Osmium tetroxide, application method

Stress-strain relationships are determined by DMA and temperature scans reveal glass transitions, crystallization and melting information. Blends of polypropylene and rubber have been studied by where the intensity of one of the two crystallization exotherms was used as a measure of the polypropylene domains and compared to the size determined by TEM cryomicrotomy and osmium tetroxide staining methods [25]. Isothermal annealing of PET above the crystallization temperature was shown to influence the morphology and increase thermal stability by combined SAXS and DSC analysis [26]. An excellent text edited by Turi [21] described the instrumentation and theory of thermal analysis and its application to thermoplastics, copolymers, thermosets, elastomers, additives and fibers. 

Inclusion in the reaction of a cooxidant serves to return the osmium to the osmium tetroxide level of oxidation and allows for the use of osmium in catalytic amounts. Various cooxidants have been used for this purpose historically, the application of sodium or potassium chlorate in this regard was first reported by Hofmann [7]. Milas and co-workers [8,9] introduced the use of hydrogen peroxide in f-butyl alcohol as an alternative to the metal chlorates. Although catalytic cis dihydroxylation by using perchlorates or hydrogen peroxide usually gives good yields of diols, it is difficult to avoid overoxidation, which with some types of olefins becomes a serious limitation to the method. Superior cooxidants that minimize overoxidation are alkaline t-butylhydroperoxide, introduced by Sharpless and Akashi [10], and tertiary amine oxides such as A - rn e t h y I rn o r p h o I i n e - A - o x i d e (NMO), introduced by VanRheenen, Kelly, and Cha (the Upjohn process) [11], A new, important addition to this list of cooxidants is potassium ferricyanide, introduced by Minato, Yamamoto, and Tsuji in 1990 [12]. 

As a consequence of the development of the N-methylmorpholine N-oxide (NMO) and later the potassium ferricyanide cooxidant systems the amounts of osmium tetroxide and chiral ligand used in the reaction could be considerably reduced. However, the method remains problematic for large-scale applications. The cooxidants for Os(VI) are expensive and large amounts of waste are produced (Table 5). Lately, several groups have addressed this problem and new reoxidation processes for osmium(VI) species have been developed. 

Burton and Elad" in an investigation of a natural product introduced a new method of osmate cleavage. A solution of the olefinic product and osmium tetroxide in dioxane was let stand for 48 hrs. and hydrogen sulfide was passed in to precipitate hlack osmium dioxide this was removed by filtration and the diol recovered from the filtrate. An application of this procedure impressive for the scale on which costly chemicals were expended is reported by Hirschmann et al. of Merck in the synthesis from prednisone (I) of A-norcortisol (9), which proved to be devoid of physiological activity. A solution of 100 g. of prednisone BMD (2) in 720 ml. of pyridine was cooled to 5° and treated with a solution of 69.9 g. of osmium tetroxide (l.l X theoryI) in 40H ml. of pyridine. Black material began to separate in 5 min. After. 5 days at room temperature the reuclion mixture was stirred into 13.4 I. of petroleum ether and the precipitate was collected, washed, and dissolved in 8 I. of dioxane. The solution was cooled In an ice bath and saturated with hydrogen sulfide. The precipitated osmium dioxide was removed, and the bulk of the solvent removed... 

Osmium tetroxide also catalyzes the oxidation of organic sulfides to sulfones with NMO or trimethylamine iV-oxide (see Osmium Tetroxide-N-MethylmorphoUne N-Oxide). In contrast, most sulfides are not oxidized with stoichiometric amounts of OSO4. Oxidations of alkynes and alcohols with OSO4 without and in the presence of cooxidants have also been reported. However, these reactions have not found wide synthetic applications because of the availability of other methods. 

Andrews [107], who stained unsaturated synthetic rubbers, and then further developed by Kato [108-110], to show the morphology of rubber modified plastics and unsaturated latex particles. The polybutadiene in ABS pol)m[ ers is not apparent in unstained cross sections in the TEM, but staining results in contrast enhancement due to increased density of the unsaturated phase. Latex particles flatten and aggregate upon drying and early attempts at hardening, such as by bromination, were not considered successful. Thirty years after its first application, the method of osmium tetroxide staining is still widely and successfully applied to unsaturated rubbers and latexes. 

Aldehydes are more easily identified than are the parent compounds, since a wide range of standards is available from commercial sources or can be prepared synthetically from other lipids. As an example of the full application of this methodology, more than 30 different bases were detected in the sphingolipids of bovine kidney [469]. Mass spectrometry can be utilised as an aid to identification of aldehydes (see also Section B above), although some workers have preferred to reduce them to fatty alcohols and then to prepare acetate or TMS ether derivatives for this purpose [624]. In addition, all the methods for the location of double bonds in fatty acids, such as ozonolysis or hydroxylation with osmium tetroxide and preparation of TMS ethers for MS, have been utilised with aldehydes prepared from sphingoid bases [464,465]. 

Today, the most common methods of observing multiphase polymers are by phase contrast OM of thin sections, TEM of stained ultrathin sections, SEM of etched or fractured surfaces, and SPM of microtomed or etched surfaces. Osmium and ruthenium tetroxide are the most commonly used stains for observation of the dispersed phases in multiphase blends, whereas other stains have more limited application. Detailed fine structure of polymers is also made... 

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