Emulsion polymerisation of styrene in the presence of hydrocarbon diluents

Results are given for the effects of four hydrocarbon diluents (benzene, toluene, ethylbenzene and cyclohexane) upon the rate of emulsion polymerisation of styrene at 50°C, upon the particle size of the resultant latex, and upon the intrinsic viscosity of the polymer produced. The aromatic diluents were found to have a much greater effect in reducing the rate of polymerisation than can be accounted for by mere dilution of the monomer. For these diluents, the rate of polymerisation is approximately third order in initial concentration of monomer in the oil phase as a whole. Cyclohexane is far less effective as a retarder of polymerisation, the rate being approximately first order in monomer concentration in the oil phase. Particle size determinations indicate that the reductions in rate are due primarily to a reduction in the rate of polymerisation at each locus, rather than to a reduction in the number of reaction loci. The intrinsic viscosity of the polymer produced tends to fall with the rate of polymerisation. These observations are consistent with the view that aromatic diluents cause severe retardation of polymerisation because they diminish the Trommsdorff ‘gel’ effect at the reaction loci. This view is in turn shown to be consistent with a reaction mechanism in which, in the case of styrene‐rich systems at least, the growing polymer particles do not attain equilibrium with the monomer droplets.