Javaid H. Khan

The regiochemistry and stereochemistry of 1,3-dipolar cycloaddition of cyclic nitrones

Abstract A comparative study of the regio- and stereo-chemical behaviour of the 1,3-dipolar cycloaddition of a series of alkenes with 1-pyrroline 1-oxide and 2,3,4,5-tetrahydropyridine 1-oxide has been carried out. The high degree of both regio- and stereochemical control observed in these reactions has been explained in terms of frontier orbital interaction, steric factors, and secondary orbital interaction in the transition state. While most common alkenes (both mono- and 1,1-di-substituted) gave 2-substituted cycloadducts, highly polarized alkene dimethyl methylenemalonate afforded mainly regioisomeric 3-substituted cycloadduct. Significant secondary orbital interaction is observed with the non-conjugated substituents, hydroxymethyl and its derivatives.

Durability of HALS-stabilized polyethylene film in a greenhouse environment

Abstract Low-density polyethylene-based greenhouse films have been monitored for outdoor weather effects by mounting the film on a model greenhouse and on aluminum exposure racks at Dhahran plastic exposure facility. The change in chemical properties was monitored by Fourier transform infrared (FTIR) spectroscopy. The thermal properties were observed by differential scanning calorimetry (DSC) and the drop in percent elongation and stress at break was monitored. It is concluded that greenhouse film mounted on the model greenhouse degraded more in comparison to the film on aluminum racks. The spectral emissions with wavelength between 7 and 14 μm are an important part of the energy losses from the soil and the plants inside a greenhouse. The partial prevention of the dissipation of this thermal energy from the greenhouse film during cool night hours creates a unique environment and influences the degradation reactions in the plastic film Pesticides, mostly sulfur- and halogen-based compounds, can also accelerate the degradation of the film.

The 1,3-dipolar cycloaddition of cyclic nitrones with 1,2-disubstituted alkenes

Abstract A comparative study of the stereochemical behaviour of the 1,3-dipolar cycloaddition of a series of 1,2-disubstituted alkenes with 1-pyrroline 1-oxide ( 1 ) and 2,3,4,5-tetrahydropyridine 1-oxide ( 2 ) has been carried out. Both the nitrones exhibit very similar stereochemical properties. Rate constants for the cycloaddition of these nitrones to several disubstituted alkenes have been determinded at 36°C by 1 H n.m.r. spectroscopy. It is found that the nitrone 1 reacts slower than 2 due to the presence of bond eclipsing strain in the transition state involving 1 .

XPS investigation of the electronic environment in selected heterogenized zirconocene catalysts

Ethylbisindenyl zirconium dichloride (Et(Ind)2ZrCl2) and the MAO methylalumoxane (MAO) co-catalyst were heterogenized on Davision silica 955 partially dehydroxylated at 275 °C, following the concept of equilibrium adsorption. The influence of MAO on the electronic environment resulting from the heterogenization was investigated using XPS. Heterogenization of Et(Ind)2ZrCl2 and MAO on the above silica generated two types of zirconocenium cations (Cation 1 and Cation 2), independent of the heterogenization methods. Based on the postulated surface chemistry, Cation 1 is presumed to be in the form of an ion-pair [SiO]−[Et(Ind)2ZrCl]+, whereas Cation 2 is presumed to be a trapped multi-coordinated crown complex of MAO. In the absence of MAO, only Cation 1 is formed. The present study provides some support for the postulated surface chemistry regarding heterogenization of Et(Ind)2ZrCl2 and MAO on silica. Copyright

Prognostic Tools for Lifetime Prediction of Aircraft Coatings: Paint Degradation

A direct interrogation, portable analysis technique (portable FT-IR) and a novel environment-monitoring profluorescent sensor for studying aircraft coating degradation have been developed. For the direct interrogation approach, a standard military aircraft paint: 459-line Anzothane flexible polyurethane (lead free) has been used to illustrate a new potential field technique to evaluate coating service lifetime, portable FT-IR. This technique allows direct analysis of chemical changes within the degrading coatings and has the potential to evaluate service lifetime when coupled with advanced statistical analysis methods (chemometrics). The degradation environment monitoring sensors are embodied in a profluorescent environment-sensitive witness patch that may be analysed in-service by a field-deployable fluorescence spectrometer. Accelerated ageing for both the paint and the witness patches has been undertaken and their capabilities as aircraft paint degradation monitors assessed.

Nonisothermal, uncontrolled homo- and copolymerization of ethylene using selected zirconocenes

Nonisothermal, uncontrolled polymerization, conducted in varying mixing regimes, offered a facile methodology to evaluate the influence of several important process development factors such as mixing, reaction exotherm, and thermal perturbations on the catalytic activity and kinetic stability, polymerization performance, and properties of the resulting polymers. Ethylene was homo- and copolymerized with hexene-1 under varying impeller speeds (hence, thermal perturbations), using Ind2ZrCl2 and Et(Ind)2ZrCl2 and the MAO cocatalyst. With respect to the effects of the above process development factors, the following was observed: The reaction exotherm profiles, tracing the polymerization history, qualitatively represented the kinetic profile and the catalytic stability. The unbridged Ind2ZrCl2 was shown to be more stable than the bridged Et(Ind)2ZrCl2. With change in the level of stirring from a diffusion-controlled regime to a nondiffusion-controlled, external gas–liquid mass-transfer resistance-free one, the reaction exotherm and the run time-average catalytic activity increased. So far as the influence of the chiral versus the achiral zirconocene structure is concerned, the copolymer composition distribution and soluble fraction generated by chiral Et(Ind)2ZrCl2 were more sensitive to the mixing conditions and thermal perturbations than were those produced by achiral Ind2ZrCl2. Et(Ind)2ZrCl2 produced higher molecular weight backbones, incorporated more hexene-1 and chain branching, and introduced less crystallinity in the copolymers than did Ind2ZrCl2. The influence of Ind2ZrCl2 on higher-weight homopolymer backbones was opposite to that of Et(Ind)2ZrCl2. Incorporation of hexene-1 significantly decreased the average molecular weights and density and increased the run-time-dependent average catalyst activity. A positive comonomer effect took place. The bulk polymer properties did not critically depend on the mixing state. Thermal perturbations broadened the polydispersity index.