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 k~t~. It begins by illustrating the stark reality that there is large and unacceptable scatter in literature values of k~t~, 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 k~t~: 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 k~t~ and reaching a more satisfactory understanding of this very important rate coefficient.
Variation of termination rate coefficient k~t~ 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 k~t~ 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.