Polymerisation stereospecifique du sulfure de propylene en phase homogene

The homogeneous polymerization of propylene sulphide, initiated by cadmium or zinc thiolates, proceeds without important termination reactions and gives “living” polymers. The molecular weight distributions of the polymers are narrow. After elimination of the metal included in macromolecules, changes of molecular weight are not observed. Zinc or cadmium atoms are not located in the middle of the chains but at one end. Different types of associations are shown by viscometric studies on living polymer solutions. It is possible to prepare stereoregular polypropylene sulphides from cadmium thiolates in benzene, tetrahydrofuran or tetrahydrothiophene solutions.     

Self-assembly of the 3-aminopropyltrimethoxysilane multilayers on Si and hysteretic current–voltage characteristics

We report the deposition of 3-aminopropyltrimethoxysilane (APTMS) multilayers on SiO_x/Si(p⁺⁺) substrates by a layer-by-layer self-assembly process. The multilayers were grafted in a glove box having nitrogen ambient with both humidity and oxygen contents <1 ppm using APTMS solutions prepared in an anhydrous toluene. Deposition of the multilayers has been carried out as a function of solution concentration and grafting time. Characterization of the multilayers using static de-ionized water contact angle, ellipsometry, X-rayphotoelectron spectroscopy and atomic force microscope measurements revealed that self-assembling of the multilayers takes place in two distinct stages: (i) the first APTMS monolayer chemisorbs on a hydroxylated oxide surface by a silanization process and, (ii) the surface amino group of the first monolayer chemisorbs the hydrolyzed silane group of other APTMS molecules present in the solution, leading to the formation of a bilayer. The second stage is a self-replicating process that results in the layer-by-layer self-assembly of the multilayers with trapped NH₃ ⁺ ions. The current–voltage characteristics of the multilayers exhibit a hysteresis effect along with a negative differential resistance, suggesting their potential application in the molecular memory devices. A possible mechanism for the observed hysteresis effect based on filling and de-filling of the NH₃ ⁺ acting as traps is presented. PACS 73.30.+y     

Asymmetric azidation-cycloaddition with open-chain peptide-based catalysts. A sequential enantioselective route to triazoles

A family of beta-substituted histidine-containing peptides has been synthesized to probe the effect of noncovalent conformational rigidification on catalyst enantioselectivity. Unambiguous enhancement of enantioselectivity in the conjugate addition of azide to alpha,beta-unsaturated carboxylate derivatives has been achieved, enabling application to a sequential asymmetric azidation/cycloaddition for the synthesis of optically enriched triazoles and triazolines.     

Analysis of a correlated queue in a communication system

A family of queues is studied for which the service time B_n of customer n and the interarrival time I_n between customers n-1 and n exhibit some sort of proportionality. The focus is on dependencies that arise naturally in the context of communication systems, where the finite speed of the communication links constrains the amount of data that can be received in a given time interval. The simple case of a deterministic proportionality relation between the service time of a customer and its preceding interarrival time is considered. This is extended to allow the addition of an independent, generally distributed overhead to the service time of each customer. Several models that capture the ON-OFF behavior of communication links in packet networks are considered. In all cases, expressions for the delay experienced by a packet in the system are provided. Numerical examples illustrate the impact of dependencies through comparison with less accurate models. The results should be of relevance to environments other than communication as well     

Equivalent capacity and its application to bandwidth allocation inhigh-speed networks

The authors propose a computationally simple approximate expression for the equivalent capacity or bandwidth requirement of both individual and multiplexed connections, based on their statistical characteristics and the desired grade-of-service (GOS). The purpose of such an expression is to provide a unified metric to represent the effective bandwidth used by connections and the corresponding effective load of network links. These link metrics can then be used for efficient bandwidth management, routing, and call control procedures aimed at optimizing network usage. While the methodology proposed can provide an exact approach to the computation of the equivalent capacity, the associated complexity makes it infeasible for real-time network traffic control applications. Hence, an approximation is required. The validity of the approximation developed is verified by comparison to both exact computations and simulation results     

Operation of short-channel depletion-mode MOS devices atliquid-nitrogen temperature

Experimental observations that depletion-mode MOS devices optimized for room temperature can also work well when immersed in liquid nitrogen are reported in which the classical impurity freeze-out effect seems to vanish on short-channel devices if the drain voltage is not too small. This is attributed to field-assisted ionization mechanisms such as the Poole-Frenkel effect, with possible enhancement by self-heating. The MINIMOS 4 device simulator was modified to introduce this effect and then to check the validity of this assumption by comparison with experimental results. To prove that it is possible to take advantage of this effect a 3-bit feedback adder, used as a benchmark circuit, has been processed in an enhancement-depletion 0.5-μm NMOS technology optimized for room temperature wherein the cooling from 300 to 77 K results in an improvement from 1 to 1.3 GHz for the maximum clock frequency of operation     

Throughput properties of fair policies in ring networks

Considers a slotted ring in which simultaneous transmission of messages by different stations is allowed, a property referred to as spatial reuse. Ring networks with spatial reuse can achieve significantly higher throughput than standard token rings but they also introduce the possibility of starvation for some nodes on the ring. To alleviate this problem, various policies have been suggested in the literature. The present objective is to characterize the node throughputs achievable by general transmission policies in ring networks with spatial reuse and then to evaluate the throughput trade-off for a class of policies that has been proposed in the literature in order to avoid starvation. Specifically, the authors study a policy that is based on the idea of allocating transmission quotas to the nodes. Each node is guaranteed transmission of his quota within a specified interval. The authors show that by appropriately allocating the quotas, policies that satisfy general optimality criteria-in particular criteria related to fairness-can be designed. They also study the asymptotic behavior of the quota policy when either the quotas or the number of nodes increase     

A His-Pro-Aib peptide that exhibits an Asx-Pro-turn-like structure

[formula: see text] Small peptides 1 and 2, which differ only in that 1 possesses a BOC-Phe residue at the N-terminus, where 2 bears a BOC-(tau-benzyl)His, were found to exhibit very different structures. In both the solid state (X-ray) and in solution (NMR and IR), peptide 1 exists as a beta-turn, whereas peptide 2 exists in a conformation that resembles the Asx-Pro motif.     

Photonic band gaps in one-dimensionally ordered cold atomic vapors

We experimentally investigate the Bragg reflection of light at one-dimensionally ordered atomic structures by using cold atoms trapped in a laser standing wave. By a fine-tuning of the periodicity, we reach the regime of multiple reflection due to the refractive index contrast between layers, yielding an unprecedented high reflectance efficiency of 80%. This result is explained by the occurrence of a photonic band gap in such systems, in accordance with previous predictions.