Termination mechanisms

These observations suggest how the terminal mechanism can be proved to apply to a copolymerization reaction if experiments exist which permit the number of sequences of a particular length to be determined. If this is possible, we should count the number of Mi s (this is given by the copolymer composition) and the number of Mi Mi and Mi Mi Mi sequences. Specified sequences, of any definite composition, of two units are called dyads those of three units, triads those of four units, tetrads those of five units, pentads and so on. Next we examine the ratio NmjMi/Nmi nd NmjMiMi/NmiMi If these are the same, then the mechanism is shown to have terminal control if not, it may be penultimate control. To prove the penultimate model it would also be necessary to count the number of Mi tetrads. If the tetrad/triad ratio were the same as the triad/dyad ratio, the penultimate model is proved. 

Use the values determined in Example 7.6 for the vinylidene chloride (M )-isobutylene (M2) systemf to calculate for various values of fi according to the terminal mechanism. Prepare a plot of the results. On the same graph, plot the following experimentally measured values of fi and ... 

The early kinetic models for copolymerization, Mayo s terminal mechanism (41) and Alfrey s penultimate model (42), did not adequately predict the behavior of SAN systems. Copolymerizations in DMF and toluene indicated that both penultimate and antepenultimate effects had to be considered (43,44). The resulting reactivity model is somewhat compHcated, since there are eight reactivity ratios to consider. 

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. 

One chain-end is typically unsaturated due to chain transfer and termination mechanisms. Mol wts can range from several hundred to several million. There is no long-chain branching unless special synthesis methods ate employed. The mol wt distribution is commonly the most probable,... 

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. 

Two relatively new techniques, matrix assisted laser desorption ionization-lime of flight mass spectrometry (MALDI-TOF) and electrospray ionization (FS1), offer new possibilities for analysis of polymers with molecular weights in the tens of thousands. PS molecular weights as high as 1.5 million have been determined by MALDI-TOF. Recent reviews on the application of these techniques to synthetic polymers include those by Ilantoif54 and Nielen.555 The methods have been much used to provide evidence for initiation and termination mechanisms in various forms of living and controlled radical polymerization.550 Some examples of the application of MALDI-TOF and ESI in end group determination are provided in Table 3.12. The table is not intended to be a comprehensive survey. 

The relative importance of combination and disproportionation in relevant model systems and in polymerizations of some common monomers is considered in Sections 5,2.2.1 and 5.2.2.2 respectively. The significance of the termination mechanism on the course of polymerization and on the properties of polymers is discussed briefly in Section 5.2.2 and is further discussed in Section 8.2. 

A substantial number of studies give information on kJkK for polymerizations of S (5.2.2.2.1) and MMA (5.2,2.2.2). There has been less work oil other systems. One of the main problems in assessing kjk lies with assessing the importance of other termination mechanisms (i.e. transfer to initiator, solvent, etc., primary radical termination). 

Evaluation of molecular weights after ultrasonic scission of high molecular weight polymers (PMMA and PS) in the presence of a radical trap has been claimed to provide evidence of the termination mechanism.1,1 However, scission gives radicals as shown in Scheme 5.10. 

Four studies suggest that k /kK has a significant temperature dependence (Table 5.5). Although not agreeing on the precise value of ktJkte, all four studies indicate that the proportion of disproportionation increases with increasing temperature. These results are at variance with model studies that suggest that kJkK is independent of temperature. It was also proposed that the preferred termination mechanism is solvent dependent and that disproportionation is favored in more polar media.161... 

The termination mechanism in MA polymerization has been variously determined to be predominantly disproportionationlj7,b7 or predominantly combination.12 6 ... 

Early reports37 157 167 suggested that termination during VAc polymerization involved predominantly disproportionation. However, these investigations did not adequately allow for the occurrence of transfer to monomer and/or polymer, which are extremely important during VAc polymerization (Sections 6.2.6.2 and 6.2.7.4 respectively). These problems were addressed by Bamford et who used the gelation technique (Section 5.2.2,2) to show that the predominant radical-radical termination mechanism is combination (25 °C). 

Studies on VC polymerization are also complicated by the fact that only a small proportion of termination events may involve radical-radical reactions. Most termination is by transfer to monomer (Sections 4.3.1.2 and 6.2.63). Early studies on the termination mechanism which do not allow for this probably overestimate the importance of disproportionation.lb8 iw... 

Park and Smith 170 attempted to allow for chain transfer in their examination of the termination mechanism during VC polymerization at 30 and 40 °C in chlorobenzene. They determined the initiator-derived ends in PVC prepared with radiolabeled AIBN and concluded that kjk = 3.0. However, questions have been raised regarding the reliability of these measurements.171 172 Atkinson et al.x72 applied the gelation technique (Section 5.2.2.2) to VC polymerization and proposed that termination involves predominantly combination. 

The efficiency of the halide- radical transformation is reported to be near quantitative. The yield of block or graft is then limited by the efficiency of the halide synthesis. Whether AB or ABA blocks are formed depends on the termination mechanism. Similar halo-compounds have been used to initiate A TRP (Section 9.4). 

Unstable structures are known to arise by chain termination. Mechanisms for radical-radical termination in MMA polymerization have been discussed in Sections 5.2.2.1.2 and 5.2.2.2.2 and these are summarized in Scheme 8.5. It is established that both disproportionation and combination occur to substantial extents. The head-to-head linkages 1 and the unsaturated chain ends 2 both constitute weak links in PMMA.26 2 "33 The presence of these groups account for... 

Both termination mechanisms have been shown to occur experimentally, the method being to examine the polymer molecules formed for fragments of initiator. In such a way polystyrene has been found to terminate mainly by combination and poly(methyl methacrylate) entirely by disproportionation at temperatures above 60 °C. 

Termination may be the result of either combination (Reaction 2.12) or disproportionation (Reaction 2.13). However, it is rarely necessary to distinguish between these two termination mechanisms, and so the rate constants are generally combined into a single rate constant, k. ... 

This agrees with experimental findings on the decomposition of acetaldehyde. The appearance of the three-halves power is a wondrous result of the quasisteady hypothesis. Half-integer kinetics are typical of free-radical systems. Example 2.6 describes a free-radical reaction with an apparent order of one-half, one, or three-halves depending on the termination mechanism. 

Solution The procedure is the same as in the acetaldehyde example. ODEs are written for each of the free-radical species, and their time derivatives are set to zero. The resulting algebraic equations are then solved for the free-radical concentrations. These values are substituted into the ODE governing RCl production. Depending on which termination mechanism is assumed, the solutions are... 

The oxidation of primary and secondary alcohols in the presence of 1-naphthylamine, 2-naphthylamine, or phenyl-1-naphthylamine is characterized by the high values of the inhibition coefficient / > 10 [1-7], Alkylperoxyl, a-ketoperoxyl radicals, and (3-hydroxyperoxyl radicals, like the peroxyl radicals derived from tertiary alcohols, appeared to be incapable of reducing the aminyl radicals formed from aromatic amines. For example, when the oxidation of tert-butanol is inhibited by 1-naphthylamine, the coefficient /is equal to 2, which coincides with the value found in the inhibited oxidation of alkanes [3], However, the addition of hydrogen peroxide to the tert-butanol getting oxidized helps to perform the cyclic chain termination mechanism (1-naphthylamine as the inhibitor, T = 393 K, cumyl peroxide as initiator, p02 = 98 kPa [8]). This is due to the participation of the formed hydroperoxyl radical in the chain termination ... 

As noted above, the duration of the retarding action of an inhibitor is directly proportional to the / value. In systems with a cyclic chain termination mechanism, the / coefficient depends on the ratio of the rate constants for two reactions, in which the inhibitor is regenerated and irreversibly consumed. In the oxidation of alcohols, aminyl radicals are consumed irreversibly via the reaction with nitroxyl radical formation (see earlier) and via the following reaction [11] ... 

Table 16.1 presents the inhibition coefficients / and the termination rate constants kn in systems with the cyclic chain termination mechanism with aromatic amines. Naturally, these are apparent rate constants, which characterize primarily the rate-limiting step of the chain termination process. 

The cross-disproportionation of nitroxyl and hydroperoxyl radicals is an exothermic reaction. For example, the enthalpies of disproportionation of TEMPO radical with H02, Me2C(0H)02, and cydo-C(,Y 10(OH)O2 radicals are equal to 109, —92, and 82 kJ mol-1, respectively. The Ee0 value for the abstraction of an H atom from the O—H bond in ROOH by a nitroxyl radical is 45.6 kJ mol 1 and AHe min = —58 kJ mol-1. Since AHe < AHe min, (see Chapter 6), the activation energy of such exothermic reactions for these reactions is low (E 0.5RT), and the rate constant correspondingly is high [31 34]. Therefore, in the systems in which hydroperoxyl, hydroxyperoxyl, and aminoperoxyl radicals participate in chain propagation, the cyclic chain termination mechanism should be realized. 

Why are the activation energies of the reactions of nitroxyl radicals with O—H bonds lower than those in their reactions with C—H bonds As in the case of the reaction of R02 with quinones, the difference in E values occurs as a result of the different triplet repulsions in TS [23]. When a TS of the O H O type is formed (the AmO + H02 reaction), the triplet repulsion is close to zero because the O—O bond in the labile compound AmOOH is very weak. Conversely, the triplet repulsion in the reaction of AmO with the C—H bond is fairly great, due to the high dissociation energy of the AmO—R bond. This accounts for the difference between the activation energies and between the rate constants for the reactions considered above. Thus, the possibility of the realization of a cyclic chain termination mechanism in the reactions of nitroxyl radicals with peroxyl radicals, incorporating O—H groups, is caused by the weak triplet repulsion in the TS of such disproportionation reactions... 

In addition to this reaction, quinones and other alkyl radical acceptors retard polymer oxidation by the reaction with alkyl radicals (see earlier). As a result, effectiveness of these inhibitors increases with the formation of hydroperoxide groups in PP. In addition, the inhibiting capacity of these antioxidants grows with hydroperoxide accumulation. The results illustrating the efficiency of the antioxidants with cyclic chain termination mechanisms in PP containing hydroperoxide groups is presented in Table 19.12. The polyatomic phenols producing quinones also possess the ability to terminate several chains. 

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