STYRENE-METHYL METHACRYLATE COPOLYMER MICROSTRUCTURE ACCORDING TO TERMINAL AND PENULTIMATE MODEL FOR FREE-RADICAL COPOLYMERIZATION

Microstructure of styrene-methyl methacrylate copolymers was determine d according to the ''bootstrap model'' description using the terminal and the penultimate model of copolymerization. The implication of non- uniqueness in determining the penultimate model reactivity ratios from the copolymer microstructure is discussed. The styrene centred triad sequences in the copolymer obtained in the free-radical copolymerizati on of styrene and methyl methacrylate in bulk at 40 degrees C were cal culated using various sets of terminal and penultimate model reactivit y ratios with very similar standard deviations. Calculations showed th at for both models, the terminal model and the penultimate model, diff erent reactivity ratios with very similar standard deviations are equa lly well suited for the quantitative description of the triad sequence distribution in the copolymer. The terminal and penultimate models pr oduce different styrene centred triad sequence distributions in the co polymer, the difference depending on the penultimate model reactivity ratios used for calculation and on the type of the styrene centred tri ad sequence. The best equivalence of the two kinetic models was found when the initial r-parameters guess used for determining the penultima te reactivity ratios by the simplex method were r(11) = r(21) = r(1) a nd r(22) = r(12) = r(2). The results suggest that the penultimate effe ct on the styrene-methyl methacrylate free-radical copolymerization sy stem is limited.