Inverse Emulsion Polymerization of Acrylamide Initiated by Oil- and Water-soluble Initiators: Effect of Emulsifier Concentration

The nature of the reaction loci in inverse emulsion polymerization of acrylamide (AAm) initiated by oil (2,2′-azoisobutyronitrile, AIBN)- and water-soluble (ammonium peroxodisulfate, APS) initiators was investigated. The rate of polymerization (Rp,max, Interval 2) increases with increasing the emulsifier concentration up to a certain critical concentration and then decreases. The rate of polymerization and the polymerization rate per particle are somewhat larger in the APS-initiated polymerization than in the AIBN-initiated one. For both initiators the polymerization rate per particle is proportional to the particle size and decreases with increasing the emulsifier concentration and the decrease is much more pronounced at the high emulsifier concentrations. The similar kinetic dependences for both initiators were discussed in terms of the formation of monomeric or oligomeric radicals with the similar nature, mobility via the reaction system and partitioning between the oil and water phases. The similar radicals carrying the initiator fragment were formed by the addition of one or two (or several) units to the primary radicals (SO4•− or 2-cyanoisopropyl). The estimated average number of radicals per particle (n̅) is much below 0.5. The low radical concentration in the polymer particles was discussed in terms of the desorption of monomeric and emulsifier radicals. The desorption of radicals was confirmed by both the Nomura model (Nmodel, k′des,N) and the Ugelstad approach (the solution of population balance for the particle with n̅, Tmodel, k′des,T). The k′des,T’s were found to be much larger than the k′des,N’s. The differences in the k′des’s were discussed in terms of the desorption (surface active) monomeric radicals (the polymerization within the outer spheres of polymer particles) and emulsifier (the chain transfer to emulsifier) radicals. The radical entry rate decreases with increasing the emulsifier concentration or decreasing the particle size.