  The reptation concept in polymer dynamics is studied for model chains with
added stiffness. The main idea of a chain diffusing inside a tube can be
transferred from fully flexible chains. However, the picture, which
renormalizes the chain of locally stiff Kuhn segments onto a flexible chain of
fewer segments fails. The entanglement length shrinks with increasing
persistence length. If entanglement length and persistence length come to the
same order of magnitude the picture of a tight tube is better suited, in which
chain segments can move only along the contour, any transverse motion being
much reduced. Thus, as stiffness increases, the monomers loose their freedom to
perform random walks inside the tube, the ``Rouse-like'' part of their
dynamics. As a result, for persistence lengths of more than five monomers no
length scale can be described by the Rouse model.
