Inferences on the mechanism of emulsion polymerisation of styrene from characterisation of the polymer end-groups†

TLDR: In this paper, the number of surface groups as determined by conductometric titration of the latex, plus the number buried within the particle, was less than the expected value of two per polymer molecule.

Abstract: Solute electrolyte and adsorbed emulsifier can be removed by ion exchange from polystyrene latices prepared with persulphate initiator. Rigorous purification of the ion-exchange resins is necessary to avoid contamination by leached polyelectrolytes. These ion-exchanged latices are stabilised by strong acid surface groups - sulphate end-groups of the polymer molecules. The number of surface groups as determined by conductometric titration of the latex, plus the number buried within the particle as determined by conductometric titration of the polymer in solution (or by X-ray fluorescence), is less than the expected value of two per polymer molecule. The other end-groups are shown to be hydroxyl groups by oxidation to the carboxyl form followed by ion exchange and titration. The total number of end-groups thus determined is close to two per molecule. As the pH of the polymerisation medium is decreased, the proportion of hydroxyl end-groups increases and that of the sulphate end-groups decreases. No carboxyl groups are detected in ion-exchanged latices prepared in a nitrogen atmosphere, independent of the nature of the emulsifier. The dye partition method gives end-group concentrations that are far lower than those determined by titration or X-ray fluorescence, probably because the formation of the dye end-group ion-pair is not complete. Finally, for latices prepared at relatively low emulsifier concentrations, the contribution of the sulphate end-groups to the surface charge density is significant and cannot be overlooked, as has been done in the past.