Homogeneous Chain Termination

Chain termination is connected with the decay of active centers, i.e. with processes that occur with the participation of atoms and radicals, but without regeneration of the latter. These processes can proceed both in the gas phase 

In the presence of inhibiting impurities or compounds that react with atoms and radicals to form saturated compounds or chemically low-active radicals, the rate of chain termination is proportional to the first power of the active center concentration (linear chain termination). In the case of atoms and simple radicals, linear chain termination occurs by three-body collision, e.g. H + O2 + M - HO2 + M. 

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One observes that the larger the surface of Mo or MoS2 the shorter the chain length v. The dependence of the function F=v0/v— v/v0, where v0 = v at [inhibitor] = 0, on the amount of inhibitor is unusual. The dependence F [inhibitor] is usual for homogeneous chain termination (see Chapter 14). Quite another dependence was observed for the heterogeneous chain termination by powders of Mo and MoS2 [1,7,9,10], namely... 

The HO2 radical is formed in the termolecular process (14). The resulting homogeneous chain termination was introduced as a hypothetical process in order to account for the second ignition limit. Later on, its validity was supported by many experiments. 

Transition-metal- and enzyme-catalyzed alkylations of ammonia and amines with alcohols and diols have been reviewed59. RuCl2(PPh3)3 is a homogeneous catalyst for the reaction of long-chain terminal alcohols with secondary amines to give tertiary amines (equation 22)60. 

Most examples of catalysis can be readily characterised as homogeneous or heterogeneous but there are examples of catalysis which overlap the two types. Consider a system in which intermediates are formed at the surface and then are desorbed into the gas phase and react there. Such intermediates might generate a chain reaction in the gas phase, i.e., chain initiation and chain termination occur at the surface but chain propagation occurs in the gas phase. 

However, the physicochemical content of the termination mechanism via free radical decay is best described by the Satterfield-Stein scheme [4], Meanwhile, the question about homogeneous dissociation of gaseous H202 is still open, because in this case OH and H02 radicals are formed in the reactor volume first and only then are the chains terminated on the walls ... 

Sherrington and coworkers161 have examined the chlorination of phenol by J-butyl hypochlorite in the presence of cross-linked polystyrenes substituted by pendant polymethylene chains terminated with anionic or cationic head groups, as well as some hydrophilic acrylic polymers, in four solvents water, methanol, 1,2-dichloroethane and xylene. The polymers exerted a significant influence on the chlorination, particularly in polar solvents. However, no changes in the regioselectivity of the chlorination, in comparison to homogeneous systems, was observed. 

The approach to the stationary state in a chain system is not instantaneous but takes a finite time which may be calculated from the kinetic mechanism if the individual rate constants for initiation and termination arc known. For the homogeneous chain reaction represented by case 1 Benson has shown how to calculate both the time ta required to reach any fraction a of the stationar37 -state concentration of X, and the fraction of reaction Fa occurring in that time. For thermal initiation ta is given by... 

Chain termination may be either heterogeneous or homogeneous in nature. At low pressure the chain carriers and branching agents may diffuse to the walls of the reactor where they are destroyed either by capture or conversion to inactive products. These heterogeneous termination reactions are either diffusion-controlled in which case the rate coefficient is given by the expression... 

Yet Chien demostrated that it would be possible to obtain polyethylene with a Q value near the theoretical 2, with the homogeneous (CjH5)2TiCl2—A1(CH3)2C1 catalytic system, only if carefully controlled pseudosteady-state conditions are employed. In fact he showed mathematically that the relatively high experimental polydispersiiy (<,) tfom 2 to 5 in function of reaction time), is a natural consequence of a polymerization kinetic model based on non stationary first order initiation, chain propagation and bimolecular chain termination by recombination. 

The rate equations considered under this heading were originally formulated on the concept of "nucleus branching" (Table 3.1), analogous to homogeneous chain reactions [5,8]. Occurrence of "branching", without restriction on multiplicative development or allowance for termination, would lead to an exponential dependence of flron t ... 

In general, a polymerization process model consists of material balances (component rate equations), energy balances, and additional set of equations to calculate polymer properties (e.g., molecular weight moment equations). The kinetic equations for a typical linear addition polymerization process include initiation or catalytic site activation, chain propagation, chain termination, and chain transfer reactions. The typical reactions that occur in a homogeneous free radical polymerization of vinyl monomers and coordination polymerization of olefins are illustrated in Table 2. 

Several types of transfer reactions which may occur in homogeneous systems terminating polymer chain growth are listed below. 

Unfortunately, later the development of two areas— homogeneous kinetics and heterogeneous catalysis—occurred almost independently, which caused serious intrinsic discrepancies. For instance, the traditional chain theory implies the participation of surfaces also in chain termination, which determines the existence of the low-pressure ignition limit. In the framework of this approach, two regimes—diffusional and kinetic—are distinguished. In the latter case the parameter that describes the process is the probability of surface decay of chain carriers per one collision. It is worth noticing, however, that this assumes only a disappearance of active species from the gas phase, without any analysis of its mechanism and even stoichiometry. This is why the heterogeneous termination reactions are usually represented in kinetic models as a formal reaction ... 

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