Amphoteric initiators suitable for emulsifier-free emulsion polymerizationand the properties of the resulting latices

The soap-free emulsion polymerizations of styrene (St) and acrylamide (AAm) or methyl methacrylate (MMA) were carried out in the presence of three kin ds of amphoteric initiators, and the polymerization kinetics and the colloi dal properties of the latices produced were emphatically investigated. It w as found that the number of carbon atoms between the amidino and carboxyl g roups in each initiator exhibited an appreciable effect on the dissociation as well as on the solubility of the initiator in water, and therefore, tha t the properties of the colloidal particles depended on the structure of th e initiators used. All the copolymerization runs, except for the polymeriza tion using 2.2 ' -azobis(N-(2-caboxyethyl)-2-methylpropionamidine) under a strongly alkali condition, gave amphoteric latices, which indicated higher critical flocculation concentrations at lower or higher pHs than at a mediu m pH. The surface charge density measured by titration for poly(St/MMA) par ticles was about 3-10 times as high as that for the poly(St/AAm) ones, thou gh these were prepared under the same conditions other than the monomer com position. The influence of the polymerization pH on both the polymerization rate and the surface charge density of the resulting latices was negligibl e even if the constants of the decomposition rate and the dissociation of t he amphoteric initiators strongly depended on the pH of the medium.