Molar mass control of poly(N-isopropylacrylamide) and poly(acrylic acid) in aqueous polymerizations initiated by redox initiators based on persulfates

Poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAA) were synthesized by aqueous radical polymerization initiated by the redox couple ammonium persulfate/sodium metabisulfite. PNIPAM was also synthesized by using a similar redox system: ammonium persulfate/2-aminoethanethiol hydrochloride (AET), the reducing agent being in this case a well known chain transfer agent. The final products were characterized by size exclusion chromatography. The influence on the weight-average molar mass ((M) over bar(w)) of several factors, such as monomer concentration and ionic strength of the reaction solution, were investigated. It was found that the concentration of ammonium persulfate does not practically affect the (M) over bar(w) of the products for both polymers investigated and independently of the reducing agent used. On the contrary, for constant monomer concentration, (M) over bar(w) is inversely proportional to the concentration of the reducing agent, either sodium metabisulfite or AET. This behaviour, combined with similar observations concerning the synthesis of polyacrylamide and other redox initiator systems containing metabisulfite as reducing agent, proves the special role of this reducing agent in the aqueous radical polymerizations.