G. Bokias et al., MOLAR-MASS CONTROL OF POLY(N-ISOPROPYLACRYLAMIDE) AND POLY(ACRYLIC ACID) IN AQUEOUS POLYMERIZATIONS INITIATED BY REDOX INITIATORS BASED ON PERSULFATES, Macromolecular chemistry and physics, 199(7), 1998, pp. 1387-1392
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 synth
esized by using a similar redox system: ammonium persulfate/2-aminoeth
anethiol 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 conc
entration and ionic strength of the reaction solution, were investigat
ed. It was found that the concentration of ammonium persulfate does no
t practically affect the (M) over bar(w) of the products for both poly
mers investigated and independently of the reducing agent used. On the
contrary, for constant monomer concentration, (M) over bar(w) is inve
rsely proportional to the concentration of the reducing agent, either
sodium metabisulfite or AET. This behaviour, combined with similar obs
ervations concerning the synthesis of polyacrylamide and other redox i
nitiator systems containing metabisulfite as reducing agent, proves th
e special role of this reducing agent in the aqueous radical polymeriz
ations.