Propagation reactions, radicals

A typical example of a nonpolymeric chain-propagating radical reaction is the anti-Markovnikov addition of hydrogen sulfide to a terminal olefin. The mechanism involves alternating abstraction and addition reactions in the propagating steps ... 

The degree of polymerization is controlled by the rate of addition of the initiator. Reaction in the presence of an initiator proceeds in two steps. First, the rate-determining decomposition of initiator to free radicals. Secondly, the addition of a monomer unit to form a chain radical, the propagation step (Fig. 2) (9). Such regeneration of the radical is characteristic of chain reactions. Some of the mote common initiators and their half-life values are Hsted in Table 3 (10). 

In the reaction of 2,3,3-triethyloxazirane (25), three radicals are involved 26, 27, and 28. Radical 26 (Fig. 1) corresponds to the chain reaction propagating radical of the previously mentioned decomposition [Eqs. (20) and (21)]. From 26 hy fragmentation an ethyl radical (27) is formed together with the acid amide. Finally, by radical attack on the oxazirane, 29 can be formed which rearranges to the... 

Like many radical reactions in the laboratory, methane chlorination requires three kinds of steps initiation, propagation, and termination. 

A radical polymerization involves free radical ends which of course do not associate and which interact only weakly with solvents. Consequently, the early investigators assumed that the course of propagation of radical polymerization is independent of the environment (see, for example, the recent monograph by Walling60). Actually, more recent studies, notably by Russell,36 showed that the nature of the solvent sometimes might considerably affect even the course of radical reactions. Therefore, unusual behavior of the propagation step might be expected in certain solvents. 

The basic Hammett scheme often does not offer a perfect correlation and a number of variants on this scheme have been proposed to better explain reactivities in radical reactions.-0 However, none of these has achieved widespread acceptance. It should also be noted that linear free energy relationships are the basis of the Q-e and Patterns of Reactivity schemes for understanding reactivities of propagating species in chain transfer and copolymerization. 

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). 

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). 

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. 

The book commences with a general introduction outlining the basic concepts. This is followed by a chapter on radical reactions that is intended to lay the theoretical ground-work for the succeeding chapters on initiation, propagation, and termination. Because of its importance, radical copolymerization is treated in a separate chapter. We then consider some of the implications of these chapters by... 

Cross-linking oligomers (mol wt -200-1000), containing multiple unsaturation, are used to promote rapid free-radical-induced propagation reactions leading to cross-linked strucmres [39]. 

In the following sections, the types of radical chain-propagating and chainbranching reactions responsible for combustion under both low- and high-temperature conditions will be discussed. 

This reaction has been the subject of a great deal of theoretical and mechanistic study, largely because of the commercial importance of the polymers to which it can give rise. Like the other radical reactions we have discussed, it can be said to involve three stages—(a) initiation, (i>) propagation, and (c) termination ... 

During the course of radical chain propagation, several types of transfer reaction can occur, leading to the interruption of the "material" chain, while allowing the "kinetic" chain to continue. "Different" end groups can appear as a result of such transfer reactions (Figure 20). 

A sequence of elementary steps of radical reaction leading to the regeneration of the original radical is called the chain cycle, whereas the particular reaction steps are the events of chain propagation. 

Mechanism II. Reaction R02 + RH occurs slowly and tertiary peroxyl radicals react more rapidly with the formation of alkoxyl radicals. Chain propagation includes the following steps ... 

Chain propagation proceeds in ester oxidation via the reaction of the ester peroxyl radical with weakest C—H bond of the ester. Is the activity of the ester peroxyl radical same as alkyl peroxyl radical Let us compare the rate constants of two different peroxyl radicals reactions with the same ester group, namely, RCH2C(0)0CH2R. 

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