Network Formation in an Orthogonally Self-Assembling System

Influence of the Glass Transition Temperature and the Density of Crosslinking Groups on the Reversibility of Diels-Alder Polymer Networks

The interest in self-healing, recyclable, and adaptable polymers is growing. This work addresses the reversibility of crosslink formation based on Diels-Alder reaction in copolymer networks containing furfuryl and maleimide groups, which represent the "diene" and the "dienophile," respectively. The copolymers are synthesized by atom transfer radical polymerization (ATRP) and free radical polymerization. The diene bearing copolymers are crosslinked either with a small molecule containing two dienophiles or with a dienophile bearing copolymer. The influence of the crosslinking temperature on the Diels-Alder reaction is analyzed. Furthermore, the influence of the glass transition temperature and the influence of the density of crosslinking groups on the thermo-reversibility of crosslinking are investigated by temperature dependent infrared spectroscopy and differential scanning calorimetry. It is shown that the reversibility of crosslinking is strongly influenced by the glass transition temperature of the system.

100th Anniversary of Macromolecular Science Viewpoint: Engineering Supramolecular Materials for Responsive Applications-Design and Functionality

Supramolecular polymers allow access to dynamic materials, where noncovalent interactions can be used to offer both enhanced material toughness and stimuli-responsiveness. The versatility of self-assembly has enabled these supramolecular motifs to be incorporated into a wide array of glassy and elastomeric materials; moreover, the interaction of these noncovalent motifs with their environment has shown to be a convenient platform for controlling material properties. In this Viewpoint, supramolecular polymers are examined through their self-assembly chemistries, approaches that can be used to control their self-assembly (e.g., covalent cross-links, nanofillers, etc.), and how the strategic application of supramolecular polymers can be used as a platform for designing the next generation of smart materials. This Viewpoint provides an overview of the aspects that have garnered interest in supramolecular polymer chemistry, while also highlighting challenges faced and innovations developed by researchers in the field.

Supramolecular polymer bottlebrushes

The field of supramolecular chemistry has long been known to generate complex materials of different sizes and shapes via the self-assembly of single or multiple low molar mass building blocks. Matching the complexity found in natural assemblies, however, remains a long-term challenge considering its precision in organizing large macromolecules into well-defined nanostructures. Nevertheless, the increasing understanding of supramolecular chemistry has paved the way to several attempts in arranging synthetic macromolecules into larger ordered structures based on non-covalent forces. This review is a first attempt to summarize the developments in this field, which focus mainly on the formation of one-dimensional, linear, cylindrical aggregates in solution with pendant polymer chains - therefore coined supramolecular polymer bottlebrushes in accordance with their covalent equivalents. Distinguishing by the different supramolecular driving forces, we first describe systems based on π-π interactions, which comprise, among others, the well-known perylene motif, but also the early attempts using cyclophanes. However, the majority of reported supramolecular polymer bottlebrushes are formed by hydrogen bonds as they can for example be found in linear and cyclic peptides, as well as so called sticker molecules containing multiple urea groups. Besides this overview on the reported motifs and their impact on the resulting morphology of the polymer nanostructures, we finally highlight the potential benefits of such non-covalent interactions and refer to promising future directions of this still mostly unrecognized field of supramolecular research.

Exploring Orthogonal Hydrogen Bonding towards Designing Organic-Salt-Based Supramolecular Gelators: Synthesis, Structures, and Anticancer Properties

A series of primary ammonium monocarboxylate (PAM) salts derived from β-alanine derivatives of pyrene and naphthalene acetic acid, along with the parent acids, were explored to probe the plausible role of orthogonal hydrogen bonding resulting from amide⋅⋅⋅amide and PAM synthons on gelation. Single-crystal X-ray diffraction (SXRD) studies were performed on two parent acids and five PAM salts in the series. The data revealed that orthogonal hydrogen bonding played an important role in gelation. Structure-property correlation based on SXRD and powder X-ray diffraction data also supported the working hypothesis upon which these gelators were designed. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and cell migration assay on a highly aggressive human breast cancer cell line, MDA-MB-231, revealed that one of the PAM salts in the series, namely, PAA.B2, displayed anticancer properties, and internalization of the gelator salt in the same cell line was confirmed by cell imaging.

Hydrogen bonding and thermoplastic elastomers - a nice couple with temperature-adjustable mechanical properties

Styrene-butadiene copolymers are modified with varying fractions of benzoic acid moieties being able to perform hydrogen bonding. This is done by using a simple synthetic approach which utilizes click chemistry. Temperature-dependent dynamic mechanical properties are studied, and it turns out that even the apparently rather simple hydrogen bonding motif has a marked impact on the material properties due to the fact that it facilitates the formation of a supramolecular polymer network. Besides a glass transition, the investigated functionalized copolymers exhibit a second endothermic transition, known as a quasi-melting. This is related to the opening of the hydrogen bonding complexes. Additionally to dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), temperature-dependent infrared (IR) spectroscopy and small angle X-ray scattering (SAXS) are used to understand the structure-property relationships.

Light induced assembly and self-sorting of silica microparticles

To tailor the properties of colloidal materials, precise control over the self-assembly of their constituents is a prerequisite. Here, we govern the assembly of silica particles by functionalization with supramolecular moieties which interact with each other via directional and reversible hydrogen bonding. Through a generally applicable synthesis protocol, two different types of self-complementary hydrogen bonding moieties, BTA- and UPy-derivatives, are anchored to silica particles. Their self-assembly is initiated by the UV-induced removal of a photolabile protecting group, allowing the formation of hydrogen bonds between tethered molecules. The light-induced assembly of BTA- and UPy-decorated colloids in single-component dispersions and colloidal self-sorting in mixed dispersions is studied. Furthermore, we demonstrate that UPy-colloids can dissasemble upon addition of traces of a competitive binder (NaPy). This work provides further insight into the utility of supramolecular handles to orchestrate the assembly of micron-sized colloids via non-oligonucleotide hydrogen-bonding units.

Controllable Supramolecular Polymerization Promoted by Host-Enhanced Photodimerization

In this letter, we report a new method of controllable supramolecular polymerization, taking advantage of host-enhanced photodimerization. The low-molecular-weight supramolecular oligomers were formed by noncovalent complexation between cucurbit[8]urils (CB[8]) and the bifunctional monomers (DBN) with Brooker's merocyanine moiety (MOED) on either end. Interestingly, when irradiated with UV light, the supramolecular oligomers could transform into supramolecular polymers with high molecular weight. The molecular weight of supramolecular polymers could be controlled by varying the irradiation time. It is highly anticipated that this work can enrich the methods on the modulation of supramolecular polymerization.

Dynamic supramolecular complexes constructed by orthogonal self-assembly

CONSPECTUS: Supramolecular complexes, including various low-molecular-mass structures and large molecular aggregates that are assembled by reversible and highly directional noncovalent interactions, have attracted more and more attention due to their fascinating and unconventional chemical and physical properties that are different from those of traditional architectures encountered by covalently linked backbones. Supramolecular complexes are by nature dynamic architectures considering the reversibility of noncovalent interactions by which small molecular monomers can assemble into specific architectures that are able to be repeatably reorganized through the assembly/disassembly processes under certain environmental factors such as temperature, concentration, and solvent conditions. The construction of supramolecular complexes by orthogonal self-assembly with different types of highly specific, noninterfering interactions is currently attracting considerable interest since they not only can dynamically self-assemble, but also can be tuned by various external stimuli through addressing each type of noncovalent interaction separately. Therefore, these dynamic supramolecular complexes, especially with external responsiveness, represent the most outstanding candidates for the future development of functional and smart materials, and even mimic the assembling process of natural systems. In this Account, we will summarize the recent advances of dynamic supramolecular complexes constructed by orthogonal self-assembly in soluiton in two sections: (1) Construction strategies for supramolecular complexes based on orthogonal self-assembly, whose dynamic behaviors with external responsiveness were not experimentally investigated but potentially existed due to the intrinsic reversibility of noncovalent bonds; (2) dynamic behaviors of multiresponsive supramolecular complexes, which were experimentally reported to exhibit reversible multi-responsiveness to external stimuli. Dynamic nature is one of intrinsic properties of supramolecular complexes constructed by self-assembly. Therefore, in the first section, we will describe the dynamic self-assembly in the construction of supramolecular complexes, but will focus on their external responsive dynamic behaviors in the second section. In addition, considering that an increasing number of supramolecular complexes constructed by biological building blocks through bio-orthogonal assembly as mimics of biological systems have been reported in recent years, in the second section we will also present some typical examples on such special dynamic biological supramolecular complexes. The final part of this Account is devoted to foreseeing the rapid development of dynamic supramolecular complexes toward applications in functional and smart materials and fundamental questions facing dynamic supramolecular complexes in the future.

Isophthalic Acid-Pyridine H-Bonding: Simplicity in the Design of Mechanically Robust Phase-Segregated Supramolecular Polymers

We report a new series of supramolecular polymeric networks based on the isophthalic acid-pyridine (IPA-Py) H-bonding motif. The IPA units were attached as end-groups to telechelic poly(ethylene-co-butylene) to create a tetrafunctional macromonomer, which was cross-linked by the addition of various bispyridines. Some of the supramolecular polymer networks thus made display surprisingly good mechanical characteristics. We show that their structure and properties are strongly influenced by the nature of the bispyridine motif and by the fact that some of the IPA-Py motifs aggregate into particularly well-defined hard phases.

Progressive macromolecular self-assembly: from biomimetic chemistry to bio-inspired materials

Macromolecular self-assembly (MSA) has been an active and fruitful research field since the 1980s, especially in this new century, which is promoted by the remarkable developments in controlled radical polymerization in polymer chemistry, etc. and driven by the demands in bio-related investigations and applications. In this review, we try to summarize the trends and recent progress in MSA in relation to biomimetic chemistry and bio-inspired materials. Our paper covers representative achievements in the fabrication of artificial building blocks for life, cell-inspired biomimetic materials, and macromolecular assemblies mimicking the functions of natural materials and their applications. It is true that the current status of the deliberately designed and obtained nano-objects based on MSA including a variety of micelles, multicompartment vesicles, and some hybrid and complex nano-objects is at their very first stage to mimic nature, but significant and encouraging progress has been made in achieving a certain similarity in morphologies or properties to that of natural ones. Such achievements also demonstrate that MSA has played an important and irreplaceable role in the grand and long-standing research of biomimetic and bio-inspired materials, the future success of which depends on mutual and persistent efforts in polymer science, material science, supramolecular chemistry, and biology.

Supramolecular gels formed from multi-component low molecular weight species

Low molecular weight supramolecular gels consist of small molecules (gelators) that in an appropriate solvent self-assemble into nano- or micro-scale network structures resulting in the formation of a gel. Most supramolecular gels consist of two parts, namely the solvent and the gelator. However, the concept of multi-component supramolecular gels, in which more than one compound is added to the solvent, offers a facile way (e.g. by changing the ratio of the different components) to tailor the properties of the gel. The simplest multi-component gels consist of two components added to the solvent and are the most widely studied to date. There are three general classes of such multi-component gels that have been investigated. The first class requires all the added components to access the gel; that is, no component forms a gel on its own. A second class uses two (or more) gelators which can either co-assemble or self-sort into distinct assemblies and the final class consists of one (or more) gelator and one (or more) non-gelling additive which can impact the assembly process of the gelator and therefore the gel's properties.

Stimuli-responsive supramolecular polymeric materials

Supramolecular materials, dynamic materials by nature, are defined as materials whose components are bridged via reversible connections and undergo spontaneous and continuous assembly/disassembly processes under specific conditions. On account of the dynamic and reversible nature of noncovalent interactions, supramolecular polymers have the ability to adapt to their environment and possess a wide range of intriguing properties, such as degradability, shape-memory, and self-healing, making them unique candidates for supramolecular materials. In this critical review, we address recent developments in supramolecular polymeric materials, which can respond to appropriate external stimuli at the fundamental level due to the existence of noncovalent interactions of the building blocks.