Hui-Qing Peng
email: hqpeng@mail.buct.edu.cn

Wenping Zhu

Wu-Jie Guo

Qingyun Li

Shixiang Ma

Christophe Bucher

Bin Liu
email: binliu@mail.buct.edu.cn

Xiaofan Ji
email: xiaofanji@hust.edu.cn

Feihe Huang
email: fhuang@zju.edu.cn

Jonathan L Sessler
email: sessler@cm.utexas.edu

Supramolecular Polymers: Recent Advances Based on the Types of Underlying Interactions

Keywords: Supramolecular polymers, Hydrogen bonds, Metal-coordination bonds, Host-guest interactions, Aromatic donor-acceptor interactions AIE, aggregation-induced emission, BTA, 1,3,5-benzenetricarboxamide, Bip, 2,6-bis(N-alkyl-benzimidazolyl)pyridine, BM, Brooker's merocyanine, BrBP,, CH3CN, acetonitrile, CD, circular dichroism, CDs, cyclodextrins, CB[n], cucurbit[n]urils, CPT, camptothecin, C6H13OH, 1 positron emission tomography, POMs, polyoxometalates, PEG, polyethylene glycol, RTP, room-temperature phosphorescence, RGD, cyclo coupling, SPNPs, supramolecular polymeric nanoparticles, -SS-, disulfide bond, Tol, toluene, TPE, tetraphenylethylene, UPy, 2-ureido

Supramolecular polymers are in broad brushstrokes self-assembled structures built up from small building blocks via the use of noncovalent interactions. In favorable cases, supramolecular polymers embody the best features of covalent polymers while displaying unique reversibility, responsiveness, adaptiveness, and stability. This has made them of interest across a wide variety of fields, running the gamut from molecular devices, to sensors, drug delivery, cell recognition, and environmentally friendly materials development. This review is concerned with the determinants that underlie supramolecular polymer construction, specifically the driving forces that have been exploited to create them.

To date, nearly the full Parthenon of known noncovalent interactions (e.g., hydrogenbonding, electrostatic interactions, charge transfer effects, and metal coordination, among others) have been exploited to create supramolecular polymers. Typically, one or more of these effects is exploited to link appropriately designed monomers. The choice of noncovalent interaction can have a significant influence on the structure and function of the resulting supramolecular polymer. Understanding the connection between the forces used to assemble supramolecular polymers and the properties of the constructs that ensue can, in our view, provide the foundation for further advances in this fast-moving field. Given the above, the organization of this review will be by interaction types. Most of the work to be reviewed will encompass literature published from 2016 to the present. This time period nevertheless includes a large number of contributions, the numericity of which reflects the fast-moving nature of the field. An overview of future challenges and opportunities for supramolecular polymers, including their formation, characterization, and applications, is also provided.

Table of Contents: 1. Introduction

Polymers play essential and ubiquitous roles in our lives. Their use in food, textiles, housing, and transportation is readily apparent even to those with no chemical training. The historical view of polymeric properties was that they resulted from the colloidal aggregation of molecules or small particles until Hermann Staudinger (1881Staudinger ( -1965) ) proposed the concept of macromolecules in 1920. [START_REF] Staudinger | Über polymerisation[END_REF] Since then, the terms polymers and macromolecules have become synonymous with each other. Many pioneers, including Carothers, Ziegler, Natta and Szwarc et al., made seminal contributions to different generations of novel macromolecules. [2][START_REF] Flory | Fundamental principles of condensation polymerization[END_REF][START_REF] Ziegler | Polymerisation von athylen und anderen olefinen[END_REF][START_REF] Natta | Von der stereospezifischen polymerisation zur asymmetrischen autokatalytischen synthese von makromolekülen nobel-vortrag am 12. dezember 1963[END_REF][START_REF] Szwarc | Living' polymers[END_REF] With the diversification of structures and properties, more sophisticated functions and practical applications of polymers are continually being developed. [START_REF] Zhu | Sustainable polymers from renewable resources[END_REF][START_REF] Berry | Celebrating 100 years of polymer science[END_REF] However, the ubiquity of polymeric materials is not without environmental consequences, including those associated with their manufacture and degradation, as well as the buildup of persistent plastic wastes. One possible means of mitigating this latter impact involves relying more heavily on the nexus between supramolecular chemistry and polymer science, viz. the development and application of supramolecular polymers. [START_REF] Greef | Supramolecular polymerization[END_REF][START_REF] Aida | Functional supramolecular polymers[END_REF][START_REF] Dhiman | Bioinspired temporal supramolecular polymerization[END_REF][START_REF] Han | Water compatible supramolecular polymers: recent progress[END_REF] The concept of supramolecular polymers is relatively new and, in many respects, was only codified in the 1990s. However, the foundation of this field rests on several earlier contributions that help highlight its intellectual connections to polymer science. In 1937, Scheibe and co-workers described phenomena associated with the dissolution of pseudoisocyanine chloride, including the appearance of new absorption bands and enhancement of the solution viscosity upon increasing concentration, which viewed in hindsight are features readily attributable to supramolecular polymerization. [START_REF] Scheibe | Polymerisation und polymere adsorption als ursache neuartiger absorptionsbanden von organischen Farbstoffen[END_REF] In 1988, Aida and co-workers reported a water-dispersible supramolecular assembly composed of amphiphilic porphyrins with polyether side chains. [START_REF] Aida | Synthesis of a novel amphiphilic porphyrin carrying water-soluble polyether side chains of controlled chain length. Formation of a cofacial molecular assembly in aqueous media[END_REF] The formation of the resulting onedimensional architectures was noted as being driven by a combination of hydrophobicity and - donor-acceptor interactions; however, the assembly was discussed within the context of amphiphiles rather than as being articulated as an example of supramolecular polymerization per se. In the same year, Ducharme and Wuest developed a strategy to amplify the strength of hydrogen-bonds via connecting 2-pyridones by appropriate spacers.

A polymeric array could then be formed by controlling the pattern of hydrogen-bond donors and acceptors. [START_REF] Ducharme | Use of hydrogen bonds to control molecular aggregation. Extensive, self-complementary arrays of donors and acceptors[END_REF] In 1990, Whitesides and co-workers arranged the hydrogen bond interactions between melamine and barbituric acid to form triply hydrogen-bonded molecular "tapes". [START_REF] Zerkowski | The design of organic structures in the solid state: hydrogen-bonded molecular "tapes[END_REF] However, the resulting structures, although driven by noncovalent interactions, were studied as crystalline solids and hence their generalized materials properties were largely ignored. In 1990, Lehn and co-workers reported a supramolecular liquid crystalline polymer, which displayed polymer-like properties, including fiber formation and gelation. [START_REF] Fouquey | Molecular recognition directed self-assembly of supramolecular liquid crystalline polymers from complementary chiral components[END_REF] These conceptual advances, coupled with seminal contributions from other pioneers in the field (i.e., the groundbreaking work of Meijer and collaborators involving the design, synthesis, and applications of functional supramolecular systems; vide infra), [START_REF] Sijbesma | Reversible polymers formed from self-complementary monomers using quadruple hydrogen bonding[END_REF][START_REF] Brunsveld | Supramolecular polymers[END_REF][START_REF] Jonkheijm | Probing the solventassisted nucleation pathway in chemical self-assembly[END_REF][START_REF] Hoeben | About supramolecular assemblies of π-conjugated systems[END_REF][START_REF] Goor | From supramolecular polymers to multi-component biomaterials[END_REF][START_REF] Su | Dilution-induced gel-sol-gel-sol transitions by competitive supramolecular pathways in water[END_REF][START_REF] Korevaar | Pathway complexity in supramolecular polymerization[END_REF] helped launch supramolecular polymers as a new research discipline lying at the crossroads between polymer science and supramolecular chemistry. [START_REF] Lehn | Supramolecular polymer chemistry-scope and perspectives[END_REF][START_REF] Moulin | Triarylamine-based supramolecular polymers: structures, dynamics, and functions[END_REF][START_REF] Yagai | Supramolecular polymers capable of controlling their topology[END_REF][START_REF] Krieg | Supramolecular polymers in aqueous media[END_REF][START_REF] Qin | Supramolecular polymer chemistry: from structural control to functional assembly[END_REF][START_REF] Hirao | Helically organized fullerene array in a supramolecular polymer main chain[END_REF] As aggregates endowed with typical polymer characteristics, supramolecular polymers are built up from small building blocks via the use of noncovalent bonds, including hydrogen-bonding, [START_REF] Sijbesma | Reversible polymers formed from self-complementary monomers using quadruple hydrogen bonding[END_REF][START_REF] Hirschberg | Helical self-assembled polymers from cooperative stacking of hydrogen-bonded pairs[END_REF][START_REF] Luo | Highly fluorescent rigid supramolecular polymeric nanowires constructed through multiple hydrogen bonds[END_REF] metal-ligand coordination, [START_REF] Lu | Supramolecular polymers and chromonic mesophases self-organized from phosphorescent cationic organoplatinum(II) complexes in water[END_REF][START_REF] Tian | Responsive supramolecular polymers based on the bis[alkynylplatinum(II)] terpyridine molecular tweezer/arene recognition motif[END_REF] electrostatic and van der Waals forces, [START_REF] Lee | Temperature-responsive hydrogels via the electrostatic interaction of amphiphilic diblock copolymers with pendant-ion groups[END_REF][START_REF] Gu | Stabilizing liquids using interfacial supramolecular polymerization[END_REF] - donor-acceptor interactions, [START_REF] Burattini | A healable supramolecular polymer blend based on aromatic π-π stacking and hydrogenbonding interactions[END_REF][START_REF] Guilleme | Noncentrosymmetric homochiral supramolecular polymers of tetrahedral subphthalocyanine molecules[END_REF][START_REF] Görl | Entropically driven self-assembly of bolaamphiphilic perylene dyes in water[END_REF] etc. [START_REF] De Greef | Supramolecular polymers[END_REF] As a consequence of the dynamic nature of noncovalent bonds, supramolecular polymers typically exhibit not only the fundamental features of conventional polymers, but also some less-common characteristics, such as easy processing, self-healing, and stimulus responsiveness. [START_REF] Cordier | Self-healing and thermoreversible rubber from supramolecular assembly[END_REF][START_REF] Wojtecki | Using the dynamic bond to access macroscopically responsive structurally dynamic polymers[END_REF][START_REF] Yan | Stimuli-responsive supramolecular polymeric materials[END_REF][START_REF] Ma | Stimuli-responsive supramolecular polymers in aqueous solution[END_REF][START_REF] Zhang | A pHresponsive supramolecular polymer gel as an enteric elastomer for use in gastric devices[END_REF][START_REF] Campanella | Self-healing in supramolecular polymers[END_REF][START_REF] Ji | Encoding, reading, and transforming information using multifluorescent supramolecular polymeric hydrogels[END_REF] One point that we see as worthy of particular emphasis is the general effect of different solvents on noncovalent bonds, which can influence greatly the formation of supramolecular polymers. For example, apolar solvents are often employed for the construction of hydrogen-bonded supramolecular polymers since water or polar solvents (e.g., DMSO) serving as hydrogen-bond donors and acceptors can disturb the hydrogen bonding between monomers. [START_REF] De Greef | Supramolecular polymers[END_REF] Likewise, the coordination between transition metal centers and organic ligands in metallosupramolecular polymers must be strong enough to overcome the influence of solvents as potentially competing ligands. [START_REF] Winter | Synthesis and characterization of metallo-supramolecular polymers[END_REF] As for the supramolecular polymers based on - stacking interactions, the binding strength between two π-conjugated moieties can be enhanced by solvophobic forces in polar or nonpolar solvents. [START_REF] Brunsveld | Supramolecular polymers[END_REF][START_REF] Chen | Self-assembled π-stacks of functional dyes in solution: structural and thermodynamic features[END_REF] With regard to supramolecular polymers constructed via crown ethers or pillararenes, the corresponding host-guest interactions are typically enhanced in organic solvents, such as chloroform, acetone and acetonitrile. However, cyclodextrin-and cucurbituril-based host-guest recognition is generally stronger in aqueous solution on account of their hydrophilic structures. Particularly, both organic solvents and water are used as suitable media for the fabrication of calixarene-based supramolecular polymers.

These attributes, and the ability to tune interactions through the choice of medium, have made supramolecular polymers attractive in the context of applications, including molecular machines, [START_REF] Du | Muscle-like supramolecular polymers: integrated motion from thousands of molecular machines[END_REF][START_REF] Goujon | Controlled solgel transitions by actuating molecular machine based dupramolecular polymers[END_REF][START_REF] Cai | Molecular-pump-enabled synthesis of a daisy chain polymer[END_REF][START_REF] Hart | Material properties and applications of mechanically interlocked polymers[END_REF] sensors, [START_REF] Ji | A supramolecular cross-linked conjugated polymer network for multiple fluorescent sensing[END_REF][START_REF] Shang | Chiral self-sorted multifunctional supramolecular biocoordination polymers and their applications in sensors[END_REF][START_REF] Sun | Tri-pillar[5]arenebased multifunctional stimuli-responsive supramolecular polymer network with conductivity, aggregation-induced emission, thermochromism, fluorescence sensing, and separation properties[END_REF][START_REF] Hu | In Situ Generation of N-Heteroaromatic Polymers: Metal-Free Multicomponent Polymerization for Photopatterning, Morphological Imaging, and Cr(VI) Sensing[END_REF] drug delivery, [START_REF] Webber | Drug delivery by supramolecular design[END_REF][START_REF] Chen | β-Cyclodextrin modified Pt(II) metallacycle-based supramolecular hyperbranched polymer assemblies for DOX delivery to liver cancer cells[END_REF] cell recognition, [START_REF] Peng | Supramolecular polymeric fluorescent nanoparticles based on quadruple hydrogen bonds[END_REF][START_REF] Zhu | Aggregation-induced emission materials with narrowed emission band by light-harvesting strategy: fluorescence and chemiluminescence imaging[END_REF][START_REF] Xie | Supramolecular biomaterials for bio-imaging and imaging-guided therapy[END_REF][START_REF] Zhou | Ultralong purely organic aqueous phosphorescence supramolecular polymer for targeted tumor cell imaging[END_REF][START_REF] Liu | In situ supramolecular polymerization-enhanced self-assembly of polymer vesicles for highly efficient photothermal therapy[END_REF] and biomedicine. [START_REF] Goor | From supramolecular polymers to multi-component biomaterials[END_REF][START_REF] Dong | Functional supramolecular polymers for biomedical applications[END_REF][START_REF] Tian | Water-dispersible and soluble porous organic polymers for biomedical applications[END_REF] Notwithstanding the seminal suggestion by Lehn and co-workers regarding the concept of supramolecular polymers in 1990, it remained a challenge to design monomeric units that would provide reversible interactions of sufficient strength to support supramolecular polymerization. In 1997, Meijer and co-workers reported 2-ureido-4-pyrimidone (UPy) as a new class of quadruple hydrogen-bonded self-complementary monomers, [START_REF] Sijbesma | Reversible polymers formed from self-complementary monomers using quadruple hydrogen bonding[END_REF] and showed that they could be used to construct supramolecular polymers with high viscosity and outstanding rheological properties. [START_REF] Kushner | A biomimetic modular polymer with tough and adaptive properties[END_REF][START_REF] Sun | Super strong and multi-reusable supramolecular epoxy hot melt adhesives[END_REF][START_REF] Choi | Self-healable antifreeze hydrogel based on dense quadruple hydrogen bonding[END_REF][START_REF] Putti | Multi-component supramolecular fibers with elastomeric properties and controlled drug release[END_REF][START_REF] Yan | Quadruple H-bonding crosslinked supramolecular polymeric materials as substrates for stretchable, antitearing, and self-healable thin film electrodes[END_REF][START_REF] Liu | Biomimetic impact protective supramolecular polymeric materials enabled by quadruple H-bonding[END_REF][START_REF] Lutz | From precision polymers to complex materials and systems[END_REF][START_REF] Dankers | A modular and supramolecular approach to bioactive scaffolds for tissue engineering[END_REF] The emergence of UPy-based polymers marked the beginning of the rapid development of supramolecular polymeric materials. Since then, research on supramolecular polymers blossomed, with key advances coming not only from Meijer, [START_REF] Hoeben | About supramolecular assemblies of π-conjugated systems[END_REF][START_REF] Goor | From supramolecular polymers to multi-component biomaterials[END_REF][START_REF] Su | Dilution-induced gel-sol-gel-sol transitions by competitive supramolecular pathways in water[END_REF][START_REF] Dankers | A modular and supramolecular approach to bioactive scaffolds for tissue engineering[END_REF][START_REF] Weyandt | Controlling the length of porphyrin supramolecular polymers via coupled equilibria and dilutioninduced supramolecular polymerization[END_REF][START_REF] Lamers | Oligodimethylsiloxane-oligoproline block co-oligomers: the interplay between aggregation and phase segregation in bulk and solution[END_REF][START_REF] Van Zee | Biasing the screw-sense of supramolecular coassemblies featuring multiple helical states[END_REF] but also Leibler, [START_REF] Cordier | Self-healing and thermoreversible rubber from supramolecular assembly[END_REF][START_REF] Cortese | Order-disorder transition in supramolecular polymers[END_REF] Zimmerman, [START_REF] Corbin | Self-association without regard to prototropy. a heterocycle that forms extremely stable quadruply hydrogen-bonded dimers[END_REF][START_REF] Ma | Supramolecular polymer chemistry: self-Assembling dendrimers using the DDA•AAD (GC-like) hydrogen bonding motif[END_REF] Craig, [START_REF] Serpe | Physical organic chemistry of supramolecular polymers[END_REF][START_REF] Xu | Scaling laws in supramolecular polymer networks[END_REF][START_REF] Kim | Self-assembly and properties of main-chain reversible polymer brushes[END_REF] Bouteiller, [START_REF] Bouteiller | Thickness transition of a rigid supramolecular polymer[END_REF][START_REF] Bellot | Thermodynamic description of bis-urea self-assembly: competition between two supramolecular polymers[END_REF][START_REF] Lortie | N′-disubstituted ureas: influence of substituents on the formation of supramolecular polymers[END_REF] Binder, [START_REF] Herbst | Self-healing polymers via supramolecular forces[END_REF][START_REF] Chen | Dynamic ordering and phase segregation in hydrogen-bonded polymers[END_REF] Zhang, [START_REF] Qin | Tough and multi-recyclable crosslinked supramolecular polyureas via incorporating noncovalent bonds into main-chains[END_REF][START_REF] Chen | Supramolecular polymerization controlled through kinetic trapping[END_REF][START_REF] Huang | Supramolecular polymerization from controllable fabrication to living polymerization[END_REF] Gibson, [START_REF] Gibson | Supramolecular pseudorotaxane polymers from complementary pairs of homoditopic molecules[END_REF][START_REF] Huang | Formation of a supramolecular hyperbranched polymer from self-organization of an AB2 monomer containing a crown cther and two paraquat moieties[END_REF][START_REF] Yamaguchi | Formation of supramolecular polymers from homoditopic molecules containing secondary ammonium ions and crown ether moieties[END_REF] Long, [START_REF] Yamauchi | Thermoreversible poly(alkyl acrylates) consisting of self-complementary multiple hydrogen bonding[END_REF][START_REF] Mather | Supramolecular triblock copolymers containing complementary nucleobase molecular recognition[END_REF] and others. [START_REF] Yang | Hydrogen bonding modules for use in supramolecular polymers[END_REF] Several of the authors of this review have also contributed to the development of the field. [START_REF] Ji | Encoding, reading, and transforming information using multifluorescent supramolecular polymeric hydrogels[END_REF][START_REF] Peng | Dramatic differences in aggregation-induced emission and supramolecular polymerizability of tetraphenylethene-based stereoisomers[END_REF][START_REF] Wang | Vapochromic behaviors of a solid-state supramolecular polymer based on exo-wall complexation of perethylated pillar[5]arene with 1,2,4,5-tetracyanobenzene[END_REF][START_REF] Zhang | Unlocking chemically encrypted information using three types of external stimuli[END_REF][START_REF] Xie | Calix[4]pyrrole-based crosslinked polymer networks for highly effective iodine adsorption from water[END_REF][START_REF] Liu | A strategy based on aggregation-induced ratiometric emission to differentiate molecular weight of supramolecular polymers[END_REF][START_REF] Zheng | Supramolecular polymers constructed by crown ether-based molecular recognition[END_REF] To date, supramolecular polymers have received widespread attention across a variety of disciplines, including those beyond the traditional frontiers of supramolecular and materials chemistry. Not surprisingly, it is a topic that has been the focus of several reviews. [START_REF] Greef | Supramolecular polymerization[END_REF][START_REF] Aida | Functional supramolecular polymers[END_REF][START_REF] Brunsveld | Supramolecular polymers[END_REF][START_REF] Zheng | Supramolecular polymers constructed by crown ether-based molecular recognition[END_REF][START_REF] Khlobystov | Supramolecular design of one-dimensional coordination polymers based on silver(I) complexes of aromatic nitrogen-donor ligands[END_REF][START_REF] Geng | Supramolecular prodrugs based on host-guest interactions[END_REF][START_REF] Wang | Cucurbituril-based supramolecular polymers for biomedical applications[END_REF] Many of these reviews bring a unique perspective to the topic. [START_REF] Heinzmann | Supramolecular polymer adhesives: advanced materials inspired by nature[END_REF][START_REF] Voorhaar | Supramolecular polymer networks: hydrogels and bulk materials[END_REF][START_REF] Hartlieb | A guide to supramolecular polymerizations[END_REF] For instance, in 2015, Zhang and co-workers reviewed different driving forces that could be exploited to prepare supramolecular polymers with various topologies and applications. [START_REF] Yang | Supramolecular polymers: historical development, preparation, characterization, and functions[END_REF] Moreover, in 2020 Aida and Meijer et al. summarized their views on supramolecular polymerization and how the concepts could be extended. [START_REF] Hashim | Supramolecular polymerization: a conceptual expansion for innovative materials[END_REF] Also in 2020, the Stupp group reviewed seminal contributions to the area, including hierarchical structures and applications of supramolecular polymeric materials. [START_REF] Clemons | Design of materials with supramolecular polymers[END_REF] That same year, Würthner and coworkers reviewed methods of controlling supramolecular polymerization by controlling the kinetics and thermodynamics of noncovalent interactions. [START_REF] Wehner | Supramolecular polymerization through kinetic pathway control and living chain growth[END_REF] In 2020, Sessler and Huang et al. highlighted functional supramolecular polymers formed from covalent polymers connected by macrocycle-based host-guest interactions. [START_REF] Xia | Functional supramolecular polymeric networks: the marriage of covalent polymers and macrocycle-based host-guest Interactions[END_REF] In 2021, the Zhang group summarized recent studies of supramolecular polymers with controllable structures and intriguing functions. [START_REF] Qin | Supramolecular polymer chemistry: from structural control to functional assembly[END_REF] Meanwhile, several contributions have noted the wide choices of monomers (e.g., biomolecules, nanoparticles, nanotubes, and nanosheets) [START_REF] Liu | An anisotropic hydrogel with electrostatic repulsion between cofacially aligned nanosheets[END_REF][START_REF] Mcmillan | Programming protein polymerization with DNA[END_REF][START_REF] Uemura | A supramolecular polymer network of graphene quantum dots[END_REF][START_REF] Liu | Self-assembled supramolecular nanotube yarn[END_REF], media (e.g., solvents, liquid crystals, bulk environments, on electrodes, and at interfaces), [START_REF] Yan | Quadruple H-bonding crosslinked supramolecular polymeric materials as substrates for stretchable, antitearing, and self-healable thin film electrodes[END_REF][START_REF] Sather | 3D printing of supramolecular polymer hydrogels with hierarchical Structure[END_REF][START_REF] Zhang | One-step fabrication of supramolecular microcapsules from microfluidic droplets[END_REF][START_REF] Craig | From ionic liquids to supramolecular polymers[END_REF][START_REF] Xu | Notch-insensitive, ultrastretchable, efficient self-healing supramolecular polymers constructed from multiphase active hydrogen bonds for electronic applications[END_REF] and mechanisms (e.g., chain-growth supramolecular polymerization, [START_REF] Kang | A rational strategy for the realization of chain-growth supramolecular polymerization[END_REF] sequence-specific supramolecular copolymerization, [START_REF] Mcmillan | Protein materials engineering with DNA[END_REF] and seeded supramolecular polymerization [START_REF] Ogi | Mechanism of selfassembly process and seeded supramolecular polymerization of perylene bisimide organogelator[END_REF][START_REF] Sarkar | Stereoselective primary and secondary nucleation events in multicomponent seeded supramolecular polymerization[END_REF]) that are available for supramolecular polymerization. Perhaps as a result, supramolecular polymers have been studied for a range of practical applications. On a more fundamental level, research efforts are moving beyond simply fabricating new constructs to studying systematically their structure-function properties, controlling their structures, and exploring their potential functions, etc. Nevertheless, the types of noncovalent interactions that underpin the construction of supramolecular polymers are increasingly being appreciated as being indispensable to a detailed understanding of supramolecular polymeric materials.

In this review, we emphasize the types of noncovalent interactions that have been and are being used to generate supramolecule polymers. This treatment is designed to extend and complement the coverage of other recent review articles. Through this lens, we provide coverage of supramolecular polymers (from 2016 to the present) built up from small building blocks based on the types of underlying interactions, including multiple hydrogen bonds, metal-coordination bonds, host-guest interactions, aromatic donor-acceptor interactions, among others. An overview of future challenges and opportunities associated with supramolecular polymers, including their formation, characterization, and application, is also provided.

Supramolecular Polymers Based on Multiple Hydrogen Bonds

Multiple Hydrogen Bonds: An Historical Perspective

Nature relies in part on hydrogen bonds to regulate the interactions between biological molecules and to impart advanced functionality. [START_REF] Watson | Molecular structure of nucleic acids: a structure for deoxyribose nucleic acid[END_REF] For decades, scientists have been inspired to take advantage of hydrogen bonds, e.g., their strength and directionality, to create artificial systems with new properties. [START_REF] Song | Efficient artificial light-harvesting system based on supramolecular peptide nanotubes in water[END_REF] In this context, it is now essentially axiomatic that under most conditions a single hydrogen bond is too weak to stabilize specific aggregates. Starting in the 1980s, chemists began using multiple hydrogen bonds to construct inclusion compounds and to control the intermolecular complexation. [START_REF] Kim | A molecular box, based on bibracchial lariat ethers having adenine and thymine sidearms, that self-assembles in water[END_REF][START_REF] Lindquist | Isolation and structure determination of the didemnenones, novel cytotoxic metabolites from tunicates[END_REF][START_REF] Chang | Molecular recognition of biologically interesting substrates: synthesis of an artificial receptor for barbiturates employing six hydrogen bonds[END_REF] In 1988, Wuest et al. designed motifs with extensive arrays of hydrogen-bond donors and acceptors with a view to amplifying their inherent binding affinities. [START_REF] Ducharme | Use of hydrogen bonds to control molecular aggregation. Extensive, self-complementary arrays of donors and acceptors[END_REF] Viewed through the lens of history, we now appreciate that the synergistic use of multiple hydrogen bonds can significantly increase the strength and directionality of the interactions they stabilize. [START_REF] Furuta | Molecular recognition via base pairing: aminecontaining, cytosine-based ditopic receptors that complex guanosine monophosphate[END_REF][START_REF] Furuta | Enhanced transport of nucleosides and nucleoside analogs with complementary base-pairing agents[END_REF] This phenomenon became the initial key to effective supramolecular polymerization. In 1990, Lehn et al. described the first example of a linear supramolecular polymer that was formed by triple hydrogen-bonded complementary units. [START_REF] Fouquey | Molecular recognition directed self-assembly of supramolecular liquid crystalline polymers from complementary chiral components[END_REF] Later on, numerous architectures emerged after the quadruple hydrogen-bonded unit UPy was reported by Meijer [START_REF] Dankers | A modular and supramolecular approach to bioactive scaffolds for tissue engineering[END_REF][START_REF] Guo | Tough Stimuli-Responsive Supramolecular Hydrogels with Hydrogen-Bonding Network Junctions[END_REF][START_REF] Abbel | White-light emitting hydrogen-bonded supramolecular copolymers based on pi-conjugated oligomers[END_REF][START_REF] Zha | End groups of functionalized siloxane oligomers direct block-copolymeric or liquid-crystalline selfassembly behavior[END_REF][START_REF] Teunissen | Regulating competing supramolecular interactions using ligand concentration[END_REF][START_REF] Ten Cate | Conformational control in the cyclization of hydrogen-bonded supramolecular polymers[END_REF][START_REF] Hosono | Forced unfolding of single-chain polymeric nanoparticles[END_REF] and coworkers; this subunit was a game changer due to its high binding constant (Ka) and synthetic accessibility. To date, multiple hydrogen-bonding motifs have been used to create a plethora of exciting structures that run the gamut from simple linear supramolecular polymers to complex polymeric materials. [START_REF] Zhu | Aggregation-induced emission materials with narrowed emission band by light-harvesting strategy: fluorescence and chemiluminescence imaging[END_REF][START_REF] Peng | Water-dispersible nanospheres of hydrogen-bonded supramolecular polymers and their application for mimicking light-harvesting systems[END_REF][START_REF] Peng | Convenient synthesis of functionalized bis-ureidopyrimidinones based on thiol-yne reaction[END_REF] Per the objectives of this review, we now summarize contributions to supramolecular polymer chemistry with a focus on binding motif.

Recent Advances in Hydrogen-Bonded Supramolecular Polymers

In 2017, Tang and co-workers reported two dramatically different supramolecular aggregates formed from pure stereoisomeric monomers; these two precursors were synthesized concurrently via a one-pot McMurry coupling reaction and then separated by simple column chromatography. [START_REF] Peng | Dramatic differences in aggregation-induced emission and supramolecular polymerizability of tetraphenylethene-based stereoisomers[END_REF] These researchers demonstrated that the differing molecular configurations influenced the supramolecular polymerization, leading to distinct fluorescence colors and morphologies in the self-assembled states produced from the monomeric stereoisomers. Of note was the finding that the (Z)-derived isomer could be used as an Hg 2+ fluorescent sensor, while its (E)-based congener formed 2D and 3D photopatterns. This work provided a useful illustration of how ostensibly similar stereogenic structures could be exploited for supramolecular polymer engineering. In 2016, the Weder group used UPy motifs to create several stimuli-responsive supramolecular polymers. In particular, they synthesized monomer M1 containing a 1,1,1tris(hydroxymethyl)propane core and three UPy groups that supported formation of the supramolecular network SP1 (Figure 1A). [START_REF] Balkenende | Optically responsive supramolecular polymer glasses[END_REF] It was shown that this material could be easily processed into supramolecular glasses possessing high stiffness and useful adhesive features. Furthermore, the SP1-based glasses were endowed with optical healing capability, a feature ascribed to the ability of the UPy units to absorb ultraviolet light and convert it into heat. Later on, the same group reported thermo-and mechano-responsive luminescent (MRL) materials prepared from M2 (Figure 1B). [START_REF] Lavrenova | Mechanoand thermoresponsive photoluminescent supramolecular polymer[END_REF] Supramolecular polymers (SP2) formed from M2 were found to form self-standing films, which exhibited different fluorescence colors under mechanical and thermal stimuli. The thermoresponsive and MRL properties of these supramolecular materials were attributed to changes in the molecular packing of the individual chromophore within the self-assembled materials. Supramolecular polymers tend to disassemble upon heating, which limits their applications under harsh conditions. In 2017, Aida et al. reported a thermally bisignate supramolecular polymerization process wherein a noncovalent polymer of generalized structure SP4 was formed on heating and cooling. [START_REF] Rao | Thermally bisignate supramolecular polymerization[END_REF] In this study, a metalloporphyrin carrying eight side-chain amide groups (M4) was allowed to self-assemble in a dodecane/CHCl3 mixture to afford a one-dimensional polymer (SP4). Particularly, C6H13OH was employed as a hydrogen-bond "scavenger". Upon cooling below 50 °C, molecules of C6H13OH cluster together and thus do not hydrogen-bond effectively with the amide groups of M4. This results in the formation of SP4 with M4 constituting the exclusive building block. As the temperature was increased from 50 to 110 °C, SP4 was found to undergo depolymerization before reassembling. It was proposed that at ~60 °C, the motifs in the M4 building block that are involved in polymer-supporting hydrogen bonds at lower temperatures interact effectively with C6H13OH, leading to self-dissociation. However, at yet higher temperatures, thermal motion leads to the detachment of the C6H13OH molecules from M4. This results in liberation of the free form of M4, which then undergoes repolymerization (Figure 3). [START_REF] Rao | Thermally bisignate supramolecular polymerization[END_REF] Based on detailed spectroscopic and theoretical analyses, the authors concluded that the supramolecular polymerization of M4 could be fit to an With the goal of fabricating programmed one-dimensional topologies, Yagai and coworkers prepared the naphthalene-and anthracene-based monomers M5-1 and M5-2. They then showed that both these monomers would undergo self-association to afford chimeric nanofibers SP5-1 and SP5-2 with unique secondary structures (Figure 4). [START_REF] Kitamoto | One-shot preparation of topologically chimeric nanofibers via a gradient supramolecular copolymerization[END_REF] Both monomers bear barbiturate units designed to stabilize hydrogen-bonded supermacrocycle ensembles (rosettes), which were expected to polymerize further as the result of - donoracceptor interactions. In practice, it was found that M5-1 stabilized the formation of the helically folded supramolecular polymer SP5-1 while self-assembly of M5-2 gave rise to linear fibers upon cooling (1.0 K min -1 ) methylcyclohexane solutions from 373 to 293 K.

Temperature-controlled supramolecular copolymerization of M5-1 and M5-2 at a molar ratio of 1:1.5 was found to produce chimeric fibers (DP = 1.6  10 3 ) with a well-defined (helical-linear-helical)-type structure (Figure 4B). It was also found that SP5-2 nucleates before SP5-1, presumably reflecting its propensity to undergo aggregation. Interactions between the M5-1-rich rosettes and the M5-2-rich fibers were then proposed to produce helical segments (Figure 4C). The formation of the chimeric structures was found to be sensitive to the cooling rate and concentrations of the monomers. Finally, it was found that the topology of the supramolecular copolymers could be varied by subjecting M5-1 to photodimerization. In 2020, Yagai et al. reported a different set of controllable supramolecular copolymers, SP6-1 and SP6-2, comprising six-membered hydrogen-bonded supramolecular rosettes built up from M6-1 and M6-2 (Figure 5). [START_REF] Aratsu | Supramolecular copolymerization driven by integrative self-sorting of hydrogen-bonded rosettes[END_REF] By cooling methylcyclohexane solutions of M6-1 and M6-2 in a 1:1 molar ratio, amorphous aggregates were produced. Per the authors' design, M6-1 and M6-2 are electron-rich and deficient, respectively; this allows them to interact via electrostatic interactions. Under conditions of equilibrium, this results in optimized electrostatic stacking between the monomers and allows for the formation of a multi-component supramolecular system, SP6-3, over the course of several days that benefit, presumably, from a maximal enthalpic gain (Figure 5C).

Access to the latter form could be accelerated by incorporating a sterically hindered monomer to suppress the formation of rosette intermediates. This study provides an example of how molecular recognition can be tuned to help preorganize a system to favor a specific supramolecular polymerization product, in loose analogy to what is achieved in natural systems. photoirradiation. These researchers appreciated that the barbiturate naphthalene derivative M6-1 could form toroidal nanofibers containing hydrogen-bonded rosettes (Figure 5). The nature of the resulting supramolecular backbone proved dependent on the spontaneous curvature resulting from - stacking between individual rosettes. The authors exploited this curvature, and the associated translational and rotational displacements, to fabricate the helically folded 1D supramolecular polymer SP7-1. This system, derived from monomer M7-1, comprises an azobenzene motif that leads to increased stacking between the rosettes (Figure 6A). [START_REF] Adhikari | Light-induced unfolding and refolding of supramolecular polymer nanofibres[END_REF] It also provides for photoresponsiveness. Partial photoisomerization of the constituent azobenzene subunit from trans to cis, was found to cause the helically folded fibers of M7-1 to be transformed into highly extended fibers characterized by misfolded structures. [154], Copyright 2021. Adapted with permission from the American Chemical Society.

All scale bars: 100 nm.

To improve the degree of internal order within the supramolecular polymer backbone, Yagai and co-workers synthesized monomer M7-2 possessing a more rigid molecular scaffold (Figure 6B). [START_REF] Prabhu | Self-folding of supramolecular polymers into bioinspired topology[END_REF] Upon polymerization, this system was found to undergo slow self-folding from randomly coiled structures to produce helically folded fibers (SP7-2).

Reversible folding/unfolding of the supramolecular polymers could be achieved by means of photoirradiation. Subsequently, this same group synthesized a diarylethene (DAE)contained barbiturate derivative M7-3, whose molecular rigidity could be tuned by ringclosing/opening photoreactions (Figure 6C). [START_REF] Fukushima | Diarylethenepowered light-induced folding of supramolecular polymers[END_REF] By cooling a hot methylcyclohexane solution of M7-3, randomly coiled supramolecular polymer fibers (SP7-3) with a uniform curvature could be obtained. Irradiating a solution of SP7-3 with UV light to induce ring closure of the DAE subunit, served to transform the SP7-3 backbone into a helicoidal conformation (SP7-3-helicoid). DAE ring-closure increases the - interactions within the fibers leading to compression of the supramolecular backbone. This, in turn, increases the intrinsic curvature of the supramolecular fiber associated with helical folding. Conversely, exposure of solutions of the SP7-3-helicoid to visible light leads to ring-opening of the ringclosed DAE and the eventual formation of supramolecular polymer fibers with randomly coiled conformations similar to those seen in SP7-3, although only after 1 week of aging.

The associated hysteretic process was attributed to interloop van der Waals interactions within SP7-3-helicoid that collectively constitute a kinetic barrier to rearrangement. This line of investigation is unique, yet may be viewed as mirroring some of the structural rearrangements of biopolymers, e.g., proteins with 3D tertiary structures. Feringa and co-workers were able to control the macroscopic properties of supramolecular polymers containing urea groups and photoisomerizable stiff-stilbene subunits. [START_REF] Xu | From photoinduced supramolecular polymerization to responsive organogels[END_REF] They found that cis-M8, a monomer with these constituents, could form intramolecular hydrogen bonds, which precluded effective supramolecular polymerization (Figure 7). Exposing the cis-M8 solution to light (385 nm) for 3 min to induce stilbene photo-isomerization produced the corresponding trans system (trans-M8), which formed a supramolecular polymer, SP8, stabilized via intermolecular hydrogen bonds. Self-assembly of trans-M8 was found to proceed per a cooperative polymerization mechanism. Using this supramolecular system, the authors were able to demonstrate both in situ gelation and gelsol phase transitions with control over the underlying phenomenon. A key aspect of supramolecular polymers is the degree of polymerization. Determining this parameter is typically required for complete structural characterization. Recently, the Ji group suggested an attractive strategy wherein the molecular weight is differentiated visually by monitoring the fluorescence color variations in hydrogen-bonded supramolecular polymers (SP9) subject to aggregation-induced emission (AIE). [START_REF] Liu | A strategy based on aggregation-induced ratiometric emission to differentiate molecular weight of supramolecular polymers[END_REF] The monomer (M9) was synthesized by linking two UPy groups to a pyrene benzohydrazonate-based AIE fluorophore. This monomer was designed to form SP9 through quadruple hydrogen bonding interactions between the constituent UPy subunits. The average DP for SP9 was found to increase from 1.00 to 293 as the M9 concentration was increased from 0.1 to 100 mM. During this increase in DP, the fluorescence color of SP9 was found to vary from dark blue to yellow-green due to aggregation of the fluorophores. This variation allowed a relationship between the DP and the nature of the SP9 fluorescence color to be established; as a consequence, the visual distinction of molecular weight in this demonstration system could be readily established.

Supramolecular Polymers Based on Metal-Coordination Bonds

Metal-Coordination Bonds: An Historical Perspective

In the 19 th century, the principles of metal-ligand bonding in transition metal complexes were unraveled by Alfred Werner, who was awarded the Nobel Prize in 1913 in recognition of his contribution to the field of coordination chemistry. [START_REF] Von Zelewsky | Stereochemistry of coordination compounds. from alfred werner to the 21st century[END_REF] In the 1950s, John C. Bailar, Jr. and co-workers pioneered polymerization through coordination, e.g., they reported copper phthalocyanine polymers with molecular weights reaching 4000 Da. [157] Subsequently, efforts were devoted to developing alternative synthetic approaches, work that resulted in coordination polymers and a broader vista for metal-based synthetic materials. [START_REF] Robin | Coordination polymer networks with O-and N-donors: What they are, why and how they are made[END_REF][START_REF] Noro | Coordination polymers constructed from transition metal ions and organic N-containing heterocyclic ligands: crystal structures and microporous properties[END_REF] Starting the 1980s,metal-coordination bonds have been increasingly exploited by supramolecular chemists who have utilized metal ions as templates to synthesize inter alia interlocked molecular structures. [START_REF] Amabilino | Interlocked and intertwined structures and superstructures[END_REF] Sauvage and his coworkers sowed the seeds for this field. [START_REF] Co | Une nouvelle famille de molecules : les metallo-catenanes[END_REF][START_REF] Co | Templated synthesis of interlocked macrocyclic ligands: the catenands[END_REF] In 2000, the Stang group highlighted the main characteristics of metal-mediated supramolecular assembly [START_REF] Leininger | Self-assembly of discrete cyclic nanostructures mediated by transition metals[END_REF] [172], Copyright 2017. Adapted with permission from the American Association for the Advancement of Science.

In order to create poly[n]catenanes, the Rowan group designed a metallosupramolecular polymer based on the coordination of Zn 2+ and 2,6-bis(N-alkylbenzimidazolyl)pyridine (Bip) in a 1:2 stoichiometry (Figure 9). [START_REF] Wu | Poly[n]catenanes: synthesis of molecular interlocked chains[END_REF] Upon the addition of Zn 2+ to a mixture of compound M10-1 and M10-2 in acetonitrile, self-assembly via the binding between Bip and Zn 2+ (Ka > 10 branched poly [START_REF] Zhu | Sustainable polymers from renewable resources[END_REF][START_REF] Berry | Celebrating 100 years of polymer science[END_REF][START_REF] Greef | Supramolecular polymerization[END_REF][START_REF] Aida | Functional supramolecular polymers[END_REF][START_REF] Dhiman | Bioinspired temporal supramolecular polymerization[END_REF][START_REF] Han | Water compatible supramolecular polymers: recent progress[END_REF][START_REF] Scheibe | Polymerisation und polymere adsorption als ursache neuartiger absorptionsbanden von organischen Farbstoffen[END_REF][START_REF] Aida | Synthesis of a novel amphiphilic porphyrin carrying water-soluble polyether side chains of controlled chain length. Formation of a cofacial molecular assembly in aqueous media[END_REF][START_REF] Ducharme | Use of hydrogen bonds to control molecular aggregation. Extensive, self-complementary arrays of donors and acceptors[END_REF][START_REF] Zerkowski | The design of organic structures in the solid state: hydrogen-bonded molecular "tapes[END_REF][START_REF] Fouquey | Molecular recognition directed self-assembly of supramolecular liquid crystalline polymers from complementary chiral components[END_REF][START_REF] Sijbesma | Reversible polymers formed from self-complementary monomers using quadruple hydrogen bonding[END_REF][START_REF] Brunsveld | Supramolecular polymers[END_REF][START_REF] Jonkheijm | Probing the solventassisted nucleation pathway in chemical self-assembly[END_REF][START_REF] Hoeben | About supramolecular assemblies of π-conjugated systems[END_REF][START_REF] Goor | From supramolecular polymers to multi-component biomaterials[END_REF][START_REF] Su | Dilution-induced gel-sol-gel-sol transitions by competitive supramolecular pathways in water[END_REF][START_REF] Korevaar | Pathway complexity in supramolecular polymerization[END_REF][START_REF] Lehn | Supramolecular polymer chemistry-scope and perspectives[END_REF][START_REF] Moulin | Triarylamine-based supramolecular polymers: structures, dynamics, and functions[END_REF]catenanes and linear poly catenanes. This work underscored in dramatic fashion the utility of metallosupramolecular polymers as steppingstones to the synthesis of sophisticated smart materials.

In separate work, Li, Tu and Hla et al. achieved visualization of a single polymer chain within metallopolymers constructed via the coordination of Ru(II) or Fe(II) to a terpyridine ligand. [START_REF] Li | Synthesis of metallopolymers and direct visualization of the single polymer chain[END_REF] These researchers synthesized hexameric and trimeric precursors bearing azide groups. Reaction with an alkyne-modified pentaethylene glycol crosslinker resulted in the formation of hexameric and trimeric polymers, respectively. The polymers were metalated with Ru(II) and Fe(II), thus introducing distinct strong and weak coordination sites, respectively. On the basis of scanning tunneling spectroscopic studies, it was concluded that the hexameric polymer would stabilize both a cyclic hexagonal structure, as well as linear chain species. In contrast, the trimeric polymer only gave rise to cyclic trigonal species. The different energy gaps of Ru(II) and Fe(II) allowed the identification of their exact positions within the structures and permitted the conformation of the metallosupramolecular polymers to be determined. In 2017, Teasdale and co-workers utilized the photocleavable monomer, M11, to synthesize metallosupramolecular polymers of generalized structure SP11 that were expected to act as photoresponsive gels (Figure 10). [START_REF] Theis | Metallosupramolecular gels that are photocleavable with visible and near-infrared irradiation[END_REF] When the gel produced by SP11

was subject to visible light (> 395 nm, 10 min) photoirradiation, a gel-sol transition occurred, a finding ascribed to the photocleavage of the ruthenium-pyridine bonds. Of particular interest was that the liquid obtained by subjecting the organogel to photoirradiation could be swollen in water to form a hydrogel, which proved photoresponsive only in the regions subjected to photoirradiation. A three-dimensional photopattern could also be obtained by engraving the hydrogel using near-infrared light.

In 2017, supramolecular polymeric materials based on the coordination of lanthanide metal cations with terpyridine ligands were reported by Liu and co-workers. [START_REF] Zhou | Tunable luminescent lanthanide supramolecular assembly based on photoreaction of anthracene[END_REF] Later on, the He group described a smart polymeric gel stabilized by means of iminodiacetate (IDA)derived Eu(III) complexation. [START_REF] Weng | Dynamic coordination of eu-Iminodiacetate to control fluorochromic response of polymer hydrogels to multistimuli[END_REF] Europium-based d-f transitions made the hydrogel luminescent, while the dynamic nature of the Eu(III)-IDA coordination provided for selfhealing. External stimuli, including heating, pH, sonication, metal ion competition, and pressure, were all found to be effective factors in tuning the Eu(III) ion luminescence within the supramolecular gel.

In spite of the proliferation of systems based on metal-ligand coordination and dynamic covalent bonds, it remains a challenge to access multiple states without the requirement of external energetic inputs. In 2018, Sessler and co-workers reported a system based on bispyridine-substituted calix [START_REF] Ziegler | Polymerisation von athylen und anderen olefinen[END_REF]pyrrole (M12) that allowed access to six different discrete states via the combined use of metal-based self-assembly and molecular recognition. [START_REF] Hirao | Control over multiple molecular states with directional changes driven by molecular recognition[END_REF] The pyridine-functionalized system M12 provides latent metal complexation sites in two of its meso positions. Addition of AgBF4 was found to promote metal coordination-based supramolecular polymerization to give SP12, in which the binding constant between M12

and Ag + was determined to be 9  10 3 M -1 in tetrahydrofuran. As inferred from a Job's plot, a 1:1 molar ratio of Ag + cations and M12 was present in SP12 (Figure 23B), in accord with the expected binding stoichiometry set by the pyridine functionality in M12. Single crystals of SP12 could be grown by evaporating a mixed chloroform/acetonitrile solution of SP12.

X-ray diffraction analysis then revealed a metal-linked spiral structure. SEM measurements further verified that SP12 formed helical polymeric assemblies characterized by widespread twisted wire-like shapes with bundles of ~ 80 polymeric chains. This research demonstrated an intriguing approach to creating information-rich functional supramolecular materials. Of no less significance, the authors demonstrated that by adding a proton source or various anionic guests, it was possible to use M12 to stabilize several capsule-type and monomeric forms, thus allowing access to six different states in total. Only one of these states, SP12, was polymeric. exist in the form of sheet-like nanostructures, SP13 is highly crystalline and possesses a hexagonal lattice structure (Figure 12B). The structural attributes of SP13, particularly its ability to stabilize pores, allowed it to be used to fabricate water transport membranes. A general drawback of supramolecular polymers is that they lack the toughness of traditional polymers. In an effort to overcome this shortcoming, Schrettl et al. utilized a low-molecular-weight trifunctional monomer (M14-1), the telechelic macromonomer (M14-2), and stoichiometric amounts of Zn(NTf2)2 to build up metallosupramolecular copolymers (SP14-1). [START_REF] Sautaux | Mechanically robust supramolecular polymer co-assemblies[END_REF] Compared with the corresponding rigid polymer and soft material (SP14-2 and SP14-3, respectively) that were assembled separately from M14-1 + Zn(NTf2)2 and M14-2 + Zn(NTf2)2 (Figure 13A), the metallosupramolecular polymer SP14-1 produced via co-assembly of the two monomers displayed greater strength, toughness, failure strain and tunable mechanical properties (cf. Figure 13B). It was also free of the macrophase separation between hard and soft domains that often plague co-assembled materials. It was further found that compositionally graded objects displaying spatially controlled mechanical behavior could be created by welding two SP14-1 blocks containing different ratios of M14-1 and M14-2.

A redox-responsive metallosupramolecular polymer (SP15) based on porphyrin-and viologen-containing tectons was reported by the Bucher group in 2016 (Figure 14). [START_REF] Kahlfuss | Electron-triggered metamorphism in porphyrin-based self-assembled coordination polymers[END_REF] The constituent monomer (M15) contains three functional units, viz. a zinc porphyrin and an imidazole ligand as self-complementary acceptors and donors, respectively, as well as a bis-viologen linker used as a redox-responsive molecular hinge. The authors established that the self-assembling properties of this trifunctional molecular tecton can be reversibly switched on and off upon controlling the redox state of the central bis(bipyridinium) hinge.

In the ground state, electrostatic repulsion between the positively charged viologens was exploited to prevent intramolecular association between the acceptor and donor at the ends of the hinge. This "open" conformation, was found to promote the formation of supramolecular polymers SP15 stabilized through intermolecular Zn-imidazole coordination bonds, an interaction for which the binding constant was determined to be 360 M -1 in CH3CN. The electrochemical or chemical reduction of the bipyridinium subunits from their cationic to cation radical forms was then found to induce a full dissociation of the polymers in favor of an intramolecularly locked monomer (Folded M15) stabilized by π-dimerization of two bipyridinium cation radicals and by intramolecular coordination of the imidazole ligand. Similar strategies making use of the dimerization ability of this bisviologen-hinge were developed to achieve redox control over the formation of coordination polymers. For instance, symmetrical systems, where both ends of the hinge were functionalized with terpyridine or bipyridine-like ligands, were reported as supporting metallopolymers in combination with a source of Zn 2+ , Fe 2+ or Pd 2+ cations. [START_REF] Abdul-Hassan | Redoxtriggered folding of self-assembled coordination polymers incorporating viologen units[END_REF][START_REF] Chowdhury | Light-controlled aggregation and gelation of viologen-based coordination polymers[END_REF][START_REF] Huang | Nitrogen-rich tetrazolo[1,5b]pyridazine: promising building block for advanced energetic materials[END_REF][START_REF] Roizard | Photoredox processes in the aggregation and gelation of electron-responsive supramolecular polymers based on viologen[END_REF][START_REF] Courtois | Redoxresponsive colloidal particles based on coordination polymers incorporating viologen units[END_REF] Photo/redox stimulation of certain of the resulting 1D coordination polymers in solution was found to trigger large scale reorganization along with a macroscopic sol/gel-type phase transition. [START_REF] Chowdhury | Light-controlled aggregation and gelation of viologen-based coordination polymers[END_REF][START_REF] Kahlfuss | Redox-induced molecular metamorphism promoting a sol/gel phase transition in a viologen-based coordination polymer[END_REF] Also, photo-or electrically responsive metallosupramolecular polymers were fabricated and used to build multiswitchable molecular logic gates. [START_REF] Chatir | Photochromic metallopolymer based on dithienylethene as a molecular calculator[END_REF] Figure 14. Chemical structure of M15, and schematic representation of the formation of metallosupramolecular polymers SP15. [START_REF] Kahlfuss | Electron-triggered metamorphism in porphyrin-based self-assembled coordination polymers[END_REF], Copyright 2016. Adapted with permission from the American Chemical Society.

A conceptually simpler simple approach was reported recently that relies on the use two identical metal coordinating units on both sides of a redox-responsive 4,4'-bipyridinium unit (Figure 15). [START_REF] Kahlfuss | Electrontriggered metamorphism in palladium-driven self-assembled architectures[END_REF] In this case, either discrete 2: Specifically, they reported a copper-based double-helical supramolecular polymer (SP17) that permitted resistive switching (Figure 16). [START_REF] Greenfield | Electrically induced mixed valence Increases the conductivity of copper helical metallopolymers[END_REF] This system was produced via the coordination interaction between M17-1, M17-2, and CuNTf2 in MeCN, Upon the exposure to electric fields, about 25% of the Cu(I) in SP17 was oxidized to Cu(II) without producing appreciable damage to the metallopolymer chain. This mixed-valence form displayed a 10-fold increase in electrical conductivity relative to the starting material, a finding attributed to the presence of partly filled dz 2 orbitals aligned along the mixedvalence copper array. It was found that this partial oxidation led to the formation of a double-helical structure to accommodate the Cu(II) centers. The need to rearrange this structure was thought to provide a kinetic barrier to re-reduction, which presumably helps account for the observation that the oxidation-induced increase in conductivity was found to persist for several years. The research highlights a new approach to controlling current flow at the nanoscale.

Supramolecular Polymers Based on Host-Guest Interactions

Classic host-guest complexes are typically stabilized by a range of non-covalent interactions, including hydro-and solvatophobic effects, hydrogen bonding, electrostatic interactions and van der Waals forces, while specificity is often guided by size complementarity. Host-guest binding has been identified in numerous biological systems, and forms the basis for a variety of molecular machines, supramolecular polymers, and selfassembled amphiphiles among other synthetic constructs. [START_REF] Geng | Supramolecular prodrugs based on host-guest interactions[END_REF][START_REF] Dong | Supramolecular polymers constructed from macrocycle-based host-guest molecular recognition motifs[END_REF][START_REF] Ma | Biomedical applications of supramolecular systems based on hostguest Interactions[END_REF][START_REF] Chen | Real-time observation of the dynamics of an individual rotaxane molecular shuttle using a single-molecule junction[END_REF] To date, most supramolecular polymers based on host-guest interactions have relied on the molecular recognition features of specific macrocyclic receptors, of which pillar[n]arenes, crown ethers, cyclodextrins, calixarenes and cucurbit[n]urils are the most widely studied. [START_REF] Li | Pillararene-based supramolecular polymers: from molecular recognition to polymeric aggregates[END_REF] Guests for these hosts have included a large variety of molecules complementary to the host cavities and whose binding supports relatively strong noncovalent interactions. Recent progress in this generalized area is discussed in this section.

Cyclodextrin-Based Supramolecular Polymers

Cyclodextrin-Based Host-Guest Interactions: An Historical Perspective

Cyclodextrins (CDs) are basket-like macromolecules that comprise glucopyranose units linked by glycosidic groups (Figure 17A). They possess a hydrophobic cavity and a hydrophilic rim and are capable of capturing organic guest molecules from aqueous environments. These time-honored hosts have been studied in applications ranging from drug delivery to catalysis and metal purification. [START_REF] Szejtli | Introduction and general overview of cyclodextrin chemistry[END_REF] Three cyclodextrins, , , and , which vary in their size (Figure 17A) have been widely studied and, as might be expected, size dependent effects are seen in their recognition chemistry. [START_REF] Rekharsky | Complexation thermodynamics of cyclodextrins[END_REF][START_REF] Crini | Review: a history of cyclodextrins[END_REF][START_REF] Hapiot | Cyclodextrins as supramolecular hosts for organometallic complexes[END_REF][START_REF] Prochowicz | Interactions of native cyclodextrins with metal Ions and inorganic nanoparticles: fertile landscape for chemistry and materials Science[END_REF] To avoid self-inclusion and head-to-head dimerization of the monomers, which would compete with the formation of higher order structures, in 2005 Harada et al. attached a guest-like subunit to the lower (wider) rim of -CD; this resulted in a supramolecular polymer (SP18) with degree of polymerization >15 (Figure 17B). [START_REF] Miyauchi | Chiral supramolecular polymers formed by host-guest interactions[END_REF] In 2018, Sollogoub and co-workers addressed these same issues (i.e., avoiding self-inclusion and dimerization) by bridging two sites on the upper (narrower) -CD rim with a bis-ammonium linker bearing an adamantane moiety to give monomer M19 (Figure 18). [START_REF] Evenou | Bridging β-cyclodextrin prevents self-Inclusion, promotes supramolecular polymerization, and promotes cooperative interaction with nucleic acids[END_REF] This bridging strategy prevented self-inclusion of the adamantane subunit within the cyclodextrin cavity.

Moreover, the two ammonium groups on the rim of the CD increased the solubility of M19 in water. On the basis of NMR spectroscopic analyses and concentration-dependent diffusion coefficient studies, the authors concluded that at millimolar concentrations M19

(Ka = 1.1  10 4 M -1 in D2O) was capable of forming supramolecular polymers of generalized structure SP19. The resulting ensembles were found to interact cooperatively with DNA and siRNA, while not inhibiting the transfection of siRNA.

Chen and Huang et al. developed the CD-based supramolecular polymer SP20 and explored some of its potential in biomedical applications. [START_REF] Yu | Supramolecular polymerbased nanomedicine: high therapeutic performance and negligible long-term immunotoxicity[END_REF] They designed and synthesized monomer M20, wherein -CD was expected to function as a host within an aqueous-compatible supramolecular polymer. The anticancer drug camptothecin (CPT) was linked to the -CD core as a guest subunit via a disulfide linker. The use of CPT was designed to support the formation of a supramolecular polymer, SP20, that was expected to function as a cancer theranostic. Specifically, it was anticipated the disulfide linker would be cleaved in the presence of glutathione (GSH) allowing for the release of CPT in environments, such as tumors, where GSH is overexpressed (Figure 19). Characterization studies, including analyses of diffusion coefficients, molecular weights and viscosity, provided support for the proposed formation of the supramolecular polymer SP20 from M20 (Ka = 1.68  10 4 M -1 in D2O). To construct a self-assembly with suitable surface properties and of a size appropriate for biomedical applications, a CPT-PEG-RGD polymer was allowed to copolymerize with M20. It was expected that the polyethylene glycol chains would improve the stability of the resulting supramolecular polymeric nanostructures in aqueous media. Likewise, the cRGDfK (an αvβ3 integrin targeting peptide) ligands were designed to impart targeting capability to the nanostructures. The authors put forward evidence that mixing the CPT-PEG-RGD polymer with M20 resulted in the formation of supramolecular polymeric nanoparticles (SNPs) with a diameter of 118 nm. These SNPs were then tested in vitro and in vivo. They were found to be internalized into HeLa cells and localized within the lysosomes. Evidence of cell apoptosis and necrosis was seen after CPT release, presumably as the result of GSH-induced cleavage of the disulfide linkers. Another monomer, CPT-PEG-NOTA, was also copolymerized with SP20. The resulting SNPs were found capable of coordinating 64 Cu, a radiotracer that allows for positron emission tomography (PET) imaging. In 2020, Liu and co-workers reported a supramolecular polymer (SP21) based on the host-guest interaction between -cyclodextrin and anthraquinone (Ka = 382 M -1 ). [START_REF] Chen | Reversible emitting anti-counterfeiting ink prepared by anthraquinone-modified β-cyclodextrin supramolecular polymer[END_REF] Under UV photoirradiation, the anthraquinone moieties of M21 in this supramolecular polymer were photo-reduced to the corresponding hydroquinone. This resulted in the appearance of a yellow color and a strong fluorescence for aqueous solutions of the polymer SP21. This photo-reduction could be reversed by bubbling air into a solution of the polymer, which led to oxidation back to the original hydroquinone-containing species. Taking advantage of interconversion between the oxidized and reduced forms the authors were able to use their smart supramolecular polymers for data encryption and decryption. Unlike numerous studies on supramolecular polymers in solution or in gel state, Dong and co-workers employed a so-called "deep eutectic solvents" strategy to develop solventfree supramolecular polymers. [START_REF] Wu | Deep eutectic supramolecular polymers: bulk supramolecular Materials[END_REF] For this study, several naturally occurring acids M23 (Figure 22A), viz. citric acid, malic acid and tartaric acid, were chosen as the solvents since their -COOH groups were expected to interact with the -OH groups of a CD via hydrogenbonds. Mixing these acids with various CDs resulted in deep eutectic supramolecular polymers (SP23) characterized by high viscosity. Flexible fibers and films were prepared from SP23, which was taken as an augury of good processability (Figure 22B). Polymers SP23 showed very strong adhesion not only to glasses and iron plates, but also to biological tissues. Good adhesion was also observed in dichloromethane and at different temperatures.

Polyoxometalates (POMs) are large, charge diffuse anions that have seen applicability in a number of fields. Wu and co-workers synthesized a hybrid cluster monomer comprising a Mn-centered POM bearing two anthracene groups at opposite sides (cf. Figure 23 for a schematic representation). [START_REF] Guan | A supramolecular approach of modified polyoxometalate polymerization and visualization of a single polymer chain[END_REF] In the presence of -CD, host-guest interactions between the CD cavity and the two anthracene moieties were found to stabilize a set of supramolecular polymers (SP24-1). Subsequent photoirradiation led to [4+4] cyclodimerization of the anthracene units held within the CD cavities. The net result was a transformation of the original supramolecular polymers (SP24-1) to the corresponding covalent polymers (SP24-2). TEM imaging revealed long linear shish kebab structures and provided evidence for a high degree of polymerization (DP 175). Although specific applications of this set of hybrid supramolecular materials have yet to be reported, we believe that this work will pave the way to the construction of useful polymetallic polymers.

Calixarene-Based Supramolecular Polymers

Calixarene-Based Host-Guest Interactions: An Historical Perspective

Calixarenes are cyclic oligomers synthesized by condensation reactions of phenols and aldehydes. They contain a hydrophobic cavity for host-guest complexation and active sites on the rim for useful chemical modifications (Figure 24A). [216] A number is often inserted between "calix" and "arene" to represent the number of subunits in the macrocycles.

Calix[n]arenes have a time-honored history dating back to pioneering work by Baeyer [START_REF] Baeyer | Ueber die verbindungen der aldehyde mit den phenolen und aromatischen kohlenwasserstoffen[END_REF] and then later Zinke and Ziegler. [START_REF] Zinke | Zur kenntnis des härtungsprozesses von phenol-formaldehydharzen, x. mitteilung[END_REF] In the 1970s, Gutsche et al. carried out seminal studies demonstrating the utility of calixarenes; he also gave them their current name based on the resemblance between calix [START_REF] Ziegler | Polymerisation von athylen und anderen olefinen[END_REF]arne in its cone conformation and a type of Greek vase called a calix crater. [START_REF] Gutsche | Calixarenes. 4. the synthesis, characterization, and properties of the calixarenes from p-tert-butylphenol[END_REF][START_REF] Izatt | Selective M+-H+ coupled transport of cations through a liquid membrane by macrocyclic calixarene ligands[END_REF] In 1997, Rebek et al. reported the first calixarene-based supramolecular polymer, which they termed "polycaps". These systems were obtained by covalently connecting two calix [START_REF] Ziegler | Polymerisation von athylen und anderen olefinen[END_REF]arenes at their lower rims and linking them noncovalently at their upper rims (Figure 24B). [START_REF] Castellano | Polycaps: reversibly formed polymeric capsules[END_REF] It was found that two cup-shaped calixarene units could come together to form a capsule capable of encapsulating a small guest molecule. In the ensuing years, several other polymers containing calixarenes were reported. [START_REF] Bügler | Interconnective host-guest complexation of β-cyclodextrin-calix[4]arene couples[END_REF] However, most did not rely on host-guest recognition involving the calixarene cavity per

Recent Advances in Calixarene-Based Supramolecular Polymers

In 2017, Sun and co-workers prepared a supramolecular polymer (SP26) based on a bisulfonato calix [START_REF] Ziegler | Polymerisation von athylen und anderen olefinen[END_REF]arene M26-3 and a protonated monomer M26-2, which contains a modified dimethylamino fluorophore (M26-1) (Figure 25). To the best of our knowledge, there are no more recent reports of calixarene-based supramolecular polymers. In a sharp contrast, amphiphilic calixarenes have been used to construct various supramolecular particles for light-harvesting, bioimaging, and targeted phototheranostics, among other applications. [START_REF] Xu | Broad-spectrum tunable photoluminescent nanomaterials constructed from a modular light-harvesting platform based on macrocyclic amphiphiles[END_REF][START_REF] Geng | Supramolecular bioimaging through signal amplification by combining indicator displacement assay with förster resonance energy transfer[END_REF][START_REF] Pan | Biomedical applications of calixarenes: state of the art and perspectives[END_REF] The paucity of calixarene-based supramolecular polymers could reflect some combination of the following factors:

1) The common calix[n]arenes (n = 4 and 5) can only chelate metal ions or form 1:1 complexes with small organic molecules owing to their limited cavity sizes; thus, to promote effective self-assembly ditopic hosts need to be synthesize, which adds to the synthetic burden associated with constructing supramolecular polymers.

2) Most n:n supramolecular polymers of calixarenes are entropy-disfavored relative to the corresponding 1:1 complexes. [START_REF] Guo | Electrochemical stimulus-responsive supramolecular polymer based on sulfonatocalixarene and viologen dimers[END_REF] Considerable time and efforts are thus required to design dedicated monomers that favor formation of stable calixarene-based supramolecular polymers.

3) Other molecular motifs are increasingly available that stabilize noncovalent interactions suitable for use in supramolecular polymerization studies. In 2020 Yan and co-workers designed and synthesized a four-arm monomer M27 containing two dialkylammonium guest units and two benzo-21-crown-7 host components and used it to create self-cross-linking supramolecular polymers of general structure SP27

stabilized by [2]pseudorotaxane linkages (Figure 26). [START_REF] Wang | A self-cross-linking supramolecular polymer network enabled by crown-ether-based molecular recognition[END_REF] At high concentrations (≥21.0 wt%), SP27 in acetonitrile was found to form gels that underwent pH-and thermo-triggered reversible gel-sol transitions. The dynamic mechanical properties of these and other gels are often described in terms of the storage modulus, G', and loss modulus, G", which reflect the elasticity and viscosity of the materials, respectively. The gel point is typically defined as the point where G' = G''. In the case of gel SP27, G' was found to exceed G" at room temperature. On the other hand, temperature dependent studies revealed a critical temperature Tc (~58 °C), at which G' and G" were equal. When T > Tc, the G" values were higher than G', indicating the destruction of the gel form, presumably as the result of breaking up the [2]pseudorotaxane linkages that serve to stabilize the gel form. In contrast, G' values greater than G'' are seen when T < Tc, reflecting reformation of the supramolecular network. These findings provide support for the proposed thermoplastic features of SP27. Furthermore, rapid glass-liquid conversion of SP27 was seen at elevated temperatures ascribed to the fast exchange nature of the dynamic host-guest interactions between the benzo-21-crown-7 and ammonium subunits. This, in turn, endowed SP27 with good processability and malleability and allowed the construction of several architectures with different macroscopic shapes.

In 2016, the Gibson group reported hyperbranched supramolecular polymers SP28

formed from monomers M28 consisting of a bis(m-phenylene)-32-crown-10 (bis-cryptand)

and linked bis-paraquat moieties bearing tris(p-t-butylphenyl)methane blocking groups (Figure 27A). In contrast with previously reported hyperbranched polymers, the blocking groups in this system were found to prevent dethreading of the guests from the pseudorotaxane units. [START_REF] Lee | Rotaxane-type hyperbranched polymers from a crown ether host and paraquat guests containing blocking groups[END_REF] In 2018, the same group designed and synthesized an array of bis-cryptand monomers based on covalently linked cryptand host units known to bind paraquat derivatives well. [START_REF] Price | Supramolecular pseudorotaxane polymers from biscryptands and bisparaquats[END_REF] Five polymers were successfully obtained by mixing different monomers. It was found that short and rigid linkers between the two subunits making up the bis-cryptand receptor provided monomers that did not favor supramolecular polymerization, presumably due to an inability to overcome the repulsive electrostatic interactions between the two paraquat guests. On the other hand, the bis-cryptand monomer, M29-2, possessing long and flexible linkers was found to complex with the specific bisparaquat M29-1 (Ka = 4.36 × 10 5 M -1 in DCM, Figure 27B). This gave the supramolecular polymer SP29 with a DP of 126 at 37 mM. In this case, a double logarithmic plot of the specific viscosity vs concentration revealed a critical polymerization concentration of 37 mM. Above 37 mM, a slope of 3.55 was seen, which was taken as evidence for the formation of supramolecular polymeric ensembles with high molecular weights. Polymers such as SP29 were found to form brittle films; however, their corresponding fibers were all strong and flexible. It was also found that the thermal properties of the polymers were influenced by the paraquat counter anions. Adapted with permission from the American Chemical Society.

The 1,3,5-benzenetricarboxamide (BTA) unit has proved to be a versatile motif for constructing ordered superstructures. [START_REF] Cantekin | Benzene-1,3,5-tricarboxamide: a versatile ordering moiety for supramolecular chemistry[END_REF] Over the past decades, many sophisticated supramolecular polymers have been produced by derivatizing BTA with other functional groups, including benzocrown ethers, porphyrins, amino acids, as well as a number of other substituents. [START_REF] Rv | Macroscopic hierarchical surface patterning of porphyrin trimers via self-assembly and dewetting[END_REF][START_REF] Van Hameren | Supramolecular porphyrin polymers in solution and at the solid-liquid interface[END_REF][START_REF] Besenius | Controlling the growth and shape of chiral supramolecular polymers in water[END_REF] For instance, the Dong and Han groups synthesized a monomer M30 carrying BTA as the core and three benzo-21-crown-7 units as the arms (Figure28A). [START_REF] Li | Preparation of cross-linked supramolecular polymers based on benzo-21-crown-7/secondary ammonium salt hostguest interactions[END_REF] The resulting species were found to self-assemble into one-dimensional supramolecular aggregates (SP30-1) in chloroform, presumably as the result of stabilizing hydrogenbonding and - interactions. Guest molecules bearing two ammonium salt groups could then be used to crosslink the supramolecular aggregates (SP30-2) to produce the corresponding supramolecular polymers. Another BTA-based supramolecular polymer formed by interactions of a benzo-21crown-7 derivative and water molecules was studied by Schalley and co-workers (Figure 28B). [START_REF] Dong | Structural water as an essential comonomer in supramolecular polymerization[END_REF] The pure triply benzo-21-crown-7-substituted BTA host (M31) appeared to be a fragile solid when dry. However, a very viscous material (SP31) was obtained after M31 was allowed to absorb water from the air. The viscosity could be tuned by controlling the water content in the samples. At appropriate water ratios relative to M31, SP31 could be easily processed into long and flexible fibers. Reflectance infrared spectroscopic studies led to the conclusion that the water molecules interact with the crown ether subunits of M31 via hydrogen-bonds. Data from broadband dielectric spectroscopy and density functional theory simulations provided support for this conclusion and led to the inference that 4 to 5 water molecules act as crosslinkers by binding in concert to two crown ethers on separate BTA subunits. These supramolecular polymeric materials (SP31) were used to glue together substrates with hydrophilic surfaces (e.g., glass and paper). The adhesives proved reusable.

Raising the temperature served to release water molecules from SP31 and undo the gluing effect.

In 2019, the Stang group reported a supramolecular polymeric adhesive (SP32) that also relied on hydrogen bonding between water molecules and crown ethers as the main stabilizing interaction (Figure 29A). [START_REF] Zhang | Formation of a supramolecular polymeric adhesive via water-participant hydrogen bond formation[END_REF] They found that the use of Pt-pyridine coordination, a key feature of their system, affected the conformation of the monomer, which reduced competitive intermolecular interactions and allowed the water molecules to approach the crown ethers.

In 2020, the Yan group demonstrated that the catalyst-free condensation of acetoacetate groups present in [2]rotaxane monomers (M33-1 and M33-2) with two selected multiamine monomers (M33-3 and M33-4) could be exploited to fabricate mechanically interlocked supramolecular network polymers (SP33-1 and SP33-2) (Figure 29B). [START_REF] Zhao | Mechanically interlocked vitrimers[END_REF] In the case of M33-1, the two acetoacetate groups were placed on the wheel and axle, respectively, while the two acetoacetate groups were present only on the axle in M33-2.

Both SP33-2 and SP33-1 demonstrated good mechanical properties with sliding of the wheel being seen under an external force. The associated energy dissipation process was ascribed to the force-induced dissociation of the crown-ether-based host-guest interactions.

When SP33-1 was treated with an excess of a monofunctional amine, it exhibited enhanced swellability that was ascribed to the destruction of the stabilizing host-guest interactions allowing for freer movement of the wheel within the network. Supramolecular polymer a differing number of glycoluril units bridged by methylene groups (Figure 30). [START_REF] Assaf | Cucurbiturils: from synthesis to high-affinity binding and catalysis[END_REF][START_REF] Barrow | Cucurbituril-based molecular recognition[END_REF] Their structural rigidity and the large number of heteroatoms they contain has made CB[n]s receptors par excellence for appropriately chosen guests. Hydrophobic interactions and charge-dipole attraction between positively charged guests and the oxygen atoms in CB [n] are thought to be key to the underlying host-guest interactions. Of particular note is the fact that CB [START_REF] Berry | Celebrating 100 years of polymer science[END_REF] is capable of forming ternary complexes by incorporating a homo-or heteroguest pair within its relatively large cavity. This is a key feature that has been widely employed to build supramolecular polymeric materials. [START_REF] Liu | Cucurbit[8]uril-based supramolecular polymers[END_REF][START_REF] Correia | Dynamic supramolecular polymers built from cucurbit[n]urils and viologens[END_REF][START_REF] Yang | Oligomeric cucurbituril complexes: from peculiar assemblies to emerging applications[END_REF] Recently, a number of researchers, including Isaacs, Day, and Tao et al. have reported other CB[n]-related cyclic macrocycles, such as CB [START_REF] Aida | Functional supramolecular polymers[END_REF], CB [START_REF] Aida | Synthesis of a novel amphiphilic porphyrin carrying water-soluble polyether side chains of controlled chain length. Formation of a cofacial molecular assembly in aqueous media[END_REF], nor-seco-CB [START_REF] Aida | Functional supramolecular polymers[END_REF], and nor-seco-CB [START_REF] Szwarc | Living' polymers[END_REF], the extending the frontiers of CB chemistry. [START_REF] Liu | Cucurbit[10]uril[END_REF][START_REF] Day | A cucurbituril-based gyroscane: a new supramolecular form[END_REF][255][START_REF] Cheng | Twisted cucurbit[14]uril[END_REF] Of these newer systems, only CB [START_REF] Aida | Functional supramolecular polymers[END_REF] has to our knowledge been exploited to create a supramolecular polymer (vide infra). Later on, the same group successfully employed CB [START_REF] Berry | Celebrating 100 years of polymer science[END_REF] as a supramolecular catalyst for covalent polymerization. [START_REF] Tang | Supramolecularly catalyzed polymerization: from consecutive dimerization to polymerization[END_REF] The key to this advance was an appreciation that CB [START_REF] Berry | Celebrating 100 years of polymer science[END_REF] could encapsulate two Brooker's merocyanine (BM) groups into its cavity and that once bound they could be subject to photodimerization. Also critical was the observation that the photodimerized product was bound less effectively than the two original BM substrates.

Thus, after photoirradiation, the dimer would be replaced from the cavity by new merocyanines, setting up the next dimerization cycle. When a bifunctional monomer carrying two BM end groups was used, the effect of photodimerization and release was the formation of a covalent polymer. This study from the Zhang group thus served to illustrate the potential of appropriately chosen CB[n] hosts in controlling polymerization.

In 2019, the Scherman group reported the formation of nanoscale assemblies from supramolecular polymers comprising CB [START_REF] Berry | Celebrating 100 years of polymer science[END_REF] and the naphthalene-containing salts M35-1 and M35-2 (Figure 32A). [START_REF] Jd | Emerging twodimensional crystallization of cucurbit[8]uril complexes: from supramolecular polymers to nanofibers[END_REF] These researchers showed that a mixture of CB [START_REF] Berry | Celebrating 100 years of polymer science[END_REF] and M35-1 in a 1:1 molar ratio formed platelet-like aggregates containing polymer chains. In contrast, the host-guest interaction of CB [START_REF] Berry | Celebrating 100 years of polymer science[END_REF] and M35-2 yielded nanofibers. Based on liquid AFM and cryo-TEM measurements, the authors suggested that the fibers were formed through a two-step self-assembly process that included first linear supramolecular polymer formation of the host-guest complexation of CB [START_REF] Berry | Celebrating 100 years of polymer science[END_REF] and M35-2 followed by laterally assembly of the polymers SP35 as the result of O … HC hydrogen-bonding interactions between the carbonyl groups and the methylene groups in CB [START_REF] Berry | Celebrating 100 years of polymer science[END_REF] (Figure 32B). The differing self-assembly behavior seen for M35-1 and M35-2 was ascribed to differences in the hydrophobichydrophilic ratio of the host-guest complexes. In operational terms it was found that M35-2/CB [START_REF] Berry | Celebrating 100 years of polymer science[END_REF] with two imidazolium groups produced higher-order self-assemblies. On the other hand, both the platelet-like aggregates and nanofibers proved to be crystalline materials with viscoelastic properties. This makes these nanostructures of interest in the context of potential applications such as, e.g., drag reducers and viscosity modifiers. This work was seminal in its time because it showed how aggregate morphologies can be controlled by modulating the hydrophobic-hydrophilic ratio of the constituent building blocks. It also advanced our understanding of CB [START_REF] Berry | Celebrating 100 years of polymer science[END_REF]-based self-assembly. In separate work, Cao and co-workers constructed cuboid and spheroid supramolecular organic frameworks from CB [START_REF] Berry | Celebrating 100 years of polymer science[END_REF] and tetraphenylethylene (TPE) precursors. [START_REF] Li | Shape-controllable and fluorescent supramolecular organic frameworks through aqueous host-guest complexation[END_REF] The use of different linkers (rigid or flexible) between the pyridinium moieties and the TPE cores led to the formation of distinct framework structures that displayed intriguing photophysical properties and stimuli-responsive behavior. These features were ascribed to the aggregation-induced emission of the constituent TPE subunits, as well as the supramolecular association/disassociation of the supramolecular networks.

CB [n] hosts have been used to enhance the room-temperature phosphorescence (RTP) of supramolecular polymeric materials. For instance, the Liu group prepared a hyaluronic acid polymer bearing 4-(4-bromophenyl)-pyridin-1-ium (BrBP) subunits. [START_REF] Zhou | Ultralong purely organic aqueous phosphorescence supramolecular polymer for targeted tumor cell imaging[END_REF] This polymer was found to undergo further cross-linking as the result of 2:1 complexation of the BrBP subunits by CB [START_REF] Berry | Celebrating 100 years of polymer science[END_REF], as well as hydrogen bonding between the main chains of the polymers.

The resulted networks exhibited a strong phosphorescence with a lifetime of 4.33 ms and a quantum yield of ca. 7.6%. These desirable RTP features were attributed to restricted motion and a corresponding reduction in the triplet state annihilation.

Recently, the same group reported another supramolecular luminescent polymer SP36 that was stabilized as the result of interactions between CB [START_REF] Berry | Celebrating 100 years of polymer science[END_REF] and an anthracene-conjugated bromophenylpyridinium salt. The binding constant between these entities was determined to be 3.13 × 10 12 M -1 in water and 6.98 × 10 12 M -1 in phosphate buffer (Figure 33). [START_REF] Yu | Photooxidation-driven purely organic room-temperature phosphorescent lysosome-targeted imaging[END_REF] SP36 with a DP of 15 was found to emit a red fluorescence that was proposed to result from a host-stabilized charge-transfer state involving the electron-rich anthryl group and the electron-deficient pyridiuium group. When the anthracene group of M36 was oxidized to the corresponding anthraquinone by means of UV photo-irradiation, SP36 was transformed to a ternary inclusion complex in which two bromophenylpyridinium units are localized in one CB [START_REF] Berry | Celebrating 100 years of polymer science[END_REF] cavity. The resulting ternary assembly gave a photoluminescent emission at 529 nm, which was assigned to a long-lived phosphorescence. According to supporting theoretical calculations, a relatively small S1-T1 energy gap and large spin orbit coupling (SOC) coefficient results in an improved intersystem crossing rate and an efficient RTP for the ternary complex. In 2006 Isaacs et al. reported the host-guest properties of nor-seco-CB [START_REF] Aida | Functional supramolecular polymers[END_REF], which remains a relatively new member of the CB[n] family. [START_REF] Huang | Nor-seco-cucurbit[10]uril exhibits homotropic allosterism[END_REF] They showed, in particular, that a single nor-seco-CB [START_REF] Aida | Functional supramolecular polymers[END_REF] can accommodate two guest molecules. In 2019, Xu and Ni al.

designed a cationic monomer (M37) bearing naphthalene groups at the ends and a phenyl group in the middle (Figure 34). [START_REF] Yang | Fabrication of nor-seco-cucurbit[10]uril based supramolecular polymers via self-sorting[END_REF] It was demonstrated that nor-seco-CB [START_REF] Aida | Functional supramolecular polymers[END_REF] and CB [START_REF] Zhu | Sustainable polymers from renewable resources[END_REF] when used as a mixture would undergo a self-sorting process in the presence of M37. In particular CB [START_REF] Zhu | Sustainable polymers from renewable resources[END_REF] was found to bind the phenyl linker, whereas the nor-seco-CB [START_REF] Aida | Functional supramolecular polymers[END_REF] was found to bind the naphthalene moieties of two separate M37 monomer thus stabilizing the formation of a supramolecular polymer. As inferred from a diffusion-ordered NMR spectroscopic analysis, the molecular weight of SP37 was estimated to be 5.6 × 10 4 Da, which is consistent with a DP = 16. At present, studies of supramolecular polymers based on CB [START_REF] Aida | Functional supramolecular polymers[END_REF] and other less-explored members of the CB[n] superfamily remain rare. We believe their further use could advance the supramolecular polymer field. In order to promote the development of cucurbit [START_REF] Aida | Functional supramolecular polymers[END_REF]uril-based supramolecular biomaterials, Xiao and co-workers prepared an AIE fluorescent SP38 comprised of the AIE molecule M38 and CB [START_REF] Aida | Functional supramolecular polymers[END_REF] and assembled via host-guest interactions (Figure 35). [START_REF] Luo | A new cucurbit[10]urilbased AIE fluorescent supramolecular polymer for cellular imaging[END_REF] In SP38, monomer M38, with four rotating ''arms'', was employed as a guest, while the relatively large CB [START_REF] Aida | Functional supramolecular polymers[END_REF] cavity was used to accommodate the arms of two separate M38 subunits through host-guest interactions, thus providing crosslinking. In the supramolecular polymer SP38, the molar ratio of M38: CB [START_REF] Aida | Functional supramolecular polymers[END_REF] is 1:2. The formation of SP38 was evidenced by thermodynamic analysis, DLS measurements, and DOSY studies.

Additionally, high content analysis system imaging of HUVEC (normal) and HeLa (cancer) cells incubated with M38 before and after treatment with CB [START_REF] Aida | Functional supramolecular polymers[END_REF] was carried out (Figure 35B). In the absence of CB [START_REF] Aida | Functional supramolecular polymers[END_REF] only a weak fluorescence signal ascribed to M38 was seen in the cytoplasm. However upon adding CB [START_REF] Aida | Functional supramolecular polymers[END_REF], particularly upon reaching a molar ratio of 1:2, an enhanced fluorescence was observed. The SP38 produced in situ as the result of this addition displayed an extraordinary blue fluorescence while displaying low bio-toxicity and excellent cytocompatibility. Due to the unique binding properties of CB [START_REF] Aida | Functional supramolecular polymers[END_REF] we believe that supramolecular systems analogous to SP38 will soon see application in numerous fields, including those associated with drug delivery and sensing. "pillar [START_REF] Natta | Von der stereospezifischen polymerisation zur asymmetrischen autokatalytischen synthese von makromolekülen nobel-vortrag am 12. dezember 1963[END_REF]arene" and reported their host-guest interactions with pyridinium and viologen derivatives. [START_REF] Ogoshi | Para-bridged symmetrical pillar[5]arenes: their lewis acid catalyzed synthesis and host-guest property[END_REF] The five hydroquinone units of pillar [START_REF] Natta | Von der stereospezifischen polymerisation zur asymmetrischen autokatalytischen synthese von makromolekülen nobel-vortrag am 12. dezember 1963[END_REF]arene are bridged by methylene groups at their 2 and 5 positions and can be easily functionalized via pre-or postmodification. Later, Meier and Cao et al. optimized the synthesis of pillar [START_REF] Natta | Von der stereospezifischen polymerisation zur asymmetrischen autokatalytischen synthese von makromolekülen nobel-vortrag am 12. dezember 1963[END_REF]arenes and achieved the isolation of pillar [START_REF] Szwarc | Living' polymers[END_REF]arenes as minor products. [START_REF] Cao | A facile and efficient preparation of pillararenes and a pillarquinone[END_REF] The distinct cavity sizes of these two classes of pillar[n]arenes (i.e., n = 5 and 6) led to investigations into their respective chemical features. One such study was carried out by the Huang group, which in 2010 reported the first pillar [START_REF] Szwarc | Living' polymers[END_REF]arene crystal structure. [START_REF] Han | DIBPillar[n]arenes (n = 5, 6): syntheses, x-ray crystal structures, and complexation with n-octyltriethyl ammonium hexafluorophosphate[END_REF] Subsequently, the same group reported the first pillararene-based supramolecular polymer (SP39) derived from a pillar [START_REF] Natta | Von der stereospezifischen polymerisation zur asymmetrischen autokatalytischen synthese von makromolekülen nobel-vortrag am 12. dezember 1963[END_REF]arene derivative monomer (M39) bearing an octyl group designed to act as a guest for the pillar [START_REF] Natta | Von der stereospezifischen polymerisation zur asymmetrischen autokatalytischen synthese von makromolekülen nobel-vortrag am 12. dezember 1963[END_REF]arene host (Figure 36A). [START_REF] Zhang | Formation of linear supramolecular polymers that is driven by C-H⋅⋅⋅π interactions in solution and in the solid state[END_REF] The authors proposed that the driving force for supramolecular polymer formation involved a combination of host-guest C-H interactions and favorable entropic effects associated with desolvation of the pillar [START_REF] Natta | Von der stereospezifischen polymerisation zur asymmetrischen autokatalytischen synthese von makromolekülen nobel-vortrag am 12. dezember 1963[END_REF]arene cavity. Since then, pillar[n]arenes (n = 5, 6) have been used to create a variety of supramolecular assemblies. [START_REF] Li | Pillararene-based supramolecular polymers: from molecular recognition to polymeric aggregates[END_REF][START_REF] Li | Pillararene-based supramolecular polymers[END_REF][START_REF] Ogoshi | Applications of pillar[n]arene-based supramolecular assemblies[END_REF][START_REF] Yang | Construction of a pillar[6]arene based water-soluble supramolecular pseudopolyrotaxane driven by cucurbit[8]uril-enhanced π-π interaction[END_REF][START_REF] Teng | A host-guest strategy for converting the photodynamic agents from a singlet oxygen generator to a superoxide radical generator[END_REF] Although larger pillar[n]arene homologues (n 7) have been known since 2012, [START_REF] Chen | Synthesis of pillar[7]arene[END_REF] studies of their host-guest interactions remain limited. Presumably, this reflects the fact that they have to date only been prepared in low yield as the result of cumbersome synthetic procedures. For insights into early development of pillar[n]arene chemistry, the reader is referred to a 2016 review by the Ogoshi group. [START_REF] Ogoshi | Pillar-shaped macrocyclic hosts pillar[n]arenes: new key players for supramolecular chemistry[END_REF]

Recent Advances in Pillararene-Based Supramolecular Polymers

Zhang and co-workers prepared a multiresponsive supramolecular polymer (SP40) based on a naphthalimide-functionalized monomer that contained pillar [START_REF] Natta | Von der stereospezifischen polymerisation zur asymmetrischen autokatalytischen synthese von makromolekülen nobel-vortrag am 12. dezember 1963[END_REF]arene (Figure 36B). [START_REF] Lin | Iodine controlled pillar[5]arene-based multiresponsive supramolecular polymer for fluorescence detection of cyanide, mercury, and cysteine[END_REF] Host-guest interactions between the pillar [START_REF] Natta | Von der stereospezifischen polymerisation zur asymmetrischen autokatalytischen synthese von makromolekülen nobel-vortrag am 12. dezember 1963[END_REF]arene (M40-1) and a pyridinium guest (M40-2) combined with donor-acceptor interactions involving the constituent naphthalimides and pillar [START_REF] Natta | Von der stereospezifischen polymerisation zur asymmetrischen autokatalytischen synthese von makromolekülen nobel-vortrag am 12. dezember 1963[END_REF]arenes, allowed these small monomers to self-assemble and form gels in DMSO/water solution (9/1, v/v). Under UV photo-irradiation, a bright yellow emission ascribed to the naphthalimide chromophores in the gel was observed. Addition of a small amount of iodine was found to quench completely the fluorescence, presumably as the result of the iodine out-competing the pyridinium moieties for the pillar [START_REF] Natta | Von der stereospezifischen polymerisation zur asymmetrischen autokatalytischen synthese von makromolekülen nobel-vortrag am 12. dezember 1963[END_REF]arene receptor sites. The redox features of iodine were then exploited. Specifically, the nonfluorescent gel produced upon exposure to iodine was shown to undergo a "turn-on" increase in the presence of CN -, Hg 2+ and L-Cys with high sensitivity. This detection capability was further verified in living cells. Adapted with permission from the American Chemical Society.

Another pillar[n]arene-based supramolecular polymeric material that proved useful in the detection of heavy metal ions was reported by Yang and co-workers. [START_REF] Dai | Supramolecular assemblyinduced emission enhancement for efficient mercury(II) detection and removal[END_REF] They synthesized a [2]biphenyl-extended pillar [START_REF] Szwarc | Living' polymers[END_REF]arene. Subsequently, this macrocyclic arene was modified by two thymine binding sites (M41-1; Figure 37). Separately, a tetraphenylethylene (TPE) chromophore M41-2 bearing two quaternary ammonium groups was synthesized. On the basis of NMR spectroscopic analyses, it was concluded that quaternary ammonium groups, such as those present in M41-2, could be included within M41-1 to form a 2:1 complex. As a result, a linear AABB-type supramolecular polymer SP41 was obtained upon mixing M41-2 with M41-1. This polymer contains numerous thymine arms and was found capable of binding Hg 2+ as the result of thymine-Hg 2+ -thymine interactions. The binding of the Hg 2+ ions to SP41 was thought restrict intramolecular rotations of the TPE chromophore, resulting in a strong Hg 2+ induced turn-on fluorescence.

Treatment with Na2S allowed removal of the Hg 2+ ions and recycling of the supramolecular polymer. The same group also reported several other fluorescent supramolecular polymers based on host-guest recognition. For example, they combined pillar [START_REF] Natta | Von der stereospezifischen polymerisation zur asymmetrischen autokatalytischen synthese von makromolekülen nobel-vortrag am 12. dezember 1963[END_REF]arene/imidazole pairings with diselenium dynamic covalent bonding to create a supramolecular polymer that proved responsive to certain reductants, such as D,L-dithiothreitol, as well as competitive guests, such as adiponitrile.[276] Efficient excitation energy transfer (EET) in pillar [START_REF] Natta | Von der stereospezifischen polymerisation zur asymmetrischen autokatalytischen synthese von makromolekülen nobel-vortrag am 12. dezember 1963[END_REF]arene-based supramolecular polymers was also utilized by the Yang group to fabricate an artificial light-harvesting system. [START_REF] Sun | Artificial lightharvesting supramolecular polymeric nanoparticles formed by pillar[5]arene-based hostguest interaction[END_REF] Here, fluorescent supramolecular polymers, SP43, were used as precursors to construct water-dispersible supramolecular polymeric nanoparticles (SPNPs) (Figure 39).

The supramolecular polymer, SP43, in turn was prepared via the self-assembly of three key monomers, M43-1, M43-2, and M43-3. The binding constant between pillar [START_REF] Natta | Von der stereospezifischen polymerisation zur asymmetrischen autokatalytischen synthese von makromolekülen nobel-vortrag am 12. dezember 1963[END_REF]arene and the guests was determined to be 1.2  10 4 M -1 in CHCl3. Within SP43 and the resulting SPNP, host-guest interactions served to shorten the distance between the chromophores thus facilitating EET. The presence of the bulky pillar [START_REF] Natta | Von der stereospezifischen polymerisation zur asymmetrischen autokatalytischen synthese von makromolekülen nobel-vortrag am 12. dezember 1963[END_REF]arene units also precluded self-quenching of the excitation energy within the SPNPs. As a result, the fluorescence quantum yield of the M43-2 fluorophores in SPNPs proved ~4.2-fold higher than in seen prior to self-assembly. A maximal energy transfer efficiency of 85% was seen at a M43-2/M43-3 molar ratio of 97:3. The antenna effect, a parameter used to quantify the light-harvesting ability of a system, was found to reach a maximum of 22 at a 99. Adapted with permission from John Wiley & Sons Inc.

Other Macrocycle-Based Supramolecular Polymers

Although the above-mentioned macrocycles have played a significant role in advancing our understanding of supramolecular polymers, there remain opportunities to exploit new host molecules in the context of materials-related supramolecular selfassembly. Of particular interest in this regard are underexplored host-guest interactions, such as those involving anion recognition, radical-radical interactions, and metallacyclebased neutral substrate binding. Recent efforts to exploit some of these less-common motifs are detailed below. [280], Copyright 2019. Adapted with permission from the American Chemical Society.

Although prior to the coverage period of this review, in 2015, Sessler, Anslyn and coworkers showed it was possible to create a self-assembled polymer as the result of anion recognition. They exploited a combination of highly acidic dicarboxylates and expanded porphyrins capable of forming 1:2 complexes with anions to produce a set of linear polymers, whose disassembly could be induced in a solvent dependent manner. This allowed the creation of a new class of solvent specific sensors. [START_REF] Zhang | Expanded porphyrin-anion supramolecular assemblies: environmentally responsive sensors for organic solvents and anions[END_REF] In 2019, the Flood group reported a new class of anion-driven supramolecular polymers based on anion-anion dimerization of phosphonate hydroxyanions within the cavity of -stacked cyanostar dimers (Figure 41A). [START_REF] Zhao | Linear supramolecular polymers driven by anion-anion dimerization of difunctional phosphonate monomers Inside cyanostar macrocycles[END_REF] The 2:2 stoichiometry of the cyanostar host M45-1 and the hydroxyanion guest allowed for the formation of linear supramolecular polymers (Figure 28B). Several diphosphonate monomers with different alkyl linkers were studied. It was found that longer alkyl linkers were more flexible, which led to greater monosomic polymerizability with an average degree of polymerization of nine emerging at 10 mM. The formation of SP45 (DP = 9) from dodecylene diphosphonates (M45-2) and the cyanostar host M45-1 was established by means of a single crystal X-ray diffraction analysis (Figure 28C). Viscosity measurements led the authors to suggest that the supramolecular polymerization in chloroform was concentration dependent with a critical polymerization concentration of 5 mM. The anion-driven polymerization process could be reversed via the addition of acid or competing anions, such as picric acid or hexyl monophosphonate.

In subsequent work, Flood and co-workers showed that an appropriate choice of monomers allowed for the construction of supramolecular homo-and copolymers with well-defined structures (Figure 42). [START_REF] Zhao | Tunable adhesion from stoichiometry-controlled and sequence-defined supramolecular polymers emerges hierarchically from cyanostar-stabilized anion-anion linkages[END_REF] In this latter study, a cyanostar, M46-1, bearing triethylene glycol chains for enhanced solubility was used. When M46-1 and phenylene diphosphate (M46-2) were mixed in a 2:1 stoichiometric ratio, SP46-1 formed, presumably as the result of strong anion-anion dimerization within the -stacked cyanostar dimers.

However, when a 1:1 ratio of the two monomers (M46-1 and M46-2) was used, alternating copolymers (SP46-2) containing both cyanostar-stabilized phosphate dimers (A) and cation-stabilized phosphate dimers (B) were obtained. Polymer SP46-1 was tested for its adhesive properties and found to support a 10-lb weight with a pull-off strength of about Recently, Stoddart and co-workers synthesized monomer M47 containing a cyclobis(paraquat-p-phenylene) ring and moieties designed to function as molecular pump at the end of a long linker (Figure 43). [START_REF] Cai | Molecular-pump-enabled synthesis of a daisy chain polymer[END_REF] When the bipyridinium groups in M47 were reduced by Zn dust, radicals were produced that stabilize radical-pair interactions; this, in turn, allowed for the construction of a supramolecular daisy chain polymer SP47 with a DP of 11. Reoxidation with Ag2SO4 led to preferential interactions with the p-phenylene units within the long chain linker resulting in the formation of a different mechanically interlocked daisy chain polymer. A key aspect of this work was the demonstration of redox control over self-assembly and the use of radical-radical interactions to stabilize one particular state. Other macrocycle hosts, such as chalcogen-and metal-containing macrocycles, have been used to create supramolecular polymers. [START_REF] Zeng | Chalcogen-bonding supramolecular polymers[END_REF] In 1990s, Fujita and coworkers and Stang et al. independently reported the preparation of platinum(II)-and palladium(II)-based macrocycles and demonstrated their utility in the context of host-guest recognition. [START_REF] Fujita | Preparation of a macrocyclic polynuclear complex, [(en)Pd(4,4'-bpy)]4(NO3)8 (en = ethylenediamine, bpy = bipyridine), which recognizes an organic molecule in aqueous media[END_REF][START_REF] Stang | Transition metal based cationic molecular boxes. self-assembly of macrocyclic Platinum(II) and Palladium(II) tetranuclear complexes[END_REF] Later, the Stang group made efforts to develop supramolecular polymers by modifying the exterior vertices of selected metallacycles to support non-covalent crosslinking interactions. [START_REF] Li | Cross-linked supramolecular polymer gels constructed from discrete multi-pillar[5]arene metallacycles and their multiple stimuli-responsive behavior[END_REF][START_REF] Zhang | Self-healing metallacycle-cored supramolecular polymers based on a metal-salen complex constructed by orthogonal metal coordination and host-guest interaction with amino acid sensing[END_REF] However, difficulties were encountered that were ascribed to the dynamic nature of the metallacycle cores. In 2019, Stang and Huang et al. synthesized a platinum(II) metallacycle M48 bearing two pyrene groups. [START_REF] Shi | Spontaneous formation of a cross-Linked supramolecular polymer both in the solid state and in solution, driven by platinum(II) metallacycle-based host-guest interactions[END_REF] In the solid state, it was found that one of the pyrene moieties on M48 could enter into the cavity of another AA-BB unit to form a host-guest complex (Figure 44). Moreover, C-H and C-HO interactions between the pyrene group and metallacycle, as well as - donor-acceptor interactions between the benzene rings on different M48 molecules, were observed that were thought to stabilize supramolecular polymerization (SP48). Following a similar molecular design strategy, the same group reported other supramolecular polymers based on host-guest interactions of platinum(II) metallacycles. [START_REF] Shi | Spontaneous supramolecular polymerization driven by discrete platinum metallacycle-based host-guest complexation[END_REF] This body of work extends the lexicon of receptors that may be considered in the design of new supramolecular macromolecular materials. Adapted with permission from the American Chemical Society.

Supramolecular Polymers Based on Aromatic Donor-Acceptor Interactions

Aromatic -/Donor-Acceptor Interactions: An Historical Perspective

Strong attractive interactions between aromatic structures have been known for many years and are widely recognized as playing a crucial role in biological systems. In 1960, Patrick and Prosser discovered a complex formed between benzene and hexafluorobenzene and attributed the underlying association to electron donor-acceptor or charge-transfer interactions. [START_REF] Patrick | A molecular complex of benzene and hexafluorobenzene[END_REF] In 1971, a crystal structure of the 1:1 complex of mesitylene and hexafluorobenzene was reported by Dahl, who emphasized the synergy between - bonding interactions and other weak intermolecular forces in stabilizing the observed stacked arrangement. [START_REF] Tor | Crystal structure of the 1:1 complex between mesitylene and hexafluorobenzene[END_REF] In the 1990s, Williams et al. suggested that the underlying complexation was driven by interactions between the positive and negative electric quadrupole moments of benzene and hexafluorobenzene. [START_REF] Williams | The molecular electric quadrupole moment and solid-state architecture[END_REF] In this same time frame (albeit slightly earlier), Sanders et al., using porphyrins as specific examples, described - interactions as being the result of - attractions that outweigh - repulsions. [START_REF] Hunter | The nature of .pi.-.pi. interactions[END_REF] Although the determinants of - interactions are still not completely understood, their importance in has been demonstrated in numerous instances. For example, the Stoddart group prepared rotaxanes and catenanes by exploiting the interactions between -electron-rich and -electron-deficient components. [START_REF] Claessens | π-π interactions in self-assembly[END_REF] In the early 2000s, reports of supramolecular polymers driven by - interactions, including electron donor-acceptor interactions began to appear. [START_REF] Adam | Fast photoconduction in the highly ordered columnar phase of a discotic liquid crystal[END_REF][START_REF] Fenniri | Helical rosette nanotubes with tunable chiroptical properties[END_REF][START_REF] Keizer | Hierarchical self-assembly of columnar aggregates[END_REF] The Iverson and the Li groups mimicked biological duplexes by donor-acceptor interaction-driven oligomers. [START_REF] Gabriel | Aromatic oligomers that form hetero duplexes in aqueous solution[END_REF][START_REF] Zhang | Dimerization of conjugated radical cations: an emerging non-covalent interaction for self-assembly[END_REF] In addition, Colquhoun and Hayes gave the first report of the generation of self-healing supramolecular polymers using a chain-folding polydiimide as the -electron acceptor and a pyrene end-capped siloxane as the -electron donor. [START_REF] Burattini | A novel self-healing supramolecular polymer system[END_REF] Also in the 1990s, Adam, et al. reported that a highly ordered arrangement of stacked disk-like organic molecules displayed a high photoinduced charge mobility. [START_REF] Adam | Fast photoconduction in the highly ordered columnar phase of a discotic liquid crystal[END_REF] These early examples have paved the development of polymeric materials wherein - interactions play a stabilizing role with many groups, including the authors,[301] contributing to the development of the field. While the packing of conjugated molecules is often a key feature of the resulting supramolecular constructs, it is important to appreciate that the term - stacking constitutes a geometric descriptor, rather than an energetic driving force per se.

Recent Advances in Supramolecular Polymers Involving Aromatic Stacking

To modulate the sequence structures of supramolecular polymers, Haino and coworkers designed a cleft-like tetraporphyrin M49-1 that was expected to interact with M49-2 via donor-acceptor interactions (Figure 45). [START_REF] Nadamoto | Supramolecular copolymerization by sequence reorganization of a supramolecular homopolymer[END_REF] On its own, M49-1 was found to associate to form a supramolecular polymer of general structure SP49-1 (Ka = 2.8  10 5 M - 1 in 1,2-dichloroethane) in chloroform solution. However, formation of SP49-1 was not observed in the presence of the electron-deficient monomer M49-2. Rather a copolymer SP49-2 derived from M49-1 and M49-2 (interacting with a Ka = 1.0  10 5 M -1 in 1,2dichloroethane) was obtained. The degree of polymerization (DP) of the supramolecular polymer was evaluated by monitoring the diffusion coefficient of its monomer and found to be concentration dependent as expected for a two-component self-assembled system. At a concentration of 30 mmol L -1 in both M49-1 and M49-2, a high degree of supramolecular polymerization, on the order of 10 3 , could be achieved. Differences in the diffusion coefficients and viscosities were seen as the ratio of M49-1 to M49-2 varied, leading the authors to suggest that different supramolecular structures were being formed. Moreover, SP49-2 was found to form highly ordered nanostructures; presumably this is a consequence of its constituent long alkyl chains, which were able to interact laterally. showing the proposed formation of SP50. [START_REF] Tian | Donor-acceptor-type supramolecular polymers derived from robust yet responsive heterodimeric tweezers[END_REF], Copyright 2016. Adapted with permission from the American Chemical Society.

In 2020, the George group used an amphiphilic, donor-acceptor monomer (M51-1) to prepare monodisperse charge-transfer supramolecular polymers (SP51) (Figure 47). [START_REF] Jalani | Transient dormant monomer states for supramolecular polymers with low dispersity[END_REF] Monomer M51-1 was synthesized by covalently linking a pyrene donor to a naphthalenediimide (NDI) acceptor using hexaethylene glycol as a flexible linker.

Presumably as the result of the strong intramolecular charge-transfer interactions between the pyrene and NDI subunits, M51-1 in an acetonitrile-water (1:1 v/v) mixture at a concentration of 50 M was found to adopt a foldameric conformation and further selfassemble into supramolecular polymers of general structure SP51 characterized by stacked bilayers. However, as formed, the self-assembled material (SP51) proved polydisperse (i.e., broad length distribution). As part of their study, the authors reduced the NDI chromophores to its radical anion (M51-2) and dianionic (M51-3) forms by treating with sodium dithionite.

Neither species was found to support strong charge-transfer interactions. As a consequence, M51-2 and M51-3 remain unfolded and exist as spherical nanoparticles. Both M51-2 and M51-3 could be oxidized back in a controlled manner to give the folded M51-1 form. The net is a kinetically controlled supramolecular polymerization giving rise to relatively monodisperse SP51 structures possessing a higher degree of polymerization than seen when M51-1 was allowed to undergo direct self-assembly. In 2019, Würthner and co-workers used a seeded living polymerization approach to prepare supramolecular copolymers with well-defined structural backbones (Figure 49). [START_REF] Wagner | Supramolecular block copolymers by seeded living polymerization of perylene bisimides[END_REF] They first prepared three perylene bisimides bearing methoxy, chlorine and methylthio groups in the so-called 1,7 bay-positions (M53-1, M53-2, and M53-3). In Copyright 2022. Adapted with permission from the American Chemical Society.

In 2020, Wang and co-workers reported a supramolecular donor-acceptor copolymer the formation of which could be controlled by modulating the organization of the chiral comonomers at the molecular level. [START_REF] Liao | Chirality-controlled supramolecular donoracceptor copolymerization with distinct energy transfer efficiency[END_REF] In this work, two structurally similar comonomers, M55-1 and M55-2, were designed, that contain π-aromatic diethynylacene core and alkyl chain peripheries linked by enantiopure N-[(1R or 1S)-phenylethyl]benzamides.

As the result of either homochiral or heterochiral supramolecular copolymerization, it proved possible to access, respectively, either a nanofiber, SP55-1, with a random arrangement of the constituents, or a nanoparticle SP55-2 with an alternate monomer arrangement. The relatively reduced steric hindrance between the stereogenic methyl groups in the heterochiral supramolecular ensemble led to an increase in the donor-acceptor complexation strength of the monomers, resulting in the formation of the alternate copolymer SP55-2. It was also found that, upon photoexcitation, M55-1 could serve as an energy donor, while M55-2 acted as an acceptor. The specifics of the resulting photoinduced energy transfer, as well as the circular polarized luminescence, could be controlled via the choice of SP55-1 vs SP55-2 at the supramolecular level. This makes structures such as SP55-1 vs SP55-2 of possible interest in the context of chiral sensing, recognition, and optoelectronics development.

Supramolecular Polymers Based on Multiple Interactions

In addition to supramolecular polymers constructed through primarily one noncovalent interaction, systems wherein more than one recognition mode is used to stabilize the system have begun to receive increasing attention. Recent developments along these lines are summarized in this section.

In 2020, the Stang group reported a set of supramolecular polymers with attractive properties that are stabilized by a combination of metal-ligand and other weak interactions. [START_REF] Yang | Hierarchical self-assembly of a pyrene-based discrete organoplatinum(II) double-metallacycle with triflate anions via hydrogen bonding and its tunable fluorescence emission[END_REF] For instance, they showed that it was possible to create a supramolecular polymer based on a combination of platinum(II)-pyridine coordination, zinc-terpyridine complexation, and crown ether-based host-guest interactions (Figure 52). In another 2020 contribution, Bai and co-workers reported two functionalized tetraphenylethene-derived monomers, M57-1 and M57-3, which bear four crown ether and four pillar [START_REF] Natta | Von der stereospezifischen polymerisation zur asymmetrischen autokatalytischen synthese von makromolekülen nobel-vortrag am 12. dezember 1963[END_REF]arene capping subunits, respectively (Figure 53). [START_REF] Li | Regulable aggregation-induced emission supramolecular polymer and gel based on self-sorting assembly[END_REF] A dialkylammonium salt tethered to a nitrile was also synthesized; it was designed to bridge M57-1 and M57-3

as the result of orthogonal binding of the nitrile and ammonium groups to the pillarene and crown ether moieties, respectively. In accord with the authors' design expectations, mixing these precursors led to the formation of the cross-linked supramolecular polymer SP57. The binding constants between M57-1 and M57-2 and between M57-2 and M57-3 in chloroform-d/acetone-d6 (2/1, v/v) were calculated to be 9.1  10 3 M -1 and 675 M -1 , respectively. It was found that SP57 could be depolymerized by adding K + as a competitive guest, but also that it would gelate at high monomer concentrations. Moreover, the tetraphenylethene cores endowed the system with aggregation-induced emission features.

In the monomeric states, the tetraphenylethene-based building blocks M57-1 and M57-3

were non-fluorescent due to presumed nonradiative decay of the respective excited states.

However, the corresponding cross-linked supramolecular networks exhibited enhanced fluorescence, a result ascribed to a reduction in molecular motions. This work thus provided an illustration of how supramolecular polymerization (or lack thereof) could be monitored via changes in aggregation-induced emission. [310], Copyright 2020. Adapted with permission from the American Chemical Society. Albertazzi and Pujals et al. designed monomer M58-1, a star-like system that contains a benzene-1,3,5-tricarboxamide (BTA) core, three amphiphilic wedges, and three azobenzene moieties as substituents (Figure 54). [START_REF] Fuentes | An azobenzene-based single-component supramolecular polymer responsive to multiple stimuli in water[END_REF] This monomer was found to selfassemble to give supramolecular polymers of general structure SP58 that the authors propose is stabilized by hydrogen-bonding between the BTA subunits, as well as hydrophobic interactions involving the amphiphilic wedges. Microscopic morphological studies of SP58 revealed long fibers with a diameter of about 6 nm. The authors also studied an ostensibly analogous monomer M58-2 bearing azobenzene units on the main backbone of the wedges; this derivative failed to self-assemble, underscoring the seemingly unique nature of M58-1 in terms of supporting supramolecular polymerization. In the case of M58-1, (E)-/(Z)-isomerization of the three azobenzene units imparted light responsiveness to the corresponding fibers SP58. By irradiating the samples alternatively with 365 nm and 455 nm light, and thus changing the proportion of the azobenzene subunits in the (E) configuration, self-assembly of M58-1 could be fine-tuned. This supramolecular system also exhibited responsiveness to temperature, ionic strength and pH, and in a reversible manner.

In 2021, Huang et al. reported a chemoresponsive supramolecular polymer (SP59) driven by host-guest complexation and metal coordination-based interactions between a bis(p-phenylene)-34-crown-10 macrocycle (M59-1) and a viologen-centered bisbenzonitrile guest (M59-2) (Figure 55). [START_REF] Wu | Chemoresponsive supramolecular polypseudorotaxanes with infinite switching capability[END_REF] Upon initial contact, the two monomers were found to form a [2]pseudorotaxane with a binding constant of 6.1  10 2 M -1 in acetone-d6 in which the nitrile groups remained accessible. SP59 was then generated via metal-ligand coordination of Ag(I) cations to the exposed nitrile groups. Specifically, it was found that the Ag atoms formed NAgN bonds characterized by an angle of 135.43 and distances of 2.203 and 2.268 Å, respectively, as inferred from a single crystal structure of SP59.

Interactions with the three O atoms on M59-1 were also inferred. When M59-1 and the Ag + cations were added in excess to an acetone solution of SP59, the supramolecular polymer was found to depolymerize to give a [START_REF] Flory | Fundamental principles of condensation polymerization[END_REF]pseudorotaxane with one Ag + coordinated to one nitrile group, one acetone molecule and three O atoms. The [START_REF] Flory | Fundamental principles of condensation polymerization[END_REF]pseudorotaxane could be converted back to the initial state via the subsequent addition of M59-2. The resulting depolymerization/repolymerization processes could be operated for a large number of cycles without apparent system fatigue. This feature could prove useful in fabricating degradable or self-healing functional materials. (DB24C8) moieties (M60-1). They then showed that the presence of a hexagonal metallacycle formed from the bis-platinum and bis(pyridinium) precursors (M60-2 and M60-3, respectively), a mechanically interlocked supramolecular network (SP60) would be stabilized as the result of threading of M60-1 through the metallacycle (Figure 56). [START_REF] Zhang | Mechanically interlocked networks cross-linked by a molecular necklace[END_REF] Owing to the multivalent interactions, rigid metallacycle skeleton, and motions involving the threaded structures, SP60 exhibited attractive mechanical properties in terms of stiffness (Young's modulus = 47.3 MPa), toughness (49.3 MJ/m 3 ), and strength (breaking stress = 12.1 MPa). This system also displayed dynamic responsiveness. Specifically, the addition of K + cations, which would compete for the crown ether moieties, led to release of the metallacycle from SP60 and a breakdown of the network. The addition of Br -was also found to decompose SP60 presumably as the result of destroying the labile Pt-N bond in the metallacycle. In 2021, Aida and Itoh et al. reported an alternating heterochiral supramolecular copolymer (SP61) prepared from two enantiomeric thiophene-fused cyclooctatetraene precursors, namely (+)-M61-1 and (-)-M61-1 (Figure 57A). [START_REF] Ueda | Alternating heterochiral supramolecular copolymerization[END_REF] When the monomers were mixed in a methylcyclohexane/dichloroethane solution in an equal molar ratio, a larger hydrodynamic diameter was recorded that was taken as evidence of supramolecular polymerization. Under these same conditions no change in the diameter as a function of time was seen for either enantiomer of the monomer ((+)-M61-1 and (-)-M61-1). A crystal structure of the polymeric assembly of M61-2, an analogue of M61-1 bearing shorter side chains, allowed this finding to be rationalized in structural terms. This structure revealed alternating hydrogen-bonded and polar π-stacked domains stabilized separately by two out of four side-chain amide groups and a one out of the 4 thiophene amide units present in (+)-M61-2 and (-)-M61-2. A computational investigation using density-functional theory led to the conclusion that two thiophene amide moieties facing one another would effectively cancel their in-plane dipole moments, resulting in the observed alternating heterochiral preference. In contrast, the computational studies led to the inference that the alternative homochiral stacking arrangement would suffer from steric repulsion between the thiophene plane and the alkylamide units, which would hamper effective supramolecular polymerization of the enantiomerically pure monomers. In 2017, the Wang group reported one-dimensional structures that could be used in optical waveguiding. [START_REF] Wang | Cooperative supramolecular polymerization of fluorescent platinum acetylides for optical waveguide applications[END_REF] These systems relied on the platinum acetylide monomer M62-1 (Figure 58). In methylcyclohexane, M62-1 displays a 2x greater fluorescence intensity than seen in chloroform, a finding that was taken as evidence that this monomer could undergo -aggregation even in moderately polar media, such as chloroform. It was also noted that M62-1 gives rise to a bisignate Cotton effect. This was ascribed to its propensity

Conclusion and Outlook

In this review, we have summarized representative recent advances in the area of supramolecular polymers (from 2016 to the present). The organization is based on the types of noncovalent interactions supporting the formation of the self-assembled material in question. The progress in this field reflects an increasing appreciation that such systems are attractive in terms of inter alia facile material fabrication, superior processing and recycling, self-healing and stimuli-responsiveness. They also show promise in a variety of potential applications including light harvesting, light waveguide generation, sensing, adsorption and separations, OLED device construction, and biomedicine development, to name a few. This progress notwithstanding, it is clear from considerations of natural oligomeric and selfassembled systems that the field of supramolecular polymers is still in its infancy, particularly with regard to the degree of complexity and functionality that can be incorporated into rationally designed materials. Challenges and opportunities in this area thus abound. Three directions that we see as worthy of particular emphasis are detailed below:

1) Development of new methods for supramolecular polymer synthesis. This encompasses both the creation of new monomers with a diversity of topologies and functions and their conversion into pre-designed supramolecular polymers. Complementing these efforts, attention should be devoted to developing or exploiting new non-covalent interactions that provide for both a high driving force for supramolecular polymerization and control over structure and function. These are points that, not surprisingly, have been emphasized in other reviews. [START_REF] Yang | Supramolecular polymers: historical development, preparation, characterization, and functions[END_REF] Nevertheless, they remain crucial if the field is to advance. We anticipate that additional supramolecular recognition motifs could be exploited to fabricate novel monomers, thus allowing in principle the creation of supramolecular polymers with novel properties. For example, halogen bonding as well as multiple interactions, including ion pair binding, could be incorporated into monomers. In addition, we believe that more effort should be devoted to optimizing the effects of the interactions that underlie current systems and their use in controlling the polymerization dynamics. As a recent example of this approach, Scherman et al. strengthened the polar- interactions of perfluorobenzene and benzene by encapsulating them within CB [START_REF] Berry | Celebrating 100 years of polymer science[END_REF]. [START_REF] Huang | Hostenhanced phenyl-perfluorophenyl polar-π interactions[END_REF][START_REF] Huang | Highly compressible glass-like supramolecular polymer networks[END_REF] By tuning dissociation kinetics of these host-enhanced polar- interactions, they were able to generate a glass-like supramolecular polymer networks exhibiting ultra-high compressibility and fast self-recovery. This work highlights a new direction in the design and construction of smart polymeric materials.

2) Characterization of supramolecular polymers: More precise determination of various self-assembled structures and pathways for their formation is expected to lead to a deeper understanding of the structure-function relationships that make supramolecular polymer so appealing in terms of both aesthetics and targeted applications. To date, UV-Vis absorption and 1 H NMR spectroscopies, diffusion-ordered NMR spectroscopy, electron microscopy, specific viscosity determination, dynamic light scattering, and differential scanning calorimetry have been widely used to verify the formation of supramolecular polymers. [START_REF] Liu | Characterization of supramolecular polymers[END_REF] However, new methods, such as small-angle neutron scattering, microcrystal electron crystallography and high level calculations, could be informative if brought to bear on the problems of supramolecular polymer characterization. The use of time-resolved optical methods, including fluorescence-and phosphorescence tools, are also appealing. Indeed, efforts to use the effects of aggregation-induced emission to characterize various dynamic processes associated with supramolecular polymers and their preparation. [START_REF] Guan | Synthesis and design of aggregation-induced emission surfactants: direct observation of micelle transitions and microemulsion droplets[END_REF][START_REF] Wang | Hierarchically compartmentalized supramolecular gels through multilevel self-sorting[END_REF][START_REF] Peng | Visualizing the Initial Step of Self-Assembly and the Phase Transition by Stereogenic Amphiphiles with Aggregation-Induced Emission[END_REF] However, extensions to other spectroscopies, including polarized luminescence and optically driven paramagnetic resonance methods, can easily be envisioned.

3) Applications of supramolecular polymers: Practical applications of supramolecular polymers are facing challenges for several reasons: i) The relatively weak nature of noncovalent bonds compared to covalent bonds leads to weaker inherent mechanical properties of supramolecular polymers. ii) The formation of supramolecular polymers with certain mechanical strength usually requires high concentrations of the building blocks in the medium; otherwise, their performances will be adversely affected.

iii) The dynamic nature of noncovalent bonds makes the resulting supramolecular polymers sensitive towards environment stimuli, including those that can induce supramolecular depolymerization.

However, this same lability, in turn, generally endows supramolecular polymers with dynamic reversibility and environmental responsiveness and has made them of interest as possible self-healing materials that may be readily recycled or used as sustainable/degradable green materials. Nevertheless, further developments along these lines are necessary if effective translation into the marketplace is to be realized. In a related vein, the design of so-called smart supramolecular polymers that produce one-dimensional fibers could lead to market opportunities in the areas of optical and electronic materials. In this context, we note that work to date serves to highlight the potential of supramolecular polymer engineering to create advanced photonic devices. [START_REF] Caram | Roomtemperature micron-scale exciton migration in a stabilized Emissive molecular aggregate[END_REF][START_REF] Rehhagen | Exciton migration in multistranded perylene bisimide J-aggregates[END_REF][START_REF] Hecht | Supramolecularly engineered J-aggregates based on perylene bisimide dyes[END_REF] Also of potential commercial interest and practical importance would be the development of supramolecular biomimetic materials and biomaterials. Here, the inherent responsiveness of supramolecular polymers, as well as an ability to tune their materials features, such as hardness, stretchiness, and toxicity, are appealing. We predict that in due course these attributes will lead to advances in diagnosis and treatment.

Figure 1 .

 1 Figure 1. (A) Chemical structure of M1, and the schematic representation of reversible

Figure 2 .

 2 Figure 2. (A) Chemical structures of M3-1 and M3-2. (B) Schematic illustration of the

Figure 3 .

 3 Figure 3. (A) Chemical structure of M4 and schematic representation of supramolecular

Figure 4 .

 4 Figure 4. (A) Molecular structures, schematic rosettes and supramolecular polymers,

Figure 5 . 3 .

 53 Figure 5. (A) Chemical structures of M6-1 and M6-2. (B) Schematic diagrams showing

Figure 6 .

 6 Figure 6. Chemical structures of (A) M7-1. [152], Copyright 2017. Adapted with

Figure 7 .

 7 Figure 7. Schematic illustrations of the reversible photoisomerization and self-assembly

Figure 8 .

 8 Figure 8. Chemical structures of M9, schematic representation of supramolecular

3. 2 .Figure 9 .

 29 Figure 9. Synthesis of poly[n]catenate via self-assembly of M10-1 and M10-2 into a

Figure 10 .

 10 Figure 10. (A) Synthesis and photocleavage of a ruthenium-containing supramolecular

Figure 11 .

 11 Figure 11. (A) Schematic illustration of SP12, a self-assembled polymer formed from

Figure 12 .

 12 Figure 12. (A) Chemical structures of M13 and protonated M13. (B) Schematic

Figure 13 .

 13 Figure 13. Chemical structures of M14-1, M14-2, and schematic showing the proposed

  2 metal: ligand (M:L) macrocycles (SP16-1) or coordination polymers (SP16-2) are formed in solution via the reaction of a bis-imidazole-tethered viologen (M16) with cis-[Pd(en)(NO3)2], trans-[Pd(CH3CN)2(Cl)2], or [Pd(CH3CN)4(BF4)2]. Electrochemical reduction of the viologen unit results either in a reversible "inflation/deflation" of the discrete 2:2 (M:L) macrocyclic assemblies (SP16-3) or dissociation in the case of the coordination polymers containing Pd(II) centers. It was proposed that the driving force for disassembly process is the formation of discrete boxshaped 2:2 (M:L) assemblies stabilized by π-dimerization of two viologen cation radicals.

Figure 15 .

 15 Figure 15. Chemical structure of M16, and schematic representation of the

Figure 16 .

 16 Figure 16. (A) Scheme illustration of SP17 formed from M17-1, M17-2, and CuNTf2 (Tf

Figure 17 . 4 . 1 . 2 .Figure 18 .

 1741218 Figure 17. (A) Scheme illustration of α-CD (n = 6), β-CD (n = 7), and γ-CD (n = 8).

Figure 19 .

 19 Figure 19. Schematic illustration showing the self-assembly of M20 leading to the

Figure 20 .

 20 Figure 20. Chemical structure of M21, and schematic representation of the

Figure 21 .

 21 Figure 21. Chemical structure of M22, and schematic representation of the corresponding

Figure 22 .

 22 Figure 22. (A) Chemical structure of M23. (B) Preparation of deep eutectic

Figure 23 .

 23 Figure 23. Chemical structure of M24, and schematic representation of the

Figure 24 . 1 (

 241 Figure 24. (A) Structural illustration of calixarenes shown in their idealized cone

[ 227 ]

 227 As inferred from the chemical shifts seen in the 1 H NMR spectrum, only a small portion of the guest molecule actually enters the host cavity. UV and fluorescent spectral studies revealed rational changes consistent with polymerization. The morphologies of the resulting supramolecular assemblies were investigated by atomic force microscopy and transmission electron microscopy. These imaging studies revealed long linear structures and provided support for the formation of calixarene-based supramolecular polymers with well-defined shapes and sizes.

Figure 25 .

 25 Figure 25. Chemical structure of M26-1, M26-2 and schematic representation of the

4. 3 . 4 . 3 . 1 . 4 . 3 . 2 .Figure 26 .

 343143226 Figure 26. Chemical structure of M27, and schematic representation of the

Figure 27 .

 27 Figure 27. (A) Chemical structures and schematic illustrations of M28, and SP28. [238],

Figure 28 .SP30- 2 .

 282 Figure 28. (A) Chemical structures and schematic illustrations of M30, and SP30-1,

SP33- 1

 1 proved reprocessable with the underlying chemical recyclability thought to stem from the exchange of the dynamic covalent (imine) bonds created by initial condensation between the monomers.

Figure 29 .

 29 Figure 29. (A) Chemical structures and schematic illustrations of M32, and SP32. [246],

Figure 30 .Figure 31 .

 3031 Figure 30. Space-filling models of CB[5]-CB[8]. [249], Copyright 2015. Adapted with

Figure 32 .

 32 Figure 32. (A) Chemical structures of CB[8], M35-1, and M35-2. (B) Schematic

Figure 33 .

 33 Figure 33. Schematic illustration of the supramolecular structures of SP36, the ternary

Figure 34 .

 34 Figure34. Schematic illustration of SP37 formed from M37, nor-seco-CB[START_REF] Aida | Functional supramolecular polymers[END_REF], and CB[START_REF] Zhu | Sustainable polymers from renewable resources[END_REF]

Figure 35 . 1 .

 351 Figure 35. (A) Schematic illustration of SP38 formed from M38 and CB[10]. (B) High

Figure 36 .

 36 Figure 36. (A) Chemical structures and schematic illustrations of M39, and SP39. [267],

Figure 37 . 1 and M42- 2

 3712 Figure 37. Chemical structures of M41-1 and M41-2, and schematic illustration of the

Figure 38 .

 38 Figure 38. Chemical structures of M42-1, M42-2 and M42-3. Also shown is a schematic

2 /M43- 3 .

 23 5:0.5 molar ratio of M43-This work is likely to inspire the design of yet-more efficient artificial lightharvesting systems via macrocycle-based self-assembly.

Figure 39 .

 39 Figure 39. Chemical structures of M43-1, M43-2 and M43-3. Also shown is a schematic

Figure 40 .

 40 Figure 40. Chemical structures of M44-1, M44-2, n-propanal, n-butanal and n-pentanal,

Figure 41 .

 41 Figure 41. (A) Design rule of the anion-directed supramolecular polymer. (B) Chemical

4. 2

 2 MPa. In contrast, SP46 displayed an adhesion strength that was only 20% of SP46-1, leading to the conclusion that linkage A is stronger than that of B. This work served to demonstrate one practical consequence of being able to modulate the nature of supramolecular polymers via a simple change in monomer stoichiometry.

Figure 42 .

 42 Figure 42. Chemical structures of the cyanostar M46-1, phenylene diphosphate M46-2,

Figure 43 .

 43 Figure 43. Chemical structures of M47 and schematic illustration of the supramolecular

Figure 44 .

 44 Figure 44. Chemical structure of M48, corresponding AA-BB unit, host-guest complex,

Figure 45 .

 45 Figure 45. Chemical structures of M49-1 and M49-2, and schematic illustration showing

Figure 46 .

 46 Figure 46. Chemical structures of M50-1, M50-2 and M50-3, and schematic illustration

Figure 47 .

 47 Figure 47. Chemical structures of M51-1, M51-2, and M51-3, and schematic illustration

Figure 48 .

 48 Figure 48. Chemical structure of M52 and schematic illustration of its proposed self-

a 2: 1 (

 1 v/v) solvent mixture of MCH and toluene (Tol) these PBIs were found to undergo thermodynamically favored supramolecular polymerization, resulting in the formation of ordered J-aggregates. These J-aggregates were found to exist as helical fibers, with the helical pitch of the M53-2-derived fibers (10 nm) being much wider than those of M53-1 or M53-3 fibers (both 5 nm). It was also found that when M53-1, M53-2, and M53-3 were subject to rapid cooling in MCH/Tol (2:1, v/v) (15 o C/min, 90 to 20 o C), metastable Haggregates were formed. When short seeds of the linear J-aggregates (produced by ultrasonication) were added to a MCH/Tol (2:1, v/v) solution of the kinetically trapped Haggregates, transformation to the corresponding thermodynamically stable supramolecular polymeric J-aggregate form occurred. Exploiting this strategy, the authors prepared A-B-A type and B-A-B type supramolecular block copolymers (SP53-1 and SP53-2) by means of a two-component seeded polymerization, wherein the same two monomers were used to produce kinetically trapped species and as transformation-promoting seeds.

Figure 49 .M54- 2 ,

 492 Figure 49. Chemical structures of the PBI monomers M53-1, M53-2, and M53-3 and

Figure 50 .

 50 Figure 50. (A) Chemical structures of monomers with boron-and nitrogen-cores. (B)

Figure 51 .

 51 Figure 51. Chemical structures of the monomers M55-1 and M55-2, as well as a

1 .

 1 [START_REF] Zhang | Self-healing heterometallic supramolecular polymers constructed by hierarchical assembly of triply orthogonal interactions with tunable photophysical properties[END_REF] These three binding modalities proved orthogonal to one another, allowing for stepwise self-assembly of a macromolecular system. In this study, a rhomboidal metallacycle M56-3 was first prepared via coordination of the platinum(II) centers on the crown ether monomer M56-2 with the dipyridyl moieties on monomer M56-1. The terminal terpyridine groups on metallacycle M56-3 were then treated with Zn(II) to afford linear supramolecular polymers of generalized structure SP56-The binding constant between M56-3 and Zn(II) was determined to be 3.8  10 4 M -1 in dichloromethane/acetonitrile. Finally, a supramolecular network SP56-2 was obtained by cross-linking these polymers by taking advantage of hostguest interactions between an added bis-ammonium salt and the benzo-21-crown-7 units present in the M56-2 subunits of SP56-1. At high concentrations, SP56-2 in appropriate solvents was found to gel, a state that necessarily endowed the system with stimuliresponsiveness and an ability to self-heal.

Figure 52 .

 52 Figure 52. Chemical structures of M56-1, M56-2, M56-3, and schematic illustration of

Figure 53 .

 53 Figure 53. (A) Chemical structures of M57-1, M57-2, M57-3, and (B) schematic

Figure 54 .

 54 Figure 54. (A) Chemical structures of M58-1, M58-2 and schematic illustration of how

Figure 55 .

 55 Figure 55. Chemical structures of M59-1, M59-2 and schematic illustrations showing the

Figure 56 .

 56 Figure 56. Chemical structures of M60-1, M60-2, and M60-3, as well as schematic

Figure 57 .

 57 Figure 57. (A) Chemical structures of (-)-M61-1, (+)-M61-1 and their supramolecular

  to self-assemble in a helical manner to form the supramolecular polymer SP62 with a DP = 242. When the amide bonds present in M62-1 were replaced by ester bonds (giving M62-2), no Cotton effect was observed. The high-molecular-weight of the ensembles formed as the result of M62-1 self-assembly allowed the authors to fabricate macroscopic fibers by electrospinning technique. These macrofibres were used for optical waveguiding applications and were found to exhibit outstanding optical waveguiding capability with low light propagation loss.

Figure 58 .

 58 Figure 58. Schematic illustration of the supramolecular polymerization of M62-1 and its

Acknowledgements

Notes

The authors declare no competing financial interest.

Acknowledgements

H.-Q. Peng thanks the National Natural Science Foundation of China (22105016, 52002015) and the Ministry of Science and Technology of China (2022YFA1505900). H.-Q. Peng is also grateful for support from the Open Fund of Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology (2019B030301003). X. Ji acknowledges initial funding from the Huazhong University of Science and Technology, where he is being supported by Fundamental Research Funds for the Central Universities (grant 2020kfyXJJS013). X. Ji is also grateful for support from the National Natural Science Foundation of China (22001087), and the Open Fund of Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology (2020MCF08). F. Huang thanks National Key Research and Development Program of China (2021YFA0910100), the National Natural Science Foundation of China (22035006), Zhejiang Provincial Natural Science Foundation of China (LD21B020001), and the Starry Night Science Fund of Zhejiang University Shanghai Institute for Advanced Study (SN-ZJU-SIAS-006) for financial support. Support from the Robert A. Welch Foundation (Chair funding F-0018 to J.L.S.) is also acknowledged with gratitude.