We develop a mesoscale polystyrene model based on atomistic simulations of oligomers using the iterative Boltzmann inversion method. The potential is optimized against the atomistic simulation until the radial distribution function generated from the mesoscale model is consistent with the atomistic simulation. The mesoscale model allows to elucidate the polymer dynamics of long chains. The dynamics of polystyrene melts are investigated at various chain lengths between 15 and 240 monomers. Mean-squared displacements are analyzed to capture the dynamics changing from the Rouse to the reptation behavior. The reorientation behavior of segmental vectors is investigated to identify the heterogeneity along the chain. Diffusion constants of various systems are calculated to verify the crossover from the unentangled to entangled dynamics. Rouse mode analysis shows that the Rouse model is applicable from chain length between 50 and 100 monomers. The entanglement length of this polystyrene model is around 85 monomers at 450 K, in fair agreement with experiments.