Critically Evaluated Termination Rate Coefficients for Free‐Radical Polymerization, 1. The Current Situation

Abstract

This is the first publication of an IUPAC‐sponsored Task Group on “Critically evaluated termination rate coefficients for free‐radical polymerization.” The paper summarizes the current situation with regard to the reliability of values of termination rate coefficients kt. It begins by illustrating the stark reality that there is large and unacceptable scatter in literature values of kt, and it is pointed out that some reasons for this are relatively easily remedied. However, the major reason for this situation is the inherent complexity of the phenomenon of termination in free‐radical polymerization. It is our impression that this complexity is only incompletely grasped by many workers in the field, and a consequence of this tendency to oversimplify is that misunderstanding of and disagreement about termination are rampant. Therefore this paper presents a full discussion of the intricacies of kt: sections deal with diffusion control, conversion dependence, chain‐length dependence, steady state and non‐steady state measurements, activation energies and activation volumes, combination and disproportionation, and theories. All the presented concepts are developed from first principles, and only rigorous, fully‐documented experimental results and theoretical investigations are cited as evidence. For this reason it can be said that this paper summarizes all that we, as a cross‐section of workers in the field, agree on about termination in free‐radical polymerization. Our discussion naturally leads to a series of recommendations regarding measurement of kt and reaching a more satisfactory understanding of this very important rate coefficient. Variation of termination rate coefficient kt with inverse absolute temperature T−1 for bulk polymerization of methyl methacrylate at ambient pressure.[6] The plot contains all tabulated values[6] (including those categorized as “recalculated”) except ones from polymerizations noted as involving solvent or above‐ambient pressures.magnified imageVariation of termination rate coefficient kt with inverse absolute temperature T−1 for bulk polymerization of methyl methacrylate at ambient pressure.[6] The plot contains all tabulated values[6] (including those categorized as “recalculated”) except ones from polymerizations noted as involving solvent or above‐ambient pressures.

The Importance of Chain‐Length Dependent Kinetics in Free‐Radical Polymerization: A Preliminary Guide

“…1, where 〈k t 〉 is the chain-length averaged termination rate coefficient and 〈k p 〉 is the chainlength averaged propagation rate coefficient for the given system. The use of a systemdependent 〈k t 〉 instead of an (incorrect) single chain-length independent value of k t in this equation seems to be generally accepted now, [1,2] but as we have shown previously and will elaborate upon in this paper, in certain cases the use of 〈k p 〉 instead of the long-chain k p value is also required. [3][4][5] [M] [I] …”

The Cutthroat Competition Between Termination and Transfer to Shape the Kinetics of Radical Polymerization

“…Making the steady-state assumption and the long-chain approximation, use of Equations (5) and (6) in Equations (2), (3) and (7) for the case of k trX = 0 (i.e., the termination limit) results in [9,14,15] (8) This equation holds strictly only for the geometric mean model, the physical basis of which is dubious for RP. [14] However, the remarkable thing about Equation (8) is that it holds qualitatively and semi-quantitatively for all models of cross-termination.…”

“…[3,4] carried out in a non-steady state, then the rate will instead yield k p /k t , still enabling k t to be easily obtained. [5,6] This has opened up hope that many of the frustrations associated with k t , a centrally important parameter, will be resolved. With this in mind, an IUPAC Task-Group looking into this broad issue was created.…”

“…[5] However it was also concluded that all the methods in Table 1 potentially should provide good k t values, as long as the user is aware of particular limitations that apply (see Table 1). This finding came as something of a surprise, because the notorious problem of excessive scatter [6] in literature values of k t was commonly assumed to arise, at least in part, from some methods of measurement simply being inherently bad techniques. There is no doubt that scatter in literature data for k t is due in no small part to naive employment of measurement methods, for example allowing a large change of conversion over the course of a k t measurement, or the choice of a poor value of k p or R init for data analysis.…”

“…By far the most notable of these is chain-length-dependent termination (CLDT). [6] The aim of the present work is to illuminate some of the most significant trends to which CLDT gives rise, and thus to reveal the rich impact that it has on k t . Once these effects are comprehended, it becomes clear why many purportedly identical k t measurements in fact were nothing of the sort, thus explaining why different values of k t were found.…”