13 июля, 2015 
Malyar The initiators used in emulsion polymerisation are exclusively of the free radical type and, of necessity, they must be water soluble. The free radicals may be generated thermally or by use of an oxidation-reduction (or redox) couple.
Hydroperoxides are often used (e. g. t-butyl hydroperoxide) particularly as a post reaction initiator to scavenge the last vestiges of unreacted monomer. This part of the process can be carried out in a maturing tank or blender once the emulsion polymer has been
discharged from the reactor. The major initiators used in commercial emulsion polymerisation are persulphates, with sodium, ammonium and potassium persulphate all being used.
Whilst initiating efficiency and half life of the persulphates vary, ammonium persulphate is preferred in practice because of its better solubility. The rate of free radical generation increases with temperature, and it is normal to employ reaction temperatures of 70-90°C when using thermal generation techniques.
When redox couples are employed the rate of free radical generation is increased compared to that provided by thermal generation at the same temperature. Therefore, when using Redox couples reaction temperatures as low as 30°C are not uncommon.
When persulphates are involved the major initiating species is the sulphate ion radical.
Figure 2-4
Persulphate Initiating Species
Typical reducing agents used in a redox couple are thiosulphates, metabisulphites and hydrosulphides. Frequently the redox couple is combined with trace quantities of a ferrous salt providing an even more rapid generation of free radicals.
S2082′ + Fe2+ ———— ► SO/+ SO/ + Fe3+
S2Oe2 + 2S203z——— ► s<o/ + 2so;
Fe3+ + 2S2032"———- ► Fe2+ + S4062"
Figure 2-5
REDOX Couple
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