Molecular Brushes Research Articles

Cylindrical molecular brushes with a loose grafting density

AbstractA cylindrical molecular brush with a low graft density was synthesized by two steps. Firstly, free radical alternating copolymerization of styrene (St) and N‐[2‐(2‐bromoisobutyryloxy)ethyl] maleimide (BiBEMI) gave a macroinitiator where pendant initiating sites for atom transfer radical polymerization (ATRP) being positioned along the backbone with an interval of four CC bonds. The backbone‐to‐be with an alternating sequence was verified by elemental analysis (EA). Secondly, grafting poly(tert‐butyl acrylate) chains from the macroinitiator by ATRP produced the novel molecular brush. Size exclusion chromatography, static light scattering (SLS), and 1H NMR and atomic force microscopy (AFM) were used to characterize the macroinitiator and the molecular brush. The results show that the backbone contains an average 730 repeat units (1 repeat unit = a pair of St and BiBEMI) and the absolute molecular weight of the brush, Mw,SLS, was 4.88 × 106 Da. The brush reveals a number average length Ln to be 96 nm under AFM observation on carbon coated mica, corresponding to the length per main chain monomer unit (Lunit) of 0.066 nm, indicating a less extended conformation due to the low grafting density. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5527–5533, 2009

Facile syntheses of cylindrical molecular brushes by a sequential RAFT and ROMP “grafting‐through” methodology

The orthogonality of RAFT and ROMP chemistries are exploited for a highly efficient “grafting through” strategy to afford cylindrical molecular brushes. The ROMP of α-norbornenyl-functionalized poly(t-butyl acrylate) macroRAFT chain transfer agents, catalyzed by a modified 2nd generation Grubbs' catalyst, achieved completion in minutes under air. Following hydrolysis, functionalizable water-soluble poly(acrylic acid) molecular brushes were afforded. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Incorporation of poly(2‐acrylamido‐2‐methyl‐N‐propanesulfonic acid) segments into block and brush copolymers by ATRP

Abstract2‐Acrylamido‐2‐methyl‐N‐propanesulfonic acid (AMPSA) was successfully polymerized via atom transfer radical polymerization (ATRP) using a copper chloride/2,2′‐bipyridine (bpy) catalyst complex afterin situneutralization of the acidic proton in AMPSA with tri(n‐butyl)amine (TBA). A 5 mol % excess of TBA was required to completely neutralize the acid and prevent protonation of the bpy ligand, as well as to avoid side reactions caused by large excess of TBA. The use of activators generated by electron transfer (AGET) ATRP with ascorbic acid as reducing agent resulted in both increased conversion of the AMPSA monomer during polymerization (up to 50% with a 0.8 [ascorbic acid]/[Cu(II)] ratio) and much shorter polymerization times (<30 min). Block copolymers and molecular brushes containing AMPSA side chains were prepared using this method, and the solution and surface behavior of these materials were investigated. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5386–5396, 2009

Cellulose-Based Dual Graft Molecular Brushes as Potential Drug Nanocarriers: Stimulus-Responsive Micelles, Self-Assembled Phase Transition Behavior, and Tunable Crystalline Morphologies

Well-defined cellulose-based dual graft molecular brushes, composed of ethyl cellulose-graft-poly(N,N-dimethylaminoethyl methacrylate)-graft-poly(epsilon-caprolactone) (EC-g-PDMAEMA-g-PCL), have been prepared by ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). Unlike other brush copolymers, the new molecular brushes show some unique physicochemical properties and multifunction due to their unique topological structures. These biocompatible copolymers self-assembled to micelles in aqueous solution. Upon pH change, the single micelles further assembled into micellar aggregates. As a result, the micelles in aqueous media could act as excellent drug nanocarriers for controlled drug release. The crystallinity and crystal morphology of the copolymers can be controlled to a certain extent by varying the length of the side chains, which may exert strong spacial restriction and, hence, affect the crystal structures.

Ion‐exchanger synthesis using reversible addition‐fragmentation chain transfer polymerization

An ion-exchanger with polyanionic molecular brushes was synthesized by a "grafting from" route based on "surface-controlled reversible addition-fragmentation chain transfer polymerization" (RAFT). The RAFT agent, PhC(S)SMgBr was covalently attached to monodisperse-porous poly(dihydroxypropyl methacrylate-co-ethylene dimethacrylate), poly(DHPM-co-EDM) particles 5.8 microm in size. The monomer, 3-sulfopropyl methacrylate (SPM), was grafted from the surface of poly(DHPM-co-EDM) particles with an immobilized chain transfer agent by the proposed RAFT protocol. The degree of polymerization of SPM (i. e. the molecular length of the polyanionic ligand) on the particles was controlled by varying the molar ratio of monomer/RAFT agent. The particles carrying polyanionic molecular brushes with different lengths were tested as packing material in the separation of proteins by ion exchange chromatography. The columns packed with the particles carrying relatively longer polyanionic ligands exhibited higher separation efficiency in the separation of four proteins. Plate heights between 130-200 microm were obtained. The ion-exchanger having poly-(SPM) ligand with lower degree of polymerization provided better peak-resolutions on applying a salt gradient with higher slope. The molecular length and the ion-exchanger group content of polyionic ligand were adjusted by controlling the degree of polymerization and the grafting density, respectively. This property allowed control of the separation performance of the ion-exchanger packing.

Novel Biodegradable Heterografted Polymer Brushes Prepared via a Chemoenzymatic Approach

AbstractBiodegradable heterografted molecular bottle brushes were prepared in two steps starting with a linear polyglycidol as a macroinitiator. First, ca. 45% of the polyglycidol hydroxy groups were grafted with ε‐caprolactone via enzymatic catalysis. Selective end capping of the hydroxy groups at the graft ends was achieved via enzymatic esterification with vinyl acetate or vinyl acrylate. Finally, poly(L‐lactide) grafts were attached to the remaining polyglycidol hydroxy groups by chemically catalyzed ring‐opening polymerization. The successful synthesis of the heterografted comb‐shaped brushes was confirmed by NMR and SEC analyses. Additionally, the feasibility of microsphere formulation was described.magnified image

Preparation of size-tunable, highly monodisperse particles by self-assembly of N-phthaloylchitosan-g-polycaprolactone molecular bottle brushes

A series of amphiphilic N-phthaloylchitosan-g-polycaprolactone (PHCS-g-PCL) molecular bottle brushes were prepared by “graft onto” method. The narrow distribution of polycaprolactone (PCL) side chains ensures that the molecular bottle brushes can self-assemble into particles with a narrow size distribution in aqueous solution. The size of the self-assembled polymeric particles can be tuned by three parameters such as PCL chain length, grafting content and the concentration of molecular bottle brushes. The tunable particles would be a promising candidate for potential application for new clean energy sources, pollution control systems, catalysis or catalyst supports, nanoscale sensors, and drug delivery tools working inside the human body.

Cylindrical Molecular Brushes of Poly(2-oxazoline)s from 2-Isopropenyl-2-oxazoline

We report on the synthesis and characterization of cylindrical molecular brushes based on poly(2-oxazoline)s (POx). The dual-functional monomer, 2-isopropenyl-2-oxazoline (IPOx), was first converted to a poly(2-isopropenyl-2-oxazoline), backbone by free radical (PIPOxR) or living anionic polymerization (PIPOxA). Quantitative reaction with methyl triflate yields a macroinitiator salt (PIPOxOTfR/A) for the preparation of molecular brushes via the grafting from approach by living cationic polymerization of 2-oxazolines (2-methyl-, 2-ethyl-, and 2-isopropyl-2-oxazoline). Characterization of the resulting molecular brushes by NMR and FTIR spectroscopy indicates a very high side chain grafting density and quantitative reactions. Visualization of adsorbed molecular brushes by AFM corroborates this assumption. Furthermore, the lower critical solution temperatures of the POx molecular brushes were determined. The transition temperatures were found to be very defined, reversible, and with no noticeable hysteresis.

Tension amplification in molecular brushes in solutions and on substrates.

Molecular bottle-brushes are highly branched macromolecules with side chains densely grafted to a long polymer backbone. The brush-like architecture allows focusing of the side-chain tension to the backbone and its amplification from the pico-Newton to nano-Newton range. The backbone tension depends on the overall molecular conformation and the surrounding environment. Here we study the relation between the tension and conformation of the molecular brushes in solutions, melts, and on substrates. In solutions, we find that the backbone tension in dense brushes with side chains attached to every backbone monomer is on the order of f(0)N(3/8) in athermal solvents, f(0)N(1/3) in theta solvents, and f(0) in poor solvents and melts, where N is the degree of polymerization of side chains, f(0) approximately equal k(B)T/b is the maximum tension in side chains, b is the Kuhn length, k(B) is Boltzmann's constant, and T is the absolute temperature. Depending on the side chain length and solvent quality, molecular brushes develop tension on the order of 10-100 pN, which is sufficient to break hydrogen bonds. Significant amplification of tension occurs upon adsorption of brushes onto a substrate. On a strongly attractive substrate, maximum tension in the brush backbone is approximately f(0)N, reaching values on the order of several nano-Newtons, which exceeds the strength of a typical covalent bond. At low grafting density and high spreading parameter, the cross-sectional profile of an adsorbed molecular brush is approximately rectangular with a thickness approximately b (A/S)1/2, where A is the Hamaker constant, and S is the spreading parameter. At a very high spreading parameter (S > A), the brush thickness saturates at monolayer approximately b. At a low spreading parameter, the cross-sectional profile of adsorbed molecular brush has a triangular tent-like shape. In the cross-over between these two opposite cases, covering a wide range of parameter space, the adsorbed molecular brush consists of two layers. Side chains in the lower layer gain surface energy due to the direct interaction with the substrate, while the second layer spreads on the top of the first layer. Scaling theory predicts that this second layer has a triangular cross-section with width R approximately N(3/5) and height h approximately N(2/5). Using self-consistent field theory we calculate the cap profile y(x) = h(1 - x2/R2)2, where x is the transverse distance from the backbone. The predicted cap shape is in excellent agreement with both computer simulation and experiment.

Degradable star polymers with high “click” functionality

AbstractDegradable polyester‐based star polymers with a high level of functionality in the arms were synthesized via the “arms first” approach using an acetylene‐functional block copolymer macroinitiator. This was achieved by using 2‐hydroxyethyl 2′‐methyl‐2′‐bromopropionate to initiate the ring‐opening polymerization (ROP) of caprolactone monomer followed by an atom transfer radical polymerization (ATRP) of a protected acetylene monomer, (trimethylsilyl)propargyl methacrylate. The hydroxyl end‐group of the resulting block copolymer macroinitiator was subsequently crosslinked under ROP conditions using a bislactone monomer, 4,4′‐bioxepanyl‐7,7′‐dione, to generate a degradable core crosslinked star (CCS) polymer with protected acetylene groups in the corona. The trimethylsilyl‐protecting groups were removed to generate a CCS polymer with an average of 1850 pendent acetylene groups located in the outer block segment of the arms. The increased functionality of this CCS polymer was demonstrated by attaching azide‐functionalized linear polystyrene via a copper (I)‐catalyzed cycloaddition reaction between the azide and acetylene groups. This resulted in a CCS polymer with “brush‐like” arm structures, the grafted segment of which could be liberated via hydrolysis of the polyester star structure to generate molecular brushes. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1485–1498, 2009

Simultaneous Block Copolymer and Magnetic Nanoparticle Assembly in Nanocomposite Films

We investigate self-assembly of nanocomposite films composed of lamellar-forming poly(styrene-b-methyl methacrylate) (PS-b-PMMA) and PMMA-grafted magnetite (Fe3O4) nanoparticles (NPs). The Fe3O4 NPs are grafted with PMMA brushes with molecular weights ranging from 2700 to 35 700 g/mol. For the NP with the lowest molecular weight brush, the morphology of the nanocomposite film depends on the NP concentration (ϕNP). At low ϕNP, the block copolymer self-assembles into mixed morphology of perpendicular lamellae (⊥Lam) and parallel lamellae (∥Lam), and the ⊥Lam are stabilized by individual NPs or small NP aggregates. The NPs can also retard the dynamics of self-assembly of block copolymer films. At high ϕNP, NPs form small aggregates which inhibit the formation of a lamellar structure. As the molecular weight of the PMMA brush increases to 13 300 or 35 700 g/mol, the Fe3O4 NPs form aggregates in the as-cast nanocomposite films, and this behavior is attributed to aggregation of NPs in the solution state. Since the size of NP aggregates is larger than the copolymer domain size, the block copolymer has to self-assemble around these aggregates. The magnetic properties of these nanocomposite films are characterized, and typical superparamagnetic behavior is observed.

Tension Amplification in Molecular Brushes in Solutions and on Substrates †

Tension Amplification in Molecular Brushes in Solutions and on Substrates †

PH-induced conformational changes of loosely grafted molecular brushes containing poly(acrylic acid) side chains

A series of water-soluble loosely grafted poly(acrylic acid) (PAA) brushes with four different grafting densities were synthesized by the “grafting from” approach using atom transfer radical polymerization (ATRP). Gel permeation chromatography (GPC) and 1H NMR spectroscopy were used to provide evidence for formation of the well-defined backbones and the resulting brush copolymers. Atomic force microscopy was used to study the conformation of adsorbed brushes as a function of pH. The adsorbed molecules undergo a globule-to-extended conformational transition as the solution is changed from acidic to basic. This transition was monitored on a mica surface by imaging individual molecules with atomic force microscopy (AFM). The conformational behavior was compared with 100%-grafted PAA brushes. Unlike the loose brushes, the 100%-grafted molecules remained fully extended in a broad range of pH values (pH=2–9) due to steric repulsion between the densely grafted side chains which is strongly enhanced upon adsorption to a substrate.

A Water-Soluble pH-Responsive Molecular Brush of Poly(N,N-dimethylaminoethyl methacrylate) Grafted Polythiophene

We report the synthesis of a new polythiophene (PT)-based molecular brush (PT-g-PDMA) by growing poly(N,N-dimethylaminoethyl methacrylate) (PDMA) chains from the PT backbone by ATRP. The polymer shows a reversible pH response in aqueous solution. A combination of AFM, light scattering, and 1H NMR measurements indicated that the polymer brush forms a more extended conformation with a decrease in pH from 8 to 2 due to the protonation of the Me2N− groups and increased repulsive interactions among the PDMA side chains, which drives the red shift of the absorption and PL spectra of the PT backbone. The good solubility of this polythiophene-based brush in a wide range of solvents is attractive for the fabrication of functional polymer composites.

Temperature- and pH-Responsive Dense Copolymer Brushes Prepared by ATRP

Dual responsive molecular brushes consisting of statistical copolymers of di(ethylene glycol) methyl ether methacrylate (MEO2MA) and either methacrylic acid (MAA) or N,N-dimethylaminoethyl methacrylate (DMAEMA) were synthesized by grafting from poly(2-(2-bromoisobutyroyloxy)ethyl methacrylate (PBIEM) macroinitiators using atom transfer radical polymerization (ATRP). Copolymer brushes with controlled composition and molecular weights ranging from Mn = 600 000 to 1 400 000 with polydispersity indexes (Mw/Mn) between 1.18 and 1.45 were obtained. The lower critical solution temperature (LCST) of MEO2MA-stat-MAA decreased with the increasing molar fraction of MAA in the copolymer in deionized water but increased in buffer solution at pH 7. At pH 9, the hydrophilicity of polymer increased with ionization of carboxylic acid to further raise the LCST. On the other hand, the LCST of MEO2MA-stat-DMAEMA copolymers increased with increasing DMAEMA content at pH 4, 7, and 9. A bottle-brush terpolymer prepared from all...

Adsorption of Molecular Brushes with Polyelectrolyte Backbones onto Oppositely Charged Surfaces: A Self-Consistent Field Theory

The two-gradient version of the Scheutjens-Fleer self-consistent field (SF-SCF) theory is employed to model the interaction between a molecular bottle brush with a polyelectrolyte backbone and neutral hydrophilic side chains and an oppositely charged surface. Our system mimics graft-copolymers with a cationic main chain (among which poly( L-lysine)- graft-poly(ethylene glycol) (PLL- g-PEG) or poly( L-lysine)- graft-polyoxazoline are well-known examples) interacting with negatively charged (metal oxide) solid surfaces. We aim to analyze the copolymer-surface interaction patterns as a function of the molecular architecture parameters. Two regimes are investigated: First, we compute the effective interaction potential versus the distance from the surface for individual bottle brush macromolecules. Here, depending on the grafting ratio and the degree of polymerization of the side chains, the interplay of electrostatic attractions of the main chain to the surface and the steric repulsion of the grafts results in different patterns in the interaction potential and, therefore, in qualitatively different adsorption behavior. In particular, we demonstrate that, at high side chain grafting density and short grafts, the molecular brushes are strongly adsorbed electrostatically onto negatively charged substrates, whereas, in the opposite case of low grafting ratio and high molecular weight of grafts, the steric repulsion of the side chains from the surface dominates the polymer-surface interaction. At intermediate grafting ratios, the adsorption/depletion scenario depends essentially on the ratio between the electrostatic screening length and the thickness of the molecular bottle brush. We further have analyzed the equilibrium adsorbed amount as a function of the macromolecular architecture. Our results are based on a detailed account of attractive and repulsive (including intermolecular in-plane) interactions, and suggest a nonmonotonic dependence of the adsorbed amount on the grafting ratio, in good agreement with the experimental studies for PLL- g-PEG adsorption onto Nb2O5 surfaces. The results of the theoretical modeling are discussed in the context of the optimization of the PLL-g-PEG molecular design parameters in order to create protein-resistant surfaces.

Pearl-Necklace Structures in Core−Shell Molecular Brushes: Experiments, Monte Carlo Simulations, and Self-Consistent Field Modeling

We present theoretical arguments and experimental evidence for a longitudinal instability in core-shell cylindrical polymer brushes with a solvophobic inner (core) block and a solvophilic outer (shell) block in selective solvents. The two-gradient self-consistent field Scheutjens-Fleer (SCF-SF) approach and Monte Carlo (MC) simulations are employed to study a conformational transition which occurs upon a decrease in the solvent strength for the inner block from theta- to poor solvent conditions. It is found that a decrease in the solvent strength for the core block leads to an instability in the cylindrically uniform structure and the appearance of longitudinal undulations in the collapsed core of the molecular brush. This result of our modeling is in excellent agreement with experimental observations on core-shell brushes poly(acrylic acid) (PAA) core and poly(n-butyl acrylate) shell, where the core forms pearl-necklace-like structures in either a bad solvent for PAA or due to complexation with multivalent ions.

Well-defined complex macromolecular architectures by anionic polymerization of styrenic single and double homo/miktoarm star-tailed macromonomers

Styrenic single and double star-tailed macromonomers were synthesized by selective reaction of living homo/miktoarm stars with the chlorosilane groups of 4-(chlorodimethylsilyl)- and 4-(dichloromethylsilyl)styrene, respectively. The in situ anionic homopolymerization of macromonomers with sec-BuLi and copolymerization with butadiene and styrene, led to single/double homo/miktoarm star-tailed molecular brushes and combs, as well as a block copolymer consisting of a linear polystyrene chain and a double miktoarm (PBd/PS) star-tailed brush-like block. Molecular characterization by size exclusion chromatography, size exclusion chromatography/two-angle laser light scattering, and NMR spectroscopy, revealed the high molecular/compositional homogeneity of all intermediate and final products. These are only a few examples of the plethora of complex architectures possible using the above macromonomers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1826–1842, 2008

ATRP Synthesis of Thermally Responsive Molecular Brushes from Oligo(ethylene oxide) Methacrylates

Molecular brushes consisting of statistical copolymers of di(ethylene glycol) methyl ether methacrylate (MEO2MA) and tri(ethylene glycol) methyl ether methacrylate (MEO3MA) were synthesized by grafting from poly(2-(2-bromoisobutyryloxy)ethyl methacrylate (PBIEM) macroinitiators using atom transfer radical polymerization (ATRP) providing copolymers with controlled composition and molecular weights ranging from Mn = 601 500 to 2 731 000 with polydispersity indexes (Mw/Mn) between 1.06 and 1.20. The lower critical solution temperature (LCST) of the brushes increased with the mole fraction of MEO3MA in the side chain, and the hysteresis between the heating and cooling cycles decreased with the length of the side chain. Brush copolymers with different graft density were also prepared, and the average hydrodynamic diameter, measured by dynamic light scattering (DLS), varied with temperature above the LCST, and the maximum diameter of the aggregates increased according to the graft density of side chain along th...

Chemoenzymatic Approach toward Heterografted Molecular Bottle Brushes

Heterografted molecular bottle brushes, i.e., poly(glycidol-graft-e-caprolactone-acetyl)-co-(glycidol-graft-methyl methacrylate) [P(G-graft-CLAC)-co-(G-graft-MMA)] and poly(glycidol-graft-e-caprolactone-acetyl)-co-(glycidol-graft-n-butyl methacrylate) [P(G-graft-CLAC)-co-(G-graft-BMA)] were prepared in two steps starting with a linear polyglycidol. In the first step an approximately 50% homografted polymer poly(glycidol-graft-e-caprolactone-acetyl)-co-glycidol [P(G-graft-CLAC)-co-G] was obtained via ring-opening polymerization of e-caprolactone using polyglycidol as a multifunctional macroinitiator and Novozyme 435 (Lipase B from Candida antarctica (CALB) immobilized on a macroporous resin) as a catalyst. Selective acetylation of the hydroxy groups at the graft ends was achieved via enzymatic acetylation with vinyl acetate, and the hydroxy groups at the backbone were acylated with 2-bromo-2-methylpropionyl bromide. Finally poly(methyl methacrylate) or poly(n-butyl methacrylate) grafts were attached by ato...