Termination by coupling

If chain tennination occurs only by coupling or combination, each polymer molecule consists of two kinetic chains which grew independently and were joined together following their mutual termination. The polymer formed is analogous to a di-chain (J = 2) condensation polymer (Chapter 5). If P represents the probability of continuation of either ehain from one of its units to the next, (1 - F) is the probability that a given unit in the chain reacts by termination and the latter can be equated to the ratio of terminated to total units. Sinee two units are involved in each termination step, one can then write 

Substitution of [M ] = Rpjkp M] from Eq. (6.12) into Eq. (6.180) and simpU cation leads to 

since each molecule eonsists of two ehains, the number-average degree of polymerization will be given by [cf. Eq. (6.175)], 

For the weight-average degree of polymerization, the following equation can be shown (Tanford, 1961) to be applicable  

The ratio in Eq. (6.184) has a limiting value of 1.5 at high polymer moleeular weights when Rp Ri and P approaehes 1. This is narrower than the distribution produeed in the absence of termination by coupling [cf. Eq. (6.177)], which is, however, quite expeeted. Since the probabiUty for coupling between same-sized propagating radicals is the same as that between different-sized radicals, polymer molecules of sizes close to the average (and hence of relatively narrow size distribution) are more likely. 

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Two other important commercial uses of initiators are in polymer cross-linking and polymer degradation. In a cross-linking reaction, atom abstraction, usually a hydrogen abstraction, occurs, followed by termination by coupling of two polymer radicals to form a covalent cross-link ... 

The main reason that the decreases as the polymerization temperature increases is the increase in the initiation and termination reactions, which leads to a decrease in the kinetic chain length (Fig. 17). At low temperature, the main termination mechanism is polystyryl radical coupling, but as the temperature increases, radical disproportionation becomes increasingly important. Termination by coupling results in higher PS than any of the other termination modes. 

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. 

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. 

Results of Melville and Bickeps have been recalculated assuming termination by coupling and using the present notation. 

Direct quantitative determination of the number of initiator fragments combined with the polymer is feasible only under very exceptional circumstances. Another useful method depends on the determination of the Molecular weight by a suitable method. The number of polymer molecules may then be calculated, and assuming termination by coupling, the number of combined, primary radicals may be considered to be twice the number of molecules, still another method for determining the efficiency depends on the reaction of the chain radicals stoichiometrically with certain inhibitors. 

The mode of termination varies with monomer and reaction conditions. While styrene macroradicals typically terminate by coupling, methyl methacrylate macroradicals terminate by coupling at temperatures below 60°C, but by disproportionation at higher temperatures. 

The kinetic chain length v is equal to DP for disproportionation termination, but DP = 2v for termination by coupling. 

Which t q)e of chain-reaction polymerization is most likely to terminate by coupling ... 

The number-average degree of polymerization X , defined as the average number of monomer molecules contained in a polymer molecule, is related to the kinetic chain length. If the propagating radicals terminate by coupling (Eq. 3-16a), a dead polymer molecule is composed of two kinetic chain lengths and... 

For termination by coupling (RIr — 0 and R, is the rate of coupling) where a polymer arises from the combination of two kinetic chains, the size distribution is narrower. The... 

In the case of styrene the polymer contains at both ends a carboxyl group (chain termination by coupling), while polymers characterized by a termination reaction by disproportionation (polyvinyl acetate, polymethyl methacrylate) contain only one carboxyl end group. The carboxyl groups in these polymers were transformed to acid chlorides and coupled with a diol, e.g. 1,6-hexanediol. The comparison of the molecular weight of the polymers before and after condensation permits to elucidate the... 

The photoimaging process occurs via a photooxidation process photo-initiated by residual transition metal impurities in the presence of oxygen and terminated by coupling of polymer-bound radicals. Photoinduced cross-linking thus requires generation of a critical, and large, concentration of free radicals. 

When the substituent R stabilizes radicals as in (A) and (C), chain scission is more likely than termination by coupling. Radicals (C) then propagate the depolymerization process with volatilization of polypropylene and polystyrene at a temperature at which these polymers would not give significant amounts of volatile products when heated alone. Moreover, unsaturated chain ends such as (B) would also initiate the volatilization process because of the thermal instability of carbon-carbon bonds in P position to a double bond (Equation 4.23). 

Finally, some termination step occurs, two of which are shown in the scheme. The most common is coupling, in which two radicals combine, leading to one larger macromolecule. Polystyrene radicals typically undergo termination by coupling. Another reaction that is common with some monomers (e.g., methyl methacrylate) is called disproportionation in which on the reaction of two radicals, a hydrogen atom transfers from one species to the other. 

Growth is terminated by coupling of radicals from two growing polymer molecules or by the reactive sites becoming buried in the polymer matrix. Polymers formed by the di-p-xylylene process have been shown (6, 7,8, 9,10, II, 13) to be living polymers and exhibit radical concentrations of 5-10 X 10 4 mole of free electrons per mole of p-xylylene. 

Styrene monomer will spontaneously or auto-polymerize and must be inhibited to prevent reaction during transport and storage. Polymerization is initiated by the generation of free radicals either by the reaction of the styrene with itself ( auto-initiation ) or by means of a peroxide initiator ( chemical initiation ). Radicals rapidly propagate by reaction with monomer and ultimately terminate by coupling with another growing radical or by transferring the radical to a small molecule to start a new chain (chain transfer). 

Equation 9.9 involves the still unknown chain-carrier concentration Cx. This concentration, and with it the final form of the rate equation, depends on the type of termination. In principle, there are three potential mechanisms of termination by coupling X + X— . .., Y + Y — . .., and X + Y — . In reactions of two... 

Termination by coupling tends to be dominated by the most plentiful chain carrier. 

Termination. As in anionic polymerization, termination by coupling or disproportionation cannot occur, leaving chain transfers as the most likely mechanisms. 

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