Initiation and Termination in Styrene Free-Radical Polymerization Initiated by Redox Initiation

Hydrogen peroxide and hydroperoxides with substituents of different size are combined with ferrous sulfate to form redox initiation systems, which are used to initiate the polymerization of styrene in emulsion. Gas chromatography and size-exclusion chromatography are used to measure the monomer conversion and the molecular weight of the polystyrene. Nuclear magnetic resonance is used to identify the characteristic structures, quantitative information is used to understand the polymerization. The results suggest that the initiation of the primary radicals directly depend on the size of the substitute, hydroxyl radical shows almost no selectivity between head-addition and tail-addition during initiation (F-hi = 47.4%). But for primary radicals with big substitute groups, for example,t-butyl and cumyl hydroperoxides, tail-addition takes advantages over head-addition during initiation (F-hi approximate to 80%). As for the termination mechanism, it mainly depends on the solubility of the peroxide in water, the interfacial area of the particle as well as the diffusion rate of the primary radical in aqueous phase. Primary termination dominates in the polymerization initiated by hydroperoxide with poor solubility in water, for example,F-pt = 75-80% fort-butyl and cumyl hydroperoxides. ButF(pt)is only 18.3% in the polymerization initiated by hydrogen peroxide (H2O2), indicating coupling termination predominately occurred because H(2)O(2)has excellent solubility in water.