Poly(sodium styrene sulfonate)-b-poly(methyl methacrylate) diblock copolymers through direct atom transfer radical polymerization: Influence of hydrophilic–hydrophobic balance on self-organization in aqueous solution

A series of poly(sodium styrene sulfonate)-b-poly(methyl methacrylate), PSSNa-b-PMMA, amphiphilic diblock copolymers have been synthesized through atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) in N,N-dimethylformamide/water mixtures, starting from a PSSNa macroinitiator. The kinetics of the polymerization was followed by ¹H NMR, while the chemical composition of the copolymers was verified by a variety of techniques, such as ¹H NMR, FTIR and TGA. The MMA content of the copolymers ranges from 0 up to 60 mol%, while the number–average molecular weight of the PSSNa macroinitiator was 9000 g/mol. The self-association of the diblock copolymers in aqueous solution was compared to the respective behavior of similar random P(SSNa-co-MMA) copolymers through optical density measurements, pyrene fluorescence probing, dynamic light scattering and surface tension measurements. It is shown that the diblock copolymers form micellar structures in water, characterized by an increasing hydrophobic character and a decreasing size as the length of the PMMA block increases. These micelle-like structures turn from surface inactive to surface active as the length of the PMMA block increases. Moreover, contrary to the MMA-rich random copolymers, the respective diblock copolymers form water insoluble polymer/surfactant complexes with cationic surfactants such as hexadecyltrimethyl ammonium bromide (HTAB), leading to materials with antimicrobial activity.     

A general method for the synthesis of bastaranes and isobastaranes: first total synthesis of bastadins 5, 10, 12, 16, 20, and 21

A general strategy for the synthesis of twenty naturally occurring bastadins (all but bastadin 3) is presented. A key retrosynthetic disconnection of the two amide bonds, common in all target molecules, bisects the macrocyclic core into two diaryl ether fragments, an alpha,omega-diamine (western part) and an alpha,omega-dicarboxylic acid (eastern part). Efficient preparation of the synthetically challenging o-mono or dibromo-substituted diaryl ether linkages was achieved employing the diaryl iodonium salt method. Regarding the western part, variations of the aliphatic chain were more efficiently secured by the preparation of two different alpha,omega-aminonitrile moieties. Cobalt boride mediated reduction of the nitrile functionality established the required diamines and, at the same time, provided the necessary variation of the aromatic-ring bromination pattern. Regarding the eastern part, two different dicarboxyl precursors had to be prepared in order to accommodate bromination-pattern variations. Coupling and subsequent macrolactamization of different combinations of these key intermediates may lead at will to any member of this family of marine natural products. Four bastaranes (bastadins 5, 10, 12 and 16) and two isobastaranes (bastadins 20 and 21) were synthesized as a demonstration of the flexibility and efficiency of the approach presented.     

Novel diblock copolymer‐grafted multiwalled carbon nanotubes via a combination of living and controlled/living surface polymerizations

Diels–Alder cycloaddition reactions were used to functionalize multiwalled carbon nanotubes (MWNTs) with 1‐benzocylcobutene‐1′‐phenylethylene (BCB‐PE) or 4‐hydroxyethylbenzocyclobutene (BCB‐EO). The covalent functionalization of the nanotubes with these initiator precursors was verified by FTIR and thermogravimetric analysis (TGA). After appropriate transformations/additions, the functionalized MWNTs were used for surface initiated anionic and ring opening polymerizations of ethylene oxide and ε‐caprolactone (ε‐CL), respectively. The OH‐end groups were transformed to isopropylbromide groups by reaction with 2‐bromoisobutyryl bromide, for subsequent atom transfer radical polymerization of styrene or 2‐dimethylaminoethyl methacrylate to afford the final diblock copolymers. ¹H NMR, differential scanning calorimetry (DSC), TGA, and transmission electron microscopy (TEM) were used for the characterization of the nanocomposite materials. TEM images showed the presence of a polymer layer around the MWNTs as well as the dissociation of MWNT bundles. Consequently, this general methodology, employing combinations of different polymerization techniques, increases the diversity of diblocks that can be grafted from MWNTs. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1104–1112, 2010     

Optical response of small closed-shell sodium clusters

Absorption spectra of closed-shell Na(2), Na(3) (+), Na(4), Na(5) (+), Na(6), Na(7) (+), and Na(8) clusters are calculated using a complex Bethe-Salpeter equation derived using a conserving linear response method. In the framework of a quasiparticle approach, we determine electron-hole correlations in the presence of an external field. The calculated results are in excellent agreement with experimental spectra, and some possible cluster geometries that occur in experiments are analyzed. The position and the broadening of the resonances in the spectra arise from a consistent treatment of the scattering and dephasing contributions in the linear response calculation. Comparison between the experimental and the theoretical results yields information about the cluster geometry, which is not accessible experimentally.     

Novel Monte Carlo scheme for systems with short-ranged interactions

We propose a Monte Carlo (MC) sampling algorithm to simulate systems of particles interacting via very short-ranged discontinuous potentials. Such models are often used to describe protein solutions or colloidal suspensions. Most normal MC algorithms fail for such systems because, at low temperatures, they tend to get trapped in local potential-energy local minima due to the short range of the pair potential. To circumvent this problem, we have devised a scheme that changes the construction of trial moves in such a way that the potential-energy difference between initial and final states drops out of the acceptance rule for the Monte Carlo trial moves. This approach allows us to simulate systems with short-ranged attraction under conditions that were unreachable up to now.     

The total synthesis of coleophomones B, C, and D

Members of the coleophomone family of natural products all possess several intriguing and challenging architectural features, as well as exhibit unusual biological activity. They, therefore, constitute attractive targets for synthesis. In this Article, we describe the total synthesis of coleophomones B (2), C (3), and D (4). The highly strained and congested 11-membered macrocycle of coleophomones B (2) and C (3) was constructed using an impressive olefin metathesis reaction. Furthermore, both of the requisite geometric isomers of the Delta(16,17) within the macrocycle could be accessed from a common precursor, facilitating a divergence that lent the coleophomone B (2)/C (3) synthesis an unusually high degree of efficiency. The synthesis of coleophomone D (4) confirmed that it exists as a dynamic mixture of isomeric forms with a different aromatic substitution pattern from the other family members.     

Attraction of iodide ions by the free water surface, revealed by simulations with a polarizable force field based on Drude oscillators

Recent theoretical and experimental studies have shown that polarizable anions, such as iodide and bromide, preferentially accumulate close to the surface of electrolyte solutions. This finding is in sharp contrast to the previously prevailing idea that salts are dielectrically excluded from the free water surface and opens up new avenues for research in specific salt effects. In this work, we have verified the ability of a recently introduced polarizable water model, SWM4-DP, to reproduce this behavior, by simulations of a NaI/water slab, corresponding to a 1.2 M solution. The water and ion polarizabilities are modeled by classical Drude oscillator particles. As revealed by the simulations, a double layer is formed close to the free water surface, with the iodide ions located closer to the interface and the sodium ions at a neighboring, interior layer. Near the surface, all solution species acquire an induced dipole moment, that is perpendicular to the surface and points toward the exterior. The double charge layer causes ordering of water at a subsurface region. Simulations with a simpler system of a single iodide ion in a water slab show that the surface position is stabilized by induced charge interactions; in contrast, the charge-dipole interactions between the iodide permanent charge and the water permanent dipole moment favor the bulk position. Thus, the polarizabilities of ion and water are essential for explaining the increased preference of iodide for the air-water interface, in accordance with other studies.     

Investigation of the incorporation of electrolyte anions in porous anodic Al2O3 films by employing a suitable probe catalytic reaction

A new method has been developed capable of describing the incorporation of electrolyte anions along the pore wall surface and across both the barrier layer and the pore wall oxide after the establishment of the steady state of growth of porous anodic Al₂O₃ where other methods cannot be applied to obtain reliable results. The knowledge of the nature/composition of anodic oxides as regards the incorporation of species like electrolyte anions is of specific importance for both the understanding of the electrochemical mechanism of oxide production and growth and the scientific and technological applications of porous anodic Al₂O₃ films. The method consists of the selection and use of a suitable catalytic probe reaction on porous anodic oxides at thicknesses varying from a value near zero up to the maximum limiting thickness and the treatment of the experimental reaction rate results by a properly developed mathematical formalism. This method was employed in anodic Al₂O₃ films prepared in H₂SO₄ anodizing electrolyte at a constant bath temperature and different current densities using as a probe reaction the decomposition of HCOOH on these oxides, which is almost exclusively a dehydration reaction, at relatively high reaction temperatures, 350 °C and 390 °C, where the effect of other species except SO₄ ²⁻ incorporated in the oxide on the reaction rate is eliminated. It has been shown that the fraction of the intercrystallite surfaces occupied by SO₄ ²⁻ follows a parabola-like distribution. It has a significant value at the pore base surface, depending on the current density, then it passes through a maximum along the pore wall surface and across both the barrier layer and the pore walls near the pore bases at positions depending on the current density and then becomes almost zero at the mouths of the pores of the oxide with the maximum limiting thickness and at both the Al₂O₃/Al interface and cell boundaries. The maximum value of the surface coverage is almost independent of the current density and is always near 1, showing an almost complete saturation of intercrystalline surfaces at these positions. The above distribution of surface coverage predicts a qualitatively similar distribution of the SO₄ ²⁻ bulk concentration across both the barrier layer and pore wall oxide around the pore bases. The method may be improved and developed further either for a more detailed investigation of the above films or to investigate films prepared in other pore-forming electrolytes. Received: 30 July 1998 / Accepted: 30 September 1998     

Characterization of the core-shell nanoparticles formed as soluble hydrogen-bonding interpolymer complexes at low pH

The formation of soluble hydrogen-bonding interpolymer complexes between poly(acrylic acid) (PAA) and poly(acrylic acid-co-2-acrylamido-2-methyl-1-propane sulfonic acid)-graft-poly(N,N-dimethylacrylamide) (P(AA-co-AMPSA)-g-PDMAM) at pH=2.0 was studied. A viscometric study showed that in semidilute solution a physical gel is formed due to the interconnection of the anionic P(AA-co-AMPSA) backbone of the graft copolymer, in a transient network, by means of the complexes formed between the PDMAM side chains of the graft copolymer and PAA. Dynamic and static light scattering measurements, in conjunction with small-angle neutron scattering measurements, suggest the formation of core-shell colloidal nanoparticles in dilute solution, comprised by an insoluble PAA/PDMAM core surrounded by an anionic P(AA-co-AMPSA) corona. Even if larger clusters are formed in semidilute solution, the size of the insoluble core remains practically stable. Atomic force microscopy performed under ambient conditions reveal that the particles collapse and flatten upon deposition on a substrate, with dimensions close to the ones of the dry hydrophobic core.