Patent ID: 8629226

Claim:
A retardation film, formed by stretching a film comprising a norbornene-based ring-opening copolymer containing a structural unit (A) represented by the following general formula (1): wherein in the formula (1), n represents an integer of any one of 0 and 1, R 1 represents an atom or a group selected from the group consisting of a hydrogen atom; halogen atoms; substituted or unsubstituted hydrocarbon groups, each of which may have at least one linking group selected from the group consisting of an oxygen atom, a nitrogen atom, a sulfur atom, and a silicon atom, and which has 1 to 30 carbon atoms; and polar groups, X 1 represents a group selected from the group consisting of a group represented by a formula: —CH═CH— and a group represented by a formula: —CH 2 CH 2 —, a and b each independently represent an integer of 0 to 6, each Y, which may be the same or different when provided in plurality in a single structural unit, represents a group selected from the group consisting of a methylene group, a carbonyl group, an oxy group, and alkylimino groups having 1 to 5 carbon atoms, and each Z, which may be the same or different when provided in plurality in a single structural unit, represents a group selected from the group consisting of a methylene group, a carbonyl group, an oxy group, and alkylimino groups having 1 to 5 carbon atoms, and a structural unit (B) represented by the following general formula (2): wherein in the formula (2), m represents an integer of any one of 0 and 1, X 2 represents a group selected from the group consisting of a group represented by a formula: —CH═CH— and a group represented by a formula: —CH 2 CH 2 —, R 2 , R 3 , R 4 , and R 5 , which may be the same or different, each represent an atom or a group selected from the group consisting of a hydrogen atom, halogen atoms, a cyano group, alkyl groups having 1 to 20 carbon atoms, alkenyl groups having 1 to 20 carbon atoms, alkylcarbonyl groups having 1 to 20 carbon atoms, and ester groups having a hydrocarbon group having 1 to 20 carbon atoms, two or more of R 2 to R 5 may be bonded to each other to form a monocyclic hydrocarbon which has 3 to 20 carbon atoms, which is not a spiro ring, and which may have an unsaturated bond, or a polycyclic hydrocarbon which has 4 to 20 carbon atoms, which is not a spiro ring, and which may have an unsaturated bond, and R 2 and R 3 , or R 4 and R 5 may together form an alkylidene group having 1 to 20 carbon atoms, wherein a content of the structural unit (A) is not less than 5 mol % but not more than 95 mol % relative to a total amount of the structural units (A) and (B), wherein the content of the structural unit (A) is more than 50 mol % but not more than 95 mol % relative to the total amount of the structural units (A) and (B), and the retardation film is a negative A retardation film.