Determination of reactivity ratios from analysis of triad fractions—analysis of the copolymerization of styrene and acrylonitrile as evidence for the penultimate model

A numerical analysis of the relationship between the composition of the monomer feed and that for the resultant copolymer (at low conversion) is the method usually adopted for determination of copolymerization reactivity ratios. However, the six triad fractions provide much more information about a copolymer than does the single copolymer composition. In most of the cases where both sets of information can be reliably obtained, the copolymer composition can be determined with slightly greater experimental accuracy than the triad fractions. Despite the somewhat lower accuracy of the individual triad fractions, in some cases the “best” model of the copolymerization and the corresponding reactivity ratios can be determined with higher precision from the triad fractions, because of the extra information provided by these data. A direct numerical analysis of the triad fractions for the styrene-acrylonitrile copolymerization in terms of the penultimate model has yielded the reactivity ratios and their joint confidence regions for the bulk polymerization at 60°. The analysis shows that the data can be satisfactorily described by the penultimate model, whereas statistical tests demonstrate that the terminal model will not adequately represent the polymerization. The reactivity ratios are determined with higher precision from the triad fractions than from the copolymer compositions.