Termination activation energy

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 ... 

Table 6.3 lists the activation energies for termination (these are overall values, not identified as to mode) of several different radicals. The rate constants for termination at 60°C are also given. We shall see in Sec. 6.6 how these constants are determined. 

Table 6.3 Rate Constants at 60 C and Activation Energies for Some Termination Reactions...

The unsaturation present at the end of the polyether chain acts as a chain terminator ia the polyurethane reaction and reduces some of the desired physical properties. Much work has been done ia iadustry to reduce unsaturation while continuing to use the same reactors and hoi ding down the cost. In a study (102) usiag 18-crown-6 ether with potassium hydroxide to polymerise PO, a rate enhancement of approximately 10 was found at 110°C and slightly higher at lower temperature. The activation energy for this process was found to be 65 kj/mol (mol ratio, r = 1.5 crown ether/KOH) compared to 78 kj/mol for the KOH-catalysed polymerisation of PO. It was also feasible to prepare a PPO with 10, 000 having narrow distribution at 40°C with added crown ether (r = 1.5) (103). The polymerisation rate under these conditions is about the same as that without crown ether at 80°C. 

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. 

E 3,p,E = activation energy for dissociation, propagation and termination respectively (KJ/mol)... 

The activation energy Ed for the decomposition of the initiators commonly used is of the order of 30 kcal., while (Ep — Etl2) assumes values from 4 to 7 kcal. Hence, insofar as it is termination-controlled, the temperature coefBcient of the degree of polymerization is negative. Similarly for thermal polymerizations, the corresponding quantity... 

Activation energies for chain termination are smaller than for chain propagation, but they are significantly greater than zero. This might not have been anticipated inasmuch as methyl radicals seem to combine in the gas phase without measurable activation energy. ... 

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