Controlled Grafting Density Research Articles

Characterization of well‐defined PHEMA‐based bottlebrushes with controlled chain length and grafting density

AbstractBottlebrushes with controlled horizontal‐ and vertical‐axis scales, which have a high density of graft chains on their main chains, are useful as a nanomaterial. The properties of bottlebrushes depend on the polymer‐type, ‐chain length, and grafting density. We fabricate well‐defined bottlebrushes based on poly(2‐hydroxyethyl methacrylate) (PHEMA) using the grafting from method. Macroinitiators with different compositions of an atom transfer radical polymerization (ATRP) initiator group are prepared, and two bottlebrushes are consequently synthesized by ATRP of HEMA with the macroinitiator: a concentrated bottlebrush with graft chains on the repeating units and a semi‐dilute bottlebrush with graft chains on approximately 10%. The hydrodynamic and gyration radii of the concentrated‐ and semi‐dilute bottlebrushes are Rh = 9.2 nm (Rg = 24.9 nm) and Rh = 69.1 nm (Rg = 130 nm), respectively. The shape factors (Rg/Rh) of these bottlebrushes indicate both solid‐rods. The virial coefficient of semi‐dilute bottlebrush, calculated from static light scattering measurements, is higher than that of concentrated bottlebrush, owing to the flexibility and solvation structure. The grafting density play a more prominent role than the graft‐chain length in the excluded volume effect of the bottlebrush. These results provide useful insights for the construction of polymethacrylate‐based bottlebrushes with specific hydrodynamic properties.

Fabrication of Surface Polymer Brushes Via Thin Film Crystallization and Solvent Annealing.

Polymer single crystals have been used as templates to synthesize polymer brushes, known as the "polymer-single-crystal-assisted-grafting-to" (PSCAGT) approach. Polymer brushes with controlled grafting densities and spatial tethering locations have been demonstrated. Previous works focused on solution crystallization, which involves large amounts of organic solvent, and the grafting density can only be tuned by varying crystallization temperatures. In this work, thin film crystallization was utilized to fabricate two-dimensional (2D) polymer crystals on flat surfaces. Subsequent chemical tethering led to polymer brushes that retained the original morphology of the crystals with high fidelity. Furthermore, we show that the grafting density of the polymer brushes fabricated using this method depends on the chain end distribution on the top/bottom surfaces of the crystal, which can be facilely controlled by annealing the crystals at various nonsolvent media. Our work broadens the scope of the PSCAGT method and provides a new route to achieve polymer brushes with controlled structures. This article is protected by copyright. All rights reserved.

Control of Vancomycin Activity through the Encapsulation and Controlled Release from a Propargyl Acrylate-Poloxamer Nanocomposite System.

Vancomycin is a potent antibacterial drug that suffers from poor bioavailability due to its poor water solubility and relatively high molecular weight. Consequently, the application of vancomycin to treat bacteria-induced disease is limited. In this study, the ability of a temperature-stimulated propargyl acrylate-poloxamer nanocomposite (PAPN) system to encapsulate and release vancomycin is investigated. A controllable encapsulation and release system can be used to not only increase and prolong the bioavailability of vancomycin but also activate vancomycin with a temperature change. The PAPN system was prepared using an emulsion polymerization of propargyl acrylate followed by a surface decoration with a poloxamer at a precisely controlled grafting density. The activity of the PAPN system loaded with vancomycin is compared to that of the free drug and unmodified propargyl acrylate nanoparticles. It is shown that the activity of the PAPN system loaded with vancomycin is comparable to that of a freshly prepared, free-floating vancomycin solution. Upon storage, the activity of the free vancomycin in solution decreases, while the PAPN system loaded with vancomycin retains its high activity. Additionally, the PAPN system is able to effectively encapsulate and deactivate vancomycin until heated above a lower critical solution temperature (LCST). At temperatures above the LCST, the PAPN system releases vancomycin restoring the activity of the drug.

Degradable cellulose‐based polymer brushes with controlled grafting densities

ABSTRACTCellulose‐based polymer brushes with variable grafting densities and low dispersity were synthesized by grafting poly(n‐butyl acrylate) (PBA) side chains from cellulose‐derived backbones via ATRP. Esterification of commercially available cellulose acetate with 2‐bromoisobutyryl bromide (2‐BiBB) in NMP provided cellulose‐based macroinitiators averaging one initiation site per double glucose unit. ATRP macroinitiators averaging up to 6 initiation sites per repeating double glucose unit were prepared by acylation of microcrystalline cellulose (MCC) in LiCl/DMAc solvent system with 2‐BiBB. A series of linear macroinitiators with narrow MWD were obtained by fractional precipitation process. The content of initiating sites was determined by elemental analysis. (Meth)acrylate side chains were then grafted from the cellulose‐based macroinitiators. The prepared cellulose‐based polymer brushes showed tunable degradation rates dependent on grafting density of the brush, following two different degradation pathways, either cleavage of the main chain or detachment of the side chains. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2426–2435

Well-Defined Alkyl Functional Poly(Styrene-co-Maleic Anhydride) Architectures as Pour Point and Viscosity Modifiers for Lubricating Oil

A series of well-defined poly(styrene-co-maleic anhydride) copolymers (PSMA) allowing controlled grafting density and distribution of long alkyl side chains (C12 and C22) was synthesized and invest...

Micellization of novel biocompatible thermo-sensitive graft copolymers based on poly(ε-caprolactone), poly(N-vinylcaprolactam) and poly(N-vinylpyrrolidone)

This study reports the metal-free synthesis, the characterization and the micellization of a novel type of amphiphilic biocompatible and thermo-sensitive graft copolymers. It comprises a hydrolytically degradable hydrophobic poly(ε-caprolactone) (PCL) backbone and water-soluble thermo-sensitive poly(N-vinylcaprolactam) (PNVCL)-based side chains. The synthesis of these PCL-g-PNVCL graft copolymers, with controlled graft density, was carried out in a three-step process: (i) preparation of a chloro-functionalized PCL backbone by metal-free ring-opening copolymerization of ε-caprolactone and α-chloro-ε-caprolactone, (ii) generation of a macro-RAFT initiator by nucleophilic substitution of the chloro groups with potassium ethyl xanthogenate, and (iii) free radical controlled RAFT-Madix polymerization of NVCL. In order to tune the LCST of these graft copolymers with the human body temperature, a RAFT copolymerization of NVCL with N-vinylpyrrolidone (NVP) was performed in another series of reactions. Such PCL-g-PNVCL copolymers with a constant graft density of 15 grafting sites, DPn,backbone = 120 and DPn,average per graft = 54 lead in aqueous medium to stable micelles having an average hydrodynamic diameter Dv of 38 nm. The spherical morphology of these micelles was observed by TEM. Their thermo-sensitivity was further confirmed and a LCST value of 32 °C was determined. The incorporation of 10 or 20 mol% NVP units in the grafts of the PCL-g-P(NVCL-co-NVP) copolymers leads to a shift of LCST to higher temperatures in the range of 38– 40 °C.

Universal Method for Producing Reduced Graphene Oxide/Gold Nanoparticles Composites with Controlled Density of Grafting and Long-Term Stability.

In this study, we report a universal approach allowing the non-covalent deposition of gold nanoparticles on reduced graphene oxide surface in a controlled fashion. We used a modified Hummers method to obtain graphene oxide, which then underwent surficial functionalization with carboxyl moieties coupled with simultaneous reduction. Nanoparticles were synthesized ex-situ and capped with a thiolated poly-ethylene glycol (PEG) ligand. The interactions between the surface of modified graphene oxide and nanoparticle ligands enabled the formation of stable hybrid graphene-nanoparticles materials in the aqueous phase. Using this technique, we were able to cover the surface of graphene with gold nanoparticles of different shapes (spheres, rods, triangles, stars, and bipyramids), broad range of sizes (from 5 nm to 100 nm) and controlled grafting densities. Moreover, materials obtained with this strategy exhibited long-term stability, which coupled with the versatility and facility of preparation, makes our technique appealing in the light of increasing demand for new graphene-based hybrid nanostructures.

Terraced and Smooth Gradient Polymer Brushes via a Polymer Single‐Crystal Assisted Grafting‐To Method

AbstractGradient polymer brushes provide a spatial gradient change in molecular characteristics of the brush, and such a change can be utilized to study structure–property relationships in a combinatorial fashion. In this study, a bottom‐up method was used to synthesize gradient polymer brushes with a predesigned and precisely controlled grafting density gradient and brush pattern. A polymer single‐crystal assisted grafting‐to (PSCAGT) method was employed where end‐functionalized polymers were grown into two‐dimensional polymer single crystals. The latter were chemically coupled to a solid substrate to form well‐defined polymer brushes. To tune the grafting density, end‐dissimilar polymers were used to co‐crystallize into one single crystal. Programmed single‐crystal growth was introduced to synthesize brushes with two different gradient architectures, that is, terraced and smooth gradient with pyramid patterns. This work demonstrates that the PSCAGT method offers a unique means to tune polymer brush nanostructure.

Terraced and Smooth Gradient Polymer Brushes via a Polymer Single-Crystal Assisted Grafting-To Method.

Gradient polymer brushes provide a spatial gradient change in molecular characteristics of the brush, and such a change can be utilized to study structure-property relationships in a combinatorial fashion. In this study, a bottom-up method was used to synthesize gradient polymer brushes with a predesigned and precisely controlled grafting density gradient and brush pattern. A polymer single-crystal assisted grafting-to (PSCAGT) method was employed where end-functionalized polymers were grown into two-dimensional polymer single crystals. The latter were chemically coupled to a solid substrate to form well-defined polymer brushes. To tune the grafting density, end-dissimilar polymers were used to co-crystallize into one single crystal. Programmed single-crystal growth was introduced to synthesize brushes with two different gradient architectures, that is, terraced and smooth gradient with pyramid patterns. This work demonstrates that the PSCAGT method offers a unique means to tune polymer brush nanostructure.

Effect of Topology and Composition on Liquid Crystal Order and Self-Assembly Performances Driven by Asynchronously Controlled Grafting Density

A series of thermo-tunable liquid crystal block copolymers (LCBCs) with well-designed architectures were successfully synthesized. Linear/star poly[4-(4-vinylphenyl)-1-butene]-block-polybutadiene (PVSt-co-PB) moieties were obtained using living anionic copolymerization of 4-(4-vinylphenyl)-1-butene with butadiene, and topological [PVSt-co-PB]-LCBCs were generated through the adherence of mesogenic moiety via facile hydrosilylation. The PVSt LC block had well-defined grafting densities of approximately 100%, 70%, and 40%, whereas the PB LC block had an asynchronously tunable grafting density. This work included comprehensive studies on their self-assembly and yielded some interesting results. The influences of topologies and compositions on the phase transition behaviors and polarized optical performances of the resulting LCBCs that were driven by asynchronously controlled grafting density were carefully illustrated. The LCBCs with controlled molecular weight (MW) and narrow PDI showed wider LC phase range...

When convergent syntheses of graft block copolymers diverge: The treachery of chemical images

ABSTRACTA synthetic strategy to produce graft block copolymers (BCPs) with controlled grafting densities using both grafting‐through and grafting‐from methods is reported. For grafting‐through, poly(4‐methylcaprolactone‐block‐d,l‐lactide) macromonomers were synthesized with a polymerizable maleimide end group. These macromonomers were copolymerized using reversible addition‐fragmentation chain‐transfer (RAFT) polymerization with styrene and various amounts of N‐ethylmaleimide to control grafting density. A kinetic study showed that the macromonomers (>10 kDa) polymerized at the same rate as N‐ethylmaleimide under RAFT conditions, suggesting that uniform grafting density could be achieved. However, incorporating more than 5–10 grafts per chain was found to be challenging and potentially limited by kinetics. A higher number of grafts per chain with controlled densities was achieved using a grafting‐from technique. Macroinitiators of styrene, N‐ethylmaleimide, and N‐(2‐hydroxyethyl)maleimide were copolymerized with a fixed density of grafting sites. Subsequently, BCPs were grown off of the macroinitiator using ring‐opening transesterification polymerization (ROTEP). © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 3097–3104

Control of Grafting Density and Distribution in Graft Polymers by Living Ring-Opening Metathesis Copolymerization.

Control over polymer sequence and architecture is crucial to both understanding structure-property relationships and designing functional materials. In pursuit of these goals, we developed a new synthetic approach that enables facile manipulation of the density and distribution of grafts in polymers via living ring-opening metathesis polymerization (ROMP). Discrete endo,exo-norbornenyl dialkylesters (dimethyl DME, diethyl DEE, di-n-butyl DBE) were strategically designed to copolymerize with a norbornene-functionalized polystyrene (PS), polylactide (PLA), or polydimethylsiloxane (PDMS) macromonomer mediated by the third-generation metathesis catalyst (G3). The small-molecule diesters act as diluents that increase the average distance between grafted side chains, generating polymers with variable grafting density. The grafting density (number of side chains/number of norbornene backbone repeats) could be straightforwardly controlled by the macromonomer/diluent feed ratio. To gain insight into the copolymer sequence and architecture, self-propagation and cross-propagation rate constants were determined according to a terminal copolymerization model. These kinetic analyses suggest that copolymerizing a macromonomer/diluent pair with evenly matched self-propagation rate constants favors randomly distributed side chains. As the disparity between macromonomer and diluent homopolymerization rates increases, the reactivity ratios depart from unity, leading to an increase in gradient tendency. To demonstrate the effectiveness of our method, an array of monodisperse polymers (PLAx-ran-DME1-x)n bearing variable grafting densities (x = 1.0, 0.75, 0.5, 0.25) and total backbone degrees of polymerization (n = 167, 133, 100, 67, 33) were synthesized. The approach disclosed in this work therefore constitutes a powerful strategy for the synthesis of polymers spanning the linear-to-bottlebrush regimes with controlled grafting density and side chain distribution, molecular attributes that dictate micro- and macroscopic properties.

Well-defined polyurethane-graft-poly(N,N-dimethylacrylamide) copolymer with a controlled graft density and grafted chain length: synthesis and its application as a Pickering emulsion

Well-defined PU-g-PDMA graft copolymers with controlled graft densities and grafted chain lengths could be facilely synthesized by combining the polyaddition reaction with the RAFT polymerization.

Self-assembly of Au nanoparticles on graphene sheets as a catalyst with controlled grafting density and high reusability

Novel catalysts based on self-assembly of Au NPs on graphene oxide sheets through silane functionalization were reported.

Tailored (Bio)Interfaces via Surface Initiated Polymerization: Control of Grafting Density and New Responsive Diblock Copolymer Brushes

SummaryPolymer brush synthesis by surface initiated polymerization (SIP) comprises a powerful strategy to tailor interfacial properties. Among others, the passivation of solid substrates against non‐specific protein adsorption or the implementation of stimuli responsive properties can be achieved. In this context controlled radical polymerization approaches are frequently employed. Here we report on our efforts in the synthesis of oligo and di(ethylene glycol) methylether methacrylate (OEGMA and DEGMA)‐based brushes with (i) controlled grafting density and (ii) new diblock copolymer structures. We assembled mixed self‐assembled monolayers (SAMs) comprising octadecanethiol (ODT), 16‐mercaptohexadecanoic (MHDA) and ω‐mercaptoundecyl bromoisobutyrate (MUBiB) on gold to initiate the controlled radical polymerization of OEGMA and DEGMA. The SAMs as well as the polymer layers obtained by atom transfer radical polymerization for a constant polymerization time were characterized by contact angle measurements with water, FTIR spectroscopy, ellipsometry and atomic force microscopy (AFM). It was found that ODT and MHDA assemble in the mixed SAMs preferentially and despite the different endgroups very similar brushes were obtained. SAMs with fractional surface coverages of the initiator ω‐mercaptoundecyl bromoisobutyrate of > 0.45 showed no further increase in brush thickness, which is in line with a mushroom to brush transition below this coverage. In addition, diblock copolymer brushes comprising POEGMA and poly(tert‐butyl acrylate) (PtBA) blocks were obtained for the first time. The PtBA block was successfully hydrolyzed to yield POEGMA‐block‐poly acrylic acid (PAA) brushes.

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.

Controlled synthesis of graft polymer through the coupling reaction between the appending β-keto ester and the terminal amine

A new method to synthesize graft polymer with controlled graft density (Ğ) through the coupling reaction between the appending β-keto ester and the terminal amine is proposed. Following this method, the backbone polymer of poly[2-(acetoacetoxy) ethyl methacrylate] (PAEMA) containing the appending β-keto ester is synthesized by reversible addition-fragmentation chain transfer polymerization and the side-chain polymer of poly(ethylene glycol) monomethyl ether containing the –NH2 terminal group (MPEG–NH2) is prepared, and then the side-chain polymer is covalently bonded to the backbone polymer by the CN bond through the coupling reaction between the appending β-keto ester and the terminal –NH2. By changing the molar ratio of the side-chain polymer and the backbone polymer, the PAEMA-g-MPEG graft polymers with Ğ ranging from 0.07 to 0.85 are prepared. The solubility/dispersion of the graft polymers in water is studied and the Ğ-dependent solubility/dispersion is demonstrated. The proposed method is anticipated to be a valid method to prepare graft polymers.

Grafting Control of Mainstay Terpyridine Self-Assembled Monolayers for the Preparation of Planar Silicon Surfaces with Variable Catalytic Loadings

Monolayers of terpyridine-derivatized silanes were self-assembled, with accurately controlled grafting densities, on single-crystal silicon surfaces. Complexation of the resulting terpyridine monolayers with Pd(OAc)(2) afforded a series of catalytic surfaces covering a full range of Pd loadings (0.14-0.85 nmol.cm(-2)) in the aim to explore their impact on catalysis methodically. X-ray reflectivity (XRR), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry (ICP-MS) were combined to afford a precise picture of the grafting density, chemical composition, and catalyst loadings of the surfaces investigated here. We report that the control of the terpyridine density and thus the control of catalytic loadings can be achieved through a fine modification of silanization concentrations, which affords surfaces with tunable catalytic activity.

Human Mesenchymal Stem Cell Behavior on Concentrated Polymer Brushes Presenting Different Surface Stiffness

Abstract The graft density of poly[poly(ethylene glycol) methacrylate] (PPEGMA) was controlled using atom transfer radical polymerization to observe the change in the differentiation behavior of stem cells because the alteration in the graft density leads to the altered surface elasticity. However, since the cells do not adhere on the high graft density polymer brush, it is impossible to observe such behavior directly. We therefore immobilized collagen onto polymer brushs with controlled graft density to obtain a surface which is effective for cell adhesion and maintains its different surface matrix elasticity. For the cell adhesion, the human mesenchymal stem cell (hMSC) adhesion and differentiation was investigated on the surface of collagen-immobilized polymer brush. It was shown that the hMSC spreading tendency was altered by the surface elasticity implying that the behavior of hMSC would be altered by the stiffness of the polymer brush.

Poly(ε‐caprolactone)‐graft‐poly(2‐(dimethylamino)ethyl methacrylate) Amphiphilic Copolymers Prepared via a Combination of ROP and ATRP: Synthesis, Characterization, and Self‐Assembly Behavior

AbstractPoly(ε‐caprolactone)‐graft‐poly(2‐(dimethylamino) ethyl methacrylate) (PCL‐g‐PDMAEMAs), a kind of amphiphilic graft copolymer, was prepared by combination of ROP and ATRP. The FTIR, 1H NMR, and GPC results indicate that well‐defined polymers with controlled graft density and length of side chain were successfully synthesized. We prepared PCL‐g‐PDMAEMA nanoparticles by employing a nanoprecipitation technique. The pH‐ and thermosensitive properties of PCL‐g‐PDMAEMA nanoparticles were investigated by 1H NMR, TEM, and DLS. It was found that the nanoparticles with an average size of 120 nm presented core–shell structure in aqueous dispersion. Furthermore, the nanoparticles are sensitive to temperature in base while not in an acidic environment.magnified image