Bimolecular initiation reaction

At low temperatures the polymerization process is only initiated by UV-, y- or x-irradiation via excitation of the monomer molecules. However, in order to understand the relatively high quantum yield obtained in the bimolecular initiation reaction of Eq. (9) we have to consider a metastable long-lived excited state M, which represents... 

Coming back to the a(C-C)/(3(C-H) primary split ratio (Table 3), it would be valuable to compare these values with that obtained either in the thermal pyrolysis of propene or in chemical activated systems. For example, in shock tube experiments (1650-2300 K), the dominant bimolecular initiation reaction leads to the C-C bond rupture, although a possible contribution of the 3(C-H) bond rupture cannot be excluded (50). This is also observed in the decomposition of hot propene formed from ethylcarbene f(E)(C3H ) s 414 kJ/mol] a(C-C)/ P(C-H) = 22 (51). Conversely, hot propene formed by the addition of singlet methylene to ethylene [(EXCsH ) s 464—492 kJ/mol) gives rise to C-H bond... 

The hypothesis of a bimolecular initiation reaction for liquid phase autoxida-tions was extended beyond cyclohexanone as a reaction partner. Also other substances featuring abstractable H-atoms are able to assist in this radical formation process. The initiation barrier was found to be linearly dependent on the C-H bond strength, ranging from 30 kcal/mol for cyclohexane to 5 kcal/mol for methyl linoleate [14, 15]. Substrates that yield autoxidation products that lack weaker C-H bonds than the substrate (e.g., ethylbenzene) do not show an exponential rate increase as the chain initiation rate is not product enhanced [16]. 

Write down the bimolecular initiation reaction of normal butane with oxygen. 

Table XIV-2.5.b gives the kinetic parameters of five categories of bimolecular initiation reactions concerning primary (p), secondary (s), tertiary (t), allylic (a) and vinylic (v) H atoms. The A factor is relative to one H atom.

Calculate the kinetic parameters for the bimolecular initiation reactions of propane. There are two reactions ... 

The production of chains depends on the concentration of initiated radicals R,, which, in turn are produced from T according to the bimolecular initiation reaction ... 

This chapter has provided a brief overview of the application of optimal control theory to the control of molecular processes. It has addressed only the theoretical aspects and approaches to the topic and has not covered the many successful experimental applications [33, 37, 164-183], arising especially from the closed-loop approach of Rabitz [32]. The basic formulae have been presented and carefully derived in Section II and Appendix A, respectively. The theory required for application to photodissociation and unimolecular dissociation processes is also discussed in Section II, while the new equations needed in this connection are derived in Appendix B. An exciting related area of coherent control which has not been treated in this review is that of the control of bimolecular chemical reactions, in which both initial and final states are continuum scattering states [7, 14, 27-29, 184-188]. 

Studies of the influence of total pressure on the initial reaction rate for pure reactants present in stoichiometric proportions provide a means of discriminating between various classes of Hoqgen-Watson models. Isolation of a class of probable models by means of plots of initial reaction rate versus total pressure, feed composition, and temperature constitutes the first step n developing a Hougen-Watson rate model. Hougen (14) has considered the influence of total pressure for unimolecular and bimolecular surface reactions the analysis that follows is adopted from his monograph. 

In initial rate studies no products need be present in the feed, and the terms in the rate expression involving the partial pressures of these species may be omitted under appropriate experimental conditions. The use of stoichiometric ratios of reactants may also cause a simplification of the rate expression. If one considers a reversible bimolecular surface reaction between species A and ,... 

The decompositions of hydroperoxides (reactions 4 and 5) that occur as a uni-or bimolecular process are the most important reactions leading to the oxidative degradation (reactions 4 and 5). The bimolecular reaction (reaction 5) takes place some time after the unimolecular initiation (reaction 4) provided that a sufficiently high concentration of hydroperoxides accumulates. In the case of oxidation in a condensed system of a solid polymer with restricted diffusional mobility of respective segments, where hydroperoxides are spread around the initial initiation site, the predominating mode of initiation of free radical oxidation is bimolecular decomposition of hydroperoxides. 

The reaction rate of Co3+ with benzaldehyde was measured in independent experiments from the consumption of Co3+ in the absence of oxygen. The rate constant of this bimolecular reaction was found to coincide with k. Thus, in this process the limiting step of initiation is the reduction of Co3+ by aldehydes, and the complete cycle of initiation reactions includes the reactions [50,51] ... 

The observation of second-order kinetics (ku) for the spectral decay of the anisole cation radical in Fig. 15 points to the disappearance of AN + - after its separation from the initially formed triad in (63). Owing to the high yields of nitroanisoles obtained, such a process can be formulated as in Scheme 11 as the bimolecular (homolytic) reaction in (64) that produces the critical Wheland intermediate in aromatic nitration according to Perrin (1977) and Ridd (1991). 

The development of a number of mass spectrometric techniques in the late 1960s paved the way for the study of bimolecular chemical reactions of ions with neutral molecules in the gas phase (see, for example, Harrison et al., 1966 McDaniel et al., 1970 Franklin, 1972). While the systems initially investigated were concerned primarily with positive ions and with relatively simple reactions, the underlying potential of such techniques was soon to be explored in a dramatic way. 

In consideration of the kinetic law obtained, Rp i0 of magnitude range, one can conclude that the common polymerization mechanism, based on bimolecular termination reactions, is no longer valid for these multifunctional systems when irradiated in condensed phase. Indeed, for conventional radical-induced polymerizations, the termination step consists of the interaction of a growing polymer radical with another radical from the initiator (R), monomer (M) or polymer (P) through recombination or disproportionation reactions ... 

The bimolecular termination reaction can be neglected at low conversions since no linear sulphide residues are present initially, enabling a simpler interpretation of initial conversion/time curves to be made. In the earlier work the concentration of active centres was equated with the initial catalyst salt concentration, but later an XH NMR method of analysis was employed (137). As in the polymerisation of tetrahydrofuran it was anticipated that both free ions and ion pairs were likely to contribute to the propagation reaction and the calculated rate constant kP.pp e t, was described by... 

Non-activated double bonds, e.g. in the allylic disulfide 1 (Fig. 10.2) in which there are no substituents in conjugation with the double bond, require high initiator concentrations in order to achieve reasonable polymerisation rates. This indicates that competition between addition of initiator radicals (R = 2-cyanoisopropyl from AIBN) to the double bond of 1 and bimolecular side reactions (e.g. bimolecular initiator radical-initiator radical reactions outside the solvent cage with rate = 2A t[R ]2 where k, is the second-order rate constant) cannot be neglected. To quantify this effect, [R ] was evaluated using the quadratic Equation 10.5 describing the steady-state approximation for R (i.e. the balance between the radical production and reaction). In Equation 10.5, [M]0 is the initial monomer concentration, k is as in Equation 10.4 (and approximately equal to the value for the addition of the cyanoisopropyl radical to 1-butene) [3] and k, = 109 dm3 mol 1 s l / is assumed to be 0.5, which is typical for azo-initiators (Section 10.2). The value of 11, for the cyanoisopropyl radicals and 1 was estimated to be less than Rpr (Equation 10.3) by factors of 0.59, 0.79 and 0.96 at 50, 60 and 70°C, respectively, at the monomer and initiator concentrations used in benzene [5] ... 

A typical effect observed in the synthesis of linear polymers by a free-radical mechanism is the auto-acceleration process. At a particular conversion, when sufficient polymer has accumulated in the system for the viscosity to reach a certain level, the rate of the bimolecular termination reaction begins to fall because of diffusional restrictions to the encounter of two chain ends. However, the initiation and growth rates are hardly affected. 

Kinetic studies A kinetic study of PA transformation was carried out in a batch reactor at 433 K in the presence of sulfolane (very polar) or dodecane (nonpolar) as solvent.[48] The initial reaction rates, the decrease in rate with time and the product distribution depend very much on the solvent polarity. The initial rates are lower in the polar sulfolane than in the nonpolar dodecane solvent. In the latter solvent, but not in sulfolane, there is a rapid decrease in the reaction rates, this decrease affecting preferentially the bimolecular formation of p-AXAP and p-HAP. As a consequence, these products are much more favoured with respect to o-HAP in sulfolane than in dodecane. The (p-AXAP + p-HAP)/o-HAP ratio is equal to 7 with sulfolane and 1 with dodecane. 

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