Radical—radical reactions

There are two especially important radical—radical reactions of alkylperoxy radicals (28) both beheved to proceed via formation of a transient tetroxide (eqs. 13 and 15). 

Acids are usually the end products of ketone oxidations (41,42,44) but vicinal diketones and hydroperoxyketones are apparent intermediates (45). Acids are readily produced from vicinal diketones, perhaps through anhydrides (via, eg, a Bayer-ViUiger reaction) (46,47). The hydroperoxyketones reportedly decompose to diketones as well as to aldehydes and acids (45). Similar products are expected from radical— radical reactions of the corresponding peroxy radical precursors. 

The blue luminescence observed during cool flames is said to arise from electronically excited formaldehyde (60,69). The high energy required indicates radical— radical reactions are producing hot molecules. Quantum yields appear to be very low (10 to 10 ) (81). Cool flames never deposit carbon, in contrast to hot flames which emit much more intense, yellowish light and may deposit carbon (82). 

Other radical reactions not covered in this chapter are mentioned in the chapters that follow. These include additions to systems other than carbon-carbon double bonds [e.g. additions to aromatic systems (Section 3.4.2.2.1) and strained ring systems (Section 4.4.2)], transfer of heteroatoms [eg. chain transfer to disulfides (Section 6.2.2.2) and halocarbons (Section 6.2.2.4)] or groups of atoms [eg. in RAFT polymerization (Section 9.5.3)], and radical-radical reactions involving heteroatom-centered radicals or metal complexes [e g. in inhibition (Sections 3.5.2 and 5.3), NMP (Section 9.3.6) and ATRP (Section 9.4)]. 

The last comprehensive review of reactions between carbon-centered radicals appeared in 1973.142 Rate constants for radical-radical reactions in the liquid phase have been tabulated by Griller.14 The area has also been reviewed by Alfassi114 and Moad and Solomon.145 Radical-radical reactions arc, in general, very exothermic and activation barriers are extremely small even for highly resonance-stabilized radicals. As a consequence, reaction rate constants often approach the diffusion-controlled limit (typically -109 M 1 s"1). 

In studies of radical-radical reactions, radicals are typically generated pairwise and the products come from both cage and encounter (non-cage) reactions. 

The concentration of monomers in the aqueous phase is usually very low. This means that there is a greater chance that the initiator-derived radicals (I ) will undergo side reactions. Processes such as radical-radical reaction involving the initiator-derived and oligomeric species, primary radical termination, and transfer to initiator can be much more significant than in bulk, solution, or suspension polymerization and initiator efficiencies in emulsion polymerization are often very low. Initiation kinetics in emulsion polymerization are defined in terms of the entry coefficient (p) - a pseudo-first order rate coefficient for particle entry. 

The most important mechanism for the decay of propagating species in radical polymerization is radical-radical reaction by combination or disproportionation as shown in Scheme 5.1. This process is sometimes simply referred to as bimolecular termination. However, this term is misleading since most chain termination processes are bimolecular reactions. 

However, while it is generally accepted that the rate of radical-radical reaction is dependent on how fast the radical centers of the propagating chains (Pp and Pj ) come together, there remains some controversy as to the diffusion mechanism(s) and/or what constitutes the rate-determining step in the diffusion process. The steps in the process as postulated by North and coworkers30 3" arc shown conceptually in Scheme 5.5. 

Even though the rate of radical-radical reaction is determined by diffusion, this docs not mean there is no selectivity in the termination step. As with small radicals (Section 2.5), self-reaction may occur by combination or disproportionation. In some cases, there are multiple pathways for combination and disproportionation. Combination involves the coupling of two radicals (Scheme 5.1). The resulting polymer chain has a molecular weight equal to the sum of the molecular weights of the reactant species. If all chains are formed from initiator-derived radicals, then the combination product will have two initiator-derived ends. Disproportionation involves the transfer of a P-hydrogen from one propagating radical to the other. This results in the formation of two polymer molecules. Both chains have one initiator-derived end. One chain has an unsaturated end, the other has a saturated end (Scheme 5.1). 

Studies on VC polymerization are also complicated by the fact that only a small proportion of termination events may involve radical-radical reactions. Most termination is by transfer to monomer (Sections 4.3.1.2 and 6.2.63). Early studies on the termination mechanism which do not allow for this probably overestimate the importance of disproportionation.lb8 iw... 

If the amount of termination by radical-radical reaction is neglected the degree of polymerization and the kinetic chain length are given by eq. 29 ... 

Chains with uttdesired functionality from termination by combination or disproportionation cannot be totally avoided. Tn attempts to prepare a monofunctional polymer, any termination by combination will give rise to a difunctional impurity. Similarly, when a difunctional polymer is required, termination by disproportionation will yield a monofunctional impurity. The amount of termination by radical-radical reactions can be minimized by using the lowest practical rate of initiation (and of polymerization). Computer modeling has been used as a means of predicting the sources of chain ends during polymerization and examining their dependence on reaction conditions (Section 7.5.612 0 J The main limitations on accuracy are the precision of rate constants which characterize the polymerization. 

Dithiols and dienes may react spontaneously to afford dithiols or dienes depending on the monomer dithiol ratio.221 However, the precise mechanism of radical formation is not known. More commonly, pholoinilialion or conventional radical initiators are employed. The initiation process requires formation of a radical to abstract from thiol or add to the diene then propagation can occur according to the steps shown in Scheme 7.17 until termination occurs by radical-radical reaction. Termination is usually written as involving the monomer-derived radicals. The process is remarkably tolerant of oxygen and impurities. The kinetics of the tbiol-ene photopolymerizalion have been studied by Bowman and... 

The ends of polymer chains are often not representative of the overall chain composition. This arises because the initiator and transfer agent-derived radicals can show a high degree of selectivity for reaction with a particular monomer type (Section 3.4). Similarly, there is specificity in chain tennination. Transfer agents show a marked preference for particular propagating species (Section 6.2.2 and 6.2.3). The kinetics of copolymerization are such that the probability for termination of a given chain by radical-radical reaction also has a marked dependence on the nature of the last added units (Section 7.4.3). 

To avoid these stability problems, it is necessary to minimize the proportion of chains that terminate by radical-radical reaction. One way of achieving this is to conduct the polymerization in the presence of an appropriate chain transfer agent. For example, if polymerization is performed in the presence of a H-donor chain transfer agent, conditions can be chosen such that most chains terminate by hydrogen-atom transfer. Bagby et al.iA examined the thermal stability of PMMA formed with dodecanethiol. These polymer chains will then possess, more... 

Though there is still debate about detailed mechanism, in each of the processes (a-c) the propagating species is believed to be a conventional propagating radical. Thus, termination by radical-radical reaction is not eliminated, though, as we shall see, with appropriate choice of reaction conditions, the significance of this process can be markedly reduced. 

A characteristic of free radicals is the bimolecular radical-radical reaction which in many cases proceeds at the diffusion-controlled limit. These radical-radical reactions can occur either between two identical radicals or between unlike radicals, the two processes being known as self-termination and cross-termination reactions, respectively. 

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