Design of materials with supramolecular polymers

Abstract

One hundred years ago Hermann Staudinger was strongly criticized by his scientific peers for his macromolecular hypothesis, but today it is hard to imagine a world without polymers. His hypothesis described polymers as macromolecules composed of large numbers of structural units connected by covalent bonds. In the 1990s the concept of supramolecular polymers emerged in the scientific literature as discrete entities of large molar mass comparable to that of classical polymers but built through non-covalent bonds among monomers. Supramolecular polymers exist in biological systems, and potentially blend the physical properties of covalent polymers with unique features such as high degrees of internal order within the polymeric structure, defined shapes, and novel dynamics. This trend article provides a summary of seminal contributions in supramolecular polymerization and provides recent examples from the Stupp laboratory to demonstrate the potential applications of an exciting class of materials composed fully or partially of supramolecular polymers. In closing, we provide our perspective on future opportunities provided by this field at the onset of a second century of polymers. It is our objective here to demonstrate that this second century could be as prosperous, if not more so, than the preceding one.

Keywords: Biomaterials; Functional polymers; Hierarchical assembly; Photocatalysis; Self-assembly; Supramolecular polymers.

Graphical abstract

Fig. 1

Examples of hierarchical supramolecular polymers with the monomer (inner circle), supramolecular polymers (middle circle), and hierarchical assembly (outer circle) in each case schematically represented. Clockwise from the top, two-dimensional (2D) polymers that form single-crystal stacks, asymmetric 0D supramolecular polymers which form polar stacks, dendron rod coil monomers that assemble into twisted nanoscale ribbons used to template CdS crystal growth, peptide amphiphile monomers that form filament-like supramolecular polymers which can bundle and interact intimately with cells, and most recently chromophore amphiphile monomers that self-assemble into crystalline supramolecular polymers with enhanced photocatalytic properties. , Copyright 1993. Reproduced with permission from the American Association for the Advancement of Science. , Copyright 1997. Reproduced with permission from the American Association for the Advancement of Science. , Copyright 2001. Reproduced with permission from the American Chemical Society. , Copyright 2002. Reproduced with permission from Wiley-VCH. , Copyright 2001. Reproduced with permission from the American Association for the Advancement of Science. , Copyright 2014. Reproduced with permission from the Nature Publishing group.

Fig. 2

Reversible hierarchical assembly of supramolecular polymers. a) Representative scanning electron microscopy (SEM) images of the hierarchical assembly with molecular graphics demonstrating molecular interaction of C₁₆V₃A₃E₃K₃ (left), C₁₆V₃A₃E₃-PEG₂-K₃ (middle), and C₁₆V₃A₃E₄-PEG₆-K₄ (right). b) Representative SEM images of C₁₆V₃A₃E₃-PEG₂-K₃ at pH 3, 7, and 10. The SEM micrograph for pH 7 has been false-colored to emphasize the hierarchical superstructure formation. c) Small-angle X-ray scattering (SAXS) patterns of C₁₆V₃A₃E₃-PEG₂-K₃ monomer annealed at i) pH 4, ii) pH 7, iii) pH 10, and iv) C₁₆V₃A₃E₃ monomer annealed at pH 7. , Copyright 2018. Reproduced with permission from the American Association for the Advancement of Science.

Fig. 3

Glycopeptide supramolecular polymer for tissue regeneration. a) cryogenic transmission electron micrograph of the sulfated glycopeptide amphiphile which forms filamentous nanofibers, with molecular graphic overlay of the assembled nanofiber. b) Confocal microscopy images of Cy-3 fluorescently labeled glycopeptide amphiphile assembly (left) and after mixing with Cy-5 labeled BMP-2 (middle). The merged image (right) demonstrates the colocalization of the protein with the supramolecular polymers. c) Representative sagittal cross-sectional images of L4-L5 posterolateral spine specimens with hematoxylin and eosin staining following treatment in a rat spinal fusion model. Insets are representative volume renderings from micro-computed tomography analysis with yellow arrows indicating bone fusion. , Copyright 2017. Reproduced with permission from the Nature Publishing Group.

Fig. 4

Crystalline supramolecular polymers for hydrogen production. a) Molecular structure and space-filling model of the chromophore amphiphile monomers. b) Cryogenic transmission electron micrograph of the crystalline supramolecular polymers which are approximately 40 nm in width; differences in contrast are a result of the ribbon orientation either face-on (white arrow) or edge-on (black arrow). c) Schematic representation of the antiparallel packing of monomers to form the interdigitated supramolecular polymer ribbons. d) Schematic representation of the porous photocatalytic hydrogel for photoinduced H₂ production containing crystalline supramolecular polymers (red), with entrapped nickel catalyst (green) and the sacrificial electron donor, ascorbic acid (blue). Water molecules have been omitted from the schematic for clarity. e) Plot of the amount of H₂ produced in the presence of the photocatalytic hydrogel. H₂ is not generated when chromophore, catalyst, sacrificial reagent or light is absent in the photocatalytic system (error bar = 1 s.d., n = 12 trials (or n = 3 for each control experiment)). f) Plot of the catalytic turnovers of H₂ produced from the β-phase helices compared to α-phase structures in the presence of a Mo₃S₁₃²⁻ proton reduction catalyst with corresponding photographs of the β-phase and α-phase gels. , Copyright 2014. Reproduced with permission from the Nature Publishing Group. , Copyright 2017. Reproduced with permission from the American Chemical Society. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

Fig. 5

Control over morphology of supramolecular polymers formed by electron donor monomers affects photovoltaic device performance. (a) Diketopyrrolopyrrole (DPP)-based asymmetric electron donor molecule design with variations in the hydrogen bonding capabilities at one end of the molecule. (b) Molecular graphic representation demonstrating the formation of supramolecular polymers using DPP monomers with amide versus ester bonds. (c) Schematic representation demonstrating how the effective competition of hydrogen bonding over extensive π−π stacking results in supramolecular polymer morphologies that lead to higher photovoltaic efficiencies. (d) Current-voltage (I-V) sweep curves of bulk heterojunction solar cells containing either the DPP ester or amide supramolecular polymers with [6,6]-phenyl-C₇₁-butyric acid methyl ester as the electron acceptor. , Copyright 2015. Reproduced with permission from the American Chemical Society.