Supramolecular polymers

Supramolecular polymers are a kind of polymers whose monomeric units hold together via highly directional and reversible non-covalent interactions.[1][2] Unlike conventional bonded polymers, supramolecular polymers engage in a variety of non-covalent interactions that define their properties. These interactions include hydrogen bonding, π-π interaction, metal coordination, and host-guest interaction.[1] Owing to the presence of these reversible noncovalent interactions, supramolecular polymers exhibit dynamic properties such as self-healing.[3]

Fig 1. An example of supramolecular polymers with quadruple hydrogen bonds.[4]

## Contents

* 1 Properties of supramolecular polymers
* 1.1 Self-healing property
* 1.2 Optoelectronic property
* 1.3 Biocompatible property
* 2 Examples of supramolecular polymers
* 2.1 Hydrogen bonding-based self-healing supramolecular polymers
* 2.2 Polymers containing coordination complexes
* 2.3 π-π interaction-based self-healing supramolecular polymers
* 2.4 Potential biomedical applications
* 2.4.1 Drug delivery
* 2.4.2 Gene transfection
* 2.4.3 Others
* 3 References

## Properties of supramolecular polymers

### Self-healing property

Due to the nature of supramolecular polymers, the noncovalent interactions make supramolecular polymers more dynamic and reversible. Such properties enable supramolecular polymers to construct a dynamic and reversible network, which are able to develop self-healing materials based on noncovalent bonds.[5] Compared with self-healing materials based on covalent bonds, these supramolecular polymers-based self-healing materials can restore the initial structure and function of polymers before being exposed to damages, and can also undergo repeating damage-heal process.

### Optoelectronic property

To achieve the light-to-charge conversion is the prerequisite step in artificial photosynthesis systems.[6] By incorporating electron donor and electron acceptor into the supramolecular polymers, a number of artificial systems, including photosynthesis system, can be constructed. Due to the existence of more than one interactions (π-π interaction, hydrogen bonding interaction and the like), electron donor and electron acceptor can be held together in a proper proximity to afford long-lived charge separated states.[6] Then a light-to-charge conversion system with faster photoinduced electron transfer and higher electron-transfer efficiency can be achieved in these artificial polymers.[2][6]

### Biocompatible property

It is quite common that biomolecules, such as DNA,[7] protein [8] and the like, come into being through various noncovalent interactions in biological system. Likewise, supramolecular polymers assembles itself by means of combination of noncovalent interactions. Such formation manner endows supramolecular polymers with features, being more sensitive to external stimuli and able to render reversibly dynamic changes in structures and functions.[9] By modifying monomeric units of supramolecular polymers with water-soluble pendants, bioactive moieties as well as biomarkers, supramolecular polymers can realize various kinds of functions and applications in biomedical field.[10] At the same time, their reversible and dynamic nature make supramolecular polymers bio-degradable,[11][12] which surmounts hard-to-degrade issue of covalent polymers and makes supramolecular polymers a promising platform for biomedical applications. Being able to degrade in biological environment lowers potential toxicity of polymers to a great extent and therefore, enhances biocompatibility of supramolecular polymers.[13][14]

Fig 2. A titin-mimicking supramolecular polymers, with quadruple hydrogen bonding between 2-ureido-4[1H]-pyrimidone (UPy) moiety.[15]

## Examples of supramolecular polymers

Three types self-healing materials are illustrated here: hydrogen bonding-based, metal coordination-based and π-π interaction-based self-healing supramolecular polymers.

### Hydrogen bonding-based self-healing supramolecular polymers

A bivalent poly(isobutylene)s (PIBs) with barbituric acid functionalized at head and tail.[16] Multiple hydrogen bonding existed between the carbonyl group and amide group of barbituric acid enable it to form a supramolecular network. In this case, the snipped small PIBs-based disks can recover itself from mechanical damage after several-hour contact at room temperature.

### Polymers containing coordination complexes

pH-control self-healing supramolecular polymers.[17] The formation of Mono-, bis- and triscatehchol-Fe3+ complexes can be manipulated by pH, of which the bis- and triscatehchol-Fe3+ complexes show elastic moduli as well as self-healing capacity. For example, the triscatehchol-Fe3+ can restore its cohesiveness and shape after being torn.

Fig 3. An example of metal coordination-based self-healing supramolecular polymer. After standing for 24 hours, the cut pieces were rejoined.[18]

### π-π interaction-based self-healing supramolecular polymers

Chain-folding polyimide and pyrenyl-end-capped chains give rise to supramolecular networks.[19]

### Potential biomedical applications

With the excellent nature in biodegradation and biocompatibility, supramolecular polymers show great potential on development of drug delivery, gene transfection and etc.[10]

#### Drug delivery

Multiple cellular stimuli could induce responses in supramolecular polymers.[9][20][10] The dynamic molecular skeletons of supramolecular polymers can be depolymerized when exposing to the external stimuli like pH in vivo. On the basis of this property, supramolecular polymers are capable of being a drug carrier. Making use of hydrogen bonding between nucleobases to induce self-assemble into pH-sensitive spherical micelles.

#### Gene transfection

Effective and low-toxic nonviral cationic vectors are highly desired in the field of gene therapy.[10] On account of the dynamic and stimuli-responsive properties, supramolecular polymers offer a cogent platform to construct vectors for gene transfection. By combining ferrocene dimer with β-cyclodextrin dimer, Aredox-control supramolecular polymers system has been proposed as a vector. In Cos-7 cells, this supramolecular polymersic vector can release enclosed DNA upon exposing to H2O2 and achieve gene transfection.[21]

#### Others

Rationally designed supramolecular polymers-based polymerss can simultaneously meet the requirements of aqueous compatibility, bio-degradability, biocompatibility, stimuli-responsiveness and other strict criterion.[10] Consequently, supramolecular polymers can be applied to the biomedical field as a robust system. Other than applications mentioned above, other important and fascinating biomedical applications, like protein delivery,[22][23] bio-imaging and diagnosis[24][25] and tissue engineering,[26][27] are also well developed.

## References

1. ↑ 1.0 1.1 Brunsveld, L.; Folmer, B. J.; Meijer, E. W.; Sijbesma, R. P., Supramolecular polymers. Chemical reviews 2001, 101 (12), 4071-98.
2. ↑ 2.0 2.1 De Greef, T. F.; Smulders, M. M.; Wolffs, M.; Schenning, A. P.; Sijbesma, R. P.; Meijer, E. W., Supramolecular polymerization. Chemical Reviews 2009, 109 (11), 5687-754.
3. ↑ Yang, L.; Tan, X.; Wang, Z.; Zhang, X., Supramolecular Polymers: Historical Development, Preparation, Characterization, and Functions. Chemical reviews 2015, 115 (15), 7196-239.
4. ↑ Cafferty, B. J.; Fialho, D. M.; Khanam, J.; Krishnamurthy, R.; Hud, N. V., Spontaneous formation and base pairing of plausible prebiotic nucleotides in water. Nature Communications 2016, 7, 11328.
5. ↑ Herbst, F.; Dohler, D.; Michael, P.; Binder, W. H., Self-healing polymers via supramolecular forces. Macromolecular rapid communications 2013, 34 (3), 203-20.
6. ↑ 6.0 6.1 6.2 Peurifoy, S. R.; Guzman, C. X.; Braunschweig, A. B., Topology, assembly, and electronics: three pillars for designing supramolecular polymers with emergent optoelectronic behavior. Polymer Chemistry 2015, 6 (31), 5529-5539.
7. ↑ Watson, J. D.; Crick, F. H., Molecular structure of nucleic acids; a structure for deoxyribose nucleic acid. Nature 1953, 171 (4356), 737-8.
8. ↑ Pauling, L.; Corey, R. B.; Branson, H. R., The structure of proteins; two hydrogen-bonded helical configurations of the polypeptide chain. Proceedings of the National Academy of Sciences of the United States of America 1951, 37 (4), 205-11.
9. ↑ 9.0 9.1 Yan, X.; Wang, F.; Zheng, B.; Huang, F., Stimuli-responsive supramolecular polymeric materials. Chemical Society Reviews 2012, 41 (18), 6042-65.
10. ↑ 10.0 10.1 10.2 10.3 10.4 Dong, R.; Zhou, Y.; Huang, X.; Zhu, X.; Lu, Y.; Shen, J., Functional supramolecular polymers for biomedical applications. Advanced materials 2015, 27 (3), 498-526.
11. ↑ Lim, Y. B.; Moon, K. S.; Lee, M., Recent advances in functional supramolecular nanostructures assembled from bioactive building blocks. Chemical Society Reviews 2009, 38 (4), 925-34.
12. ↑ Petkau-Milroy, K.; Brunsveld, L., Supramolecular chemical biology; bioactive synthetic self-assemblies. Organic & biomolecular chemistry 2013, 11 (2), 219-32.
13. ↑ Li, J.; Li, X.; Ni, X.; Wang, X.; Li, H.; Leong, K. W., Self-assembled supramolecular hydrogels formed by biodegradable PEO-PHB-PEO triblock copolymers and alpha-cyclodextrin for controlled drug delivery. Biomaterials 2006, 27 (22), 4132-40.
14. ↑ Appel, E. A.; del Barrio, J.; Loh, X. J.; Scherman, O. A., Supramolecular polymeric hydrogels. Chemical Society Reviews 2012, 41 (18), 6195-214.
15. ↑ Kushner, A. M.; Vossler, J. D.; Williams, G. A.; Guan, Z., A biomimetic modular polymer with tough and adaptive properties. Journal of the American Chemical Society 2009, 131 (25), 8766-8.
16. ↑ Herbst, F.; Seiffert, S.; Binder, W. H., Dynamic supramolecular poly(isobutylene)s for self-healing materials. Polymer Chemistry 2012, 3 (11), 3084-3092.
17. ↑ Holten-Andersen, N.; Harrington, M. J.; Birkedal, H.; Lee, B. P.; Messersmith, P. B.; Lee, K. Y.; Waite, J. H., pH-induced metal-ligand cross-links inspired by mussel yield self-healing polymer networks with near-covalent elastic moduli. Proceedings of the National Academy of Sciences of the United States of America 2011, 108 (7), 2651-5.
18. ↑ Nakahata, M.; Takashima, Y.; Yamaguchi, H.; Harada, A., Redox-responsive self-healing materials formed from host-guest polymers. Nature Communications 2011, 2, 511.
19. ↑ Burattini, S.; Colquhoun, H. M.; Fox, J. D.; Friedmann, D.; Greenland, B. W.; Harris, P. J.; Hayes, W.; Mackay, M. E.; Rowan, S. J., A self-repairing, supramolecular polymer system: healability as a consequence of donor-acceptor pi-pi stacking interactions. Chemical communications 2009, (44), 6717-9.
20. ↑ Ma, X.; Tian, H., Stimuli-responsive supramolecular polymers in aqueous solution. Accounts of chemical research 2014, 47 (7), 1971-81.
21. ↑ Dong, R.; Su, Y.; Yu, S.; Zhou, Y.; Lu, Y.; Zhu, X., A redox-responsive cationic supramolecular polymer constructed from small molecules as a promising gene vector. Chemical communications 2013, 49 (84), 9845-7.
22. ↑ Kameta, N.; Masuda, M.; Mizuno, G.; Morii, N.; Shimizu, T., Supramolecular nanotube endo sensing for a guest protein. Small 2008, 4 (5), 561-5.
23. ↑ Kameta, N.; Yoshida, K.; Masuda, M.; Shimizu, T., Supramolecular Nanotube Hydrogels: Remarkable Resistance Effect of Confined Proteins to Denaturants. Chemistry of Materials 2009, 21 (24), 5892-5898.
24. ↑ Janib, S. M.; Moses, A. S.; MacKay, J. A., Imaging and drug delivery using theranostic nanoparticles. Advanced drug delivery reviews 2010, 62 (11), 1052-63.
25. ↑ Barreto, J. A.; O'Malley, W.; Kubeil, M.; Graham, B.; Stephan, H.; Spiccia, L., Nanomaterials: applications in cancer imaging and therapy. Advanced materials 2011, 23 (12), H18-40.
26. ↑ Shah, R. N.; Shah, N. A.; Del Rosario Lim, M. M.; Hsieh, C.; Nuber, G.; Stupp, S. I., Supramolecular design of self-assembling nanofibers for cartilage regeneration. Proceedings of the National Academy of Sciences of the United States of America 2010, 107 (8), 3293-8.
27. ↑ Dankers, P. Y.; Harmsen, M. C.; Brouwer, L. A.; van Luyn, M. J.; Meijer, E. W., A modular and supramolecular approach to bioactive scaffolds for tissue engineering. Nature Materials 2005, 4 (7), 568-74.

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