Molecular Bottlebrushes Research Articles

Phototunable Supersoft Elastomers using Coumarin Functionalized Molecular Bottlebrushes for Cell-Surface Interactions Study

Phototunable supersoft elastomers were prepared by photo-cross-linking molecular bottlebrushes with poly(n-butyl acrylate) (PBA) side chains. Bottlebrushes were synthesized by atom transfer radical polymerization (ATRP) and Br-chain end functionalities were replaced by photo-cross-linkable coumarin units. Photo-cross-linking and photoscission of molecular bottlebrushes resulted in tunable elastomeric behavior quantified by dynamic mechanical analysis. The cell–substrate interactions, tuned by UV-light, depended on surface morphology and softness that could be as low as several kilopascals.

Structural Evolution of Polylactide Molecular Bottlebrushes: Kinetics Study by Size Exclusion Chromatography, Small Angle Neutron Scattering, and Simulations.

Structural evolution from poly(lactide) (PLA) macromonomer to resultant PLA molecular bottlebrush during ring opening metathesis polymerization (ROMP) was investigated for the first time by combining size exclusion chromatography (SEC), small-angle neutron scattering (SANS), and coarse-grained molecular dynamics (CG-MD) simulations. Multiple aliquots were collected at various reaction times during ROMP and subsequently analyzed by SEC and SANS. These complementary techniques enable the understanding of systematic changes in conversion, molecular weight and dispersity as well as structural details of PLA molecular bottlebrushes. CG-MD simulation not only predicts the experimental observations, but it also provides further insight into the analysis and interpretation of data obtained in SEC and SANS experiments. We find that PLA molecular bottlebrushes undergo three conformational transitions with increasing conversion (i.e., increasing the backbone length): (1) from an elongated to a globular shape due to longer side chain at low conversion, (2) from a globular to an elongated shape at intermediate conversion caused by excluded volume of PLA side chain, and (3) the saturation of contour length at high conversion due to chain transfer reactions.

Shifting Electronic Structure by Inherent Tension in Molecular Bottlebrushes with Polythiophene Backbones.

Bottlebrush macromolecules can be regarded as molecular tensile machines, where tension is self-generated along the backbone due to steric repulsion between densely grafted side chains. This intrinsic tension is amplified upon adsorption of bottlebrush molecules onto a substrate and increases with grafting density, side chain length, and strength of adhesion to the substrate. To investigate the effects of tension on the electronic structure of polythiophene (PT), bottlebrush macromolecules were prepared by grafting poly(n-butyl acrylate) (PBA) side chains from PT macroinitiators by atom transfer radical polymerization (ATRP). The fluorescence spectra of submonolayers of PT bottlebrushes were measured on a Langmuir-Blodgett (LB) trough with the backbone tension adjusted by controlling the side-chain length, surface pressure, and chemical composition of a substrate. The wavelength of maximum emission has initially red-shifted, followed by a blue-shift as the backbone tension increases from 0 to 2.5 nN, which agrees with DFT calculations. The red-shift is ascribed to an increase in the conjugation length due to the extension of the PT backbone at lower force regime (0-1.0 nN), while the blue-shift is attributed to deformations of bond lengths and angles in the backbone at higher force regime (1.0-2.5 nN).

Polymer bottlebrushes with a redox responsive backbone feel the heat: synthesis and characterization of dual responsive poly(ferrocenylsilane)s with PNIPAM side chains

Molecular bottlebrushes, possessing a redox-responsive poly(ferrocenylsilane) (PFS) backbone and temperature-responsive poly(N-isopropylacrylamide) (PNIPAM) side chains, distributed homogeneously or as a gradient along the PFS main chain, were synthesized. Attachment of the PNIPAM chains via azide–alkyne click chemistry, or grafting from a PFS macroinitiator backbone by ATRP, resulted in cylindrical shaped molecular bottlebrushes. We found the bottlebrushes to be both redox and temperature responsive, with little influence of one responsiveness on the other. In an aqueous environment above 32 °C the bottlebrushes collapsed to 70% of their original size due to the temperature sensitive side chains, and reversibly recovered their initial size upon cooling as revealed by Dynamic Light Scattering (DLS). Cyclic voltammograms showed electrochemical behavior typical of well solvated, single PFS chains. The backbone of the deposited molecules was in close proximity to the highly ordered pyrolytic graphite (HOPG) electrode surface and was accessible to the supporting electrolyte owing to the presence of the hydrophilic PNIPAM side chains. The bottlebrush molecules were deposited on HOPG surfaces for direct molecular visualization by atomic force microscopy (AFM). Molecular size data obtained by DLS and AFM showed good agreement. The bottlebrushes, reported here, are excellent candidates as addressable components for future devices e.g. to carry and deliver molecular payloads.

Disk-Like Micelles with a Highly Ordered Pattern from Molecular Bottlebrushes.

Self-assembling amorphous macromolecules into nanoparticles with a highly ordered internal structure and a defined shape is always a big challenge. Herein we present formation of soft disk-like micelles by hexagonally packing AbBA amphiphilic triblock copolymers, whose bB block is the molecular bottlebrush with densely grafted poly(t-butyl acrylate)-b-polystyrene (PBA-b-PS) branches and A block is poly(N-(2-methacryloyloxyethyl)pyrrolidone) (PNMEP). In a selective solvent of A segment, it was found that the AbBA molecular bottlebrushes with stiffened middle blocks self-assembled two-dimensionally into disks with a uniform thickness of about 33 nm and a diameter of hundreds of nanometers. A hexagonal pattern of molecular bottlebrushes aligned perpendicularly to the disk plane with a periodic spacing of 9 nm was visualized by TEM through selective staining the PS shells of brushes. The aggregation number of AbBA molecular bottlebrushes in each disk was counted directly from the stained TEM image.

Functionalized Molecular Bottlebrushes

A three-step synthetic strategy is established for the preparation of functionalized bottlebrush copolymers. In this scheme, the highly efficient nature of thiol-epoxy coupling chemistry is employed for the attachment of thiol terminated poly(ethylene glycol) (PEG) polymers (0.18, 0.8, and 2 kDa) to the poly(glycidyl methacrylate) (PGMA) backbone (25 and 46 kDa). This coupling reaction resulted in the formation of water-soluble bottlebrush copolymers (50–426 kDa) with grafting densities ranging from 88 to 97%. The coupling process also produced reactive hydroxyl groups in the vicinity of the polymer backbone. These hydroxyl groups could be functionalized with pyrene (a fluorescent probe) or biotin (a biological ligand) molecules through an esterification reaction. Therefore, fluorescent/biorelevant bivalent bottlebrushes could be obtained in three linear synthetic steps starting from a commercially available monomer. The prepared polymers displayed structure dependent thermal and optical properties, and single bottlebrushes could be visualized with the help of atomic force microscopy (AFM).

One-Pot Synthesis of Molecular Bottle-Brush Functionalized Single-Walled Carbon Nanotubes with Superior Dispersibility in Water

Molecular bottle-brush functionalized single-walled carbon nanotubes (SWCNTs) with superior dispersibility in water are prepared by a one-pot synthetic methodology. Elongating the main-chain and side-chain length of molecular bottle-brushes can further increase SWCNT dispersibility. They show significant enhancement of SWCNT dispersibility up to four times higher than those of linear molecular functionalized SWCNTs.

Exploring Quality in Gradient Copolymers

Quality of gradient copolymers is evaluated by atomic force microscopy (AFM) and correlated with molecular weight distribution (MWD) values. ARGET ATRP is employed with decreasing levels of catalyst concentrations to generate copolymers with increasing M¯w/M¯n values. The copolymers are transformed into molecular bottlebrushes to enable imaging and analysis of individual molecules by AFM. The average height (cross-sectional) profile of all bottlebrushes agrees with the instantaneous composition (ICHEMA-TMS ) of the analogous copolymer backbone, as determined by (1) H NMR. The copolymer synthesized with 500 ppm of catalyst exhibits more narrow distributions of both brush height and backbone length when analyzed as a bottlebrush by AFM. Correspondingly, the copolymers synthesized with lower catalyst concentrations yield bottlebrushes with broader height and length distribution. These results establish MWD values as an excellent trait to assess quality within gradient copolymers.

Poly(3-hexylthiophene) Molecular Bottlebrushes via Ring-Opening Metathesis Polymerization: Macromolecular Architecture Enhanced Aggregation.

We report a facile synthetic strategy based on a grafting through approach to prepare well-defined molecular bottlebrushes composed of regioregular poly(3-hexylthiophene) (rr-P3HT) as the conjugated polymeric side chain. To this end, the exo-norbornenyl-functionalized P3HT macromonomer was synthesized by Kumada catalyst transfer polycondensation (KCTP) followed by postpolymerization modifications, and the resulting conjugated macromonomer was successfully polymerized by ring-opening metathesis polymerization (ROMP) in a controlled manner. The P3HT molecular bottlebrushes display an unprecedented strong physical aggregation upon drying during recovery, as verified by several analyses of the solution and solid states. This remarkably strong aggregation behavior is attributed to a significant enhancement in the number of π-π interactions between grafted P3HT side chains, brought about due to the bottlebrush architecture. This behavior is qualitatively supported by coarse-grained molecular dynamics simulations.

DNA-Grafted Polypeptide Molecular Bottlebrush Prepared via Ring-Opening Polymerization and Click Chemistry

A new type of DNA grafted polypeptide molecular brush was synthesized via a combination of ring-opening polymerization (ROP) and click chemistry. This conjugation method provides an easy and efficient approach to obtain a hybrid DNA-grafted polypeptide molecular bottlebrush. The structure and assembly behaviors of this hybrid brush were investigated using electrophoresis, UV–vis spectroscopy, transmission electron microscopy (TEM), and atomic force microscopy (AFM). Hierarchical supramolecular assemblies can be obtained through hybridization of two kinds of polypeptide-g-DNA molecular bottlebrushes containing complementary DNA side chains. We further demonstrated that such polypeptide-g-DNA can be hybridized with ds-DNA and DNA-grafted gold nanoparticles to form a supermolecular bottlebrush and hybrid bottlebrush, respectively. In addition, DNA-polypeptide hydrogel can be prepared by hybridization of polypeptide-g-DNA with a linker-ds-DNA, which contains the complementary “sticky ends” to serve as cross-l...

Synthesis and Characterization of Molecular Bottlebrushes Prepared by Iron-Based ATRP

Molecular bottlebrushes with hydrophobic poly(n-butyl acrylate) or polystyrene and hydrophilic poly(di(ethylene glycol) ethyl ether acrylate)) side chains were successfully synthesized by grafting from a poly(2-(2-bromoisobutyryloxy)ethyl methacrylate) macroinitiator using iron-based atom transfer radical polymerization (ATRP). Iron(II) bromide, iron(III) bromide, and tetrabutylammonium bromide catalyst was employed for an ATRP grafting-from reaction, resulting in brush macromolecules with a narrow molecular weight distribution (Mw/Mn = 1.18–1.28). Molecular weights measured by multiangle laser light scattering correlates well with the theoretical values for all bottlebrushes. Imaging of individual bottlebrushes by atomic force microscopy exhibited a wormlike conformation. Initiation efficiencies were calculated by cleaving the side chains by alcoholysis and then injecting to gel permeation chromatography. The initiation efficiencies were ca. 80–95%, showing relatively high values for a grafting from poly...

Molecular Bottlebrushes: New Opportunities in Nanomaterials Fabrication.

This Viewpoint highlights the ability of molecular bottlebrushes to serve as highly tunable building blocks for creating nanostructured materials via molecular templating, solution aggregation, and melt self-assembly. Recent achievements in the synthesis of discrete nano-objects, micellar structures, and periodic nanomaterials from bottlebrush copolymers are emphasized, and future opportunities in this area of polymer science are briefly discussed.

Efficient synthesis and self-assembly of hetero-grafted amphiphilic polypepide bottlebrushes

A new type of hetero-grafted molecular bottlebrush with polypeptide as backbone was synthesized using graft-onto strategy. Poly(γ-propargyl-L-glutamate) (PPLG) as backbone was firstly synthesized via ring-opening polymerization (ROP) of γ-propargyl-L-glutamate (PLG) N-carboxyanhydride (NCA). Next, polystyrene-N3 (PS-N3) and monomethoxy poly(ethylene glycol)-N3 (mPEG-N3) as side chains were grafted onto the PPLG backbone using copper-catalyzed click reaction, which afforded good grafting density and efficiency. Two polypeptide bottlebrushes with PS-to-mPEG molar ratio at 70/30 and 30/70 were prepared. The self-assembly behaviors of these two polypeptide bottlebrushes were investigated using the cosolvent method, and their supramolecular structures were characterized using light scattering (LS) and electron microscopy.

Synthesis of Molecular Bottlebrushes by Atom Transfer Radical Polymerization with ppm Amounts of Cu Catalyst.

Molecular bottlebrushes were prepared by ICAR (initiators for continuous activator regeneration) atom transfer radical polymerization (ATRP) and supplemental activator and reducing agent (SARA) ATRP in the presence of 50 ppm Cu-based catalyst. Poly(n-butyl acrylate) (PBA) side chains were grafted from a polymethacrylate backbone resulting in well-defined molecular bottlebrushes. Imaging of individual bottlebrush macromolecules by atomic force microscopy corroborated the targeted degrees of polymerization of the backbone and side chains. Initiation efficiency was determined by cleaving the side chains to be around 50%.

Preparation of Polymeric Nanoscale Networks from Cylindrical Molecular Bottlebrushes

The design and control of polymeric nanoscale network structures at the molecular level remains a challenging issue. Here we construct a novel type of polymeric nanoscale networks with a unique microporous nanofiber unit employing the intra/interbrush carbonyl cross-linking of polystyrene side chains for well-defined cylindrical polystyrene molecular bottlebrushes. The size of the side chains plays a vital role in the tuning of nanostructure of networks at the molecular level. We also show that the as-prepared polymeric nanoscale networks exhibit high specific adsorption capacity per unit surface area because of the synergistic effect of their unique hierarchical porous structures. Our strategy represents a new avenue for the network unit topology and provides a new application for molecular bottlebrushes in nanotechnology.

Synthesis of Amphiphilic Poly(N-vinylpyrrolidone)-b-poly(vinyl acetate) Molecular Bottlebrushes.

Well-defined molecular bottlebrushes with poly(N-vinylpyrrolidone) and poly(N-vinylpyrrolidone)-b-poly(vinyl acetate) (PNVP-b-PVOAc) side chains were prepared via a combination of atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer (RAFT). A macro chain transfer agent poly(2-((2-ethylxanthatepropanoyl)oxy)ethyl methacrylate) (PXPEM) was prepared by attaching xanthate chain transfer agents onto each monomeric unit of poly(2-hydroxyethyl methacrylate). Subsequently, a RAFT polymerization procedure was used to synthesize molecular bottlebrushes with PNVP side chains with controlled molecular weight and low polydispersity by grafting from the PXPEM backbone. The side chains were then chain extended with PVOAc, yielding a bottlebrush macromolecule with PNVP-b-PVOAc side chains. The comb-like shape of the chain extended bottlebrushes was confirmed by atomic force microscopy (AFM).

Molecular Bottlebrushes with Polypeptide Backbone Prepared via Ring‐Opening Polymerization of NCA and ATRP

A new type of molecular bottlebrush with poly-L-lysine (PLL) as backbone was synthesized via ROP followed by ATRP. A Nϵ-bromoisobutyryl functionalized Nα-CBZ-L-lysine was firstly synthesized and converted in polymerizable α-amino acid N-carboxyanhydride (NCA), which was then polymerized using Ni(0) transition metal complex to give well-defined bromo-functionalized homopolypeptide (PBrLL), from which we prepared two types of polypeptide bottlebrushes with polystyrene and poly(oligoethylene glycol methacrylate) as side-chains. PBrLL macroinitiator was demonstrated to have high initiation efficiency for ATRP, which allowed good control over side-chain length. CD and FTIR characterization revealed that both PBrLL macroinitiator and PLL backbone of bottlebrushes adopted α-helical conformation in appropriate solvents.

PH-Responsive Fluorescent Molecular Bottlebrushes Prepared by Atom Transfer Radical Polymerization

Fluorescein O-methacrylate was copolymerized with n-butyl acrylate by atom transfer radical polymerization in a “grafting from” reaction with a multifunctional linear macroinitiator to form pH responsive fluorescent bottlebrushes. The brush-like structure of the synthesized macromolecules was confirmed through molecular imaging by atomic force microscopy. NMR spectroscopy showed that the fluorescent units were successfully incorporated into the polymer. The synthesized bottlebrushes displayed highly fluorescent properties under basic conditions, yet showed no fluorescence under neutral or acidic conditions. The fluorescence could be turned on and off by changing the pH of the solution. These bottlebrush molecules could have potential applications in molecular imaging.

Core−Shell Molecular Bottlebrushes with Helical Polypeptide Backbone: Synthesis, Characterization, and Solution Conformations

A series of core−shell molecular bottlebrushes with helical polypeptide backbone and polylactide-b-poly(ethylene glycol) block copolymer side chains [PPLG-g-(PLA-b-PEG)] have been synthesized via a grafting-to method. The structurally well-defined side chains with low molecular weight distribution (PDI < 1.03) have been synthesized by ring-opening polymerization (ROP) of dl-lactides using PEG-OH macroinitiator and AlEt3 catalyst. Postpolymerization modification by esterification enables the quantitative installation of an alkynyl functionality at the block polymer chain ends. Grafting of alkynyl-terminated PLA-b-PEG onto poly(γ-azidopropyl-l-glutamate) (PAPLG) via copper-mediated [2 + 3] alkyne−azide 1,3-dipolar cycloaddition yields the targeted molecular bottlebrushes with high grafting density under mild conditions. A combination of the Langmuir−Blodgett (LB) technique with atomic force microscopy (AFM) has allowed for direct imaging of the synthesized molecular brushes and characterization of their mol...

Focusing bond tension in bottle-brush macromolecules during spreading

The backbone of molecular bottle-brushes undergoes spontaneous degradation upon spreading on a solid substrate. The self-generated tension in the brush backbone is ascribed to steric repulsion between the densely grafted side chains. The paper discusses two approaches for controlling the bond-scission process on the molecular and macroscopic scales, respectively. On the molecular scale, the tension linearly increases with the distance from the backbone ends and attains its maximum value in the middle section of the backbone. When the backbone becomes shorter than the side chains, the tension is focused precisely on the central bond resulting in the predominant mid-chain fracture of the brush backbone. On the macroscopic scale, addition of a linear polymer to a melt of molecular bottle-brushes alters the film pressure and thus allows controllable positioning of the fracture zone within a spreading film.