  Based on non-equilibrium molecular dynamics simulations of entangled polymer
melts, a recent Letter [Phys. Rev. Lett. $\textbf{121}$, 047801 (2018),
arXiv:1806.09509] claims that the rising extensional stress is quantitatively
consistent with the decreasing entropy of chains at the equilibrium
entanglement length. We point out that exactly the opposite is true: the
intrachain entropic stress arising from individual entanglement strands
generally does not agree with the total "macroscopic" stress. The conclusion of
the Letter is based on an incomplete and questionable analysis of a limited
range of the simulation trajectory. The opposite conclusion should have been
drawn from their data, had they examined the full simulation trajectory in a
proper way.
