Unexpected entropy-driven ring-opening polymerization in a reversible supramolecular system

Visible-Light Driven Control Over Triply and Quadruply Hydrogen-Bonded Supramolecular Assemblies

Supramolecular polymers offer tremendous potential to produce new "smart" materials, however, there remains a need to develop systems that are responsive to external stimuli. In this work, visible-light responsive hydrogen-bonded supramolecular polymers comprising photoresponsive supramolecular synthons (I-III) consisting of two hydrogen bonding motifs (HBMs) connected by a central ortho-tetrafluorinated azobenzene have been characterized by DOSY NMR and viscometry. Comparison of different hydrogen-bonding motifs reveals that assembly in the low and high concentration regimes is strongly influenced by the strength of association between the HBMs. I, Incorporating a triply hydrogen-bonded heterodimer, was found to exhibit concentration dependent switching between a monomeric pseudo-cycle and supramolecular oligomer through intermolecular hydrogen bonding interactions between the HBMs. II, Based on the same photoresponsive scaffold, and incorporating a quadruply hydrogen-bonded homodimer was found to form a supramolecular polymer which was dependent upon the ring-chain equilibrium and thus dependent upon both concentration and photochemical stimulus. Finally, III, incorporating a quadruply hydrogen-bonded heterodimer represents the first photoswitchable AB type hydrogen-bonded supramolecular polymer. Depending on the concentration and photostationary state, four different assemblies dominate for both monomers II and III, demonstrating the ability to control supramolecular assembly and physical properties triggered by light.

Predicting Ion Sequestration in Charged Polymers with the Steepest-Entropy-Ascent Quantum Thermodynamic Framework

The steepest-entropy-ascent quantum thermodynamic framework is used to investigate the effectiveness of multi-chain polyethyleneimine-methylenephosphonic acid in sequestering rare-earth ions (Eu3+) from aqueous solutions. The framework applies a thermodynamic equation of motion to a discrete energy eigenstructure to model the binding kinetics of europium ions to reactive sites of the polymer chains. The energy eigenstructure is generated using a non-Markovian Monte Carlo model that estimates energy level degeneracies. The equation of motion is used to determine the occupation probability of each energy level, describing the unique path through thermodynamic state space by which the polymer system sequesters rare-earth ions from solution. A second Monte Carlo simulation is conducted to relate the kinetic path in state space to physical descriptors associated with the polymer, including the radius of gyration, tortuosity, and Eu-neighbor distribution functions. These descriptors are used to visualize the evolution of the polymer during the sequestration process. The fraction of sequestered Eu3+ ions depends upon the total energy of the system, with lower energy resulting in greater sequestration. The kinetics of the overall sequestration are dependent on the steepest-entropy-ascent principle used by the equation of motion to generate a unique kinetic path from an initial non-equilibrium state.

Synthesis and Emergent Photophysical Properties of Diketopyrrolopyrrole-Based Supramolecular Self-Assembly

Diketopyrrolopyrrole (DPP)-based molecular semiconductors exhibit intriguing optical and charge transport properties. Herein, we rationally design a series of electronically identical but structurally distinct Hamilton receptor (HR)-based supramolecular assembly of DPP. The HR endows supramolecular assemblies via hydrogen bonding with enhanced structural ordering and excitonic couplings. The mechanism of supramolecular self-assembly was probed by diffusion ordered spectroscopy (DOSY) nuclear magnetic resonance (NMR) and solid-state IR spectroscopy studies. We investigated the morphology of self-assembly, photophysical and electrochemical properties and compared them with the identical DPP molecular structures without HRs. The microstructure of self-assembly was probed with atomic force microscopy in thin films. Subsequently, the influence of solid-state packing was studied by single-crystal X-ray diffraction. The single-crystal structure of HR-TDPP-C20 reveals slipped stack arrangements between the two neighboring chromophores with π-π stacking distance and slip angle of 3.55 Å and 35.4°, respectively. Notably, the slight torsional angle of 1° between thiophene and lactam rings and small π-π stacking distance suggest a significant intermolecular coupling between thiophene (D) and lactam (A) rings. This intramolecular coupling between two π-π chromophore stacks manifests in their optical properties. In this manuscript, we report rational design and synthesis of supramolecular self-assembly of DPP with a collection of compelling structural and optical properties.

Supramolecular polymers with reversed viscosity/temperature profile for application in motor oils

We report novel supramolecular polymers, which possess a reversed viscosity/temperature profile. To this end, we developed a series of ditopic monomers featuring two self-complementary binding sites, either the guanidiniocarbonyl pyrrole carboxylic acid (GCP) or the aminopyridine carbonyl pyrrole carboxylic acid (ACP). At low temperatures, small cyclic structures are formed. However, at elevated temperatures, a ring–chain transformation leads to the formation of a supramolecular polymer. We demonstrate that this effect is dependent on the concentration of the solution and on the polarity of the solvent. This effect can counteract the loss of viscosity of the solvent at elevated temperatures, thus opening an application of our systems as viscosity index improvers (VIIs) in working fluids. This was tested for different motor oils and led to the identification of one compound as a promising VII.

Preparation of multi-temperature responsive elastomers by generating ionic networks in 1,2-polybutadiene using an anionic melting method

The development of reversible networks in elastomers provided unique inspiration for the design of advanced polymers with excellent properties. In this paper, we adopted an anionic melting method to introduce carboxylate groups into 1,2-polybutadiene (1,2-PB), using maleic anhydride as a modifier, and sodium hydride (NaH), calcium hydride (CaH2), and lithium aluminum hydride (LiAlH4) as metallization reagents. Na-Based, Ca-based, and Li/Al-based ionic bond networks were constructed in the covalently crosslinked 1,2-PB. The effects of the electronegativity and valence of the metal ions on the strength and reversible temperature of the ionic network were studied. Payne effect was shown by rheological tests, demonstrating the interactions between the ionic networks and rubber chains. The reforming temperature for these ionic networks was studied by stress-relaxation analysis, and shape memory experiments were performed based on these temperatures. This concept provides novel inspiration for the design of high-performance and temperature-adaptive elastomers.

Supramolecular Polymers Capable of Controlling Their Topology

One important class of supramolecular materials is one-dimensionally elongated supramolecular polymers, in which monomers are associated by reversible intermolecular interactions, yielding a fibrous morphology. Unlike frequently reported conventional supramolecular polymers based on, for instance, host-guest interactions, those composed of one-dimensionally stacked π-conjugated molecules can be encoded with high degrees of internal order by cooperative association of the rigid aromatic monomers, endowing such supramolecular polymers with extraordinary properties and functionality. However, their internal order has not yet been exploited to manipulate the complex landscape of well-defined states of the supramolecular polymer backbone, which may induce new functionalities beyond the intrinsic properties of the backbones. This Account will focus on the inceptive phase of our research on supramolecular polymers with high degrees of internal order able to impart intrinsic curvature to their backbones. Initially, we developed a naphthalene molecule functionalized with barbituric acid, which forms uniform toroidal short fibers with diameters of approximately 16 nm via the formation of hydrogen-bonded cyclic hexamers (rosettes). As we thought the uniformity of the toroid size to arise from the intrinsic curvature generated upon stacking of the rosettes, we exploited this intrinsic curvature to design continuously curved extended supramolecular polymers by extension of such molecular π-systems. The intrinsic curvature produced by the monomers with more expanded π-systems indeed gave us access to higher-order structures (topologies) ranging from randomly folded to helically folded coils in extended supramolecular polymers. We will discuss the kinetic aspects of the generation of intrinsic curvature for topology control, including the formation of toroidal structures resulting from ring-closing processes. For extended supramolecular polymers with well-defined topologies, we will discuss manipulation of a complex landscape of well-defined states by external stimuli. The incorporation of a photoresponsive azobenzene chromophore in the original naphthalene molecular scaffold allowed us to reversibly destroy or recover the curvature of the main chain through trans- cis photoisomerization. By means of this photocontrollable curvature, we have demonstrated light-induced unfolding of helically folded structures into entirely stretched structures. Furthermore, a direct extension of the π-conjugated core provided us with access to unprecedented supramolecular polymers with emergent time-dependent topology transitions. Molecules with a naphthalene core conjugated with two phenylene units kinetically afforded supramolecular polymers that consist of helically folded and misfolded domains. Upon aging the supramolecular polymer solution, we observed spontaneous folding of the misfolded domains in a time scale of days, eventually obtaining a supramolecular polymer topology analogous to the tertiary structure of proteins. These supramolecular polymers with unrivaled and active topologies provide new prospects for supramolecular polymers as one-dimensional nanomaterials.

Ring-opening supramolecular polymerization controlled by orthogonal non-covalent interactions

The π–π interaction has been successfully utilized to orthogonally regulate the supramolecular polymerization driven by quadruple hydrogen bonding.

Fluorescent supramolecular polymers with aggregation induced emission properties

This review summarizes the recent developments in AIE fluorescent supramolecular polymeric materials based on different types of intermolecular noncovalent interactions, and their wide ranging applications as chemical sensors, organic electronic materials, bio-imaging agents and so on.

4-Sulfocalix[4]arene/Cucurbit[7]uril-Based Supramolecular Assemblies through the Outer Surface Interactions of Cucurbit[n]uril

Upon mixing of aqueous solutions of the freely soluble building blocks cucurbit[7]uril (Q[7]) and 4-sulfocalix[4]arene (SC[4]A), white microcrystals instantly separate in near-quantitative yield. The driving force for this assembly is suggested to be the outer-surface interaction of the Q[n]. Dynamic light scattering, scanning electron microscopy, and NMR (diffusion-ordered NMR spectroscopy) analyses have confirmed the supramolecular aggregation of Q[7] and SC[4]A. Titration ¹H NMR spectroscopy and isothermal titration calorimetry have shown that the interaction ratio of Q[7] and SC[4]A is close to 3:1. Moreover, the Q[7]/SC[4]A-based supramolecular assembly can accommodate molecules of some volatile compounds or luminescent dyes. Thus, this work offers a simple and highly efficient means of preparing adsorbent or solid fluorescent materials.

Readily functionalized AAA-DDD triply hydrogen-bonded motifs

Herein we present a new, readily functionalized AAA-DDD hydrogen bond array. A novel AAA monomeric unit (3a-b) was obtained from a two-step synthetic procedure starting with 2-aminonicotinaldehyde via microwave radiation (overall yield of 52-66%). 1H NMR and fluorescence spectroscopy confirmed the complexation event with a calculated association constant of 1.57 × 107 M-1. Likewise, the usefulness of this triple hydrogen bond motif in supramolecular polymerization was demonstrated through viscosity measurements in a crosslinked supramolecular alternating copolymer.

Progress in bio-inspired sacrificial bonds in artificial polymeric materials

Mimicking natural structures has been highly pursued in the fabrication of synthetic polymeric materials due to its potential in breaking the bottlenecks in mechanical properties and extending the applications of polymeric materials. Recently, it has been revealed that the energy dissipating mechanisms via sacrificial bonds are among the important factors which account for strong and tough attributes of natural materials. Great progress in synthesis of polymeric materials consisting of sacrificial bonds has been achieved. The present review aims at (1) summarizing progress in the mechanics and chemistry of sacrificial bond bearing polymers, (2) describing the mechanisms of sacrificial bonds in strengthening/toughening polymers based on studies by single-molecule force spectroscopy, chromophore incorporation and constitutive laws, (3) presenting synthesis methods for sacrificial bonding including dual-crosslink, dual/multiple-network, and sacrificial interfaces, (4) discussing the important advances in engineering sacrificial bonding into hydrogels, biomimetic structures and elastomers, and (5) suggesting future works on molecular simulation, viscoelasticity, construction of sacrificial interfaces and sacrificial bonds with high dissociative temperature. It is hoped that this review will provide guidance for further development of sacrificial bonding strategies in polymeric materials.

Model-driven engineering of supramolecular buffering by multivalency

The ability of chemists to regulate the concentration of molecules is extremely important. However, as reactions are slowly superseded by more complex reaction networks, new ways of regulating molecular concentrations are needed. Recently, we described a system in which the concentration of a monovalent molecule with catalytic activity was buffered over a wide concentration range by its binding to a divalent molecule. Guided by model predictions, we are able to experimentally optimize the system by increasing the valency of the buffer, with even-numbered valencies displaying superior buffering capabilities. These results allow us to understand and gain more control over the activities of molecules in complex molecular systems, thereby obtaining insights into natural systems as well as creating adaptive artificial systems.

A supramolecular system in which the concentration of a molecule is buffered over several orders of magnitude is presented. Molecular buffering is achieved as a result of competition in a ring–chain equilibrium of multivalent ureidopyrimidinone monomers and a monovalent naphthyridine molecule which acts as an end-capper. While we previously only considered divalent ureidopyrimidinone monomers we now present a model-driven engineering approach to improve molecular buffering using multivalent ring–chain systems. Our theoretical models reveal an odd–even effect where even-valent molecules show superior buffering capabilities. Furthermore, we predict that supramolecular buffering can be significantly improved using a tetravalent instead of a divalent molecule, since the tetravalent molecule can form two intramolecular rings with different “stabilities” due to statistical effects. Our model predictions are validated against experimental ¹H NMR data, demonstrating that model-driven engineering has considerable potential in supramolecular chemistry.

Fluorescent Supramolecular Polymeric Materials

Fluorescent supramolecular polymeric materials are rising stars in the field of fluorescent materials not only because of the inherent optoelectronic properties originating from their chromophores, but also due to the fascinating stimuli-responsiveness and reversibility coming from their noncovalent connections. Especially, these noncovalent connections influence the fluorescence properties of the chromophores because their state of aggregation and energy transfer can be regulated by the assembly-disassembly process. Considering these unique properties, fluorescent supramolecular polymeric materials have facilitated the evolution of new materials useful for applications in fluorescent sensors, probes, as imaging agents in biological systems, light-emitting diodes, and organic electronic devices. In this Review, fluorescent supramolecular polymeric materials are classified depending on the types of main driving forces for supramolecular polymerization, including multiple hydrogen bonding, electrostatic interactions, π-π stacking interactions, metal-coordination, van der Waals interactions and host-guest interactions. Through the summary of the studies about fluorescent supramolecular polymeric materials, the status quo of this research field is assessed. Based on existing challenges, directions for the future development of this field are furnished.

Synthesis of Polyamide Dendrimers bearing Multiple Hydrogen Bonding Parts on the Periphery

A series of polyamide dendrimers (GO-G2) having carboxy1 groups on the periphery were prepared by the divergent method. The reaction of G 18NH2 dendrimer with glutaric acid anhydride gave polyamide dendrimer (Gi-8COOH). The structure of polyamide dendrimers were confirmed by IR, 1H, 13C NMR, MALDI-TOF mass, and elemental analysis. Poly(4-vinylpyridine) (P4VP) was prepared by atom transfer radical polymerization of 4-vinylpyridine. A hydrogen bonded GI-8COOH/P4VP 1/8 (molar ratio) complex was prepared by mixing GIA8COOH with P4VP in methanol, followed by removing the solvent. IR spectrum of Gl-8COOH/P4VP 1/8 complex showed the characteristic absorptions at 2550 and 1950 cm-1 due to the hydrogen bonding. A glass transition temperature (Tg) of GI-8COOH/P4VP 1/8 complex was observed at 177°C by DSC measurement, whereas the Tg values of G 1-8COOH and P4VP were at 147 and 138°C, respectively.

Surprising impact of remote groups on the folding--unfolding and dimer-chain equilibria of bifunctional H-bonding unimers

Oligoamide 1, consisting of two H-bonding units linked by a trimethylene linker, was previously found to form a very stable, folded dimer. In this work, replacing the side chains and end groups of 1 led to derivatives that show the surprising impact of end groups on the folding and dimer-chain equilibria of the resultant molecules.

Hyperbranched supramolecular polymer constructed from twisted cucurbit[14]uril and porphyrin via host–guest interactions

A novel supramolecular polymer was constructed between Q[14] host and 5,10,15,20-tetrakis(N-butyl-4-pyridinium)porphyrin tetrabromide guest depending on the host–guest interactions, representing the first cucurbit[14]uril-involved host–guest supramolecular polymer.

A supramolecular ladder polymer prepared by hydrogen bonding-mediated self-assembly of a metallomacrocycle

A defect-free supramolecular ladder polymer was prepared by H-bond-mediated self-assembly of a metallocycle 1 as determined by NMR, viscometry and dynamic laser light scattering studies.

A cross-linked supramolecular polymer constructed from pillar[5]arene and porphyrine via host–guest interactions

A novel supramolecular polymer, DMeP5@TImPor, was successfully constructed via host–guest interactions between the host molecule, bis-(methoxy-pillar[5]arene) (DMeP5), and the guest molecule, 5,10,15,20-tetrakis{butoxy-4-(1H-imidazol-1-yl)}porphyrine (TImPor).

Unprecedented cucurbituril-based ternary host–guest supramolecular polymers mediated through included alkyl chains

Novel ternary host–guest supramolecular polymers constructed from Q[8] and 1′,1′′-(alkylene-1,4-diyl)bis(1-butyl)-4,4′-(bipyridine-1,1′-diium)bromide with included alkyl chains as the mediating functional groups have been fabricated and structurally characterized for the first time.

Characterization of supramolecular polymers

Supramolecular polymers are made of monomers that are held together by noncovalent interactions. This is the reason for the wide range of novel properties, such as reversibility and responses to stimuli, exhibited by supramolecular polymers. A range of supramolecular polymerization methods have been developed leading to a number of novel supramolecular materials. However, standard techniques for the characterization of supramolecular polymers have yet to be established. The dynamic nature of supramolecular polymers makes them difficult to be fully characterized using conventional polymer techniques. This tutorial review summarizes various methods for characterizing supramolecular polymers, including theoretical estimation, size exclusion chromatography, viscometry, light scattering, vapor pressure osmometry, mass spectrometry, NMR spectroscopy, scanning probe microscopy, electron microscopy, and atomic force microscopy-based single molecule force spectroscopy. Each of these methods has its own particular advantages and disadvantages. Most of the methods are used to characterize the supramolecular polymer chain itself. However, some of the methods can be used to study the self-assembled state formed by supramolecular polymers. The characterization of a supramolecular polymer cannot be realized with a single method; a convincing conclusion relies on the combination of several different techniques.

Highly stable chiral (A)6-B supramolecular copolymers: a multivalency-based self-assembly process

A novel type of chiral layered supramolecular copolymer with high molecular weight has been assembled from a hydrogen bonded C(6)-symmetric zinc porphyrin hexamer and chiral C(3)-symmetric pyridine hexadentate linkers driven by multivalent zinc porphyrin-pyridine coordination. UV-vis, circular dichroism, and static light scattering experiments revealed that the formation of the layered supramolecular copolymers is at first dynamically controlled and then becomes thermodynamically controlled.

Thermoresponsive dynamers: thermally induced, reversible chain elongation of amphiphilic poly(acylhydrazones)

A nanostructured poly(acylhydrazone), which is reversibly formed in acidic aqueous solution from di(aldehyde) and di(acylhydrazine) monomers with appended hexaglyme groups, was found to display lower critical solution (LCS) behavior. Remarkably, under acidic conditions in which polymerization is reversible, large and reversible molecular weight (M(w)) increases were observed in response to elevated temperatures, both below and above the LCS temperature. No variation in M(w) was evident under neutral and alkaline conditions, in which the acylhydrazone condensation is essentially irreversible. Results of turbidometry studies, size-exclusion chromatography-multiangle laser light scattering (SEC-MALLS), and transmission electron microscopy (TEM) suggest that heating the polymer below the LCS temperature leads to polymer growth with preservation of the characteristic nanostructured morphology, whereas the onset of the microphase separated state causes a fundamental change in morphology, in which the polymer chains aggregate into larger bundles and fibers. van't Hoff analysis of a small molecule model system indicates that the acylhydrazone condensation is enthalpy driven (ΔH(eq) = -8.2 ± 0.2 kcal·mol(-1) and ΔS(eq) = -11.1 ± 0.4 = cal·mol(-1)·K(-1)), which suggests that the observed polymer growth with temperature is not a consequence of the intrinsic thermodynamics of the intermonomer linkage but is likely the result of the thermoresponsive characteristics conferred by the multiple hexaglyme groups. The system described displays double control of the dynamer state by two orthogonal agents, heat and protons (pH). It also represents a prototype for dynamic materials displaying multiple control adaptive behavior.

Dynamers: Dynamic Molecular and Supramolecular Polymers

Dynamers are defined as constitutional dynamic polymers, i.e. polymeric entities whose monomeric components are linked through reversible connections and have therefore the capacity to modify their constitution by exchange and reshuffling of their components. They may be either of supramolecular or molecular nature depending on whether the connections are non-covalent interactions or reversible covalent bonds. They are formed respectively either by polyassociation with interactional recognition or by polycondensation with functional recognition between the connecting subunits. Both types are illustrated by specific examples implementing hydrogen bonding on one hand and formation of imine-type bonds on the other. The dynamic properties confer to dynamers the ability to undergo adaptation and driven evolution under the effect of external chemical or physical triggers. Dynamers thus are constitutional dynamic materials resulting from the application of the principles of constitutional dynamic chemistry to polymer science.

Interplay Between Cyclization and Polymerization in Ditopic Cavitand Monomers

Quinoxaline kite velcrands were functionalized with a ureidopyrimidone moiety at the lower rim, in order to combine multiple hydrogen bonding and solvophobic interactions to build dual-coded supramolecular structures. The orthogonality of the selected binding motifs has been demonstrated by NMR studies, as well as the reversible nature of the obtained assemblies. At low concentration, the presence of a ring-chain equilibrium has been established by 1H NMR experiments, and confirmed by static and dynamic light scattering measurements.