  We compare the Rubinstein-Duke model for reptation to a model where the
boundary dynamics is modified by calculating the viscosity of polymer melts.
The question is investigated whether the viscosity is determined by details of
the dynamics of the polymer ends or by the stretching of the polymer. To this
end the dependence of the viscosity on the particle density of the lattice gas
models which can be identified by the stretching is determined. We show that
the influence of the stretching of the polymer on the absolute value of the
viscosity in the scaling limit of of very long chains is much bigger than the
influence of the boundary dynamics, whereas the corrections of the scaling of
the viscosity depends significantly on the details of the boundary dynamics.
