Bimolecular termination processe

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. 

The polymerizations (a) and (b) owe their success to what has become known as the persistent radical effect."1 Simply stated when a transient radical and a persistent radical are simultaneously generated, the cross reaction between the transient and persistent radicals will be favored over self-reaction of the transient radical. Self-reaction of the transient radicals leads to a build up in the concentration of the persistent species w hich favors cross termination with the persistent radical over homotermination. The hoinolermination reaction is thus self-suppressing. The effect can be generalized to a persistent species effect to embrace ATRP and other mechanisms mentioned in Sections 9.3 and 9.4. Many aspects of the kinetics of the processes discussed under (a) and (b) are similar,21 the difference being that (b) involves a bimolecular activation process. 

Like all controlled radical polymerization processes, ATRP relies on a rapid equilibration between a very small concentration of active radical sites and a much larger concentration of dormant species, in order to reduce the potential for bimolecular termination (Scheme 3). The radicals are generated via a reversible process catalyzed by a transition metal complex with a suitable redox manifold. An organic initiator (many initiators have been used but halides are the most common), homolytically transfers its halogen atom to the metal center, thereby raising its oxidation state. The radical species thus formed may then undergo addition to one or more vinyl monomer units before the halide is transferred back from the metal. The reader is directed to several comprehensive reviews of this field for more detailed information. 

The observed methane generation points to a plausible I —> III or II - III transformation, but it does not distinguish which of the structures (II or III) is the metathesis-active carbene. This matter is mechanistically significant with regard to the chain termination process. Type III may terminate by a bimolecular dimerization sequence as in Eq. (11), or it may convert to a 7r-olefin complex via an uncommon 1,2-hydride shift ... 

A typical effect observed in the synthesis of linear polymers by a free-radical mechanism is the auto-acceleration process. At a particular conversion, when sufficient polymer has accumulated in the system for the viscosity to reach a certain level, the rate of the bimolecular termination reaction begins to fall because of diffusional restrictions to the encounter of two chain ends. However, the initiation and growth rates are hardly affected. 

These chains cannot undergo bimolecular termination and so grow unhindered until a second free radical enters the latex particle. This environment is manifestly different from that in other growing latex particles and from that in the bulk system. This argument also explains why compartmentalization has no effect on the MWD if termination is by chain transfer because the chains containing one free radical can still undergo the transfer process, just as they do in the bulk system. 

In the early stages of autoxidations, hydroperoxide concentrations are low and chain initiation is inefficient. Under these conditions, Mn(II) and Co(II) can act as inhibitors by scavenging alkylperoxy radicals [reaction (278)]. Competition in the termination step between the usual bimolecular termination of peroxy radicals and their reaction with metal complexes can affect the chain length of the autoxidation. The expression for the chain length in a process involving bimolecular termination of peroxy radicals is... 

At high temperatures, both simplifications and complications of the above mechanism occur. Bimolecular initiation processes (involving at least one unsaturated molecule) cannot be excluded (see, for example, ref. 15). Transfer processes must be included since chains are no longer long. H abstraction from alkenes generates not only allylic type radicals, but also vinylic type radicals. As the temperature increases, allylic type radicals become thermally unstable. As the activation energy of unimolecular fissions of radicals is much higher than that of bimolecular processes such as metatheses, when the temperature increases the relative concentration of the p- radicals, compared with that of the thermally stable / and Y- radicals, decreases. Therefore, termination processes involve mainly / radicals (except for H- radicals, because they are very reactive and recombine in a third-order process) and Y-radicals. Finally, the addition of the most concentrated / and Y- radicals to unsaturated molecules can play a role, because this process is followed by a very fast unimolecular fission. For reasons of size limitation, the addition of radicals (e.g. H- and CH3-) will mainly be considered. Of course, the above a priori hypotheses about relative radical concentrations or reaction rates must be checked a posteriori, when numerical calculations have been carried out. 

In processes based on reversible termination, like NMCRP and ATRP (Sect. 4.4.2), a species is added which minimizes bimolecular termination by reversible coupling. In NMCRP this species is a nitroxide. The mechanism of nitroxide-mediated CRP is based on the reversible activation of dormant polymer chains (Pn-T) as shown in Scheme 1. This additional reaction step in the free-radical polymerization provides the living character and controls the molecular weight distribution. 

The early kinetic model by Smith and Ewart was based on Harkin s mechanistic understanding of the batch process. The particle population balances were written for a stationary state assuming that the rate of formation of particles with n radicals equals the rate of their disappearance (see equation at the bottom of this page). Where / , is the rate of radical entry into a particle (m /sec) is the rate constant for radical exit (m/sec) S is the particle surface area (m ) ktp is the rate constant for bimolecular termination in the particles (m /sec) and o is the particle volume. According to Smith and Ewart three limiting cases can be identified ... 

The kinetic chain length is thus inversely dependent on the radical concentration [Eq. (6.123)] or the polymerization rate [Eq. (6.124)]. This is of great practical significance as it shows that any attempt to increase the rate of polymerization by increasing the radical concentration will be only at the expense of producing smaller size polymer molecules. Equations (6.123) and (6.124) are applicable for all cases of bimolecular termination irrespective of the exact mechanism (combination or disproportionation) and also irrespective of the nature of the initiation process. Thus, for any monomer the Idnetic chain length will be Independent of whether the polymerization is initiated by thermal, redox, or photochemical means, or of the initiator used, if the [M ] or Rp is the same. 

Termination. Just as peroxy radicals are key to the propagation sequence, so the bimolecular recombination of these radicals is the major termination process in the unstabilized polymer. The existence of an intermediate tetroxide has been established in solution (25). Several factors influence the competitive pathways of subsequent decomposition to form alcohols, ketone and singlet oxygen or to form alkoxy radicals which can couple before separation from the reaction center to form a peroxide. This latter process is a route to crosslinking in the case of polymeric peroxy radicals. The effect of steric control, viscosity and temperature have been studied in solution. However, in the solid phase the rates of bimolecular processes which require the mutual diffusion of the reactant groups will be limited by the diffusion process. As a standard, we have assumed a value close to that determined from oxygen absorption (26) and by ESR spectra (27) for oxidized polypropylene films. 

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