Block polymers of isocyanates and vinyl monomers by homogeneous anionic polymerization

The “living” polymer method was used to prepare block polymers of vinyl monomers and isocyanates at low temperatures in toluene–tetrahydrofuran mixtures. Vinyl monomers and diisocyanates, which have one hindered isocyanate group, as in 2,4-toluene diisocyanate, form block polymers which contain pendant reactive isocyanate groups. These block polymers can be crosslinked with water, diols, diamines, etc. The polymerization is apparently limited to block polymer formation, since the polyisocyanate anion is incapable of initiating the polymerization of common vinyl monomers.     

Assessing the effects of homosexuals and bisexuals on the intrinsic dynamics of HIV/AIDS in heterosexual settings

A deterministic compartmental sex-structured HIV/AIDS model for assessing the effects of homosexuals and bisexuals on the intrinsic dynamics of the disease in heterosexual settings in which homosexuality and bisexuality issues have remained taboo is presented. The epidemic threshold and equilibria for the model are determined and stabilities are investigated. Comprehensive qualitative analysis of the model including invariance of solutions and permanence are carried out. The epidemic threshold known as the basic reproductive number suggests that heterosexuality, homosexuality, and bisexuality influence the growth of the epidemic in HIV/AIDS affected populations and the partial reproductive number (homosexuality induced or heterosexuality and bisexuality induced) with the larger value influences the overall dynamics of the epidemic in a setting. Numerical simulations of the model show that as long as one of the partial reproductive numbers is greater than unity, the disease will exist in the population. We conclude from the study that homosexuality and bisexuality enlarge the epidemic in a heterosexual setting. The theoretical study highlights the need to carry out substantial research to map homosexuals and bisexuals as it has remained unclear as to what extent this group has contributed to the epidemic in heterosexual settings especially in southern Africa, which has remained the epidemiological locus of the epidemic.     

Crystallographic effects in low energy ion scattering due to local ion-atom neutralisation

New experiments on 1 keV ₄He⁺ ion scattering from Ni {100} and Ni {100} (√2 × √2)R45°?O surfaces show azimuthal anisotropies attributable to variations in ion neutralisation probability for different ion trajectories relative to the position of the surface atoms. These effects are shown to be compatible with a simple localised ion-atom neutralisation mechanism. The results indicate that local neutralization is an important process in substrate shadowing in low energy ion scattering studies of adsorbate structures.     

Hyperfine interactions in the microwave spectrum of 2-propen-1-Imine (vinylimine)

A high resolution study of the microwave spectrum of 2-propen-1-imine has yielded values for ¹⁴N hyperfine coupling constants for both the trans-syn and trans-anti isomers. In the latter case, evidence was found for magnetic spin-rotation interaction. Rotational and centrifugal distortion constants of improved accuracy have been derived in the course of the analysis.     

Observations on the chemistry of the iodoxy group

In the presence of an appropriate catalyst, iodoxy-arenes act as oxidants towards a variety of diverse functional groups     

Conformational analysis of alkyl aryl ethers and alkyl aryl sulphides by photoelectron spectroscopy

Photoelectron spectroscopy shows that in t-butoxybenzenes, but not in methoxy-, ethoxy- or isopropoxybenzenes, there is a loss of p-π orbital overlap. A similar effect is observed in 2,6-dimethylalkoxybenzenes. In alkyl aryl sulphides two conformers predominate, the one with maximum p-π overlap and the other with reduced overlap. The importance of the less conjugated conformer increases monotonously through the series hydrogen, methyl, ethyl, isopropyl and t-butyl in alkyl phenyl sulphides.     

The high-resolution microwave spectrum of cyclopropyl cyanide

The quadrupole hyperfine structure of six selected rotational transitions of cyclopropyl cyanide has been analyzed. The derived coupling constants are χ_aa = ?3.453(11) MHz and χ_bb = 1.759(16) MHz. The analysis yielded improved center frequencies for the multiplets and, in turn, led to slightly revised rotational constants: A = 15786.068(22), B = 3465.112(7), C = 3286.252(10) MHz; D_N = 1.18(30) kHz.     

The total synthesis of d1-dihydrocallitrisin

Bromodiene $\text{2}$ is converted to $\text{dl}$-dihydrocallitrisin ($\text{1}$), a sesquiterpene lactone recently isolated from the heartwood of $\text{Callitris}$$\text{columellaris}$.     

Power requirement for the mixing of thixotropic liquids

A procedure has recently been proposed by Godfrey et al. [1] for the calculation of the variation of power consumption with time when a thixotropic liquid is agitated from rest using an impeller which rotates at constant speed. This procedure requires a knowledge of the power requirement for Newtonian and time-independent non-Newtonian liquids together with viscometric data for the thixotropic liquid obtained under constant shear rate conditions.Experimental work to test the procedure has been carried out in a 0.126-m-diameter cylindrical vessel with anchor, helical ribbon and helical screw impellers. Power consumption data were obtained for a range of Newtonian and time-independent non-Newtonian liquids and this was then used to make predictions of the measured power input to the thixotropic liquids: salad cream, tomato ketchup, yoghurt, paint and Laponite solutions.The agreement between experiment and theory was usually better than 10%. However, for the case of the helical screws rotating in Laponite it was observed that regions existed close to the vessel wall where there was no fluid circulation. In such cases, the predicted power input was greatly in excess of the measured value. This is not a serious limitation of the predictive procedure since efficient industrial mixers would keep the entire fluid in circulation.