Molecular Radicals Radical Reactions

Rate constants for the self-reactions of a number of tertiary and secondary peroxy radicals have been determined by electron spin resonance spectroscopy. The pre-exponential factors for these reactions are in the normal range for bi-molecular radical-radical reactions (109 to 1011 M"1 sec 1). Differences in the rate constants for different peroxy radicals arise primarily from differences in the activation energies of their self reactions. These activation energies can be large for some tertiary peroxy radicals (—10 kcal. per mole). The significance of these results as they relate to the mechanism of the self reactions of tertiary and secondary peroxy radicals is discussed. Rate constants for chain termination in oxidizing hydrocarbons are summarized. 

Even though the rate of radical-radical reaction is determined by diffusion, this docs not mean there is no selectivity in the termination step. As with small radicals (Section 2.5), self-reaction may occur by combination or disproportionation. In some cases, there are multiple pathways for combination and disproportionation. Combination involves the coupling of two radicals (Scheme 5.1). The resulting polymer chain has a molecular weight equal to the sum of the molecular weights of the reactant species. If all chains are formed from initiator-derived radicals, then the combination product will have two initiator-derived ends. Disproportionation involves the transfer of a P-hydrogen from one propagating radical to the other. This results in the formation of two polymer molecules. Both chains have one initiator-derived end. One chain has an unsaturated end, the other has a saturated end (Scheme 5.1). 

The semiquinone radicals are produced by base-induced oxidation of 1,4-dihydroxy-benzene (hydroquinone) or 1,2-dihydroxybenzene (catechol) by molecular oxygen, present in dissolved form. Radical concentration will increase over a period of time as the oxidation reaction proceeds and then decay as radical-radical reaction and other processes destroy the anions. Rates for these processes will depend on temperature, concentration of the dihydroxybenzene, and other parameters, so some experimentation may be necessary to obtain optimal spectra. 

Table 3.1 shows the effect of solvent polarity on four different nucleophilic substitution reactions. Creation or destruction of charge gives the biggest effects spreading or dispersal of charge as in the second and third examples in the table gives smaller effects. Molecular and radical reactions do not involve charge build-up in the transition state, and are little affected by solvents thus a check for the presence or absence of a solvent effect often allows a distinction to be made between radical or molecular mecha-... 

These semi-global processes are followed by various radical-radical reactions in the gas phase involving ClOx (x = 0 - 4) which can be formed by reactions of molecular and radical species in the system. 

Before application of the above kinetic data for reactions of low molecular radicals to reactions of macroradicals in polyoldins, a difference between reaction coiKlitions during determination of the values of the above rate constants and those in cross-linking should be accounted for. 

At high conversion, the growth of viscosity hinders the termination by radical-radical reactions, but it has less effect on the propagation reaction, because diffusion of small monomer molecules is not so much affected by viscosity. Hence, both the rate of polymerization and the molecular weight of the polymer show an increase, there is auto acceleration at high conversion. This effect is known as gel-effect or Tommsdorf-effect, which is usually accompanied by a sudden rise of temperature (Swallow 1973). Such effect was observed in many systems, for instance in the y-ray induced steady-state bulk polymerization of methyl methacrylate (Luthjens et al. 2001). 

The ejq)erimental facts currently available permit us to assume that the decomposition of pol3rvinyl chloride under an energy influence proceeds in an interconnected manner, according to a mechanism including ionic-molecular and radical reactions. The ratio of the rates of these reactions depends on the chemical structure of the polymer, reactivity of the medium, and degree of decomposition of the polymer. 

The proposed scheme makes it possible to more profoundly disclose the mechanism of the decomposition of polyvinyl chloride, since it considers the interrelationship between the ionic-molecular and radical reactions that occur during thermal and thermooxidative decomposition of the polymer. 

I do not intend to dwell here on the techniques, usually involving molecular beams and/or tunable lasers, which are needed in experiments that yield dynamical data. However, it is worth emphasising the additional complexities which arise in both kinetics and dynamics experiments when the reagents are both reactive free radicals. Consequently, experiments that involve the stable free radicals NO, NOj and 0 have provided especially important information about radical-radical reactions. 

However, the subsequent transformations of these radicals are less evident. Kinetic simulations of the DMTM process in the framework of the mechanism developed by Vedeneev with co-workers [63—66], have demonstrated that an important role is played not only by the interaction of this radical with molecular products to form peroxides, the decomposition of which provides an effective branching in this reaction — no less important are the radical—radical reactions involving CH3O2. 

Laser photolysis of a precursor may also be used to generate a reagent. In a crossed-beam study of the D + FI2 reaction [24], a hypertliennal beam of deuterium atoms (0.5 to 1 eV translational energy) was prepared by 248 mn photolysis of DI. This preparation method has been widely used for the preparation of molecular free radicals, both in beams and in experiments in a cell, with laser detection of the products. Laser photolysis as a method to prepare reagents in experiments in which the products are optically detected is fiirtlier discussed below. 

The three-step mechanism for free-radical polymerization represented by reactions (6.A)-(6.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it complicates the neat picture presented until now. On the other hand, this additional reaction can be turned into an asset in actual polymer practice. One of the consequences of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer without necessarily affecting the rate of polymerization. 

The BDE theory does not explain all observed experimental results. Addition reactions are not adequately handled at all, mosdy owing to steric and electronic effects in the transition state. Thus it is important to consider both the reactivities of the radical and the intended coreactant or environment in any attempt to predict the course of a radical reaction (18). AppHcation of frontier molecular orbital theory may be more appropriate to explain certain reactions (19). 

The main industrial use of alkyl peroxyesters is in the initiation of free-radical chain reactions, primarily for vinyl monomer polymerizations. Decomposition of unsymmetrical diperoxyesters, in which the two peroxyester functions decompose at different rates, results in the formation of polymers of enhanced molecular weights, presumably due to chain extension by sequential initiation (204). 

Reactions with Parting of Radicals. The one-electron oxidation of cationic dyes yields a corresponding radical dication. The stabihty of the radicals depends on the molecular stmcture and concentration of the radical particles. They are susceptible to radical—radical dimerization at unsubstituted, even-membered methine carbon atoms (77) (Fig. 6). 

Instead of radical reactions, models based on molecular reactions have been proposed for the cracking of simple alkanes and Hquid feeds like naphtha and gas oil (40—42). However, the vaUdity of these models is limited, and caimot be extrapolated outside the range with confidence. With sophisticated algorithms and high speed computers available, this molecular reaction approach is not recommended. 

With the introduction of Gear s algorithm (25) for integration of stiff differential equations, the complete set of continuity equations describing the evolution of radical and molecular species can be solved even with a personal computer. Many models incorporating radical reactions have been pubHshed. 

Selective chlorination of the 3-position of thietane 1,1-dioxide may be a consequence of hydrogen atom abstraction by a chlorine atom. Such reactions of chlorine atoms are believed to be influenced by polar effects, preferential hydrogen abstraction occurring remotely from an electron withdrawing group. The free radical chain reaction may be propagated by attack of the 3-thietanyl 1,1-dioxide radical on molecular chlorine. 

The presence of oxygen can modify the course of a fiee-radical chain reaction if a radical intermediate is diverted by reaction with molecular oxygen. The oxygen molecule, with its two unpaired electrons, is extremely reactive toward most free-radical intermediates. The product which is formed is a reactive peroxyl radical, which can propagate a chain reaction leading to oxygen-containing products. 

Bonds may also be broken symmetrically such that each atom retains one electron of the pair that formed the covalent bond. This odd electron is not paired like all the other electrons of the atom, i.e. it does not have a partner of opposite spin. Atoms possessing odd unpaired electrons are termed free radicals and are indicated by a dot alongside the atomic or molecular structure. The chlorination of methane (see later) to produce methyl chloride (CH3CI) is a typical free-radical reaction ... 

The trapped radicals, most of which are presumably polymeric species, have been used to initiate graft copolymerization [127,128]. For this purpose, the irradiated polymer is brought into contact with a monomer that can diffuse into the polymer and thus reach the trapped radical sites. This reaction is assumed to lead almost exclusively to graft copolymer and to very little homopolymer since it can be conducted at low temperature, thus minimizing thermal initiation and chain transfer processes. Moreover, low-molecular weight radicals, which would initiate homopolymerization, are not expected to remain trapped at ordinary temperatures. Accordingly, irradiation at low temperatures increases the grafting yield [129]. 

Molecular orbital calculations (ah initio or semiempirical methods) are also often used to provide a description of radical species and their reactions. High levels of theory are required to provide reliable data. However, rapid advances in computer power and computational methods are seeing these methods more widely used and with greater success (for leading references on the application of theory to describe radical addition reactions, see Section 1.2.7). 

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