses. The carbon spectra pattern of the polymeric materials are very similar to the ones of the starting D-NNH2 dendrimers evidencing the intactness of their chemical structure upon multicondensation. This is exemplified for P6 in Figure 2. The most pronounced difference comes from the resonance of the methylated nitrogen groups (N−CH3) of the methylhydrazine, for which the rotational freedom in the starting dendrimer affords three distinguished signals (at 33.59, 39.38, and 43.05 ppm for 3-NNH2). Upon cross-linking, a broadening in the signal peaks of the methylated nitrogen is observed, which is consistent with the reduced mobility of the chains in the network that do not allow to discriminate different (N−CH3) signals. The carbon belonging to CHN resonates between 130 and 134 ppm for the starting D-NNH2 and the polymeric materials. Nevertheless, the disappearance of CO resonance
Figure 2. 13C CP MAS NMR of dendrimer 3-NNH2 and polymer P6.
at 190 ppm of the starting aldehyde-terminated dendrimers corroborates the condensation reaction between D-CHO and D-NNH2, in consistency with FTIR analysis (see Supporting Information). Solid-state 31P NMR brings additional evidence for the modification occurring within the material skeletal network. The typical variations observed during the synthesis of the polymeric materials are illustrated on Figure 3 for P1 and P5, resulting from chemical condensation of 1-NNH2 and 1-CHO and from 3-NNH2 and 1-CHO, respectively. The preparation of P1 (Figure 3, left) results in a chemical shift of the signal of the core initially observed at 29.80 and 36.42 ppm for the starting 1-NNH2 to 17.31 ppm, typically assignable to P−N−NHC. The phosphorus signal observed at 9.33 in the P1 is assignable to the phosphorus core belonging to 1-CHO units. In the case of P5, three signals are observed at 11.42, 60.09, and 68.37 ppm for the starting 3-NNH2. The first one correspond to the resonance of the core (∼9 ppm), which overlaps with that of the branching unit Ph2PN of the Ph2PN−PS. The second resonance (appearing as a shoulder at 60.09 ppm) corresponds to the phosphorus environments PS in the branching points and the last one correspond 68.37 ppm to the PS−N(Me)NH 2 located near to the surface. Upon condensation, the signal at 68.37 ppm disappears and only one signal is observed at 60 ppm. This displacement is in agreement with the shift previously reported in the literature from 71 to 56 ppm for similar reactions.51 Aiming to further enlarge the chemical functionality of these polymeric materials, we turned our attention to the reticular synthesis implying methyl-hydrazinated D-NNH2 and a rigid benzyl-viologen terminated dialdehyde (S1 in Supporting Information). The choice of this cationic bipyridinium motif was motivated by its versatile use and outstanding reactivity (electro-redox transferring agent,52 biologically active ingredient,53−55 anion and metallic exchanger,56 discrete assembler,57 and efficient stabilizer for nanoparticulates57−59) which could extend the practical use of our polymeric materials in various domains. Recently, covalent organic framework bearing viologen units has been reported by means of Sonoghasira cross-coupling and was shown to find practical applications for CO2 capture and heterogeneous catalysis.60 We consequently embarked to install the selected photoactive viologen within the material framework. Similarly to the E
DOI: 10.1021/acs.macromol.6b01365 Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. 31P CP MAS NMR of (a) dendrimer 1-NNH2 and polymer P1 and (b) dendrimer 3-NNH2 and polymer P5.
Scheme 3. Preparation of Branching Polymeric Material by Fusing Hydrazine Terminated Dendrimers and ViologenFunctionalized Dialdehyde
Figure 4. CP MAS NMR of dendrimer 1-NNH2, Vio-CHO, and polymer P8: (a) 13C CP MAS NMR and (b) 31P CP MAS NMR.
above-described condensations, 1-NNH2, 2-NNH2, and 3-NNH2 were subjected to condensation with Vio-CHO (Scheme 3). After 48 h of stirring at room temperature, the resulting three
materials were obtained as orange solid powder. Expectedly, these materials were insoluble in common organic solvents. F
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Figure 5. CP MAS NMR of dendrimer 1-NNH2, 2-COOH, and polymer P12: (a) 13C CP MAS NMR and (b) 31P CP MAS NMR. 13
DRIFT analysis confirms the disappearance of the terminal groups involved in the condensation, namely the methyl hydrazine at 3288 and 3324 cm−1 and the aldehyde groups at 1698 cm−1 (S2 in Supporting Information). In solid-state 13C NMR, the observation of the typical signal at 63.76−64.43 ppm ascribed to the benzylic carbon bridged nitrogen (CH2−N) confirms the presence of viologen units within the newly created material network. In solid-state 31P NMR, the persistence of the signal at −144 ppm assignable to the anionic PF6− further corroborates the successful anchorage of the viologen units and highlights the tolerance of this synthetic strategy toward ionic structures. In a complementary fashion, a new signal appears at 19.57 ppm attributed to the variation in the chemical environment of N3P3 core upon hydrazine to aldehyde condensation (Figure 4). DRUV analysis of the solid material displays the typical absorption band of the viologen units centered at 220 nm57,59 (S5 in Supporting Information). Polymerization by Peptide-Coupling Reaction. Having elucidated the success of this rational design in accessing novel three-dimensional, insoluble polymeric materials, we turned our attention to the investigation of another synthetic pathway aiming to broaden as possible the library of functional candidates and to diversify the nature of the link. We therefore selected terminal carboxylic building blocks to be coupled with the above-mentioned methylhydrazine-terminated dendrimers by means of peptide coupling that classically affords stable −CO−NH− amide bonds. Because the peptide coupling is challenging, activation of the carboxylic fragment of 1-COOH and 2-COOH was achieved by a carbodiimide agent before the addition of 1-NNH2 with gentle stirring for 2 days at ambient temperature. The isolated materials were expectedly insoluble in common organic solvents. DRIFT analysis corroborated the consumption of the terminal functional groups involved in these condensations. The intensity of both NH2 and the carbonyl groups, respectively at 3171 and 3288 cm−1 for the former and at 1688 cm−1 for the latter, disappeared on behalf of a new amide bond appearing at 1644 cm−1 (S2 in Supporting Information). Solid-state 13C and 31P NMR displayed typical signals of the starting 1-NNH2 core and those of the carboxylic dendrimer as exemplified on Figure 5 for the synthesis of P12. A slight shift of the carbonyl groups (from 170 to ∼167 ppm) is observed in
C NMR, pointing to the successful substitution of carboxylic butters (COOH) by novel amide linkers (CO−NH) (Figure 5, left). Nicely, 31P NMR allowed discrimination of the signals of the hydrazine-terminated dendrimer core from the one of the carboxylic dendrimer as the phosphorus of 1-NNH2 are linked to nitrogen (∼29 ppm) while those of 2-COOH are bridged to oxygen atoms (∼9 ppm). In P12 final material, the two signals are observed as well as the signal of the branching units (PS) at 61.78 ppm. The reduced intensity of the signal of 2-COOH is reminiscent of the reduction experienced for cyclophosphazene-based dendrimers of highest generations, in perfect consistency with the increased molecular weight and ramification in these solid materials that causes an occlusion of the phosphorus core inside of the structure. The signal becomes under these circumstances hardly detectable by NMR analysis. Textural Analysis. Beside the chemical diversity of these macromolecular objects, one of the greatest interest in drawing these soft building blocks to the realm of solid materials is their textural engineering. Given the similarities of these cross-linked dendritic units with metal−organic framework (MOF) and rigid covalent organic frameworks (COF) materials, a question raised on whether these condensed materials feature crystallinity and porosity? Although the packing of molecules tend normally to form layered, nonporous network, the sterical hindrance and the directional polymerization axis afford, not systematically, however, porous and void space within the orthogonally created network.61,62 To get insight into the network organization, scanning and transmission electronic microscopies (SEM and TEM) were performed on polymeric materials. SEM reveals the presence of homogeneously shaped nanospheres with in the whole an average size ranging from 100 to 300 nm (Figure 6 and S6 in Supporting Information). Notably, the nanospheres were formed whatever the approach used: Schiff-base formation and peptide coupling. These regular nanospheres are of paramount interest in several domains including chromatography, catalysis with organic monolithic bodies, and polymer imprinting technology. To further assess the openness of the materials framework, these solid materials were subjected to nitrogen sorption studies. While P1 features significant porosity as illustrated by G
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Figure 6. Scanning electron microscopy pictures of (a) P1, (b) P2, (c) P6, and (d) P7.
its measured specific surface area (SBET = 181 m2 g−1), the condensation implying more bulky units affords marginally porous materials for which the measured surface area did not exceed 27 m2 g−1 (Table 1 and S7 in Supporting Information). Nitrogen sorption isotherm profiles of these materials were typical of solids with macroporous domain or interporosity as the adsorbed volume rises only at very high pressures.63 This speculates that the origin of the porosity is the voids between the spherical particles (interparticles) while each elementary particle is compact and devoid from porosity. Wide-angle X-ray diffraction displays only minor broad peaks evidencing the amorphous nature of the material framework, in contrast to crystalline MOF’s and some rigid COF’s built from smaller, rigid organic precursors (S8 in Supporting Information). TEM analysis provides a clear picture of the organization of these building blocks at the nanoscale. The presence of homogeneous dark spots indicates that each individual nanosphere is built from an infinite of primary nanoparticulates of 2 nm in size (Figure 7a,b and S9a in Supporting Information). The presence of such smaller nano-objects is a testament to the wellcontrolled nucleation and growth process during the polymeric condensation of the two antagonistic monomeric units. In some cases, layered structures are observed at the edge of the spheres or even well distributed throughout the shell, which may reflect to some extent the above-mentioned ordering (Figure 7c and S9b in Supporting Information). At a very high magnification, plans with lamellar structure are also visible, thereby confirming the nanoscale ordering seen in these materials (Figure 7d and S9c in Supporting Information).
Figure 7. Transmission electron microscopy pictures of (a) P5 and (b) P3 showing the formation of small nanoparticulates. (c) P5 where the arrows show layered stacks at the edge and a core−shell like morphology. (d) P5: TEM shows the presence of plans with lamellar structure.
phosphorus layers (up to 8 phosphorus for P7), shaped as nanospheres with intervoid porosity and slight nanoscale ordering. Notably, this new approach is not restricted to phosphorus-based dendrimers and having in mind the unlimited nanostructures available in the literature, a large library of more regular, hyperbranched polymer-like architectures referred herein as “polydendrimers” can be designed and structurally tailored. Our future work focuses on expanding the scope to organic and silicon-based dendrimers and on tuning their multimodal porosity (micro-, meso-, and macroporosity) as well as their application in host−guest interactions and polymer-imprinting biotechnologies.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01365. Synthesis and chemical structures of dendrimers, DRIFT spectra, NMR spectra, DRUV spectra, SEM analysis, nitrogen sorption analysis, DRX analysis, and TEM analysis of polymers (PDF)
4. CONCLUSION Hitherto, only linear polymers featuring dendrimers in their side chains are known. By contrast, the truly three-dimensional giant architectures have eluded synthetic polymer chemistry. We consequently report herein for the first time an easy access to a novel class of macromolecular architectures called “covalent polydendrimer frameworks”. We illustrate this approach by fusing both classical and onion-peel phosphorus dendrimers in single spherical tectonic nanostructures. Our concept follows an easy design implying greener metal-free route, zero waste with only water and nitrogen as byproducts. Schiff-base formation and peptide coupling provide access to solid macromolecular materials featuring different types of
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (A.E.K.). *E-mail:
[email protected] (M.B.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS UEMF is warmly acknowledged for the financial support of this project. A.E.K. is grateful to Fondation Balard for mobility funding. H
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