Block Copolymer Side Chains Research Articles

Synthesis and Antibiotic Activity of Chitosan-Based Comb-like Co-Polypeptides

Infections caused by multidrug-resistant Gram-negative bacteria have been named one of the most urgent global health threats due to antimicrobial resistance. Considerable efforts have been made to develop new antibiotic drugs and investigate the mechanism of resistance. Recently, Anti-Microbial Peptides (AMPs) have served as a paradigm in the design of novel drugs that are active against multidrug-resistant organisms. AMPs are rapid-acting, potent, possess an unusually broad spectrum of activity, and have shown efficacy as topical agents. Unlike traditional therapeutics that interfere with essential bacterial enzymes, AMPs interact with microbial membranes through electrostatic interactions and physically damage cell integrity. However, naturally occurring AMPs have limited selectivity and modest efficacy. Therefore, recent efforts have focused on the development of synthetic AMP analogs with optimal pharmacodynamics and an ideal selectivity profile. Hence, this work explores the development of novel antimicrobial agents which mimic the structure of graft copolymers and mirror the mode of action of AMPs. A family of polymers comprised of chitosan backbone and AMP side chains were synthesized via the ring-opening polymerization of the N-carboxyanhydride of l-lysine and l-leucine. The polymerization was initiated from the functional groups of chitosan. The derivatives with random- and block-copolymer side chains were explored as drug targets. These graft copolymer systems exhibited activity against clinically significant pathogens and disrupted biofilm formation. Our studies highlight the potential of chitosan-graft-polypeptide structures in biomedical applications.

Styrene-Based Poly(ethylene oxide) Side-Chain Block Copolymers as Solid Polymer Electrolytes for High-Voltage Lithium-Metal Batteries.

Herein, we report the design of styrene-based poly(ethylene oxide) (PEO) side-chain block copolymers featuring a microphase separation and their application as solid polymer electrolytes in high-voltage lithium-metal batteries. A straightforward synthesis was established, overcoming typical drawbacks of PEO block copolymers prepared by anionic polymerization or ester-based PEO side-chain copolymers. Both the PEO side-chain length and the LiTFSI content were varied, and the underlying relationships were elucidated in view of polymer compositions with high ionic conductivity. Subsequently, a selected composition was subjected to further analyses, including phase-separated morphology, providing not only excellent self-standing films with intrinsic mechanical stability but also the ability to suppress lithium dendrite growth as well as good flexibility, wettability, and good contacts with the electrodes. Furthermore, good thermal and electrochemical stability was demonstrated. To do so, linear sweep and cyclic voltammetry, lithium plating/stripping tests, and galvanostatic overcharging using high-voltage cathodes were conducted, demonstrating stable lithium-metal interfaces and a high oxidative stability of around 4.75 V. Consequently, cycling of Li||NMC622 cells did not exhibit commonly observed rapid cell failure or voltage noise associated with PEO-based electrolytes in Li||NMC622 cells, attributed to the high mechanical stability. A comprehensive view is provided, highlighting that the combination of PEO and high-voltage cathodes is not impossible per se.

Shape-Changing Bottlebrush Polymers.

Bottlebrush polymers (BBPs), composed of relatively short polymeric side chains densely grafted on a polymer backbone, exhibit many unique characteristics and hold promise for a variety of applications. This Perspective focuses on environmentally induced shape-changing behavior of BBPs at interface and in solution, particularly worm/star-globule shape transitions. While BBPs with a single type of homopolymer or random copolymer side chains have been shown to undergo pronounced worm-to-globule shape changes in response to external stimuli, the collapsed brushes are unstable and prone to aggregation. By introducing a second, solvophilic polymer into the side chains, either as a distinct type of side chain or as the outer block of block copolymer side chains, the collapsed brushes not only are stabilized but also create unimolecular micellar nanostructures, which can be used for, e.g., encapsulation and delivery of substances. The current challenges in the design, synthesis, and characterization of stimuli-responsive shape-changing BBPs are discussed.

Self-Assembly of Molecular Brushes with Polyimide Backbone and Amphiphilic Block Copolymer Side Chains in Selective Solvents.

Three-component molecular brushes with a polyimide backbone and amphiphilic block copolymer side chains with different contents of the “inner” hydrophilic (poly(methacrylic acid)) and “outer” hydrophobic (poly(methyl methacrylate)) blocks were synthesized and characterized by molecular hydrodynamics and optics methods in solutions of chloroform, dimethylformamide, tetrahydrofuran and ethanol. The peculiarity of the studied polymers is the amphiphilic structure of the grafted chains. The molar masses of the molecular brushes were determined by static and dynamic light scattering in chloroform in which polymers form molecularly disperse solutions. Spontaneous self-assembly of macromolecules was detected in dimethylformamide, tetrahydrofuran and ethanol. The aggregates size depended on the thermodynamic quality of the solvent as well as on the macromolecular architectural parameters. In dimethylformamide and tetrahydrofuran, the distribution of hydrodynamic radii of aggregates was bimodal, while in ethanol, it was unimodal. Moreover, in ethanol, an increase in the poly(methyl methacrylate) content caused a decrease in the hydrodynamic radius of aggregates. A significant difference in the nature of the blocks included in the brushes determines the selectivity of the used solvents, since their thermodynamic quality with respect to the blocks is different. The macromolecules of the studied graft copolymers tend to self-organization in selective solvents with formation of a core–shell structure with an insoluble solvophobic core surrounded by the solvophilic shell of side chains.

Impact of bottlebrush chain architecture on Tg‐confinement and fragility‐confinement effects enabled by thermo‐cleavable bottlebrush polymers synthesized by radical coupling and atom transfer radical polymerization

AbstractThermo‐cleavable bottlebrush polymers were synthesized by a facile grafting‐to method via radical coupling and atom transfer radical polymerization (ATRP) without small‐molecule synthesis involved. Bottlebrushes were achieved by coupling backbones of poly(4‐methacryloyloxy‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl), which contain nitroxide radicals, and ATRP‐synthesized side chains, which can be halogen‐abstracted to generate carbon‐centered radicals. Bottlebrushes were prepared using homopolymer or block copolymer side chains. Alkoxyamine covalent bonds resulting from radical coupling are thermo‐reversible at high temperature, and grafting density may be tuned by annealing of post‐synthesis bottlebrush, with the bottlebrush regime going from loose bottlebrush to dense comb and loose comb. The effects of confinement on the Tg and fragility of films of polystyrene bottlebrush were studied by ellipsometry; comparisons were made to thermally cleaved linear components obtained directly after annealing. Relative to linear polymer, bottlebrush topology reduces bulk fragility and suppresses Tg‐ and fragility‐confinement effects. The correlation between the strengths of the confinement effects is consistent with other film studies of linear and non‐linear polymers and supports the notion that fragility is a fundamental property underlying perturbations to Tg. Besides providing a platform for advancing fundamental scientific understanding, our synthetic strategy may afford novel applications of bottlebrushes via incorporated dynamic chemistry.

Shell of amphiphilic molecular bottlebrush matters as unimolecular micelle

Molecular bottlebrushes (MBBs) with a hydrophobic core and a hydrophilic shell may exist as well-defined unimolecular nanoparticles in water. However, they may also aggregate to form large particles if the structure of the hydrophilic and hydrophobic segments fail to be rationally controlled. Herein, a series of core-shell amphiphilic MBBs with a poly (ε-caprolactone) (PCL) backbone bearing two block copolymer side chains, poly (butyl acrylate)-b-poly ((oligo (ethylene glycol) acrylate) (PBA-b-POEGA) and poly (butyl acrylate)-b-poly (ethylene glycol) (PBA-b-PEG), were synthesized through grafting-from and grafting-onto strategies. Their hydrophilic shells are different in topology: POEGA is comblike whereas PEG is linear. The stability of the two kinds of MBB with the similar composition as unimolecular nanoparticles in water and their encapsulation capacity for guest-molecule were evaluated. Compared to the MBB with a linear PEG shell, those with comblike POEGA side chains remained stable against aggregation as demonstrated by atomic force microscopy and dynamic light scattering measurement. The MBB with a POEGA shell also displayed a much higher encapsulating capacity to a model guest molecule, pyrene, by hydrophobic interaction. This study provides guidance to the rational preparation of MBBs for applications as molecular vehicles.

Synthesis and characterization of a new hydrogen bonded side chain liquid crystal block copolymer and investigation of electrical properties

Abstract A new PEG-containing liquid crystalline side chain block copolymer HBC-PEG550 was prepared from poly(ethylene glycol)-b-poly(2-(diethylamino) ethyl methacrylate copolymer (BC-PEG550) and 8-(4-cyanobiphenyl-4′-oxy) octan-1-ol (LC8) by molecular self-assembly processes via hydrogen bond formation between amine group of HBC-PEG550 and hydroxyl group of the LC8. The formation of H bond was confirmed by using FTIR spectroscopy. The liquid crystalline behavior of the HBC-PEG550 was investigated by differential scanning calorimeter (DSC) and polarized optical microscopy. Dielectric properties of HBC-PEG550 and HBC-PEG550 has been studied by impedance spectroscopy. Real and imaginary parts of complex dielectric constant, impedance, and energy loss tangent factor for HBC-PEG550 and HBC-PEG550 have been characterized in the frequency range of 100 Hz–15 MHz.

Preparation of ZnO hybrid nanoparticles by ATRP

Zinc oxide (ZnO) is a wide bandgap semiconductor material that has attracted widespread interest as particle filler in polymer nanocomposite materials. However, its applications have been hindered by the limited dispersibility and surface-modification techniques. Herein, three distinct approaches for the synthesis of polymer-tethered ZnO hybrid materials are compared in terms of uniformity and yield of the particle-brush product: “grafting-from”, “grafting-onto”, and “grafted-copolymer template” methods. In the “grafting-from” method, pristine ZnO nanoparticles (NP) were first functionalized with atom transfer radical polymerization (ATRP) initiators followed by grafting-from process to form poly(methyl methacrylate) (PMMA) or poly(styrene-co-acrylonitrile) (PSAN) tethered polymer chains. In the “grafting-onto” method, PMMA-b-PAA (poly[acrylic acid]) and PSAN-b-PAA diblock copolymers were prepared and attached onto the surface of ZnO NPs using sonication bath. For the “grafted-copolymer template” method, PSAN-b-PtBA-Br (poly[tert-butyl acrylate]-Br) macroinitiators were crosslinked with divinylbenzene (DVB) to form PSAN-b-PtBA-PDVB core-shell star polymers. After hydrolysis to form PSAN-b-PAA-PDVB star polymers, the functional stars were used as polymer templates for the synthesis of ZnO NPs within the PAA-core of the stars. Core-shell molecular bottlebrushes with PAA-b-PS block-copolymer side chains were also used as anisotropic analogues of star template to prepared worm-like ZnO particles. Several ZnO precursors, zinc nitrite, zinc 2-ethylhexanoate, and zinc acetate were evaluated as precursors of ZnO. Conditions were identified that enable the synthesis of polymer-tethered ZnO with excellent size uniformity and dispersion characteristics using the star-template method.

Preparation of titania nanoparticles with tunable anisotropy and branched structures from core–shell molecular bottlebrushes

Abstract Core–shell molecular bottlebrushes with various topologies and with poly (tert-butyl acrylate)-b-polystyrene (PtBA-b-PS) block copolymer side chains were synthesized via atom transfer radical polymerization (ATRP). After hydrolysis of the PtBA segments and their conversion to poly (acrylic acid) (PAA), the materials were used as templates to prepare titania nanoparticles. Transmission electron microscopy (TEM) images showed that the particles retained the characteristic shape of the templates, including branches and cylindrical geometry. The cylindrical dimensions, i.e. diameter and length, were controlled by the number average degrees of polymerization of the PAA side-chain block and the polymethacrylate backbone, respectively. Thermogravimetric analysis (TGA) showed the weight fraction of inorganic content ranged from 17% to 35%, depending on the ratio of the length of PAA and PS blocks.

Highly conductive side chain block copolymer anion exchange membranes.

Block copolymers based on poly(styrene) having pendent trimethyl styrenylbutyl ammonium (with four carbon ring-ionic group alkyl linkers) or benzyltrimethyl ammonium groups with a methylene bridge between the ring and ionic group were synthesized by reversible addition-fragmentation radical (RAFT) polymerization as anion exchange membranes (AEMs). The C4 side chain polymer showed a 17% increase in Cl(-) conductivity of 33.7 mS cm(-1) compared to the benzyltrimethyl ammonium sample (28.9 mS cm(-1)) under the same conditions (IEC = 3.20 meq. g(-1), hydration number, λ = ∼7.0, cast from DMF/1-propanol (v/v = 3 : 1), relative humidity = 95%). As confirmed by small angle X-ray scattering (SAXS), the side chain block copolymers with tethered ammonium cations showed well-defined lamellar morphologies and a significant reduction in interdomain spacing compared to benzyltrimethyl ammonium containing block copolymers. The chemical stabilities of the block copolymers were evaluated under severe, accelerated conditions, and degradation was observed by (1)H NMR. The block copolymer with C4 side chain trimethyl styrenylbutyl ammonium motifs displayed slightly improved stability compared to that of a benzyltrimethyl ammonium-based AEM at 80 °C in 1 M NaOD aqueous solution for 30 days.

Anion Conductive Multiblock Copolymers with Long Side Chains for AEM Fuel Cells

Anion Exchange Membrane (AEM) fuel cells are a possible route to overcoming fundamental issues with acid-based fuel cells. The issues include the high cost of platinum catalysts, complex water transport, and sluggish electrochemical reactions, etc. However, the performance of AEM fuel cells is not as good as that of Proton Exchange Membrane (PEM) fuel cells because of the limitations of current anion exchange membranes, such as low ionic conductivity, poor stability at high pH, and high water uptake. A series of novel multiblock copoly(arylene ether)s with long side chains were synthesized for AEM fuel cells. The multiblock copolymers were synthesized by polycondensation of separately-prepared hydroxyl-terminated oligomers with fluoro-terminated oligomers. The resulting multiblock structure, in which one block is hydrophilic and one block is hydrophobic, was designed to ensure a nanophase-separated morphology. Multiblock copolymers with different block lengths and ion exchange capacities (IEC) were synthesized to maximize ion conductivity and explore the relationship between chemical structures and properties in anion exchange membranes. Furthermore, copolymers with long side chains exhibited higher stability in high pH environment than their counterparts without long side chains. The degradation mechanism was also studied for the long side chain block copolymers. Financial support from US Office of the Deputy Assistant Secretary of the Army for Defense Exports and Cooperation (DASA-DE&C) is gratefully acknowledged.

Synthesis and Properties of a Multiblock Copolymer with Long Side Chains for Anion Exchange Membrane Fuel Cells

Anion Exchange Membrane (AEM) fuel cells are a possible route to overcoming fundamental issues with acid-based fuel cells. The issues include the high cost of platinum catalysts, complex water transport, and sluggish electrochemical reactions, etc. However, the performance of AEM fuel cells is not as good as that of Proton Exchange Membrane (PEM) fuel cells because of the limitations of current anion exchange membranes, such as low ionic conductivity, poor stability at high pH, and high water uptake. A series of novel multiblock copoly(arylene ether)s with long side chains were synthesized for AEM fuel cells. The multiblock copolymers were synthesized by polycondensation of separately-prepared hydroxyl-terminated oligomers with fluoro-terminated oligomers. The resulting multiblock structure, in which one block is hydrophilic and one block is hydrophobic, was designed to ensure a nanophase-separated morphology. Multiblock copolymers with different block lengths and ion exchange capacities (IEC) were synthesized to maximize ion conductivity and explore the relationship between chemical structures and properties in anion exchange membranes. Furthermore, copolymers with long side chains exhibited higher stability in high pH environment than their counterparts without long side chains. The degradation mechanism was also studied for the long side chain block copolymers. Financial support from US Office of the Deputy Assistant Secretary of the Army for Defense Exports and Cooperation (DASA-DE&C) is gratefully acknowledged.

Two decades of molecular brushes by ATRP

This review presents the development of brush copolymers as macromolecular engineering materials prepared by atom transfer radical polymerization (ATRP) and reported by the Matyjaszewski's group during the last twenty years. The graft structures combine different segments which allowed to precisely design properties of macromolecules to target various applications. The first molecular brushes contained homopolymer side chains, which were extended by other segments yielding core–shell macromolecules with block copolymer side chains. Many variants of brushes, densely vs loosely grafted copolymers, statistical and gradient distribution of grafts, block and statistical graft structures, homografted vs heterografted copolymers, have been designed. The semigrafted copolymers combine linear and brush topologies, whereas block brushes consist of two different grafted segments. These are also known the brushes with much more complex architectures, such as star-like brushes. Examples of various types of brushes, properties and their potential applications are discussed.

Synthesis and characterization of hydrogen bonded liquid crystalline block copolymers based on a Schiff base moiety

Two new series of supramolecular liquid crystalline block copolymers have been synthesized. The first, is based on a backbone that contains carboxylic acid groups. The second is a side chain block copolymer exhibiting hydrogen bonding between the carboxylic acid and the pyridine moiety of a simple low molar mass molecule. These copolymers contain two different flexible spacer lengths, the first fixed at two methylene groups, while the other spacer varies from two to seven groups. The phase behavior and transitional properties of these series have been analyzed by optical polarized microscopy and differential scanning calorimetry, while the structures and properties of the prepared compounds were determined by spectroscopic techniques, elemental analysis, thermogravimetric analysis, and powdered X-ray diffraction.

Rod‐Like Nano‐Light Harvester

Imitating the natural "energy cascade" architecture, we present a single-molecular rod-like nano-light harvester (NLH) based on a cylindrical polymer brush. Block copolymer side chains carrying (9,9-diethylfluoren-2-yl)methyl methacrylate units as light absorbing antennae (energy donors) are tethered to a linear polymer backbone containing 9-anthracenemethyl methacrylate units as emitting groups (energy acceptors). These NLHs exhibit very efficient energy absorption and transfer. Moreover, we manipulate the energy transfer by tuning the donor-acceptor distance.

Thermoresponsive Poly(2‐Oxazoline) Molecular Brushes by Living Ionic Polymerization: Modulation of the Cloud Point by Random and Block Copolymer Pendant Chains

AbstractMolecular brushes (MBs) of poly(2‐oxazoline)s were prepared by living anionic polymerization of 2‐isopropenyl‐2‐oxazoline to form the backbone and living cationic ring‐opening polymerization of 2‐n‐propyl‐2‐oxazoline and 2‐methyl‐2‐oxazoline to form random and block copolymers. Their aqueous solutions displayed a distinct thermoresponsive behavior as a function of the side‐chain composition and sequence. The cloud point (CP) of MBs with random copolymer side chains is a linear function of the hydrophilic monomer content and can be modulated in a wide range. For MBs with block copolymer side chains, it was found that the block sequence had a strong and surprising effect on the CP. While MBs with a distal hydrophobic block had a CP at 70 °C, MBs with hydrophilic outer blocks already precipitated at 32 °C.

Gel formation in a mixture of a block copolymer and a nematic liquid crystal

The viscoelastic properties of a binary mixture of a mesogenic side-chain block copolymer in a low molecular weight nematic liquid crystal are studied for mass concentrations ranging from the diluted regime up to a liquid crystalline gel state at about 3%. In the gel state, the system does not flow, exhibits a polydomain structure on a microscopic level, and strongly scatters light. Below the gelation point, the system is homogeneous and behaves like a usual nematic, so the continuum theory of liquid crystals can be applied for interpreting the experimental data. Using the dynamic Fréedericksz transition technique, the dependence of the splay elastic constant and the rotational viscosity on the polymer concentration have been obtained. Comparing the dynamic behavior of block copolymer solutions with the respective homopolymer solutions reveals that, above a mass concentration of 1%, self-assembling of the block copolymer chain segments in clusters occurred, resulting in a gel state at higher concentrations. The effective cluster size is estimated as a function of the concentration, and a scaling-law behavior near the sol-gel transition is confirmed. This technique may serve as an alternative method for determining the gelation point.

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...

Synthesis and Self-Assembly of Amphiphilic Asymmetric Macromolecular Brushes

Two well-defined amphiphilic asymmetric macromolecular brushes, one bearing hydrophilic poly(ethylene glycol) (PEO) and hydrophobic polystyrene (PS) side chains on poly(glycidyl methacrylate) (PGMA) backbone and the other bearing pendant PEO and poly(styrene-block-N-isopropylacrylamide) (PS-b-PNIPAM) block copolymer side chains, were synthesized by grafting from approach based on a combination of click chemistry and in situ reversible addition−fragmentation chain transfer (RAFT) polymerization. PGMA backbone was synthesized by atom transfer radical polymerization (ATRP), and a polymer backbone with pendant hydroxyl and azide groups (PGMA−OH/N3) was obtained after ring-opening reaction of the epoxide rings on PGMA. RAFT chain transfer agent (CTA) was introduced to the polymer backbone by facile click reaction between alkyne-terminated RAFT CTA and PGMA−OH/N3. PEO side chains were grafted onto the polymer backbone by esterification between carboxyl end group of PEO and hydroxyl group on the polymer backbone...

Crystallization of Molecular Brushes with Block Copolymer Side Chains

A series of three cylindrical molecular brushes with poly(e-caprolactone)-b-poly(n-butyl acrylate) side chains were investigated with regard to the effect of the intramolecular confinement on the crystallization behavior of the PCL core block. While the length of the PCL block was maintained (nPCL = 50), the degree of polymerization of the PBA corona block, prepared by atom transfer radical polymerization, was varied from nPBA = 0 (PCL brush) to nPBA = 52 and 163. The crystallization behavior of the studied polymers was shown to be different in thin films and bulk samples. In thin films, the brushlike macromolecules were fully extended and remained segregated due to the steric repulsion of the adsorbed PBA corona. Under the constraint of the backbone extension, crystallization of the PCL core led to formation of a characteristic spinelike morphology. In bulk samples, the core−shell confinement did not prohibit breakout crystallization, leading to the formation of a spherulitic morphology similar to linear...