Poly(lactide)‐based supramolecular polymers driven by self‐complementary quadruple hydrogen bonds: construction, crystallization and mechanical properties

A new method of supramolecular emulsion interfacial polymerization is developed, which can be used to fabricate supramolecular polymeric nanospheres. We designed a water-soluble monomer containing two maleimide end groups, acting as both building block and surfactant, and an oil-soluble supramonomer bearing two thiol groups connected by quadruple hydrogen bonds. With the assist of ultrasonication, hollow nanospheres can be controllably prepared by thiol-maleimide reaction of two monomers at the emulsion interface, which exhibit good stability and dynamic property. In addition, the encapsulated guest molecules could be controllably released from the supramolecular polymeric nanospheres, owing to their stimuli-responsiveness. It is anticipated that this approach will enrich the methodology of supramolecular polymerization and can be applied to constructing supramolecular materials with controllable structures and functions.

Fractionated crystallization and fractionated melting behaviors of poly(ethylene glycol) induced by poly(lactide) stereocomplex in their block copolymers and blends

Owing to the mastery exhibited by Nature in integrating both covalent and noncovalent interactions in a highly efficient manner, the quest to construct polymeric systems that rival not only the precision and fidelity but also the structure of natural systems has remained a daunting challenge. Supramolecular chemists have long endeavored to control the interplay between covalent and noncovalent bond formation, so as to examine and fully comprehend how function is predicated on self-assembly. The ability to reliably control polymer self-assembly is essential to generate "smart" materials and has the potential to tailor polymer properties (i.e., viscosity, electronic properties) through fine-tuning the noncovalent interactions that comprise the polymer architecture. In this context, supramolecular polymers have a distinct advantage over fully covalent systems in that they are dynamically modular, since noncovalent recognition motifs can be engineered to either impart a desired functionality within the overall architecture or provide a designed bias for the self-assembly process. In this Account, we describe engineering principles being developed and pursued by our group that exploit the orthogonal nature of noncovalent interactions, such as hydrogen bonding, metal coordination, and Coulombic interactions, to direct the self-assembly of functionalized macromolecules, resulting in the formation of supramolecular polymers. To begin, we describe our efforts to fabricate a modular poly(norbornene)-based scaffold via ring-opening metathesis polymerization (ROMP), wherein pendant molecular recognition elements based upon nucleobase-mimicking elements (e.g., thymine, diaminotriazine) or SCS-Pd(II) pincer were integrated within covalent monofunctional or symmetrically functionalized polymers. The simple polymer backbones exhibited reliable self-assembly with complementary polymers or small molecules. Within these systems, we applied successful protecting group strategies and template polymerizations to enhance the control afforded by ROMP. Main-chain-functionalized alternating block polymers based upon SCS-Pd(II) pincer-pyridine motifs were achieved through the combined exploitation of bimetallic initiators and supramolecularly functionalized terminators. Our initial design principles led to the successful fabrication of both main-chain- and side-chain-functionalized poly(norbornenes) via ROMP. Utilizing all of these techniques in concert led to engineering orthogonality while achieving complexity through the installation of multiple supramolecular motifs within the side chain, main chain, or both in our polymer systems. The exploitation and modification of design principles based upon functional ROMP initiators and terminators has resulted in the first synthesis of main-chain heterotelechelic polymers that self-assemble into A/B/C supramolecular triblock polymers composed of orthogonal cyanuric acid-Hamilton wedge and SCS-Pd(II) pincer-pyridine motifs. Furthermore, supramolecular A/B/A triblock copolymers were realized through the amalgamation of functionalized monomers, ROMP initiators, and terminators. To date, this ROMP-fabricated system represents the only known method to afford polymer main chains and side chains studded with orthogonal motifs. We end by discussing the impetus to attain functional materials via orthogonal self-assembly. Collectively, our studies suggest that combining covalent and noncovalent bonds in a well-defined and precise manner is an essential design element to achieve complex architectures. The results discussed in this Account illustrate the finesse associated with engineering orthogonal interactions within supramolecular systems and are considered essential steps toward developing complex biomimetic materials with high precision and fidelity.

Heterostructured nanomaterials composed of heterogeneous zones with dramatically different chemical and/or physical properties have attracted significant attention. However, achieving controllable synthesis of heterostructures integrated with segments of different stiffness and morphologies has been highly challenging, limiting their applications. Herein, a site-selective electrostatic co-assembly strategy was developed to rationally fabricate topological heterostructures (THSs) containing rigid and flexible segments with different chemical compositions and morphologies. Cationic organometallic complexes have been employed to prepare 1D crystalline supramolecular polymers (SPs), followed by electrostatic co-assembly with oppositely charged block copolymers (BCPs) of poly(ethylene glycol)-block-poly(acrylic acid). It has been found that the BCPs could interact and co-assemble with the complexes in SPs in situ at some specific sites bearing positively charged groups to form nanofiber-like moieties. The co-assembled nanofibers are connected to the intact parts of SPs, resulting in THSs composed of nanofibers and intact parts of SPs. The position of the defect sites on SPs and the dimension of SPs could be finely regulated by performing thermal annealing and/or sonication treatments, further enriching the architectures of the co-assembled THSs. This study unveils new topologies that exhibit enhanced higher-order structural complexity in the realm of supramolecular nanostructures, enabling the exploration of their properties and applications.

Breaking point: The unbinding behavior of single supramolecular polymers formed through quadruple hydrogen bonds between 2-ureido-4(1H)-pyrimidinone (UPy) units was studied by AFM-based single-molecule force spectroscopy. The interactions between individual UPy moieties and the stretching of polyethylene glycol (PEG) spacers in reversible supramolecular polymer chains were unveiled by AFM force-displacement measurements.

Heterostructured nanomaterials composed of heterogeneous zones with dramatically different chemical and/or physical properties have attracted significant attention. However, achieving controllable synthesis of heterostructures integrated with segments of different stiffness and morphologies has been highly challenging, limiting their applications. Herein, a site‐selective electrostatic co‐assembly strategy was developed to rationally fabricate topological heterostructures (THSs) containing rigid and flexible segments with different chemical compositions and morphologies. Cationic organometallic complexes have been employed to prepare 1D crystalline supramolecular polymers (SPs), followed by electrostatic co‐assembly with oppositely charged block copolymers (BCPs) of poly(ethylene glycol)‐block‐poly(acrylic acid). It has been found that the BCPs could interact and co‐assemble with the complexes in SPs in situ at some specific sites bearing positively charged groups to form nanofiber‐like moieties. The co‐assembled nanofibers are connected to the intact parts of SPs, resulting in THSs composed of nanofibers and intact parts of SPs. The position of the defect sites on SPs and the dimension of SPs could be finely regulated by performing thermal annealing and/or sonication treatments, further enriching the architectures of the co‐assembled THSs. This study unveils new topologies that exhibit enhanced higher‐order structural complexity in the realm of supramolecular nanostructures, enabling the exploration of their properties and applications.

Conventional amphiphilic block copolymers are macromolecules consisting of at least a hydrophilic segment covalently attached to a hydrophobic segment. They are unique and versatile building blocks in supramolecular polymer chemistry, both for the generation of highly organized, self-assembled structures and for the structural control of material interfaces. In the absence of solvents, the phase-behavior of block-copolymers is sparingly described. In aqueous solutions block copolymers self-assemble into nanoscopic objects. These structures are gaining more and more attention for technical formulations. Self-assembly is simply induced by dissolving dry powder of amphiphilic copolymers in water. This self-assembly process, however, cannot be described solely based on bulk phase behavior and very few experimental phase diagrams of block copolymer/water mixtures have been reported. In order to control, predict and eventually achieve a comprehensive understanding of this process and in particular the formation of vesicles, worm/rod-like micelles and spherical micelles it is important to systematically investigate the phase behavior of block copolymers across the whole water concentration range. We therefore undertook investigations of the self-assembly of two block copolymers. Preliminary encapsulation studies reveal their potential for applications relevant for drug delivery. 
\nThe concentration profile of the identified amphiphilic block-copolymers diluted in aqueous solution was examined across the whole concentration range with four poly(ethylene oxide)-block-poly(γ-methyl-ε-caprolactone) (PEO-b-PMCL) block copolymers keeping the hydrophilic block length constant but varying the hydrophobic lengths. Bulk polymers did not display any ordered morphology. With increasing water concentrations the polymers underwent transitions from lamellar phases to packed vesicles and eventually all polymers self-assembled into vesicles in dilute aqueous solutions. At high concentration the largest polymer organized further into an inverse hexagonal phase prior to self-assembly into a lamellar phase. The smallest block-copolymer also self-aggregated into rod-like micelles and exhibited a hexagonal phase between the packing of the rod-like micelles/vesicles and the lamellar phase.
\nThe concentration profile of phases assembled by amphiphilic block-copolymers diluted in aqueous solution was examined as well across the whole concentration range with three poly(isobutylene)-block-poly(ethylene oxide) block copolymers having the same hydrophobic block length but varying in their hydrophilic lengths. In contrast to PEO-b-PMCL these polymers self-aggregated depending on the hydrophilic length into micelles, worm-like micelles and for the shortest polymer into vesicles. Thus, increasing the polymer concentration lead to a different phase behavior. The largest polymer which solely formed micelles, underwent transitions from packed micelles with a few packed worm-like micelles to a hexagonal phase and finally into a lamellar phase, in contrary to the middle length polymer, which self-aggregated into worm-like micelles. The best to our knowledge, it is the first time that an experimental phase diagram is built such that it displays the formation of worm/rod-like micelles. With the composition studied in here, the worms, with increasing polymer concentration, pack and undergo afterwards into a hexagonal and finally a lamellar phase has been observed. For the shortest polymer the phase behavior was similar to that of vesicles self-assembled by PEO-b-PMCL. In contrast, the pure polymer formed an inverse hexagonal phase.
\nIt was eventually shown that PEO23-PMCl32 can be used to encapsulate both small and large molecules. By adjusting the pH towards acidic conditions the responsiveness of the vesicles could be triggered to release the encapsulated material.

A set of fluorene oligomers has been synthesized by stepwise palladium-catalyzed (Suzuki) couplings of fluorene monomers. Ureidopyrimidinones (UPy), functional groups that can dimerize via quadruple hydrogen bonds, were attached to both ends of the oligofluorenes. The resulting bis-UPy-terminated oligomers self-assemble into supramolecular chain polymers. For comparison, oligofluorenes of the same oligomer lengths but without terminal hydrogen-bonding groups were synthesized. Chains of hydrogen-bonded fluorenes can be simply endcapped by a variety of chain stoppers, molecules that have one UPy group. In this manner, we have endcapped the hydrogen-bonded fluorene chains with either oligo(p-phenylenevinylene) or perylene bisimide. Energy-transfer experiments in solution and the solid state demonstrate that oligofluorenes can donate energy to a variety of energy acceptors, but that this energy transfer occurs most effectively when the donor fluorene is hydrogen-bonded to the acceptor.

The development of well-defined three-dimensional supramolecular polymers presents significant challenges, particularly in achieving crystalline state structures. This study addresses this challenge by presenting the construction of a crystalline three-dimensional supramolecular polymer through the self-assembly of clamparene (CLP) and a naphthalene diimide derivative (NDIOH) in the solid state. The hierarchical self-assembly progresses from one-dimensional linear supramolecular polymers to two-dimensional supramolecular polymers and ultimately to a crystalline three-dimensional supramolecular polymer. Moreover, the prepared crystalline three-dimensional supramolecular polymer demonstrates effective photothermal conversion. This work advances the understanding and design of functional three-dimensional supramolecular polymers in the crystalline state.

A well-defined quadruple hydrogen bonding strategy involving dimerization of 2-ureido-4[1H]-pyrimidone (UPy) units is innovatively designed to prepare polyureas with high overall mechanical properties. Three polyureas containing different amounts of UPy units were synthesized by replacing a portion of isophorone diisocyanate (IPDI) with a UPy-derived diisocyanate. The formation of quadruple hydrogen bonds in hard segments via UPy dimers was confirmed by nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The mechanical properties of the polyureas were evaluated by uniaxial tensile testing. Compared to the polyurea without UPy units, remarkable improvements in Young’s modulus, tensile strength, and toughness were simultaneously achieved when UPy units were incorporated. The mechanism behind the strong strengthening effect rooted in the stronger intermolecular forces among hard segments brought by the quadruple hydrogen bonds, which were stronger than the inherent bidentate and monodentate hydrogen bonds among urea groups, and the slower soft segmental dynamics reaveled by both increased Tg and relaxation time of the soft segments. The mechanism behind the strong toughening effect was ascribed to more effective energy dissipation brought by the quadruple hydrogen bonds that served as stronger sacrificial bonds upon deformation. This work may offer new insight into the design of polyurea elastomers with comprehensively improved mechanical properties.

Supramolecular polymers (SMPs) have quite different crystallization behavior from the conventional polymers, because of the confinement effects of supramolecular units. Crystallization of SMPs undergoes in a “confined” and “dynamic” manner. Herein, we selected the 2-ureido-4[1H]-pyrimidione (UPy)-bonded poly(l-lactic acid) (PLLA) as a model SMP and investigated the crystallization kinetics, polymorphic crystalline structure and phase transition of supramolecular PLLAs (SM-PLLAs). SM-PLLAs were synthesized by the end functionalization of hydroxyl-terminated 2- and 3-arm PLLA precursors with different molecular weights. Crystallization rate and crystallinity of SM-PLLAs are strongly depressed in both nonisothermal and isothermal crystallizations, as compared to the nonfunctionalized PLLA precursors. Crystalline structure of SM-PLLAs is sensitive to the crystallization temperature (Tc). A low Tc (75–95 °C) facilitates the formation of metastable β crystals of PLLA in SM-PLLAs; while a high Tc (100–130 °C) fa...

A facile method for preparing supramolecular waterborne polyurethane (WPU) based on quadruple hydrogen bonds (4H‐WPU) is reported. Herein, 4H‐WPU with quadruple hydrogen bonds in main chain were synthesized by poly (1,4‐buthylene‐neopentylene adipate glycol) as soft segment, 2‐ureido‐4[1H]‐pyrimidinone (UPy) functionalized monomer, isophorone diisocyanate, 2,2‐Bis(hydroxymethyl)propionic acid (hydrophilic monomer), isophorondiamine, triethylamine (neutralizer), and monoethanolamine (blocking agent) as hard segment. The molecular weight of 4H‐WPU was controlled around 18,000 consistently. The properties of 4H‐WPU with different content of hydrophilic group, hard segment, and UPy units were characterized and the results could provide the reference for preparing supramolecular WPU with high mechanical and self‐healing performance while maintaining dispersion stability. The tensile strength of 4H‐WPU was 10 times of the blank sample. The healing time of scratched 4H‐WPU film was inversely proportional to the content of UPy‐ functionalized monomer and the shortest healing time is 2.5 hr at 80°C. Mechanical performance of healed films can be restored to more than 90%.

It is still challenging and attractive to prepare polyurethane (PU) materials with excellent self-healing ability while improving their mechanical properties and high ductility. Here, a multifunctional linear PU supramolecular elastomer was successfully prepared by introducing a cross-linking network of quadruple hydrogen bonds and thermo-reversible Diels–Alder bonds and rigid ring structure to the linear backbone. The results exhibited that the obtained PU elastomer displayed a high tensile strength (6.30 MPa), elongation (1957.84%), toughness (84.48 MJ/m3), and excellent repair efficiency (93.33%). The quadruple hydrogen bonds from 5-(2-hydroxyethyl)-6-methyl-2-aminouracil and thermo-reversible Diels–Alder bonds from the conjugated reaction of 4,4′-bismaleimide diphenylmethane with furfuryl alcohol, due to its synergetic dual reversible bonds, formed the PU elastomer that possessed excellent mechanical, self-healing, shape recovery, and reprocessing properties. The prepared multifunctional PU can be used as a substrate for flexible conductive materials or as conductive composite material with conductive materials, which can self-repair many times when the surface is damaged, can be recycled, and greatly improve the service life of the material. Therefore, the prepared multifunctional high-performance self-healing PU materials have potential applications in several fields.

Quasi block copolymers noncovalent bonded by stereocomplex crystals

Several series of new b-nylon 6-b-polyimide-b-nylon 6 triblock copolymers were synthesized via a condensation polymerization of polyimide segments and anionic polymerization of the nylon 6 components. Crystallization and phase behavior of these triblock copolymers have been studied with varying polyimide block molecular weights and copolymer compositions as well as chemical structures (and therefore, different solubility parameters) of the polyimide components. Furthermore, in-situ and solution blends have also been prepared in order to compare the phase behaviors of the triblock copolymers. Wide angle X-ray diffraction experiments indicate that in these triblock copolymers, only nylon 6 components crystallize and polyimides do not take part in the crystals. From isothermal crystallization kinetics studies carried out in differential scanning calorimetry experiments, the miscibility of the polyimide components with nylon 6 critically affects the crystallization rate of these triblock copolymers. When the polyimide composition is low and its solubility parameter is similar to that of the nylon 6 components, the polyimides mix with the amorphous part of the nylon 6 components and are located in the inter-lamellar region. This can be identified by monitoring the change of α-relaxation temperature and activation energy measured using a dynamic mechanical method and by examining the crystalline lamellar morphology under small angle X-ray scattering experiments. Microphase separation can also be recognized by transmission electron microscopy using an OsO4 vapor staining technique. As the difference of solubility parameters between these two components as well as the block molecular weight of the polyimides increase, microphase separation between the two components in the triblock copolymers can be found.