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Conversion of Supramolecular Clusters to Macromolecular Objects

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Science  22 Jan 1999:
Vol. 283, Issue 5401, pp. 523-526
DOI: 10.1126/science.283.5401.523

Abstract

In a reaction proceeding within a nanoscopic volume, supramolecular clusters were transformed to polymer objects while retaining their shape and size. Spatial isolation of the cross-linkable blocks of oligobutadiene that were involved in this stitching reaction was achieved by self-assembly of the molecules that made up the clusters. Thermal activation of cross-linking yielded macromolecules (molecular weight of 70,000) with a narrow size distribution that was similar to that of the supramolecular clusters. The macromolecules obtained have an anisotropic shape (2 nanometers by 8 nanometers), as determined by electron microscopy and small-angle x-ray scattering, and form materials that exhibit a liquid crystalline state.

A key difference between synthetic polymers and proteins is the well-defined folded shape of proteins in solution; polymers, with their simpler composition, tend to exhibit highly symmetrical nanostructures (1, 2). Polymers with more sophisticated functions might be realized if synthetic pathways were found to create macromolecules with distinct and permanent shapes (regardless of state) or to create low-symmetry objects that were different from highly symmetrical dendrimers (3). Molecular object polymers (1, 4) have a permanent chemical map on their surfaces and pack in specific ways to form condensed phases. Self-assembly is a rapid pathway from small molecules to supramolecular nanometer-sized objects that cannot be synthesized by conventional chemical reactions (5,6). We recently synthesized triblock rodcoil molecules that self-assemble into mushroom-shaped nanostructures (5), which exist in the solid state but could disassemble in solution or in the melt. Here, we report on the synthesis of soluble nanostructures with a molecular weight (MW) of ∼70,000 through cross-linking of supramolecular clusters. The covalent polymers obtained are highly anisotropic, with an aspect ratio of 4, and have a characteristic shape that resembles a mushroom. Furthermore, the precursor molecular aggregates and the resulting macromolecules appear identical when viewed with an electron microscope, which suggests that this cross-linking reaction is essentially a size- and shape-invariant chemical transformation.

Our first approach to creating shape-invariant molecular objects was to synthesize molecules that organized into two-dimensional (2D) structures with the proper orientation to react (4). Cross-linkable groups in these supramolecular structures offer the possibility of covalently locking the aggregate's original shape, but in many cases, such reactions resulted in either insoluble materials or in polymer products with very broad MW distributions (1). This is due to the difficulty of controlling the planar dimensions (xy) of cross-linkable regions in these 2D structures. Thus, it is important to control the size of cross-linkable regions by preassembled nanostructures in a way that limits the volume of reaction and the extent of interconnection. This goal requires spatial isolation of cross-linkable segments within one supramolecular unit.

We synthesized triblock rodcoil molecules that had the structure shown in Scheme 1. The synthesis and complete characterization of this molecule will be reported elsewhere (7). Rodcoil molecules 1 are composed of three different blocks: a block of atactic oligostyrene; a block of oligobutadiene with a random sequence of 1,4- and 1,2-addition units, based on nuclear magnetic resonance (NMR) data; and a rodlike block that is identical in all of the molecules. The butadiene block serves as a segment in which carbon-carbon double bonds can generate cross-links at high temperatures (8–12). The aromatic identical segments in all molecules are strongly driven to aggregate and crystallize (partly because of π-π overlap between biphenyl units). However, the structural diversity of the triblock molecules prevents the formation of three-dimensional crystals. As previously suggested (5), the oligostyrene segment provides steric elements that limit the size of the aggregates. An important feature of molecules 1 is their CF3-terminus [as opposed to the OH-terminus of the molecule reported in (5)], which has a very large dipole moment and is also an extremely hydrophobic functional group (13). The CF3 group also appears to provide an additional driving force for self-assembly.

Figure 1

GPC traces of precursor 1 samples that were annealed for different periods of time. Each trace shows the peak corresponding to 1(M̅ n = 3200 andM̅ w/M̅ n= 1.08). Molecular weight and polydispersity of the macromolecular product (relative to polystyrene standards) were found to beM̅ n= 63,000 andM̅ w/M̅ n= 1.07, after 5 hours;M̅ n= 67,800 andM̅ w/M̅ n= 1.11, after 8 hours;M̅ n= 72,600 andM̅ w/M̅ n= 1.21, after 11 hours; andM̅ n = 75,200 andM̅ w/M̅ n= 1.35, after 20 hours.

Figure 2

(left). GPC traces of precursor 1 (blue curve) and the high molar mass product (red curve) isolated by flash chromatography after 8 hours. The molecular representation of the macromolecule contains 20 molecules of precursor 1 stitched by covalent bonds through butadiene blocks.

Figure 3

(right). (A) A TEM micrograph at high magnification of a film that was formed by regularly sized and shaped supramolecular clusters composed of triblock molecules1. (B) A TEM micrograph at high magnification of a film that was formed by the isolated molecular objects. The wide-angle electron diffraction patterns are shown in the upper right corner of (A) and (B). The contrast in the image is produced by osmium tetroxide that stains double bonds in polybutadiene segments and also by the crystalline order among rod segments based on the observed diffraction pattern. The diffraction contrast should be substantial, considering the intensity of the Debye rings in the pattern. (C) An optical micrograph between crossed polarizers showing the smectic to isotropic phase transition of precursor 1 at 330°C. (D) An optical micrograph between crossed polarizers showing the smectic texture of the macromolecular product at 420°C.

Figure 4

Two-dimensional projections of molecules linked in the supramolecular cluster (right) based on a random walk computer simulation. The stitching conversion (SC), which is the percentage of covalently linked molecules in the cluster, increases with the number of reactive layers. The number of layers is equal to the number of butadiene structural units in precursor molecules (left).

Scheme 1

Triblock rodcoil molecules 1.

Annealing precursor 1 in a liquid crystalline (LC) phase at 250°C (14) under a nitrogen atmosphere for several hours resulted in the formation of a product with a high molar mass, a narrow polydispersity, and a MW of ∼70,000. Gel permeation chromatography (GPC) traces (Fig. 1) of samples that were annealed for different periods of time and then dissolved in tetrahydrofuran (THF) (1 weight %) show that annealing time strongly affects the yield, which can be as high as 80 weight % after annealing for 20 hours. A slight increase in MW and polydispersity [the ratio of weight average (M̅ w) to number average (M̅ n) MWs] occurred with further annealing. However, the calculated degree of polymerization with polystyrene standards remained constant at 22 ± 2 (MW ≈70,000). Moreover, the material was completely soluble (even after 20 hours of annealing), and the formation of an insoluble gel-fraction was not observed. The GPC traces also show that products with a MW less than that of the precursor do not form, suggesting that decomposition reactions do not occur. The use of conventional flash chromatography resulted in the complete isolation of the new product because it has a considerably lower retention factor relative to that of precursor 1 in most organic solvents. The GPC traces of starting material and isolated product reveal narrow polydispersities, which are on the order of 1.08 and 1.11, respectively (see Fig. 2).

Small-angle x-ray scattering (SAXS) revealed that both the monomeric precursor 1 and the isolated macromolecular product have layered structures in the solid state with a primaryd spacing of 81 Å. Nonlinear optical experiments showed that supramolecular clusters have a noncentrosymmetric structure (15), and therefore the molecules must pack in a parallel (mushroomlike) manner and not in an antiparallel (dumbbell-like) manner. Most important, the layer periodicity and narrow polydispersity suggest a strong correlation between the covalent structure of the product and that of its supramolecular precursor. Moreover, the polymerization is apparently confined to the volume of the supramolecular cluster and is therefore nanoscopic, connecting a nearly discrete number of molecules. Because precursor 1 is in an LC phase at the polymerization temperature, the spatial isolation of cross-linkable blocks that is required to form well-defined objects must prevail in the mesophase.

Transmission electron microscopy (TEM) studies of triblock molecules1 revealed a solid state structure consisting of aggregates ∼2 nm in diameter (Fig. 3A). Other similar molecules that were synthesized in our laboratory also formed aggregates but had larger dimensions (5, 16). Similar structures of 2 nm were observed for the isolated polymer object material (Fig. 3B), which indicate that aggregation and rod packing were not altered by polymerization. The thickness of layers (8 nm) revealed by SAXS is four times the diameter of the aggregates (2 nm). Thus, the supramolecular units are highly anisometric in shape (8 nm by 2 nm). More important, the electron diffraction patterns show the crystal's primary spacing to be 4.5 Å, which is presumably the distance between biphenyl groups. On the basis of this number and the cross section of nanostructures observed by TEM (20 Å), we estimated that there are (on average) 23 ± 2 molecules in each supramolecular aggregate. On the basis of the electron diffraction pattern, we found that the rod axes in the cluster are normal to the layers and are perpendicular to the plane of the TEM micrographs (before and after polymerization). The combination of SAXS, nonlinear optical experiments, and TEM suggests that triblock molecules1 pack into the mushroom-shaped nanostructures that lackD h symmetry (5).

The observed clusters are smaller by about a factor of 4 in comparison with those described in (5). However, their precursor differs chemically in a subtle way (a CF3-terminus versus an OH-terminus). In order to understand this difference, we synthesized and studied more than 20 analogous molecules with varying chemical structure (7), many of which had the same structure as that of precursor 1, except for the terminal group. This slight structural change resulted in a remarkable difference in properties, and none of the analogous rodcoils could be converted to well-defined molecular objects. Annealing of these molecules leads to products that are either insoluble or have very broad polydispersity (M̅ w/M̅ n> 8). However, rodcoil molecules with a CF3-terminal group but with a different length of either the coil or rod segments have shown similar reactive behavior. Their GPC traces revealed the formation of high-molar-mass products with a narrow polydispersity within the range from 1.12 to 1.25 and MWs of up to 100,000. These data suggest that the CF3 group has an important role in the observed phenomenon. We think that the small size of the aggregates, possibly as a result of molecular geometry or dipole moment, may result in the observed nanoscopic reaction. Substitution of the CF3 group by other termini (such as methyl, carboxyl, hydroxyl, vinyl, phenyl, and chromone) causes a substantial change in the wide-angle electron diffraction pattern. This pattern shows a much higher degree of order in the nanosized aggregates of molecules with a CF3-terminus. As a result, the spatial isolation of the butadiene block is only achieved in rodcoils with a CF3-terminus, thus preventing the interconnection of nanostructures.

Nuclear magnetic resonance (NMR) measurements (17) revealed that conversion of the clusters into molecular objects by cross-linking is accompanied by a 70% loss of unsaturated bonds. According to the mechanism proposed for this reaction (8–12), each butadiene structural unit can produce only one cross-link. On the basis of this information, we ran a multilayered random walk simulation of the nanoscopic reaction using a program that was developed previously in our laboratory (1). The top view of the 2D projections of linked molecules for different numbers of layers is shown in Fig. 4. Matching the experimental observation, the cluster used in the simulation contained 25 molecules (5 molecule by 5 molecule lattice) and became completely stitched by covalent bonds with only six layers of reactive groups (corresponding to six butadiene units per molecule). Because the average number of layers in the experimental system is nine, the conversion of supramolecular clusters into polymer objects is feasible, on the basis of the simulation.

Additional information confirming the structure of the polymer objects was obtained by 1H NMR experiments (17). On the basis of integration of the NMR spectrum of the cross-linked product, we estimated that annealing causes the formation of six cross-links per molecule of precursor 1. This result implies that the cross-linking density within each macromolecule is very high and that this feature should contribute to the shape invariance of the formed objects. Furthermore, the relaxation time T 1 of the biphenyl protons increased from 1.5 to 2.9 s after polymerization, and this T 1 value is considerably greater than that of the aliphatic protons (1.1 s) and styrenic aromatic protons (1.4 s), whose T 1values after polymerization remained constant within experimental error. The observation suggests that, even in solution, rod segments have the strongest interaction within the object. This interaction should also contribute to the shape persistence of the covalent objects as the solid material is dissolved.

Dynamic light scattering experiments for solutions in THF (0.1 weight %) only revealed evidence of nanostructures after cross-linking. Conversion of molecules 1 into covalent polymers results in a strong stabilization of the LC phase of the system. The initial precursor undergoes a phase transition from the smectic to the isotropic state at 330°C (Fig. 3C), whereas the polymerized objects form a highly birefringent melt at 160°C, and isotropization is never observed before chemical decomposition at 430°C (Fig. 3D). Differential scanning calorimetry indicated that, contrary to precursor 1, the molecular object polymers do not exhibit any first-order phase transitions. The observation suggests that these mushroom-shaped objects cannot be stable in the isotropic state, possibly because their particular shape and surface chemical map would create an isotropic liquid of high free volume and fluctuations between favorable and very unfavorable contacts.

In nature, there is a mushroom-shaped object that is formed by a heptameric aggregate of protein molecules (18). This supramolecular structure, known as α-hemolysin, can implant itself into cell membranes, and it is synthesized by the human pathogenStaphylococcus aureus. This molecular aggregate also contains a pore to transport molecules and ions across cell membranes (19). In contrast to α-hemolysin, our cross-linked aggregate could function as an “undenaturable” molecular object polymer, and with proper modification, it could serve as a macromolecule that modifies cell behavior or creates interesting materials.

  • * To whom correspondence should be addressed. E-mail: s-stupp{at}nwu.edu

  • Present address: Department of Materials Science and Engineering and Department of Chemistry, Northwestern University, Evanston, IL 60208, USA.

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