  Molecular dynamics simulations are performed on a system of model linear
polymers to look at the violations of Stokes-Einstein (SE) and
Stokes-Einstein-Debye (SED) relations near the mode coupling theory transition
temperature $T_c$ at three (one higher and two lower) densities. At low
temperatures, both lower density systems show stable gas-supercooled-liquid
coexistence whereas the higher density system is homogeneous. We show that
monomer density relaxation exhibits SE violation for all three densities,
whereas molecular density relaxation shows a weak violation of the SE relation
near $T_c$ in both lower density systems. This study identifies disparity in
monomer mobility and observation of jumplike motion in the typical monomer
trajectories resulting in the SE violations. In addition to the SE violation, a
weak SED violation is observed in the gas-supercooled-liquid coexisting domains
of the lower densities. Both lower density systems also show a decoupling of
translational and rotational dynamics in this polymer system.
