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     

Supramolecular Polymerization from Controllable Fabrication to Living Polymerization

Supramolecular polymers have attracted plenty of interest in the scientific community; however, developing controllable methods of supramolecular polymerization remains a serious challenge. This article reviews some recent developments of methods for supramolecular polymerization from controllable fabrication to living polymerization. Three facile methods with general applicability for controllable fabrication of supramolecular polymers have been established recently: the first method is a self‐sorting approach by manipulating ring–chain equilibrium based on noncovalent control over rigidity of monomers; the second is covalent polymerization from supramonomers formed by noncovalent interactions; and the third is supramolecular interfacial polymerization. More excitingly, living supramolecular polymerization has been achieved by two elegant strategies, including seeded supramolecular polymerization under pathway complexity control and chain‐growth supramolecular polymerization by metastable monomers. It is anticipated that this review may provide some guidance for precise fabrication of supramolecular polymers, leading to the construction of supramolecular polymeric materials with controllable architectures and functions.     

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.     

Biocomplementary interaction behavior in DNA‐like and RNA‐like polymers

A series of nucleobased polymers and copolymers were synthesized through atom transfer radical polymerization (ATRP). Biocomplementary DNA‐ and RNA‐like supramolecular complexes are formed in dilute DMSO solution through nucleobase recognition. ¹H NMR titration studies of these complexes in CDCl₃ indicated that thymine‐adenine (T‐A) and uracil‐adenine (U‐A) complexes form rapidly on the NMR time scale with high association constants (up to 534 and 671 M^–1, respectively) and result in significant T_g increase. WAXD and differential scanning calorimetry analyzes in the bulk state indicate the presence of highly physical cross‐linked structures and provide further details into the nature of the self‐assembly of these systems. Furthermore, this study is of discussion on the difference in the hydrogen bond strength between T‐A and U‐A base pairs within polymer systems, indicating that the strength of hydrogen bonds in RNA U‐A pairs is stronger than that in DNA T‐A base pairs. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6388–6395, 2009     

A Homotelechelic bis‐terpyridine macroligand: One‐step synthesis and its metallo‐supramolecular self‐assembly

A homotelechelic macroligand bearing two 2,2′:6′,2″‐terpyridin‐4′‐yl units, as chain ends, is used as building block for the preparation of a linear metallo‐supramolecular chain‐extended polymer. The macroligand has been prepared by nitroxide‐mediated polymerization (NMP) of styrene using a bis‐terpyridine‐functionalized NMP initiator. The controlled character of the NMP process has been confirmed by detailed characterization of the polymer by size‐exclusion chromatography, nuclear magnetic resonance spectroscopy as well as mass spectrometry. Subsequently, the self‐assembly with Fe^II ions into the chain‐extended metallopolymer and the disassembly thereof, in the presence of a strong competitive ligand, has been studied by UV–vis absorption spectroscopy and diffusion‐ordered NMR spectroscopy. The reversibility of the formation of the metallo‐supramolecular material, when addressed by external stimuli, could be proven. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Small molecule self‐assembly in polymer matrices

Using small molecules in polymer matrices is common in applications such as (i) plasticizing polymers to modify the glass transition and mechanical properties and (ii) dispersion of photoactive or electroactive small molecules in polymer matrices in organic‐electronic devices Aggregation of these small molecules and phase separation leading to crystallization often cannot be morphologically controlled. If these are designed with self‐assembling codes such as hydrogen bonding or aromatic interactions, their phase separation behavior would be distinctly different. This review summarizes the studies on morphologies in such situations, such as (i) sub‐surface assembly in polymer matrices, (ii) controlled polymerization‐induced phase separation to create polymer blends, (iii) using the polymer to direct the assembly of small molecules in liquid crystalline devices, (iv) functionalizing a polymer with self‐assembling small molecules to cause organo‐gelation which the polymer itself would not by itself, and (v) using such systems as templates to create porous polymer structures. Organic–inorganic hybrids using polymers as templates for nanostructures and imprinted porous membranes is an emerging area. Since self‐assembly is one of the dominating area of research with respect to both small molecules, polymers as well as the combination of the two, this review summarizes the studies on the aforementioned topics. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 451–478     

Self‐assembly of metallo‐supramolecular block copolymers in thin films

The self‐assembly of a metallo‐supramolecular PS‐[Ru]‐PEO block copolymer, where ‐[Ru]‐ is a bis‐2,2′:6′,2″‐terpyridine‐ruthenium(II) complex, in thin films was investigated. Metallo‐supramolecular copolymers exhibit a different behavior as compared to their covalent counterparts. The presence of the charged complex at the junction of the two blocks has a strong impact on the self‐assembly, effecting the orientation of the cylinders and ordering process. Poly(ethylene oxide) cylinders oriented normal to the film surface are obtained directly regardless of the experimental conditions over a wide range of thicknesses. Exposure to polar solvent vapors can be used to improve the lateral ordering of the cylindrical microdomains. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4719–4724, 2008     

Synthesis,self‐assembly and reversible healing of supramolecular perfluoropolyethers

The synthesis and thermomechanical properties of a novel class of self‐healing perfluoropolyethers (PFPEs) is reported. By decoration of 2‐ureido‐4[1H]‐pyrimidone end groups on the termini of low molar mass PFPE, the formation of supramolecular polymers and networks held together via hydrogen bonding associations was achieved. These novel supramolecular polymer materials exhibit a combination of enhanced modulus and elasticity, along with self‐healing properties, where rapid self‐healing time was demonstrated using dynamic rheological measurements. These types of supramolecular PFPEs are anticipated to be useful for a number of emerging areas in lubrication. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3598–3606     

Dendronized supramolecular polymers self‐assembled from dendritic ionic liquids

The synthesis, structural, and retrostructural analysis of a library of self‐assembling dendrons containing triethyl and tripropyl ammonium, pyridinium and 3‐methylimidazolium chloride, tetrafluoroborate, and hexafluorophosphate at their apex are reported. These dendritic ionic liquids self‐assemble into supramolecular columns or spheres which self‐organize into 2D hexagonal or rectangular and 3D cubic or tetragonal liquid crystalline and crystalline lattices. Structural analysis by X‐ray diffraction experiments demonstrated the self‐assembly of supramolecular dendrimers containing columnar and spherical nanoscale ionic liquid reactors segregated in their core. Both in the supramolecular columns and spheres the noncovalent interactions mediated by the ionic liquid provide a supramolecular polymer and therefore, these assemblies represent a new class of dendronized supramolecular polymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4165–4193, 2009     

Polymer science with transition metals and main group elements: Towards functional,supramolecular inorganic polymeric materials

Inorganic polymers are relatively unexplored because the efficient formation of macromolecular chains from atoms of transition metals and main group elements has presented a synthetic challenge. Nevertheless, these materials offer exciting opportunities for accessing properties that are significantly different from and which therefore complement those available with the well‐established organic systems. Inorganic block copolymers are of particular interest for the generation of functional, nanoscale supramolecular architectures and hierarchical assemblies using self‐assembly processes. This article focuses on research in my group over the past decade, which has targeted the development of new and controlled routes to inorganic polymers and their subsequent use in forming supramolecular materials as well as studies of their properties and applications. The use of ring‐opening polymerization (ROP) and transition‐metal‐catalyzed polycondensation approaches are illustrated. Controlled ROP procedures have been developed that allow access to polyferrocene block copolymers that self‐assemble into interesting nanoscopic architectures such as cylinders and superstructures such as flowers. The future prospects for inorganic polymer science are discussed, and a growing emphasis on the study of supramolecular inorganic polymeric materials is predicted. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 179–191, 2002     

Synthesis of supramolecular precision polymers: Crystallization under conformational constraints

Placing artificial folding elements into precision polymers is an important strategy to systematically study structure formation in self‐assembly, particularly in the semicrystalline state. To this purpose, a series of precision polymers bearing either a N‐ protected or N‐ unprotected diaminopyridine (DAP) unit after every 16th, 18th, and 20th carbon as well as a urea unit after every 20th carbon along a polyethylene‐like polymer were synthesized via acyclic diene metathesis polymerization and subsequent hydrogenation. The polymers thus contain either H‐bonds (urea/DAP), π–π‐elements (DAP), or no H‐bonds (respective N? protected urea/DAP‐units) in their main chain, able to consequently study the crystallization behavior under influence of such supramolecular moieties. Therefore, the thermal properties and crystallization behavior were analyzed via differential scanning calorimetry (DSC) as well as wide angle X‐ray diffraction. The obtained crystalline polymer is influenced by the different supramolecular interactions existing between adjacent polymer chains and the varying defect size exerted by the incorporated functional groups. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 3736–3748     

Polymerization of supramonomers: A new way for fabricating supramolecular polymers and materials

Supramolecular polymers and materials are attracting more and more attention nowadays due to their dynamic properties such as reversibility, stimuli-responsiveness and self-healing. Conventionally, bifunctional or multi-functional monomers are first covalently synthesized, followed by the supramolecular complexation to form supramolecular polymers and materials. Recently, we have proposed the supramonomer concept to construct supramolecular polymers and materials in a different way. Supramonomers are bifunctional or multi-functional monomers fabricated by noncovalent synthesis, but can undergo traditional covalent polymerization. In this highlight article, we will summarize and discuss the fabrication of supramonomer and covalent polymerization methods of supramonomers; fabrication of multi-responsive supramolecular polymers from supramonomers; and fabrication of supramolecular materials from supramonomers. It is highly anticipated that the supramonomer concept will enrich the methodology towards supramolecular polymers and materials. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 604–609     

Supramolecular Polymerization Promoted and Controlled through Self‐Sorting

A new method in which supramolecular polymerization is promoted and controlled through self‐sorting is reported. The bifunctional monomer containing p‐phenylene and naphthalene moieties was prepared. Supramolecular polymerization is promoted by selective recognition between the p‐phenylene group and cucurbit[7]uril (CB[7]), and 2:1 complexation of the naphthalene groups with cucurbit[8]uril (CB[8]). The process can be controlled by tuning the CB[7] content. This development will enrich the field of supramolecular polymers with important advances towards the realization of molecular‐weight and structural control.     

Polymerization of cyclic imino ethers: From its discovery to the present state of the art

Topics concerning the cationic ring‐opening polymerization of cyclic imino ethers and functional material production based on the resulting polymers are reviewed. Cyclic imino ethers are readily subjected to isomerization polymerization via cationic initiators. Mechanistic studies have provided a new concept, electrophilic polymerization. Double isomerization polymerization and no‐catalyst alternating copolymerization are interesting examples that show characteristics of the ring opening of cyclic imino ethers. The living polymerization of these monomers affords precisely controlled polymeric materials. Through the use of the unique properties of the product polymers, various functional polymeric materials, such as polymeric nonionic surfactants, compatibilizers, hydrogels, stabilizers for dispersion polymerization, biocatalyst modifiers, and supramolecular assemblies, have been developed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 192–209, 2002     

Synthesis of amphiphilic homopolymers and their self‐assembly into acid‐responsive polymeric micelles

This contribution presents a strategy for preparing amphiphilic homopolymers as building blocks for self‐assembly into supramolecular nanostructures. The synthesis begins with norbornene monomers containing oligoethylene glycols on the side chains. Ring‐opening metathesis polymerization of the monomers and subsequent dihydroxylation afford water‐soluble dihydroxylated poly(norbornene)s (PNBs). Amphiphilic modifications of the hydrophilic PNBs can be achieved by reacting 1,2‐diols on the backbones with hydrophobic dodecanals to form acetal linkages. The self‐assembly of the resulting amphiphilic PNB homopolymers affords polymeric micelles whose morphologies can be tuned by breaking the acetal linkages under acidic conditions. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3804–3808     

Highly Ordered Nanoporous Films from Supramolecular Diblock Copolymers with Hydrogen‐Bonding Junctions

We designed efficient precursors that combine complementary associative groups with exceptional binding affinities and thiocarbonylthio moieties enabling precise RAFT polymerization. Well defined PS and PMMA supramolecular polymers with molecular weights up to 30 kg mol^?1 are synthesized and shown to form highly stable supramolecular diblock copolymers (BCPs) when mixed, in non‐polar solvents or in the bulk. Hierarchical self‐assembly of such supramolecular BCPs by thermal annealing affords morphologies with excellent lateral order, comparable to features expected from covalent diblock copolymer analogues. Simple washing of the resulting materials with protic solvents disrupts the supramolecular association and selectively dissolves one polymer, affording a straightforward process for preparing well‐ordered nanoporous materials without resorting to crosslinking or invasive chemical degradations.     

Self‐Discriminating Termination of Chiral Supramolecular Polymerization: Tuning the Length of Nanofibers

Directing the supramolecular polymerization towards a preferred type of organization is extremely important in the design of functional soft materials. Proposed herein is a simple methodology to tune the length and optical chirality of supramolecular polymers formed from a chiral bichromophoric binaphthalene by the control of enantiomeric excess (ee). The enantiopure compound gave thin fibers longer than a few microns, while the racemic mixture favored the formation of nanoparticles. The thermodynamic study unveils that the heterochiral assembly gets preference over the homochiral assembly. The stronger heterochiral binding over homochiral one terminated the elongation of fibrous assembly, thus leading to a control over the length of fibers in the nonracemic mixtures. The supramolecular polymerization driven by π–π interactions highlights the effect of the geometry of a twisted π‐core on this self‐sorting assembly.     

Fabrication of Supramolecular Semiconductor Block Copolymers by Ring‐Opening Metathesis Polymerization

ω‐Telechelic poly(p‐phenylene vinylene) species (PPVs) are prepared by living ring‐opening metathesis polymerization of a [2.2]paracyclophane‐1,9‐diene in the presence of Hoveyda–Grubbs 2nd generation initiator, with terminating agents based on N¹,N³‐bis(6‐butyramidopyridin‐2‐yl)‐5‐hydroxyisophthalamide (Hamilton wedge), cyanuric acid, Pd^II–SCS‐pincer, or pyridine moieties installing the supramolecular motifs. The resultant telechelic polymers are self‐assembled into supramolecular block copolymers (BCPs) via metal coordination or hydrogen bonding and analyzed by ¹H NMR spectroscopy. The optical properties are examined, whereby individual PPVs exhibit similar properties regardless of the nature of the end group. Upon self‐assembly, different behaviors emerge: the hydrogen‐bonding BCP behaves similarly to the parent PPVs whereas the metallosupramolecular BCP demonstrates a hypsochromic shift and a more intense emission owing to the suppression of aggregation. These results demonstrate that directional self‐assembly can be a facile method to construct BCPs with semiconducting networks, while combating solubility and aggregation.     

New stage in living cationic polymerization: An array of effective Lewis acid catalysts and fast living polymerization in seconds

Our recent extensive research on Lewis acid catalysts with a weak base for the cationic polymerization of vinyl ethers led to unprecedented living reaction systems: fast living polymerization within 1–3 s; a wide choice of metal halides containing Al, Sn, Fe, Ti, Zr, Hf, Zn, Ga, In, Si, Ge, and Bi; and heterogeneously catalyzed living polymerization with Fe₂O₃. The use of added bases for the stabilization of the propagating carbocation and the appropriate selection of Lewis acid catalysts were crucial to the success of such new types of living polymerizations. In addition, the base‐stabilized living polymerization allowed the quantitative synthesis of star‐shaped polymers with a narrow molecular weight distribution via polymer‐linking reactions and the precision synthesis and self‐assembly of stimuli‐responsive block copolymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1801–1813, 2007.     

Strategies toward well‐defined polymer nanoparticles inspired by nature: Chemistry versus versatility

Polymeric nanoparticles are promising delivery platforms for various biomedical applications. One of the main challenges toward the development of therapeutic nanoparticles is the premature disassembly and release of the encapsulated drug. Among the different strategies to enhance the kinetic stability of polymeric nanoparticles, shell‐ and core‐crosslinking have been shown to provide robust character, while creating a suitable environment for encapsulation of a wide range of therapeutics, including hydrophilic, hydrophobic, metallic, and small and large biomolecules, with gating of their release as well. The versatility of shell‐ and core‐crosslinked nanoparticles is driven from the ease by which the structures of the shell‐ and core‐forming polymers and crosslinkers can be modified. In addition, postmodification with cell‐recognition moieties, grafting of antibiofouling polymers, or chemical degradation of the core to yield nanocages allow the use of these robust nanostructures as “smart” nanocarriers. The building principles of these multifunctional nanoparticles borrow analogy from the synthesis, supramolecular assembly, stabilization, and dynamic activity of the naturally driven biological nanoparticles such as proteins, lipoproteins, and viruses. In this review, the chemistry involved during the buildup from small molecules to polymers to covalently stabilized nanoscopic objects is detailed, with contrast of the strategies of the supramolecular assembly of polymer building blocks followed by intramicellar stabilization into shell‐, core‐, or core–shell‐crosslinked knedel‐like nanoparticles versus polymerization of polymers into nanoscopic molecular brushes followed by further intramolecular covalent stabilization events. The rational design of shell‐crosslinked knedel‐like nanoparticles is then elaborated for therapeutic packaging and delivery, with emphasis on the polymer chemistry aspects to accomplish the synthesis of such nanoparticulate systems. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012