Vinyl peroxy

The overall reaction and product stoichiometries for the degradation of chloroalkene substrates by O2 - in DMF are summarized in Table 7-1.20 Within the limits of a reaction time of 10 min or less, chloroethene, frflws-1,2-dichloroethene, Aldrin, and Dieldrin are not oxidized by O2 - in DMF. A reasonable mechanism for these oxidations is an initial nucleophilic addition of superoxide to the chloroalkenes [e.g., tetrachloroethene (Scheme (7-7)]. Subsequent loss of chloride ion would give a vinyl peroxy radical, which can cyclize and decompose to a chloroacyl radical and phosgene. t These would undergo subsequent facile reactions with O2 - to give bicarbonate and chloride ions. 

Further reaction of the vinyl peroxy radical would be analogous to that shown in Scheme 7-7. This sequence would lead to the ketones [RC(O)R] that are observed as products (Table 7-1). 

In the present work we attempt to gain a better understanding of the destruction mechanism of dibenzofuranyl + O2 system as well as the reaction pathways important in its oxygen-free decomposition. The difficulties encountered now are related to the large size of aromatic species contained in the dibenzofuranyl + O2 system making high level calculations very costly or not possible. The approach we have taken to circumvent the problem is to reduce the system to the smallest representative unit and then to proceed with the computation of the thermochemical properties (Figure 1.1). Based on this approach dibenzofuranyl peroxy radical (A) can be concentrated around the phenyl peroxy radical reaction system (B), which itself can be reduced to vinyl peroxy radical system (C). 

Figure 1.1 Dibenzofuranyl-peroxy / phenyl-peroxy / vinyl-peroxy systems...

Explosions involving flammable gases, vapours and dusts are diseussed in Chapter 6. In addition, eertain ehemieals may explode as a result of violent self-reaetion or deeomposition when subjeeted to meehanieal shoek, frietion, heat, light or eatalytie eontaminants. Substanees eontaining the atomie groupings listed in Table 7.7 are thermodynamieally unstable, or explosive. They inelude aeetylides and aeetylenie eompounds, partieular nitrogen eompounds, e.g. azides and fulminates, peroxy eompounds and vinyl eompounds. These unstable moieties ean be elassified further as in Table 7.8 for peroxides. Table 7.9 lists a seleetion of potentially-explosive eompounds. 

The theory of radiation-induced grafting has received extensive treatment. The direct effect of ionizing radiation in material is to produce active radical sites. A material s sensitivity to radiation ionization is reflected in its G value, which represents the number of radicals in a specific type (e.g., peroxy or allyl) produced in the material per 100 eV of energy absorbed. For example, the G value of poly(vinyl chloride) is 10-15, of PE is 6-8, and of polystyrene is 1.5-3. Regarding monomers, the G value of methyl methacrylate is 11.5, of acrylonitrile is 5.6, and of styrene is >0.69. 

Polyaddition reactions based on isocyanate-terminated poly(ethylene glycol)s and subsequent block copolymerization with styrene monomer were utilized for the impregnation of wood [54]. Hazer [55] prepared block copolymers containing poly(ethylene adipate) and po-ly(peroxy carbamate) by an addition of the respective isocyanate-terminated prepolymers to polyazoesters. By both bulk and solution polymerization and subsequent thermal polymerization in the presence of a vinyl monomer, multiblock copolymers could be formed. 

The stereochemistry of the vinyl ether is retained during its reaction with the a-peroxy lactone (41) which leads to a l,4-dioxan-2-one <96JOC8432>. 

DSC can be used effectively in the isothermal mode as well. In this case, the container with the sample is inserted into the DSC preheated to the desired test temperature. This type of experiment should be performed to examine systems for induction periods that occur with autocatalytic reactions and with inhibitor depletion reactions. (Reactions with induction periods can give misleading results in the DSC operated with increasing temperature scans.) Autocatalytic reactions are those whose rates are proportional to the concentration of one or more of the reaction products. Some hydroperoxides and peroxy esters exhibit autocatalytic decomposition. Inhibitor depletion can be a serious problem with certain vinyl monomers, such as styrene and acrylic acid, that can initiate polymerization at ambient temperatures and then selfheat into runaways. Isothermal DSC tests can be used to determine a time to runaway that is related to the inhibitor concentration. 

Trialkyl derivatives of boron, and in fact many other molecules such as boroxines with carbon-boron bonds, react readily with oxygen. The initial products are peroxy derivatives with BOOR bonds, which tend to react further to form borate esters. The ease of the initial reaction is shown by the fact that reported examples of vinyl polymerization induced by trialkyl borons require oxygen and are actually radical processes induced by the boron oxygen reaction or intermediate peroxides (7). 

Tsubokawa et al. (12-14) have introduced radical sources of azo or peroxy groups by another methods, and successively conducted the radical polymerization of vinyl compounds, such as styrene or methyl methacrylate, to give polymer-grafted particles see Reaction (3). The grafting by the radical polymerization of methyl methacrylate, initiated from a peroxy group introduced on silica, takes place at relatively high efficiency, compared with those from azo group-introduced particles. 

A peroxy acid mediated oxidative rearrangement of 2-alkoxy-3,4-dihydro-2//- pyrans affords 5-alkoxytetrahydrofuran-2-carbaldehydes (79JCS(Pi)847>. This reaction pathway was used in developing a method for the synthesis of optically active monoalkylfurans. (S)-2-Ethoxy-5-s-butyl-3,4-dihydro-2//-pyran (319), obtained through a cycloaddition reaction of (S)-2-s-butylacrolein to ethyl vinyl ether, was converted to (S)-2-s-butyl-5-ethoxytetrahydrofuran-2-carbaldehyde (320) (Scheme 85). 

The synthesis of epoxy ethers by peroxy acid treatment of suitable vinylic ethers, on the other hand, is complicated by the acid sensitivity of epoxy ethers. For example, Bergmann and Mk>keley1Ss claimed in 1921 to have prepared 1 -ethoxy-1 (2 -epoxyethane by the oxidation of ethyl vinyl ether with perbenzoic aoid, bat B years later modified their structure to a dioxone type of dimer.186 In 1 B0 Mous-seron and co-wcrkere1168-1184 reported the preparation of an epoxy ether from 1 -ethoxy-1 -eydohexene, but 4 years later Stevens and Taznma164 showed the compound obtained in this oxidation, not to have the structure initially assigned to it. 

Light-sensitive carboxylate complexes of Fe,n have been used to generate vinyl polymerization initiators in relief-imaging systems. The Fe11 generated in light-struck areas produces polymerization-initiating hydroxyl radicals when treated with peroxy compounds.244... 

Vinyl ethers, such as EtOCH=CH2, EtOCH=CHMe, EtOCH=CHOEt, etc. (with or without an additional oxygen function), have been shown to react with a-peroxy lactones (54) to give mainly the products of stereoselective cycloaddition (55) contaminated by epoxides (56).57... 

As shown in Figure 1.2, the solvent strength of supercritical carbon dioxide approaches that of hydrocarbons or halocarbons. As a solvent, C02 is often compared to fluorinated solvents. In general, most nonpolar molecules are soluble in C02, while most polar compounds and polymers are insoluble (Hyatt, 1984). High vapor pressure fluids (e.g., acetone, methanol, ethers), many vinyl monomers (e.g., acrylates, styrenics, and olefins), free-radical initiators (e.g., azo- and peroxy-based initiators), and fluorocarbons are soluble in liquid and supercritical C02. Water and highly ionic compounds, however, are fairly insoluble in C02 (King et al., 1992 Lowry and Erickson, 1927). Only two classes of polymers, siloxane-based polymers and amorphous fluoropolymers, are soluble in C02 at relatively mild conditions (T < 100 °C and P < 350 bar) (DeSimone et al., 1992, 1994 McHugh and Krukonis, 1994). 

The previous concepts may be illustrated with the experimental determination of the evolution of reaction rate, measured by DSC at T = 60°C, for the copolymerization of methyl methacrylate (MMA) with variable amounts of ethylene glycol dimethacrylate (EGDMA), a vinyl-divinyl system (Sun et al., 1997). The reaction was initiated with 2,5-dimethyl-2,5-bis(2-ethylhexanoyl)peroxy hexane. 

It is all true. Evidence that a silyl group stabilizes the S 2 transition state comes from the reactions of the epoxides of vinyl silanes. These compounds can be made stereospecifically with one equivalent of a buffered peroxy-acid such as m-CPBA. Epoxidation is as easy as the epoxidation of simple alkenes. You will see in a moment why acid must be avoided. 

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