Termination of chain

Diacyl peroxides are particularly prone to induced decomposition (Scheme 3.16). Transfer to initiator is of greatest importance for polymerizations taken to high conversion or when the ratio of initiator to monomer is high. It has been shown that, during the polymerization of S initiated by BPO, transfer to initiator can be the major pathway for the termination of chains.7,41... 

The inhibition method has found wide usage as a means for determining the rate at which chain radicals are introduced into the system either by an initiator or by illumination. It is, however, open to criticism on the ground that some of the inhibitor may be consumed by primary radicals and, hence, that actual chain radicals will not be differentiated from primary radicals some of which would not initiate chains in the absence of the inhibitor. This possibility is rendered unlikely by the very low concentration of inhibitor (10 to 10 molar). The concentration of monomer is at least 10 times that of the inhibitor, yet the reaction rate constant for addition of the primary radical to monomer may be less than that for combination with inhibitor by only a factor of 10 to 10 Hence most of the primary radicals may be expected to react with monomer even in the presence of inhibitor, the action of the latter being confined principally to the termination of chain radicals of very short length. ... 

Such a mechanism is open to serious objections both on theoretical and experimental grounds. Cationic polymerizations usually are conducted in media of low dielectric constant in which the indicated separation of charge, and its subsequent increase as monomer adds to the chain, would require a considerable energy. Moreover, termination of chains growing in this manner would be a second-order process involving two independent centers such as occurs in free radical polymerizations. Experimental evidence indicates a termination process of lower order (see below). Finally, it appears doubtful that a halide catalyst is effective without a co-catalyst such as water, alcohol, or acetic acid. This is quite definitely true for isobutylene, and it may hold also for other monomers as well. 

The critical value of a at which the formation of an infinite network becomes possible can be deduced as follows If the branching unit is trifunctional, as in Fig. 61, each chain which terminates in a branch unit is succeeded by two more chains. If both of these terminate in branch units, four more chains are reproduced, and so on. If less than an even chance that each chain will lead to a branch unit and thus to two more chains there is a greater than even chance that it will end at an unreacted functional group. Under these circumstances the network cannot possibly continue indefinitely. Eventually termination of chains must outweigh continuation of the network through branching. Consequently, when a < 1/2 all molecular structures must be limited, i.e., finite, in size. 

Evidence for a competing disproportionation mechanism (see Figure 1) for the termination of chain ends is provided by the combined presence of the peaks from 4 and 5 in the MALDI-TOF mass spectrum of this PMMA polymer (see Figure 6) [10]. Confirmation of the presence of the unsaturated and saturated chain ends, arising from disproportionation, was obtained by means of and 13C NMR spectroscopy, respectively [11]. 

Radical polymerizations have three important reaction steps in common chain initiation, chain propagation, and chain termination. For the termination of chain radicals several mechanisms are possible. Since the lifetime of a radical is usually less than 1 s, radicals are continuously generated and terminated. Each propagating radical can add a finite number of monomers between its initiation and termination. If a divinyl monomer is in the monomer mixture, the reaction kinetics changes drastically. In this case, a dead polymer chain may grow again as a macroradical, when its pendant vinyl groups react with radicals, and the size of the macromolecule increases until it extends over the whole available volume. 

Note that methacrylonitrile lacks an acidic hydrogen for self-termination of chains. 

For termination of the long chain, these free radicals can combine in different ways to form polythene. One mode of termination of chain is shown as under ... 

Let us for the moment disregard chain transfer reactions. Radical polymerization then consists of three component reactions initiation, propagation of the polymer chains, and termination of chain growth. The rate of primary radical formation, v, by decomposition of the initiator I, may be written ... 

The termination of chain growth can also occur both in the gas phase and at the polymer surface. In the gas phase, free radicals are lost by reaction with both hydrogen atoms and other free radicals. The kinetics of these processes are given by... 

Stabilization and Termination of Chain Growth by Ring Formation... 

Because the purity of the monomers used affects the molecular weight of the polyester 39), we can assume that proton donors whether added to the monomer mixture or formed during copolymerization, participate in termination or transfer reactions. Termination of chain growth may occur by reaction of the growing chain end with proton donors according to Eqs. (77) and (78). 

The relationships of oxidation potential to radical reactivity index Sr and nucleophilic reactivity index Sn illustrated in Figure 4 are very similar to those with antioxidation and antiozonization, where the maximum values were observed at 0.4 and 0.25 volt. Therefore, antioxidation seems to proceed by a radical mechanism in contrast to the nucleophilic type of antiozonization. Indeed, the antioxidation effect of amines toward NR, SBR, BR, and HR is well correlated with radical reactivity as shown in Figures 5-8. The protection of SBR solution by amines from oxidative degradation and the termination of chain reaction in the oxygen-Tetralin system are also shown as functions of Sr in Figures 9 and 10. 

The mechanisms and resulting kinetic equations are shown in Figure 4. Other mechanisms are possible as well as modifications of these—e.g., disproportion termination of chain reactions, and condensation between unlike monomers. The left sides of the equations represent the reactor operator (note that all resulting differential equations are nonlinear because of the second-order propagation and termination reactions). To this is added the complexity of considering separate equations for the thousands of separate species frequently required to define completely commercially useful polymers. Solution by direct application of classical techniques is impractical or impossible in most cases even direct numerical solution is often difficult. Simplifying assumptions or special mathematical techniques must be used (described below in the calculations of MWD). 

Mechanism of action Upon entry into the host cell, didanosine is biotransformed into ddATP through a series of reactions that involve phosphorylation of the ddl, amination to ddAMP and further phosphorylation. The resulting ddATP is incorporated into the DNA chain like AZT, causing termination of chain elongation. 

The reasons for the appearance of the flash are not yet fully clear it is also unclear why the flash can be supressed by introducing certain additives to the powder (probably catalytic termination of chain reactions). It is certain that muzzle flash is promoted by the high temperature of the combustion gases, the high gas pressure and the high velocity of the gas emerging from the muzzle. 

It seems to be necessary, in order to account for the influence of the wall on the first explosion limit, to postulate termination of chains on the wall. From other evidence (which we shall discuss later in a few specific cases) on the slow rates outside the explosion peninsula it appears equally necessary to postulate chain initiation at the walls. Where surfaces such as the walls play an important role in chemical reactions, we must expect to find that in a certain range of reaction conditions the diffusion to and from such surfaces may exert a limiting effect on the rates of chemical reactions. If the reactions arc taking place in the volume of the vessel in competition with reactions at the wall, then we may expect to find concentration gradients within the volume of the vessel. ... 

In a thermal study, Morris and Pease used vessels with Pyrex walls both clean and coated with potassium chloride, the temperature being 180 °C. The reaction was followed by measuring the hydrogen with a gas buret before and after the reaction. Earlier work by Pease had suggested that surfaces were very important in the initiation and termination of chain carriers and Morris and Pease attempted to establish the mechanism of these steps. 

Using the technique of differential calorimetry, Kapralova has found evidence that, the initiation and termination of chains in 1,2-dichloroethane pyrolysis occurs on the reaction vessel walls instead of in the homogeneous gas phase. It has also beenproposed - that the inhibitory effect of propene may be associated with its absorption on the vessel walls causing a reduction in the rate of the initiation of chains on the surface. The mechanism of chain initiation and the action of inhibitors and sensitisers in 1,2-dichloroethane pyrolysis have been further discussed in papers by Kapralova and Semenov > Kitabatake and Onouchi and Smolyan . 

The antiviral nucleosides aciclovir and ganciclovir are also converted to their respective nucleoside triphosphates in the cytoplasm of infected cells. They proceed to inhibit viral DNA replication either by inhibition of the DNA polymerase or by incorporation into DNA with subsequent termination of chain extension. Finally the anti-HIV drug AZT acts in an analogous manner, being converted to the corresponding triphosphate and inhibiting viral RNA synthesis by the HIV reverse transcriptase. 

Thus the molecular weight of the polymer gives an indication of the rate of termination of chains relative to propagation. 

Problem 1.3 Styrene monomer containing 0.02% (by wt.) benzoyl peroxide initiator was reacted until all the initiator was consumed. If at this stage 22% of the monomer remained unreacted, calculate the average degree of polymerization of the polymer formed. Assume 100% efficiency of the initiator (i.e., all initiator molecules are actually consumed in polymer formation) and termination of chain radicals by coupling alone. 

Case 1. No degradative chain transfer. The termination of chain radicals occurs by combination with primary radicals and by the normal processes of mutual deactivation. In this case, Eq. (P6.32.7) simplifies to... 

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