We generalize the force-level Elastically Collective Nonlinear Langevin Equation theory of supercooled molecular liquid dynamics to polymer melts based on mapping chains to disconnected and noninterpenetrating Kuhn-sized spheres. This allows first-principles, no adjustable parameter calculations to be performed for the temperature-dependent mean segmental relaxation time of chemically diverse van der Waals polymers over a wide range of molecular weights. Despite the simplicity of the mapping, the theory does a good job of a priori predicting the glass transition temperature (Tg), the dynamic fragility, and full temperature dependence of the α-relaxation time for some high molecular weight polymers and the chain length dependence of Tg as the consequence of the molecular weight dependence of backbone stiffness. The minimalist model does not capture the unusually low and high fragilities of certain long chain polymers which are not typical of van der Waals molecular liquids. This seems likely due to the sim...