Application of the monomer reactivity ratios to the kinetic‐model discrimination and the solvent‐effect determination for the styrene/acrylonitrile monomer system

The impact of reactivity ratios determined with the Nelder and Mead simplex method on the kinetic‐model discrimination and the solvent‐effect determination for the styrene/acrylonitrile monomer system was investigated. For the monomer system, the penultimate unit effect was inversely proportional to the polarity of the solvent: acetonitrile < N,N‐dimethylformamide < methyl ethyl ketone < toluene. Quantitatively, the penultimate unit effect could be correlated with an absolute value of the difference between the standard deviation of the reactivity ratios determined for the terminal and penultimate models. By application of the F test, the penultimate model was justified for copolymerization in toluene. The conclusion was less certain for polymerization in methyl ethyl ketone. With a scanning procedure based on the simplex method, it was found that an equivalent representation of the copolymer‐composition data could be achieved with multiple sets of penultimate‐model reactivity ratios. However, the relationship between the triad‐sequence distribution and copolymer composition depended on the reactivity‐ratio set chosen for the microstructure determination. The microstructure calculated with the penultimate‐model reactivity ratios determined with the simplex method from the initial guess (r₁₁ = r₁, r₂₁ = 1/r₂, r₂₂ = r₂, r₁₂ = 1/r₁) did not obey the general “bootstrap effect” rule. This observation still requires some theoretical interpretation. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 846–854, 2000