J. Phys. Chem. 1984, 88, 3665-3669 x axis of the nitroxide principal axis system of magnetic interaction in the given temperature range. In both cases the plots log R l , , l vs. 1/T are approximately linear. The rotational diffusion may then be described as an activated process with the activation energies Ell,l= 10-16 kJ/mol, which are higher than the activation energy of the viscous flow of ethyl acetate ( E , = 8 kJ/ mol),'9 and with reasonable values of the preexponential factors R m l l , l . =1 X 101*-1 X l O I 3 rad/s. As expected, the rotational diffusion of the free nitroxide is more rapid than the rotational diffusion of the same nitroxide bound to the polymer as the spin-label at all the temperatures where both data are available.
Conclusions Data obtained in the analysis of the slow-motional spectra of polymer IIb in the ethyl acetate solution indicate that the approximately isotropic rotational diffusion of the PMMA chain segment may be described as an activated process with the activation energy Es = 24 !d/mol in this temperature range. The temperature dependence of the more rapid internal rotational diffusion is weak in this temperature range, probably because of the steric hindrance caused by the neighboring PMMA chain segments, which may primarily limit the internal rotation about the C,-NH bond when Rs
