Polymer glasses can be either ductile, undergoing yielding and necking, or brittle, suffering from crazing and brittle fracture. Until recently, there was no satisfactory molecular-level interpretation of these processes. In this work, carefully designed polystyrene glass systems with new deformation protocol have been modeled using all-atom molecular dynamics simulation. The effect of chain networking in terms of yielding taking place in the presence of long chains embedded in short chains is investigated. The results are compared with a recently proposed molecular model, which suggests that the structural integrity, necessary to prevent brittle fracture, is mainly due to chain networking.