Diacyl peroxides, organic peroxide initiators

See related DIACYL PEROXIDES, ORGANIC azides, static initiation incidents... 

Peroxides. See also Inorganic peroxides Organic peroxides acid hydrolysis of, 23 459 diacyl, 24 282-284 explosive, 20 569-573 formation of, 20 577 as free-radical initiators, 24 279-293 organomercury-containing, 23 445 potassium salts of, 18 478 silylation and, 22 703 stereoisomers of, 28 459 as vulcanizing agents, 22 795 ... 

Qiu et al. [11] reported that the aromatic tertiary amine with an electron-rich group on the N atom would favor nucleophilic displacement and thus increase the rate of decomposition of diacyl peroxide with the result of increasing the rate of polymerization (Table 1). They also pointed out that in the MMA polymerization using organic peroxide initiator alone the order of the rate of polymerization Rp is as follows ... 

In the polymerization of acrylic monomers by bulk, suspension, or in organic solution, the most common initiators are diacyl peroxide (e.g., dibenzoyl peroxide supplied as a paste in water) or azo compounds (e.g., 2,2 -azobisisobutyronitrile). For emulsion or aqueous solution polymerizations, sodium persulfate by itself or in combination with bisulfites or a host of other reducing agents may be used. 

The solution, having been made stable in the can, must now be made reactive on the coated surface. In the case of unsaturated polyesters, this is done by introducing factor (3) in relatively large quantities. This is an initiator (p. 65). In thermally cured, as opposed to radiation cured finishes, the substance chosen to decompose and produce free radicals is an organic peroxide. A solution of this material in unreactive solvent forms the activator pack of the two-pack finish. The amount of solvent is chosen to give a suitable finish/activator mixing ratio, e.g. 10 1 by volume. The ketone, diacyl and hydroperoxide types are most frequently used. 

The family of dialkyl peroxides includes dicumyl peroxide, which accounts for one-third of the volume of dialkyls world-wide and is the workhorse of this family of peroxides. Dicumyl peroxide is commonly used as a catalyst in polyester resin systans and for cross-linking polyethylene. Benzoyl peroxide is the most common of the diacyl peroxides. It is also used as a catalyst for curing polyester resins. Hydroperoxides are generally used as a raw material to produce other organic peroxides. The most common peroxides in this family include cumene hydroperoxide and t-butyl hydroperoxide. Ketone peroxides are mixtures of peroxides and hydroperoxides that are commonly used for room-temperature curing of polyester resins. Methyl ethyl ketone peroxide (MEKP) is the major product in this family. Peroxydicarbonates are largely used to initiate polymerization of polyvinyl chloride (PVC). 

For the past ten years we have been using free-radical systems to study the chemistry of organic solids. This work originated with studies of the photolysis of solid azoalkanes in P. D. Bartlett s laboratory at Harvard. U,2 ) Much of our effort has been devoted to epr spectroscopy of radical pairs generated in low concentration by photolyzing single crystals of such standard radical initiators as azoalkanes and diacyl peroxides. In several cases this tool has allowed us to determine where molecules in a solid move during chemical reaction and how easily they move. (3 )... 

Organic peroxides act through the splitting of the —0—0— bond into free radicals, thereby initiating the polymerization or crosslinking of monomers or polymers. Their exceptionally broad line includes diacyl peroxides, dialkyl peroxides, hydroperoxides, ketone peroxides, peroxyketals, peroxydicarbonates, and peroxyesters. The last two are particularly important in PVC resin manufacture as initiators in the polymerization of vinyl chloride monomer. 

The initiator usually is created in a redox reaction that follows a free radical pathway. Common reactions involve the reduction of an organic peroxide by some reducing agent such as an amine or an ion capable of undergoing a one-electron transfer reaction. One example of an efficient free radical initiator generating reaction is the reduction of diacyl peroxides such as benzoyl peroxide (BPO) by tertiary aromatic amines such as N,N-dimethyl aniline (DMA). 

This is called an initiation step. It may happen spontaneously or may be induced by heat or light (see the discussion on p. 279), depending on the type of bond. Peroxides, including hydrogen peroxide, dialkyl, diacyl, and alkyl acyl peroxides, and peroxyacids are the most common source of free radicals induced spontaneously or by heat, but other organic compounds with low-energy bonds, such as azo compounds, are also used. Molecules that are cleaved by light are most often chlorine, bromine, and various ketones (see Chapter 7). Radicals can also be formed... 

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