  We investigate the molecular origin of shear-thinning in melts of flexible,
semi flexible and rigid polymers with coarse-grained simulations of a sheared
polymer melt. Polymer alignment, stretching and tumbling modes or suppression
of the latter all contribute to understanding how macroscopic flow properties
emerge from the molecular level. By performing simulations of single chains in
a shear flow, we identify which of these phenomena are of collective nature and
arise through inter chain interactions and which are already present in dilute
systems. Building upon these microscopic simulations we identify by means of
the Irving-Kirkwood formula the corresponding macroscopic stress tensor for a
non-Newtonian polymer fluid. Shear-thinning effects in polymer melts are also
demonstrated by macroscopic simulations of a channel flow. The latter have been
obtained by the discontinuous Galerkin method approximating macroscopic polymer
flows. Our study confirms the influence of microscopic details in the molecular
structure of polymers such as chain flexibility on macroscopic polymer flows.
