Energy of propagation

Activation energy of propagation reaction in the Arrhenius equation... 

By the described kinetic analysis, the activation energy of initiation [reaction (40)], which is otherwise accessible only with difficulty, could also be estimated. A value about twice that of the activation energy of propagation was found ( 74kJmol-1, or 66 kJ mol-1, Ep = 32.6 kJ mol-1 [16]). 

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

An Arrhenius plot was created to determine the activation energy of propagation using sec-BuLi/t-BuP (Figure 8, Table 1). 

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). 

From the above discussion it follows first that all unsaturated compounds are potential monomers. Second, if a carbon atom of a double bond is attached to two different groups, the monomer is a building block for a stereoregular polymer. Third, it follows that the polymerizability of a potential monomer depends on the free energy of propagation. Note that if the reaction is thermodynamically unfavorable at room temperature it may still be feasible at much lower temperatures. Finally, the kinetics of polymerization must be considered, because, even though the thermodynamics are favorable, the rate of reaction may be too slow for synthesis to be practical. Of course, the rate problem may be eliminated with the right catalyst. 

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.

The energetic criterion, according to which the crack development into a mechanically solicited composite releases a certain amount of elastic energy that is stocked in material and can be evaluated. When its level attains a critical value, G, the cracks will propagate until the material s fracture. The critical energy of propagation represents an intricate characteristic of material, and in the case of brittle fracture, the Griffith criterion can be ex-... 

The most unsophisticated interpretation of the flexed bend impact test is that it is a measure of the energy required to propagate the crack across the specimen, irrespective of whether the specimen is notched or unnotched. Notch sensitivity is ignored and only the energy of propagation is involved. In this case... 

This is a significant analysis since it enables the true values of the activation energies of propagation for various systems to be determined. 

Coincidental or not, also in a PLP study of several methacrylates, the ethyl monomer EMA was found to have the highest activation energy of propagation [15, 28]. (2) Because of the high uncertainty in the exact values of the Mark-Houwink coefficients of EA, the corresponding confidence intervals of equations 4.12 to 4.14 have not been calculated. 

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... 

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... 

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