Exit in the emulsion polymerization of vinyl acetate

In most emulsion polymerizations the exhaustion of the separate monomer phase is followed by a decrease in conversion rate due to the decrease in monomer concentration in the latex particles. The unusual feature of vinyl acetate is that the rate remains constant throughout most of the period of decreasing monomer concentration. There are two postulated mechanisms consistent with the observed kinetics. Both require that kex = ktr Cp and this can be tested with a gamma radiolysis experiment. (kex = exit rate coefficient, ktr = rate coefficient for transfer to monomer and Cp monomer concentration in particles). An automatic tracking device (developed by David Sangster) measures the meniscus height in a dilatometer every three seconds. When the dilatometer is removed from the gamma source initiation by radiolysis stops allowing kex to be determined directly from the relaxation data. One aspect of relaxation experiments which is often overlooked is the effect of the heat of reaction on the temperature of the reaction mixture. In order to compensate for this effect the temperature and contraction of the reaction mixture were monitored (out of source) while the temperature of the waterjacket was lowered through the range appropriate to the particular experiment. These data were used to construct a calibration curve which, together with a sensitive temperature probe in the dilatometer, was used to correct the conversion/time data obtained during the relaxation experiment