Abstract A number of diacylperoxides, peroxydicarbonates, peresters and dialkylperoxides with various substituents were chosen to investigate their homolytic decomposition under pressures of up to 3000 bar and high temperatures of up to 200°C, discontinuously in steel autoclaves, in an optical high pressure cell and in flow processes. The measurements, carried out in isododecane as solvent with a peroxide concentration of 1% by weight show that pressure has a strong influence on the decomposition: the half‐life time at 3000 bar is twice that at normal pressure. The pressure dependence, which varies according to the structure of the peroxide and the decomposition mechanism, is interpreted by means of the activation volumes. On the basis of the results, recommendations are made on the choice of peroxides suitable as polymerization initiators for application at high pressures and temperatures.
Abstract The thermal decomposition of the bifunctional peroxide 2,2-bis(tertbuty1 peroxy)butane was investigated in a flow reactor at temperatures of 135 to 212°C and pressures of up to 2000 bar. The concentration of the peroxide in isododecane was varied between 1 and 22% by weight. The rate of decomposition measured was used to determine the energy of activation (EA = 151.1 ± 3 kJ/mol) and the activation volume (Δv” = 22.3 mL/mol). Up to a peroxide concentration of 5% by weight, the decomposition takes place according to a firat-order rate equation. Above this concentration, induced decomposition takes effect. Gas chromatographic analysis showed that the liquid decomposition products consisted mainly of acetone and tert-butyl alcohol apart from methyl ethyl ketone, propionic acid methyl ester, acetic acid ethyl ester, tert-butyl methyl ether, and oligomers of the solvent. The gaseous decomposition products consisted of methane and carbon dioxide as well as small quantities of ethane, propane, and ethylene. A theoretical explanation of the mechanism of decomposition is postulated which explains the spectrum of these components and its changes as a function of the peroxide concentration, the residence time, the temperature, and the pressure.
Abstract The first part of this publication dealt with the effect of synthesis conditions on short and long‐chain branching of low density polyethylene; this second part reports on the effect on the molar mass, the molar mass distribution and the polydispersity that can be determined from these parameters. The average molar mass was measured with a membrane osmometer and the molar mass distribution by gel permeation chromatography, using the same samples as those used for the branching analysis. These samples were produced in continuously operating stirred autoclaves under widely differing polymerization conditions. The number‐average molar mass falls as the polymerization temperature and the initiator concentration rises and the pressure decreases. When the polymerization temperature is high, the molar mass distribution is narrower and less asymmetrical. When the polymerization temperature is low, there are broader, more asymmetrical distributions with a larger higher molar mass fraction. The polydispersity calculated from the molar mass distribution falls accordingly as the polymerization temperature rises. The polyethylene samples produced in a squat stirred autoclave have a higher polydispersity than those produced in a narrow autoclave. These findings have been confirmed on commercial products produced in various types of reactors.
Abstract To initiate the high-pressure polymerization of ethylene, oxygen is used together with organic peroxides in a number of tubular reactor processes. Since molecular oxygen is capable of promoting or inhibiting radical polymerization, depending on the reaction conditions chosen, controlled experiments were carried out to clarify these aspects of high pressure ethylene polymerization. In continuous polymerization tests carried out at 1700 bar and temperatures between 110 and 320°C, conversions were determined with tert-amyl perneodecanoate and di-tert-butyl peroxide initiation in the presence of various quantities of oxygen. Batch tests using a photo-initiator together with oxygen were also carried out. A comparison with polymerizations under conditions of careful elimination of oxygen shows no effect on the peroxide-initiated polymerization up to temperatures of 160 to 170°C. Although oxygen is an initiator at higher temperatures, the conversions obtained from the simultaneous addition of controlled quantities of oxygen and organic peroxides is lower than that obtained by adding together the conversions from the separate polymerizations.
Abstract Both organic peroxides and molecular oxygen can be used for the high-pressure polymerization of ethylene. In order to answer the question if the product quality is influenced by the choice of the initiator, polyethylenes were produced in a stirred autoclave under 1700 bar at temperatures of 200–320°C with DTBP, oxygen, and with mixtures of both initiators. The products were compared for density, short- and long-chain branching, the average molar mass, and the molar mass distribution. Except for a slight increase in the number of long-chain branches, and as a consequence of this a slightly broader molar mass distribution, no significant changes of product properties could be noted. There are no objections against the combined use of peroxide and oxygen; for example, when the conversion from oxygen to peroxide takes place.
To the Editors: We read the recent paper by Trivino-Duran et al1 with great interest. As mentioned by the authors, point prevalences for tinea capitis may vary according to the region under examination. Several countries (including Spain) have reported a rising incidence of scalp ringworm cases.2,3 For instance, in the United Kingdom, dermatology departments in large urban areas have documented rates of positive scalp isolates up to 20 times higher than previous baseline rates.4 To obtain data from Germany, we conducted a cross-sectional study among 5- to 6-year-old preschool children from Augsburg, Germany in 1996. The study was embedded in the multicentric international study for risk assessment of indoor and outdoor air pollution on allergy and eczema morbidity (MIRIAM). After parental informed consent was obtained, 1000 unselected preschoolers participating in the medical school entrance examination received a detailed dermatologic examination of the whole body. Additionally a brush sampling method was used to identify fungal organisms located at the scalp region. After cleansing with 70% alcoholic solution, a sterile tooth brush was run over the scalp ≈10 times to collect infected scales and hair debris. Afterward the brush was pressed into Sabouraud's dextrose agar containing chloramphenicol and cycloheximide. Cultures were read after 4 weeks of incubation at 26°C and identified by using recognized methods for morphologic classification. Among the 1000 examined preschoolers, clinical signs of tinea capitis were completely absent. Analysis of cultures revealed the presence of Trichophyton terrestre in a single case. This unexpected low prevalence of overt tinea capitis and asymptomatic carriers is in contrast to figures from the United States, where prevalence rates range between 3 and 8%,2 although also prevalence rates of <1% have been mentioned, at least for the Kansas region in the setting of an urban pediatric clinic.5 Our findings are similar to the results obtained by Cuétara et al from Madrid6 and now Trivino-Duran et al from Barcelona,1 adding hereby more epidemiologic data to this important infectious disease. Matthias Möhrenschlager, MD Harald Bruckbauer, MD Hans P. Seidl, PhD Johannes Ring, MD, PhD Heidelore Hofmann, MD Department of Dermatology and Allergy Biederstein Technical University of Munich Munich, Germany
Abstract In order to further our knowledge about the effect of synthesis conditions on the structure and properties of low density polyethylene, a large number of polymer samples was prepared at temperatures between 100 and 300°C and at pressures from 1100 to 2000 bar. A continuously operating polymerization plant with 3 agitated autoclaves each of different geometry was available for the synthesis. The aim was to use various organic peroxides and oxygen in order to find out whether the properties of the polyethylene produced also depend on the initiator. An analysis of the polymers revealed that high temperatures promote the formation of both short and long side chains which are characteristic of low density polyethylene. High pressures reduce the amount of branching. The type of initiator was not found to have any effect but a change in reactor geometry affects the long chain branching because of the accompanying change in temperature distribution.
Chemokinrezeptoren dürften in der Pathogenese des malignen Melanoms eine bedeutende Rolle spielen. Studien haben gezeigt, dass die Expression gewisser Chemokinrezeptoren wie CXCR3, CXCR4 und CCR7 mit erhöhter Motilität und Invasion von Melanomzellen einhergeht. Wir verglichen die Expressionsprofile von Chemokinrezeptoren in Melanozyten, Melanomzelllinien, kutanen Metastasen des malignen Melanoms, Keratinozyten der menschlichen Vorhaut, Endothelzellen und peripherem Blut mittels RT-PCR. Kryoschnitte (25µm) von gefrorenem Gewebe, das von 10 Patienten mit kutanen Metastasen stammte, wurden homogenisiert und anschließend RNA isoliert. CXCR4, CXCR5, CCR1 und CCR10 wurde von Melanozyten exprimiert. Melanomzellen exprimierten CXCR3, CXCR4, CXCR5, CCR1, CCR6, CCR7 und CCR10, wobei CXCR1, CXCR2, CCR2, CCR3, CCR4 und CCR5 in unterschiedlichem Ausmaße exprimiert wurden. Im Gegensatz zu Melanomzelllinien zeigten alle kutanen Metastasen zusätzlich eine konstante Expression von CXCR1, CXCR2 und CCR2. Die Expression von Chemokinrezeptoren in Keratinozyten war begrenzt auf CXCR3, CXCR4, CXCR5 und CCR1.
Abstract The suitability of difunctional organic peroxides for the synthesis of low density polyethylene (LDPE) was examined with a view to improve the results. Polymerization tests were carried out in a stirrer autoclave pilot plant using three different difunctional compounds with different levels of thermal stability at a pressure of 1700 bar and temperatures of between 180 and 290°C. The conversion and the specific peroxide consumption were measured and the average molar masses of the polymers obtained were determined. The solubility of the peroxides in isododecane was also examined.
