Crystallization, cold polymerization

For installations in cold climates, as well as applications in which prod-net freezing, polymerization, or crystallization is a concern, the flame arrester and piping shonld be insnlated and, and where reqnired, heat... 

Cyclodimer 3 proved to be somewhat difficult to manipulate, thus contributing to the complexity of its characterization. The bowed diacetylenic linkages revealed in the X-ray data impart surprising physical characteristics to the molecule. The energy-rich hydrocarbon was sufficiently strained that it decomposed explosively upon grinding (i. e. preparing a Nujol mull) or when heated above 80°C. At room temperature, crystals blackened within a few days and apparently auto-polymerized, even when stored under vacuum in the dark. Only dilute solutions of 3 in benzene or pyridine were fairly stable over time, especially when stored cold under an inert atmosphere. 

When wax crystal modifiers are added to cold fuel, even to fuel well above its cloud point, modifiers may not dissolve properly. The polymeric nature of wax crystal modifiers makes them quite viscous at low temperatures. Additive suppliers will often provide modifiers in a highly dilute form (i.e., 10% or 20% solution), so they will remain fluid at low temperatures. 

In addition to refining techniques, compounds identified as wax crystal modifiers are available for use in contending with the effects of wax in fuels. Wax crystal modifiers, also called pour point depressants or cold flow improvers, are typically polymeric compounds which have the ability to crystallize with fuel wax as it forms. By co-crystallizing with wax, the modifiers typically effect a change in the size, shape, and conformation of wax crystals. Other wax crystal modifiers function by dispersing or inhibiting the nucleation or growth of wax crystals within a fuel or oil. 

If it is warmed in aqueous solution, the nitrosoamide breaks down into water and nitrogen, and the cyanamide polymerizes to dicy-andiamide. The evaporation of the solution yields crystals of the latter substance. A cold aqueous solution of nitrosoguanidine acidified with hydrochloric acid yields nitrous acid, and may be used for the introduction of a nitroso group into dimethylaniline or some similar. substance which is soluble in the acidified aqueous liquid. 

Perfluorodistibatriptycene is unaffected by cold, concentrated nitric acid but dissolves with oxidation on boiling (ff) dilution of the solution with water results in the deposition of Sb2(C6F4)3(N03)2(0H)2 as needlelike, colorless crystals. This nitrate slowly hydrolyzes on repeated crystallization from water to give Sb2(C6F4)3(OH)4 attempts to dehydrate this tetrahydroxo derivative to the above polymeric oxide have so far failed. The stereochemistry of these Sb(V)-substituted triptycenes would be of great interest but all the crystals yet studied have proved to be twinned. 

From most solvents Ss crystallizes as orthorhombic a-Ss. Monoclinic pSg is stable above 96 °C and is usually obtained by cooling liquid sulfur slowly below the triple-point temperature of 115 °C. At 25 crystals of pSg convert to polycrystalline a-Ss in less than 1 h but are stable for several weeks at temperatures below -20 C [27]. y-S is metastable at all temperatmes and occasionally crystallizes by chance, for example from ethanolic solutions of ammonium polysulfide [28], by decomposition of copper ethybcanthate [29] or in the preparation of bis(dialkylthiophosphoryl)disulfane [30]. Smpris-ingly, y-Ss occurs also naturally as the minerd rosickyite. Fmthermore, 7-8 is a component of stretched plastic sulfur which is obtained by quenching liquid sulfur from 350 C to 20 °C (in cold water) and stretching the fibers obtained in the direction of their axes. According to an X-ray diffraction study, this fibrous sulfur consists of helical polymeric sulfur chains (S[Pg.7]

Some solutes may cause inhibition, i.e., reduce growth at quite low concentrations. Several, generally polymeric, substances can adsorb onto ice crystals and thereby reduce the freezing rate. This includes many proteins. Especially the antifreeze peptides, found in many plants and cold-blooded animals that have been subjected to cold stress, strongly decrease growth rate at fairly low concentrations. The presence of solutes and particulate material can also affect the shape of the ice crystals. 

Sulfur nitride polymer (polysulfur nitride, polythiazyl) [-(SN)-]n. First synthesized in 1910 but ignored for six decades, this covalent polymer has been restudied and found to have the physical and electrical properties of a metal. It is formed by passing the vapor of (SN)4 over a catalyst that cracks it to (SN)2, which is condensed on a cold surface where it spontaneously polymerizes into crystalline form. The crystals are malleable and can be cold-worked into thin sheets or fibers under pressure, having an electrical conductivity similar to that of mercury, and superconducting near 0 K. 

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