Plane stress Fracture toughness of amorphous films of poly(ethylene-2,6-naphthalate) (PEN) with various molecular weights (MW characterized by the intrinsic viscosity, IV) was determined by the essential work of fracture (EWF) concept using tensile-loaded deeply double-edge notched (DDEN-T) specimens. These PENs met the basic requirement of the EWF concept: full ligament yielding, which was marked by a load drop in the force-displacement curves of the DDEN-T specimens, preceded the crack growth. This "load mark" allowed us to partition between yielding and necking. The yielding-related EWF terms were not affected by the MW of the resins and thus served for the comparison of the results. It is argued that the EWF response is governed by the entanglement network density. The role of entanglements was substantiated by showing that the "plastic" zone developed via cold drawing and not by true plastic deformation. On the other hand, MW influenced the necking related EWF terms, High MW PENs failed by stable crack growth, whereas low MW resins experienced unstable crack growth (more exactly a transition From stable to unstable crack growth) in the necking phase. This was traced to the load distribution capacity of the long entangled chains. Attempts were also made to estimate the essential and non-essential work of fracture parameters and their constituents From uniaxial tensile rests performed on dumbbells.