Molecular dynamics simulations are performed for poly(propylene oxide) with molecular masses between M = 104 and 5795 g/mol using an atomistic force field. From atomic trajectories extending well into the nanoseconds regime, we calculate rank-two orientational correlation functions, providing access to segmental motion, to free Rouse dynamics, and even to the onset of entanglement dynamics, depending on the molecular mass. The simulation results are directly compared with experimental data for poly(propylene glycol) from field-cycling NMR relaxometry. We find that simulation and experiment are in very good agreement for high values of M. For low values of M, some deviations result from the fact that the present analysis of the simulation results focuses on intramolecular behavior while the experimental data are influenced by both intramolecular and intermolecular relaxation contributions, particularly at longer time scales. Exploiting that the computational data allow us to separately study polymer motions at different positions along the polymer backbone, it is shown that free Rouse dynamics and constrained Rouse dynamics are modified for a few and a few dozen monomers at the chain ends, respectively. We discuss implications of such chain-end effects for the interpretation of experimental results, which are obtained from an ensemble average over all monomers along the backbone.