Coupling terminates radical chains

Soluble Co compounds are generally employed in the autoxidation of hydrocarbons, i.e., the oxidation with O2 as the oxidant. In neat hydrocarbons, low concentrations of Co compounds accelerate the autoxidation since the Co2+/Co3+ couple is excellent for decomposing alkyl hydroperoxides and thus initiates free radical chain reactions. However, at high conversions, the Co may be deactivated by formation of insoluble clusters with side products of the hydrocarbon autoxidation. Moreover, high concentrations of a Co compound may actually inhibit the reaction because Co also terminates radical chains by reaction with ROO radicals ... 

The free-radical chain reaction may also be terminated by coupling of two carbon-radical species. As solvent carbon tetrachloride is commonly used, where the A-bromosuccinimide is badly soluble. Progress of reaction is then indicated by the decrease of the amount of precipitated NBS and the formation of the succinimide that floats on the surface of the organic liquid layer. 

The number of reported reactions in which the radical derived from the decomposition of AIBN plays a role in the termination process has increased considerably. Often these reactions are not radical chain reactions, since the initiator is used in stoichiometric amounts. A few examples of rearomatization of cyclohexadienyl radicals by disproportionation have been reported herein. Below are some other examples, where the phenyl selenide 61 reacts with (TMSfsSiH (3 equiv), AIBN (1.2 equiv) in refluxing benzene for 24 h to give the coupling product of radicals 63 and 64 in good yields (Scheme 9).i24,i25 these cases,... 

When an aqueous phase radical enters the polymer particles it becomes a polymer phase radical, which reacts with a monomer molecule starting a propagating polymer chain. This chain may be stopped by chain transfer to monomer, by chain transfer to agent or it may terminate by coupling. Small radicals in the particle may also desorb from or reenter the particle. In a batch reactor. Interval I indicates the new particle formation period, Interval II particle growth with no new particles, and Interval III the absence of monomer droplets. 

Unlike ionic polymerizations, the termination of the growing free radical chains usually occurs by coupling of two macroradicals. Thus, the kinetic chain length (v) is equal to DP/2. The chemical and kinetic equations for bimolecular termination are shown below (Equations 6.17 and 6.18). 

For classical free radical polymerizations the rate of propagation is proportional to the concentration of monomer and the square root of the initiator concentration. Termination usually occurs through a coupling of two live radical chains but can occur through disproportionation. The rate of termination for coupling is directly proportional to initiator concentration. The DP is directly proportional to monomer concentration and inversely proportional to the square root of the initiator concentration. 

The theoretical molecular weight distributions for cationic chain polymerizations are the same as those described in Sec. 3-11 for radical chain polymerizations terminating by reactions in which each propagating chain is converted to one dead polymer molecule, that is, not including the formation of a dead polymer molecule by bimolecular coupling of two propagating chains. Equations 2-86 through 2-89, 2-27, 2-96, and 2-97 withp defined by Eq. 3-185... 

Termination (by radical coupling, disproportionation, or chain transfer) ... 

The radical chain will form a terminated macromolecule either by coupling with another radical or by disproportionation, or by hydrogen abstraction from the polymerisation diluent. Note that monomolecular termination, leading to Mt-H species according to scheme (7), which is characteristic of... 

The use of hydrogen as terminal reductant has been accomplished by its activation with transition metal complexes. The resulting weak M-H bonds can be used in both radical generation and reduction through HAT. In this manner, conceptually novel radical chain reactions, such as hydrogen mediated cyclizations, or metal catalyzed processes with coupled catalytic cycles for radical generation and reduction, have been realized. The latter transformations are especially attractive for enantioselective synthesis. 

The reason why FR polymerizations are not living is that growing polymer radicals interact with each other, resulting in chain termination. There are several modes whereby polystyryl radicals become terminated, including radical coupling, disproportionation, and chain transfer. Total elimination of these bimolecular processes from an FR polymerization is impossible. However, if one can keep the FR concentration very low, the rate of these termination... 

With the recent development of living radical polymerization, the problem of gel formation during radical polymerization possibly can be controlled. This is because termination by radical chain coupling is virtually eliminated. Thus Hawker reported the preparation of soluble hyperbranched polystyrene using alkoxyamine IV as a living radical polymerization initiator [12]. 

So far we have taken for granted that initiation produces a chain carrier and that termination occurs by coupling of the chain carriers. Such behavior is the norm, but there are exceptions, owing to the ability of free radicals to transmit their... 

In the literature there is a lack of consensus on terminology regarding "termination." We follow Kennedy and Marechal [38] Termination irretrievably ends the kinetic chain (e.g., by coupling, disproportionation, or chain transfer to produce an inactive radical) chain breaking ends the growth of the respective polymer radical by whatever mechanism without necessarily terminating the kinetic chain, which, upon chain transfer, may or may not continue on another molecule. 

The radical chain length depends on the mechanism of chain breaking. The most common of these is coupling. Here, the chain-breaking (and termination) rate is... 

In copolymerization, several different combinations of initiation and termination mechanisms are possible, giving rise to a variety of different polymerization rate equations. Only two cases will be singled out here free-radical copolymerization with termination by coupling, and ionic polymerization with termination by chain transfer to a deactivating agent or impurity. For other combinations, the derivation of rate equations follows along the same lines. 

Termination occurs by radical coupling or disproportionation. Chain branching occurs. 

The corrqrlexity of the reaction heme conadered stems from the variety of radicals involved in the initiation and reinitiation steps (e. g., two different radical initiators, benzoyloxy and phenyl three different sites of attack in EPTM drains see Sect. 3.2.a), and in termination ste] vdiidr indude couplings between growing chains, rubber and solvent radicals. 

---