Primary structure of poly(N-isopropylacrylamide) synthesized by radical polymerization. Effects of polymerization solvents

The mean-square radius of gyration < S-2 >, second virial coefficient A(2), and intrinsic viscosity [eta] were determined in methanol at 25.0 degrees C for two kinds of poly(N-isopropylacrylamide) (PNIPA) synthesized by radical polymerization in tert-butanol and benzene by the use of azobis(isobutyronitrile) as an initiator. In all cases of the three quantities, it is found that the value is smaller for the PNIPA synthesized in benzene than for that synthesized in tert-butanol and the difference between the two kinds of PNIPA increases with increasing the weight-average molecular weight M-w The average chain dimension of the former PNIPA should then be smaller than that of the latter. Since the two kinds of PNIPA were shown to have the same stereochemical composition (and also the same chain end group), the difference in the average chain dimension may be regarded as arising from the difference in the primary structure, i.e., the number of branch points. Considering the fact that the average dimension of a given polymer chain in general decreases with increasing the number of branch points, it may be concluded that the number is larger in the former PNIPA than in the latter. The behavior of the interpenetration function psi and the Flory-Fox factor Phi as functions of M-w is also examined, confirming the conclusion.