Hybrid Bonding Bottlebrush Polymers Grafted from a Supramolecular Polymer Backbone

Bottlebrush polymers, macromolecules consisting of dense polymer side chains grafted from a central polymer backbone, have unique properties resulting from this well-defined molecular architecture. With the advent of controlled radical polymerization techniques, access to these architectures has become more readily available. However, synthetic challenges remain, including the need for intermediate purification, the use of toxic solvents, and challenges with achieving long bottlebrush architectures due to backbone entanglements. Herein, we report hybrid bonding bottlebrush polymers (systems integrating covalent and noncovalent bonding of structural units) consisting of poly(sodium 4-styrenesulfonate) (p(NaSS)) brushes grafted from a peptide amphiphile (PA) supramolecular polymer backbone. This was achieved using photoinitiated electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization in water. The structure of the hybrid bonding bottlebrush architecture was characterized using cryogenic transmission electron microscopy, and its properties were probed using rheological measurements. We observed that hybrid bonding bottlebrush polymers were able to organize into block architectures containing domains with high brush grafting density and others with no observable brushes. This finding is possibly a result of dynamic behavior unique to supramolecular polymer backbones, enabling molecular exchange or translational diffusion of monomers along the length of the assemblies. The hybrid bottlebrush polymers exhibited higher solution viscosity at moderate shear, protected supramolecular polymer backbones from disassembly at high shear, and supported self-healing capabilities, depending on grafting densities. Our results demonstrate an opportunity for novel properties in easily synthesized bottlebrush polymer architectures built with supramolecular polymers that might be useful in biomedical applications or for aqueous lubrication.

Influence of Polymer Side Chain Size and Backbone Length on the Self-Assembly of Supramolecular Polymer Bottlebrushes

Hydrogen bonds are a versatile tool for creating fibrous, bottlebrush-like assemblies of polymeric building blocks. However, a delicate balance of forces exists between the steric repulsion of the polymer chains and these directed supramolecular forces. In this work we have systematically investigated the influence of structural parameters of the attached polymers on the assembly behaviour of benzene trisurea (BTU) and benzene tris(phenylalanine) (BTP) conjugates in water. Polymers with increasing main chain lengths and different side chain sizes were prepared by reversible addition-fragmentation chain-transfer (RAFT) polymerization of hydroxyethyl acrylate (HEA), tri(ethylene glycol) methyl ether acrylate (TEGA) and oligo(ethylene glycol) methyl ether acrylate (OEGA). The resulting structures were analyzed using small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Both BTU and BTP formed fibres with PHEA attached, but a transition to spherical morphologies was observed at degrees of polymerisation (DP) of 70 and above. Overall, the main chain length appeared to be a dominating factor in inducing morphology transitions. Increasing the side chain size generally had a similar effect but mainly impeded any aggregation as is the case of POEGA. Interestingly, BTP conjugates still formed fibres, suggesting that the stronger intermolecular interactions can compensate partially for the steric repulsion.

Elucidating Sequence-Assembly Relationships for Bilingual PNA Biopolymers

Nucleic acids and proteins possess encoded "languages" that can be used for information storage or to direct function. However, each biopolymer is limited to encoding its respective "language." Using a peptide nucleic acid (PNA) scaffold, nucleobase and amino acid residues can be installed on a singular backbone, enabling a single biopolymer to encode both languages. Our laboratory previously reported the development of a "bilingual" PNA biopolymer that incorporates a sequence-specific nucleic acid code interspersed with hydrophobic (alanine) and hydrophilic (lysine) amino acid residues at defined positions to produce amphiphilic character. We observed the amphiphilic amino acid residues directing the biopolymer to undergo self-assembly into micelle-like structures, while the nucleic acid recognition was harnessed for disassembly. Herein, we report a series of bilingual PNA sequences having amino acid residues with varying lengths, functional group charges, hydrophobicities, and spacings to elucidate the effect of these parameters on micelle assembly and nucleic acid recognition. Negative charges in the hydrophilic block or increased bulkiness of the hydrophobic side chains led to assembly into similarly sized micelles; however, the negative charge additionally led to increased critical micelle concentration. Upon PNA sequence truncation to decrease the spacing between side chains, the biopolymers remained capable of self-assembling but formed smaller structures. Characterization of disassembly revealed that each variant retained sequence recognition capabilities and stimuli-responsive disassembly. Together, these data show that the amino acid and nucleic acid sequences of amphiphilic bilingual biopolymers can be customized to finely tune the assembly and disassembly properties, which has implications for applications such as the encapsulation and delivery of cargo for therapeutics.

Micelle and Nanotape Formation of Benzene Tricarboxamide Analogues with Selective Cancer Cell Cytotoxicity

Analogues of benzene-1,3,5-tricarboxamide bearing combinations of different alkyl chains (dodecyl to octadecyl) and ester-linked PEG (polyethylene glycol) chains are shown to self-assemble into either micelles or nanotapes in aqueous solution, depending on the architecture (number of alkyl vs PEG chains). The cytotoxicity to cells is selectively greater for breast cancer cells than fibroblast controls in a dose-dependent manner. The compounds show strong stability, retaining their self-assembled structures at low pH (relevant to acidic tumor conditions) and in buffer and cell culture media.

Reincarnation of the Analytical Ultracentrifuge: Emerging Opportunities for Nanomedicine

The analytical ultracentrifuge (AUC) and the modern field of analytical ultracentrifugation found its inception approximately a century ago. We highlight the scope of its major experimental opportunities as a transport-based method, contemporary and up-and-coming investigation potential for polymers, polymer-drug conjugates, polymer assemblies, as well as medical nanoparticles. Special focus lies on molar mass estimates of unimeric polymeric species, self-assemblies in solution, and (co)localization of multicomponent systems in solution alongside the material-biofluid interactions. We close with present challenges and incentives for future research.

Adjusting the length of supramolecular polymer bottlebrushes by top-down approaches

Controlling the length of one-dimensional (1D) polymer nanostructures remains a key challenge on the way toward the applications of these structures. Here, we demonstrate that top-down processing facilitates a straightforward adjustment of the length of polyethylene oxide (PEO)-based supramolecular polymer bottlebrushes (SPBs) in aqueous solutions. These cylindrical structures self-assemble via directional hydrogen bonds formed by benzenetrisurea (BTU) or benzenetrispeptide (BTP) motifs located within the hydrophobic core of the fiber. A slow transition from different organic solvents to water leads first to the formation of µm-long fibers, which can subsequently be fragmented by ultrasonication or dual asymmetric centrifugation. The latter allows for a better adjustment of applied shear stresses, and thus enables access to differently sized fragments depending on time and rotation rate. Extended sonication and scission analysis further allowed an estimation of tensile strengths of around 16 MPa for both the BTU and BTP systems. In combination with the high kinetic stability of these SPBs, the applied top-down methods represent an easily implementable technique toward 1D polymer nanostructures with an adjustable length in the range of interest for perspective biomedical applications.

Kinetically Controlling the Length of Self-Assembled Polymer Nanofibers Formed by Intermolecular Hydrogen Bonds

Strong directional hydrogen bonds represent a suitable supramolecular force to drive the one-dimensional (1D) aqueous self-assembly of polymeric amphiphiles resulting in cylindrical polymer brushes. However, our understanding of the kinetics in these assembly processes is still limited. We here demonstrate that the obtained morphologies for our recently reported benzene tris-urea and tris-peptide conjugates are strongly pathway-dependent. A controlled transfer from solutions in organic solvents to aqueous environments enabled a rate-dependent formation of kinetically trapped but stable nanostructures ranging from small cylindrical or spherical objects (<50 nm) to remarkably large fibers (>2 μm). A detailed analysis of the underlying assembly mechanism revealed a cooperative nature despite the steric demands of the polymers. Nucleation is induced by hydrophobic interactions crossing a critical water content, followed by an elongation process due to the strong hydrogen bonds. These findings open an interesting new pathway to control the length of 1D polymer nanostructures.

Overcoming the Necessity of a Lateral Aggregation in the Formation of Supramolecular Polymer Bottlebrushes in Water

The assembly of supramolecular polymer bottlebrushes in aqueous systems is, in most cases, associated with a lateral aggregation of the supramolecular building blocks in addition to their axial stacking. Here, it is demonstrated that this limitation can be overcome by attaching three polymer chains to a central supramolecular unit that possesses a sufficiently high number of hydrogen bonding units to compensate for the increased steric strain. Therefore, a 1,3,5-benzenetrisurea-polyethylene oxide conjugate is modified with different peptide units located next to the urea groups which should facilitate self-assembly in water. For a single amino acid per arm, spherical micelles are obtained for all three tested amino acids (alanine, leucine, and phenylalanine) featuring different hydrophobicities. Only a slight increase in size and solution stability of spherical micelles is observed with increasing hydrophobicity of amino acid unit. In contrast, introducing two amino acid units per arm and thus increasing the number of hydrogen bonds per unimer molecule results in the formation of cylindrical structures, that is, supramolecular polymer bottlebrushes, despite a suppressed lateral aggregation. Consequently, it can be concluded that the number of hydrogen bonds has a more profound impact on the resulting solution morphology than the hydrophobicity of the amino acid unit.