Cloud points in aqueous solutions of poly(N-isopropylacrylamide) synthesized by aqueous redox polymerization

The mean-square radius of gyration, second virial coefficient and intrinsic viscosity were determined in methanol at 25.0 degrees C for poly(N-isopropylacrylamide) (PNIPA) samples in the range of weight-average molecular weight M-w from 1.04 x 10(5) to 2.65 x 10(6), which were synthesized by aqueous redox polymerization using a redox catalyst consisting of ammonium persulfate and sodium metabisulfite. Analyses of the quantities showed that the samples have almost the same degree of branching as that of PNIPA samples previously synthesized by radical polymerization using azobis(isobutyronitrile) as an initiator in tert-butanol. The cloud point was also determined in aqueous solutions of two PNIPA samples with M-w 1.04 x 10(5) and 1.86 x 10(5) and compared with the results of the previous samples. It was found that the cloud point is definitely higher for the present samples than for the previous ones, and, moreover, the M-w dependences of the cloud point for the two kinds of samples are opposite to each other. As the two kinds of samples have almost the same stereochemical composition and degree of branching, such difference in the behavior of the cloud point is considered to arise from the difference in their chain-end groups. Polymer Journal (2011) 43, 164-170; doi: 10.1038/pj.2010.121; published online 15 December 2010