Molecular Brushes Research Articles

Synthesis and self-assembly of a DNA molecular brush.

We report herein on the polymer-crystallization-assisted thiol-ene photosynthesis of an amphiphilic comb/graft DNA copolymer, or molecular brush, composed of a hydrophobic poly(2-oxazoline) backbone and hydrophilic short single-stranded nucleic acid grafts. Coupling efficiencies are above 60% and thus higher as compared with the straight solid-phase-supported synthesis of amphiphilic DNA block copolymers. The DNA molecular brushes self-assemble into sub-micron-sized spherical structures in water as evidenced by light scattering as well as atomic force and electron microscopy imaging. The nucleotide sequences remain functional, as assessed by UV and fluorescence spectroscopy subsequent to isoindol synthesis at the surface of the structures. The determination of a vesicular morphology is supported by encapsulation and subsequent spectroscopy monitoring of the release of a water-soluble dye and spectroscopic quantification of the hybridization efficiency (30% in average) of the functional nucleic acid strands engaged in structure formation: about one-half of the nucleotide sequences are available for hybridization, whereas the other half are hindered within the self-assembled structure. Because speciation between complementary and non complementary sequences in the medium could be ascertained by confocal laser scanning microscopy, the stable self-assembled molecular brushes demonstrate the potential for sensing applications.

On the pH sensitive optoelectronic properties of amphiphilic reduced graphene oxide via grafting of poly(dimethylaminoethyl methacrylate): a signature of p- and n-type doping

Poly(N,N′-dimethylaminoethyl methacraylate) (PDMAEMA) functionalized reduced graphene oxide (rGO) is synthesized by atom transfer radical polymerization, followed by attachment to rGO via diazonium coupling. The rGO-PDMAEMA (RGP) is characterized by 1H NMR, UV-Vis, FTIR and Raman spectroscopy. TEM and AFM studies demonstrate the formation of molecular brushes of PDMAEMA chains over rGO surface, and the TGA thermograms indicate 55 wt% grafting of PDMAEMA. RGP is dispersible in the widest spectrum of solvents from CCl4 to water [solubility parameter (δp + δh), 0.6 to 58]. RGP exhibits strongly pH dependent fluorescence properties: at pH 4, it exhibits two emission peaks, but at pH 7 and pH 9.2, a single and broad emission peak is observed. Two emission peaks at pH 4 are attributed to radiative decay of excitons to two kinds of holes in the rGO originating from illumination and p-type doping, which is also characterized by a blue shift of the Raman D band. Moreover, n-type doping of RGP at pH 9.2 is also evident due to a similar Raman shift. The dc-conductivity of RGP at pH 4 is 2 orders higher than that of pH 9.2 and the I–V characteristic curve at pH 7 exhibits a bimodal NDR property with a rectification ratio of 5.5. The bimodal NDR property is explained with a model using density of states and polaronic band.

Synthesis of degradable molecular brushes via a combination of ring‐opening polymerization and click chemistry

ABSTRACTAcid‐degradable molecular brushes with polycarbonate backbone and densely grafted side chains (∼1.9 SCs per backbone repeating unit) were synthesized for the first time using the grafting‐onto method. Extremely efficient copper‐catalyzed azide‐alkyne cycloaddition click reactions between the polycarbonate backbone containing two pendant azido groups per backbone unit and alkynyl‐terminated poly (methyl acrylate) (ay‐PMA72, average degree of polymerization DP = 72) SCs were demonstrated to finish in 10 min with a quantitative conversion of the azido groups. Similar grafting efficiencies were also achieved when using alkynyl‐terminated polystyrene (ay‐PS), poly(ethylene oxide) (ay‐PEO), and poly (t‐butyl acrylate)‐b‐polystyrene (ay‐PtBA‐b‐PS) to successfully prepare molecular brushes with high grafting density (>1.8 SCs per backbone repeating unit). Under acidic condition, the polycarbonate backbones were completely degradable and the final degraded product of the molecular brushes was a linear polymer chain with molecular weight two times of the SCs. When a mixture of hydrophobic ay‐PS and hydrophilic ay‐PEO chains was used, amphiphilic heterobrushes PC‐g‐(PS‐co‐PEO) were synthesized, which could self‐assemble into micelles or vesicles in selective solvents, depending on the ratio of the two SCs in the brush. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 239–248

Synthesis and photochemical properties of spiropyran graft and star polymers obtained by ‘click’ chemistry

The synthesis of photo-responsive polymers with high density of spiropyran (SP) groups has been investigated. Initially the ATRP synthesis of star-shaped polymer using acrylated benzospiropyran (BSP) was attempted but results suggested an incompatibility of ATRP with BSP preventing the formation of polymers or polymer architectures with a higher chain length or density of spiropyran side chains. Alternatively, copper catalyzed cycloaddition (click chemistry) was successfully employed to obtain spiropyran functionalized polymers. Well-defined poly(6-benzospiropyran hexylmethacrylate)s bearing a BSP moiety on the side chain of each unit of the polymer with linear, star-like and molecular brush architectures and narrow molecular weight distributions were successfully synthesized combining ATRP and click chemistry. All polymers exhibited conventional photochromism as confirmed by absorption and emission spectroscopy. The linear and star-like polymers showed similar behavior with comparable ring-closure kinetics and photostability, while the kinetics of the molecular brush architecture was two times slower and its photostability significantly greater.

Poly(ε-caprolactone)-block-poly(N-vinyl pyrrolidone) diblock copolymers grafted from macrocyclic oligomeric silsesquioxane

Poly(ε-caprolactone)-block-poly(N-vinyl pyrrolidone) diblock copolymers grafted from macrocyclic oligomeric silsesquioxane (MOSS) (denoted MOSS[PCL-b-PVPy]12) were synthesized via the sequential polymerizations involving ring-opening polymerization (ROP) of ε-caprolactone (CL) and RAFT/MADIX polymerization of N-vinyl pyrrolidone (NVP). The organic-inorganic brush-like diblock copolymers were characterized by means of nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography (GPC). Small angle X-ray scattering (SAXS) showed that all the MOSS[PCL-b-PVPy]12 was microphase-separated in the amorphous state. The microphase-separated morphologies were quite dependent on the length of PVPy blocks and the crystallization behavior of PCL subchains was significantly affected by the lengths of PVPy subchains. In aqueous solutions, the MOSS[PCL-b-PVPy]12 can be self-assembled into the polymeric micelles as evidenced by dynamic light scattering (DLS) and transmission election microscopy (TEM). The critical micelle concentrations of the brush-like diblock copolymers increased with increasing the lengths of PVPy blocks. It is proposed that the stability of the micellar cores was increased with the macrocyclic molecular brush structure of the diblock copolymers and the formation of the MOSS aggregates via MOSS–MOSS interactions.

Conformations of molecular brushes based on polyimide and poly(methyl methacrylate) in selective solvents: Experiment and computer simulation

The effect of solvent quality with respect to main and side chains on the conformations of molecular brushes is investigated by the methods of molecular hydrodynamics and optics as well as by computer simulation. Copolymers with a polyimide backbone and poly(methyl methacrylate) side chains are studied in solvents featuring strongly different thermodynamic qualities (chloroform, ethyl acetate, and 3-heptanone). The studied samples have close total molecular masses and backbone lengths but differ in grafting densities and lengths of side chains: a brush with densely grafted and relatively short side chains versus a brush with loosely grafted but very long side chains. For both types of brushes, similar changes in hydrodynamic behavior with improvement in the solvent quality are found experimentally. Computer simulation shows that these changes have different origins. In the former case, macromolecules are elongated, and their volumes grow simultaneously, while in the latter case, the shape of the macromolecules remains close to spherical and the changes in hydrodynamic parameters are mainly due to an increase in the volume of macromolecules.

Selective transport control on molecular velcro made from intrinsically disordered proteins.

The selectivity and speed of many biological transport processes transpire from a 'reduction of dimensionality' that confines diffusion to one or two dimensions instead of three. This behaviour remains highly sought after on polymeric surfaces as a means to expedite diffusional search processes in molecular engineered systems. Here, we have reconstituted the two-dimensional diffusion of colloidal particles on a molecular brush surface. The surface is composed of phenylalanine-glycine nucleoporins (FG Nups)--intrinsically disordered proteins that facilitate selective transport through nuclear pore complexes in eukaryotic cells. Local and ensemble-level experiments involving optical trapping using a photonic force microscope and particle tracking by video microscopy, respectively, reveal that 1-µm-sized colloidal particles bearing nuclear transport receptors called karyopherins can exhibit behaviour that varies from highly localized to unhindered two-dimensional diffusion. Particle diffusivity is controlled by varying the amount of free karyopherins in solution, which modulates the multivalency of Kap-binding sites within the molecular brush. We conclude that the FG Nups resemble stimuli-responsive molecular 'velcro', which can impart 'reduction of dimensionality' as a means of biomimetic transport control in artificial environments.

ISARA ATRP of methacrylic acid neutralized by simple amines yielding linear polymers and anionic molecular brushes

ISARA ATRP of the neutralized methacrylic acid (MAA) initiated by low-molecular ATRP initiator or a polymer bearing multiple ATRP initiating groups is reported demonstrating successful synthesis of linear polymers and anionic molecular brushes. To make suitable for polymerization by ATRP and solubilize in common organic solvents, methacrylic acid (MAA) was neutralized by simple amines ethylamine (MEA), diethylamine (DEA) or triethylamine (TEA). It was shown by cyclic voltammetry measurements that only TEA was proper neutralizing amine did not affecting or minimally affecting stability of the catalyst CuII/Me6TREN and enabling to carry out controlled polymerization of MAA by ATRP. ISARA ATRP of MAA neutralized by TEA was slow but well controllable enabling to synthesize PMAA with low polydispersity (PDI 1.10–1.25). ISARA ATRP of the neutralized MAA from the polymethacrylate-type backbone carrying multiple ATRP initiating groups resulted in graft copolymers with short to medium-length side chains of PMAA (DP up to 65). As determined by SEC with triple detection, anionic molecular brushes were characterized by unimodal MWD and reasonably low polydispersity index (Mw/Mn 1.2–1.4).

Thermoresponsive Polyphosphazene-Based Molecular Brushes by Living Cationic Polymerization.

A series of polyphosphazenes with molecular brush type structures have been prepared with controlled molecular weights and narrow polydispersities. The polymers show lower critical solution temperatures (LCST) between 18 and 90 °C, which can be easily tailored by choice of side-substituent to suit the required application. A temperature triggered self-assembly is observed to give stable colloidal aggregates with dimensions in the region of 100-300 nm.

Atom transfer radical polymerization as a powerful tool in the synthesis of molecular brushes

Macromolecules with complex architectures such as molecular brushes (MBs) have received much attention over the last two decades. This interest is largely due to their potential applications in many fields, such as drug delivery and electronics, among others. The synthesis of such complex macromolecules can only be achieved via the use of controlled polymerization techniques such as atom transfer radical polymerization (ATRP). This review highlights the use of ATRP in the synthesis of MBs. © 2014 Society of Chemical Industry

Synthesis by RAFT polymerization and properties of anionic cylindrical molecular brushes bearing poly(acrylic acid) side chains

RAFT polymerization of acrylic acid (AA) in the presence of low-molecular chain transfer agent (2-(butylthiocarbonothioylthio) propanoic acid and polyacrylate-based backbone bearing multiple butylthiocarbonothioylthio moieties is reported demonstrating successful synthesis of linear pAA with low polydispersity and anionic cylindrical molecular brushes, respectively. Twofold excess of butylthiocarbonothioylthio moieties attached to polyacrylate-based backbone over the initiator was optimal enabling to synthesize graft copolymers with rather low polydispersity (PDI 1.23–1.36) and reasonably high DP of pAA side chains (up to 25). Anionic cylindrical molecular brushes were characterized by SEC with triple detection, DLS, 1H NMR spectroscopy, and potentiometric titration. Thermal transitions and thermal stability of the brush polymers bearing pAA side chains were determined by DSC and TGA.

Synthesis and self-assembly behavior of organic–inorganic macrocyclic molecular brushes composed of macrocyclic oligomeric silsesquioxane and poly(N-isopropylacrylamide)

Macrocyclic molecular brushes composed of macrocyclic oligomeric silsesquioxanes and poly(N-isopropylacrylamide) were synthesized via the ATRP approach; they displayed self-assembly behavior.

Well-defined polyethylene molecular brushes by polyhomologation and ring opening metathesis polymerization

Well-defined polyethylene molecular brushes were synthesized, for the first time, by combining polyhomologation and ring opening metathesis polymerization (ROMP).

Stereocomplex Formation of Densely Grafted Brush Polymers.

In this report, we explore the capability of macromolecules to interdigitate into densely grafted molecular brush copolymers. We demonstrate that by using the tendency for stereocomplexation between poly(l-lactide) and poly(d-lactide) as a driving force complementary linear polymers and brush copolymers can form a stereocomplex. However, stereocomplex formation between complementary brush copolymers is restricted and only partially observed when the side chains are of a critical molecular weight.

A new type of monodisperse porous, hydrophilic microspheres with reactive chloroalkyl functionality: synthesis and derivatization properties

A seeded polymerization method based on a new functional monomer, 3-chloro-2-hydroxypropyl methacrylate (HPMA-Cl), was proposed for the synthesis of a new type of monodisperse porous, hydrophilic microspheres with reactive character. By applying the method, poly(3-chloro-2-hydroxypropyl methacrylate-co-ethylene dimethacrylate) (poly(HPMA-Cl-co-EDMA)) microspheres in the range of 4–7 μm, with specific surface areas between 2 and 146 m2/g, were obtained. The microspheres are hydrophilic in nature due to the hydroxyl groups and are easily derivatizable due to the reactive chloropropyl moiety. Ligands in the form of small molecules carrying hydrophobic alkyl or hydrophilic ion exchanger groups were covalently attached onto the microspheres via simple and one-pot reactions via their chloropropyl functionality. Using the same functionality, click chemistry and surface-initiated atom transfer radical polymerization were also applied for the generation of triazole ring and zwitterionic molecular brushes on the microspheres, respectively. Poly(HPMA-Cl-co-EDMA) microspheres seem to be a promising hydrophilic reactive material particularly for the synthesis of ion exchanger resins and chromatographic stationary phases.

Entropic templating

The entropic repulsion between tethered molecular brushes at fluid interfaces templates the mixing of otherwise incompatible macromolecules.

Reversible 2D/3D Coatings from Zipper‐Assembly of Block Copolymer Micelles

A zipper-assembly of micelles into 2D/3D coatings is reported. Block copolymer micelles that incorporate a poly(n-octadecyl methacrylate) block are zipped and unzipped on demand onto a PMMA surface. The mechanism implies an interdigitation of molecular brushes enabling 15 nm micelles to be assembled into monolayers or multilayers.

Rare Earth Metal-Mediated Group-Transfer Polymerization: From Defined Polymer Microstructures to High-Precision Nano-Scaled Objects

Poly(2-isopropenyl-2-oxazoline) (PIPOx) and poly(2-vinylpyridine) (P2VP) have been efficiently synthesized using bis(cyclopentadienyl)methylytterbium (Cp2YbMe) as catalyst. The polymerizations of 2-isopropenyl-2-oxazoline (IPOx) and 2-vinylpyridine (2VP) follow a living group-transfer polymerization (GTP) mechanism, allowing a precise molecular-weight control of both polymers with very narrow molecular-weight distribution. The GTP of IPOx and 2VP occurs via N coordination at the rare earth metal center, which has rarely been reported previously. The relative coordination strength of different monomers at the ytterbium center is determined by copolymerization investigations to be in the order of DEVP > MMA > IPOx > 2VP. In combination with living cationic ring-opening polymerization, PIPOx is converted to molecular brushes with defined backbone and poly(2-oxazoline) side chains using the grafting-from method.

Molecular Mobility of Molecular Brushes With a Polyimide Backbone and Polymethylmethacrylate Side Chains of Different Lengths

The molecular mobility of polyimide (PI) brushes, i.e., graft-copolymers with a polyimide backbone and PMMA side chains of different lengths, is studied by the dielectric spectroscopy method. Three dipolar polarization regions are found in the temperature range from –150 to 200°C: β-, β1, and α-processes. The β-process is due to a local mobility of polar groups of the PI backbone in the glassy state. Ester groups of polymethylmethacrylate (PMMA) side chains are responsible for two forms of the molecular mobility: a local mobility in the glassy state (β1-process) and a cooperative segmental motion of the skeletons of the PMMA side chains (α-process). The parameters of the β- and β1-processes are weakly dependent on the side chain's length. For the α-process, the molecular mobility of the PMMA side chains decreases with increasing their length.

Protein Adsorption on Well-Characterized Polyethylene Oxide Brushes on Gold: Dependence on Molecular Weight and Grafting Density

The adsorption of lysozyme protein was measured ex situ on well-characterized gold surfaces coated by end-tethered polyethylene oxide brushes of various molecular weights and controlled grafting densities. The adsorbed amount of protein for different molecular weight brushes was found to collapse onto one master curve when plotted against brush coverage. We interpret this relationship in terms of a model involving site-blocking of the adsorption of proteins at the substrate and discuss the role of the physical attraction of PEO segments to gold. We account for our observation of a simple exponential relationship between protein adsorption and normalized brush coverage with a simple protein adsorption model. In contrast to other studies in similar systems, we do not observe protein adsorption on brushes at high grafting density, and we suggest that this discrepancy may be due to the solubility effects of salt upon the brushes, influencing their protein binding affinity, in the limit of high grafting density and high brush volume fraction.