Thermal homolysis

The alkyl peroxyesters undergo homolysis, thermally and photochemically, to generate free radicals (168,213,229—232) ... 

A variety of compounds have been created by chemists to initiate radical reactions. These initiators can be used not only in addition reactions, but also radical substitutions and polymerizations. The common design principle is the incorporation of a weak bond that undergoes homolysis thermally or with light. Organic peroxides and azo compounds are common examples that can decompose by both means (Eqs. 10.41-10.43). The decomposition of benzoyl peroxide (a common light-activated acne medicine Eq. 10.42) leads to phenyl radical and carbon dioxide. 

In discussing mechanism (5.F) in the last chapter we noted that the entrapment of two reactive species in the same solvent cage may be considered a transition state in the reaction of these species. Reactions such as the thermal homolysis of peroxides and azo compounds result in the formation of two radicals already trapped together in a cage that promotes direct recombination, as with the 2-cyanopropyl radicals from 2,2 -azobisisobutyronitrile (AIBN),... 

Hydroperoxides are photo- and thermally sensitive and undergo initial oxygen—oxygen bond homolysis, and they are readily attacked by free radicals undergoing induced decompositions (eqs. 8—10). 

Chemical Properties. Acychc di-Z f/-alkyl peroxides efftciendy generate alkoxy free radicals by thermal or photolytic homolysis. 

Thermal decomposition of dihydroperoxides results in initial homolysis of an oxygen—oxygen bond foUowed by carbon—oxygen and carbon—carbon bond cleavages to yield mixtures of carbonyl compounds (ketones, aldehydes), esters, carboxyHc acids, hydrocarbons, and hydrogen peroxide. 

The radical X is formed by homolysis of the X—R bond either thermally or photolytically. In the reactions of alcohols with lead tetraacetate evidence suggests that the X—R bond (X = 0, R = Pb(OAc)3) has ionic character. In this case the oxy radical is formed by a one electron transfer (thermally or photochemically induced) from oxygen to lead. 

The S-S linkage of disulfides and the C-S linkage of certain sulfides can undergo photoinduced homolysis. The low reactivity of the sulfur-centered radicals in addition or abstraction processes means that primary radical termination can be a complication. The disulfides may also be extremely susceptible to transfer to initiator (Ci for 88 is ca 0.5, Sections 6.2.2.2 and 9.3.2). However, these features are used to advantage when the disulfides are used as initiators in the synthesis of tel ec he lies295 or in living radical polymerizations. 96 The most common initiators in this context are the dithiuram disulfides (88) which are both thermal and photochemical initiators. The corresponding monosulfides [e.g. (89)J are thermally stable but can be used as photoinitiators. The chemistry of these initiators is discussed in more detail in Section 9.3.2. 

The Mayo mechanism involves a thermal Diels-AIder reaction between two molecules of S to generate the adduct 95 which donates a hydrogen atom to another molecule of S to give the initiating radicals 96 and 97. The driving force for the molecule assisted homolysis is provided by formation of an aromatic ring. The Diels-AIder intermediate 95 has never been isolated. However, related compounds have been synthesized and shown to initiate S polymerization."110... 

The C-S bond of the sulfide end groups can be relatively weak and susceptible to thermal and photo- or radical-induced homolysis. This means that certain disulfides [for example 7-9] may act as iniferters in living radical polymerization and they can be used as precursors to block copolymers (Sections 7.5.1 and 9.3.2). 

As in the case of PS (Section 8.2.1) polymers formed by living radical polymerization (NMP, ATRP, RAFT) have thermally unstable labile chain ends. Although PMMA can be prepared by NMP, it is made difficult by the incidence of cross disproportionation.42 Thermal elimination, possibly by a homolysis-cross disproportionation mechanism, provides a route to narrow polydispersity macromonomers.43 Chemistries for end group replacement have been devised in the case of polymers formed by NMP (Section 9.3.6), ATRP (Section 9.4) and RAFT (Section 9.5.3). 

Sulfones are thermally very stable compounds, diaryl derivatives being more stable than alkyl aryl sulfones which, in turn, are more stable than dialkyl sulfones allyl and benzyl substituents facilitate the homolysis by lowering the C—S bond dissociation energy17. Arylazo aryl sulfones, on heating in neutral or weakly basic media at 100°C, yield an aryl and arenesulfonyl radical pair via a reversible one-bond fission followed by dediazoni-ation of the aryldiazenyl radical (see Scheme 2 below)20. However, photolysis provides a relatively easy method for generating sulfonyl radicals from compounds containing the S02 moiety. 

The rates of radical-forming thermal decomposition of four families of free radical initiators can be predicted from a sum of transition state and reactant state effects. The four families of initiators are trarw-symmetric bisalkyl diazenes,trans-phenyl, alkyl diazenes, peresters and hydrocarbons (carbon-carbon bond homolysis). Transition state effects are calculated by the HMD pi- delocalization energies of the alkyl radicals formed in the reactions. Reactant state effects are estimated from standard steric parameters. For each family of initiators, linear energy relationships have been created for calculating the rates at which members of the family decompose at given temperatures. These numerical relationships should be useful for predicting rates of decomposition for potential new initiators for the free radical polymerization of vinyl monomers under extraordinary conditions. 

Carbocation-carbanion zwitterionic intermediates were proposed for the thermal cleavage of several cyclic compounds. In most of these reactions the ionically dissociating bond belongs to one of four strained ring systems, i.e. cyclopropane (13), cyclobutane (14), cyclobutene (15) or norbornadiene (16). The mechanism is distinguished from the formation of a diradical intermediate through homolysis in terms of solvent and substituent effects... 

Bis(aryl)cobalt(II) compounds have been prepared by reaction of R MgX (where R = C6H6 Cl n = 2-4) with Co(PR3)2Cl2.203 They undergo both thermally and oxidatively induced decomposition, with the corresponding biphenyl a product. The reactions of alkyl-cobalt complexes have been reviewed recently, and include thermolysis, photolysis, oxidation, and reduction reactions.25 Homolysis of the Co—C bond is a feature of reactions. 

Radical Polymerization. Radical chain polymerization involves initiation, propagation, and termination. Consider the polymerization of ethylene. Initiation typically involves thermal homolysis of an initiator such as benzoyl peroxide... 

In the case of cobalt ions, the inverse reaction of Co111 reduction with hydroperoxide occurs also rather rapidly (see Table 10.3). The efficiency of redox catalysis is especially pronounced if we compare the rates of thermal homolysis of hydroperoxide with the rates of its decomposition in the presence of ions, for example, cobalt decomposes 1,1-dimethylethyl hydroperoxide in a chlorobenzene solution with the rate constant kd = 3.6 x 1012exp(—138.0/ RT) = 9.0 x 10—13 s—1 (293 K). The catalytic decay of hydroperoxide with the concentration [Co2+] = 10 4M occurs with the effective rate constant Vff=VA[Co2+] = 2.2 x 10 6 s— thus, the specific decomposition rates differ by six orders of magnitude, and this difference can be increased by increasing the catalyst concentration. The kinetic difference between the homolysis of the O—O bond and redox decomposition of ROOH is reasoned by the... 

The generation of the benzoyloxyl radical relies on the thermal or photoinitiated decomposition [reaction (49)] of dibenzoyl peroxide (DBPO). An early study (Janzen et al., 1972) showed that the kinetics of the thermal reaction between DBPO and PBN in benzene to give PhCOO-PBN" could be followed by monitoring [PhCOO-PBN ] from 38°C and upwards. The reaction was first order in [DBPO] and zero order in [PBN], and the rate constants evaluated for the homolysis of the 0—0 bond in DBPO (k = 3.7 x 10-8 s-1 at 38°C) agreed well with those of other studies at higher temperatures. Thus in benzene the homolytic decomposition mechanism of DBPO seems to prevail. 

Thermal cleavage of C—C bonds has been studied in cyclopropanes and cyclophanes, and particularly extensively in highly substituted alkanes. Riichardt and his school discovered a linear correlation between the experimental activation enthalpy for the homolysis of the weakest bond in overcrowded ethanes and the strain in the ground states of these molecules in accordance to MM2 calculations (284). 

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