Star-shaped Macromolecules Research Articles

Pervaporation Desulfurization of a Thiophene/n-Octane Mixture Using PPO Membranes Modified with Hybrid Star-Shaped Macromolecules

The presence of sulfur-containing impurities in petroleum naphtha causes a decrease in the cost and quality of fuels, a reduction in the service life of car engines, and an increase in the amount of harmful emissions into the atmosphere. Pervaporation is an alternative cost-effective fuel desulfurization method. In this study, hybrid membranes based on poly(2,6-dimethyl-1,4-phenylene oxide) and a star-shaped modifier for the pervaporation separation of a thiophene/n-octane model mixture is developed. Hybrid star-shaped macromolecules comprising six polystyrene arms and six poly(2-vinylpyridine)-block-poly(tert-butyl methacrylate) diblock copolymer arms grafted onto a common fullerene C60 central core are used as the modifier. Membrane structure is analyzed by scanning electron microscopy and atomic force microscopy. Thermal properties are studied by differential scanning calorimetry. The separation properties of the membranes are determined at low thiophene concentrations (up to 0.08 wt %). It is shown that the introduction of a star-shaped modifier leads to an increase in the extraction efficiency of sulfur-containing impurities from octane, which is the main fuel component.

Design of Linear and Star-Shaped Macromolecular Organic Semiconductors for Photonic Applications.

One of the most desirable and advantageous attributes of organic materials chemistry is the ability to tune the molecular structure to achieve targeted physical properties. This can be performed to achieve specific values for the ionization potential or electron affinity of the material, the absorption and emission characteristics, charge transport properties, phase behavior, solubility, processability, and many other properties, which in turn can help push the limits of performance in organic semiconductor devices. A striking example is the ability to make subtle structural changes to a conjugated macromolecule to vary the absorption and emission properties of a generic chemical structure. In this Account, we demonstrate that target properties for specific photonic applications can be achieved from different types of semiconductor structures, namely, monodisperse star-shaped molecules, complex linear macromolecules, and conjugated polymers. The most appropriate material for any single application inevitably demands consideration of a trade-off of various properties; in this Account, we focus on applications such as organic lasers, electrogenerated chemiluminescence, hybrid light emitting diodes, and visible light communications. In terms of synthesis, atom and step economies are also important. The star-shaped structures consist of a core unit with 3 or 4 functional connection points, to which can be attached conjugated oligomers of varying length and composition. This strategy follows a convergent synthetic pathway and allows the isolation of target macromolecules in good yield, high purity, and absolute reproducibility. It is a versatile approach, providing a wide choice of constituent molecular units and therefore varying properties, while the products share many of the desirable attributes of polymers. Constructing linear conjugated macromolecules with multifunctionality can lead to complex synthetic routes and lower atom and step economies, inferior processability, and lower thermal or chemical stability, but these materials can be designed to provide a range of different targeted physical properties. Conventional conjugated polymers, as the third type of structure, often feature so-called "champion" properties. The synthetic challenge is mainly concerned with monomer synthesis, but the final polymerization sequence can be hard to control, leading to variable molecular weights and polydispersities and some degree of inconsistency in the properties of the same material between different synthetic batches. If a champion characteristic persists between samples, then the variation of other properties between batches can be tolerable, depending on the target application. In the case of polymers, we have chosen to study PPV-type polymers with bulky side groups that provide protection of their conjugated backbone from π-π stacking interactions. These polymers exhibit high photoluminescence quantum yields (PLQYs) in films and short radiative lifetimes and are an important benchmark to monodisperse star-shaped systems in terms of different absorption/emission regions. This Account therefore outlines the advantages and special features of monodisperse star-shaped macromolecules for photonic applications but also considers the two alternative classes of materials and highlights the pros and cons of each class of conjugated structure.

Sequence-defined multifunctional polyethers via liquid-phase synthesis with molecular sieving.

Synthetic chemists have devoted tremendous effort towards the production of precision synthetic polymers with defined sequences and specific functions. However, the creation of a general technology that enables precise control over monomer sequence, with efficient isolation of the target polymers, is highly challenging. Here, we report a robust strategy for the production of sequence-defined synthetic polymers through a combination of liquid-phase synthesis and selective molecular sieving. The polymer is assembled in solution with real-time monitoring to ensure couplings proceed to completion, on a three-armed star-shaped macromolecule to maximize efficiency during the molecular sieving process. This approach is applied to the construction of sequence-defined polyethers, with side-arms at precisely defined locations that can undergo site-selective modification after polymerization. Using this versatile strategy, we have introduced structural and functional diversity into sequence-defined polyethers, unlocking their potential for real-life applications in nanotechnology, healthcare and information storage.

Hybrid Gas Separation Membranes Containing Star-Shaped Polystyrene with the Fullerene (C60) Core

A physicochemical study of novel hybrid polymer membranes based on polyphenylene oxide with a star-shaped modifier incorporated into the matrix has been conducted, and the transport properties of the membranes in the gas separation process have been studied. Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) has been selected as the polymer matrix because of the low cost and high mechanical strength of this material. Star-shaped macromolecules (up to 5 wt %) containing six polystyrene arms grafted onto a fullerene(C60) central core have been used as the filler. The structure and physical properties of the resulting membranes have been characterized by scanning electron microscopy, membrane density measurements, differential scanning calorimetry, and thermogravimetric analysis. Film surface has been studied by contact angle measurements. The gas separation properties of the membranes have been studied by the barometric method for the following individual gases: H2, O2, N2, and CH4. Data on the separation properties have been plotted as a Robeson diagram to compare with published data. It has been shown that the incorporation of star-shaped polystyrene into the PPO matrix leads to an improvement of the separation efficiency for selected gas pairs and an increase in selectivity compared with that of the unmodified membrane.

On Forgotten Topological Indices of Some Dendrimers Structure.

A series of previously conducted experiments pertaining to various chemicals and drugs uncover a natural linkage between the molecular structures and the bio-medical and pharmacological characteristics. The forgotten topological index computed for the molecular structures of various chemical compounds and drugs has proven significant in medical and pharmaceutical fields by predicting biological features of new chemical compounds and drugs. A topological index can be considered as the transformation of chemical structure into a real number. Dendrimers are highly-branched star-shaped macromolecules with nanometer-scale dimensions. Dendrimers are defined by three components: a central core, an interior dendritic structure (the branches), and an exterior surface with functional surface groups. In this paper, we determine forgotten topological indices of poly(propyl) ether imine, porphyrin, and zinc–porphyrin dendrimers.

On Topological Indices of Certain Dendrimer Structures

Abstract A topological index can be considered as transformation of chemical structure in to real number. In QSAR/QSPR study, physicochemical properties and topological indices such as Randić, Zagreb, atom-bond connectivity ABC, and geometric-arithmetic GA index are used to predict the bioactivity of chemical compounds. Dendrimers are highly branched, star-shaped macromolecules with nanometer-scale dimensions. Dendrimers are defined by three components: a central core, an interior dendritic structure (the branches), and an exterior surface with functional surface groups. In this paper we determine generalised Randić, general Zagreb, general sum-connectivity indices of poly(propyl) ether imine, porphyrin, and zinc-Porphyrin dendrimers. We also compute ABC and GA indices of these families of dendrimers.

Dynamics of Star-Shaped Polystyrene Molecules: From Arm Retraction to Cooperativity

The frequency ω dependent storage G′(ω) and loss G″(ω) moduli of star-shaped polystyrene (SPS) molecules of a range of functionalities f and molecular weights per arm Ma were measured under small amplitude oscillatory shear conditions. Star-shaped macromolecules are composed of an inner region, core, where the chain segments are stretched and the “packing” density is higher than that of the outer region, corona. The frequency dependencies of G′(ω) and G″(ω) for low functionality molecules (f < 8) with long arms Ma are well described by the model of Milner and McLeish, indicating that the translational dynamics are facilitated by an arm retraction mechanism. With increasing values of f and decreasing Ma the model fails—the arm retraction process is no longer valid—due largely to the increasing size of the core in relation to the overall size of the molecule. The molecules exhibit evidence of spatial structural order due to entropic, intermolecular interactions, and the translational dynamics of these molecules occur via a cooperative process, akin to that of soft colloids, for sufficiently large values of f and small Ma. The overall dynamics may be summarized in a diagram delineating different mechanisms that facilitate flow as a function of f and Ma.

Design of blue fluorescence emitter star-shaped macromolecules based on pyrene and anthracene

Star-shaped macromolecules decorated with fluorescence moieties such as pyrene and anthracene have been developed, which exhibit aggregation enhanced excimer emission (AEEE). The presented star-shaped macromolecules showed only weak emissions in the visible region in dilute solution, due to the predominance of the monomer emission, which has its maximum in the UV-A region of the spectrum. However, the strong blue visible excimer-like emission is generated in the aggregated state, by the use of poorly solubilizing solvents or high concentration. The solution of these macromolecules emitted blue light centred at 475 and 474 nm with pyrene and anthracene moieties respectively. In addition, emission efficiency increases were observed with increasing water fraction. The unique phenomenon of AEEE was verified by determining the excimer-monomer emission intensities ratio (IE/IM). TEM was used to detect the difference between the aggregation states in various THF/water ratios. The molecular structures of the fluorescent star were investigated using (MMFF).

C60 fullerene-containing polymer stars in mixed matrix membranes

Star-shaped macromolecules with C60 fullerene branching centers were used as modifiers of poly(2.6-dimethyl1.4-phenylene oxide) matrix to obtain mixed matrix membranes. Two types of star modifiers were synthesized i) six-arms star with six polystyrene arms grafted onto C60 center and ii) twelve-arms hybrid star with six polystyrene arms and six copolymer poly(2-vinylpyridine)-block-poly(tert-butylmethacrylate) arms grafted onto C60 center. The membrane structures were studied by scanning electron microscopy. The transport properties of the membranes were determined by using sorption and pervaporation tests toward methanol and ethylene glycol over a wide concentration range. All membranes showed high affinity for methanol. The separation factor reached its maximum level at 5 wt% modifier concentration in the membrane. Polar hybrid arms were shown to change the membranes’ morphologies and considerably improve their transport properties.

Charge-controlled nano-structuring in partially collapsed star-shaped macromolecules.

Hydrophobic polyelectrolytes exhibit intra-molecular nano-scale self-organization instead of macroscopic phase separation because of the interplay between short-range hydrophobic attraction and long-range electrostatic repulsion. We aim to unravel how the morphology of the intra-molecular nanostructures can be controlled through the topology of the macromolecule on one hand and by adjustable ionization on the other hand. Specifically, we focus on hydrophobic star-branched polyelectrolytes, composed of either strong or weak acidic monomers. While both collapse in a globule when uncharged, and expand to full stretching of arms at high ionization, they exhibit quite different intermediate scenarios. For the strong ones, we observe the formation of bundles of arms as the main structural motif, and for the weak ones the intramolecular micelle-like structure is found at the same overall charge of the macromolecule. Here intramolecular disproportionation leaves some arms in a collapsed virtually neutral core, while others are substantially ionized and stretched in the corona.

Polymer films of nanoscale thickness: linear chain and star-shaped macromolecular architectures

Abstract

Star-Shaped Macromolecules with the Core of Hexakis-(fluoren-2-yl)benzene and the Periphery of Pyridine: Synthesis and Application as Solution-Processable Electron-Transport Materials.

Three new star-shaped macromolecules with hexakis(fluoren-2-yl)benzene as the core and pyridine as the periphery (2Py-HFB, 3Py-HFB, and 4Py-HFB) are synthesized and characterized. The synthetic conditions of octacarbonyldicobat-catalyzed cycloaddition reaction for different alkyne precursors are investigated. The coordination interaction between the pyridine ring of alkyne precursor and the cobalt catalyst may result in very low yield of the cyclotrimerization product. However, with the increase of the catalyst loading, the yields of the intermediates of cyclopentadienone are enhanced. Then, the desired cyclotrimerization products can be obtained by the Diels-Alder reactions of cyclopentadienone with acetylene in good yield. These new compounds exhibit good thermal stability and favorable electron affinity. By using the new compounds as electron-transporting materials, all-solution-processed phosphorescent organic light-emitting devices (OLEDs) show good performance with a maximum current efficiency of 5.6 cd A(-1) and maximum external quantum efficiency of 4.68%.

Vitrification of Thin Polymer Films: From Linear Chain to Soft Colloid-like Behavior

We show that the vitrification of star-shaped polystyrene (PS), of functionality f and molecular weight per arm Mwarm, thin films supported by silicon oxide, SiOx, is strongly dependent on Mwarm and f. When f is small, the vitrification behavior is similar to that of linear-chain PS where the average glass transition, Tg, decreases with decreasing film thickness (ΔTg < 0). However, for sufficiently large f and small Mwarm, Tg becomes independent of film thickness (ΔTg ≈ 0). In this region, where ΔTg ≈ 0, the star-shaped macromolecules self-assemble into ordered, periodic structures, similar to that of soft spheres or colloids, as revealed by simulations and experiments. This is identified as the soft-colloidal region. The transition from the linear-chain-like to the soft-colloidal-like region occurs over an intermediate range of functionalities and arm lengths; throughout this transition range ΔTg > 0. We show that the overall vitrification behavior of these thin film star-shaped polymers is due to competing entropic interactions associated with changes in f and Mwarm. The vitrification behavior of thin star-shaped PS films on SiOx is summarized in terms of a “diagram of states”.

Mixed matrix membranes with hybrid star-shaped macromolecules for mono- and dihydric alcohols pervaporation

Mixed matrix membranes were obtained by including hybrid star-shaped macromolecules (HSM) in a matrix of polyphenylene oxide (PPO). HSM filler was a twelve-arms star with six polystyrene arms and six diblock copolymer poly(2-vinylpyridine)-b-poly(tert-butylmethacrylate) arms grafted onto a fullerene C60 center. The PPO/HSM membranes structure was studied by scanning electron microscopy and the energy dispersive spectrometry. Mass transfer through PPO/HSM membranes was studied by sorption and pervaporation tests toward methanol and ethylene glycol. In the pervaporation of methanol – ethylene glycol mixture over the concentration range of 5–30wt.% methanol in feed, all membranes showed high affinity to methanol and produced permeate concentrated in methanol. The total flux through the membrane increased with the growth of both the HSM content in membrane and the methanol concentration in the feed. Separation factor achieved maximal magnitude at the filler concentration in membrane equal to 5wt.% HSP. To evaluate intrinsic properties of the penetrant – membrane system, permeability and selectivity were calculated. The dependence of methanol permeability on methanol concentration in the feed was opposite to that of the flux. It was shown that PPO/HSM membranes exhibit the best properties when the feed enriched with ethylene glycol.

Molecular and structural characterization of hybrid poly(ethylene oxide)-polyhedral oligomeric silesquioxanes star-shaped macromolecules.

Octafunctionalized spherosilsesquioxanes (Q8M8(H)), decorated with Si-H functions, could be used to design, by coupling via hydrosilylation with α-methoxy-ω-undecenyl poly(ethylene oxide)s (PEOs), organic-inorganic nanocomposite structures. (1)H, (13)C, and (29)Si NMR; size exclusion chromatography; and Fourier transfrom infrared spectroscopy were used to follow the grafting reaction and determine the molar mass and the functionality of the different species. Hybrid star-shaped poly(ethylene oxide)s of precise molar mass and functionality could be isolated by fractional precipitation of the raw reaction product. Absolute molar masses of the purified star-shaped PEOs, calculated with the assumption of a functionality of 8, were comparable when measured by light scattering in methanol and by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Small-angle X-ray scattering was employed to determine their molecular and structural characteristics, representing the versatility and innovative aspect to this study. Both differential scanning calorimetry and optical microscopy were utilized to elaborate and analyze the thermal properties and crystallization, respectively, of the hybrid stars. Further ongoing work is being carried out currently to investigate and foresee the use of longer PEO branches onto the core.

Refractive Indexes of Porous Thin Films Prepared From Organic-templated Polymethylsilsesquioxanes

The rapidly developing microelectronics industry has shown a strong demand for high performance materials to improve device performance [1,2]. Nanoporous silicate thin films are considered as important materials for advanced electronics. The formation of air-filled nanopores is a key technology for preparing insulating materials [3-12]. Polymethylsilsesquioxane (PMSSQ), with the empirical formula of (CH3-SiO1.5)n, is a potential candidate for low dielectric spin-on glass [13]. It has the characteristics of low dielectric constant ranging from 2.7 to 2.9, high thermal stability, low moisture uptake, good adhesion to metal, and excellent mechanical properties [11,14]. In addition, star-shaped macromolecules, such as hyperbranched poly(ecaprolactone) (PCL), are generally used as an organic template to generate nanometer-sized pores in the matrix polymer [11,15]. The organic templates dispersed in the polymer matrix are required to completely decompose at a relatively low temperature range (200°C ~ 400°C), and after the thermal calcination process, the derived nanopores need to be well distributed as a closedcell structure in the polymer matrix. The air-filled nanopores in a thin film can be generated by a method that disperses organic templates into an insulating matrix material, and decomposes them at elevated temperature. The mechanical strength of an insulating material will undoubtedly decrease, and the size and distribution of the as-made pores cannot be uniform, due to the organic templates, which leave pores behind from the thermal decomposition. Moreover, if the pores agglomerate, the insulating material may break down at low voltage. Therefore, the thermal decomposition and dispersibility of the organic templates in matrix polymers, and control of the nanopore generation, should be suitably considered for the structural design of organic templates. In order to effectively generate and thoroughly distribute nanopores in the whole matrix film, the organic templates should be completely dispersed in matrix polymers, and the miJung-Soo Kim and Dae-Geun Nam Energy Component and Material R&BD Group, Korea Institute of Industrial Technology, Busan 618-757, Korea

ChemInform Abstract: Review: Mixed‐Valent Metallodendrimers: Design and Functions

AbstractReview: 56 refs.

Wetting of Macromolecules: From Linear Chain to Soft Colloid-Like Behavior

The ability to control the wetting properties of a polymeric liquid on a given surface is important for several emerging technological applications including protective coatings, lubricants and sensors. Here we show that star-shaped polystyrene (PS) molecules exhibit notably different wetting properties than their linear analogues of the same chemical structure and their wetting properties can be controlled through changes of their functionality f (number of arms per molecule). Unlike linear chains, the wetting of star-shaped macromolecules is determined by the competition between entropic forces. Wetting is enhanced due to reductions in the loss of entropy upon adsorption of the stars with increasing f; soft colloidal-like entropic repulsion effects suppress the ability of the stars to exhibit a high degree of surface adsorption and to efficiently pack, for sufficiently large values of f. This phenomenon is manifested in the existence of a minimum in the macroscopic contact angle and other related microscopic parameters embodied in the effective interface potential as a function of f.

Cathepsin B Degradable Star-Shaped Peptidic Macromolecules for Delivery of 2-Methoxyestradiol

2-Methoxyestradiol (2ME), a natural metabolite of estradiol, has antiproliferative and antiangiogenic activity. However, its clinical success is limited due to poor water solubility and poor pharmacokinetic parameters suggesting the need for a delivery vehicle. In this study we evaluated cathepsin B degradable star-shaped peptidic macromolecules (SPMs) that can potentially be used to create higher generation and high molecular weight peptidic polymer as delivery vehicle of 2ME. Two peptidic macromolecules having positively charged amine (ASPM) or negatively charged carboxyl surface groups (CSPM) were synthesized and evaluated for their degradation in the presence of cathepsin B and stability in the presence of neutral or acidic buffer and serum. Both ASPM and CSPM degraded rapidly in the presence of cathepsin B. Both were stable in neutral and acidic buffer whereas only CSPM exhibited substantial stability in the presence of serum. Both macromolecules were nontoxic toward breast cancer cells whereas 2ME-containing macromolecules exhibited antiproliferative activity in the micromolar range. Overall, results from the current study indicate that tetrapeptide GFLG can be used to create star-shaped macromolecules that are degraded in the presence of cathepsin B and have the potential to be developed as delivery vehicles of 2ME.

ChemInform Abstract: Star‐Shaped Polymers with the Fullerene C60 Branching Center

AbstractReview: syntheses, functionalization and properties; 92 refs.