The Effect of Nonreactive Additives on the Kinetics of Emulsion Polymerization

Abstract

Various substances such as plasticizers and other hydrocarbons are sometimes added to the monomer in an emulsion polymerization. The effect on the kinetics of the seeded emulsion polymerization of styrene in the presence of various 'non-reactive' hydrocarbons as low molecular weight additives have been investigated. By the use of seed latex, the number of particles was kept constant throughout the experiments, so that the effect of other parameters on the rate could be determined. All the additives decreased the polymerization rate more than could be accounted for by dilution alone. In the case of the water-insoluble alkanes n-tetracosane and n-octadecane, the decrease in rate could be attributed to a reduction in the monomer concentration in the latex particles, [M]p, resulting from a lowering of the activity of the monomer in the monomer droplets. No alkane is transported to the polymerizing particles. Theoretical equations have been developed for calculating [M]p at equilibrium swelling under these conditions. The values are in good agreement with those calculated from Smith-Ewart case 2 kinetics. Independent measurements confirm that the average number of radicals per particle, ñ = ½ in these experiments. When the hydrocarbon additives were of lower molecular weight, e.g. n-pentane and ethylbenzene, they are capable of transport through the aqueous phase and of swelling the latex particles. Ethyl benzene has been shown to be indistinguishable from styrene in its swelling behavior. In these cases the depression in the rate of polymerization comes about from not only dilution of the monomer, but also from chain transfer with the diluent as it accumulates in the particles. Chain transfer leads to formation of small radicals which may diffuse out of the particles, lowering the value of ñ. These conclusions differ from those of earlier workers on similar systems: Owen, McLemore, Liu, Seymour and Tinnerman suggested phase separation within the particles may be responsible, but they did not control nor measure their particle number concentrations. Blackley and Haynes invoked the Trommsdorff "gel" effect, but calculation shows that their kinetics follow Smith-Ewart Case 1 where this cannot occur.