Cycloaddition-Promoted Self-Assembly of a Polymer into Well-Defined β Sheets and Hierarchical Nanofibrils

Self-Assembly of Repetitive Segment and Random Segment Polymer Architectures

Recent advances in chemical synthesis have created new methodologies for synthesizing sequence-controlled synthetic polymers, but rational design of monomer sequence for desired properties remains challenging. In this work, we synthesize periodic polymers with repetitive segments using a sequence-controlled ring-opening metathesis polymerization (ROMP) method, which draws inspiration from proteins containing repetitive sequence motifs. The repetitive segment architecture is shown to dramatically affect the self-assembly behavior of these materials. Our results show that polymers with identical repetitive sequences assemble into uniform spherical nanoparticles after thermal annealing, whereas copolymers with random placement of segments with different sequences exhibit disordered assemblies without a well-defined morphology. Overall, these results bring a new understanding to the role of periodic repetitive sequences in polymer assembly.

Bioinspired Polyurethane Using Multifunctional Block Modules with Synergistic Dynamic Bonds

Nature embraces an intriguing strategy to create high-performance biomaterials, such as spider silk which presents an unparalleled combination of stiffness, tensile strength, and toughness via hierarchical structures. However, to fabricate synthetic polymers with such excellent properties remains a challenging task. Inspired by the integration of multiblock backbone and densely H-bonding assemblies in spider silk as well as the delicate iron-catecholate complexes in mussel byssus, we proposed a novel molecular design with multifunctional block modules to obtain polymer materials that exhibit excellent mechanical property, self-healing ability, and reprocessability. It was achieved by introducing reversible iron-catechol (DOPA-Fe³⁺) cross-links and quadruple H-bonds bearing 2-ureido-4-[1H]-pyrimidinone (UPy) dimers as multifunctional blocks into a segmented polyurethane backbone with urethane blocks and semicrystalline polycaprolactone (PCL) blocks. These two types of dynamic cross-linking knots served as the sacrificial bonds to dissipate energy efficiently under external stress burden, endowing the dual physical cross-linked networks with increased toughness and breaking elongation. Moreover, the DOPA-Fe³⁺ complexes could increase the crystallization of PCL, leading to remarkably enhanced Young's modulus and tensile strength. Solid-state NMR revealed the formation of quadruple H-bonds in UPy dimers and the presence of DOPA-Fe³⁺ complexes, which restricted the mobility of the mobile phase and enhanced the crystallinity of the PCL domain. This work provides a feasible way to develop bioinspired materials with self-healable and reprocessable features, in addition to balanced enhancement of both stiffness and toughness.

Reversible and rapid pH-regulated self-assembly of a poly(ethylene glycol)-peptide bioconjugate

The use of external triggers to manipulate the secondary structure of self-assembling peptides conjugated to flexible synthetic polymers is a challenging problem, particularly in terms of reversibility. Here, we demonstrate, for the first time, sustained rapid and reversible, pH-regulated self-assembly of the peptide ELELELELELF (EL-5F) and its conjugates with 2 and 5 kDa poly(ethylene glycol) (EL-5F-PEG-2K and EL-5F-PEG-5K). Circular dichroism indicated the formation of β-sheet structures at pH < 5.9, 5.8, and 5.4 and disassembly to random coils above those pH values for EL-5F, EL-5F-PEG-2K, and EL-5F-PEG-5K, respectively. β-sheets were confirmed by the thioflavin T assay, while transmission electron microscopy revealed the existence of extended fibrillar structures below the above pH values. pH-induced secondary structure conversion was reproducible for over 15 cycles, even at salt concentrations of up to 200 mM NaCl, and was quantitatively related to the pH. Self-supporting hydrogelation after self-assembly was observed at concentrations as low as 0.2 wt %, which is 15-fold lower than previously reported concentrations. This simple approach to mediate reversible self-assembly of EL-5F-PEG bioconjugates is expected to offer novel functionality relevant to drug delivery and bioseparation systems.

Modular design in natural and biomimetic soft materials

Under eons of evolutionary and environmental pressure, biological systems have developed strong and lightweight peptide-based polymeric materials by using the 20 naturally occurring amino acids as principal monomeric units. These materials outperform their man-made counterparts in the following ways: 1) multifunctionality/tunability, 2) adaptability/stimuli-responsiveness, 3) synthesis and processing under ambient and aqueous conditions, and 4) recyclability and biodegradability. The universal design strategy that affords these advanced properties involves "bottom-up" synthesis and modular, hierarchical organization both within and across multiple length-scales. The field of "biomimicry"-elucidating and co-opting nature's basic material design principles and molecular building blocks-is rapidly evolving. This Review describes what has been discovered about the structure and molecular mechanisms of natural polymeric materials, as well as the progress towards synthetic "mimics" of these remarkable systems.