Bottlebrush polymers: From controlled synthesis, self-assembly, properties to applications

Colloidal Polymer-Templated Formation of Inorganic Nanocrystals and their Emerging Applications

Inorganic nanocrystals possess unique physicochemical properties compared to their bulk counterparts. Stabilizing agents are commonly used for the preparation of inorganic nanocrystals with controllable properties. Particularly, colloidal polymers have emerged as general and robust templates for in situ formation and confinement of inorganic nanocrystals. In addition to templating and stabilizing inorganic nanocrystals, colloidal polymers can tailor their physicochemical properties such as size, shape, structure, composition, surface chemistry, and so on. By incorporating functional groups into colloidal polymers, desired functions can be integrated with inorganic nanocrystals, advancing their potential applications. Here, recent advances in the colloidal polymer-templated formation of inorganic nanocrystals are reviewed. Seven types of colloidal polymers, including dendrimer, polymer micelle, stare-like block polymer, bottlebrush polymer, spherical polyelectrolyte brush, microgel, and single-chain nanoparticle, have been extensively applied for the synthesis of inorganic nanocrystals. Different strategies for the development of these colloidal polymer-templated inorganic nanocrystals are summarized. Then, their emerging applications in the fields of catalysis, biomedicine, solar cells, sensing, light-emitting diodes, and lithium-ion batteries are highlighted. Last, the remaining issues and future directions are discussed. This review will stimulate the development and application of colloidal polymer-templated inorganic nanocrystals.

Water-soluble polyphosphonate-based bottlebrush copolymers via aqueous ring-opening metathesis polymerization

Ring-opening metathesis polymerization (ROMP) is a versatile method for synthesizing complex macromolecules from various functional monomers. In this work, we report the synthesis of water-soluble and degradable bottlebrush polymers, based on polyphosphoesters (PPEs) via ROMP. First, PPE-macromonomers were synthesized via organocatalytic anionic ring-opening polymerization of 2-ethyl-2-oxo-1,3,2-dioxaphospholane using N-(hydroxyethyl)-cis-5-norbornene-exo-2,3-dicarboximide as the initiator and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the catalyst. The resulting norbornene-based macromonomers had degrees of polymerization (DPn) ranging from 25 to 243 and narrow molar mass dispersity (Đ ≤ 1.10). Subsequently, these macromonomers were used in ROMP with the Grubbs 3rd-generation bispyridyl complex (Ru-G3) to produce a library of well-defined bottlebrush polymers. The ROMP was carried out either in dioxane or in aqueous conditions, resulting in well-defined and water-soluble bottlebrush PPEs. Furthermore, a two-step protocol was employed to synthesize double hydrophilic diblock bottlebrush copolymers via ROMP in water at neutral pH-values. This general protocol enabled the direct combination of PPEs with ROMP to synthesize well-defined bottlebrush polymers and block copolymers in water. Degradation of the PPE side chains was proven resulting in low molar mass degradation products only. The biocompatible and biodegradable nature of PPEs makes this pathway promising for designing novel biomedical drug carriers or viscosity modifiers, as well as many other potential applications.

Nanostructures, Linear Rheological Responses, and Tunable Mechanical Properties of Microphase-Separated Cellulose-graft-Diblock Bottlebrush Copolymer Elastomers

A series of cellulose-graft-diblock bottlebrush copolymer elastomers (cellulose-graft-poly(n-butyl acrylate)-block-poly(methyl methacrylate) (Cell-g-PBA-b-PMMA)) with short side chains were synthesized via successive atom transfer radical polymerization (ATRP) to study the influence of varying compositions and lengths of the graft diblock side chains on microphase morphologies and properties. The microphase-separated morphologies from misaligned spheres to cylinders were observed by atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS) measurements. These bottlebrush copolymer elastomers possessed thermal stability and enhanced mechanical properties because the PMMA outer block could self-assemble into hard microdomains, which served as physical cross-links. The viscoelastic responses of these bottlebrush copolymers within the linear viscoelastic (LVE) regime were carried out by the oscillatory shear rheology. The time-temperature superposition (tTs) principle was applied to construct the master curves of the dynamic moduli, and the sequential relaxation of dense bottlebrush copolymers with different PMMA hard outer block lengths was analyzed. The rheological behaviors in this work could be utilized to build up the connection of microstructures and properties for the application of these bottlebrush copolymers as high-performance thermoplastic elastomers.

Synthesis of Bottlebrush Polymers by Z-/E-Specific Living Ring-Opening Metathesis Polymerization, Exhibiting Different Thermal Properties

Synthesis of bottlebrush polymers (BBPs) and block copolymers by Z-/E-specific living ring-opening metathesis polymerization (ROMP) of N-substituted-norbornene-2,3-dicarboximides containing long alkyl chains (n-octadecyl, n-tetradecyl, etc.) has been attained by the vanadium(V)-alkylidene catalysts V(CHSiMe3)(ArN)[OC(CF3)3](PMe3)2 [Ar = 2,6-Cl2C6H3 (1), C6F5 (2)] and V(CHSiMe3)(2,6-F2C6H3N)(OC6Cl5)(PMe3)2 (3). The ROMPs using 1 afforded the BBPs with exclusive Z selectivity (98 to >99% cis) even at high temperature (up to 80 °C) in the presence of PMe3, whereas the ROMPs using 3 gave the BBPs with high E selectivity (90% trans). These ROMPs proceeded in a living manner (even at 80 °C using 1), affording various (amphiphilic) block copolymers while maintaining high E/Z selectivity. The resultant Z- and E-selective BBPs especially prepared from N-(n-octadecyl)norbornene-2,3-dicarboximide possessed different melting temperatures due to different degrees of interpolymer alkyl side chain interaction (side chain crystallization).

Trends in the Synthesis of Polymer Nano- and Microscale Materials for Bio-Related Applications

Polymeric nano- and microscale materials bear significant potential in manifold applications related to biomedicine. This is owed not only to the large chemical diversity of the constituent polymers, but also to the various morphologies these materials can achieve, ranging from simple particles to intricate self-assembled structures. Modern synthetic polymer chemistry permits the tuning of many physicochemical parameters affecting the behavior of polymeric nano- and microscale materials in the biological context. In this Perspective, an overview of the synthetic principles underlying the modern preparation of these materials is provided, aiming to demonstrate how advances in and ingenious implementations of polymer chemistry fuel a range of applications, both present and prospective.

Protein-Inspired Control over Synthetic Polymer Folding for Structured Functional Nanoparticles in Water

The folding of proteins into functional nanoparticles with defined 3D structures has inspired chemists to create simple synthetic systems mimicking protein properties. The folding of polymers into nanoparticles in water proceeds via different strategies, resulting in the global compaction of the polymer chain. Herein, we review the different methods available to control the conformation of synthetic polymers and collapse/fold them into structured, functional nanoparticles, such as hydrophobic collapse, supramolecular self-assembly, and covalent cross-linking. A comparison is made between the design principles of protein folding to synthetic polymer folding and the formation of structured nanocompartments in water, highlighting similarities and differences in design and function. We also focus on the importance of structure for functional stability and diverse applications in complex media and cellular environments.

Degradable Bottlebrush Polypeptides and the Impact of their Architecture on Cell Uptake, Pharmacokinetics, and Biodistribution In Vivo

Bottlebrush polymers are highly promising as unimolecular nanomedicines due to their unique control over the critical parameters of size, shape and chemical function. However, since they are prepared from biopersistent carbon backbones, most known bottlebrush polymers are non-degradable and thus unsuitable for systemic therapeutic administration. Herein, we report the design and synthesis of novel poly(organo)phosphazene-g-poly(α-glutamate) (PPz-g-PGA) bottlebrush polymers with exceptional control over their structure and molecular dimensions (Dh ≈ 15-50 nm). These single macromolecules show outstanding aqueous solubility, ultra-high multivalency and biodegradability, making them ideal as nanomedicines. While well-established in polymer therapeutics, it has hitherto not been possible to prepare defined single macromolecules of PGA in these nanosized dimensions. A direct correlation was observed between the macromolecular dimensions of the bottlebrush polymers and their intracellular uptake in CT26 colon cancer cells. Furthermore, the bottlebrush macromolecular structure visibly enhanced the pharmacokinetics by reducing renal clearance and extending plasma half-lives. Real-time analysis of the biodistribution dynamics showed architecture-driven organ distribution and enhanced tumor accumulation. This work, therefore, introduces a robust, controlled synthesis route to bottlebrush polypeptides, overcoming limitations of current polymer-based nanomedicines and, in doing so, offers valuable insights into the influence of architecture on the in vivo performance of nanomedicines.

Design of Microstructure-Engineered Polymers for Energy and Environmental Conservation

With the ever-growing demand for sustainability, designing polymeric materials using readily accessible feedstocks provides potential solutions to address the challenges in energy and environmental conservation. Complementing the prevailing strategy of varying chemical composition, engineering microstructures of polymer chains by precisely controlling their chain length distribution, main chain regio-/stereoregularity, monomer or segment sequence, and architecture creates a powerful toolbox to rapidly access diversified material properties. In this Perspective, we lay out recent advances in utilizing appropriately designed polymers in a wide range of applications such as plastic recycling, water purification, and solar energy storage and conversion. With decoupled structural parameters, these studies have established various microstructure-function relationships. Given the progress outlined here, we envision that the microstructure-engineering strategy will accelerate the design and optimization of polymeric materials to meet sustainability criteria.

Facile and Universal Defect Engineering Toward Highly Stable Carbon-Based Polymer Brushes with High Grafting Density

Carbon-based polymer brushes (CBPBs) are an important class of functional polymer materials, which synergistically combine the advantageous properties of both carbons and polymers. However, the conventional fabrication procedures of CBPBs involve tedious multistep modification, including preoxidation of carbon substrates, introduction of initiating groups, and subsequent graft polymerization. In this study, a simple yet versatile defect-engineering strategy is proposed for the efficient synthesis of high-grafting-density CBPBs with highly stable CC linkages via free radical polymerization. This strategy involves the introduction and removal of nitrogen heteroatoms in the carbon skeletons via a simple temperature-Fmed heat treatment, leading to the formation of numerous carbon defects (e.g., pentagons, heptagons, and octagons) with reactive C=C bonds in the carbon substrates. The as-proposed methodology enables the facile fabrication of CBPBs with various carbon substrates and polymers. More importantly, the highly grafted polymer chains in the resulting CBPBs are tethered with the carbon skeletons by robust CC bonds, which can endure strong acid and alkali environments. These interesting findings will shed new light on the well-orchestrated design of CBPBs and broaden their applications in various areas with fascinating performances.

Conformation of a Comb-like Chain in Solution: Effect of Backbone Rigidity

We study the effect of backbone rigidity on the conformation of comb-like chains in dilute solution by using Brownian dynamics simulations. Our results demonstrate that the backbone rigidity can control the effect of side chains on the conformation of comb-like chains; that is, the relative strength of the excluded-volume interactions from backbone monomer-graft and graft-graft to backbone monomer-monomer gradually weakens with the increase of backbone rigidity. Only when the rigidity of the backbone tends to be flexible and the grafting density is high is the effect of excluded volume of graft-graft on the conformation of comb-like chains significant enough, and other cases can be ignored. Our results show that the radius of gyration of comb-like chains and the persistence length of the backbone are exponentially related to the stretching factor, where the power exponent exhibits an increase with the increase of the strength of bending energy. These finds provide new insights for characterizing the structure properties of comb-like chains.

Synthesis, Characterization, Conformation in Solution, and Thermoresponsiveness of Polymer Brushes of methoxy[oligo (propylene glycol)-block-oligo(ethylene glycol)]methacrylate and N-[3-(dimethylamino)propyl]methacrylamide Obtained via RAFT Polymerization

The thermo- and pH-responsive polymer brushes based on methoxy[oligo(propyleneglycol)8-block-oligo(ethyleneglycol)8]methacrylate with different concentrations of N-[3-(dimethylamino)propyl]methacrylamide (from 0% to 20%) were synthesized via RAFT polymerization. The “grafting-through” approach was used to prepare the low-molar-mass dispersion samples (Mw/Mn ≈ 1.3). Molar masses and hydrodynamic characteristics were obtained using static and dynamic light scattering and viscometry. The solvents used were acetonitrile, DMFA, and water. The molar masses of the prepared samples ranged from 40,000 to 60,000 g·mol–1. The macromolecules of these polymer brushes were modeled using a prolate revolution ellipsoid or a cylinder with spherical ends. In water, micelle-like aggregates were formed. Critical micelle concentrations decreased with the content of N-[3-(dimethylamino)propyl]methacrylamide. Molecular brushes demonstrated thermo- and pH-responsiveness in water–salt solutions. It was shown that at a given molecular mass and at close pH values, the increase in the number of N-[3-(dimethylamino)propyl]methacrylamide units led to an increase in phase separation temperatures.

Self-Aggregation in Aqueous Media of Amphiphilic Diblock and Random Block Copolymers Composed of Monomers with Long Side Chains

Amphiphilic diblock copolymers and hydrophobically modified random block copolymers can self-assemble into different structures in a selective solvent. The formed structures depend on the copolymer properties, such as the ratio between the hydrophilic and the hydrophobic segments and their nature. In this work, we characterize by cryogenic transmission electron microscopy (cryo-TEM) and dynamic light scattering (DLS) the amphiphilic copolymers poly(2-dimethylamino ethyl methacrylate)-b-poly(lauryl methacrylate) (PDMAEMA-b-PLMA) and their quaternized derivatives QPDMAEMA-b-PLMA at different ratios between the hydrophilic and the hydrophobic segments. We present the various structures formed by these copolymers, including spherical and cylindrical micelles, as well as unilamellar and multilamellar vesicles. We also examined by these methods the random diblock copolymers poly(2-(dimethylamino) ethyl methacrylate)-b-poly(oligo(ethylene glycol) methyl ether methacrylate) (P(DMAEMA-co-Q_6/12DMAEMA)-b-POEGMA), which are partially hydrophobically modified by iodohexane (Q₆) or iodododecane (Q₁₂). The polymers with a small POEGMA block did not form any specific nanostructure, while a polymer with a larger POEGMA block formed spherical and cylindrical micelles. This nanostructural characterization could lead to the efficient design and use of these polymers as carriers of hydrophobic or hydrophilic compounds for biomedical applications.

Supramolecular Framework Constructed by Dendritic Nanopolymer for Stable Flexible Perovskite Resistive Random-Access Memory

The 3D supramolecular framework (3D-SF) is constructed in this work through the hydrogen bond assisted self-assembly of spherical dendritic nanopolymer to regulate the flexibility, stability, and resistive switching (RS) performance of perovskite resistive random-access memory (RRAM). Herein, the 3D-SF network acts as the perovskite crystallization template to regulate the perovskite crystallization process due to its coordination interaction of functional groups with the perovskite grains, presenting the uniform, pinhole-free, and compact perovskite morphology for stable flexible RRAM. The 3D-SF network in situ stays at the perovskite intergranular boundaries to crosslink the perovskite grains. The RS performance of 3D-SF-modified perovskite RRAM device is evidently improved to the ON/OFF ratio of 10⁵ , the cycle number of 500 times, and the data retention time of 10⁴ s. The 50-days exposure of unencapsulated RRAM device at ambient environment still makes the ON/OFF ratio to be kept at ≈10⁴ , indicating the potential of long-term stable multilevel storage in the high-density data storage. The bending action under different radius also does not change the RS performance due to the excellent bending-resistant ability of 3D-SF-modified perovskite film. This work explores a novel polymer additive strategy to construct the 3D supramolecular framework for stable flexible perovskite optoelectronic devices.

Loose Semirigid Aromatic Polyester Bottle Brushes at Poly(2-isopropyl-2-oxazoline) Side Chains of Various Lengths: Behavior in Solutions and Thermoresponsiveness

A polycondensation aromatic polyester with an oxygen spacer was synthesized and used as a macroinitiator for the grafting of linear poly(2-isopropyl-2-oxazoline) (PiPrOx) by the cationic polymerization method. The length of the thermosensitive side chains was varied by the initiator:monomer ratio. Using methods of molecular hydrodynamics, light scattering and turbidimetry, the copolymers were studied in organic solvents and in water. The molecular characteristics of the main chain and graft copolymers, the polymerization degree of side chains and their grafting density have been determined. The equilibrium rigidity of the macroinitiator and the conformations of grafted macromolecules were evaluated. In selective solvents, they take on a star-like conformation or aggregate depending on the degree of shielding of the main chain by side chains. The thermoresponsiveness of graft copolymers in aqueous solutions was studied, and their LCST were estimated. The results are compared with data for graft copolymers composed of PiPrOx side chains and flexible or rigid chain backbones of aromatic polyester type.

Hairy Gels: A Computational Study

We present results of MD and MC simulations of the equilibrium properties of swelling gels with comb-like or bottlebrush subchains and compare them to scaling-theory predictions. In accordance with theory, the simulation results demonstrate that swelling coefficient of the gel increases as a function of the polymerization degree of the main chains and exhibits a very weak maximum (or is virtually constant) as a function of the polymerization degree and grafting density of side chains. The bulk osmotic modulus passes through a shallow minimum as the polymerization degree of the side chains increases. This minimum is attributed to the onset of overlap of side chains belonging to different bottlebrush strands in the swollen gel.

Theoretical advances in molecular bottlebrushes and comblike (co)polymers: solutions, gels, and self-assembly

We present an overview of state-of-the-art theory of (i) conformational properties of molecular bottlebrushes in solution, (ii) self-assembly of di- and triblock copolymers comprising comb-shaped and bottlebrush blocks in solutions and melts, and (iii) cross-linked and self-assembled gels with bottlebrush subchains. We demonstrate how theoretical models enable quantitative prediction and interpretation of experimental results and provide rational guidance for design of new materials with physical properties tunable by architecture of constituent bottlebrush blocks.

The Impact of Polymerization Chemistry on the Mechanical Properties of Poly(dimethylsiloxane) Bottlebrush Elastomers

We compare the low-strain mechanical properties of bottlebrush elastomers (BBEs) synthesized using ring-opening metathesis and free radical polymerization. Through comparison of experimentally measured elastic moduli and those predicted by an ideal, affine model, we evaluate the efficiency of our networks in forming stress-supporting strands. This comparison allowed us to develop a structural efficiency ratio that facilitates the prediction of mechanical properties relative to polymerization chemistry (e.g., softer BBEs when polymerizing under dilute conditions). This work highlights the impact that polymerization chemistry has on the structural efficiency ratio and the resultant mechanical properties of BBEs with identical side chains, providing another "knob" by which to control polymer network properties.

Structure of Self-Initiated Photopolymerized Films: A Comparison of Models

Self-initiated photografting and photopolymerization (SI-PGP) uses UV illumination to graft polymers to surfaces without additional photoinitiators using the monomers as initiators, "inimers". A wider use of this method is obstructed by a lack of understanding of the resulting, presumably heterogeneous, polymer structure and of the parallel degradation under continuous UV illumination. We have used neutron reflectometry to investigate the structure of hydrated SI-PGP-prepared poly(HEMA-co-PEG₁₀MA) (poly(2-hydroxyethyl methacrylate-co-(ethylene glycol)₁₀ methacrylate)) films and compared parabolic, sigmoidal, and Gaussian models for the polymer volume fraction distributions. Results from fitting these models to the data suggest that either model can be used to approximate the volume fraction profile to similar accuracy. In addition, a second layer of deuterated poly(methacrylic acid) (poly(dMAA)) was grafted over the existing poly(HEMA-co-PEG₁₀MA) layer, and the resulting double-grafted films were also studied by neutron reflectometry to shed light on the UV-polymerization process and the inevitable UV-induced degradation which competes with the grafting.

Liquid crystal droplet design by using pseudopeptidic bottlebrush polymer additives

Liquid crystal (LC) droplets are promising candidates for sensing applications due to their high sensitivity to surface anchoring changes, resulting in readily detectable optical effects. Herein, we have designed and synthesized amino acid-based bottlebrush polymers and investigated their impact on LC director configurations in the droplets. The pseudopeptidic bottlebrush polymers with an aromatic (phenyl) and aliphatic appendages are synthesized using ring-opening metathesis polymerization (ROMP). Polymer dispersed liquid crystal (PDLC) samples are prepared by employing pseudopeptidic bottlebrush polymers and 4-cyano-4'-pentylbiphenyl (5CB) LC via solvent-induced phase separation (SIPS) technique. Due to π-π stacking, the phenyl group favours radial configuration, whereas the repulsion between 5CB and aliphatic groups induces molecular alignment leading to bipolar droplet arrangement. The impact of various pendant groups attached to the polymer on the prepared PDLC sample's surface characteristics and free energy components is illustrated. The sensing capability of 5CB dispersed in pseudopeptidic bottlebrush polymers for various pH solutions is investigated using polarizing optical microscopy (POM). The PDLC samples are moderately permeable to water and sensitive to different pH solutions. The results demonstrate a simplified and straightforward approach for preparing LC-based biosensors and chemical sensors.

Temperature-Responsive Polymer Brush Coatings for Advanced Biomedical Applications

Modern biomedical technologies predict the application of materials and devices that not only can comply effectively with specific requirements, but also enable remote control of their functions. One of the most prospective materials for these advanced biomedical applications are materials based on temperature-responsive polymer brush coatings (TRPBCs). In this review, methods for the fabrication and characterization of TRPBCs are summarized, and possibilities for their application, as well as the advantages and disadvantages of the TRPBCs, are presented in detail. Special attention is paid to the mechanisms of thermo-responsibility of the TRPBCs. Applications of TRPBCs for temperature-switchable bacteria killing, temperature-controlled protein adsorption, cell culture, and temperature-controlled adhesion/detachment of cells and tissues are considered. The specific criteria required for the desired biomedical applications of TRPBCs are presented and discussed.

Anionic polymers amplify electrokinetic perfusion through extracellular matrices

Electrical stimulation (ES) promotes healing of chronic epidermal wounds and delays degeneration of articular cartilage. Despite electrotherapeutic treatment of these non-excitable tissues, the mechanisms by which ES promotes repair are unknown. We hypothesize that a beneficial role of ES is dependent on electrokinetic perfusion in the extracellular space and that it mimics the effects of interstitial flow. In vivo, the extracellular space contains mixtures of extracellular proteins and negatively charged glycosaminoglycans and proteoglycans surrounding cells. While these anionic macromolecules promote water retention and increase mechanical support under compression, in the presence of ES they should also enhance electro-osmotic flow (EOF) to a greater extent than proteins alone. To test this hypothesis, we compare EOF rates between artificial matrices of gelatin (denatured collagen) with matrices of gelatin mixed with anionic polymers to mimic endogenous charged macromolecules. We report that addition of anionic polymers amplifies EOF and that a matrix comprised of 0.5% polyacrylate and 1.5% gelatin generates EOF with similar rates to those reported in cartilage. The enhanced EOF reduces mortality of cells at lower applied voltage compared to gelatin matrices alone. We also use modeling to describe the range of thermal changes that occur during these electrokinetic experiments and during electrokinetic perfusion of soft tissues. We conclude that the negative charge density of native extracellular matrices promotes electrokinetic perfusion during electrical therapies in soft tissues and may promote survival of artificial tissues and organs prior to vascularization and during transplantation.

A Field Guide to Optimizing Peptoid Synthesis

N-Substituted glycines (peptoids) are a class of peptidomimetic molecules used as materials for health, environmental, and drug delivery applications. Automated solid-phase synthesis is the most widely used approach for preparing polypeptoids, with a range of published protocols and modifications for selected synthetic targets. Simultaneously, emerging solution-phase syntheses are being leveraged to overcome limitations in solid-phase synthesis and access high-molecular weight polypeptoids. This Perspective aims to outline strategies for the optimization of both solid- and solution-phase synthesis, provide technical considerations for robotic synthesizers, and offer an outlook on advances in synthetic methodologies. The solid-phase synthesis sections explore steps for protocol optimization, accessing complex side chains, and adaptation to robotic synthesizers; the sections on solution-phase synthesis cover the selection of initiators, side chain compatibility, and strategies for controlling polymerization efficiency and scale. This text acts as a "field guide" for researchers aiming to leverage the flexibility and adaptability of peptoids in their research.

Cluster Size of Amylopectin and Nanosized Amylopectin Fragments Characterized by Pyrene Excimer Formation

Amylopectin from waxy corn and the three nanosized amylopectin fragments (NAFs)—NAF(56), NAF(20), and NAF(8)—from waxy corn starch with a hydrodynamic diameter of 227, 56, 20, and 8 nm, respectively, were randomly labeled with 1-pyrenebutyric acid. The efficiency of these pyrene-labeled amylopectin-based polysaccharides (Py-AbPS) for pyrene excimer formation (PEF) upon diffusive encounter between an excited and a ground-state pyrene increased with increasing concentration of unlabeled NAF(56) in Py-AbPS dispersions in DMSO. Fluorescence decay analysis of the Py-AbPS dispersions in DMSO prepared with increasing [NAF(56)] yielded the maximum number (N_blob^exp) of anhydroglucose units (AGUs) separating two pyrene-labeled AGUs while still allowing PEF. Comparison of N_blob^exp with N_blob^theo, obtained by conducting molecular mechanics optimizations on helical oligosaccharide constructs with HyperChem, led to a relationship between the interhelical distance (d_h-h) in a cluster of oligosaccharide helices, [NAF(56)], and the number of helices in a cluster. It was found that the AbPSs were composed of building blocks made of 3.5 (±0.9) helices that self-assembled into increasingly larger clusters with increasing [NAF(56)]. The ability of PEF-based experiments to yield the cluster size of AbPSs provides a new experimental means to probe the interior of AbPSs at the molecular level.

Synthesis of well-defined linear-bottlebrush-linear triblock copolymer towards architecturally-tunable soft materials

Linear-bottlebrush-linear (LBBL) triblock copolymers are emerging systems for topologically-tunable elastic materials. In this paper, a new synthetic methodology is presented to synthesize LBBL polystyrene-block-bottlebrushpolydimethylsiloxane-block-polystyrene (PS-b-bbPDMS-b-PS) triblock copolymer via the "grafting onto" approach where the precursors are individually synthesized through living anionic polymerization and selective coupling reaction. In this two-step approach, polystyrene-block-polymethylvinylsiloxane (PS-b-PMVS) diblock copolymer with a low dispersity couples with another living PS block to form PS-b-PMVS-b-PS triblock copolymer. Secondly, this is followed by grafting of separately prepared monohydride-terminated PDMS chains with controllable grafting density through a hydrosilylation reaction. In addition to fully tunable architectural parameters, this approach permits a quantitative determination of the ratio of diblock and triblock bottlebrush copolymers and consistency between batches, highlighting the feasibility for scaled-up production. These LBBL triblock copolymers self-assemble into soft, low-modulus thermoplastic elastomers, and the precise knowledge of the composition is crucial for correlating microstructure to mechanical properties.

Concentration-Driven Self-Assembly of PS-b-PLA Bottlebrush Diblock Copolymers in Solution

Bottlebrush polymers are a class of semiflexible, hierarchical macromolecules with unique potential for shape-, architecture-, and composition-based structure–property design. It is now well-established that in dilute to semidilute solution, bottlebrush homopolymers adopt a wormlike conformation, which decreases in extension (persistence length) as the concentration and molecular overlap increase. By comparison, the solution phase self-assembly of bottlebrush diblock copolymers (BBCP) in a good solvent remains poorly understood, despite critical relevance for solution processing of ordered phases and photonic crystals. In this work, we combine small-angle X-ray scattering, coarse-grained simulation, and polymer synthesis to map the equilibrium phase behavior and conformation of a set of large, nearly symmetric PS-b-PLA bottlebrush diblock copolymers in toluene. Three BBCP are synthesized, with side chains of number-averaged molecular weights of 4500 (PS) and 4200 g/mol (PLA) and total backbone degrees of polymerization of 100, 255, and 400 repeat units. The grafting density is one side chain per backbone repeat unit. With increasing concentration in solution, all three polymers progress through a similar structural transition: from dispersed, wormlike chains with concentration-dependent (decreasing) extension, through the onset of disordered PS/PLA compositional fluctuations, to the formation of a long-range ordered lamellar phase. With increasing concentration in the microphase-separated regimes, the domain spacing increases as individual chains partially re-extend due to block immiscibility. Increases in the backbone degree of polymerization lead to changes in the scattering profiles which are consistent with the increased segregation strength. Coarse-grained simulations using an implicit side-chain model are performed, and concentration-dependent self-assembly behavior is qualitatively matched to experiments. Finally, using the polymer with the largest backbone length, we demonstrate that lamellar phases develop a well-defined photonic band gap in solution, which can be tuned across the visible spectrum by varying polymer concentration.

Toward Mechanical Recycling of Polystyrene/Polypropylene Blends with Bottlebrush-Modified Graphene Oxide as a Compatibilizer

The compatibilization of immiscible polystyrene/polypropylene (PS/PP) blends with virgin graphene oxide (GO-V) and GO modified with a bottlebrush reactive copolymer layer (GO-P) is reported. This practically important blend was chosen since, currently, PS and PP are recycled to a very low degree. The amphiphilic bottlebrush copolymer synthesized by us contained hydrophobic and hydrophilic side chains and was attached to the GO nanosheets via epoxy functionality. The GO modification and the introduction of GO into the blend were conducted from water. Thus, the introduction of the compatibilizing nanomaterial can be conducted during the mechanical recycling washing stage in a real-world situation. The final blend was prepared via melt mixing using an extruder. We examined the influence of GO modification and the mixing order on the blends' morphology, rheology, and mechanical properties. Thermodynamic calculations predicted a higher interfacial activity of GO nanosheets in PS/PP/GO-P blends than that in PS/PP/GO-V blends. The morphological and rheological study assessed this prediction. It was demonstrated that the bottlebrush-modified GO-P sheets were readily driven to the PS/PP interphase. The mechanical measurements showed enhanced mechanical properties for PS/PP/GO-P blends, especially for those in which GO was first premixed with PS.

Influences of the Periodicity in Molecular Architecture on the Phase Diagrams and Microphase Transitions of the Janus Double-Brush Copolymer with a Loose Graft

The backbone of the Janus double-brush copolymer may break during long-term service, but whether this breakage affects the self-assembled phase state and microphase transitions of the material is still unknown. For the Janus double-brush copolymers with a periodicity in molecular architecture ranging from 1 to 10, the influences of the architectural periodicity on their phase diagrams and order–disorder transitions (ODT) were investigated by the self-consistent mean field theory (SCFT). In total, nine microphases with long-range order were found. By comparing the phase diagrams between copolymers of different periodicity, a decrease in periodicity or breakage along the copolymer backbone had nearly no influence on the phase diagrams unless the periodicity was too short to be smaller than 3. For copolymers with neutral backbones, a decrease in periodicity or breakage along the copolymer backbone reduced the critical segregation strengths of the whole copolymer at ODT. The equations for the critical segregation strengths at ODT, the architectural periodicity, and the volume fraction of the backbone were established for the Janus double-brush copolymers. The theoretical calculations were consistent with the previous theoretical, experimental, and simulation results.

Bioinspired Bottlebrush Polymers for Aqueous Boundary Lubrication

An extremely efficient lubrication system is achieved in synovial joints by means of bio-lubricants and sophisticated nanostructured surfaces that work together. Molecular bottlebrush structures play crucial roles for this superior tribosystem. For example, lubricin is an important bio-lubricant, and aggrecan associated with hyaluronan is important for the mechanical response of cartilage. Inspired by nature, synthetic bottlebrush polymers have been developed and excellent aqueous boundary lubrication has been achieved. In this review, we summarize recent experimental investigations of the interfacial lubrication properties of surfaces coated with bottlebrush bio-lubricants and bioinspired bottlebrush polymers. We also discuss recent advances in understanding intermolecular synergy in aqueous lubrication including natural and synthetic polymers. Finally, opportunities and challenges in developing efficient aqueous boundary lubrication systems are outlined.

Linking microscopic structural changes and macroscopic mechanical responses in a near-ideal bottlebrush elastomer under uniaxial deformation

Bottlebrush (BB) elastomers, in which load-bearing network strands are densely grafted with side chains, are gaining much attention due to their unique mechanical properties. Herein, we used in situ small-angle X-ray scattering coupled with tensile tests to investigate the microscopic structural changes induced in a model BB elastomer with a controlled network structure under uniaxial deformation. The model BB elastomer was synthesized by end-linking a monodisperse star-shaped BB polymer, which ensured a controlled network structure. The BB elastomer exhibited both significant strain stiffening and backbone chain alignment under uniaxial loading, and these properties were not observed in an analogous side chain-free elastomer and gel. It was also found that the side chains in the BB elastomer did not show any sign of chain orientation even when the attached backbone chain was aligned in the stretching direction. These observations highlighted the roles of side chains: they were structurally disordered at the segment level but their steric repulsion made the backbone chain aligned and overstretched.

Construction of Enzyme-Responsive Micelles Based on Theranostic Zwitterionic Conjugated Bottlebrush Copolymers with Brush-on-Brush Architecture for Cell Imaging and Anticancer Drug Delivery

Bottlebrush copolymers with different chemical structures and compositions as well as diverse architectures represent an important kind of material for various applications, such as biomedical devices. To our knowledge, zwitterionic conjugated bottlebrush copolymers integrating fluorescence imaging and tumor microenvironment-specific responsiveness for efficient intracellular drug release have been rarely reported, likely because of the lack of an efficient synthetic approach. For this purpose, in this study, we reported the successful preparation of well-defined theranostic zwitterionic bottlebrush copolymers with unique brush-on-brush architecture. Specifically, the bottlebrush copolymers were composed of a fluorescent backbone of polyfluorene derivate (PFONPN) possessing the fluorescence resonance energy transfer with doxorubicin (DOX), primary brushes of poly(2-hydroxyethyl methacrylate) (PHEMA), and secondary graft brushes of an enzyme-degradable polytyrosine (PTyr) block as well as a zwitterionic poly(oligo (ethylene glycol) monomethyl ether methacrylate-co-sulfobetaine methacrylate) (P(OEGMA-co-SBMA)) chain with super hydrophilicity and highly antifouling ability via elegant integration of Suzuki coupling, NCA ROP and ATRP techniques. Notably, the resulting bottlebrush copolymer, PFONPN9-g-(PHEMA15-g-(PTyr16-b-P(OEGMA6-co-SBMA6)2)) (P2) with a lower MW ratio of the hydrophobic side chains of PTyr and hydrophilic side chains of P(OEGMA-co-SBMA) could self-assemble into stabilized unimolecular micelles in an aqueous phase. The resulting unimolecular micelles showed a fluorescence quantum yield of 3.9% that is mainly affected by the pendant phenol groups of PTyr side chains and a drug-loading content (DLC) of approximately 15.4% and entrapment efficiency (EE) of 90.6% for DOX, higher than the other micelle analogs, because of the efficient supramolecular interactions of π–π stacking between the PTyr blocks and drug molecules, as well as the moderate hydrophilic chain length. The fluorescence of the PFONPN backbone enables fluorescence resonance energy transfer (FRET) with DOX and visualization of intracellular trafficking of the theranostic micelles. Most importantly, the drug-loaded micelles showed accelerated drug release in the presence of proteinase K because of the enzyme-triggered degradation of PTyr blocks and subsequent deshielding of P(OEGMA-co-SBMA) corona for micelle destruction. Taken together, we developed an efficient approach for the synthesis of enzyme-responsive theranostic zwitterionic conjugated bottlebrush copolymers with a brush-on-brush architecture, and the resulting theranostic micelles with high DLC and tumor microenvironment-specific responsiveness represent a novel nanoplatform for simultaneous cell image and drug delivery.

Rigid-to-Flexible Transition in a Molecular Brush in a Good Solvent at a Semidilute Concentration

The structures of a molecular brush in a good solvent are investigated using synchrotron small-angle X-ray scattering in a wide range of concentrations. The brush under study, PiPOx₂₃₉-g-PnPrOx₁₄, features a relatively long poly(2-isopropenyl-2-oxazoline) (PiPOx) backbone and short poly(2-n-propyl-2-oxazoline) (PnPrOx) side chains. As a solvent, ethanol is used. By model fitting, the overall size and the persistence length as well as the interaction length and interaction strength are determined. At this, the interplay between form and structure factor is taken into account. The conformation of the molecular brush is traced upon increasing the solution concentration, and a rigid-to-flexible transition is found near the overlap concentration. Finally, the results of computer simulations of the molecular brush solutions confirm the experimental results.

Reconfiguration and Reorganization of Bottlebrush Polymer Surfactants

Bottlebrush random copolymers (BRCPs), having randomly distributed hydrophilic and hydrophobic side chains, are shown to reconfigure into hydrophilic-rich and hydrophobic-rich conformations at liquid-liquid interfaces to reduce interfacial energy. Both the degree of polymerization (N_BB ) and extent of grafting in these BRCPs were found to impact surface coverage and assembly kinetics. The time-dependence of the interfacial tension is described as the sum of two exponential relaxation functions characterizing BRCP diffusion, interfacial adsorption, and reorganization. Interfacial tension (γ) and fluorescence recovery after photobleaching (FRAP) results showed that higher molecular weight BRCPs require longer time to adsorb to the water-oil interface, but less time for interfacial reorganization. Overall, this work describes fundamental principles of BRCP assembly at liquid-liquid interfaces, with implications pertaining to polymer design with enhanced understanding of emulsification, adhesion, and related properties in fluids and at interfaces.

Synthesis and characterization of poly(ethylene glycol) bottlebrush networks via ring-opening metathesis polymerization

Herein it is reported how the overlap concentration (C*) can be used to overcome crosslinking due to diol impurities in commercial PEG, allowing for the synthesize of bottlebrush polymers with good control over molecular weight. Additionally, PEG-based bottlebrush networks are synthesized via ROMP, attaining high conversions with minimal sol fractions (<2%). The crystallinity and mechanical properties of these networks are then further altered by solvent swelling with phosphate buffer solution (PBS) and 1-ethyl-3-methylimidazolium ethyl sulfate/DCM cosolvents. The syntheses reported here highlight the potential of the bottlebrush network architecture for use in the rational design of new materials.

Molecular polymer bottlebrushes in nanomedicine: therapeutic and diagnostic applications

Molecular polymer bottlebrushes are densely grafted, individual macromolecules with nanoscale proportions. The last decade has seen an increased focus on this material class, especially in nanomedicine and for biomedical applications. This Feature Article provides an overview of major developments in this area to highlight the many opportunities that these polymer architectures bring to nano-bio research. The article covers aspects of bottlebrush synthesis and summarises their use in drug and gene delivery, imaging, as theranostics and as prototype materials to correlate nanoparticle structure and composition to biological function and behaviour. Areas for future research in this area are discussed.