Birefringent film and polarizer

A birefringent film comprising an ester-based polymer having at least a repeating unit represented by the following formula (I):

• wherein A and B each independently represents a hydrogen atom, a halogen atom, an alkyl group having 1 to 6 carbon atoms, or a substituted or unsubstituted aryl group;
• a and b each independently represents any one of integers of 0 to 4;
• R1 represents a hydrogen atom, a linear or branched alkyl group having 1 to 10 carbon atoms, or a substituted or unsubstituted aryl group;
• R2 represents a linear or branched alkyl group having 2 to 10 carbon atoms;
• R3 to R6 each independently represents a hydrogen atom, a halogen atom, a linear or branched alkyl group having 1 to 6 carbon atoms, or a substituted or unsubstituted aryl group;
• and n represents an integer of 2 or more.