To explore the relationship between microscopic structure and viscoelastic properties of polyurea, a coarsegrained (CG) model is developed by a structure matching method and validated against experiments conducted on a controlled, benchmark material. Using the Green-Kubo method, the relaxation function is computed from the autocorrelation of the stress tensor, sampled over equilibrium MD simulations, and mapped to a real time scale established by matching self-diffusion rates of atomistic and CG models. Master curves computed from the predicted stress relaxation function are then compared with dynamic mechanical analysis experiments mapped to a wide frequency range by time-temperature superposition, as well as measurements of ultrasonic shear wave propagation. Computational simulations from monodisperse and polydisperse configurations, representative of the benchmark polyurea, show excellent agreement with the experimental measurements over a multidecade range of loading frequency. V C 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 797-810 KEYWORDS: coarse-grained molecular dynamics; mechanical properties; polyurea INTRODUCTION Knowledge of the connections between chemistry, structure, and properties is needed to develop improved polymers with a materials-by-design approach. Computational models offer promise in identifying these relationships, but unfortunately they typically lack predictive capability beyond a small range of properties. Molecular dynamics (MD) simulations can provide tremendous insight into how the fine details of chemistry, chain architecture, and microstructure affect many physical properties; however, they face well-known limitations in both time and length scales. The goal of this work is to develop coarse-grained (CG) models that enable molecular simulations to reach more representative time and length scales to investigate the viscoelastic properties of polyurea, a thermorheologically complex block copolymer, for which theoretical rheological models are difficult to apply.