Disproportionation, termination mechanism

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 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. 

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... 

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... 

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. ... 

In addition to disproportionation, another mechanism of the alkylhyroxyperoxyl radical chain termination is recombination [38,39],... 

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... 

The free radical polymerization of HPMA in the presence of mercaptans involves two different initiation mechanisms (Scheme 2) [26]. One is the initiation by RS radicals from chain transfer agent the other appears to be the direct initiation by the primary isobutyronitrile (IBN) radicals formed by the decomposition of AIBN [27]. The RS are formed by either the free radical transfer reaction of alkyl mercaptans with the IBN radicals or the chain transfer reaction of an active polymer chain with the mercaptans. The initiation by the RS radicals produces the ST polymers with a functional group at one end of the polymer chain. The initiation by IBN radicals leads to nonfunctional polymer chains with an IBN end group. The presence of the polymers with IBN end groups effects the purity and the functionality of ST polymers. As expected, the production of nonfunctionalized polymer chains is affected by reaction conditions. The polymerization is mainly terminated by chain transfer reaction with the mercaptans, but other termination mechanisms, such as disproportionation and recombination, take place depending on the reaction conditions [26]. 

For the limit of x — 1, the polydispersity is (almost) the same as the one obtained for chainwise polymerizations, provided that the termination mechanism is chain transfer or disproportionation and primary chains are long enough so that q — 1. However, in the former case polydispersity increases continuously with conversion, while in the latter it gets a value close to 2 from the very beginning of reaction. 

Figure 3.22 Fraction of tetrafunctional crosslinks formed in the course of an A2 + A4 chainwise copolymerization (af = 0.01, q = 0.999), with two different termination mechanisms (E, = 0 represents the case of a termination by chain transfer or disproportionation, while , = 1 represents the case of a termination by combination).

This expression fits the observed kinetics, and so mechanism 3 involving a disproportionation termination step is a possible mechanism for this reaction. 

Polystyrene. The polymerization of styrene is most commonly done under free radical conditions. Peroxides are used to initiate the reaction at low temperatures. At 100°C styrene acts as its own initiator. Below 80°C the termination mechanism primarily involves combination of radicals. Above 80°C both disproportionation and chain transfer with the Diels-Alder dimer are important. 

Thus f, (m) is the (unnormalized) length distribution of inactive chains formed by disproportionation, particularly in systems where disproportionation represents an exclusive or predominating termination mechanism. f2(m) corresponds to the (unnormalized) length distribution of macroradicals. 

CC = C2H5 ). Initiation breaks the ethane carbon-carbon bond, which is weaker than the carbon-hydrogen bonds. The most plentiful free radical under most conditions of interest is QHj- [30], so that coupling of two of these to butane should be the dominant termination mechanism, probably accompanied to a small extent by disproportionation to ethene and ethane [31,32] ... 

Termination by disproportionation. In Example 10.3, coupling of two polymer radicals was assumed to be the only termination mechanism, as is indeed essentially true for polymerization of styrene [34], However, various other mechanisms may contribute to termination or even dominate it. The most common of these is disproportionation, mainly observed for tertiary and other sterically hindered free radicals [35]. An example is methyl methacrylate [34] (see reaction 10.26 below). In disproportionation, two polymer radicals react with one another, transferring a... 

Free-radical polymerization requires initiation to produce free radicals that link up with monomer molecules to produce reactive centers. Additional monomer molecules are then added successively at these centers. In this way, a small family of polymer radicals acts as an assembly line to produce "dead" polymer. The most common termination mechanisms are reactions of two polymer radicals with one another, either by coupling to yield one larger dead polymer molecule or, more rarely, by disproportionation to convert... 

The more reactive the radicals, the more propagation steps occur before the chain reaction is brought to an end by a termination reaction. As with propagation reactions, there are often many possible termination reactions, some of which, as we have seen above, produce intermediates or starting materials of the reaction. This multiplicity of possible reactions makes radical reactions very difficult to study kinetically. One possible termination reaction consists of disproportionation. Suggest what would be the resultant products of the disproportionation termination reaction between two ethyl radicals, and further suggest a mechanism for this reaction. 

A second termination mechanism is disproportionation, shown by the following equation ... 

It is very clear that if the initiator has hydroxyl groups, and if the termination takes place exclusively by recombination then a polymeric diol is obtained [2, 3], which is ideal for polyurethane. If the termination takes place by disproportionation, only monofunctional compounds are obtained, which cannot be used in PU. The vinylic and dienic monomers used in practice have various termination mechanisms. Some monomers give only recombination reactions, such as styrene, acrylates (methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethyl hexyl acrylate), acrylonitrile and butadiene. Other monomers give both mechanisms of termination, around 65-75% disproportionation and 25-35% recombination, such as methacrylates (methyl methacrylate, ethyl methacrylate, butyl methacrylate etc.), substituted styrenes and other monomers [2, 3, 4]. 

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