Four-arm Star Research Articles

The influence of the structure of terminal groups and cores on the properties of schiff base star-shaped liquid crystals

ABSTRACT A series of star-shaped liquid crystal (LC), respectively, based on phloroglucinol, pentaerythritol and glucose as cores, rod-shaped molecules (MeOSBAA, MeSBAA and ClSBAA) containing a schiff base as mesogenic mesogen were synthesised. The molecular structures were confirmed by FT-IR and 1H-NMR techniques. The LC textures and thermal phase behaviours were investigated by polarising optical microscopy and differential scanning calorimetry. The results showed that MeOSBAA and ClSBAA were thermally enantiotropic liquid crystals. Three-arm star LC macromolecules X-SBA3E and four-arm star LC macromolecules X-SBA4E were found to display enantiotropic nematic liquid crystals behaviour. While the five-arm star-shaped chiral LC molecules X-SBA5E exhibited enantiotropic chiral nematic phase.

Synthesis of poly(styrene)-b-poly(2-vinyl pyridine) four-arm star block copolymers via ATRP and their self-assembly behaviors

In this study, synthesis of poly (styrene)-b-poly (2-vinyl pyridine) four-arm star block copolymers ((PS-b-P2VP)4 sBCPs) possessing high interaction parameter (χ) feature was investigated. To obtain well-defined sBCPs, a challenge on ATRP of pyridine-containing monomers is foreseeable. We thus scrutinized the domino effect from PS macroinitiators (MIs) having different halogen chain ends (i.e., (PS–X)4, where X = Br/Cl) to the subsequent chain extensions. As evident from the model reactions of benzyl halides (BzX, where X = Br/Cl) and 2-methylpyridine (2 MP), the combination of BzCl and 2 MP can significantly suppress the substitution side reaction. Consequently, well-defined (PS-b-P2VP)4 sBCPs were obtained (Mn = ca. 58 k–82 k with PDI <1.5). From the analyses of small angle X-Ray scattering (SAXS), transmission electron microscopy (TEM), and atomic force microscope (AFM), self-assembly behaviors of the obtaining sBCPs were facilely observed due to the high χ feature, mainly including cylinder and lamellae morphologies, in nanoscale of approximately 30 nm.

Correction to Supramolecular Four-Armed Star A2B2 Copolymer (Miktoarm Star) via Host–Guest Complexation and Nitroxide-Mediated Radical Polymerization

Correction to Supramolecular Four-Armed Star A₂B₂ Copolymer (Miktoarm Star) via Host–Guest Complexation and Nitroxide-Mediated Radical Polymerization

Supramolecular Four-Armed Star A2B2 Copolymer (Miktoarm Star) via Host–Guest Complexation and Nitroxide-Mediated Radical Polymerization

A supramolecular four-armed star copolymer was prepared from the pseudorotaxane complex of a crown-centered polystyrene and a bis(2,2,6,6-tetramethylpiperidinyl-1-oxy) viologen compound and n-butyl methacrylate (BMA) by nitroxide-mediated polymerization of the latter. Solvent fractionation with dioxane/THF/MeOH removed a small amount of a high molecular weight poly(BMA) homopolymer. The molecular weight increase from its monomodal size exclusion chromatographic trace gives direct evidence of the four-armed star copolymer formation; there is no residual starting polystyrene. NMR spectroscopy shows the two components of the copolymer and confirms the presence of the central [2]rotaxane unit. Differential scanning calorimetry displays two glass-transition temperatures, which correspond closely to those of polystyrene and poly(BMA).

Synthesis of pH-Sensitive Macromolecular Micelles from Amphiphilic Star Copolymers for Drug Delivery

Stimuli-responsive copolymers are a significant class of smart materials extensively studied for drug delivery systems. Two novel amphiphilic four-arm star copolymers poly(ε-caprolactone)-b-poly(hydroxyethyl methacrylate) (4sPCL-b-PHMEA) and poly(ε-caprolactone)-b-poly(diethylaminoethyl methacrylate)-b-poly(hydroxyethyl methacrylate (4sPCL-b-PDEAM-b-PHEMA) with pH-responsive were designed and synthesized by a combination of ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). The structure of the copolymers and the particle size of macromolecular micelles were investigated by FT-IR and DLS, and the controlled release kinetics of two copolymer micelles was detected using adriamycin (DOX), a hydrophobic antitumor drug. The results showed that, the both micelles have appropriate particle size and can be used for DOX carrier, the DLC of 4sPCL-b-PHEMA and 4sPCL-b-PDEAM-b-PHEMA micelles were 21.30% and 27.25%, respectively. The release process showed that, comparing to the diblock star copolymer, triblock star copolymer had more significant pH-sensitivity. The cumulative release percentage of DOX in triblock micelles gradually increased with the decrease of pH. The cumulative release percentage of 14 h at pH 2.2 was about 80%, which had significant pH-sensitivity. A pH-sensitive four-arm star copolymer was synthesized, which can be used as drug delivery system for further research.

Controlled Arrangement of Gold Nanoparticles on Planar Surfaces via Constrained Dewetting of Surface-Grafted RAFT Polymer.

Linear and four-arm star polystyrene samples prepared by RAFT polymerization were grafted to gold surfaces directly via their thiocarbonylthio-end groups. Nanoscale polymer patterns were subsequently formed via constrained dewetting. The patterned polymer films then served as a template for the precise arrangement of gold nanoparticles in a monolayer with a well-defined and regular structure. Using star polymers as a linker between the planar gold surface and the particles, the structural stability of the arranged particles can be further enhanced. The surface-bound nanocomposite films made of polymer and nanoparticles can also reversibly switch their nanostructures by simple wetting or dewetting treatment.

Microstructural characterization of a star-linear polymer blend under shear flow by using rheo-SANS

We present an investigation into the dynamic relaxation mechanisms of a polybutadiene blend composed of a four-arm star (10 wt. %) and a linear polymer matrix in the presence of an applied shear flow. Our focus was the response of the star polymer, which cannot be unambiguously assessed via linear viscoelastic measurements since the signature of the star polymer can barely be detected due to the dominant contribution of the linear matrix. By utilizing small-angle neutron scattering (SANS) coupled with a Couette shear device and a deuterated matrix polymer, we investigated the dynamics of the minority star component of the blend. Our results confirm that the stars deform anisotropically with increasing shear rate. We have compared the SANS data with predictions from the well-established scattering adaptation of the state-of-the-art tube model for entangled linear polymer melts undergoing shear, i.e., Graham, Likhtman, Milner, and McLeish (GLaMM) approach, appropriately modified following earlier studies in order to apply to the star. This modified model, GLaMM-R, includes the physics necessary to understand stress relaxation in both the linear and nonlinear flow regimes, i.e., contour length fluctuations, constraint release, convective constraint release, and chain retraction. The full scattering signal is due to the minority star component and, although the contribution of the linear chains is hidden from the neutron scattering, they still influence the star polymer molecular dynamics, with the applied shear rate ranging from approximately 8 to 24 s−1, below the inverse relaxation time of the linear component. This study provides another confirmation that the combination of rheology and neutron scattering is an indispensable tool for investigating the nonlinear dynamics of complex polymeric systems.

Smart micelles self-assembled from four-arm star polymers as potential drug carriers for pH-triggered DOX release

A series of amphiphilic four-arm star polymers poly(e-caprolactone)-b-poly(2-(diethylamino)ethylmethacrylate) (4AS-PCL-b-PDMAEMA) were developed by a combination of ring opening polymerization (ROP) and continuous activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP). The molecular structures of the copolymers were confirmed with 1H NMR, FT-IR and gel permeation chromatography (GPC). The critical micelle concentration (CMC) values of the star polymers in aqueous solution were extremely low (3.0–4.1 mg/L).In aqueous solution, the copolymers self-assemble into blank and doxorubicin (DOX)-loaded micelles with an average size of 56–150 nm measured by scanning electron microscopy (SEM) and dynamic light scattering (DLS), which consist of outer shell with blocks PDMAEMA and inner core with blocks PCL. The core block PCL was used to contain drug molecules and shell block PDMAEMA could protonize responding to acidic condition. The in vitro release behavior showed that the micelle structure remains basically unchanged, and the drug cumulative release was only 24.5% at pH 7.4, and at pH 5.0, PDMAEMA is protonated and the micelles swell, which accelerates the release rate of DOX and the cumulative release is higher than pH 7.4. Through the semi-empirical model, the mechanism of drug release process was analyzed, and the mechanism of drug release from polymer micelles was further explored. The in vitro cytotoxicity indicated that the 4AS-PCL-b-PDMAEMA materials had low toxicity, but the DOX-loaded micelles had an obvious inhibitory effect on the HepG2 cells. The results indicate the four-arm star polymers 4AS-PCL-b-PDMAEMA may be a potential drug carriers for pH-triggered DOX release.

Dual-Responsive Cross-Linked Micelles from Amphiphilic Four-Arm Star Copolymers with Different Block Ratios for Triggering DOX Release

The four-arm star copolymers poly(methacrylic acid)-poly(2-hydroxyethyl methacrylate-disulfide~)-poly(poly(ethylene glycol) methyl ether methacrylate) (4AS-PMAAx-(PHEMA-SS~)y-PPEGMAz) with four different block ratios were synthesized and could self-assembled into cross-linked polymer micelles for the exploration of the structure-property relationship. The cross-linked polymer micelles in aqueous solution had low critical micelle concentration (CMC) values (1.9-4.6 mg/L), which exhibited better stability than non-cross-linked micelles. The CMC value decreased with the increase of the length of inner PMAA core and hydrophobic PHEMA cross-linked middle layer. The blank and doxorubicin (DOX)-loaded micelles with different block ratios were prepared by dialysis with the particle sizes of 120-240 nm. The longer inner PMAA core and cross-linked middle layer enhanced the drug loading content (DLC) results and led to relatively bigger particle sizes of polymer micelles. The in vitro DOX release data revealed that DOX-loaded micelles had low DOX cumulative release percentages of 18-37% after 110 h at pH 7.4, but up to 83-90% when introducing reductant GSH at pH 5.0. The 4AS-PMAA21.2-(PHEMASS≈)13.1-PPEGMA5.1 micelles with the longest PMAA core had the largest cumulative release of 90.1%. The DOX release process and mechanism of the micelles at different conditions fitted well with the semi-empirical equation. Overall, the results demonstrated that the block ratios and pH/redox-responsiveness of these four-arm star copolymers could be well-controlled and their self-assembled cross-linked micelles as anticancer drug carrier system could be improved by optimizing the different ratios.

Synthesis of zinc-porphyrin based multi-arm star Poly(N-isopropylacrylamide) and the effect of topology on photocatalytic performance

Zinc-porphyrin based multi-arm star polymers (ZnTHPP-(PNIPAM-Br)n) with poly(N-isopropylacrylamide) as n = 1–4 arms respectively were successfully prepared via atom transfer radical polymerization (ATRP) initiated by four zinc porphyrin initiators (ZnTHPP-nBr) with n = 1–4 initiator groups accordingly, and these structures of star-shaped polymers were characterized by FT-IR, UV–Vis, 1H NMR and GPC. They were used as photocatalyst for the degradation of Rhodamine B(RhB) under visible-light irradiation to study the effect of topology on photocatalytic activity, it was found as the number of arms increased, the photocatalytic activity and stability of the star polymer increased. The four-arm star polymer (ZnTHPP-(PNIPAM-Br)4) was shown the best photocatalytic performance and stability, could be recycled by warming.

R-RAFT or Z-RAFT? Well-Defined Star Block Copolymer Nano-Objects Prepared by RAFT-Mediated Polymerization-Induced Self-Assembly

Reversible addition-fragmentation chain transfer (RAFT)-mediated polymerization-induced self-assembly (PISA) has served as a versatile platform for the large-scale preparation of block copolymer nano-objects with a diverse set of morphologies. However, almost all PISA formulations are focused on the syntheses of linear block copolymers rather than star block copolymers. Owing to the asymmetrical structure of the RAFT agent, herein, we report a direct comparison between Z-RAFT (the nonfragmenting group attached to the core) and R-RAFT (the fragmenting group attached to the core) strategies for preparing well-defined star block copolymer nano-objects via RAFT-mediated PISA. We showed that the Z-RAFT strategy is a more suitable strategy for the preparation of star block copolymer nano-objects without compromising the control over the molecular weight and morphology. A binary mixture of Z-type and R-type tetrafunctional macro-RAFT agents was used to control the morphologies of star block copolymer assemblies. The effect of numbers of the RAFT group on polymerization kinetics and morphologies of block copolymer nano-objects was also investigated in detail. Finally, (ABC)4 four-arm star triblock copolymer vesicles were prepared by seeded RAFT dispersion polymerization of solvophobic and solvophilic monomers using (AB)4 four-arm star diblock copolymer vesicles as seeds. This research not only expands the scope for preparing well-defined star block copolymer nano-objects but also provides important insights into the effect of the polymer architecture on RAFT-mediated PISA.

Formation of well-organized, concentric-ringed spherulites of four-arm star symmetric PEO-b-PCL via confined evaporative crystallization

Toluene solvent-assisted topology confinement facilitates PCL block templated rhythmic crystallization into concentric-ringed spherulites of star symmetric P(EO2.5k-b-CL2.7k)4.

Assessing the Range of Validity of Current Tube Models through Analysis of a Comprehensive Set of Star–Linear 1,4-Polybutadiene Polymer Blends

We blend newly synthesized nearly monodisperse four-arm star 1,4-polybutadienes with various well-entangled linear polymers, confirming the conclusions in Desai et al. [ Macromolecules201649 (13)49...

Effect of Macromonomer Branching on Structural Features and Solution Properties of Long-Subchain Hyperbranched Polymers: The Case of Four-Arm Star Macromonomers

Following our recent work on AB3 macromonomer systems (Macromolecules, 2019, 52, 1065–1082), this work focuses on AB4 systems to further explore the effect of macromonomer branching on structural features and solution properties of long-subchain hyperbranched polymers (LHPs). Compared with ABn macromonomers (n < 3), our results reveal that AB4 macromonomers tend to undergo less interchain coupling but more intrachain self-cyclization during “click” polyaddition process. Via the triple-detection SEC characterization of resultant LHPs, both intrinsic viscosity ([η]) and radius of gyration (Rg) are found to be scaled to the macromonomer molar mass (Mmacro) and the total molar mass (Mhyper) as [η]=Kη,AB4MhyperνMmacroμ (ν ≈ 0.40, μ ≈ 0.25, and Kη, AB4 ≈ 1.72 × 10–2 mL/g) and Rg=HR,AB4MhyperαMmacroβ (α ≈ 0.52, β ≈ 0.20, and HR, AB4 ≈ 1.54 × 10–3 nm). The obtained results clearly demonstrate that (i) the self-similar behavior of LHPs is almost independent of the detailed structures of the macromonomer (AB2, AB3, and AB4) and (ii) the AB4 system presents Mmacro-dependent solution properties (μ ≈ 0.25 and β ≈ 0.20), which is similar to the result for the AB2 system but totally different from the observed Mmacro independence for the AB3 system (μ ≈ 0 ≈ β). A combination of experimental observation and Langevin dynamics simulation of ABn dendrimers and LHPs unambiguously verifies our previously proposed conjecture; that is, the unique synergistic effect of segment back-folding and segment interpenetration exists only in moderately branched LHP systems but not in highly branched ABn LHP systems (n > 4).

Carboxylate Salts as Ideal Initiators for the Metal-Free Copolymerization of CO2 with Epoxides: Synthesis of Well-Defined Polycarbonates Diols and Polyols

Tetrabutylammonium carbonate (TBAC) which is obtained by treating CO2 with tetrabutylammonium hydroxide is shown to perform as an ideal difunctional initiator for the copolymerization of carbon dioxide (CO2) and propylene oxide (PO) in the presence of triethylborane (TEB). In this system, CO2 thus serves as the initiating moiety of its own copolymerization with epoxides when used in the form of a carbonate salt. Based on this remarkable result, mono-, tri-, and tetrafunctional ammonium carboxylate initiators and also other difunctional carboxylate initiators were synthesized and used for the synthesis of well-defined ω-hydroxyl-polycarbonates with linear and star structures. Well-defined telechelics, three- and four-armed star samples of molar mass varying from 1 kg/mol to 10 kg/mol, with around 95% carbonate content, were successfully synthesized. The structure of the obtained polycarbonate ω-polyols were characterized by 1H NMR, MALDI-TOF, and GPC. The terminal hydroxyl functionality of polycarbonate diol was further used for polycondensation with diisocyanates to afford polyurethanes. Finally, taking TBAC as an example, the recyclability of this ammonium-based initiator is demonstrated for the preparation of polycarbonate diols.

Isomeric and structural effects in polymer cononsolvent systems

Thermoresponsive poly (N-isopropylacrylamide) (PNIPAM) has many analogs that exhibit cononsolvency behavior in mixtures of water and an alcohol. Cononsolvency is characterized by a combination of good solvents for a polymer that results in decreased solubility. In this work, the cononsolvency behavior of linear PNIPAM, four-arm (4f) star PNIPAM, and linear poly(N-n-propylacrylamide) (PNnPAM), with terminal groups that vary in hydrophobicity, were investigated in mixtures of water and propanol. Polymers were synthesized by RAFT polymerization and subsequently functionalized with one pot aminolysis/thiol-ene chemistry. Turbidimetry measurements and dynamic light scattering (DLS) were used to study the cononsolvency behavior by determining the critical solution temperature (Tc). The measurements show that the size and shape of the hydrophobic region of both the solvent and n-alkyl acrylamide monomer affect Tc and the phase transition behavior. The findings suggest methods to impart multiresponsiveness to soft material systems.

Two-dimensional scattering patterns of coarse-grained molecular dynamics model of filled polymer gels during uniaxial expansion

In this study, coarse-grained molecular dynamics (CGMD) simulations of polymer network gels filled with spherical nanoparticles (NPs) were performed, and two-dimensional (2D) small-angle scattering patterns of the NPs during uniaxial elongation were examined. To create filled polymer network gels, mixtures of polymer chains, four-arm star chains, and NPs, and pseudo-reaction processes were considered. Active sites of the polymer chains, four-arm star chains, and surface particles of the NPs were reacted to create bonds. After this pseudo-reaction process, it was evaluated whether the obtained polymer network was a sol or gel. The gel polymer network obtained in this study was not percolated as a graph but was able to transmit force through catenane-like linkages. This paper named this gel as “topological-percolation.” Further, 2D scattering patterns (2DSPs) were calculated using three-dimensional fast Fourier transformation for efficient computing on a large-scale system. The obtained 2DSPs were quite different for the sol and gel, and depended on the properties of the crosslinked polymer networks. It is confirmed that changes in 2DSPs can be predicted by CGMD simulations because different patterns can be observed with different pre-polymer distributions, different chain lengths, and different chain densities.

Characterization of the Kinetics and Mechanism of Degradation of Human Mesenchymal Stem Cell-Laden Poly(ethylene glycol) Hydrogels.

Human mesenchymal stem cells (hMSCs) are motile cells that migrate from their native niche to wounded sites where they regulate inflammation during healing. New materials are being developed as hMSC delivery platforms to enhance wound healing. To act as an effective wound healing material, the hydrogel must degrade at the same rate as tissue regeneration, while maintaining a high cell viability. This work determines the kinetics and mechanism of cell-mediated degradation in hMSC-laden poly(ethylene glycol) (PEG) hydrogels. We use a well-established hydrogel scaffold that is composed of a backbone of four-arm star PEG functionalized with norbornene that is cross-linked with a matrix metalloproteinase (MMP) degradable peptide. This peptide sequence is cleaved by cell-secreted MMPs, which allow hMSCs to actively degrade the hydrogel during motility. Three mechanisms of degradation are characterized: hydrolytic, noncellular enzymatic and cell-mediated degradation. We use bulk rheology to characterize hydrogel material properties and quantify degradation throughout the entire reaction. Hydrolysis and noncellular enzymatic degradation are first characterized in hydrogels without hMSCs, and follow first-order and Michaelis-Menten kinetics, respectively. A high cell viability is measured in hMSC-laden hydrogels, even after shearing on the rheometer. After confirming hMSC viability, bulk rheology characterizes cell-mediated degradation. When comparing cell-mediated degradation to noncellular degradation mechanisms, cell-mediated degradation is dominated by enzymatic degradation. This indicates hydrogels with hMSCs are degraded primarily due to cell-secreted MMPs and very little network structure is lost due to hydrolysis. Modeling cell-mediated degradation provides an estimate of the initial concentration of MMPs secreted by hMSCs. By changing the concentration of hMSCs, we determine the initial MMP concentration increases with increasing hMSC concentration. This work characterizes the rate and mechanism of scaffold degradation, giving new insight into the design of these materials as implantable scaffolds.

Enzymatic Degradation of Star Poly(ε-Caprolactone) with Different Central Units.

Four-arm star poly(ε-caprolactone) with a central poly(ethylene glycol) PEG unit bridged with 2,2-bis(methyl) propionic acid, (PCL)2-b-PEG-b-(PCL)2, and six-arm star PCL homopolymer with a central dipentaerythritol units were hydrolysed using a lipase from Pseudomonas cepacia and the Thermobifida cellulosilytica cutinase Thc_Cut1. For comparative analysis, Y-shaped copolymers containing methylated PEG bridged with bisMPA, MePEG-(PCL)2, and linear triblock copolymers PCL-b-PEG-b-PCL were also subjected to enzymatic hydrolysis. The hydrophilic nature of the polymers was determined using contact angle analysis, showing that a higher PEG content exhibited a lower contact angle and higher surface wettability. Enzymatic hydrolysis was monitored by % mass loss, scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). A higher rate of mass loss was found for lipase catalysed hydrolysis of those polymers with the highest PEG content, leading to significant surface erosion and increase in crystallinity within the first two days. Liquid chromatography (LC) and size exclusion chromatography (SEC) of samples incubated with the cutinase showed a significant decrease in molecular weight, increase in dispersity, and release of ε-CL monomer units after 6 h of incubation.

Anomalous Confinement Slows Surface Fluctuations of Star Polymer Melt Films.

The unusually large film thickness at which confinement effects manifest themselves in surface fluctuations of unentangled four-arm star polymers has been defined using film thicknesses from 10Rg to 107Rg. For 15k four-arm star polystyrene (SPS), confinement appears at a thickness between 112 nm (40Rg) and 72 nm (26Rg), which is remarkably larger than the thicknesses at which confinement appears for unentangled 6k linear (<15 nm, <7Rg) and 6k and 14k cyclic (24 and 22 nm, respectively) polystyrenes. Data for 15k star films can be rationalized using a two-layer model with a 17 nm (6Rg) thick highly viscous layer at the substrate, which is significantly thicker than the 1Rg thick "irreversibly adsorbed" layer. For a 29 nm (10Rg) thick film, more striking confinement occurs due to the combined influence of both interfaces. These results underscore the extraordinary role long-chain branching plays in dictating surface fluctuations of thin films.