Modified copolyesters and improved multilayer reflective films

A multilayered polymer film, comprising:

• a plurality of first layers comprising a first copolyester, the first copolyester being semicrystalline and birefringent, the first copolyester comprising carboxylate subunits and glycol subunits, wherein 70 to 100 mol % of the carboxylate subunits are first carboxylate subunits, the first carboxylate subunits being naphthalate or terephthalate subunits, 0 to 30 mol % of the carboxylate subunits are first comonomer carboxylate subunits, 70 to 100 mol % of the glycol subunits are first glycol subunits, 0 to 30 mol % of the glycol subunits are first comonomer glycol subunits, and at least 0.5 mol % of the combined carboxylate and glycol subunits of the first copolyester are first comonomer carboxylate subunits, first comonomer glycol subunits, or a combination thereof;
• and
• a plurality of second optical layers comprising a second polymer having an in-plane birefringence of about 0.04 or less, as measured with 632.8 nm light, after the multilayered polymer film has been formed;
• the multilayered polymer film being formed by drawing the first and second layers in at least one draw direction to a particular draw ratio,
• wherein, after drawing, the first layers of the multilayered polymer film have an index of refraction in the draw direction which, when measured with 632.8 nm light, is at least 0.02 units less than an index of refraction in the draw direction of a polyethylene naphthalate layer with the same in-plane birefringence for 632.8 nm light,
• wherein one in-plane index of refraction of the first copolyester is substantially equal to one in-plane index of refraction of the second polymer after the multilayered polymer film has been formed.