  A current focus in studies of the glass transition is the role of dynamic
heterogeneities. Although these efforts may clarify the origin of the
spectacular change in properties of liquids approaching vitrification, we point
out that a seemingly related phenomenon, thermorheological complexity in
polymers, must involve different mechanisms. In particular, as seen from
consideration of various properties involving the chain dynamics, averaging
over different length and time scales cannot offer a resolution to the problem
of thermorheological complexity.
