Styrene-methyl methacrylate copolymer microstructure according to terminal and penultimate model for free-radical copolymerization

Microstructure of styrene-methyl methacrylate copolymers was determined 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 copolymerization of styrene and methyl methacrylate in bulk at 40 degrees C were calculated using various sets of terminal and penultimate model reactivity ratios with very similar standard deviations. Calculations showed that for both models, the terminal model and the penultimate model, different reactivity ratios with very similar standard deviations are equally well suited for the quantitative description of the triad sequence distribution in the copolymer. The terminal and penultimate models produce different styrene centred triad sequence distributions in the copolymer, the difference depending on the penultimate model reactivity ratios used for calculation and on the type of the styrene centred triad sequence. The best equivalence of the two kinetic models was found when the initial r-parameters guess used for determining the penultimate reactivity ratios by the simplex method were r(11) = r(21) = r(1) and r(22) = r(12) = r(2). The results suggest that the penultimate effect on the styrene-methyl methacrylate free-radical copolymerization system is limited.