Termination by transfer

This involves the removal of the radical from the propagating chain and the transfer of the radical to another chemical species. 

Both combination and disproportionation reactions result in the extinction of radicals. However, in the case of transfer reactions, the radical is not destroyed, but merely removed from the propagating chain and transferred to another species. The new radical may, dependent upon its stability, act to initiate the propagation of another polymer chain. 

Transfer reactions result in the termination of a growing chain, but not the extinction of the radical. 

The species AB in the example above, can be a solvent, a polymer, a modifier added specifically as a chain transfer agent, or an inhibitor. 

If the radical B does initiate polymerisation, then the overall rate of propagation is unaffected by the transfer reaction. However, the molecular weight of the polymer formed under these conditions, will be lower than that formed without the occurrence of transfer reactions. 

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Transfer to initiator can be a major complication in polymerizations initiated by diacyl peroxides. The importance of the process typically increases with monomer conversion and the consequent increase in the [initiator] [monomer] ratio.9 105160 162 In BPO initiated S polymerization, transfer to initiator may be lire major chain termination mechanism. For bulk S polymerization with 0.1 M BPO at 60 °C up to 75% of chains are terminated by transfer to initiator or primary radical termination (<75% conversion).7 A further consequence of the high incidence of chain transfer is that high conversion PS formed with BPO initiator tends to have a much narrower molecular weight distribution than that prepared with other initiators (e.g. AIBN) under similar conditions. 

The portion of the polymer consisting of molecules terminated by transfer will conform to the most probable distribution, its average degree of polymerization being... 

FIGURE 6 Speculative mechanism of Crl hydrocarbon biosynthesis from fatty acid hydroperoxides in algae. Homolytic cleavage of the hydroperoxide is assumed to give an allyl radical, which cyclizes to the thermolabile (1S,2R)-cyclopropane. The sequence is terminated by transfer of a hydrogen radical from C(16) to the -X-0 function. The cyclopropane rearranges to (6S)-ectocarpene as shown in Figure 4. 

The Mayo equation (Equation 6.42) that gives positive slopes when the data is plotted (such as Figure 6.3) is the reciprocal relationship derived from the expression cited earlier. The ratio of the rate of cessation or termination by transfer to the rate of propagation is called the chain transfer constant (Cs). 

In the polymerization reaction the monomer is coordinated at a vacant site of the metal, followed by insertion into the metal-carbon bond (Fig. 9.5-5). Finally, the chain-propagation is terminated by transfer of a hydrogen atom in a P-position to the metal or to the coordinated monomer. 

Chain growth continues at a rate dependent on the concentrations of monomer [M] and of active sites [MJ. Monomer exponents in the range 1.3 to 1.5 or higher had been observed (110, 123, 127) especially at low [M], but first order dependence has now been established over a broad range of [M] (21). A stationary level of [M ] is reached rapidly and is typically of the order of 10-8 molar. Chains grow rapidly by successive monomer additions until the polymer chain is terminated by transfer or by reaction with another radical. The rate constant for propagation (ft2) at 60° in DMF is 1960 m-1 Is-1 (16), which is a comparatively high value [see Table 1 and ref. (76)]. On the other hand it is only about one-tenth of that found for acrylonitrile in aqueous systems (Table 6)... 

The reaction mixture contained in a total volume of 1 mL 50 mM Tris-HC1 buffer (pH 8.0), 100 tnM MgG2, 3.8 mAf L-a,/3-dihydroxyisovaleric add (sodium salt), and crude extract containing 0.5 mg of protein. The reaction mixture was incubated for various times at 37°C, and the reaction was terminated by transferring 100 L of the assay mixture to a 200 fiL centrifuge tube containing 20 / L of 50% trichloroacetic acid. Predpitated proteins were removed by centrifugation. Then 50 fiL of the supemate was mixed with... 

In chain breaking exclusively by chain transfer to a deactivating transfer agent, Tr, total monomer consumption per unit time is again the sum of the initiation and propagation rates. However, of the monomer radicals M- produced by initiation and link-up, some are lost by chain transfer to the agent only a fraction P ends up as polymer, where P is the probability that a radical will add monomer rather than be terminated by transfer ... 

In many free-radical polymerizations, the molecular weight of the polymer produced is lower than that predicted from Eq. (6-64). This is because the growth of macroradicals in these systems was terminated by transfer of an atom to the macroradical from some other species in the reaction mixture. The donor species itself becomes a radical in the process, and the kinetic chain is not terminated if this new radical can add monomer. Although the rate of monomer consumption may not be altered by this change of radical site, the initial macroradical will have ceased to grow and its size is less than it would have been in the absence of the atom transfer process. These reactions are called chain transfer processes. They can be classified as varieties of propagation reactions (Section 6.3.2). 

Transfer of an anion from the counterion occurs to varying degrees depending on the stability of the counterion. Thus, counterions such as (PFe)"" and (SbCle) have little tendency to bring about termination by transfer of a halide ion, while counterions of aluminum and tin show appreciable transfer tendencies others such as (BF4)" and (FeCU) are intermediate in behavior. 

Taking into account also the termination by transfer to adventitious water, besides transfer to NH3, the number-average degree of polymerization is given by... 

As a new propagating species is generated each time a growing chain is terminated by transfer to monomer, many polymer molecules can result for each molecule of catalyst-cocatalyst complex initially formed. 

Chain Transfer and Termination There are a variety of reactions by which a propagating cationic chain may terminate by transferring its activity. Some of these reactions are analogous to those observed in cationic polymerization of alkenes (Chapter 8). Chain transfer to polymer is a common method of chain termination. Such a reaction in cationic polymerization of tetrahydrofuran is shown as an example in Fig. 10.1. Note that the chain transfer occurs by the same type of reaction that is involved in propagation described above and it leads to regeneration of the propagating species. Therefore, the kinetic chain is not affected and the overall effect is only the broadening of MWD. 

Ideally, free-radical polymerization involves three basic steps initiation, propagation, and termination, as discussed above. However, a fourth step, called chain transfer, is usually involved. In chain-transfer reactions, a growing polymer chain is deactivated or terminated by transferring its growth activity to a previously inactive species, as illustrated in Equation 2.17. 

In theory, the chain could continue to propagate imtil all the monomer in the system has been consumed, but for the fact that free radicals are particularly reactive species and interact as quickly as possible to form inactive covalent bonds. This means that short chains are produced if the radical concentration is high because the probabihty of radical interaction is correspondingly high, and the radical concentration should be kept small if long chains are required. Termination of chains can take place in several ways by (1) the interaction of two active chain ends, (2) the reaction of an active chain end with an initiator radical, (3) termination by transfer of the active center to another molecule which may be solvent, initiator, or monomer, or (4) interaction with impurities (e.g., oxygen) or inhibitors. 

The Inifer process developed by Kennedy can be used to functionalize vinyl monomers via a cationic route by initiating a polymerization with an alkyl halide-boron trichloride mixture R BCl. The termination by transfer to an alkyl halide leaves a halide-terminated polymer. This can be transformed to a hydroxyl terminal unit via the sequence (1) dehydrohalogenation, (2) hydroboration, and (3) oxidation and hydrolysis (Equation 5.24). These co-functional blocks may be coupled to form diblock copolymers using standard reaction techniques, e.g., diisocyanate will couple cohydroxy and co-amine blocks together. Direct reactions can also occur, and co-acid chlorides combine readily with co-hydroxy units. 

The degree of polymerization can be derived by dividing the propagation rate by the sum of the rates of terminations (by transfer) as follows ... 

On the other hand, transfer reactions are quite frequent. In aliphatic olefins, the growth of an individual chain is terminated by transfer to monomer with the formation of an unsaturated end group ... 

In this equation, DP and DP are the degrees of polymerization of the polymer with and without chain transfer agent, respectively, [5] is the concentration of chain transfer agent, [M] is the monomer concentration, and C is the chain transfer constant or the ratio of the rate of termination by transfer to the rate of propagation. This constant is related to relative bond strengths in the chain transfer agent and the stability of the new free radical produced. 

In addition to chain transfer processes arising from the P-H elimination reaction, the growing polymer chain can also be terminated by transfer reactions to alkylaluminum, MAO and monomer, all of which lead to a decrease of the molecular weights of sPS. In the presence of TIB A, the chain transfer reaction to aluminum is the dominant chain transfer reaction instead of P-H elimination. No unsaturated end group could be detected in the obtained polystyrene after deactivation with acidic... 

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