A Facile Route to Synthesize Well‐Defined Polybutadiene DendriMacs

A facile route to synthesize well‐defined polybutadiene (PBd) DendriMacs is described. In a highly modified approach to that recently published for the synthesis of polystyrene DendriMacs, a G1 polybutadiene (PBd) DendriMac has been synthesized in three simple and high yielding steps. The first step involves the synthesis of a three‐arm PBd mikto arm star in which one arm has a terminal hydroxy group introduced by the use of a protected functionalized initiator. Following fractionation of the star, the hydroxy group is deprotected and converted into an alkyl bromide moiety before coupling the star to a trifunctional core (1,1,1‐tris(4‐hydroxylphenyl)ethane) by a Williamson coupling reaction catalyzed by cesium carbonate to yield a G1 PBd DendriMac.

     

Cross-polarized wave generation in the UV region

We demonstrate experimentally the generation of cross-polarized femtosecond pulses in BaF2 crystal in the UV region. We show that unsaturated cross-polarized wave generation in the UV is six times more efficient than in the visible region, and we deduce the corresponding wavelength dispersion of the third-order nonlinearity.     

Local enrichment and its nonlocal consequences for victim-exploiter metapopulations

The stabilizing effects of local enrichment are revisited. Diffusively coupled host-parasitoid and predator-prey metapopulations are shown to admit a stable fixed point, limit cycle or stable torus with a rich bifurcation structure. A linear toy model that yields many of the basic qualitative features of this system is presented. The further nonlinear complications are analyzed in the framework of the marginally stable Lotka-Volterra model, and the continuous time analog of the unstable, host-parasitoid Nicholson-Bailey model. The dependence of the results on the migration rate and level of spatial variations is examined, and the possibility of “nonlocal” effect of enrichment, where local enrichment induces stable oscillations at a distance, is studied. A simple method for basic estimation of the relative importance of this effect in experimental systems is presented and exemplified.     

The Endomorphisms of a Finite Chain Form a Rees Congruence Semigroup

Rees congruence semigroup , is one for which every non-identity congruence is a Rees congruence. We show that the widely studied semigroup of endomorphisms of a finite chain is a Rees congruence semigroup. Communicated by     

Evaluation of propagation rate constants for the free radical polymerization of methacrylonitrile by pulsed laser photolysis

Propagation rate constants for the free radical polymerization of methacrylonitrile (MAN) have been obtained by pulsed laser photolysis (PLP). The temperature dependence of the propagation rate constants indicates a frequency factor of 10^{(6,43 ± 0,26)} L · mol⁻¹ · s⁻¹ and an activation energy of 29,7 ± 1,5 kJ · mol⁻¹. These parameters suggest that the relatively slow rate of propagation in MAN polymerization in relation to other common monomers (methyl methacrylate, styrene) can be attributed to the relative steric bulk and stability of the propagation species.     

Spatial versus quantum confinement in porous amorphous silicon nanostructures

Light-emitting porous amorphous silicon has been produced by anodization in HF of hydrogenated amorphous silicon films. The maximal thickness of the porous films is limited by the onset of an instability which results in the formation of large channels short-circuiting the amorphous layer. This is due to the high resistivity of the amorphous silicon films as compared to that of the electrolyte. Confinement effects on the electron wavefunction are analyzed in situ using photoluminescence measurements in hydrofluoric acid and compared to those observed in porous crystalline silicon. For crystalline silicon, a huge blue shift of the photoluminescence is observable upon reducing the size of the structures by photo-etch, showing clear evidence of quantum confinement effects in this material. No shift has been observed when carrying out the same experiment with amorphous silicon. This indicates that the extent of the wavefunction in the bandtail states involved in luminescence is too small to be sensitive to confinement down to the minimum sizes of our porous material ( 3 nm). Measurements of the width and the temperature dependence of the photoluminescence demonstrate that the Urbach energy does not change upon increasing the porosity, i.e., upon decreasing the size of the a-Si:H nanostructures, in contradiction with what has been reported in ultrathin a-Si:H multilayers. Received: 3 August 1998     

Determination of molecular weight distributions of novolac resins by gel permeation chromatography

The number- and weight-average molecular weights of several statistical and high ortho novolac resins were determined using gel permeation chromatography (GPC). The standards used were pure compounds having between 2 and 12 phenol units bridged via methylene linkages. Three series of compounds were studied: (i) those with methylene linkages substituted in only the ortho positions relative to the phenolic hydroxyl group; (ii) those in which all para positions, together with sufficient ortho positions, were used to synthesize the compounds; and (iii) those in which the methylene linkages were substituted at a mixture of ortho and para positions. Such compounds, having known molecular architecture and units similar to the segments of industrial novolac resins, provide for a more exact measurement of the molecular weight than do the commonly used poly(styrene) standards. Using these new standards the number average molecular weights of the resins determined by GPC were in good agreement with the average molecular weight obtained by ¹H-NMR spectra of the resins, particularly for low molecular weight resins. GPC analysis of higher molecular weight resins tends to underestimate the molecular weights because of complications introduced by hydrogen bonding. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1399–1407, 1997