Critically evaluated rate coefficients in radical polymerization-7. Secondary-radical propagation rate coefficients for methyl acrylate in the bulk

Propagation rate coefficient (k_p) data for radical polymerization of methyl acrylate (MA) in the bulk are critically evaluated and a benchmark dataset is put forward by a task-group of the IUPAC Subcommittee on Modeling of Polymerization Kinetics and Processes. This dataset comprises previously published results from three laboratories as well as new data from a fourth laboratory. Not only do all these values of k_p fulfill the recommended consistency checks for reliability, they are also all in excellent agreement with each other. Data have been obtained employing the technique of pulsed-laser polymerization (PLP) coupled with molar-mass determination by size-exclusion chromatography (SEC), where PLP has been carried out at pulse-repetition rates of up to 500 Hz, enabling reliable k_p to be obtained through to 60 °C. The best-fit-and therefore recommended-Arrhenius parameters are activation energy E_A = 17.3 kJ mol^-1 and pre-exponential (frequency) factor A = 1.41 × 10⁷ L mol ^-1 s^-1. These hold for secondary-radical propagation of MA, and may be used to calculate effective propagation rate coefficients for MA in situations where there is a significant population of mid-chain radicals resulting from backbiting, as will be the case at technically relevant temperatures. The benchmark dataset reveals that k_p values for MA obtained using PLP in conjunction with MALDI-ToF mass spectrometry are accurate. They also confirm, through comparison with previously obtained benchmark k _p values for n-butyl acrylate, methyl methacrylate and n-butyl methacrylate, that there seems to be identical family-type behavior in n-alkyl acrylates as in n-alkyl methacrylates. Specifically, k_p for the n-butyl member of each family is about 20% higher than for the corresponding methyl member, an effect that appears to be entropic in origin. Furthermore, each family is characterized by an approximately constant E_A, where the value is 5 kJ mol^-1 lower for acrylates.