The kinetics of free radical polymerizing systems at low conversion, 3. On the variation of the termination rate coefficient with monomer and with temperature

<jats:title>Abstract</jats:title><jats:p>This paper shows that variations with temperature and with monomer of overall termination rate coefficients, denoted 〈<jats:italic>k</jats:italic><jats:sub>t</jats:sub>〉, are not what they seem. This is important because such variations, as determined experimentally, are usually taken as making direct statements about the actual termination reaction. For example, it has always been assumed that the activation energy of 〈<jats:italic>k</jats:italic><jats:sub>t</jats:sub>〉, signified <jats:italic>E</jats:italic><jats:sub>a</jats:sub> (〈<jats:italic>k</jats:italic><jats:sub>t</jats:sub>〉), reflects the activation energy of the rate determining step behind the termination reaction. However it is shown that purely as a consequence of termination rate coefficients being chain length dependent, this is not so. In fact, simply given chain length dependent termination, <jats:italic>E</jats:italic><jats:sub>a</jats:sub> (〈<jats:italic>k</jats:italic><jats:sub>t</jats:sub>〉) will be a function of the means of initiation, of the initiator itself, of the initiator concentration [I], and of whether [I] is held constant or varied with temperature. These results not only suggest interesting experiments for microscopic probing of the termination process, but they also help explain the wide variety of experimental <jats:italic>E</jats:italic><jats:sub>a</jats:sub>(〈<jats:italic>k</jats:italic><jats:sub>t</jats:sub>〉) found in the literature. Much literature data concerning 〈<jats:italic>k</jats:italic><jats:sub>t</jats:sub>〉 is also made sense of in the second half of this paper. Here variations of 〈<jats:italic>k</jats:italic><jats:sub>t</jats:sub>〉 with monomer are examined, and it is shown that in order to explain these, it is likely that variations of rates of propagation and of transfer are often of major importance. Also considered are the effects on 〈<jats:italic>k</jats:italic><jats:sub>t</jats:sub>〉 of diffusion coefficient variations, these of course being directly related to the rate coefficients of the actual termination process. Interesting results, ones relevant to experimental data interpretation, again emerge. For all these calculations a termination model appropriate for very low conversion bulk and solution polymerizations is used, and steady state 〈<jats:italic>k</jats:italic><jats:sub>t</jats:sub>〉 computed. However the principles that emerge are general, and their relevance is contingent only upon termination being chain length dependent. It is therefore to be envisaged that these principles will assist in explaining far more actual polymerization data than has been looked at here.</jats:p>