Polyester copolymeric materials include 40 to 51 mol % substituted naphthalate units, such as dimethyl-2,6-naphthalene dicarboxylate units, 10 to 40 mol % ethylene units, and 10 to 40 mol % hexane units. The polyester copolymers can be used to prepare multi-layer optical films by coextrusion and/or co-stretching. The copolyester polymeric materials have desirable optical properties and permit thermal processing at lower temperatures.