Ziegler–Natta polymerization of butadiene: Kinetic and molecular weight study

The effect of reaction conditions on conversion and molecular weight in the polymerization of butadiene initiated by cobalt octoate/diethylaluminum chloride/water was studied. The polymerization exhibited complex kinetics with respect to cobalt concentration. The molecular weight, as a function of conversion, was investigated. Chain transfer to aluminum alkyl was of only minor importance.     

Crystallisation of Titania Prepared via the Peroxo and Normal Sol Gel Routes

Titania prepared via the peroxo route (PT) was found to have a higher condensation rate (lower crystallization temperature) and smaller particle size than that produced via the normal sol gel route (NT). These two factors play important roles in determining the resultant crystal phases (anatase, rutile) after thermal treatment. The dependency of resultant crystal phase on condensation rate and particle size has been investigated using the PT and NT. It was found that condensation rate is the most important parameter in determining the crystal phase of the titania formed.     

The transformation of CsCl type ⇋ NaCl type. Part II: Orientation relations in phase transformations CsCl type → NaCl type in ammonium halides

The polymorphic transformation CsCl type → NaCl type in ammonium chloride, bromide, and iodide is shown to be topotaxial and orientation relations derived by X-ray diffraction are reported for NH₄Cl and NH₄Br. These orientation relations are similar to those maintained in the reverse transformation NaCl type → CsCl type. At least the commonest relation can result from cooperative displacements. Transformed crystals of NH₄I and NH₄Br store stresses and imperfections sufficient to modify significantly the ideal thermodynamics of transformation.     

Anionic polymerization of isoprene in tetrahydrofuran: A study of isomerization reactions and ion-pair propagation rates

The ion-pair propagation rates for isoprene have been determined in tetrahydrofuran with counter-ions lithium and sodium. Because of the instability of the carbonions of diene monomers in this solvent, a parallel spectroscopic study was carried out to check for the occurrence of side-reactions. With Li⁺ counter-ion, the anions were found to be stable up to 0° but with Na⁺, instability was noticeable even at ?40° and may be significant at even lower temperatures. For these reasons, the lithium ion-pair rates show normal behaviour but those for sodium are difficult to interpret.     

An enzyme library approach to biocatalysis: development of nitrilases for enantioselective production of carboxylic acid derivatives

The discovery, from Nature, of a large and diverse set of nitrilases is reported. The utility of this nitrilase library for identifying enzymes that catalyze efficient production of valuable hydroxy carboxylic acid derivatives is demonstrated. Unprecedented enantioselectivity and substrate scope are highlighted for three newly discovered and distinct nitrilases. For example, a wide array of (R)-mandelic acid derivatives and analogues were produced with high rates, yields, and enantiomeric excesses (95-99% ee). We also have found nitrilases that provide direct access to (S)-phenyllactic acid and other aryllactic acid derivatives, again with high yields and enantioselectivities. Finally, different nitrilases have been discovered that catalyze enantiotopic hydrolysis of 3-hydroxyglutaronitrile to afford either enantiomer of 4-cyano-3-hydroxybutyric acid with high enantiomeric excesses (>95% ee). The first enzymes are reported that effect this transformation to furnish the (R)-4-cyano-3-hydroxybutyric acid which is a precursor to the blockbuster drug Lipitor.     

Influence of Surfactant and Humidity on Sol-Gel Macroporous Organosilicate Coatings

In the preparation of macroporous hydrophobic organosilicate films using methyltriethoxysilane (MTES) as precursor, the effects of surfactant addition, surfactant properties and atmospheric humidity were explored. As films dried, preferential evaporation of the ethanol resulted in an increase of the relative water content. This led to development of phase separation between the hydrophobic gel and the aqueous liquid and ultimately the formation of macropores. In the presence of surfactant, surfactant adsorption at the aqueous phase/gel interface affected the extent of phase separation therefore the resulting pores. Span 20 surfactant (HLB = 8.6) has lower compatibility with the aqueous phase than Tween 20 (HLB = 16.7) and effectively increases the hydrophobicity of the gel phase leading to the formation of larger pores. An increase in Span 20 content from 2 wt.% to 5 wt.% also increased pore size. Film porosity also increased significantly with humidity inside the coating chamber. It would appear that the increased porosity is a result of increased phase separation caused by reduced water evaporation at the higher humidity. Highly macroporous (up to 80% porosity), reproducible and uniform films were obtained by incorporating Span 20 surfactant into the coating solutions and performing dip coating at 80% relative humidity.     

NMR-on of182Ta and183Ta in Fe

Nuclear magnetic resonance has been observed on radioactive¹⁸²Ta and¹⁸³Ta oriented at low temperature in an Fe host, by detection of the change in spatial anisotropy of γ-rays emitted during nuclear decay. By measuring the resonant frequencies of¹⁸³Ta in four different applied magnetic fields the nuclear magnetic moment and hyperfine field have been deduced. These are: $$|\mu \left( {{}^{183}Ta; I = \tfrac{\user2{7}}{\user2{2}}} \right)| = 2.28(3)\mu _{\rm N} and B_{hf} \left( {Ta\underline {Fe} at 0 K} \right) = - 67.2(1.3)T$$ . The spin of the ground state of¹⁸²Ta has been determined asI=3 by comparing resonance results with those obtained in a thermal equilibrium nuclear orientation study. The ratio of the resonant frequencies observed for¹⁸²Ta and¹⁸³Ta at one applied field value yields a magnetic moment for the former of $$|\mu \left( {{}^{182}Ta; I = \user2{3}} \right)| = 2.91(3)\mu _{\rm N} $$ . The spin lattice relaxation time for¹⁸³TaFe (0.12 at% Ta) at 18 mK in an applied field of 0.5 T has been found to be 40(10) s.     

One‐Electron Oxidation of [M(PtBu3)2] (M=Pd,Pt): Isolation of Monomeric [Pd(PtBu3)2]+ and Redox‐Promoted C−H Bond Cyclometalation

Oxidation of zero‐valent phosphine complexes [M(P^tBu₃)₂] (M=Pd, Pt) has been investigated in 1,2‐difluorobenzene solution using cyclic voltammetry and subsequently using the ferrocenium cation as a chemical redox agent. In the case of palladium, a mononuclear paramagnetic Pd^I derivative was readily isolated from solution and fully characterized (EPR, X‐ray crystallography). While in situ electrochemical measurements are consistent with initial one‐electron oxidation, the heavier congener undergoes C?H bond cyclometalation and ultimately affords the 14 valence‐electron Pt^II complex [Pt(κ²_PC‐P^tBu₂CMe₂CH₂)(P^tBu₃)]⁺ with concomitant formation of [Pt(P^tBu₃)₂H]⁺.