Grafted Side Chains Research Articles

Molecular Tensile Machines: Anti-Arrhenius Cleavage of Disulfide Bonds

Molecular tensile machines are bottlebrush molecules where tension along the backbone is self-generated due to steric repulsion between the densely grafted side chains. Upon adsorption onto a substrate, this intrinsic tension is amplified to the nanonewton range depending on the side chain length, grafting density, and interaction with the substrate. In this paper, bottlebrushes with a disulfide linker in the middle of the backbone were designed to study the effect of force and temperature on the scission of an individual disulfide bond. The scission process was monitored on molecular length scales by atomic force microscopy. The scission rate constant has been shown to increase exponentially with bond tension but decrease with temperature. This anti-Arrhenius behavior is ascribed to the decrease of substrate surface energy upon heating, which overpowers the corresponding effects of thermal energy and temperature dependent pre-exponential factor. Quantitative analysis using the force-modified Arrhenius and transition state theory (TST) equations, respectively, was conducted to determine the dissociation energy, maximum rupture force, and activation barrier of a disulfide bond under tension.

Intramolecular mobility of side chains of poly(methacrylic acid) in regularly grafted copolyimides in solution

A “dark” copolyimide with regularly grafted side chains of polymethacrylic acid and a luminescence-labeled copolyimide containing an anthracene label covalently attached to side chains with an average polycondensation degree of the polyimide backbone of ∼16 and an average polymerization degree of side poly(methacrylic acid) chains of ∼100 are synthesized. Relaxation times τIMM characterizing the mobility of parts of side chains in solvents of different thermodynamic qualities for the backbone and side chains are determined through the polarized luminescence method. It is shown that, in a “common” solvent for the backbone and side chains, the values of τIMM are close to those characterizing the mobility of linear polymethacrylate chains. In selective solvents, changes in τIMM are related to not only changes in intramolecular interactions but also changes in the heterogeneity of the dynamic characteristics of parts of grafted chains arranged at different distances from the grafting point.

Molecular Properties and Aggregation of Porphyrin-modified Polysiloxane in Solutions

Molecular properties and aggregation behavior of a polysiloxane with grafted side chain groups consisting of asymmetrical hydroxy-functionalized porphyrin were studied in dilute solutions. In order to understand the mechanism of aggregation of the polymeric porphyrin derivatives, several analyses of chloroform solutions were performed permitting comparison of the data obtained by means of different techniques. Molar mass and hydrodynamic size of the macromolecules were obtained using the methods of analytical ultracentrifugation, isothermal translational diffusion, and viscometry. Long distance interactions between macromolecules in dilute solutions were detected with static light scattering. With increasing the concentration of the solutions, the electronic and fluorescence spectra bands assigned to generation of H and J aggregates became apparent. The organization of aggregates, which seemed to be favored by chloroform, was visualized by atomic force microscopy images that displayed sponge-like morphology, small triangular particles, characteristic of H aggregates, and nano or micro-structured ring geometries obtained by the side-by-side J-process, coexisting together.

Interfacial interaction between comb-like copolymer dispersants and tricalcium silicate

Two commercial comb-like copolymer dispersants (also called polycarboxylate-based superplasticisers, hereinafter called PCs) were employed to investigate their interaction with tricalcium silicate (C3S). It is determined that when tricalcium silicate dissolves in deionised water, it releases a large amount of cations (mainly Ca2+). Those cations adsorb the surface of initially negatively charged tricalcium silicate and finally reach saturation. Through this mechanism, tricalcium silicate attains a slightly positive zeta potential. Thus, it is emphasised that with a larger amount of cations present in the synthetic pore solution, higher zeta potential values are measured. Owing to the positively charged layer of adsorbed cations, anionic PC adsorption occurs by chelating with calcium ions. Ultimately, the adsorbed PCs impart dispersity owing to its steric hindrance from hydrophilic graft side chains. Moreover, the introduced PCs lead to a strong retardation of tricalcium silicate hydration, and consequently have a significant effect on the initial dispersity of tricalcium silicate paste. Results are presented for a better understanding of the interaction between PCs and tricalcium silicate.

Constructing well-defined star graft copolymers

This review highlights representative efforts to construct well-defined star graft copolymers over the past decade. Star graft copolymers, consisting of multiple arms connected to a central core as the backbone, and branched side chains grafted from the arms, possess a more complex topological architecture than traditional graft copolymers and star polymers bearing linear arms. According to the distinction of grafting density, star graft copolymers can be divided into typical star graft copolymers with loosely grafted side chains, and star brush polymers with densely grafted side chains. For the synthesis of both types, there are great difficulties in achieving precise control over their topology, microstructure, and composition. Especially for the preparation of star brush polymers, their high grafting densities and steric hindrance also bring more challenges. Through the combination of two tactics for preparing star polymers, the diverse strategies employed in synthesizing graft copolymers, and a great variety of controlled/living polymerization techniques, a series of star graft polymers has been obtained. Thanks to the giant size and high compactness of star brush polymers, the unique hierarchical self-organization behavior of these highly branched star polymers will pave the way for their potential use in the fields of drug delivery, bio-catalysis, super soft elastomers, and templates for hybrid nanomaterials, acting as unimolecular micelles.

A new precursor for conducting polymer-based brush interfaces with electroactivity in aqueous solution

We present the synthesis of a novel conducting polymer (CP) incorporating both pyrrole and thiophene units in its monomer, which is also substituted with an initiator for grafting of sidechains by atom-transfer radical polymerisation (ATRP). The precursor monomer for the CP macroinitiator, 2-(2,5-di(pyrrol-2-yl)thiophen-3-yl)ethyl 2-bromopropanoate) (PyThon) is very readily electropolymerised at low monomer concentrations and low applied potentials. Density functional theory (DFT) predictions of ionisation potentials and spin-charge distribution for PyThon are in good agreement with these experimental results. We present also the grafting of sidechains from electropolymerised PolyPyThon (PPyThon) to yield surface-confined polymer brushes. Functionalisation with polystyrene (PS), poly(2,3,4,5,6-pentafluorostyrene) (PFS) and poly(ethylene glycol) methyl ether acrylate (PEGMEA) is demonstrated and confirmed by FT-IR and water contact angle measurements. These PPyThon-based molecular brushes are electroactive in both water and acetonitrile, and show evidence of changes in surface conformation related to the redox state of the CP. The growth of human fibroblasts on PPyThon films is also demonstrated, indicating good biocompatibility of the polymer. We conceive PPyThon-based molecular brushes as a substrate for electrical stimuli-responsive surfaces with application particularly in the biomedical field.

Soft Elastomers via Introduction of Poly(butyl acrylate) "Diluent" to Poly(hydroxyethyl acrylate)-Based Gel Networks.

We report a new strategy for the synthesis of stable and well-defined supersoft elastomers. First, four-arm star-like polymers, poly(trimethylsilyloxyethyl acrylate), were synthesized and cross-linked to form a relatively uniform polymer gel network. Second, short poly(n-butyl acrylate) side chains were grown from the initiating sites along the network backbone via atom transfer radical polymerization. These soft side chains act as low molecular weight "diluent" that "swells" the cross-linked polyHEA network, but cannot be leached from it. Using this strategy, materials with shear modulus less than 5 kPa were prepared and the effect of grafted side chain fraction on their mechanical properties was explored.

Modification of side chain terminals of PEGylated molecular bottle brushes—A toolbar of molecular nanoobjects

Water soluble molecular brush with poly(glycidyl methacrylate) (PGMA) as the main chain and mono-allyl terminated poly(ethylene glycol) (PEG) as the densely grafted side chains was efficiently synthesized via grafting-onto approach by copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. Then highly efficient post-modification of the ends of the side chains was realized through UV light induced thiol–ene radical addition. Thus polymer brushes with various side chain terminal functional groups including hydroxyl group, carboxyl group, amino group, amino acid, sulfonic group and glucose unit were obtained.

Study on the nonisothermal crystallization kinetics of ternary-monomer solid-phase graft copolymer of polypropylene

In order to discover how the grafted side chains affect the crystallization of the graft copolymers, the ternary-monomer solid-phase graft copolymers of polypropylene (PP; PPSM) were synthesized by solid-phase grafting of maleic anhydride, styrene and butyl acrylate onto PP. The Jeziorny and Mo methods were used to investigate the nonisothermal crystallization kinetics and polarizing microscope was used to study the crystalline morphology of PPSM. Results show that the grafted side chains not only promoted the heteronucleation of PP and reduced the size of spherulites but also hindered the growth of crystals resulting in the formation of imperfect crystallization region in PP. Due to the hindrance of the grafted side chains on the crystallization, the nonisothermal crystallization energy (ΔE) of PPSM calculated by the Kissinger equation is evidently higher than that of PP.

A novel poly(p‐phenylene) containing alternating poly(perfluorooctylethyl acrylate‐co‐methyl methacrylate) and polystyrene grafts by combination of atom transfer radical polymerization and Suzuki coupling processes

AbstractA poly(p‐phenylene) (PP), carrying perfectly alternating, well‐defined poly(perfluorooctylethyl acrylate‐co‐methyl methacrylate) [P(FEA‐co‐MMA)] and polystyrene (PS) side chain grafts, was synthesized by the combination of atom transfer radical polymerization (ATRP) and Suzuki cross‐coupling processes. First, dibromobenzene and diboronic ester functional macromonomers of P(FEA‐co‐MMA) and PS, respectively, were prepared by ATRP. In the second step, PP with lateral alternating P(FEA‐co‐MMA) and PS chains was synthesized by a Suzuki coupling reaction in the presence of Pd(PPh3)4 catalyst. The wetting behavior of the polymers was studied by measurements of the static contact angle θ of thin films (200−400 nm thickness) using water and n‐hexadecane as wetting liquids. The obtained fluorinated PP showed high static contact angles with both interrogating liquids, exhibiting simultaneously hydrophobic (θw = 111°) and lipophobic (θh = 67°) properties. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Microphase separation in a melt of graft copolymers formed by blocks with different grafting densities

The weak-segregation phase behavior of a monodisperse melt of a binary graft copolymer the macromolecules of which consist of two blocks with different densities of grafting of side chains is investigated. The length of the side chains in each block is assumed to be the same. Analysis of the structure factor of the system indicates the possibility of formation of two-scale structures in the melt. The realized scales differ significantly from each other and correspond to a phase separation between the blocks and between the monomeric units that form the repeating fragment of the graft copolymer. Classification diagrams are constructed, which describe the scale of the structures formed in the melt at different values of the parameters of the chemical structure of the macromolecules.

Molecular brushes with extreme grafted side chain densities

A series of densely grafted poly(n-butyl acrylate) (PBA) molecular brushes with four different grafting densities were synthesized by the “grafting-from” approach using atom transfer radical polymerization (ATRP). A novel monomer, isopropylidene-2,2-Bis(methoxy)propionic hydroxyethylmethacrylate (IMPHMA), was synthesized and copolymerized with methyl methacrylate (MMA) under different monomer feed ratios to yield a series of linear poly(methyl methacrylate-stat-IMAPA), [PMMA-s-(PIMPHMA)]. The resulting copolymers were deprotected and transformed to macroinitiators, [PMMA-s-(PHEMA-IMPHMA-Br)]. n-Butyl acrylate (BA) was grafted from these macroinitiators to yield a series of molecular brushes, [PMMA-s-{(PIMPHMA)-g-PBA}], with various side chain lengths. Molecular brushes were characterized by gel permeation chromatography (GPC) and 1H NMR. PBA side chains were cleaved by acid hydrolysis, and the resulting linear PBA polymers were characterized by GPC to study initiation efficiency during the synthesis of molecular brushes. The initiation efficiency increased with polymerization time and decreased with macroinitiators that had more initiation sites. Atomic force microscopy (AFM) measurements demonstrated the characteristic molecular structure by resolving individual brush molecules.

Switchable surfaces of electroactive polymer brushes grafted from polythiophene ATRP-macroinitiator

Brush copolymers based on polyterthiophene have been synthesized with the aim of using them as electrically responsive layers in smart surfaces. A novel terthiophene monomer, which is polymerizable both chemically and electrochemically, was used to polymerize polyterthiophene backbones as precursors for grafting of sidechains. The precursor polymer is equipped with a bromoester, making it suitable for use as a macroinitiator for Atom Transfer Radical Polymerization (ATRP) of polystyrene-b-polyacrylate branches. In this paper we present the synthesis and characterization of this conductive poly(3′-(ethyl-2-bromopropionate)-2,2′:5′,2″-terthiophene)-g-(polystyrene-b-poly acrylic acid) (PBrTTh-g-(PS-b-PAA)) brush copolymer and demonstrate its reversible switching between conducting and insulating states, with accompanying changes in physical properties. We characterize morphological changes driven by the electrochemical switch with scanning electron microscopy and observe a microscale wetting–dewetting transition of brush copolymer films on the electrode.

Coil-to-rod conformational transition and single chain structure of graft copolymer by tuning the graft density

The chain conformation and individual chain structures of the graft copolymers ethyl cellulose grafting poly (acrylic acid) (EC-g-PAA) were investigated by laser light scattering (LLS) and atom force microscopy (AFM). The EC-g-PAA graft copolymers with two different side chain lengths and side chain grafting densities were synthesized via atom transfer radical polymerization (ATRP) from ethyl cellulose as the backbone. The graft copolymer molecules are adsorbed on the mica surface, and the observed single molecule structures by AFM reflect the molecular conformation in solution. An increase of the graft density of the graft copolymer induces the conformational transition due to the repulsion between side chains from coil to rod conformation. The observed disclike and rodlike single chain structures response to the coil and rod conformations, respectively. The results provide a direct visual experimental evidence of chain conformational and single chain structural transitions for graft copolymers in common solvents induced by graft density.

Ultra low dielectric constant hybrid films via side chain grafting reaction of poly(ether ether ketone) and phosphotungstic acid

The novel ultra low dielectric constant poly(ether ether ketone) hybrid films in which nanoscale phosphotungstic acid (PWA) clusters were grafted onto the side chains of poly(ether ether ketone) containing (3-trifluoromethyl) phenyl groups and carboxyl groups (PEEK–CF3–COOH) in the presence the silane coupling agent γ-aminopropyltriethoxysilane (KH-550) were prepared and characterized. The chemical structures of the PWA/PEEK–CF3–COOH hybrid films were confirmed by FT-IR spectroscopy and 1H-NMR. The analysis of wide-angle X-ray diffraction (WAXD) indicated that the PWA clusters could not form crystalline structures in the PEEK–CF3–COOH matrix, and the investigation of the scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) (W-mapping) revealed that the PWA particles were well dispersed in the PEEK–CF3–COOH matrix with nanoscale. The influence of the incorporation of PWA particles by side chain grafting reaction on the properties of PEEK–CF3–COOH was studied. The dielectric constants of the PWA/PEEK–CF3–COOH hybrid films were remarkably lower than that of the neat PEEK–CF3–COOH film and the lowest value their dielectric constant could achieve was as low as 1.96 (1 MHz) when the content of grafted PWA reached 10 wt%. Meanwhile, the hybrid films still retain the excellent thermal and mechanical properties as well as the good transparency.

Preparation of epoxy functionalized PP with unique structure and its post‐ring open reaction

AbstractThis article discusses a convenient chemical approach named in situ chlorination graft copolymerization (ISCGC) to prepare epoxy functionalized isotactic polypropylene (iPP) with a unique structure. This method was carried out in the gas‐solid state, and chlorine was used as radical initiator as well as terminate agent. The effect of influence factors on the structure of the functionalized PP was investigated, and it was determined by levels of grafted GMA moieties. The results showed that PP modified with GMA could obtain higher grafting level. Crystallinity and mechanical properties of the functionalized PP were also investigated. For the purpose of researching the reactivity of epoxy groups located at the grafted side chains, trichloroacetic acid and hydroxyl‐terminated butadiene–acrylonitrile rubber (HTBN) were used as models to explore the ring open reaction of the epoxy groups of the functionalized PP. The structure and property of the modified PP that has undergone post‐ring open reaction was characterized by FTIR, dynamic property analysis. The results indicated that epoxy groups of the graft side chains have successfully undergone ring open reaction in the presence of carbonyl and hydroxyl groups. Additionally, the compatibilization of PP with HTBN is enhanced after PP was modified by GMA. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Synthesis and characterization of thermosensitive interpenetrating polymer networks based on N-isopropylacrylamide/N-acryloxysuccinimide, crosslinked with polylysine, grafted onto polypropylene

Interpenetrating polymer networks (IPNs) based on poly (N-isopropylacrylamide), (PNIPAAm) and poly (N-acryloxysuccinimide) (PNAS), grafted onto polypropylene (PP), were synthesized in three consecutive steps using ionizing radiation in the first and second steps and chemical reaction in third one. In the first step a thermosensitive graft copolymer of NIPAAm onto PP film was obtained by gamma radiation with a 60Co source. The grafted side chains of PNIPAAm were then crosslinked with gamma radiation to give net-[PP-g-NIPAAm]. The secondary network was obtained in situ by chemical crosslinking between PNAS and polylysine (pLys). The PP-g-IPNs exhibited the lower critical solution temperature (LCST) at around 32 °C. Based on its thermoreversible behavior, this system could be used for immobilization of biomolecules. The phase transition temperature (LCST) and network properties of the IPNs were measured by swelling behavior. Additional characterization by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and infrared (FTIR-ATR) determinations are reported.

Synthesis and Characterization of Cyclic Brush-Like Polymers by N-Heterocyclic Carbene-Mediated Zwitterionic Polymerization of N-Propargyl N-Carboxyanhydride and the Grafting-to Approach

Cyclic brush-like polymers were synthesized by tandem organo-mediated zwitterionic polymerization and a grafting-to approach. The cyclic polymer backbone, consisting of poly(N-propargylglycine) (c-PNPG), was synthesized by an N-heterocyclic carbene (NHC)-mediated zwitterionic ring-opening polymerization of N-propargyl N-carboxyanhydride. The polymerization proceeds in a quasi-living manner, allowing access to c-PNPG of well-defined chain length. The cyclic architecture of the polymers was verified by size exclusion chromatography (SEC) and mass spectroscopy (MS), as well as scanning probe characterization. Poly(ethylene glycol) functionalized with azido end-groups was subsequently grafted onto the c-PNPG by the copper-mediated azide/alkyne cycloaddition reaction (CuAAC). The side chain grafting density was determined by 1H NMR spectroscopy and SEC analysis. The grafting efficiency is low (<19%) when the cyclic backbone is comprised of a c-PNPG homopolymer. The efficiency can be significantly improved (up ...

Computational Design of High-Affinity Epitope Scaffolds by Backbone Grafting of a Linear Epitope

Computational grafting of functional motifs onto scaffold proteins is a promising way to engineer novel proteins with pre-specified functionalities. Typically, protein grafting involves the transplantation of protein side chains from a functional motif onto structurally homologous regions of scaffold proteins. Using this approach, we previously transplanted the human immunodeficiency virus 2F5 and 4E10 epitopes onto heterologous proteins to design novel “epitope-scaffold” antigens. However, side-chain grafting is limited by the availability of scaffolds with compatible backbone for a given epitope structure and offers no route to modify backbone structure to improve mimicry or binding affinity. To address this, we report here a new and more aggressive computational method—backbone grafting of linear motifs—that transplants the backbone and side chains of linear functional motifs onto scaffold proteins. To test this method, we first used side-chain grafting to design new 2F5 epitope scaffolds with improved biophysical characteristics. We then independently transplanted the 2F5 epitope onto three of the same parent scaffolds using the newly developed backbone grafting procedure. Crystal structures of side-chain and backbone grafting designs showed close agreement with both the computational models and the desired epitope structure. In two cases, backbone grafting scaffolds bound antibody 2F5 with 30- and 9-fold higher affinity than corresponding side-chain grafting designs. These results demonstrate that flexible backbone methods for epitope grafting can significantly improve binding affinities over those achieved by fixed backbone methods alone. Backbone grafting of linear motifs is a general method to transplant functional motifs when backbone remodeling of the target scaffold is necessary.

Molecular Tensile Machines: Intrinsic Acceleration of Disulfide Reduction by Dithiothreitol

Significant tension on the order of 1 nN is self-generated along the backbone of bottlebrush macromolecules due to steric repulsion between densely grafted side chains. The 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. These molecules were employed as miniature tensile machines to study the effect of mechanical force on the kinetics of disulfide reduction by dithiothreitol (DTT). For this purpose, bottlebrush macromolecules containing a disulfide linker in the middle of the backbone were synthesized by atom transfer radical polymerization (ATRP). The scission reaction was monitored through molecular imaging by atomic force microscopy (AFM). The scission rate constant increases linearly with the concentration of DTT and exponentially with mechanical tension along the disulfide bond. Moreover, the rate constant at zero force is found to be significantly lower than the reduction rate constant in bulk solution, which suggests an acidic composition of the water surface with pH = 3.7. This work demonstrates the ability of branched macromolecules to accelerate chemical reactions at specific covalent bonds without applying an external force.