Intermediate radical termination

Let us analyze the system behavior in terms of dimensionless concentrations at different values of such key parameters. In Figure 2, the log-log plot of the dimensionless concentrations of radicals (r) and intermediate radicals (q) versus the non-dimensional reaction time (t) is shown for the case a < 1 and in the absence of intermediate radical termination (sr = 0). Looking at the radical concentration, two well-distinct regions can be identified in the first one, the radical concentration grows with slope one while in the second one a steady-state value is established. This takes place at dimensional time t2 — l/ ktR ) (corresponding to T2 of Figure 1), which corresponds to the characteristic time of the termination reactions. It can be easily proved that such a behavior is equal to that of a non-living polymeriza-... 

If intermediate radicals are consumed in side reactions, such as combination or disproportionation with another radical species, this will also cause retardation. That so-called intermediate radical termination might complicate RAFT polymerization was first proposed by Monteiro and de Brouwer in 2001. " That a polystyrene intermediate radical can combine with a polystyrene propagating radical to form a stable three-armed star has been demonstrated. However, attempts to detect these species in the expeeted concentrations in polymerization have failed. 

Modem real time instmmental methods permit analyses of unstable transient species and the free-radical intermediates as well. These methods have gready expanded the scope and power of VPO studies, but important basic questions remain unresolved. Another complication is the role of surface. Peroxide decompositions and radical termination reactions can occur on a surface so that, depending on circumstances, surfaces can have either an inhibiting or accelerating effect. Each surface has varying amounts of adventitious contaminants and also accumulates deposits during reaction. Thus no two surfaces are exactly alike and each changes with time. 

Isocyanides can be reduced to the corresponding hydrocarbons by (TMS)3SiH. The reaction can be considered as a smooth route for the deamination of primary amines. An example is given in Reaction (20). The key step for these chain reactions is expected to be the fragmentation of the intermediate radical derived from the fast addition of (TMSlsSi radical to the terminal carbon atom. 

The efficiency of the chosen inhibitor can be expressed by the ratio F7[In H]. This ratio does not depend on the antioxidant concentration if the latter terminates the chains and the intermediate radical In does not propagate through the chains (see later). 

GICs, via the one-electron process in both cases. According to Shu et al., these intermediate species underwent further single-electron reduction and produce Li2C03 and propylene gas, while alkyl carbonates are generated via radical termination as shown in Schemes 1 and yss zn become the major ingredients in the surface film. 

The use of zeolites can overcome many of these limitations and provide new controlled entries into these oxidized hydrocarbons and new materials. For example, some of the most valuable industrial intermediates are terminally oxidized hydrocarbons, snch as n-hexanol or adipic acid, that are not readily available in free-radical chain processes. The ability of zeolites to function as shape-selective catalysts can, in principle, be used to restrict access, by reactant or transition state selectivity, to sites not normally attacked by oxidants [3]. 

The oxidative degradation represented by the foregoing reactions is referred to as peroxidation. Peroxidation can lead to rapid development of rancidity in fats and oils. However, the presence of a small amount of tocopherol inhibits this decomposition, presumably by trapping the intermediate radicals in the form of the more stable tocopherol radicals (Eq. 15-54), which may dimerize or react with other radicals to terminate the chain. 

Trifluoromethyl isocyanide traps the bis(trifluoromethyI)nitroxyl radical at the terminal carbon atom with the formation of an intermediate radical that decomposes spontaneously to give trifluoromethyl isocyanate as the end product (Table 14).238... 

The composition of the reaction products (1 1 adducts) needs further clarification. In the case of terminal olefins the anti-Markovnikov 1 1 addition product is almost the only 1 1 adduct, whereas the isomeric amide is formed in minute amounts only. Markovnikov-additions of free radicals to olefins have been observed in other cases too as side products (28). The point of initial attack in the free radical addition to an olefin of the type RCH=CH2 is at the terminal carbon. The intermediate radical (I) produced by this process (anti-Markovnikov) has a higher degree of resonance stabilization than the alternative radical (II) (4, 78). This means that in the present reaction,... 

What would it take to achieve better control over radical polymerization so that, for example, block copolymers could be prepared Remember that the key to making block copolymers anionically is the living nature of the intermediate—chain termination does not compete with initiation and propagation. Could we design a free radical system in which we could turn off termination reactions until we wanted them ... 

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