Polyimidines. XI. Polyimidines from 3,3′,4,4′-benzophenonetetracarboxylic dianhydride and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride

Two new bis(benzylidenephthalide)monomers were synthesized by melt condensation of phenylacetic acid with 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) and with 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA). A mixture of three isomers for each monomer was obtained and polymerized with diamines to produce new polyimidines. Polymerizations were conducted with m-xylylenediamine (MXDA) or 4,4′-oxydianiline (ODA) in quantitative yields for the undehydrated intermediate. Inherent viscosities ranged from 0.17 to 0.35 dL/g in N,N-dimethylformamide (DMF) or N-methyl-2-pyrrolidone (NMP). These intermediate poly(hydroxylactams) were thermally dehydrated to polyimidines which exhibited a 10% weight loss, as high as 546°C in nitrogen. Inherent viscosities of the dehydrated (cured) polyimidines ranged from 0.14 to 0.20 dL/g in NMP. Brittle films could be cast from NMP solutions.     

Siloxane/silane‐crosslinked systems from supercritical carbon dioxide: II. Pendant phenyl poly(carbosilane/siloxane)s

New silicone‐containing polymers with crosslinkable units have been synthesized by hydrosilation polymerization in both toluene and supercritical carbon dioxide (70°C, 3000 psi) catalyzed by platinum‐divinyltetramethyldisiloxane (Pt‐DVTMS). It was found that high molecular polymers were obtained in both toluene and supercritical carbon dioxide. The polymers were characterized by FTIR, NMR, GPC, TGA, and DSC. The molecular weights of these polymers ranged from 9000 to 39,000. With further hydrolysis and thermal curing, the molecular weight can be increased significantly. Comparison of the properties between reactions in toluene versus supercritical carbon dioxide indicated that the green solvent is a usable alternative for hydrosilation polymerization. The new polymers synthesized in either toluene or supercritical carbon dioxide are thermally stable, ranged from 350 to 488°C. Copyright © 2008 John Wiley & Sons, Ltd.     

Experiments with a high-density positronium gas

We have created a high-density gas of interacting positronium (Ps) atoms by irradiating a thin film of nanoporous silica with intense positron bursts and measured the Ps lifetime using a new single-shot technique. When the positrons were compressed to 3.3 x 10(10) cm-2, the apparent intensity of the orthopositronium lifetime component was found to decrease by 33%. We believe this is due to a combination of spin exchange quenching and PS2 molecule formation associated with colliding pairs of oppositely polarized triplet positronium atoms. Our data imply an effective cross section for this process of 2.9 x 10(-14) cm-2.     

Doppler broadening of in-flight positron annihilation radiation due to electron momentum.

We report the first observation of electron momentum contributions to the Doppler broadening of radiation produced by in-flight two-photon annihilation in solids. In these experiments an approximately 2.5 MeV positron beam impinged on thin polyethylene, aluminum, and gold targets. Since energetic positrons easily penetrate the nuclear Coulomb potential and do not cause a strong charge polarization, the experimental annihilation line shapes agree well with calculations based on a simple independent-particle model. Moreover, annihilations with the deepest core electrons are greatly enhanced.     

糖化学中锡介入的选择性反应

本文从糖锡络合物结构出发,揭示了锡介入方法进行选择性反应的原因.并对该方法在糖化学各类不同类型的反应如酰化,烃化,氧化,磺化所表现的选择性进行了详细的论述.     

Sol–gel matrices for controlled release: from macro to nano using emulsion polymerisation

By combining sol–gel technology with emulsion chemistry, it is possible to produce spherical particles with a designed microstructure based on a judicious choice of solvent/surfactant and sol–gel reaction parameters. When an active molecule is located in the aqueous droplet of a water-in-oil (W/O) emulsion, encapsulation occurs as the silicon precursors polymerise to build an oxide cage around the active species. By changing the solvent–surfactant combination, the particle size can be varied from 10 nm to 100 μm. The size of the particles is controlled by the size of the emulsion droplet, which acts as a nano-reactor for the sol–gel reaction. The release profiles can be tailored, independently of the particle size, by controlling the internal structure of the particles: pore volume, pore size, tortuosity, and surface chemistry (e.g. by introduction of trialkoxysilane). This can be easily achieved by controlling sol–gel processing parameters such as the water-to-alkoxide ratio, pH, alkoxide concentration, ageing, drying time and temperature. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.