Synthesis of New Avenalumic Carboxamide Derivatives in the Ferulic Series

A seven-step synthesis of (2E,4E)-5-[4-hydroxy-3-methoxyphenyl]penta-2,4-dienoic acid from ferulic acid was developed. The use of a natural by-product found in rice bran as a raw material provided the ferulic acid vinylogous in an overall high yield. This compound will be useful for the preparation of a wide variety of avenalumic carboxamide derivatives.     

Burstiness and spreading on temporal networks

We discuss how spreading processes on temporal networks are impacted by the shape of their inter-event time distributions. Through simple mathematical arguments and toy examples, we find that the key factor is the ordering in which events take place, a property that tends to be affected by the bulk of the distributions and not only by their tail, as usually considered in the literature. We show that a detailed modeling of the temporal patterns observed in complex networks can change dramatically the properties of a spreading process, such as the ergodicity of a random walk process or the persistence of an epidemic.     

Viscoelasticity of low molecular weight polymers and the transition to the entangled regime

The viscoelasticity of unentangled polystyrene melts has been investigated in terms of terminals parameters: zero-shear viscosity, steady-state compliance and relaxation spectrum. The Rouse model applies well for molecular weights lower than the average molecular weight between entanglements, providing that one takes into account the proper variations of the radius of gyration. Moreover, local motions at the scale of Kuhn segments have to be considered in order to describe correctly the relaxation modes intermediate between the terminal zone and the glassy plateau. On the other hand, reptation models are commonly used for describing the entangled regime. We propose an expression of the shear modulus which accounts not only for the terminal modes (reptation, tube length fluctuations and tube renewal), but also for the relaxation modes responsible for the plateau zone and the transition of the glassy plateau. A crossover region between the unentangled and untangled regimes is located around . When the molecular weight increases, a shift transfer of Rouse modes towards reptation modes occurs. That leads to a continuity of the expression of the shear modulus over the entire range of molecular weights. Received: 29 December 1997 Accepted: 27 July 1998     

Functionalization of Biphenylcarbazole (CBP) with Siloxane-Hybrid Chains for Solvent-Free Liquid Materials

We report herein the synthesis of siloxane-functionalized CBP molecules (4,4′-bis(carbazole)-1,1′-biphenyl) for liquid optoelectronic applications. The room-temperature liquid state is obtained through a convenient functionalization of the molecules with heptamethyltrisiloxane chains via hydrosilylation of alkenyl spacers. The synthesis comprises screening of metal-catalyzed methodologies to introduce alkenyl linkers into carbazoles (Stille and Suzuki Miyaura cross-couplings), incorporate the alkenylcarbazoles to dihalobiphenyls (Ullmann coupling), and finally introduce the siloxane chains. The used conditions allowed the synthesis of the target compounds, despite the high reactivity of the alkenyl moieties bound to π-conjugated systems toward undesired side reactions such as polymerization, isomerization, and hydrogenation. The features of these solvent-free liquid CBP derivatives make them potentially interesting for fluidic optoelectronic applications.     

The stability of SiC coating and SiO2/SiC multilayer on the surface of graphite for HTGRs at normal service condition

The stability of SiC coating in helium with a low concentration of O₂, CO₂, and H₂O is a key factor for their application in improvement of oxidation resistance of graphite for high temperature gas-cooled reactors (HTGRs). Through thermodynamic analysis, it is found that the influence factor controlling the critical temperature of passive oxidation for SiC is partial pressure of active gas in helium; the critical temperature of passive oxidation for SiC increases with the partial pressure of O₂, CO₂, and H₂O, SiC is prone to undergo active oxidation in He–CO₂ and He–H₂O system. SiO₂/SiC multilayer coating can improve the oxidation resistance of graphite at higher temperature than SiC coating does under normal operation condition for HTGRs.     

Étude des conditions d'accès au tétrazolo-1,2,3,4-[5′,1′:6,1] triazino-1,2,4-[4,5-b]indazole et au thioxo-3 triazolo-1,2,4-[2′,1′:6,1] triazino-1,2,4-[4,5-b]indazole

The triazinoindazolones 8 and 9 were synthesized by rearrangement of the oxadiazoles 5 and 6 or by cyclising the indazolic ethoxymethylidene-hydrazides 2 and 3. The triazinoindazolethiones 17 and 18 which were obtained by thionation of the triazinoindazolones 8 and 9 constitute the intermediates to the hydrazinoindazoles 19 and 20. The tetrrazolotriazinoindazole 21 was obtained by cyclising the hydrazinoindazole 19. The potassium indazolyldithiocarbazate 12 has been used for the synthesis of 4-thioxotriazinoindazolone 10 and 4-amino-3-thioxotriazolylindazole 15. The cyclisation of the latter gave the 3-thioxotriazinoindazolone 16. The structure of the derivatives was confirmed by ¹H-nmr.