Other -alkoxy radicals

even if /-amyloxy radicals (101) show similar specificity for addition i. v abstraction to /-butoxy radicals, abstraction will be of lesser importance. The reason is that most /-amyloxy radicals do not react directly with monomer. They undergo [3-seission and initiation is mainly by ethyl radicals. Ethyl radicals are much more selective and give addition rather than abstraction. This behavior has led to /-amyl peroxides and peroxyesters being promoted as superior to the corresponding /-butyl derivatives as polymerization initiators.  

The rate constant of (i-scission of cumyloxy radicals (102) is also significantly 

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B. Hydrogen Atom Abstraction is the other alkoxy-radical-like reaction of triplet n,7i carbonyls. The substrate can be another molecule, in which case a pair of radicals is formed or an accessible hydrogen in the same molecule, in which case a diradical is formed. 

After isomerisation occurs either oxidation (with formation of C3H7CHO) or an addition reaction with oxygen, forming the radical O2C4H8OH which in turn can isomerise. It is obvious that this mechanism has to be checked for other alkoxy radicals. 

Alkoxy radicals, such as those produced in reaction 4, can be vigorous hydrogen abstractors and may produce alcohols (eq. 12), but they can undergo other reactions as well. 

Because high temperatures are required to decompose diaLkyl peroxides at useful rates, P-scission of the resulting alkoxy radicals is more rapid and more extensive than for most other peroxide types. When methyl radicals are produced from alkoxy radicals, the diaLkyl peroxide precursors are very good initiators for cross-linking, grafting, and degradation reactions. When higher alkyl radicals such as ethyl radicals are produced, the diaLkyl peroxides are useful in vinyl monomer polymerizations. 

Other miscellaneous compounds that have been used as inhibitors are sulfur and certain sulfur compounds (qv), picryUiydrazyl derivatives, carbon black, and a number of soluble transition-metal salts (151). Both inhibition and acceleration have been reported for styrene polymerized in the presence of oxygen. The complexity of this system has been clearly demonstrated (152). The key reaction is the alternating copolymerization of styrene with oxygen to produce a polyperoxide, which at above 100°C decomposes to initiating alkoxy radicals. Therefore, depending on the temperature, oxygen can inhibit or accelerate the rate of polymerization. 

Dialkyl peroxydicarbonates have been reported as low temperature sources of alkoxy radicals (Scheme 3.30)lfMJfb and these radicals may be formed in relatively inert media. However, it is established, for primary and secondary peroxydicarbonates, that the rate of loss of carbon dioxide is slow compared to the rate of addition to most monomers or reaction with other substrates.186,187 Thus, in polymerizations carried out with diisopropyl peroxydicarbonate (47), chains will be initiated by isopropoxycarbonyloxy (48) rather than isopropoxy radicals (49) (see 3.4.2.2).188... 

Hydroperoxides react with transition metals in lower oxidation states (TiJ, Fe", Cu+, etc.) and a variety of other oxidants to give an alkoxy radical and hydroxide anion (Scheme 3.38)46 224,22"... 

The chemistry of atkoxycarbonyloxy radicals in many ways parallels that of the aroyloxy radicals (e.g. benzoyloxy, see 3.4.2.2.1). Products attributable to the reactions of alkoxy radicals generally arc not observed. This indicates that the rate of p-scission is slow relative to the rate of addition to monomers or other... 

We next examine the possible fates of the alkoxy radical produced as a result of hydroperoxide fragmentation. It should be noted that the other fragment produced in this process, an hydroxy radical (not shown), would be an extremely reactive species. Since it is not attached to a polymer chain end, it is also capable of more readily diffusing through the polymer matrix than most of the radicals discussed to this point. This also makes the photo-oxidation of the glycol potentially more destructive. 

The alkoxy radical of Scheme 18.3 (upper reaction) could scission to produce the same carboxyl radical as seen in the Norrish type 1 path (Scheme 18.1, path A) discussed above. As such, it is an additional source of CO2 but not taken into account in the report by Day and Wiles [25], Not reported but still obvious, the other fragment of this scission is an aliphatic aldehyde that could also have been one of the aldehyde carbonyl IR signals reported [11, 25], Hydrolysis of this chain end would yield the reported glyoxal [21],... 

It should additionally be noted that a number of the paths of the schemes above have received some confirmation in a number of literature reports dealing with the photolysis and photo-oxidation of other polyesters [32-35], Because these reports investigated poly(butylene terephthalate) (PBT), poly(ethylene naphthalate) and poly(butylene naphthalate), however, they may not have direct application to understanding of the processes involved in PET and PECT and so have not been discussed in this present chapter. All do contain support for the formation of radicals leading to CO and C02 evolution, as well as the hydrogen abstraction at glycolic carbons to form hydroperoxides which then decompose to form alkoxy radicals and the hydroxyl radical. These species then were postulated to undergo further reaction consistent with what we have proposed above. 

Similar chlorine atom elimination has been observed for other chlorine-containing alkoxy radicals (e.g., see Wu and Carr, 1992 and Bhatnagar and Carr,... 

Other examples of an attack on C from an alkoxy radical at C20 are recorded in the literature for instance, in the progesterone series the oxime (7) obtained from the photolysis of the corresponding nitrite was converted to the nitrile (8) J Oxidation and acid hydrolysis of the nitrile (8) gave 3,20-diketo-4-pregnen-18-oic acid (9) identical with a... 

The crucial step in all of these photochemical reactions is, of course, the formation of alkoxy radicals. When molecular environments do not permit the Barton-type reaction to proceed in the usual fashion, the alkoxy radicals decompose by other available paths. For example, in a... 

The oxygen atom of the activated alkoxy radical and the hydrogen to be abstracted and subsequently replaced by X, form two adjacent corners of the six-membered transition state. The major application of the Barton reaction have been in the synthesis of steroids particularly with compounds involving functionalization of Clg and C,9 which is difficult to achieve in other ways. 

The (n,n excited state of a ketone has electrophilic character, similar to that associated with alkoxy radicals, and it is not surprising that these excited states readily attack carbon-carbon multiple bonds. The overall reaction that normally ensues is a cycloaddition, giving a four-membered oxygen heterocycle—an oxetane from an alkene addend (4.62), or an oxete from an alkyne addend (4.63). Some oxetanes are of interest in their own right, but many are useful intermediates in the synthesis of other compounds. 

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