Self‐Assembly and Covalent Fixation for Topological Polymer Chemistry

A novel methodology (electrostatic self‐assembly and covalent fixation) has been proposed for designing various nonlinear polymer topologies, including monocyclic and polycyclic polymers, cyclic macromonomers and cyclic telechelics (kyklo‐telechelics), a‐ring‐with‐a‐branch topology polymers and polymeric topological isomers, as well as branched model polymers, such as star polymers and polymacromonomers. Thus, new telechelic polymer precursors having a moderately strained cyclic onium salt group as single or multiple end groups and carrying multifunctional carboxylates as the counterions were prepared through an ion‐exchange reaction. A variety of electrostatic self‐assemblies of these polymer precursors, formed particularly in dilute organic solution, was then subjected to heat in order to convert the ionic interactions into covalent linkages by ring‐opening reaction, and to produce topologically unique, nonlinear polymer architectures in high efficiency.     

Dynamic covalent polymers

This Highlight presents an overview of the rapidly growing field of dynamic covalent polymers. This class of polymers combines intrinsic reversibility with the robustness of covalent bonds, thus enabling formation of mechanically stable, polymer‐based materials that are responsive to external stimuli. It will be discussed how the inherent dynamic nature of the dynamic covalent bonds on the molecular level can be translated to the macroscopic level of the polymer, giving access to a range of applications, such as stimuli‐responsive or self‐healing materials. A primary distinction will be made based on the type of dynamic covalent bond employed, while a secondary distinction will be based on the consideration whether the dynamic covalent bond is used in the main chain of the polymer or whether it is used to allow side chain modification of the polymer. Emphasis will be on the chemistry of the dynamic covalent bonds present in the polymer, in particular in relation to how the specific (dynamic) features of the bond impart functionality to the polymer material, and to the conditions under which this dynamic behavior is manifested. © 2016 The Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3551–3577.     

Controlling supramolecular polymerization through multicomponent self‐assembly

The self‐assembly into supramolecular polymers is a process driven by reversible non‐covalent interactions between monomers, and gives access to materials applications incorporating mechanical, biological, optical or electronic functionalities. Compared to the achievements in precision polymer synthesis via living and controlled covalent polymerization processes, supramolecular chemists have only just learned how to developed strategies that allow similar control over polymer length, (co)monomer sequence and morphology (random, alternating or blocked ordering). This highlight article discusses the unique opportunities that arise when coassembling multicomponent supramolecular polymers, and focusses on four strategies in order to control the polymer architecture, size, stability and its stimuli‐responsive properties: (1) end‐capping of supramolecular polymers, (2) biomimetic templated polymerization, (3) controlled selectivity and reactivity in supramolecular copolymerization, and (4) living supramolecular polymerization. In contrast to the traditional focus on equilibrium systems, our emphasis is also on the manipulation of self‐assembly kinetics of synthetic supramolecular systems. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 34–78     

Preparation of few‐layer two‐dimensional polymers by self‐assembly of bola‐amphiphilic small molecules

Two bola‐amphiphilic small molecules, based on the diphenylanthracene skeleton structure, namely, BASM‐1 and its functionalized small molecule BASM‐2 , were designed and synthesized. The self‐assembly behavior and mechanism of these two molecules in aqueous solution were studied. The supramolecular two‐dimensional (2D) layer and the covalent 2D polymers were, respectively, prepared by these two molecules. What is more, the transverse size of the covalent 2D polymer laminates increased with the extension of the polymerization time. Atomic force microscopy results showed that both free‐standing single‐layer 2D polymers and few layer laminates with two to three molecular layers were obtained. So our work provides a simple and efficient method for directly preparing independent both supramolecular 2D polymers and covalent 2D polymers in liquid phase which is of great significance. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1748–1755     

Synthesis of Polymeric Topological Isomers through Electrostatic Self‐Assembly and Covalent Fixation with Star Telechelic Precursors

Summary: A pair of macromolecular constitutional isomers having topologically distinctive, dicyclic constructions, that is, θ‐shaped and manacle‐shaped polymers, has been synthesized from a polymer self‐assembly, comprised of three‐armed star poly(tetrahydrofuran) [poly(THF)] having N‐phenylpyrrolidinium salt end groups carrying dicarboxylate counteranions ( 1S / 2d ). The presence of the two constitutional polymeric isomers was confirmed by means of a reversed‐phase liquid chromatography (RPC) technique. Moreover, size exclusion chromatography (SEC) showed that a major component possesses notably larger hydrodynamic volume than the others, and is assignable as a manacle‐shaped isomer while a minor component is assigned as a θ‐shaped isomer. The statistics of the covalent‐linking process of 1S / 2d were consistent with the other experimental results.

An “electrostatic self‐assembly and covalent fixation” process of a trifunctional star‐shaped precursor gives rise to θ‐shaped and manacle‐shaped polymers.     

Supramolecular Polymerizations

Unimers of both natural and synthetic origin self‐assemble into linear, helical, columnar, planar and three‐dimensional structures depending upon the functionality of supramolecular interactions. Recent reports describing the mechanism of formation, properties and possible applications of these systems are critically reviewed. The assembling of one‐dimensional systems produces equilibrium polymers showing a length distribution and a degree of polymerization that may far exceed that of typical condensation polymers. Their growth may occur by a step‐by‐step process akin to polycondensation, and by cooperative processes such as helical growth or growth coupled to liquid crystallinity. Of particular interest are functional systems based on the coupling of a chemical reaction to supramolecular polymerization, and systems based on a covalent polymer hosted within the cavity of a supramolecular one. The assembly of two and three‐dimensional systems occurs through a process akin to crystallization. The supramolecular organization of amphiphiles such as block copolymers is currently well described by the mean‐field theory of unstable modes in homogeneous melts. An alternative, less sophisticated approach considers the growth of specifically designed building blocks. Possible applications are in areas that expand the uses of covalent polymers, electrochemical and photonic devices, ion‐selective channels, separation processes, microengines mimicking the performance of biological systems, storage of sequential information, biocompatible and patterned surfaces, sensors. A classification including additional systems that have been described as supramolecular polymers is presented.     

Emulsion procedures for thermally reversible covalent crosslinking of polymers

Quaternization and dequaternization of tertiary amine compounds were employed to obtain thermally reversible ionene networks from aqueous colloidal polymer dispersions prepared via emulsion polymerization. Chlorine‐functionalized polymers prepared via the emulsion copolymerization of styrene (St), butylacrylate (BA), or both with chloromethylstyrene, and amino‐functionalized polymers prepared via the emulsion copolymerization of St, BA, or both with 2‐(dimethylamino)ethylacrylate or 4‐vinylpyridine, were reacted without polymer separation, with a ditertiaryamine crosslinker and a dihalide crosslinker, respectively, to obtain crosslinked polymers. Crosslinked polymers were also obtained via the reaction of a chlorine‐functionalized polymer dispersion with an amino‐functionalized polymer dispersion or via the drying of the polymer blend prepared from the two kinds of dispersions. Reactive solubility experiments, flowability investigations (by thermocompression at ca. 215 °C), IR, and ¹H NMR analyses of the obtained crosslinked polymers indicated that the generated ionene bridges dequaternized on heating and requaternized on cooling. In comparison with solution crosslinking, no organic solvent was employed, and simple procedures were required for the preparation of the thermally reversible covalent crosslinked polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4373–4384, 2000     

Design of a Tunable Self‐Oscillating Polymer with Ureido and Ru(bpy)3 Moieties

An upper critical solution temperature (UCST)‐type self‐oscillating polymer was designed that exhibited rhythmic soluble–insoluble changes induced by the Belousov–Zhabotinsky (BZ) reaction. The target polymers were prepared by conjugating Ru(bpy)₃, a catalyst for the BZ reaction, to ureido‐containing poly(allylamine‐co‐allylurea) (PAU) copolymers. The Ru(bpy)₃‐conjugated PAUs exhibited a UCST‐type phase‐transition behavior, and the solubility of the polymer changed in response to the alternation in the valency of Ru(bpy)₃. The ureido content influences the temperature range of self‐oscillation, and the oscillation occurred at higher temperatures than conventional LCST‐type self‐oscillating polymers. Furthermore, the self‐oscillating behavior of the Ru‐PAU could be regulated by addition of urea, which is a unique tuning strategy. We envision that novel self‐oscillating polymers with widely tunable soluble‐insoluble behaviors can be rationally designed based these UCST‐type polymers.     

Synergistic Covalent and Supramolecular Polymers for Mechanically Robust but Dynamic Materials

Nature has engineered delicate synergistic covalent and supramolecular polymers (CSPs) to achieve advanced life functions, such as the thin filaments that assist in muscle contraction. Constructing artificial synergistic CSP materials with bioinspired mechanically adaptive features, however, represents a challenging goal. Here, we report an artificial CSP system to illustrate the integration of a covalent polymer (CP) and a supramolecular polymer (SP) in a synergistic fashion, along with the emergence of notable mechanical and dynamic properties which are unattainable when the two polymers are formed individually. The synergistic effect relies on the peculiar network structures of the SP and CPs, which endow the resultant CSPs with overall improved mechanical performance in terms of the stiffness, strength, stretchability, toughness, and elastic recovery. Moreover, the dynamic properties of the SP, including self‐healing, stimuli‐responsiveness, and reprocessing, are also retained in the CSPs, thus leading to their application as programmable and tunable materials.     

Recent Advances in Self‐Oscillating Polymer Material Systems

In 1996, we first reported self‐oscillating polymer gels exhibiting autonomous swelling‐deswelling oscillations driven by the Belousov‐Zhabotinsky reaction. In contrast to conventional stimuli‐responsive gels, the self‐oscillating gel can autonomously and periodically change its volume in a closed solution without any external stimuli. Since the first report, the novel concept of self‐oscillating gels has been expanded into various polymer and gel systems. Herein, we summarize recent advances in self‐oscillating polymers and gels.     

Synthesis and Qualitative Analysis of BACy and Its Self‐polymer

Many synthetic strategies of a reversible cross‐linker N,N′‐bis(acryloyl)cystamine (BACy) involve the typical condensation between the amino group of cystamine and the acyl group of acryloyl chloride in the mixed‐phase solvent system. In this study, the synthesis of BACy was performed in pure organic phase during the whole process. The yield and purity of synthesized BACy were comparable to those from aqueous/organic phase procedures. In addition, polymerization of BACy was also carried out by free radical reaction to prepare the self‐polymer and hydrogel which were characterized with FT‐IR, DSC and UV/VIS spectrophotometer. Notably, the BACy and its self‐polymer were both cleavable when exposed to the reducing agents, i.e. 1,4‐dithiothreitol (DTT) and 2‐mercaptoethanol (β‐ME). Interestingly, the reduced product of BACy contains vinyl and thiol groups, which could be further applied to the co‐polymerization with other monomeric units. On the other hand, carefully controlled reduction of BACy self‐polymer may be used to create the modified polymers with available thiol‐end groups for further chemistry. Together, our study provides modified procedure for BACy synthesis and characteristics of BACy self‐polymer and hydrogel. Further application of BACy and its self‐polymer in developing polymers with additional functionality is anticipated.     

Self‐Templated Stepwise Synthesis of Monodispersed Nanoscale Metalated Covalent Organic Polymers for In Vivo Bioimaging and Photothermal Therapy

Size‐ and shape‐controlled growth of nanoscale microporous organic polymers (MOPs) is a big challenge scientists are confronted with; meanwhile, rendering these materials for in vivo biomedical applications is still scarce. In this study, a monodispersed nanometalated covalent organic polymer (MCOP, M=Fe, Gd) with sizes around 120 nm was prepared by a self‐templated two‐step solution‐phase synthesis method. The metal ions (Fe³⁺, Gd³⁺) played important roles in generating a small particle size and in the functionalization of the products during the reaction with p ‐phenylenediamine (Pa). The resultant Fe‐Pa complex was used as a template for the subsequent formation of MCOP following the Schiff base reaction with 1,3,5‐triformylphloroglucinol (Tp). A high tumor suppression efficiency for this Pa‐based COP is reported for the first time. This study demonstrates the potential use of MCOP as a photothermal agent for photothermal therapy (PTT) and also provides an alternative route to fabricate nano‐sized MCOPs.     

Macromolecular Recognition and Macroscopic Interactions by Cyclodextrins

Herein macromolecular recognition by cyclodextrins (CDs) is summarized. Recognition of macromolecules by CDs is classified as main‐chain recognition or side‐chain recognition. We found that CDs form inclusion complexes with various polymers with high selectivity. Polyrotaxanes in which many CDs are entrapped in a polymer chain were prepared. Tubular polymers were prepared from the polyrotaxanes. CDs were found to recognize side‐chains of polymers selectively. CD host polymers were found to form gels with guest polymers in water. These gels showed self‐healing properties. When azobenzene was used as a guest, the gel showed sol‐gel transition by photoirradiation. When ferrocene was used, redox‐responsive gels were obtained. Macroscopic self‐assembly through molecular recognition has been discovered. Photoswitchable gel association and dissociation have been observed.     

Entropy‐Driven Selectivity for Chain Scission: Where Macromolecules Cleave

We show that, all other conditions being equal, bond cleavage in the middle of molecules is entropically much more favored than bond cleavage at the end. Multiple experimental and theoretical approaches have been used to study the selectivity for bond cleavage or dissociation in the middle versus the end of both covalent and supramolecular adducts and the extensive implications for other fields of chemistry including, e.g., chain transfer, polymer degradation, and control agent addition are discussed. The observed effects, which are a consequence of the underlying entropic factors, were predicted on the basis of simple theoretical models and demonstrated via high‐temperature (HT) NMR spectroscopy of self‐assembled supramolecular diblock systems as well as temperature‐dependent size‐exclusion chromatography (TD SEC) of covalently bonded Diels–Alder step‐growth polymers.     

A versatile method for cyclic polymers from conjugated and unconjugated vinyl monomers

A versatile method was introduced to prepare cyclic polymers from both conjugated and unconjugated vinyl monomers. It was developed on the combination of the RAFT polymerization and the self‐accelerating double strain‐promoted azide‐alkyne click (DSPAAC) reaction. In this approach, a switchable chain transfer agent 1 was designed to have hydroxyl terminals and a functional pyridinyl group. The protonation and deprotonation of pyridinyl group endowed the chain transfer agent 1 with a switchable control capability to RAFT polymerization of both conjugated and unconjugated vinyl monomers. Based on this, RAFT polymerization and the following hydroxyl end group modification were used to prepare various azide‐terminated linear polymers including polystyrene, poly(N‐vinylcarbazole), and polystyrene‐block‐poly(N‐vinylcarbazole). Using sym‐dibenzo‐1,5‐cyclooctadiene‐3,7‐diyne (DBA) as small linkers, the corresponding cyclic polymers were then prepared via the DSPAAC reaction between DBA and azide terminals of the linear precursors. Due to the self‐accelerating property of DSPAAC reaction, this bimolecular ring‐closing reaction could efficiently produce the pure cyclic polymers using excess molar amounts of DBA to linear polymer precursors. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 1811–1820     

Toughening a Self‐Healable Supramolecular Polymer by Ionic Cluster‐Enhanced Iron‐Carboxylate Complexes

Supramolecular polymers that can heal themselves automatically usually exhibit weakness in mechanical toughness and stretchability. Here we exploit a toughening strategy for a dynamic dry supramolecular network by introducing ionic cluster‐enhanced iron‐carboxylate complexes. The resulting dry supramolecular network simultaneous exhibits tough mechanical strength, high stretchability, self‐healing ability, and processability at room temperature. The excellent performance of these distinct supramolecular polymers is attributed to the hierarchical existence of four types of dynamic combinations in the high‐density dry network, including dynamic covalent disulfide bonds, noncovalent H‐bonds, iron‐carboxylate complexes and ionic clustering interactions. The extremely facile preparation method of this self‐healing polymer offers prospects for high‐performance low‐cost material among others for coatings and wearable devices.     

Fusion of Different Crosslinked Polymers Based on Dynamic Disulfide Exchange

Crosslinked polymers (CLPs) exhibit exceptional mechanical properties as well as good chemical and solvent resistance. However, their reprocessing, recycling, and modification remain difficult. One promising approach to overcome this limitation is to introduce dynamic covalent bonds that enable chain‐exchange reactions and network‐structure rearrangements in identical polymer networks (A–A fusion), resulting in self‐healing and reprocessing properties. Reported here is the fusion of two distinct polymer networks (A–B fusion) by the dynamic behavior of bis(2,2,6,6‐tetramethylpiperidin‐1‐yl)disulfide (BiTEMPS) at the interface between different CLPs. The appearance, swelling behavior, and mechanical properties of the fused samples indicate exchange reactions of the BiTEMPS units and the formation of topological bonds at the interface, commensurate with the generation of a CLP that exhibits tunable properties.     

Self‐Propagating Association of Zwitterionic Polymers Initiated by Ionene Polymers

A self‐propagating association of zwitterionic polymers is observed when a small amount of x,y‐ionene bromide (x = 3 or 6; y = 3, 4, 6, 10 or 12) polymer is added to aqueous solutions of zwitterionic polymer, poly[3‐dimethyl(methacryloyloxyethyl)ammoniumpropanesulfonate] (PDMAPS), to give large amount of PDMAPS precipitate. The self‐propagating association initiated by ionene polymers is explained in terms of the electrostatic interaction between the ionene polymers and the zwitterionic polymers whereupon the geometry of the charges on the polymer chains plays an important role.     

Efficient Homodifunctional Bimolecular Ring‐Closure Method for Cyclic Polymers by Combining RAFT and Self‐Accelerating Click Reaction

An efficient metal‐free homodifunctional bimolecular ring‐closure method is developed for the formation of cyclic polymers by combining reversible addition‐fragmentation chain transfer (RAFT) polymerization and self‐accelerating click reaction. In this approach, α,ω‐homodifunctional linear polymers with azide terminals are prepared by RAFT polymerization and postmodification of polymer chain end groups. By virtue of sym‐dibenzo‐1,5‐cyclooctadiene‐3,7‐diyne (DBA) as small linkers, well‐defined cyclic polymers are then prepared using the self‐accelerating double strain‐promoted azide–alkyne click (DSPAAC) reaction to ring‐close the azide end‐functionalized homodifunctional linear polymer precursors. Due to the self‐accelerating property of DSPAAC ring‐closing reaction, this novel method eliminates the requirement of equimolar amounts of telechelic polymers and small linkers in traditional bimolecular ring‐closure methods. It facilitates this method to efficiently and conveniently produce varied pure cyclic polymers by employing an excess molar amount of DBA small linkers.

     

Synthesis of Responsive Two‐Dimensional Polymers via Self‐Assembled DNA Networks

Despite a growing interest in two‐dimensional polymers, their rational synthesis remains a challenge. The solution‐phase synthesis of a two‐dimensional polymer is reported. A DNA‐based monomer self‐assembles into a supramolecular network, which is further converted into the covalently linked two‐dimensional polymer by anthracene dimerization. The polymers appear as uniform monolayers, as shown by AFM and TEM imaging. Furthermore, they exhibit a pronounced solvent responsivity. The results demonstrate the value of DNA‐controlled self‐assembly for the formation of two‐dimensional polymers in solution.