Activation energy, of propagation and termination

Activation Energies of Propagation and Termination in Free Radical Polymerization... 

II / 418 ACTIVATION ENERGIES OF PROPAGATION AND TERMINATION IN FREE RADICAL POLYMERIZATION TABLE 3. cont d... 

Termination can also occur by the reaction of polymer free radicals with primary initiator radicals (called primary termination) or free-radical scavenging species, especially oxygen. Activation energies for propagation and termination for some typical monomers are listed in Table 2. 

The zero activation energy for radiation initiation, E., leads to some interesting practical consequences. The overall activation for the rate, E = E + 1/2 E - 1/2 E, where E and E are the activation energies for propagation and termination, respectively. Since E. for most catalytic initiation is about 30 and E close to zero, this leads to a value of about 22 kcal per mole compared x ith about 7 kcal per mole for radiation for v/hich E.=0. The practical advantages of this were referred to earlier. 

In the bulk polymerization of styrene by ultraviolet radiation, the initial polymerization rate and degree of polymerization are 1.3x10 mol/L-s and 260, respectively, at 30°C. What will be the corresponding values for polymerization at 80°C The activation energies for propagation and termination of polystyryl radicals are 26 and 8.0 W/mol. What assumption, if any, is made in this calculation ... 

Bengough, W. J., and H. W. Melville A thermocouple method of folloving the non-stationary state of chemical reactions. 2. The evaluation of velocity coefficients and energies of activation for the propagation and termination reactions for the initial and later stages of the polymerization of vinyl acetate. Proc. Roy. Soc. A 230, 429 (1955). 

In contrast to chain transfer to solvent which would be prevalent from the initial stages of a polymerization due to the solvent s high concentration, chain transfer to polymer often does not compete noticeably with propagation until the end of the polymerization when monomer is depleted. In addition, chain transfer and termination reactions generally have higher activation energies than propagation, and therefore can be... 

The temperature of reaction for cationic polymerization is usually kept very low. The rate of initiation is largely insensitive to temperature, so the rates of propagation and termination alone determine the temperature dependence of the polymerization. Thus the observed activation energy, will be just the difference between propagation and termination activation energies ... 

Values for activation energies of propagation, initiation, and termination are given in Table 6 for polychlorotrifluoroethylene, poly(a-methylstyrene), polystyrene, and poly(methyl methacrylate), assuming a random initiation mechanism. The energy for combination Et is assumed to be zero in the gas phase, hut in a viscous condensed medium it is considerably above zero, about 83.7 kJ/mol (20 kcal/mol) has been reported for poly(methyl methacrylate) (63). Termination is a diffusion-controlled process, and the activation energy for the diffiision of... 

Even though the absolute rate constant for reactions between propagating species may be determined largely by diffusion, this does not mean that there is no specificity in the termination process or that the activation energies for combination and disproportionation are zero or the same. It simply means that this chemistry is not involved in the rate-determining step of the termination process. 

Transfer reaction to the monomer, leading to the insertion of an unsaturated end group, is an important reaction in cationic chain polymerisation. As the activation energies of both termination and transfer reactions are higher than that of the propagation step, cationic chain polymerisation can only lead to high molecular masses when undertaken at low temperatures, typically — 100°C. 

The traditional chain oxidation with chain propagation via the reaction RO/ + RH occurs at a sufficiently elevated temperature when chain propagation is more rapid than chain termination (see earlier discussion). The main molecular product of this reaction is hydroperoxide. When tertiary peroxyl radicals react more rapidly in the reaction R02 + R02 with formation of alkoxyl radicals than in the reaction R02 + RH, the mechanism of oxidation changes. Alkoxyl radicals are very reactive. They react with parent hydrocarbon and alcohols formed as primary products of hydrocarbon chain oxidation. As we see, alkoxyl radicals decompose with production of carbonyl compounds. The activation energy of their decomposition is higher than the reaction with hydrocarbons (see earlier discussion). As a result, heating of the system leads to conditions when the alkoxyl radical decomposition occurs more rapidly than the abstraction of the hydrogen atom from the hydrocarbon. The new chain mechanism of the hydrocarbon oxidation occurs under such conditions, with chain... 

Since the reactants (R02 ketone) and the transition state have a polar character, they are solvated in a polar solvent. Hence polar solvents influence the rate constants of the chain propagation and termination reactions. This problem was studied for reactions of oxidized butanone-2 by Zaikov [81-86]. It was observed that kp slightly varies from one solvent to another. On the contrary, kt changes more than ten times from one solvent to another. The solvent influences the activation energy and pre-exponential factor of these two reactions (see Table 8.16). 

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. 

Nair et al. studied the kinetics of the polymerization of MMA at 60-95 °C using N,1SP-diethyl-NjW-di(hydroxyethyl)thiuram disulfide (30a) as the thermal in-iferter [142]. The dependence of the iniferter concentration on the polymerization rate was examined. The chain transfer constant of the propagating radical of MMA to 30a was determined to be 0.23-0.46 at 60-95 °C, resulting in the activation energy of 37.6 kj/mol for the chain transfer. Other derivatives 30b-30d were also prepared and used to derive telechelic polymers with the terminal phosphorus, amino, and other functional aromatic groups [143-145]. Thermal polymerization was also investigated with the end-functional poly(St) and poly(MMA) which were prepared using the iniferter 13 [146]. 

Typical energies of activation for propagation and termination are given in Table 6.2 and typical free radical kinetic values in Table 6.3. 

The rate of oxidation decreases with dilution of methyl ethyl ketone by a nonpolar solvent—namely, benzene. The rate constants for both chain propagation and chain termination also drop (Table I), and the activation energies increase (Table II) because the elementary reactions of chain propagation and termination represent interaction between two dipoles occurring at a rate which depends on the dielectric constant of the medium. 

Use the data below for the activation energies of some typical addition polymerization reactions at 60° C to calculate an average activation energy for propagation, termination, and initiator decomposition. Each person should calculate one of the three average activation energies. 

Reactions (3.95) and (3.96) are the propagation steps, although the former is very rapid, with a negligible activation energy, whereas the latter is slower, with an activation energy of about 1.7 kJ/mol (7 kcal/mol). Three types of termination reactions ensue, all of which are very rapid, with activation energies of less than 1 kJ/mol (3 kcal/mol) ... 

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