Activation energy, of propagation and

Fig. 1. Change in energy of a polsmaerization/depolymerization equilibrium along the reaction coordinate, showing the relation of the activation energies of propagation and depropagation and the heat of polymerization.

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... 

Importantly, the intercepts of the 1 jxn versus 1/[M] plots shown in the figure are the same for all temperatures, that is, the activation energies of transfer and propagation are equal in the —30° C to —50° C range. Hence, transfer has no effect on the change of molecular weight with temperature in this region. 

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... 

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 ... 

Table 4.2 The activation energy of propagation reaction (Rp) and transfer reaction (Rtr) in PO and EO anionic polymerisation ...

Thus, the activation energy of propagation for the. sec-BuLi/r-BuP4 initiating system was determined to be Ep = 45 kJ/mol with a frequency exponent of log(A) = 6.7. A comparison with literature values for other counterions shows that the homopolymerization of EO in the presence of the sec-BuL /t-BuP4 initiating system has a lower activation energy and frequency factor than conventional counterions, e.g. Cs, (Table 2). 

Depropagation n. The sequential chain scission step during depolymerization responsible for the formation of monomer. Has a lower activation energy than propagation and hence is favored at high temperatures. 

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... 

S. Korcek, J.H.B. Chenier, J.A. Howard, K.U Ingold. Absolute rate constants for hydrocarbon autoxidation. XXL Activation energies for propagation and the correlation of propagation rate constants with carbon-hydrogen bond strengths. Canadian Journal of Chemistry 50(14), 2167-2174 (1972). 

---