Polymer Structure Dependent Hierarchy in PolyMOC Gels

Article pubs.acs.org/Macromolecules

Polymer Structure Dependent Hierarchy in PolyMOC Gels Aleksandr V. Zhukhovitskiy,† Julia Zhao,† Mingjiang Zhong,† Eric G. Keeler,†,‡ Eric A. Alt,† Paul Teichen,†,‡ Robert G. Griffin,†,‡ Michael J. A. Hore,§ Adam P. Willard,† and Jeremiah A. Johnson*,† †

Department of Chemistry and ‡Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States § Department of Macromolecular Science and Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States S Supporting Information *

ABSTRACT: Polymer gels are often very soft due to their low branch functionality ( f) and the inevitable presence of defects (e.g., primary loops or dangling chains). Polymer metal−organic cage (polyMOC) gels are a relatively new class of supramolecular gels with precisely defined junction structures made possible by subcomponent assembly of nanoscale MOCs connected by polymer chains. Herein, we report that variation of the molecular weight and architecture of the polymer component of polyMOCs provides an entry into gels with ultra-high f. For example, materials with f ∼ 9− 12, i.e., ∼ 9−12 polymer chains connect each MOC within the gel network, are realized. As a consequence of their increased f, these gels display exemplary mechanical properties at low concentrations (down to 240 μM) of metal ions and only 5.4−5.9 wt % of polymer. Furthermore, X-ray and neutron scattering pointed to an additional level of structural hierarchy that arises from the assembly of M12L24 MOCs into clusters. The relationships between polymer and polyMOC network structure revealed here will facilitate the design of high-performance polyMOCs.

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INTRODUCTION Polymer gels1 are ubiquitous materials whose utility (e.g., in biological systems2 or biomedical applications3−7) stems in part from their viscoelasticity. Supramolecular polymer gelse.g., metallogels, linked through dynamic and reversible metal− ligand coordinationare particularly promising for a number of emerging applications.6−8 However, conventional metallogels9−15 with high fluid content typically suffer from limited stiffness and high sensitivity to defects due to the low branch functionality (f) of their noncovalent junctions, which consist of a single metal ion that links a small number, typically ∼4 or fewer, of polymer chains. Moreover, introduction and control of structural hierarchy are challenging in conventional metallogels. One approach to tune and enhance f in metallogels and simultaneously address structural hierarchy involves the use of subcomponent metallosupramolecular assembly16−22 to construct MxLy junctions; this relatively new strategy has already produced novel metallogels,23−30 as well as block copolymer hybrid materials,31 with a range of unprecedented properties. For example, we presented polymer metal−organic cage (polyMOC) gels based on pyridine−palladium M12L24 cage32 or M2L4 paddlewheel self-assembly (Figure 1A).29 These materials were constructed from short poly(ethylene glycol) (PEG) building blocks (2.2 kDa PEG) with a radius of gyration (Rg) similar to the radius of the M12L24 MOCs (1.7−1.8 nm)29,32 (Figure 1A). Thus, though the M12L24 polyMOC © XXXX American Chemical Society

could, in principle, have f = 24 (i.e., 24 polymers attached to each network junction), such a high f value would require an unfavorable crowding of the M12L24 MOCs. In the real network, a bias toward the formation of loop defects relieved this crowding. The resulting polyMOC gels proved robust and defect-tolerant because they possessed an unprecedented combination of high f (∼4) and a high fraction of primary loop defects (Figure 1A).7 Additionally, the presence of MOC junctions embedded within a polymer network afforded intrinsic structural hierarchy in these polyMOC gels. Seeking to harness the M12L24 MOCs to create materials with previously unattainable f values, we envisioned that the use of longer polymer chains or tetra-arm polymers would dramatically reduce the bias toward loop formation and thereby favor inter-MOC connections (Figure 1B). Furthermore, in typical polymer gels, increasing the polymer length necessarily reduces the stiffness by reducing the cross-link density. We reasoned that it would be possible to offset this decrease in MOC density with a reduction in elastically ineffective loops, which would enable the formation of mechanically robust polyMOCs using very low metal concentrations. Thus, we began this study with a simple yet fundamental question: how does the polymer chain Received: July 26, 2016 Revised: August 23, 2016

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Figure 1. Illustrations of polymer networks features examined in the previous (A) and the current (B) studies. Elastically active chains are shown in blue, primary loop defects in red, ligands in black and green, and Pd(II) ions in yellow.

named by adding “g” or “xg”, respectively, before the name of the corresponding macromer: e.g., the gel derived from P2.7k is named gP2.7k and the xerogel xgP2.7k. The polyMOC gels were analyzed by 1H magic angle spinning (MAS) NMR (Figure 3 and Figure S1), which offered valuable insights into the junction composition as a function of the macromer identity. The spectra indicated that the M2L4 paddlewheel junctions formed in all gMMW polyMOCs (Figure S1), while much larger MOC junctions formed for the gPMW

length and architecture affect the network structure in polyMOC gels? To address this question, we synthesized two sets of linear homotelechelic PEG macromers of varying molecular weight (MW) whose bis-pyridyl termini were designed to assemble with Pd(II) ions into either M12L24 (PMW) or M2L4 (MMW) MOCs (Figure 2). Two analogous 4-arm macromers were

Figure 2. Macromers used in this work. MW = number-average molecular weight, P = para-bis-pyridyl, M = meta-bis-pyridyl.

synthesized (PStar and MStar; Figure 2). In all cases, benzylic alcohol derivatives of the appropriate para- or meta-bis-pyridyl ligand were attached to the ends of carboxylic acid-terminated PEG via standard carbodiimide coupling (see Synthetic Procedures in the Supporting Information).33 1H nuclear magnetic resonance (NMR) spectra and gel permeation chromatography (GPC) results indicated ≥95% end-group fidelity (see Spectra section in the Supporting Information). PolyMOC gels were prepared by mixing perdeuterated dimethyl sulfoxide (DMSO-d6) solutions of macromers and Pd(NO3)2·2H2O at room temperature followed by annealing under argon for 4 h at 80 °C;29 5.4 wt % of polymer (or 5.4− 5.9 wt % network including the Pd(NO3)2·2H2O) was used for all gels (see Synthetic Procedures in Supporting Information). Thus, as the macromer MW increased, the amount of metal used, and the MOC concentration, proportionally decreased. For example, the polyMOC gel prepared from P22.2k had only 240 μM of metal. In the subsequent discussion, gels and xerogels (obtained by evaporating the solvent from the gel) are

Figure 3. 1H MAS NMR (ω0H/2π = 500 MHz, DMSO-d6, 20 °C) of gPMW gels. B

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Figure 4. (A) SAXS/WAXS analysis of gPMW. Inset: comparison of the mid- to high-q regions of the X-ray scattering data for gP2.7k and g*P3.0k indicating the impact of endohedral functionalization on peaks a/a′ and b/b′. (B) SANS analysis of M12L24-linked polyMOC gels, with the fits indicated by curves of darker color; location of the d-spacing peak was indicated for each sample. The data are offset for clarity (see Figures in the Supporting Information for original data) (C) The relationship between the d-spacing (with the cage diameter subtracted), as determined from SAXS/WAXS and SANS of the polyMOC gels and xerogels and the MW of the PEG portion of the macromers (i.e., excluding the ligands; the tetraarm macromer (in gPStar) was treated as a pair of linear polymers with PEG MW equal to 1/2 that of the tetra-arm). The calculated maximum spacing of uniformly distributed MOCs and minimum spacing of solvent-free, densely packed MOCs are shown as the purple and blue lines, respectively (the blue shaded region represents the minimum spacing in the case that the MOC dense phase retains some solvent). The red shaded region represents MOC spacing which is governed by the preferred end-to-end distance (R0) of the polymer chain based on the relation R0 = b(Nα), where b and N are the Kuhn length and number of Kuhn segments, respectively. The upper bound of the red region assumes the exponent α = 0.58 for PEG in a good solvent, the lower bound of the region assumes the exponent α = 0.50 for a melt, and the red line assumes an exponent that falls between that of a melt and of a well-solvated polymer.

the limited q-range, and/or the very low MOC concentration in this material. While it is possible that the physical origin of peak “b” for gP11.6k is different from that of peaks “b” in the other polyMOCs, its absence in the SAXS/WAXS of analogous polyMOC gM11.5k (Figure S3) supports our inclusion of this feature in the pattern of peaks “b” (vide inf ra). To facilitate the assignment of peaks a and b, we first compared them to the SAXS/WAXS profile of a variant of gP2.7k (g*P3.0k) with endohedral “n-hexyl” functionality (see structure of corresponding macromer *P3.ok in Figure 2 and see Supporting Information for its synthesis). In g*P3.0k, the PEG length between junctions is identical to gP2.7k, and the ligands have the same bite angle, which controls the MOC stroichiometry; however, the n-hexyl groups bound to the ligands of *P3.ok point inside the MOC junctions and their crowding is expected to slightly expand the MOC. Indeed, polyMOC g*P3.0k exhibited analogous SAXS/WAXS features to gP2.7k, with the critical caveat that both peaks (a′ and b′) were shifted to slightly lower q (0.26 and 0.089 Å, respectively), which corresponds to a small expansion of the features for g*P3.0k (Figure 4A, inset). The junction dependence (by comparison of gP2.7k and g*P3.0k) and MW independence (by comparison of the gPMW series) of peaks a/a′ suggested that they were related to the MOC form factor. Indeed, peak a, which was more prominent in the xerogel analogue of gP2.7k (xgP2.7k), was fit well with a sphere form factor with radius of 1.7 nm (Figure S4). In the paddlewheel xerogel xgM2.7k (vide inf ra), the corresponding feature was fit with a sphere form factor with a radius of 0.53 nm (Figure S5). The fact that these data recapitulate not only the expected diameters of the M2L4 and M12L24 MOCs but also the subtle cage expansion expected from endohedral functionalization supports our peak assignment and is strong evidence for the presence of M2L4 and M12L24 MOCs in the corresponding polyMOC gels. Peak b was assigned to the scattering feature associated with inter-MOC separation. This assignment is based on the fact that the position of peak b corresponds to our expectation of

polyMOCs (Figure 3). In the case of gM11.5k, gM22.5k, and gMStar, nitrate or chloride anion (likely from residual Cl− salt in the macromer) encapsulation inside the paddlewheel gave rise to two sets of peaks in the aromatic region of the 1H MAS NMR spectra (Figures S1 and S2); since the paddlewheel junction geometry was unchanged, we neglected this observation in our subsequent mechanical and scattering analyses (vide inf ra). In the case of the gPMW polyMOCs, the peaks in the aromatic region of the NMR spectra (Figure 3) were broad relative to the gMMW polyMOCs; they became progressively broader with increasing MW, likely due to decreasing mobility34 as expected for higher-f junctions. Furthermore, small and relatively narrow peaks appeared along with the broad peaks in the aromatic region for gP11.6k and gP22.2k. Addition of 1.1 equiv of Pd(NO3)2·2H2O suppressed these peaks for gP11.6k but did not completely eliminate them in the case of gP22.2k (Figure 3). Their relatively narrow line width and chemical shifts (8.7 and 7.8 ppm) were consistent with uncoordinated ligands29 and indicated the presence of dangling chains in gP22.2k (roughly 10% based on integration of the peak at 8.7 ppm relative to the PEG backbone). The origin of these dangling chains is likely related to the exceedingly low concentration of the MOCs and the equilibrium nature of the MOC formation. To further study the size/stoichiometry of the MOC junctions and determine the inter-MOC distances (DJ) within these gels, we turned to small-/wide-angle X-ray scattering (SAXS/WAXS). The SAXS/WAXS profiles of gP2.7k, gP7.3k, gP11.6k, and gPStar exhibited a broad peak “a”, which was located at q = 0.32 Å−1, and another peak “b”, which shifted from 0.108 Å−1 (for gP2.7k) to progressively lower q with increasing MW (Figure 4A)note that gPStar fits within this trend when PStar is considered as a linked pair of linear macromers. The intensity of peak a waned with increasing MW; it was no longer visible for gP22.2k. Additionally, peak b broadened for gP11.6k and was not observed for gP22.2k, potentially due to further broadening, C

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Figure 5. (A) Examples of frequency sweeps for gP11.6 and gM11.5, a pair of representative new polyMOC gels. (B) G′ (black) and G″ (red) values measured at 5 rad s−1 for gPMW/Star and gMMW/Star, where MW refers to the molecular weight of single elastically active chains linking neighboring junctions. For gPStar and gMStar, MW of a single arm between two cross-links = 0.25·(macromer MW). Error bars represent standard errors. (C) Plot of rheometry-derived (black) and SANS-derived (red) branch functionality f as a function of MW for the same gels (gPStar and gMStar were treated as above in (B)). Error bars represent standard errors for rheometry-derived f and standard deviations for SANS-derived f.

remaining unchanged on cooling to 25 °C (Figure S8). The polymer R0 was computed to be 3.6 nm, which matched the expected R0 = 3.7 nm when treated as a real chain in the melt36 (see Methods in the Supporting Information). Meanwhile, R0 in xgP2.7k at 120 °C was 1.4 nm (Figure S9), consistent with further contraction of the MOC-dense regions upon complete desolvation. Small-angle neutron scattering (SANS) (Figures 4B, S10 and S11) provided an independent verification that the cage diameters were ∼3.5 nm in gP2.7k and its higher MW analogues and ∼1.1 nm in gM2.7k and its analogues (see Methods in the Supporting Information). These values were consistent with M12L24 and M2L4 junctions, respectively. Additionally, the presence of a MW-dependent peak was also observed in the SANS data (Figure 4B), although, intriguingly, the corresponding d-spacing exhibited a different dependence on MW compared to the SAXS/WAXS data (vide inf ra)likely, a consequence of the greater degree to which SAXS/WAXS is able to probe the intercage distance within cage-dense clusters compared to SANS. A more detailed examination of the MW dependence of the DJ parameter afforded further information about the polyMOC gel microstructure. Generally speaking, for a gel with a fixed volume and a given macromer MW, the interjunction spacing should be bounded by two limiting scaling relationships. The interjunction spacing is maximal when the junctions are uniformly distributed to fill the entire sample volume. We denote the position of this maximal spacing as dmax, which is indicated as a purple line in Figure 4C. On the other hand, the interjunction spacing is minimal if the macromers segregate into dense regions of polymer-linked MOCs, in which case the interjunction spacing is governed by the density of the melt-like phase. We denote the position of this minimal spacing as dmin, which is indicated as a blue line in Figure 4C (see Computation of d-spacings in the Supporting Information for details). This latter case corresponds physically to the conditions of the paddlewheel xerogels xgMMW, which exhibit peak positions whose scaling is in reasonable agreement with dmin. An offset between the xerogel data and the dmin curve is potentially due to the retention of some solvent or a greater-than-estimated excluded volume of the cage junctions. The blue shaded region assumes varying degrees of solvent retention (see Computation of d-spacings in the Supporting Information for further

the center-to-center distance between neighboring MOCs, and that it shifts to lower values of q (larger characteristic length, d = 2π/q) with increased MW. By assuming that the position of peak b reports on the characteristic center-to-center distance of adjacent network junctions, DJ, we can infer some details about the microscopic network structure of the polyMOC gels. A simple physical analysis of the measured values of DJ provides evidence that these polyMOCs are internally segregated into high- and low-MOC density regions, which suggests another level of structural hierarchy beyond the MOC cages. The q-value of 0.108 Å−1 for peak b for gP2.7k translated to a DJ of 5.8 nm, consistent with the M12L24 cage diameter (3.5 nm) and the end-to-end distance (R0 = 2.3 nm) of the polymer linker. Notably, this PEG R0 was significantly reduced compared to the theoretical value (root-mean-square R0 = 4.8 nm for the PEG-“diacyl” linker in P2.7k treated as a real chain in a good solvent; 35,36 see Methods in the Supporting Information). Moreover, a DJ of ∼10.6 nm (see Supporting Information) would be expected if the M12L24 junctions were isolated and homogeneously distributed in the polyMOC gel. These disparities between the observed and the predicted R0 and DJ values pointed to the assembly of M12L24 cages into cage-dense clusters. Consistent with this conclusion, the SAXS/ WAXS also revealed that DJ in these gels exhibited a strong dependence on MW (Figure 4A) but relative insensitivity to junction/macromer concentration for the same MW (Figure S6). Self-assembly of polymer-free M12L24 MOCs into a cagedense phase in DMSO is precedented,37 as is clustering of covalent junctions in PEG-based gels38together, these examples provide a basis for understanding the analogous MOC junction segregation phenomena observed in our polyMOC gels. In contrast to the M12L24 polyMOC gels, the SAXS/WAXS profiles of M2L4 paddlewheel materials were essentially featureless, with the exception of the amorphous halo (Figure S3). This fact was presumed to be due to diminished contrast between the low-stoichiometry junctions and the surrounding network/solvent. Indeed, evaporation of the solvent over 5 min at 120 °C revealed a MW-dependent peak “c”, which was, by analogy to the M12L24 polyMOCs, assigned to DJ as well as a shoulder at higher q (d, Figure S7) consistent with a sphere form factor with a radius of 0.53 nm (Figure S5, vide supra). The DJ of xgM2.7k was determined to be 4.7 nm at 120 °C, D

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M2L4 polyMOC gels, f increased steadily but much less steeply from 2.25 ± 0.01 (gM2.7k) to 3.0 ± 0.1 (gP22.5k) (Figure 5C). Additionally, gPStar and gMStar derived from tetra-arm macromers displayed significantly higher f for the MOC junctions compared to polyMOC gels with the same junction type and similar arm MW: 7.7 ± 0.4 for gPStar and 2.7 ± 0.1 for gMStar (Figure 5C). This observation is consistent with the fact that even if two arms of the tetra-arm macromer are connected to the same MOC (analogous to a primary loop for a linear polymer), the branched geometry of the star polymer renders this connection elastically effective. We also estimated f in our polyMOC gels by analyzing the SANS data with a new “loopy core-chain model” that explicitly describes the polyMOC gels as a dispersion of spherical cores grafted with a collection of linear chains and primary loops (note that higher-order topological defects and trapped entanglements are neglected). The only free parameter in this model is the number of primary loops, since other variables can be constrained (e.g., the maximum number of chains connecting two MOCs is 24 and the maximum number of primary loops is 12 in M12L24 gels). Note that in this model the tetra-arm macromers were treated as pairs of linear polymer chains, each with one-half the MW of the macromer. The loopy core-chain model revealed that for the M12L24 polyMOCs the average loop fraction per junction progressively diminishedand therefore f increasedwith greater macromer MW (Figure 5C). Thus, for gP2.7k, f = 7.12 ± 0.04, which increased to 8.54 ± 0.08 for gP7.3k and then to 9.36 ± 0.08 for gP11.6k; further approximate doubling of MW (gP22.2k) did not significantly change the primary loop content but led to a decrease in f to 6.94 ± 0.12 when ∼10% dangling chains (vide supra) were taken into account. For the M2L4-linked polyMOC gels, f did not display a particular dependence on MW (Figure 5C), consistent with the absence in the case of gM2.7k of a bias toward loop formation that existed in the crowded network of gP2.7k.29 Finally, gPStar displayed an f of 9.58 ± 0.08, significantly above what might be expected for the same MW in the absence of covalent tetra-functional cross-links in the macromer. Phantom network theory, as used here, does not separate the contribution to the gels’ G′ from polymer entanglement, which becomes important at higher MW.45 Consequently, rheometry is expected to overestimate f of MOC junctions for higher values of MW. On the other hand, the inability of our SANS model to discriminate between primary loops and catenated loops or trapped entanglements would lead to an underestimation of f. With these qualifications in mind, the values of f derived from SANS and rheometry are in reasonably good agreement, though it should be noted that neither of these approaches has been rigorously compared to unambiguously accurate methods for primary loop quantitation, which are currently not amenable to our polyMOCs.44,46−49 Nonetheless, these methods jointly establish that metallogels with recordhigh f (∼9−12) (e.g., gP11.6k and gPStar) can be attained through a combination of high-stoichiometry MOC junctions and optimally long and/or branched polymer chains. Also of note is the fact that despite relatively low concentrations of Pd2+ (∼6 mM for gP11.6k and 13.5 mM for gPStar) and M12L24 junctions (∼0.5 mM for gP11.6k and ∼1 mM for gPStar), these polyMOC gels displayed exemplary G′ values (6.0 ± 0.6 kPa and 13.4 ± 0.9, respectively), greater than or comparable to what has been reported for conventional metallogels with much

discussion). If we assume that interjunction spacing is governed by the preferred end-to-end statistics of the polymer linkers, then we arrive at a third, intermediate, scaling regime. We denote the position of this polymer-led spacing as dpoly, which is indicated as a red line in Figure 4C. The red shaded region indicates the range between an ideal polymer chain (in a melt) and a chain in a good solvent. The scaling indicated by dpoly represents a minimally strained polymer network. The details of how we generated the scaling relationships shown in Figure 4C are described in the Supporting Information. When dpoly falls below dmax, the system will be driven to alleviate network strain by segregating into junction-dense regions (with characteristic interjunction spacing given by dpoly) and corresponding junction-sparse regions. The likely existence of trapped entanglements, especially in the M12L24 systems with longer chains, could also contribute to this junction segregation. As discussed above, related segregation of junctions in covalent and nanocomposite polymer networks is not without precedent.38,39 Indeed, we speculate that the data point for gP11.6k that lies above the dmax curve, assuming that it corresponds to DJ (vide supra), represents a characteristic length scale for such segregated regions. Thus, collectively, the SAXS and SANS results provide indirect complementary evidence that the presence of M12L24 MOC junctions induces an additional level of hierarchy due to cage assembly into cagedense regions that are absent in M2L4-linked gels. Having shown that the macromer structure and MW induces significant changes in the corresponding polyMOC structure, we turned to rheometry to gain insight into the average branch functionality, f, of these polyMOC gels. Given f for a defect-free network ( f ideal), f for the real network can be estimated by analysis of the shear storage modulus (G′).29 G′ and loss moduli G″ at 5 rad s−1 were measured via frequency sweeps in oscillatory rheometry (Figure 5A,B, Figures S12 and S13, Table S1); deviations in the absolute values of G′ from previously reported values for gP2.7k and gM2.7k29 were ascribed to differences in the experimental setup (see Methods in the Supporting Information). The G′ of both types of polyMOCs decreased with increased MW (proportional to the drop in junction concentration), but notably, for all gels derived from linear macromers, M12L24-based polyMOC gels exhibited 1.6− 2.6 times greater G′ than the corresponding M2L4 linked ones (Figure 5A,B). Moreover, the G″ values were also substantially higher for polyMOC gels with M12L24 junctions compared to M2L4 oneseven in the case of tetra-arm macromers, when the two types of gels had virtually identical G′ (Figure 5A,B). This disparity indicates that the former polyMOCs have an enhanced ability to dissipate stress, possibly due to the presence of a greater number of topological defects.36,40 Lastly, the increased G″ observed for gP22.2k compared to gP11.6k is consistent with the appearance of dangling chain defects40 in the former as observed by MAS NMR (vide supra). The branch functionality f in the polyMOC gels was estimated from the rheometry data through the application of phantom network theory,36,41−44 as in our previous report;29 a modified, more general equation was applied in the case of gPStar and gMStar because their network contained alternating junctions of two types: covalent with f = 4 and coordination with f ideal = 24 or 4 (Table S2; see Methods in the Supporting Information for details of computations). Rheometry-derived f values for the M12L24 polyMOCs increased with MW from 5.9 ± 0.4 (gP2.7k) to 9.9 ± 0.3 (gP7.3k) and plateaued at ∼12 (12.6 ± 1.0 for gP11.6k and 11.8 ± 0.3 for gP22.2k). Meanwhile in the E

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used in this work. This research also used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DEAC02-06CH11357. We also thank Dr. X. Zuo for assistance with X-ray scattering experiment setup. Lastly, we are grateful to Prof. S. Buchwald’s group for a generous donation of XPhos precatalyst and to Vivian Tian for assistance with the synthesis of compound 2 (Supporting Information). R.G.G. thanks the National Institutes of Health (EB-002026) for support of the MAS NMR spectroscopy work.

higher polymer (10−11 wt %) and/or metal content (up to ∼50 mM).9,10,14,15

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CONCLUSIONS The effect of high-stoichiometry junctions on G′ in our Pd-bispyridyl-derived polyMOC gels is analogous to the enhancements afforded by the use of clay nanosheets,50−52 silica, or iron oxide nanoparticles53−55 as cross-linking modalities in nanocomposite gels. However, compared to these examples, polyMOC gels offer superior precision in junction assembly, which lends greater control over numerous levels of structural hierarchy: (1) endohedral contents of the MOCs, (2) the MOCs themselves, with their “exohedral” functionality, including looped, dangling, and elastically active polymer chains, (3) assemblies of MOCs,37 and (4) the average structure of the network on the microscopic scale. Such structural hierarchy, which is found more frequently in biological gels2 than in synthetic ones, has far-reaching implications in the design of functional materials. However, the critical connections among all of these levels of hierarchy are only beginning to be elucidated. Particularly intriguing is the nature of MOC segregation within polyMOC gels: although the assembly of polymer-free M12L24 cages has been investigated in detail,37 it does not necessarily directly translate to polymer networks. Thus, our X-ray and neutron scattering studies provide the first evidence for this process in polyMOC gels, and this phenomenon will be further elucidated in forthcoming experimental and computational studies.

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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.6b01607. Materials and methods, detailed synthetic and characterization procedures, calculations, supplementary figures, spectra, and tables (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.A.J.). Present Addresses

A.V.Z.: Department of Chemistry, University of California, Berkeley, Berkeley, CA 94720. M.Z.: Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06511. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.A.J. thanks the National Science Foundation (NSF) (CHE1334703 and CHE-1351646) and the MIT Energy Initiative for support of this work. A.V.Z. thanks the Department of Defense National Defense Science and Engineering Graduate program and Intel for graduate fellowships that helped to support this work. This work made use of MIT DCIF Shared Experimental Facilities (National Institutes of Health, 1S10RR013886-01; NSF, CHE-0234877), and we are grateful to Dr. Li Li (DCIF) for HRMS data acquisition. We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the neutron research facilities F

DOI: 10.1021/acs.macromol.6b01607 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b01607 Macromolecules XXXX, XXX, XXX−XXX