Multiarm Star Research Articles

Rapid Synthesis and MALDI‐ToF Characterization of Poly(ethylene oxide) Multiarm Star Polymers

AbstractMultiarm PEO star polymers with a purely aliphatic polyether structure have been synthesized using hyperbranched polyglycerol (PG) with different molecular weights as a multifunctional initiator. Different degrees of deprotonation of the initiator were studied with respect to molecular weight control. The results show that the degree of deprotonation is a crucial parameter for the synthesis of well‐defined polymers with controlled molecular weights. Partial deprotonation of the PG hydroxyl groups (5–8%) was proven to represent an optimum for the synthesis of star polymers with molecular masses close to the theoretical values. Molecular weights of the stars ranged between 9 000 and 30 000 g · mol−1. MALDI‐ToF spectra confirmed that the PEO arms in the star polymers possess homogeneous lengths.magnified image

Dendrimer-based coatings for surface modification of polyamide reverse osmosis membranes

The first use of dendrimer nanotechnology in the modification of reverse osmosis (RO) membranes is reported. The effects of dendrimer surface coatings on the advancing water contact angle (i.e., coatings’ hydrophilicity), permeate flux and % salt rejection of commercial polyamide membranes were studied. The membranes were coated by in situ crosslinking of amine-functional polyamidoamine (PAMAM) dendrimers and PAMAM–polyethylene glycol (PAMAM–PEG) multi-arm stars with difunctional PEG crosslinkers. The resulting coatings significantly reduced contact angles of membrane surfaces without affecting their % salt rejection and only moderately reducing their permeate fluxes. Lower contact angles indicated more hydrophilic membranes with the potential for increased resistance to fouling by hydrophobic foulants, such as biofoulants and organic pollutants.

Multiarm star block and multiarm star mixed-block copolymers via azide-alkyne click reaction

The synthesis of multiarm star block (and mixed-block) copolymers are efficiently prepared by using Cu(I) catalyzed azide-alkyne click reaction and the arm-first approach. α-Silyl protected alkyne polystyrene (α-silyl-alkyne-PS) was prepared by ATRP of styrene (St) and used as macroinitiator in a crosslinking reaction with divinyl benzene to successfully give multiarm star homopolymer with alkyne periphery. Linear azide end-functionalized poly(ethylene glycol) (PEG-N3) and poly (tert-butyl acrylate) (PtBA-N3) were simply clicked with the multiarm star polymer described earlier to form star block or mixed-block copolymers in N,N-dimethyl formamide at room temperature for 24 h. Obtained multiarm star block and mixed-block copolymers were identified by using 1H NMR, GPC, triple detection-GPC, atomic force microscopy, and dynamic light scattering measurements. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 99–108, 2010

Supramacromolecular chemistry: Self-assembly of polystyrene-based multi-armed pseudorotaxane star polymers from multi-topic C60derivatives

The author of this article has notified us that Schemes 2 and 3 are incorrect. The corrected Schemes are now shown below. We apologize for any inconvenience this error may have caused.

Modulating the guest encapsulation and release properties of multi‐arm star polyethylenimine‐block‐poly(ε‐caprolactone)

The guest encapsulation and release properties of the amphiphilic multi-arm star polyethylenimine-block-poly(ε-caprolactone) copolymer as nanocapsule could be tuned finely. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Sea stars from Middle Jurassic lagerstätte of La Voulte-sur-Rhône (Ardèche, France)

The Callovian Lagerstätte of La Voulte-sur-Rhône represents deep (bathyal) environments at the bottom of a submarine rocky slope. Three sea star body fossils partially replaced with pyrite were collected from clay beds. A small, stellate specimen is assigned to Terminaster cancriformis, a species already known from Jurassic clay deposits of Germany, Switzerland and England. the morphology of Terminaster illustrates mostly plesiomorphic features for post-Paleozoic sea stars, and the genus may be considered either as a paedomorphic Zoroasteridae or a stem group Forcipulatacea. Two specimens of a large multiarmed sea star are assigned to Decacuminaster n. gen. solaris n. sp., characterized by a lack of abactinal plates and an axial skeleton typical of the Velatida. the two genera Xandarodaster and Plesiastropecten are close to Decacuminaster and must be transferred to the Velatida. in modern oceans, the association of a multiarmed Velatida and a zoroasterid-like species would be typical of bathyal/abyssal environments, which suggests that these groups explored deep sea environments early in their history.

Biodegradable and biocompatible multi-arm star amphiphilic block copolymer as a carrier for hydrophobic drug delivery

Multi-arm star amphiphilic block copolymers (SABCs) with approximately 32 arms were synthesized and characterized for drug delivery applications. A hyperbranched polyester, boltorn ® H40 (H40), was used as the macroinitiator for the ring-opening polymerization of ɛ-caprolactone (ɛ-CL). The resulting multi-arm H40-poly(ɛ-caprolactone) (H40-PCL-OH) was further reacted with carboxyl terminated methoxy poly(ethylene glycol) (MPEG-COOH) to form H40-PCL- b-MPEG copolymers. The resulting SABCs were characterized by 1H NMR spectroscopy and gel permeation chromatography (GPC). The critical aggregation concentration (CAC) of H40-PCL- b-MPEG was 3.8 mg/L as determined by fluorescence spectrophotometry. Below the CAC, stable unimolecular micelles were formed with an average diameter of 18 nm as measured by TEM. Above the CAC, unimolecular micelles exhibited agglomeration with an average diameter of 98 nm. The hydrodynamic diameter of these agglomerates was found to be 122 nm, as measured by dynamic light scattering (DLS). The drug loading efficacy of the H40-PCL- b-MPEG micelles was 26 wt%. Drug release study showed an initial burst followed by a sustained release of the entrapped hydrophobic model drug, 5-fluorouracil, over a period of 9–140 h. These results indicate that the H40-PCL- b-MPEG micelles have great potential as hydrophobic drug delivery carriers.

Structure and Viscoelasticity of Mixed Micelles Formed by Poly(ethylene oxide) End Capped with Alkyl Groups of Different Length

Poly(ethylene oxide) (PEO) end capped with an alkyl group is a highly asymmetric diblock copolymer that forms spherical micelles in aqueous solution resembling multiarm star polymers. The effect of varying the length of the alkyl end group on the structure and viscoelasticity was investigated for pure and mixed micelle suspensions. The aggregation number (p) of the micelles increased and the critical association concentration (CAC) decreased with increasing the length of the end group. At high concentrations a discontinuous reversible liquid-solid transition was observed below a critical temperature (Tc) that increased with increasing length of the end group. Mixing end-capped PEO with different alkyl lengths led first to formation of the micelles by polymers with the lowest CAC into which the other polymers were incorporated when the concentration was increased. The viscoelastic properties at high concentrations are the same for pure systems and mixtures with the same average length of the alkyl end group.

Multiarm star block copolymers via Diels‐Alder click reaction

AbstractTwo types of multiarm star block copolymers: (polystyrene)m‐poly(divinylbenzene)‐poly(methyl methacrylate)n, (PS)m‐polyDVB‐(PMMA)n and (polystyrene)m‐poly(divinylbenzene)‐poly(tert‐butyl acrylate)k, (PS)m‐polyDVB‐(PtBA)k were successfully prepared via a combination of cross‐linking and Diels–Alder click reactions based on “arm‐first” methodology. For this purpose, multiarm star polymer with anthracene functionality as reactive periphery groups was prepared by a cross‐linking reaction of divinyl benzene using α‐anthracene end functionalized polystyrene (PS‐Anth) as a macroinitiator. Thus, obtained multiarm star polymer was then reacted with furan protected maleimide‐end functionalized polymers: PMMA‐MI or PtBA‐MI at reflux temperature of toluene for 48 h resulting in the corresponding multiarm star block copolymers via Diels–Alder click reaction. The multiarm star and multiarm star block copolymers were characterized by using 1H NMR, SEC, Viscotek triple detection SEC (TD‐SEC) and UV. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 178–187, 2009

Influence of some reaction parameters on the synthesis of the first generation of stars with arms composed of poly(oxyethylene glycol)

The syntheses of the multiarm stars resulting from the reaction of oligoalcohols with diepoxides have been studied. MPEG 550,750, and 2000 [MPEG = monomethyl ether of poly(oxyethylene glycol)] as precursors of arms and diglycidyl ethers of ethylene (DGEG) or neopentyl glycol (DGNG) as the core forming diepoxides were used. The increase in M n , polydispersity (MWD) and the number of arms (f) in stars with the increase in the concentration of reagents and [diepoxide] o / [MPEG] o ratio, and the decrease in temperature were observed. The final content of unreacted MPEG lowers with increasing concentration of reagents, proportion of the alkoxide groups in the initiator ([RO - ]/[RO - + ROH]), length of MPEG chain, and [diepoxide] o /[MPEG] o ratio and also with decreasing reaction temperature.

Preparation of star block copolymers with polystyrene‐block‐poly(ethylene oxide) as side chains on hyperbranched polyglycerol core by combination of ATRP with atom transfer nitroxide radical coupling reaction

AbstractThe star block copolymers with polystyrene‐block‐poly(ethylene oxide) (PS‐b‐PEO) as side chains and hyperbranched polyglycerol (HPG) as core were synthesized by combination of atom transfer radical polymerization (ATRP) with the “atom transfer nitroxide radical coupling” (“ATNRC”) reaction. The multiarm PS with bromide end groups originated from the HPG core (HPG‐g‐(PS‐Br)n) was synthesized by ATRP first, and the heterofunctional PEO with α‐2,2,6,6‐tetramethylpiperidinyl‐1‐oxy group and ω‐hydroxyl group (TEMPO‐PEO) was prepared by anionic polymerization separately using 4‐hydroxyl‐2,2,6,6‐tetramethylpiperidinyl‐1‐oxy (HTEMPO) as parents compound. Then ATNRC reaction was conducted between the TEMPO groups in PEO and bromide groups in HPG‐g‐(PS‐Br)n in the presence of CuBr and pentamethyldiethylenetriamine (PMDETA). The obtained star block copolymers and intermediates were characterized by gel permeation chromatography, nuclear magnetic resonance spectroscopy, fourier transform‐infrared in detail. Those results showed that the efficiency of ATNRC in the preparation of multiarm star polymers was satisfactory (>90%) even if the density of coupling cites on HPG was high. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6754–6761, 2008

Effect of the degree of autotomy on feeding, growth, and reproductive capacity in the multi-armed sea star Heliaster helianthus

The term autotomy refers to the process by which some species lose limbs or parts of limbs in response to adverse biotic or abiotic conditions, as for example, predation or abnormally high temperatures. The multi-armed sea star Heliaster helianthus is a key predator of the intertidal and the shallow rocky subtidal communities of north-central Chile. Natural populations of this sea star have been found with up to 60% of the individuals showing some degree of autotomy. The present study evaluated the effects of autotomy on feeding rate and growth of juvenile and adult H. helianthus after experimentally induced autotomy of 17% and 33% of their arms, as well as on the energy content of the pyloric caeca and gonads of adults during the reproductive period. Experimental juvenile sea stars were maintained and fed in the laboratory over a period of five months and adult sea stars for one month, Intact individuals were maintained as parallel controls. The results showed that juveniles undergoing 33% autotomy decreased their feeding rates, and as a consequence showed lowered net individual growth. In contrast, adults with 17% and 33% autotomy showed marked reductions in feeding. The results showed that autotomized adults had between five and seven times lower contents of carbohydrates, lipids, and proteins (and thus energetic content) in their pyloric caeca and gonads. The loss of the arms not only decreased the capacity for feeding in sea stars, but also allocated energy away from growth and reproduction into the process of regeneration of arms. This suggests that autotomy reduces the fitness of H. helianthus. Growth was reduced in the juveniles, while adults became limited in their ability to store energy which then limited their reproductive potential. Finally, based on the important effect of autotomy on reducing the feeding capacity of H. helianthus, the role of this sea star as a predator in the environment may be strongly affected.

N-Heterocyclic carbene catalysis - from simple organic reactions to polymerization of cyclic esters

This contribution aims at reporting on very recent advances achieved in controlled ring-opening polymerization of dilactones as promoted by easily accessible N-heterocyclic carbenes (NHCs). Even though carbenes have been known as very unstable species for long, more recently discovered NHC derivatives proved to display higher stability making their handling much easier and allowing their use as efficient catalysts in various organic reactions. Remarkably, these metal-free catalysts display high reactivity in synthesis of aliphatic polyesters such as polylactides and poly(β-butyrolactone)s all characterized by well-tailored compositions, molecular weight, tacticity, and end-group fidelity while presenting narrow to very narrow molecular weight distribution. Interestingly, novel macromolecular architectures based on such aliphatic polyester building blocks have been made available, e.g. multi-armed star shaped copolyesters, H-shaped copolyesters or macrocyclic polyesters paving the way to new biomedical applications for this family of important biocompatible and biodegradable aliphatic polyesters.

Aging, Yielding, and Shear Banding in Soft Colloidal Glasses

Soft colloidal interactions in colloidal glasses are modeled using suspensions of multiarm star polymers. Using a preshearing protocol that ensures a reproducible initial state ("rejuvenation" of the system), we report here the evolution of the flow curve from monotonically increasing to one dominated by a stress plateau, demonstrating a corresponding shear-banded state. Phenomenological understanding is provided through a scalar model that describes the free-energy landscape.

Core-shell type multiarm star poly(ε-caprolactone) with high molecular weight hyperbranched polyethylenimine as core: Synthesis, characterization and encapsulation properties

Core-shell type multiarm star copolymers with poly(ε-caprolactone) (PCL) as shells and hyperbranched polyethylenimine (PEI) as core have been successfully prepared by the Sn(Oct) 2 catalyzed ring-opening polymerization of ε-caprolactone (CL) using high molecular weigh PEIs directly as macroinitiators. The initiation efficiency is in the range of 91–95% for PEI with M n = 10 4 (PEI10K) and only around 60% for PEI with M n = 2.5 × 10 4 (PEI25K), leading to star polymers with an average arm number in the range of 155–276. The thermal property of the obtained multiarm star polymers were also investigated by DSC. The melting and crystallization temperatures of the star polymers increase as the PCL arm length increases when the PEI core is fixed. The fusion enthalpy, crystalline enthalpy and degree of crystallinity values of the star polymers with PEI10K core are less than those with PEI1.8K core. Due to the polarity difference between PCL arm and PEI core, the resulting multiarm star polymers can act as inverted micellar nanocapsules capable of extracting and encapsulating water soluble guests. Increasing the size and polarity of the hydrophilic PEI core of the star nanocapsules are two effective ways to enhance their hydrophilic guest encapsulation capacity. Increasing the hydrophobic PCL arm length can increase the molar ratio, whereas reduce the weight ratio of the encapsulated hydrophilic guests to the star nanocapsules. Unexpectedly, the obtained nanocapsules can entrap the bigger size hydrophilic congo red guests more than the smaller size methyl orange.

A thermodynamically consistent model for the thixotropic behavior of concentrated star polymer suspensions

Concentrated multiarm star polymer solutions are used in this work as prototype, well defined, systems that clearly exhibit viscoplasticity and thixotropy. A thermodynamically consistent approach is used here to develop a continuum rheological model for these systems that is novel for thixotropic systems and has considerable advantages over previously postulated models: The basic rheological model is constructed from the beginning in a materially objective and flow type-independent form, in contrast to previous continuum models for thixotropy that typically are suitable only for shear flows. It is also consistent with non-equilibrium thermodynamics. The model is based on a Johnson–Segalman viscoelastic constitutive equation with a variable non-affine parameter. The non-affine parameter is considered representative of the structure and the aggregations between the particles in the system: as the deformation rate increases, those aggregations are assumed to be destroyed more and more, thus leading to discontinuities in the uniform viscoelastic deformation of the constituent multiarm polymer particles. This is described phenomenologically in the proposed model by a consistent increase in the non-affine parameter value from zero, which was taken to represent the virgin structure (where the deformation was assumed to be perfectly affine), to a maximum value at high rates, representing a concentrated suspension of viscoelastic particles. Simultaneously to those changes in the non-affine parameter, which are described by an empirical kinetic equation, the characteristic relaxation time of the system is also assumed to change dramatically: from infinity, at static equilibrium, to a minimum value characterizing the residual viscoelasticity of the fully broken-down medium. This behavior is modeled by assuming the relaxation time to be inversely proportional to the non-affine parameter. In addition to this viscoelastic contribution to the stress, a purely viscous contribution is considered modeled by a standard power law expression of the rate of strain. The model has been applied to transient and steady simple shear flows. The results show good semi-quantitative agreement with the experiments performed with the model multiarm star polymers.

NMR Diffusion and Relaxation Studies of the Encapsulation of Fragrances by Amphiphilic Multiarm Star Block Copolymers

Self-diffusion NMR spectroscopy and relaxometry have been employed to study fragrance encapsulation in water-soluble, amphiphilic star block copolymers. Diffusion coefficients of four different fragrance molecules in the free form and in the presence of the polymer have been determined and used to calculate the effective degree of encapsulation. In dilute aqueous solutions between 65% and 99% of the guest molecules are trapped inside the polymer. The degree of encapsulation depends on the hydrophobicity of the guest molecule, expressed by the octanol/water partitioning coefficient (log POW), where high log POW molecules are nearly quantitatively dissolved in the polymer. The fragrance molecules are mainly located in the hydrophobic core of the polymer, which is tightly packed, whereas the hydrophilic shell is flexible and takes up only a small percentage. Proton longitudinal (T1) and transverse (T2) relaxation times of the fragrance molecules are significantly reduced in the presence of the polymer indicating slower rotational correlation times due to microsolubilization in the hydrophobic core.

Influence of Adding Unfunctionalized PEO on the Viscoelasticity and the Structure of Dense Polymeric Micelle Solutions Formed by Hydrophobically End-Capped PEO

Hydrophobically end-capped poly(ethylene oxide) (PEO) is a highly asymmetric diblock copolymer that forms spherical micelles in water resembling multiarm star polymers. The dynamic mechanical prope...

Novel well‐defined star homopolymers and star‐block copolymers of poly(n‐hexyl isocyanate) by anionic polymerization

AbstractAnionic polymerization high‐vacuum techniques and appropriate multifunctional initiators/additives were employed for the synthesis of novel star structures of poly(n‐hexyl isocyanate) (PHIC). A new trifunctional initiator prepared by the reaction of tris(4‐isocyanatophenyl)methane with benzyl sodium was used for the synthesis of three‐arm star PHIC. Divinyl benzene and the core‐first or the arm‐first/core‐first (in‐out) approach were utilized for the synthesis of multiarm star homopolymers, (PHIC)n, star‐block copolymers, (PHIC‐b‐PI)n, and miktoarm star copolymers, (PS)n(PHIC)n, where PS is polystyrene. The molecular characteristics obtained by size‐exclusion chromatography, equipped with refractive index and two‐angle light scattering detectors, nuclear magnetic resonance, spectroscopy, and dilute solution viscometry showed that well‐defined structures were synthesized in this study. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2387–2399, 2007

Anisotropic Wetting Characteristics on Submicrometer-Scale Periodic Grooved Surface

Submicrometer-scale periodic structures consisting of parallel grooves were prepared on azobenzene-containing multiarm star polymer films by laser interference. The wetting characteristics on the patterned surfaces were studied by contact angle measurements. Macroscopic distortion of water drops was found on such small-scale surface structures, and the contact angles measured from the direction parallel to the grooves were larger than those measured from the perpendicular direction. A thermodynamic model was developed to calculate the change in the surface free energy as a function of the instantaneous contact angle when the three-phase contact line (TPCL) moves along the two orthogonal directions. It was found that the fluctuations, i.e., energy barriers, on the energy versus contact angle curves are crucial to the analysis of wetting anisotropy and contact angle hysteresis. The calculated advancing and receding contact angles from the energy versus contact angle curves were in good agreement with those measured experimentally. Furthermore, with the groove depth increasing, both the degree of wetting anisotropy and the contact angle hysteresis perpendicular to the grooves increased as a result of the increase in the energy barrier. The theoretical critical value of the groove depth, above which the anisotropic wetting appears, was determined to be 16 nm for the grooved surface with a wavelength of 396 nm. On the other hand, the effect of the groove wavelength on the contact angle hysteresis perpendicular to the grooves was also interpreted on the basis of the thermodynamic model. That is, with the wavelength decreasing, the contact angle hysteresis increased due to the increase in the number of energy barriers. These results may provide theoretical evidence for the design and application of anisotropic wetting surface.