Peroxides, free radical initiators

Problem 15.44 Propanal reacts with 1-butene in the presence of uv or free-radical initiators (peroxides, sources of RO ) to give CHjCHjCOCHjCH CHjCH,. Give steps for a likely mechanism. ... 

For products needing high optical quality, syrups are produced by careful heating with precise stirring of monomer containing a small amount (0.02 to 0.1%) of a soluble free-radical initiator (peroxides, etc.) until a molasses-like consistency is achieved. 

The observation that there was a compensatory increase in the activity of CuZnSOD in the Mn-deficient mice exposed to 03 suggests that the increase in the activities of this enzyme is in part substrate-induced, and strongly supports the hypothesis that the increase in lung SOD activity is an important response to 03 exposure. Thus if the net increase in lung SOD activity is limited by the nutritional status of the animal, then excessive lung damage may occur due to free radical-initiated peroxidations. 

The isoprostanes are a unique series of prostaglandin-like compounds formed in vivo via a nonenzymatic mechanism involving the free radical-initiated peroxidation of arachidonic acid. This article summarizes our current knowledge of these compounds. Herein, a historical account of their discovery and the mechanism of their formation are described. Methods by which these compounds can be analyzed and quantified are also discussed, and the use of these molecules as biomarkers of in vivo oxidant stress is summarized. In addition to being accurate indices of lipid peroxidation, some isoprostanes possess potent biological activity. This activity will be discussed in detail. Finally, in more recent years, isoprostane-like compounds have been shown to be formed from polyunsaturated fatty acids, including eicosapentaenoic acid and docosahexaenoic acid. These findings will be summarized as well. 

The majority of commercial methacrylic ester polymers are produced by free-radical initiators. Peroxides and azo compounds ftinction as t5ipical initiators for this type of polymerization. Other possible routes for producing methacrylic polymers with radicals include photoinitiation and radiation-induced polymerization. Both Y ray and electron-beam radiation have been employed in the production of methacrylic ester polymers (36-38). At constant temperature, there is a first-order dependence of the polymerization rate on monomer concentration and a one-half-order dependence on initiator concentration. Rate data for the polymerization of various methacrylic monomers using the azo compoimd 2,2 -azobisisobut5ironitrile [78-67-1] (AIBN) are shown in Table 8. 

Hydrogen sulfide adds to olefins and unsaturated compounds to give thiols in good to excellent yields. The reaction is usually very rapid and is catalyzed either by free radical initiators (peroxides or ultraviolet light) or ionic types of catalysts such as transition metals, sulfuric acid, or even free sulfur. The free radical initiated reaction addition gives anti-MarkovnikolT addition products [Eq. (10)] whereas the ionic catalyzed reaction gives Markovnikoff addition products [Eq. (11)]. Both types of reactions work well in the laboratory and are also practiced commercially. 

Polymerization reactions. There are two broad types of polymerization reactions, those which involve a termination step and those which do not. An example that involves a termination step is free-radical polymerization of an alkene molecule. The polymerization requires a free radical from an initiator compound such as a peroxide. The initiator breaks down to form a free radical (e.g., CH3 or OH), which attaches to a molecule of alkene and in so doing generates another free radical. Consider the polymerization of vinyl chloride from a free-radical initiator R. An initiation step first occurs ... 

Step 1 Homolytic dissociation of a peroxide produces alkoxy radicals that serve as free radical initiators... 

PEROXIDES AND PEROXIDE COMPOUNDS - ORGANIC PEROXIDES] pol 18) -as initiators [INITIATORS - FREE-RADICAL INITIATORS] pol 14)... 

Usually, free-radical initiators such as azo compounds or peroxides are used to initiate the polymerization of acrylic monomers. Photochemical (72—74) and radiation-initiated (75) polymerizations are also well known. At a constant temperature, the initial rate of the bulk or solution radical polymerization of acrylic monomers is first order with respect to monomer concentration and one-half order with respect to the initiator concentration. Rate data for polymerization of several common acrylic monomers initiated with 2,2 -azobisisobutyronittile (AIBN) [78-67-1] have been determined and are shown in Table 6. The table also includes heats of polymerization and volume percent shrinkage data. 

LDPE, also known as high pressure polyethylene, is produced at pressures ranging from 82—276 MPa (800—2725 atm). Operating at 132—332°C, it may be produced by either a tubular or a stirred autoclave reactor. Reaction is sustained by continuously injecting free-radical initiators, such as peroxides, oxygen, or a combination of both, to the reactor feed. 

Commercially available MEKP formulations are mixtures of the dihydroperoxide (1), where X = OOH R = H, R = methyl, and R = ethyl (2,2-dihydroperoxybutane [2625-67 ]) and dialkyl peroxide (2), where X = OOH, Y = OOH, R = methyl, and R = ethyl (di(2-hydroperoxy-2-butyl) peroxide [126-76-1J). These formulations are widely used as free-radical initiators in the metal-promoted cure of unsaturated polyester resins at about 20°C. 

Thermal decomposition of hydroxyalkyl hydroperoxyalkyl peroxides produces mixtures of starting carbonyl compounds, mono- and dicarboxyHc acids, cycHc diperoxides, carbon dioxide, and water. One specific hydroxyalkyl hydroperoxyalkyl peroxide from cyclohexanone (2, X = OH, Y = OOH) is a soHd that is produced commercially as a free-radical initiator and bleaching agent (see Table 5). On controlled decomposition, it forms 1,12-dodecanedioic acid (150). 

Miscellaneous Reactions. Epoxy compounds yield chlorosubstituted carbonates (45). The reaction of chloroformates with hydrogen peroxide or metal peroxides results in the formation of peroxydicarbonates that are used as free-radical initiators of polymerization of vinyl chloride, ethylene, and other unsaturated monomers (46,47). 

The chlorination of toluene in the absence of catalysts that promote nuclear substitution occurs preferentially in the side chain. The reaction is promoted by free-radical initiators such as ultraviolet light or peroxides. Chlorination takes place in a stepwise manner and can be controlled to give good yields of the intermediate chlorination products. Small amounts of sequestering agents are sometimes used to remove trace amounts of heavy-metal ions that cause ring chlorination. 

Polyesters. Unsaturated polyester resins based on DCPD, maleic anhydride, and glycols have been manufactured for many years. At least four ways of incorporating DCPD into these resins have been described (45). The resins are mixed with a cross-linking compound, usually styrene, and final polymerization is accompHshed via a free-radical initiator such as methyl ethyl ketone peroxide. 

There has also been a trend in recent years to the use of free-radical initiators that decompose more rapidly than the traditional initiators such as caproyl and lauryl peroxides. Currently used initiators include peroxydicarbonates, t-butylperpiva-late, azobis-(2,4-dimethylvaleronitrile) and acetyl cyclohexylsulphonyl peroxide. 

Work should be on a scale of <0.5 g for novel but potentially explosive material until the hazards have been fully evaluated and <5 g for established, commercially available, substances such as peroxide free-radical initiators. 

Benzoyl peroxide is a commonly used free-radical initiator. It has the formula O O... 

Acrylamide is polymerized by the conventional free radical initiators, e.g., peroxides [27,28], redox pairs [29-33], and azo compounds [34]. Electro-chemical initi-... 

Random copolymer—Distribution of the monomer units does not follow any definite sequence. These are produced in bulk, aqueous, suspension, or emulsion using free radical initiators of the peroxide type or redox systems. 

Free radical initiators can polymerize olefmic compounds. These chemical compounds have a weak covalent bond that breaks easily into two free radicals when subjected to heat. Peroxides, hydroperoxides and azo compounds are commonly used. For example, heating peroxybenzoic acid forms two free radicals, which can initiate the polymerization reaction ... 

The polymerization of vinyl monomers on the surface of silica can be induced also by free radical initiators such as azo-bis-isobutyronitrile (AIBN), di-tert-butylperoxide, benzoyl peroxide etc. The selection of initiator type and method of its introduction in polymerizable systems are determined by the nature of monomers and tasks of investigations. Usually, the following procedures are used ... 

Reaction of Unsaturated Compounds with Dialkyl Phosphites The diesters of phosphorous acid are somewhat resistant to oxidation and used as intermediates for the reaction with olefins forming phosphonic acids. The reaction takes place in the presence of free radical initiators, such as di-/-bu-tyl peroxide [104,105]. 

For a system employing a free radical initiator (i.e. a peroxide or azo compound ... 

---