Functional End Groups Research Articles

Self-Assembly of Mixtures of Telechelic and Monofunctional Amphiphilic Polymers in Water: From Clusters to Flowerlike Micelles

We study the self-assembly of mixtures of n-alkyl mono- and difunctionalized poly(ethylene oxide) (PEO) chains in the dilute concentration regime. The monofunctional PEOs were prepared by living anionic polymerization with varying n-alkyl length (n = 14, 16, 22, 28) and constant PEO molecular weight of 5 kg/mol. The difunctional materials were obtained through end-to-end coupling of two of the monofunctionalized PEOs via their terminal hydroxyl groups. The chosen synthetic pathway yields well-defined model compounds with narrow molecular weight distribution and complete end-group functionalization. By using both small-angle neutron scattering (SANS) and dynamic light scattering (DLS) combined with theoretical data modeling, we have systematically investigated both the global and inner structure of the self-assembled micellar structures. For short n-alkyl chain-ends, we find a formation of clustered micelles with a finite size whereas, intriguingly, at longer n-alkyls, we observe a crossover to flowerlike ...

Chain-length dependent interfacial immunoreaction kinetics on self-assembled monolayers revealed by surface-enhanced infrared absorption spectroscopy

Self-assembled monolayer (SAM) has been extensively applied as ideal interface layer for construction of biosensors. Its chain length and end functional groups determine the physical and chemical properties of the modified surfaces, which will affect the performance of constructed biosensors. Herein, we studied the influence of chain length of n-alkanethiols SAMs on the immunoreaction kinetics employing attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). Antibody (rabbit immunoglobulin) is assembled on carboxyl terminated SAMs of n-alkanethiols with different chain lengths (n = 3, 6, 11, 16). The whole fabrication steps of the immunoassay can be monitored in situ by the ATR-SEIRAS. From the time-dependent SEIRA spectra, the interfacial immunoreaction kinetics between the immobilized antibody and antigen (goat anti-rabbit immunoglobulin) can be evaluated. We found that the immunoreaction became faster with increasing the chain length of SAMs. This chain length dependent kinetics might be attributed to different orientations of the assembled antibody caused by different packing densities of SAMs. The present research offers a sensing platform to evaluate immunoassay kinetics and provides fundamentals for construction of immunoassay with high performance.

Influence of Heterocyclic Spacer and End Substitution on Hole Transporting Properties Based on Triphenylamine Derivatives: Theoretical Investigation

Hybrid organic–inorganic halide–perovskite based solar cells have achieved outstanding progress, approaching one of the most competitive photovoltaic technologies. One of the hot topics is to develop inexpensive and efficient hole transporting materials to improve the performance of devices for practical applications. In this paper, we theoretically design a series of hole transporting materials based on triphenylamine backbone through varying the spacer and the end substitution. The properties of frontier molecular orbital, ionization potential, reorganization energy, and charge mobility have been calculated and analyzed. The results show that the spacer and the end functional groups strongly influence the molecular geometry, stacking, electron density distribution, and especially hole mobility. The best hole transporting material with furan as spacer and hydroxyl or methoxyl as substitution is proposed due to its highest hole transporting mobility induced by the planar conformation and tight π–π stackin...

Alginate modified with maleimide-terminated PEG as drug carriers with enhanced mucoadhesion

The goal of this study was to generate a new mucoadhesive carbohydrate-based delivery system composed of alginate (Alg) backbone covalently attached to polyethylene glycol (PEG) modified with a unique functional end-group (maleimide). The immobilization of PEG-maleimide chains significantly improved the mucoadhesion properties attributed to thioether bonds creation via Michael-type addition and hydrogen bonding with the mucus glycoproteins. Mucoadhesion studies using tensile and rotating cylinder assays revealed a 3.6-fold enhanced detachment force and a 2.8-fold enhanced retention time compared to the unmodified polymer, respectively. Additional indirect studies confirmed the presence of polymer-mucus glycoproteins interactions.Drug release experiments were used to evaluate the release profiles from Alg-PEG-maleimide tablets in comparison to Alg and Alg-SH tablets. Viability studies of normal human dermal fibroblasts cells depicted the non-toxic nature of Alg-PEG-maleimide. Overall, our studies disclose that PEG-maleimide substitutions on other biocompatible polymers can lead to the development of useful biomaterials for diverse biomedical applications.

PH-sensitivity and Conformation Change of the N-terminal Methacrylated Peptide VK20

ABSTRACTN-terminal methacrylation of peptide MAX1, which is capable of conformational changes by variation of the pH, results in a peptide, named VK20. Increasing the reactivity of this terminal group enables further coupling reactions or chemical modifications of the peptide. However, this end group functionalization may influence the ability of conformational changes of VK20, as well as its properties. In this paper, the influence of pH on the transition between random coil and ß-sheet conformation of VK20, including the transition kinetics, were investigated. At pH values of 9 and higher, the kinetics of ß-sheet formation increased for VK20, compared to MAX1. The self-assembly into ß-sheets recognized by the formation of a physically crosslinked gel was furthermore indicated by a significant increase of G’. An increase in pH (from 9 to 9.5) led to a faster gelation of the peptide VK20. Simultaneously, G’ was increased from 460 ± 70 Pa (at pH 9) to 1520 ± 180 Pa (at pH 9.5). At the nanoscale, the gel showed a highly interconnected fibrillary network structure with uniform fibril widths of approximately 3.4 ± 0.5 nm (N=30). The recovery of the peptide conformation back to random coil resulted in the dissolution of the gel, whereby the kinetics of the recovery depended on the pH. Conclusively, the ability of MAX1 to undergo conformational changes was not affected by N-terminal methacrylation whereas the kinetics of pH-sensitive ß-sheet formations has been increased.

Two-arm PCL and PLLA macrophotoinitiators with benzoin end-functional groups by combination of ROP and click chemistry and their use in the synthesis of A2B2 type miktoarm star copolymers

Novel and well defined macrophotoinitiators with two-arm poly(ε−caprolactone) (PCL) and poly(l-lactide) (PLLA) chains bearing two benzoin photofunctionalities (BPI) at the connection point of the polymer chains have been synthesized by combination of ring opening polymerization (ROP) and click chemistry. Initially, dibromo end-functionalized two-arm PCL, (PCL)2-(Br)2, and PLLA, (PLLA)2-(Br)2, were synthesized by ROP of ε−caprolactone and l-lactide monomers in bulk, using 2,2-bis(bromomethyl)-1,3-propanediol as the initiator and stannous-2-ethylhexanoate, (Sn(Oct)2), as the catalyst. Then, the terminal bromines of the polymer arms were converted into azide groups with sodium azide (NaN3). Subsequently, well-defined two-arm PCL and PLLA macrophotoinitiators with benzoin end functional groups [(PCL)2-(BPI)2 and (PLLA)2-(BPI)2] were obtained from the click reaction between diazido end-functionalized (PCL)2-(N3)2 and (PLLA)2-(N3)2 polymers and an alkyne-terminated benzoin photoinitiator (BPI-alkyne) which was synthesized from benzoin and propargyl bromide. The 1H NMR, FT-IR, GPC, UV, and fluorescence spectroscopic studies revealed that two-arm poly(ε−caprolactone) and poly(l-lactide) having benzoin photofunctional groups bound to the linking point of the chains were obtained. These polymers were then used as macrophotoinitiators for the photoinduced free radical promoted cationic polymerization of cyclohexene oxide (CHO) monomer to synthesize A2B2 type miktoarm star copolymers comprised of PCL or PLLA as A blocks and PCHO as B blocks.

Effects of Functional Groups of Materials on Nonspecific Adhesion and Chondrogenic Induction of Mesenchymal Stem Cells on Free and Micropatterned Surfaces.

Functional groups of materials are known to affect cell behaviors, yet the corresponding effect on stem cell differentiation is always coupled with that of cell spreading; it is thus unclear whether the chemical groups influence cell differentiation directly or via cell spreading indirectly. Herein we used a unique surface patterning technique to decouple the corresponding effects. Mesenchymal stem cells (MSCs) derived from bone marrow were seeded on surfaces coated with alkanethiols with one of four functional end groups (-CH3, -OH, -COOH, and -NH2) and underwent 9 days of chondrogenic induction. The measurements of quartz crystal microbalance with dissipation confirmed less proteins adsorbed from the cell culture media on the neutral -CH3 and -OH surfaces than on the charged -COOH and -NH2 surfaces. The neutral surfaces exhibited less cell spreading and higher extents of chondrogenic differentiation than the charged surfaces, according to the characterizations of immunofluorescence staining and quantitative real-time polymerase chain reaction. We further used a transfer lithography technique to prepare patterned surfaces on nonfouling poly(ethylene glycol) hydrogels to localize single MSCs on microislands with self-assembly monolayers of different alkanethiols, under given microisland areas and thus well-defined spreading areas of cells. While small microislands were always beneficial for chondrogenic induction, we found that the type of functional groups had no significant effect on chondrogenic induction under the given cell spreading areas, implying that the chemical groups influence cell differentiation only indirectly. Our results hence illustrate that functional groups regulate stem cell differentiation via tuning protein adsorption and then nonspecific cell adhesion and thus cell spreading.

Supramolecular H-bonded three-arm star polymers by efficient combination of RAFT polymerization and thio-bromo “click” reaction

Efficient and versatile “click”-based post-polymerization tools for the straightforward modification of especially polymers with sophisticated architectures play a significant role in polymer chemistry. We here report on the synthesis of supramolecular three-arm star polymers by a combination of RAFT polymerization and the thio-bromo “click” reaction, consequently exploring the potential of this particular thiol-halogen “click” reactions as post-polymerization tool. Therefore, two new trivalent chain transfer agents (CTAs) bearing either trithiocarbonate or dithiobenzoate groups were designed and investigated for the RAFT polymerization of n-butyl acrylate (nBA) and styrene. Kinetic investigations of CTA/monomer combinations were performed to obtain star-shaped poly(n-butyl acrylate)s (PnBA) and poly(styrene)s (PS) with adjustable molecular weights and well controllable polydispersities in the range of 1.13–1.30. The capped thiol-groups were released by aminolysis and quantitatively converted into barbiturate or imidazolium end-groups, allowing to prepare functionalized supramolecular star polymers via this particular thio-bromo “click” reaction. Thereby, the best results for aminolysis were not obtained in a one-pot reaction as expected but in a two step process, while acetonitrile has been shown to be essential for the success of the reaction in contrast to the pKa-value of the investigated bases. As proven via NMR spectroscopy and mass spectrometry all desired supramolecular star polymers were obtained with an exceptional high extend of end-group functionality (>95%) underscoring the potential of this specific “click”-based post-polymerization modification tool in combination with RAFT polymerization.

Formation of Polyrotaxane Particles via Template Assembly.

In this study, we report a versatile method to assemble tunable poly(ethylene glycol) (PEG)-based polyrotaxane (PRX) particles and capsules. By threading α-cyclodextrins (αCDs) onto PEG chains physically adsorbed onto template particles and subsequently dissolving the templates, PRX replica particles and hollow capsules are formed. This approach overcomes issues related to CD steric hindrance, and also reduces the multiple processing steps often associated with PRX-based particle formation. By simple variation of the molecular weight and end-group functionality of the PEG, we show that the rate of particle degradation as well as the stability of the particles can be tuned. We also demonstrate the loading and release of model (drug) compounds, achieving burst and controlled release of the compounds. It is envisaged that this approach will provide a flexible platform for the engineering of a diverse range of PRX-based particles, enabling PRX materials to be further explored in various applications.

Synthesis and post-polymerisation ligations of PEG-based hyperbranched polymers for RNA conjugation via reversible disulfide linkage

The synthesis of architectural polymers with multiple reactive functionalities offers significant promise as platform technologies for development of nano-medicines that require hybrid biomolecule-nanomaterial components. However, there can often be a mismatch in compatibility between the conditions required for the coupling chemistry, while maintaining stability of the biomolecule. This leads to decreased yields and poor functional fidelity. In this report, we describe the synthesis of hyperbranched polymers, where reversible addition fragmentation chaintransfer (RAFT) polymerization is used to control chain molecular weight, end group functionality and the final size of the hyperbranched polymer. Through optimization of the reaction conditions, we demonstrate that branched polymers with controlled size can be synthesized. The subsequent modification of the end-groups within the branched polymer through coupling to small oligonucleotides is then systematically investigated as a function of coupling chemistry. We demonstrate that to achieve the highest degree of coupling, chain extension of the end-group away from the sterically-hindered core of the polymer is required, and that the use of strained alkyne-azide coupling reactions appear to show the highest level of efficiency under the conditions studied. Indeed, when mixed attachment of both fluorescent dye molecules and oligos is attempted under these conditions, almost quantitative end-group modification is achieved. Overall, we highlight the importance of choosing compatible chemistries that allow efficient coupling of biomolecules to synthetic substrates under mild conditions to achieve optimal reaction performance.

Analysis using size exclusion chromatography of poly(N-isopropyl acrylamide) using methanol as an eluent

Size Exclusion Chromatography is traditionally carried out in either aqueous or non-polar solvents. A system to present molar mass distributions of polymers using methanol as a mobile phase is presented. This is shown to be a suitable system for determining the molar mass distributions poly(N-isopropylacrylamide)s (PNIPAM); a polymer class that is often difficult to analyze by size exclusion chromatography. DOSY NMR was used to provide intrinsic viscosity data that was used in conjunction with a viscometric detector to provide absolute calibration. Then the utility of the system was shown by providing the absolute molar mass distributions of dispersed highly branched PNIPAM with biologically functional end groups.

Design and Synthesis of Dendrimers with Facile Surface Group Functionalization, and an Evaluation of Their Bactericidal Efficacy.

We report a versatile divergent methodology to construct dendrimers from a tetrafunctional core, utilizing the robust copper(I) catalyzed alkyne-azide cycloaddition (CuAAC, “click”) reaction for both dendrimer synthesis and post-synthesis functionalization. Dendrimers of generations 1–3 with 8–32 protected or free OH and acetylene surface groups, were synthesized using building blocks that included acetylene- or azide-terminated molecules with carboxylic acid or diol end groups, respectively. The acetylene surface groups were subsequently used to covalently link cationic amino groups. A preliminary evaluation indicated that the generation one dendrimer with terminal NH3+ groups was the most effective bactericide, and it was more potent than several previously studied dendrimers. Our results suggest that size, functional end groups and hydrophilicity are important parameters to consider in designing efficient antimicrobial dendrimers.

End-Functionalized Palladium SCS Pincer Polymers via Controlled Radical Polymerizations.

A direct and facile route toward semitelechelic polymers, end-functionalized with palladated sulfur-carbon-sulfur pincer (PdII -pincer) complexes is reported that avoids any post-polymerization step. Key to our methodology is the combination of reversible addition-fragmentation chain-transfer (RAFT) polymerization with functionalized chain-transfer agents. This strategy yields Pd end-group-functionalized materials with monomodal molar mass dispersities (Đ) of 1.18-1.44. The RAFT polymerization is investigated using a PdII -pincer chain-transfer agent for three classes of monomers: styrene, tert-butyl acrylate, and N-isopropylacrylamide. The ensuing PdII -pincer end-functionalized polymers are analyzed using 1 H NMR spectroscopy, gel-permeation chromatography, and elemental analysis. The RAFT polymerization methodology provides a direct pathway for the fabrication of PdII -pincer functionalized polymers with complete end-group functionalization.

Interfacial Energy-Controlled Top Coats for Gyroid/Cylinder Phase Transitions of Polystyrene-block-polydimethylsiloxane Block Copolymer Thin Films.

Block copolymers (BCPs) with a high Flory-Huggins interaction parameter (χ) can form well-defined sub-10 nm periodic structures and can be used as a template for fabrication of various functional nanostructures. However, the large difference of surface energy between the blocks commonly found in high-χ BCPs makes it challenging to stabilize a useful gyroid morphology in thin film form. Here, we used an interfacial-energy-tailored top-coat on a blended film of a polystyrene-block-polydimethylsiloxane (PS-b-PDMS) BCP and a low-molecular-weight PDMS homopolymer with a hydrophilic end functional group. The top coat consisted of a random mixture of 40% hydrolyzed poly(vinyl acetate)-random-poly(vinly alcohol) (PVA-r-PVAc, PVA40) and PVAc homopolymer. At the optimized top-coat composition, gyroid nanostructures with sub-10 nm strut width were achieved down to ∼125 nm film thickness, which is only 3 times the lattice parameter of the gyroid structure. This is in marked contrast with a mixed morphology of gyroid and cylinders obtained for other compositions of the top coat. Self-consistent field theoretic simulations were used to understand the effect of the interfacial energy between the top coat and BCP/homopolymer blends on the phase transition behavior of the BCP/homopolymer films.

Thermal properties of hyperbranched polyesters

Hyperbranched poly(ester)s (HBPE) possess attributes for applications in a number of areas which include platforms for controlled release of actives and as rheology modifiers and coalescent agents in coatings. For the latter applications, properties such as the glass transition temperature and thermal stability are important. A number of HBPEs were produced with varying composition, molecular weight, degree of branching and end-group functionality in order to determine the effect of each on the HBPE thermal and rheological properties. These materials were synthesized from trimethylolpropane and the difunctional acids; adipic, phthalic and terephthalic. The chemical structure of the resulting HBPEs was fully characterized, particularly by SEC and NMR, and the thermal properties by DSC and TG. The glass transition temperature was found to depend primarily on HBPE composition and end-group functionality. The prominent feature of the thermal degradation for the hydroxyl-capped HBPE was dehydrative ether formation while that for carboxyl-capped polymers was decarboxylation.

Novel hydrophobic macromonomers for potential amphiphilic block copolymers

Oligomers of 2-methyl nylon3 (2mN3) and 3-methyl nylon3 (3mN3) were synthesized by base-catalyzed hydrogen transfer polymerization (HTP) of methacrylamide and crotonamide, respectively. The detailed structural analyses of 2mN3 and 3mN3 were performed using MALDI-MS, 1H-NMR, elemental analysis and several end-group analyses to ascertain polymerization mechanism and exact chemical structures of final products. The structural analyses revealed that (1) base-catalyzed HTP of methacrylamide and crotonamide follows the sequence of: hydrogen abstraction from amide group of monomer by basic catalyst (NatBuO), addition of monomeric units to the anionic center, intramolecular hydrogen migration and finally, termination by hydrogen transfer from protonated catalyst (tBuOH) to anionic end-group, (2) both oligomeric products have olefinic chain-ends resulting from the initiation mechanism. The specific behavior of basic catalyst leads to the formation of olefinic chain-ends that are apt to possible end-group functionalization. Since the functional end-groups of a well-defined macromonomer are of importance in terms of chain extension, grafting, chemical modification, click chemistry, monolayer surface modification, etc., it was aimed to create more reactive functional end-groups by epoxylation and bromination. Disappearance of the signals belonging to the olefinic protons in 1H-NMR spectra of modified oligomers and existence of bromine and epoxy adducts in MALDI MS spectra of the modified oligomers were attributed to end-group modification as intended. Hence, four novel macromonomers of polyamidic backbone with functional chain-ends were synthesized successfully.

Scalable Platform for Structured and Hybrid Soft Nanocolloids by Continuous Precipitation in a Confined Environment.

Geometrically structured polymer nanocolloids, including Janus nanocolloids, have been widely investigated for their unique properties, which are derived from their anisotropy. Controlled surface decoration with inorganic nanoparticles could induce another level of functionality into structured nanocolloids that could enable applications in fields ranging from rewriteable electronics to biphasic catalysis. Here, we demonstrate flash nanoprecipitation (FNP) as a one-step, scalable process platform for manufacturing hybrid polymer-inorganic nanocolloids in which one phase is selectively decorated with a metal nanocatalyst by tuning the molecular interactions between the feed ingredients during the process. For instance, by modifying the polymer end-group functionality, we document the ability to tune the location of the metal nanocatalyst, including placement at the nanocolloid circumference. Moreover, the addition of molecular additives is shown to transform the Janus nanocolloid structure from spherical to dumbbell or snowman while maintaining the ability to control the nanocatalyst location. In considering the flexibility and continuous nature of the FNP process, it offers an industrial-scale platform for the manufacturing of nanomaterials that are anticipated to impact many technologies.

Catalyst-free “click” functionalization of polymer brushes preserves antifouling properties enabling detection in blood plasma

Progress in biosensors for clinical detection critically relies on modifications of the transducer surface to prevent non-specific adsorption from matrix components (i.e. antifouling) while supporting biomolecular recognition elements to capture the analyte. Such combination of properties presents a significant challenge. Hierarchically structured polymer brushes comprising an antifouling polymer bottom block and a functionalizable top block are proposed as a promising strategy to achieve this goal. We employed the catalyst-free strain-promoted alkyne–azide cycloaddition (SPAAC) “click” reaction to biofunctionalize antifouling polymer brushes without impairing their resistance to fouling. The functionalization was performed on the side chains along the top polymer block or only on the end-groups of the polymer brush. The immobilized amounts of bioreceptors (streptavidin followed by biotin-conjugated proteins) and the resistance to fouling from blood plasma of the surfaces obtained were evaluated via surface plasmon resonance. The end group functionalization approach resulted in very low immobilization of bioreceptor. On the other hand, the side group modification of a top polymer block led to immobilization of 83% of a monolayer of streptavidin. Following binding of a biotin-conjugated antibody (66 ng cm−2) the functionalized layer was able to reduce the fouling from undiluted human blood plasma by 89% in comparison with bare gold. Finally, the functionalized hierarchical polymer brushes were applied to the label-free detection of a model analyte in diluted human blood plasma, highlighting the potential for translation to medical applications.

What Controls the Structure and the Linear and Nonlinear Rheological Properties of Dense, Dynamic Supramolecular Polymer Networks?

We investigated a series of telechelic polyisobutylenes, previously shown to exhibit self-healing, by means of small-angle X-ray scattering and rheology. All samples form a dense, dynamic network of interconnected micelles resulting from aggregation of the functional groups and leading to viscoelastic behavior. The dynamic character of this network manifests itself in the appearance of terminal flow at long time scales. While the elastic properties are distinctly molecular weight dependent, the terminal relaxation time is controlled by the functional end groups. The yielding properties under large deformation during startup shear experiments can be understood by a model of stress activation of the dynamic bonds. Stress relaxation experiments help to separate the nonlinear response into two contributions: a fast collapse of the network and a slow relaxation, happening on the time scale of the terminal relaxation. The latter is also known to control self-healing of the collapsed structure.

Induction of Chirality in β-Turn Mimetic Polymer Conjugates via Postpolymerization “Click” Coupling

Chiral and achiral β-turn mimetic polymer conjugates were synthesized, and their chiroptical properties were investigated. Chiral helical poly(n-hexyl isocyanate)s (PHICs) with alkyne functional end groups (4.0–5.4 kDa) were synthesized via coordination polymerization using either a chiral or an achiral alkyne-functional organotitanium catalyst. The obtained helical polymers were coupled with a chiral β-turn mimetic structure using the copper-catalyzed azide/alkyne cycloaddition (CuAAC) reaction, in turn allowing to tune the distance between the PHIC-helix and the chiral center from 7 to 14 A. The successful linkage of the PHIC-helix to the β-turn mimetic was confirmed by size exclusion chromatography (SEC), high performance liquid chromatography (HPLC), 1H NMR spectroscopy, and MALDI-ToF mass spectrometry, proving the attachment of either one or two PHIC-helices to the central β-turn. Circular dichroism (CD) spectra confirmed chirality induction from the chiral β-turn mimetic structure to the polymer bac...