Copolymerization of styrene and acrylonitrile in aqueous dispersion

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Abstract
The bead copolymerization of styrene‐acrylonitrile, in the presence of 1‐azobis‐1‐phenylethane, was investigated at high temperature (90–100°C.). The initial distribution of acrylonitrile between styrene and the aqueous phase was determined and the change of this distribution during the copolymerization. The concentration of acrylonitrile in the aqueous phase remains fairly constant up to 20–25% conversion, then it decreases rapidly up to about 90% conversion. The over‐all rate of copolymerization following an induction period, remains constant up to about 30–35% conversion, then it increases slightly only to decrease rapidly at about 90% conversion. In a system of this type the mole fraction of acrylonitrile in the oil phase increases through the polymerization, partly because acrylonitrile reacts at a lower relative rate, partly because the acrylonitrile dissolved in the aqueous phase diffuses continuously into the beads. The reactivities of styrene and acrylonitrile in aqueous dispersion are the same as in bulk if the partition of acrylonitrile between the two phases is taken into account. The initial rates of copolymerization, in bulk, increase with the acrylonitrile content of the mixture. At high conversion, a marked acceleration in rates takes place. This autoacceleration is due to the increased viscosity of the medium and to the variation of the comonomer composition with conversion.