Kinetic Mechanism Of Miniemulsion Polymerization Initiated By Oil-Soluble Initiator

The kinetic mechanism of miniemulsion polymerization initiated by oil-soluble initiators,2,2’-azobis(isobutyronitrile)(AIBN), was clarified via carrying out scientific research theoretically and experimentally. Isobutyronitrile (IBN) was selected as a stable substitute for the2-cyano-2-propyl radical originating from AIBN to study the partition behavior of this radical in miniemulsion polymerization. The ideal aqueous phase radical trapper (Fremy’s salt), which was used for the research of primary radical desorption in miniemulsion polymerization, was screened out from four candidates according to their efficiency of radical trapping and effects on the stability of miniemulsion polymerization. Fremy’s salt was used to determine the desorption rate of primary radical experimentally and a mathematical model based on Einstein-Smoluchowski equation and two-film theory was set up to predict the desorption rate theoretically. A "pseudo-bulk" kinetic mathematical model describing the kinetics of miniemulsion polymerization initiated by oil-soluble initiator was set up according to the desorption rate model of (primary) radical and the discrete analysis about the reaction process of two newly formed primary radicals in the latex particle.The partition coefficient for IBN was found to be10.98for the styrene/water system, which was much smaller than that of AIBN. The partition coefficient was not affected by the temperature, hexadecane content, styrene/water ratio or the pH value of aqueous phase. The Henry’s law constant of IBN in styrene was found to be4.10kPa according to the relationship between partition coefficient and Henry’s law constant. For the polystyrene/water system, the partition coefficient was found to be4.34, which was smaller than that in styrene/water system. The partition coefficient, which may fall from10.98to4.34with the content of styrene in the latex particle increasing, was not affected by the latex particles size or the surfactant type. Predictions based on a published kinetic model and the newly measured partition coefficient were compared with published experimental data and prediction based on the partition coefficient of AIBN. The model prediction based on the partition coefficient of IBN agreed well with the published experimental data, it meant that the partition coefficient of primary radical should be used in theoretical calculations rather than that of initiator.Miniemulsion polymerizations with different aqueous phase radical trappers were carried out to screen out the ideal trapper. Sodium nitrite made the miniemulsion polymerization quite unstable and could not trap all of the radicals in the aqueous phase; Ferric chloride may trap almost all the aqueous phase radicals, however, the strong acid reaction condition and the large size of latex particle made ferric chloride unsuitable for typical miniemulsion polymerization; Acrylamide may trap small amount of aqueous phase radicals, but the oligomer radicals formed from trapping could still be captured by the latex particle; Fremy’s salt could trap all of the aqueous phase radicals and not affect the stability of miniemulsion polymerization.The theoretical predictions, which combined Einstein-Smoluchowski equation and two film theory, agreed well with the aqueous phase radical trapping experimental results according to aqueous phase mass balance. It is found that the content of monomer in latex particle, particle size, surfactant layer, aqueous phase resistance and initiator type all affected the desorption rate of primary radical. It was also found that the thickness of stagnant film around the latex particle was much smaller than the diameter of latex particle.The reliability of "pseudo-bulk" kinetic model of miniemulsion polymerization initiated by oil-soluble initiator was confirmed by comparing the model-predicted with experimental results on both polymerization rate and molecular weight of polymer, in which the effects of different variable factors including the particle size, surfactant layer, initiator type, and aqueous phase inhibitor were considered. It was found that the effective radicals in the miniemulsion polymerization initiated by oil-soluble initiators were originated from the primary radicals desorbed from the latex particles and the contribution of small amount of initiator dissolved in the aqueous phase to the polymerization could be ignored. The single radical was formed by absorption of aqueous phase radicals, however, the mechanism was found to be different from miniemulsion polymerization initiated by water-soluble initiators. The kinetic model was also simplified according to the fact that the miniemulsion polymerization obeys the "0-1" mechanism while the accuracy of model prediction was as well as before.