Bottlebrush Polymers Research Articles

Multicompartment Micro- and Nanoparticles Using Supramolecular Assembly of Core-Shell Bottlebrush Polymers.

Multicompartment particles have been produced to date by the self-assembly of linear multiblock polymers. Besides the large diversity of structures that can be obtained with this approach, these are highly sensitive to dilution and environmental factors. Here we show that using core-shell bottlebrush polymers with a hydrophobic polyester core as starting materials it is possible to create compartmentalized particles from the micrometer size down to the molecular scale. These polymers can be used as building blocks to create multicompartment particles and networks via a self-assembly process. The polymers can encapsulate active compounds and slowly degrade in water into polymeric micelles, making them promising materials for drug delivery applications.

Solution Structure and Scaling Laws of Cylindrical and Tapered Bottlebrush Polymers

Solution Structure and Scaling Laws of Cylindrical and Tapered Bottlebrush Polymers

Blending Bottlebrush Polymers for Compensation of Orientation Birefringence of Polystyrene for Use as an Optical Polymer

Blending Bottlebrush Polymers for Compensation of Orientation Birefringence of Polystyrene for Use as an Optical Polymer

Regulating the Conformational and Macroscopic Properties of Inorganic Nanowires by Polymer Grafts

AbstractUltrathin inorganic nanowires are an emerging class of building blocks for creating functional materials and devices, but there remains challenging to fine‐tune the structure and property of the nanowires at the molecular level. Here, this work shows that ultrathin polymer‐grafted gold nanowires (PG‐AuNWs) can exhibit bottlebrush polymer‐like physical behaviors and macroscopic properties. The hybrid PG‐AuNWs in solutions show polymer‐like viscoelasticity which can be finely regulated by controlling the structural parameters (e.g., length of the AuNWs, molecular weight of grafted polymers) of the PG‐AuNWs, the environmental conditions (e.g., temperature, solvent compositions), and the aging time. Furthermore, this work unravels at the molecular level that the conformational properties (e.g., contour length, persistence length, and mean‐squared radius of gyration) of the PG‐AuNWs can be correlated to their structural parameters following similar power‐law relations as molecular bottlebrush polymers. This work bridges the fundamental gap between polymer‐like inorganic nanowires and bottlebrush polymers, thus accelerating the development of new nanowire‐based functional hybrid materials and devices.

Anchor Group Bottlebrush Polymers as Oil Additive Friction Modifiers.

Surface-tethered polymers have been shown to be an efficient lubrication strategy for boundary and mixed lubrication by providing a solvated film between solid surfaces. We have assessed the performance of various graft copolymers as friction modifier additives in oil and revealed important structure-property relationships for this application. The polymers consisted of an oil-soluble, grafted poly(lauryl acrylate) segment and a polar, linear poly(4-acryloylmorpholine) anchor group. Reversible addition-fragmentation chain transfer polymerization was used to access various architectures with control of the grafting density and position of the anchor group. Macrotribological studies displayed promising results with ≈50% reduction in friction coefficient at low polymer treatment rates. QCM-D experiments, neutron reflectometry, small-angle neutron scattering, and atomic force microscopy were used to gather detailed information on these polymers' surface adsorption characteristics, film structure, and solution behavior.

Relationship between Graft Density and the Dilute Solution Structure of Bottlebrush Polymers: An Inter-chemistry Comparison and Scaling Analysis

Relationship between Graft Density and the Dilute Solution Structure of Bottlebrush Polymers: An Inter-chemistry Comparison and Scaling Analysis

PolyGraft 1.0: A program for molecular structure and topology generation of polymer-grafted hybrid nanostructures.

Polymer-grafted hybrid materials have been ubiquitously employed in various engineering applications. The design of these hybrid materials with superior performances requires a molecularly detailed understanding of the structure and dynamics of the polymer brushes and their interactions with the grafting substrate. Molecular dynamics (MD) simulations are very well suited for the study of these materials which can provide molecular insights into the effects of polymer composition and length, grafting density, substrate composition and curvatures, and nanoconfinement. However, few existing tools are available to generate such systems, which would otherwise reduce the barrier of preparation for such systems to enable high throughput simulations. Here polyGraft, a general, flexible, and easy to use Python program, is introduced for automated generation of molecular structure and topology of polymer grafted hybrid materials for MD simulations purposes, ranging from polymer brushes grafted to hard substrates, to densely grafted bottlebrush polymers. polyGraft is openly accessible on GitHub (https://github.com/nanogchen/polyGraft).

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).

Bottlebrush Polymers at Liquid Interfaces: Assembly Dynamics, Mechanical Properties, and All-Liquid Printed Constructs.

Bottlebrush polymer surfactants (BPSs), formed by the interfacial interactions between bottlebrush polymers (BPs) with poly(acrylic acid) side chains dissolved in an aqueous phase and amine-functionalized ligands dissolved in the oil phase, assemble and bind strongly to the fluid-fluid interface. The ratio between NBB (backbone degree of polymerization) and NSC (side chain degree of polymerization) defines the initial assembly kinetics, interface packing efficiency, and stress relaxation. The equilibrium interfacial tension (γ) increases when NBB < NSC, but decreases when NBB ≫ NSC, correlating to a pronounced change in the effective shape of the BPs from being spherical to worm-like structures. The apparent surface coverage (ASC), i.e., the interfacial packing efficiency, decreases as NBB increases. The dripping-to-jetting transition of an injected polymer solution, as well as fluorescence recovery after photobleaching experiments, revealed faster initial assembly kinetics for BPs with higher NBB. Euler buckling of BPS assemblies with different NBB values was used to characterize the stress relaxation behavior and bending modulus. The stress relaxation behavior was directly related to the ASC, reflecting the strong influence of macromolecular shape on packing efficiency. The bending modulus of BPSs decreases for NBB < NSC, but increased when NBB ≫ NSC, showing the effect of molecular architecture and multisite anchoring. All-liquid printed constructs with lower NBB BPs yielded more stable structured liquids, underscoring the importance of macromolecular packing efficiency at fluid interfaces. Overall, this work elucidates fundamental relationships between nanoscopic structures and macroscopic properties associated with various bottlebrush polymer architectures, which translate to the stabilization of all-fluidic printed constructs.

Highly Conductive Polyoxanorbornene-Based Polymer Electrolyte for Lithium-Metal Batteries.

This present study illustrates the synthesis and preparation of polyoxanorbornene-based bottlebrush polymers with poly(ethylene oxide) (PEO) side chains by ring-opening metathesis polymerization for solid polymer electrolytes (SPE). In addition to the conductive PEO side chains, the polyoxanorbornene backbones may act as another ion conductor to further promote Li-ion movement within the SPE matrix. These results suggest that these bottlebrush polymer electrolytes provide impressively high ionic conductivity of 7.12 × 10-4 S cm-1 at room temperature and excellent electrochemical performance, including high-rate capabilities and cycling stability when paired with a Li metal anode and a LiFePO4 cathode. The new design paradigm, which has dual ionic conductive pathways, provides an unexplored avenue for inventing new SPEs and emphasizes the importance of molecular engineering to develop highly stable and conductive polymer electrolytes for lithium-metal batteries (LMB).

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.

Specific Clearance of Lipopolysaccharide from Blood Based on Peptide Bottlebrush Polymer for Sepsis Therapy.

Lipopolysaccharide (LPS) is the primary bacterial toxin that is vital to the pathogenesis and progression of sepsis associated with extremely high morbidity and mortality worldwide. However, specific clearance of LPS from circulating blood is highly challenging because of the structural complexity and its variation between/within bacterial species. Herein, a robust strategy based on phage display screening and hemocompatible peptide bottlebrush polymer design for specific clearance of targeted LPS from circulating blood is proposed. Using LPS extracted from Escherichia coli as an example, a novel peptide (HWKAVNWLKPWT) with high affinity (KD < 1.0 nм), specificity, and neutralization activity (95.9 ± 0.1%) against the targeted LPS is discovered via iterative affinity selection coupled with endotoxin detoxificationscreening. A hemocompatible bottlebrush polymer bearing the short peptide [poly(PEGMEA-co-PEP-1)] exhibits high LPS selectivity to reduce circulating LPS level from 2.63 ± 0.01 to 0.78 ± 0.05 EU mL-1 in sepsis rabbits via extracorporeal hemoperfusion (LPS clearance ratio > 70%), reversing the LPS-induced leukocytopenia and multiple organ damages significantly. This work provides a universal paradigm for developing a highly selective hemoadsorbent library fully covering the LPS family, which is promising to create a new era of precision medicine in sepsis therapy.

Synthetic Star Nanoengineered Antimicrobial Polymers as Antibiofilm Agents: Bacterial Membrane Disruption and Cell Aggregation.

Antimicrobial resistance has become a worldwide issue, with multiresistant bacterial strains emerging at an alarming rate. Multivalent antimicrobial polymer architectures such as bottle brush or star polymers have shown great potential, as they could lead to enhanced binding and interaction with the bacterial cell membrane. In this study, a library of amphiphilic star copolymers and their linear copolymer equivalents, based on acrylamide monomers, were synthesized via RAFT polymerization. Their monomer distribution and molecular weight were varied. Subsequently, their antimicrobial activity toward a Gram-negative bacterium (Pseudomonas aeruginosa PA14) and a Gram-positive bacterium (Staphylococcus aureus USA300) and their hemocompatibility were investigated. The statistical star copolymer, S-SP25, showed an improved antimicrobial activity compared to its linear equivalent againstP. aeruginosaPA14. The star architecture enhanced its antimicrobial activity, causing bacterial cell aggregation, as revealed via electron microscopy. However, it also induced increased red blood cell aggregation compared to its linear equivalents. Changing/shifting the position of the cationic block to the core of the structure prevents the cell aggregation effect while maintaining a potent antimicrobial activity for the smallest star copolymer. Finally, this compound showed antibiofilm properties against a robust in vitro biofilm model.

Facilely Control Grafting Density and Side Chain Composition of Bottlebrush Polymer.

Control over polymer architecture and composition is essential for disclosing structure-property relationships and developing high-performance materials. Herein, a new method is successfully developed to synthesize bottlebrush polymer (BP) with controllable graft density and side chain composition by "grafting-from" strategy using in situ halogen exchange and reversible chain transfer catalyzed polymerization (RTCP). The main chain of the BP is first synthesized by the polymerization of methacrylates containing alkyl bromide as a side group. Then, the alkyl bromine is quantitatively converted to alkyl iodide with sodium iodide (NaI) via in situ halogen exchange to efficiently initiate the RTCP of methacrylates. By adjusting the input amount of NaI and monomers in sequence, BP named PBPEMA-g-PMMA/PBzMA/PPEGMEMA which contains three different kinds of polymer side chains including hydrophilic PPEGMEMA, hydrophobic PMMA, and PBzMA is synthesized with narrow molecular weight distribution (Mw /Mn ≤1.36). The grafting density and the chain length of each polymer side chain are well controlled by the addition of NaI in batches and following RTCP. Moreover, the obtained BP self-assembled into spherical vesicles in aqueous with hydrophilic coronal structure, core region, and the hydrophobic wall between the former two, which enables to wrap hydrophobic pyrene and hydrophilic Rhodamine 6G separately or simultaneously.

Influence of the Norbornene Anchor Group in Ru-Mediated Ring-Opening Metathesis Polymerization: Synthesis of Bottlebrush Polymers

Ring-opening metathesis polymerization (ROMP) mediated by Grubbs’ third-generation catalyst [G3, (H2IMes)(Cl)2(pyr)2RuCHPh] is widely used to make bottlebrush polymers by polymerization of a macromonomer (MM), typically a low molecular weight polymer functionalized with a norbornene. Termed the grafting-through method, this strategy requires a high degree of living character (“livingness”) to form well-defined bottlebrush polymers. Here, we studied how various anchor groups, the series of atoms connecting the polymerizable norbornene unit to the polymer side-chain, affect livingness in ROMP in a series of exo-norbornene polystyrene MMs. First, we calculated the HOMO and HOMO/LUMO gap energies of MM structures containing five different anchor groups using density functional theory methods, finding that these energies spanned a range of 10 kcal/mol. We then performed kinetics experiments on each MM with target backbone degrees of polymerization (Nbb) of 100 to measure the propagation rate constant (kp,obs) under identical conditions. A positive correlation between the HOMO energy and measured kp,obs values emerged, revealing a 7-fold variation in kp,obs values across the five MMs, suggesting different degrees of livingness among the anchor groups. A series of studies targeting Nbb values ranging from 100 to 2000 further highlighted these differences: The MMs with high kp,obs values reached higher conversions at high target Nbb values with lower dispersities (D̵) than the MMs with lower kp,obs values. Finally, we evaluated the synthesis of bottlebrush pentablock copolymers using the MMs at the two extremes by injecting an MM aliquot into a catalyst solution five consecutive times, allowing for polymerization of each block before the next injection. MM conversion at each step was higher, and the D̵ values for each block were lower for the MM with the highest kp anchor group compared to the lowest kp anchor group. Taken together, these studies highlight how the anchor group dramatically affects both kp and livingness in ROMP, which is crucial for the synthesis of precise bottlebrush (co)polymers.

Tailoring the Wettability and Substrate Adherence of Thin Polymer Films with Surface-Segregating Bottlebrush Copolymer Additives.

We developed "reactive" bottlebrush polymers based on styrene (S) and t-butyl acrylate (tBA) as additives for polystyrene (PS) coatings. The bottlebrush polymers spontaneously bloom to both the air and substrate interfaces during solution casting. While neat PS films are hydrophobic and poorly adhere to the native oxide on clean silicon wafers, the hydrophilicity and substrate adherence of bottlebrush-incorporating PS films can be tailored through the thermally activated deprotection of tBA to produce acrylic acid (AA) and acrylic anhydride (AH). A critical design parameter is the manner by which tBA is incorporated into the bottlebrush: When the bottlebrush side chains are copolymers of S and tBA, the extent of deprotection is extremely low, even after prolonged thermal annealing at elevated temperature. However, when the bottlebrush contains a mixture of poly(t-butyl acrylate) (PtBA) and PS side chains, nearly all tBA is converted to AA and AH. Consequently, using the "mixed-chain" bottlebrush design with thermal processing and appropriate conditioning, the water contact angle is reduced from over 90° on unmodified PS down to 75° on bottlebrush-incorporating PS films, and the substrate adherence is improved in proportion to the extent of tBA deprotection.

Cyclic vs Linear Bottlebrush Polymers in Solution: Side-Chain Length Effect

Bottlebrush polymers (BBPs) of different architecture are of considerable interest for a broad range of applications, including nanomedicine, electronics, and self-healing materials. Using atomistic molecular dynamics simulations, we investigate and compare the structural and hydration properties of cyclic and linear poly(vinyl alcohol)-graft-poly(ethylene oxide) (PVA-g-PEONsc) BBPs in aqueous solution as functions of PEO side-chain length, Nsc. We find that overall cyclic BBPs are smaller than the corresponding linear BBPs and their shape changes from donutlike to disklike to starlike with increasing side-chain length, while linear BBPs vary in shape from an expanded coil to a rod/cylinder. The radius of gyration of cyclic BBPs increases with an increase of the side-chain length at a somewhat slower rate than the linear BBPs but follows the same scaling Rg ∼ Nsc0.58 in the limit of long side chains. For short grafts, we determine that the persistence length, lp, for both cyclic and linear BBPs increases in a similar manner following an lp ∼ Nsc0.54 dependence. In the long side-chain limit, the persistence length (lp) of cyclic BBP saturates, while for linear BBPs lp strongly increases following the expected scaling relation lp ∼ Nsc15/8. We propose a scaling model that shows that the fraction of side-chains located inside the cyclic BBPs depends on the radius of the backbone ring and significantly decreases with an increase of graft length. For both cyclic and linear BBPs, the hydrogen bonding between PEO side chains and water is somewhat reduced near the backbone, where local chain stretching is observed, while reaching full hydration on the periphery. Overall, the hydration shell within 1 nm of the cyclic BBP backbone is found to be more dynamically stable compared to linear BBPs.

On-Demand Cross-Linkable Bottlebrush Polymers for Voltage-Driven Artificial Muscles.

Dielectric elastomer actuators (DEAs) generate motion resembling natural muscles in reliability, adaptability, elongation, and frequency of operation. They are highly attractive in implantable soft robots or artificial organs. However, many applications of such devices are hindered by the high driving voltage required for operation, which exceeds the safety threshold for the human body. Although the driving voltage can be reduced by decreasing the thickness and the elastic modulus, soft materials are prone to electromechanical instability (EMI), which causes dielectric breakdown. The elastomers made by cross-linking bottlebrush polymers are promising for achieving DEAs that suppress EMI. In previous work, they were chemically cross-linked using an in situ free-radical UV-induced polymerization, which is oxygen-sensitive and does not allow the formation of thin films. Therefore, the respective actuators were operated at voltages above 4000 V. Herein, macromonomers that can be polymerized by ring-opening metathesis polymerization and subsequently cross-linked via a UV-induced thiol-ene click reaction are developed. They allow us to fast cross-link defect-free thin films with a thickness below 100 μm. The dielectric films give up to 12% lateral actuation at 1000 V and survive more than 10,000 cycles at frequencies up to 10 Hz. The easy and efficient preparation approach of the defect-free thin films under air provides easy accessibility to bottlebrush polymeric materials for future research. Additionally, the desired properties, actuation under low voltage, and long lifetime revealed the potential of the developed materials in soft robotic implantable devices. Furthermore, the C-C double bonds in the polymer backbone allow for chemical modification with polar groups and increase the materials' dielectric permittivity to a value of 5.5, which is the highest value of dielectric permittivity for a cross-linked bottlebrush polymer.

Self-Healable, Stretchable, and Super-Soft Bottlebrush Polyester Elastomers for Highly Sensitive Flexible Sensors

While much effort has been devoted to the creation of super-soft elastomers, there is a dearth of research aimed at self-healable and super-soft renewable elastomers. Herein, self-healing and super-soft renewable elastomers are developed based on bottlebrush architecture design and dynamic associative transesterification. The bottlebrush polymer precursors are generated by ring-opening metathesis polymerization (ROMP) of the norbornene-terminated macromonomer poly(ε-caprolactone-co-l-lactide) (PCLLA) (NB-PCLLA). After introducing alkyne groups onto the end of PCLLA side chains, the bottlebrush polymers are cross-linked to achieve elastomer networks via a UV-induced “click” thiol-yne reaction between the terminal alkyne groups and the dithiothreitol (DTT) additive. The resulting materials exhibit a shear modulus as low as 17 kPa, high stretchability with breaking strains as large as 315%, and a self-healing efficiency of 95.6% at an elevated temperature of 80 °C. Taking the super-softness advantage, flexible sensor devices are assembled based on the prepared elastomer, which are highly sensitive to deformation with a gauge factor of 0.347. This work demonstrates that high-performance biodegradable materials with unique properties that conventional linear polymers impossibly possess can be easily constructed by molecular architecture design using bottlebrush polymers as building blocks.

Star-to-Bottlebrush Transition in Extensional and Shear Deformation of Unentangled Polymer Melts

A series of model poly((±)-lactide) (PLA) graft copolymers was synthesized by ring-opening metathesis polymerization and used to probe the star-to-bottlebrush transition in shear and extensional flows. Ten samples with backbone degrees of polymerization 11 ≤ Nbb ≤ 420 were investigated using small-amplitude oscillatory shear (SAOS) and extensional rheometry measurements. Each contained one PLA side chain of length Nsc = 72 per two backbone repeating units on average (graft density of z = 0.5). The star-like to bottlebrush transition was identified at Nbb = 50–69 using SAOS. In extension, melt strain hardening is absent in the star-like melts (Nbb ≤ 50) but is prominent in the bottlebrush limit (Nbb > 69). The onset of melt strain hardening occurs at a time scale equivalent to the Rouse time of the backbone. A molecular interpretation of these results builds upon recent conjectures related to strain-induced increases in interchain friction in bottlebrush polymers. These findings will be useful in designing bottlebrush melts that strain harden, which is critical in various types of processing methods involving extensional flows, including foaming, 3D printing, and film-blowing.