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.

Summary: A novel method for measuring termination rate coefficients, kt, in free‐radical polymerization is presented. A single laser pulse is used to instantaneously produce photoinitiator‐derived radicals. During subsequent polymerization, radical concentration is monitored by time‐resolved electron spin resonance (ESR) spectroscopy. The size of the free radicals, which exhibits a narrow distribution increases linearly with time t, which allows the chain‐length dependence of kt to be deduced. The method will be illustrated using dodecyl methacrylate polymerization as an example.

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Termination rate coefficients, kt, of alkyl acrylate and alkyl methacrylate homopolymerizations at 40°C and pressures of 1000 and 2000 bar have been measured up to high degrees of monomer conversion using the time-resolved single-pulse-pulsed-laser polymerization (SP-PLP) technique. The chain-length dependence (CLD) of k t has been deduced from SP-PLP data by adopting the power-law model, kt ) k t 0 i -R , where i is the chain length. For methacrylates at low degrees of monomer conversion, R is close to the theoretically predicted value of 0.16. At conversions above 20% the exponent R increases significantly with increasing conversion. This effect becomes particularly pronounced in the gel effect region, where R, e.g. for MMA, reaches values close to unity. In the case of acrylates with small alkyl ester side chain, such as methyl acrylate, R is also close to 0.16 at low conversions and increases toward higher conversions. In the case of acrylates with larger alkyl ester side chain, such as dodecyl acrylate and 2-ethylhexyl acrylate, however, R is close to 0.4 even at low degrees of monomer conversion. The latter effect is strongly indicative of intramolecular chain transfer, which generates significant amounts of midchain radicals in the system. The fact that such transfer processes take place is supported by SP-PLP data on alkyl acrylates polymerized in mixtures with supercritical carbon dioxide.