Masa-aki Kakimoto

Hyperbranched polymers: a promising new class of materials

Abstract Hyperbranched polymers are highly branched macromolecules that are prepared through a one-step polymerization process. Many kinds of hyperbranched polymers have been investigated as novel dendritic macromolecules. The general concepts, syntheses and the properties of hyperbranched polymers are reviewed in this article. The polymerization reactions are classified into three categories: (1) step-growth polycondensation of AB x monomers; and (2) self-condensing vinyl polymerization of AB ∗ monomers; (3) multibranching ring-opening polymerization of latent AB x monomers. Hyperbranched polymers are generally composed of dendritic, linear and terminal units and a degree of branching (DB) helps to describe their structures. It has been shown that most of the hyperbranched polymers possess some of the unique properties exhibit dendritic macromolecules, such as low viscosity, good solubility, and multi-functionality. Owing to multi-functionality, physical properties such as solubility in solvents and the glass transition temperature can be controlled by the chemical modification of the end functional groups (endcapping reactions). Applications of designed hyperbranched polymers to specific fields are also described.

Feature article. Preparation of new polyimide–silica hybrid materials via the sol–gel process

After a description of the preparation of polymer-metal oxide hybrid materials via the sol-gel method, recent results on the preparation of new polyimide-silica hybrid materials are reviewed. Tetraethoxysilane (TEOS) can be hydrolysed and polycondensed to form a three-dimensional silica-gel network in a solution of polyamic acids, the precursor polymers of polyimides, and N,N-dimethylacetamide (DMAc). Films of the polyamic acid-silica gel mixtures can be cast onto glass plates, and can then be heated to 300 °C to convert the component matrix polymers to polyimides. Five kinds of polyamic acid have been designed and prepared. One of them was the usual polyamic acid, having no connection site with silica, whereas others had ethoxysilyl groups in the polymer backbone. The polyimide-silica hybrid films containing 70 wt.% of silica were self-standing and tough. The transparent-opaque changing points shifted to higher silica content with increasing ethoxysilyl-group content of the matrix polyimides. The silica particles were dispersed homogeneously in the matrix polyimides, as observed by scanning electron microscopy. The size of the silica particles increased with increasing silica content, and decreased with increasing ethoxysilyl content. Results of the dynamic-mechanical analysis indicate that movement of the polyimide chain in the matrix is restricted, especially when the polyimides contain higher amounts of ethoxysilyl. The tensile strength of the hybrid films showed no decrease with increasing silica content, whereas the tensile modulus increased in all cases except that where the matrix polyimide did not contain any ethoxysilyl groups.

Selective Aerobic Oxidation of Benzylic Alcohols Catalyzed by Carbon‐Based Catalysts: A Nonmetallic Oxidation System

Selective oxidation is of great importance in organic synthesis, and green, efficient, selective oxidation techniques are highly sought after in the chemical industry. In particular, selective oxidation of alcohols is a fundamental transformation in the synthesis of fine chemicals. Recently, aerobic oxidation processes have received increasing attention, as oxygen or air are used as the terminal oxidant to replace the stoichiometric metal oxides such as chromates and manganese oxides. The use of oxygen has great benefits from both economic and green chemistry viewpoints, because oxygen is relatively cheap and produces water as the only byproduct. Accordingly, varied heterogeneous and homogenous catalysts derived from precious metals have been developed for this purpose, for example, Pt, Ru, Pd, Au, and Ag. However, because of their rarity and high price, precious metals are impractical for industrial use. Despite this, few efforts have been made to seek out nonprecious metal catalysts. One example is the 2,2,6,6-tetramethylpiperidine1-oxyl (TEMPO)/Br2/NaNO2 aerobic oxidation system reported by Liu et al. , in which TEMPO was used as a homogeneous catalyst, and the in situ generated NO played a crucial role in the activation of O2. Our interests are in the exploration of the potential of carbon-based materials to be applied as metal-free catalysts for selective aerobic oxidation of alcohols. Several studies of carbon catalyst materials have already revealed the ability to replace metal-based catalysts in several important transformations, such as the Friedel–Crafts reaction and the oxidative dehydrogenation of aromatic hydrocarbons and alkanes. Interestingly, carbon catalysts with an ordered nanoshell structure or doped with heteroatoms have been reported to possess surprisingly high oxygen reduction reaction (ORR) activities. However, to date there is no report on the selective aerobic oxidation of alcohols using the same catalyst. Herein, we present a novel protocol: nitric acid assisted carbon-catalyzed oxidation System (NACOS), by which benzyl alcohol and its substituted derivatives can be smoothly oxidized by oxygen under mild reaction conditions, affording high conversions and good selectivities into the corresponding aldehydes. The nanoshell carbon (NSC) used in this investigation was prepared by pyrolization of a blend of phenol resin and iron phthalocyanine, similar to methods reported by Ozaki et al. , and subsequent acid washes using concentrated HCl. The iron metal on the carbon surface can be thoroughly removed by acid washes, and the metal-free surface was characterized by energy dispersive X-ray spectroscopy (EDS) (Figure 1a) and X-ray photoelectron spectroscopy (XPS; Table 4). The specific surface area of NSC was 330 m g , determined by N2 adsorption. Initial experiments with benzyl alcohol showed that the NSC alone could not activate oxygen and no alcohol conversion was observed. It was postulated that the catalyst might take effect in the presence of an additive, so several acid and base additives were tested. Among all the tested additives, concentrated nitric acid

Novel Bipolar Bathophenanthroline Containing Hosts for Highly Efficient Phosphorescent OLEDs

The electronic structures of eight bathophenanthroline derivatives were elucidated by DFT calculations, and four representatives of which CZBP, m-CZBP, m-TPAP, and BPABP were synthesized and employed as the hosts to afford highly efficient phosphorescent OLEDs. The calculated molecular orbital energies agree well with the experimental results, which further demonstrates that the localization of HOMO and LUMO at the respective hole- and electron-transporting moieties is desirable in bipolar molecular designs.

New fluorine-containing ferroelectric liquid crystal compounds showing tristable switching

Abstract To understand the real cause of stabilization of the third state and to assure the possible application of the tristable switching, we have studied the effect of chiral centers on the appearance of the third state, synthesizing (R)-(+)- and (S)-(–)-4-(1-trifluoromethylalkoxy(n)carbonyl) phenyl 4′-alkoxy(m)biphenyl-4-carboxylate where m = 8, 9, 10, or 12 and n = 6 or 8; note that the chiral centre has a trifluoromethyl group. By observing light transmittance and switching current responses to a triangular voltage wave and by taking stroboscopic micrographs, we have confirmed that all the materials synthesized show the tristable switching in wide temperature ranges; in particular, the materials with m = 10 or 12 and n = 6 have the stabilized third state even at temperatures below 30°C.

Synthesis and properties of polyurethane elastomers crosslinked with amine-terminated AB2-type hyperbranched polyamides

Abstract Amine-terminated AB 2 -type hyperbranched polyamides of different molecular weights were prepared from 3,5-bis-(4-aminophenoxy)benzoic acid (AB 2 monomer) by fractional precipitation technique and characterized by FTIR, 1 H-NMR spectroscopies, DSC and GPC techniques. The degree of branching (DB) of hyperbranched polymers (HBP) was determined using 13 C-NMR spectroscopy and it was found that the value increased with decrease in molecular weight of polymer considered. As the molecular weight distribution was narrow, the approximate number of end functional groups of each HBP was conveniently calculated. Three polymers were selected and used as crosslinkers in the preparation of polyurethanes. The incorporation of hyperbranched polyamide into the polyurethane chains was confirmed using FTIR and 1 H-NMR spectroscopic techniques. Among the range studied (1–6%), it was found that high tensile strength is attained with 1% of HBP. It was also found that the tensile strength decreases with increase in number of end functional groups and decrease in DB of HBP. However, glass transition temperatures and thermal stability of polyurethanes crosslinked with up to 6% of HBP, above which gelation occurred, were not affected and similar to the blank polymer prepared without AB 2 polymer.

Electronic structure of N-doped graphene with native point defects

Nitrogen doping in graphene has important implications in graphene-based devices and catalysts. We have performed the density functional theory calculations to study the electronic structures of N-doped graphene with vacancies and Stone-Wales defect. Our results show that monovacancies in graphene act as hole dopants and that two substitutional N dopants are needed to compensate for the hole introduced by a monovacancy. On the other hand, divacancy does not produce any free carriers. Interestingly, a single N dopant at divacancy acts as an acceptor rather than a donor. The interference between native point defect and N dopant strongly modifies the role of N doping regarding the free carrier production in the bulk pi bands. For some of the defects and N dopant-defect complexes, localized defect pi states are partially occupied. Discussion on the possibility of spin polarization in such cases is given. We also present qualitative arguments on the electronic structures based on the local bond picture. We have analyzed the 1s-related x-ray photoemission and adsorption spectroscopy spectra of N dopants at vacancies and Stone-Wales defect in connection with the experimental ones. We also discuss characteristic scanning tunneling microscope (STM) images originating from the electronic and structural modifications by the N dopant-defect complexes. STM imaging for small negative bias voltage will provide important information about possible active sites for oxygen reduction reaction.

Interplay between nitrogen dopants and native point defects in graphene

To understand the interaction between nitrogen dopants and native point defects in graphene, we have studied the energetic stability of N-doped graphene with vacancies and Stone-Wales (SW) defect by performing the density functional theory calculations. Our results show that N substitution energetically prefers to occur at the carbon atoms near the defects, especially for those sites with larger bond shortening, indicating that the defect-induced strain plays an important role in the stability of N dopants in defective graphene. In the presence of monovacancy, the most stable position for N dopant is the pyridinelike configuration, while for other point defects studied (SW defect and divacancies) N prefers a site in the pentagonal ring. The effect of native point defects on N dopants is quite strong: While the N doping is endothermic in defect-free graphene, it becomes exothermic for defective graphene. Our results imply that the native point defect and N dopant attract each other, i.e., cooperative effect, which means that substitutional N dopants would increase the probability of point defect generationandviceversa.OurfindingsaresupportedbyrecentexperimentalstudiesontheNdopingofgraphene. Furthermore we point out possibilities of aggregation of multiple N dopants near native point defects. Finally we make brief comments on the effect of Fe adsorption on the stability of N dopant aggregation.

Pt-free cathode catalysts prepared via multi-step pyrolysis of Fe phthalocyanine and phenolic resin for fuel cells.

Pt-free cathode catalysts for polymer electrolyte membrane fuel cells have been prepared by multi-step pyrolysis of FePc and PhRs, in the best of which show extensively high initial cell performance and good durability compared to other present precious-metal-free cathode catalysts to date.

Synthesis and characterization of new diphenylfluorene‐based aromatic polyazomethines

New diphenylfluorene-based aromatic polyazomethines were synthesized via polycondensation of aromatic diamines with aromatic dialdehydes. The highest inherent viscosity of polyazomethines was obtained for solution polycondensation in m-cresol at room temperature under reduced pressure. The aromatic polyazomethines have inherent viscosities of 0.26-0.36 dL/g. All the polyazomethines are amorphous and most of them are highly soluble in various organic solvents, especially in tetrahydrofuran. Their glass transition temperatures (T g ) range from 249 to 335 C, and the temperatures of 10% weight loss are as high as 490∼535°C in air, indicating that these aromatic polyazomethines have high T g and excellent thermal stability.