Chain initiation, propagation, termination

Know the meaning of substitution reaction, halogenation, chlorination, bromination, free-radical chain reaction, chain initiation, propagation, termination, combustion. 

LDPE polymerization reaction consists of various elementary reactions such as initiation, propagation, termination, chain transfer to polymer and monomer, p-scission and so forth [1-3], By using the rate expression of each elementary reaction in our previous work [4], we can construct the equations for the rate of formation of each component. 

A major difference between both kind of syntheses lies in polymerisation kinetics in the former case there is a chain mechanism (initiation, propagation, termination), in the latter case there is no chain reaction and each step is equiprobable. Main comparison points are highlighted in Table 4 [9]. 

Five different types of rate constants are of concern in radical chain polymerization—those for initiation, propagation, termination, chain transfer, and inhibition. The use of polymerization data under steady-state conditions allows the evaluation of only the initiation rate constant kd (or kt for thermal initiation). The ratio kp/k J2 or kp/kl can be obtained from Eq. 3-25, since Rp, Rj, and [M] are measurable. Similarly, the chain-transfer constant k /kp and the inhibition constant kz/kp can be obtained by any one of several methods discussed. However, the evaluation of the individual kp, k ktr, and kz values under steady-state conditions requires the accurate determination of the propagating radical concentration. This would allow the determination of kp from Eq. 3-22 followed by the calculation of kt, kIr, and kz from the ratios kp/ltj2, ktr/kp, and kz/kp. 

The oxidation of methane (as well as other hydrocarbons) proceeds via a chain branching mechanism. Each of the stages of the chain branding mechanism initiation, propagation termination, may be affected to a different degree by relatively slight changes in ambient conditions under which oxidation takes place. 

Exercise 4-16 a. Write equations to show reasonable radical-chain initiation, propagation, and termination steps in the monobromination of 2-methylbutane shown above. Explain clearly why the products of chain termination are obtained in trace amounts only. 

Exercise 13-12 Formulate chain initiation, propagation, and termination steps for the polymerization of 1,3-butadiene by a mixture of 1,2 and 1,4 addition using a peroxide catalyst. Consider carefully possible structures for the growing-chain radical. Show the expected structure of the polymer and calculate AH° for the polymerization reaction. 

The Catalyst System Eleven years ago, Kaminsky invented a novel olefin polymerization catalyst derived from Cp2ZrCl2 (Cp = 7 -5-C5H5) and methylaluminoxane (1), a result that has stimulated intense interest in synthesis and reactions of metallocenium ions. Important questions still remain, however, regarding the nature of the Kaminsky catalyst. These include (1) what is methylaluminoxane and how does it interact with Cp2ZrMe2 to initiate polymerization and (2.) what are the mechanisms of chain initiation, propagation, transfer and termination A collateral question is how these steps may be controlled. 

This table illustrates that even for small molecules, the chain initiation, propagation and termination reactions are extensive. All possible products can be formed, but again the direct conversion reactions are absent, namely propane to propylene and hydrogen, or propane to ethylene and methane. 

The mechanism of the polymerization reaction is presumed to be essentially that of a homogeneous bulk or solution free-radical polymerization. The concern is exclusively with the polymerization by double-bond opening of carbon compounds that contain at least one caibon-carbon double bond. The reactive species that propagates to produce the polymer chain is a free radical formed by opening of the rc-bond of the carbon-carbon double bond. The basic steps of the polymerization reaction are initiation, propagation, termination (by various means), and various transra reactions. Tbe structure of the polymer produced is determined by the balance of the propagation, termination, and transfer reactions. 

Blake attributed the induction period to a surface poison, which he identified as water, and the initiation was considered as a possible surface free radical mechanism. Neither considered the structures involved in the processes of chain initiation, propagation and termination. 

Ionic-polymerization Kinetics. The kinetics of ionic polymerization share some common principles with that of the free-radical reaction. Both are based on the basic steps of initiation, propagation, termination, and chain transfer, and in both the ultimate average molecular weight depends on the ratio of the reaction rates of propagation and termination. There are, however, important differences. In ionic polymerization the termination step appears to be unimolecular, while it is bimolecular in free-radical type polymerization. The dependence of the kinetic scheme of the reaction on the various parameters is therefore different in the two reactions. Likewise, the fact that a cocatalyst has to be brought into the ionic reaction scheme has to be taken into account. 

The Hj/CO ratio in the synthesis gas was varied from 1 1 to 2 1 to 4 1. This resulted in 36, 72, and 99 percent conversion for CO and 95, 99.9, and 99.9 percent conversion of C-labeled ethene. The proportion of labeled ethene converted to methane increased from 2.7 to 5.1 to 8.8 percent with increasing H2/CO ratio. The olefinic portion of the hydrocarbon products decreased with increasing H2/CO ratio. For ethene, the amount of the labeled compound that was hydrogenated to labeled ethane was 66.6, 70.6, and 71.7 percent, respectively. The amount of in C3 products was 20 to 30 percent for the three experiments. This incorporation of label into C3. products may occur by three processes chain initiation, propagation, or termination. 

Formulate chain initiation, propagation, and termination steps for the polymerization of butadiene by a peroxide... 

Because of the nature of the active species, coordination polymerization has been classified as ionic polymerization, which follows the polyaddition mechanism s characteristic steps, in the growing of the polymeric chain initiation, propagation, and termination. As for the initiation step, the ionic active species is produced by the reaction between the catalyst and cocatalyst. Usually, the catalysts are actually precursor catalysts or precatalysts, which become the real cationic active species after the activation or reaction with the cocatalyst (Fig. 5.8). 

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