The influence of phenylsilane on the syndiotactic polymerization of styrene with η5‐pentamethylcyclopentadienyl titanium trifluoride

The syndiotactic polystyrene polymerization activity of a fluorinated half‐sandwich complex, η⁵‐pentamethylcyclopentadienyl titanium trifluoride (Cp*TiF₃), in the presence of relatively low amounts of methylalumoxane (MAO; MAO/Cp*TiF₃ molar ratio = 200/1) and triisobutylaluminum, is significantly increased by the addition of phenylsilane in molar ratios to Cp*TiF₃ ranging from about 300/1 to 600/1, if the phenylsilane is added to the monomer. Lower amounts of phenylsilane, such as a 100/1 molar ratio to Cp*TiF₃, lead to a reduced polymerization activity in comparison with styrene without phenylsilane. A prereaction of phenylsilane with the catalyst mixture shows a behavior that is strongly dependent on the storage time of the composition and the temperature. A storage time of about 16 h is sufficient to reduce the polymerization conversion to about half of the original value. The results are discussed on the basis of a chain‐transfer reaction with phenylsilane and several catalyst complexes of different stabilities and activities, including an alkylation product of phenylsilane. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3476–3485, 2000     

Synthesis and photophysical studies of new pyrenylamino acids

Two new pyrenylamino acid derivatives were synthesized from β-bromodehydroalanine derivatives in good yields using addition and elimination reactions.     

Influence of the catalyst on monomer insertion in the syndiospecific copolymerization of styrene and para‐methylstyrene

The effect of the kind of transition‐metal catalyst on the extent of comonomer insertion in the syndiospecific complex‐coordinative copolymerization of styrene and para‐methylstyrene has been investigated. The results for the influence of the polymerization conditions have shown that there is no real difference between solution copolymerization in toluene and solvent‐free styrene copolymerization in bulk, with respect to the reactivity ratio for para‐methylstyrene (r₂), under comparable conditions in the presence of methylaluminoxane and triisobutylaluminum and at low polymerization conversions. All the investigated catalysts lead to a preferred incorporation of para‐methylstyrene into the polymer chain in comparison with styrene and over the whole range of monomer compositions. The increasing capability of the different catalysts to provide copolymers with enhanced para‐methylstyrene concentrations can be summarized by the increasing r₂ values for the copolymerization in bulk as follows: η⁵‐pentamethylcyclopentadienyl titanium trichloride < η⁵‐octahydrofluorenyl titanium trimethoxide < η⁵‐octahydrofluorenyl titanium tristrifluoroacetate < η⁵‐cyclopentadienyl titanium(N,N‐dicyclohexylamido)dichloride < η⁵‐cyclopentadienyl titanium trichloride. For a correlation between the catalyst structure and the comonomer insertion, the catalysts can be described by electronic effects (electrostatic charge of the transition‐metal atom) and steric effects (minimum structural cone angle). The results show that the steric properties of the transition‐metal complexes have the most important effect on the insertion of para‐methylstyrene into the copolymer. If the minimum structural cone angle of the ligand of the transition‐metal catalyst decreases, the incorporation of the comonomer para‐methylstyrene increases significantly. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2061–2067, 2005     

Disentangling the K-complex of kaonic hydrogen with DEAR

The DEAR (DAΦNE Exotic Atom Research) experiment at the new φ-factory DAΦNE of Laboratori Nazionali di Frascati aims for a 1% measurement of the line shift of the K-complex due to strong interaction in kaonic hydrogen. This presentation is meant to prove that the high statistics and good resolution obtainable with DEAR will be able to disentangle the kaonic hydrogen K-complex lines (a cascade unconstrained fit), obtaining in this way constraints for what concerns the cascade parameters and, consequently, information regarding the physical processes involved in the kaonic hydrogen atomic cascade. This revised version was published online in August 2006 with corrections to the Cover Date.     

On the existence of frames

This paper deals with two topics, namely, frames and pairwise balanced designs (PBD's). Frames, which were introduced by W.D. Wallis for the construction of (skew) Room squares, are shown to exist for most orders congruent to 1 (mod 4). This result relies heavily on the existence of PBD's since the set F = {v | there is a frame of order v] is shown to be PBD-closed. By employing a generalization of the usual recursive construction for PBD's, it is shown that $\text{B}${5, 9, 13, 17}?$\text{B}${5, 9, 13}∪{69, 77, 97, 137, 237, 277, 317, 377, 569}?{n | n  1 (mod 4), n>0}?{29, 33, 49, 57, 93, 129, 133}, where $\text{B}$(K) denotes the set of orders of PBD's of index one having block-sizes from the set K. Frames of orders 5, 9, 13 and 17 are exhibited which immediately implies that F?$\text{B}${5, 9, 13, 17}. D.R. Stinson and W.D. Wallis have shown that {29, 49}?F. Thus there is a frame of order υ for every positive integer υ congruent to 1 (mod 4) with the possible exceptions of υ ? {33, 57, 93, 133}.     

Spectral holeburning properties ofR′ color centers in LiF: dependence on doping and irradiation processes

The effects of doping and radiation-damage processes on the persistent spectral hole burning properties ofR color centers in LiF were studied using current tuned GaAlAs diode-laser derivative spectroscopy. Dopants used were Mg, Ni and Co and irradiation was done by x-rays and neutrons. The holes were alwys burned near the center of the zerophonon line. In most cases the experimentally determined hole lineshapes including side holes could be satisfactorily fit by a phenomenological lineshape model with three adjustable parameters. The amplitudes of the side holes relative to the main hole were found to increase with increasing radiation damage.     

Die reduktive Spaltung von Azofarbstoffen durch Dihydrochinoxaline

The kinetics of the reduction of substituted azobenzenes by dihydroquinoxalines have been measured. Azo-compounds with electron-donating substituents in o- or p-position are reduced to the amines in neutral or acid solution, whereby two molecules of the reducing agent are consumed. If such substituents are absent, the reaction stops at the hydrazo stage. This is also the case with cleaveable azo-compounds in strongly alcaline solution. Our kinetic measurements of the reductive cleavage reaction of methylorange are in agreement with only one mechanism, which is discussed in detail. The very high reactivity of protonated methylorange may be attributed to the formation of a mesomeric quinoid structure. Knowing the pH-dependence of the reaction rate allows the determination of the pK-value of the extremely unstable hydrazo stage of methylorange. The structure of the dihydroquinoxaline has a strong influence on ΔH? and ΔS?, and therefore on the reaction rate.     

Elektrochemische Untersuchungen über das enantiomerenselektive Verhalten von chiralen Ionophoren in flüssigen Membranen

Electrochemical Investigations of the Enantiomer Selectivity of Chiral Ionophores in Liquid Membranes The enantiomer selectivity of a series of chiral neutral ionophores was investigated by electrochemical methods using ionophore-solvent-polymer membranes and chiral ammonium salts as substrates. The potentiometrically determined enantiomer selectivities are in good agreement with the results of electrodialytic transport studies.