Segmental diffusion in long-chain polymer melts has been measured by f
ield-gradient NMR. An enlarged set of data and an improved data analys
is leads to a modification of our previously published (Macromolecules
1994, 27, 4274) view. We now come to the following conclusions: In mo
nodisperse polydimethylsiloxane (PDMS) melts, the self part of the int
ermediate scattering function stays almost Gaussian, that is S(Q,t) si
milar to exp[-Q(2)D(app)(t)t] far t > 5 x 10(-3) s with an apparent se
lf-diffusion coefficient whose time dependence is, within experimental
accuracy, in accord with the reptation model. This is also the ease f
or the molecular weight dependence of the long-time self-diffusion coe
fficient D-infinity. However, the observed cross-over fi om anomalous
to long time diffusion cannot be understood quantitatively within the
strict reptation model.