JP7633822B2 - Copolymer, its manufacturing method, and film - Google Patents

Description

The present invention relates to a copolymer, a method for producing the copolymer, and a film.

Copolymers containing structural units derived from α-methylene lactone have excellent transparency, heat resistance, and optical isotropy, and are expected to be used in optical applications. For example, Patent Document 1 describes that films and the like that are molded bodies of copolymers (resins) containing structural units derived from a specific α-methylene lactone are suitable for use as optical components.

Patent Document 2 also describes a method for producing a copolymer containing structural units derived from α-methylene lactone, which includes a step of continuously supplying each component, including α-methylene lactone, to a reaction tank and polymerizing the components by a bulk polymerization method, and a step of continuously separating and removing volatiles from the reaction mixture.

JP 2008-179813 A JP 2008-291138 A

In copolymers containing structural units derived from α-methylene lactone, the elastic modulus of the copolymer tends to improve by increasing the proportion of structural units derived from α-methylene lactone in the copolymer. However, according to the studies of the present inventors, in conventional manufacturing methods for copolymers containing structural units derived from α-methylene lactone, increasing the proportion of α-methylene lactone as a monomer produces a homopolymer of α-methylene lactone, and the transparency may decrease due to the difference in refractive index between the homopolymer and the resulting copolymer containing structural units derived from α-methylene lactone.

The main objective of the present invention is to provide a method for producing a copolymer containing structural units derived from α-methylene lactone, which can improve the transparency of the resulting copolymer.

The present invention provides a method for producing a copolymer according to the following items [1] to [6], a copolymer according to the following item [7], and a film according to the following items [8] and [9].
[1] A method for producing a copolymer containing a structural unit derived from α-methylene lactone and a structural unit derived from an alkyl (meth)acrylate having an alkyl group having 1 to 6 carbon atoms, the method comprising: a first step of preparing a mixed liquid containing α-methylene lactone, an alkyl (meth)acrylate, and a solvent; and a second step of heating the mixed liquid and further adding α-methylene lactone dropwise to the heated mixed liquid, wherein the mass ratio of the α-methylene lactone added dropwise to the mixed liquid in the second step is 10 to 80 mass% relative to the total mass of the α-methylene lactone contained in the mixed liquid in the first step and the α-methylene lactone added dropwise to the mixed liquid in the second step.
[2] The method for producing a copolymer according to [1], wherein the conversion rate of α-methylene lactone at the end of the dropwise addition in the second step is greater than the conversion rate of alkyl (meth)acrylate.
[3] The method for producing a copolymer according to [1] or [2], wherein the conversion rate of α-methylene lactone is 70% or more upon completion of the dropwise addition in the second step.
[4] The method for producing a copolymer according to any one of [1] to [3], wherein the difference between the conversion rate of α-methylene lactone and the conversion rate of alkyl (meth)acrylate at the end of the dropwise addition in the second step is more than 0% and not more than 15%.
[5] The method for producing a copolymer according to any one of [1] to [4], wherein the content of structural units derived from α-methylene lactone in the copolymer is 10 mass% or more.
[6] The method for producing a copolymer according to any one of [1] to [5], wherein the α-methylene lactone is α-methylene-γ-butyrolactone or α-methylene-δ-valerolactone.
[7] A copolymer comprising a structural unit derived from α-methylene lactone and a structural unit derived from an alkyl (meth)acrylate having an alkyl group having 1 to 6 carbon atoms, the copolymer having an internal haze of 1.0% or less per 100 μm thickness when made into a film.
[8] A film comprising the copolymer according to [7].
[9] The film according to [8], having an elastic modulus of 4.5 GPa or more.

According to the present invention, there is provided a method for producing a copolymer containing structural units derived from α-methylene lactone, which can improve the transparency of the resulting copolymer. The present invention also provides a copolymer obtained by such a production method. Furthermore, the present invention also provides a film using the copolymer.

The following describes in detail the embodiments of the present invention. However, the present invention is not limited to the following embodiments.

In this specification, the term "resin (composition)" is a broader concept than "(co)polymer." A resin may be composed of, for example, one or more (co)polymers, and may contain additives other than (co)polymers, such as antioxidants, as necessary.

[Method of producing copolymer]
A method for producing a copolymer according to one embodiment is a method for producing a copolymer that contains a structural unit derived from α-methylene lactone and a structural unit derived from an alkyl (meth)acrylate having an alkyl group having 1 to 6 carbon atoms.

The structural unit derived from α-methylene lactone is formed by polymerization of α-methylene lactone in which a methylene group is bonded to the carbon at the α position. The specific structure of the structural unit derived from α-methylene lactone is not particularly limited. The number of ring members of the lactone is not particularly limited, but is preferably a 5-membered ring (γ-lactone) or 6-membered ring (δ-lactone) because the ring structure is highly stable and higher surface strength can be obtained based on this high stability.

Specific examples of α-methylene lactones that are 5- or 6-membered rings are α-methylene-γ-butyrolactone and α-methylene-δ-valerolactone. These may have a substituent.

The structural unit derived from α-methylene lactone is preferably a structural unit having the structure shown in the following formula (1).

In formula (1), R ¹ to R ⁴ are each independently a hydrogen atom or a hydrocarbon group having 1 to 18 carbon atoms.

The structural unit having the structure shown in formula (1) can be formed by polymerization of a monomer containing α-methylene-γ-butyrolactone shown in the following formula (2).

In formula (2), R ¹ to R ⁴ are each independently a hydrogen atom or a hydrocarbon group having 1 to 18 carbon atoms.

The hydrocarbon group is an aliphatic hydrocarbon group or an aromatic hydrocarbon group. The aliphatic hydrocarbon group is, for example, an alkyl group. The number of carbon atoms in the alkyl group is preferably 1 to 10, more preferably 1 to 8. The alkyl group may be linear or branched, or may be cyclic. Examples of the alkyl group include a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, a t-butyl group, an n-pentyl group, an n-hexyl group, a cyclopentyl group, and a cyclohexyl group.

The aromatic hydrocarbon group is not particularly limited, and may include, for example, a heterocyclic structure. Examples of the aromatic hydrocarbon group include a phenyl group, a tolyl group, and a benzyl group.

R ¹ to R ⁴ are preferably each independently a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and more preferably all are a hydrogen atom.

The content of the structural unit derived from α-methylene lactone in the copolymer is preferably 10% by mass or more, more preferably 15% by mass or more, even more preferably 20% by mass or more, and particularly preferably 25% by mass or more from the viewpoint of further improving the heat resistance, and is preferably 60% by mass or less, more preferably 55% by mass or less, even more preferably 50% by mass or less, and particularly preferably 45% by mass or less from the viewpoint of further improving the transparency of the film. The content of each structural unit in the copolymer can be determined by dissolving the copolymer in a heavy solvent, measuring ¹ H-NMR, and calculating the area ratio of the peak corresponding to each structural unit. In addition, in the production method of the copolymer described later, the mass ratio of the amount of α-methylene lactone used (total amount used) to the total amount of all monomers used may be the same as the range of the content of the structural unit derived from α-methylene lactone in the copolymer described above.

The structural units derived from alkyl (meth)acrylates having an alkyl group with 1 to 6 carbon atoms are formed by polymerization of alkyl (meth)acrylates. Examples of the alkyl group with 1 to 6 carbon atoms in the alkyl (meth)acrylates include methyl, ethyl, n-propyl, i-propyl, n-butyl, t-butyl, n-pentyl, n-hexyl, cyclopentyl, and cyclohexyl groups. These may be used alone or in combination of two or more.

The number of carbon atoms in the alkyl group in the alkyl (meth)acrylate is preferably 1 to 3, more preferably 1 or 2, and even more preferably 1.

The number of carbon atoms in the alkyl group in the alkyl methacrylate is preferably 1 to 3, more preferably 1 or 2, and even more preferably 1.

The content of the alkyl (meth)acrylate-derived structural units in the copolymer is preferably 40% by mass or more, more preferably 45% by mass or more, even more preferably 50% by mass or more, and particularly preferably 55% by mass or more, from the viewpoint of further improving transparency, and is preferably 90% by mass or less, more preferably 85% by mass or less, even more preferably 80% by mass or less, and particularly preferably 75% by mass or less, from the viewpoint of further improving heat resistance. In the production method of the copolymer described below, the mass ratio of the amount of alkyl (meth)acrylate used to the total amount of all monomers used may be the same as the range of the content of the alkyl (meth)acrylate-derived structural units in the copolymer described above.

The copolymer may contain structural units of other monomers other than the structural units derived from α-methylene lactone and the structural units derived from alkyl (meth)acrylate having an alkyl group with 1 to 6 carbon atoms. Specific examples include structural units derived from monomers such as benzyl (meth)acrylate, chloromethyl (meth)acrylate, 2-chloroethyl (meth)acrylate, styrene, vinyl toluene, α-methylstyrene, acrylonitrile, methyl vinyl ketone, ethylene, propylene, and vinyl acetate. These may be used alone or in combination of two or more.

The content of the other monomer units in the copolymer is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less. In the copolymer production method described below, the mass ratio of the amount of the other monomers used to the total amount of all monomers used may be the same as the range of the content of the other monomer units in the copolymer described above.

The method for producing the copolymer of this embodiment includes a first step of preparing a mixed liquid containing α-methylene lactone, an alkyl (meth)acrylate, and a solvent, and a second step of heating the mixed liquid and further dropping α-methylene lactone into the heated mixed liquid.

The mass ratio of the α-methylene lactone dripped into the mixture in the first step (hereinafter sometimes referred to as "initial α-methylene lactone") is 10 to 80 mass% relative to the total mass of the α-methylene lactone contained in the mixture in the first step (hereinafter sometimes referred to as "dropped α-methylene lactone").

By adjusting the mass ratio of the dripped α-methylene lactone to a specified range, it is possible to improve the transparency of the resulting copolymer. The inventors believe that the cause of coloration (reduction in transparency) is the formation of a homopolymer of α-methylene lactone. The inventors believe that by adjusting the mass ratio of the dripped α-methylene lactone to a specified range, it is possible to promote the formation of the copolymer while suppressing the formation of homopolymer of α-methylene lactone, thereby reducing the coloration of the copolymer and improving its transparency.

(First step)
In this step, a mixed liquid containing α-methylene lactone (initial α-methylene lactone), alkyl (meth)acrylate, and a solvent is prepared.

The mass ratio of the initial α-methylene lactone to the total mass of the initial α-methylene lactone and the dropped α-methylene lactone is 20 to 90 mass%, preferably 30 to 85 mass%, and more preferably 35 to 80 mass%, from the viewpoint of improving the transparency of the copolymer.

The solvent may be any solvent that is generally used in the field of copolymer production. The solvent is not particularly limited, but is preferably a solvent that satisfies either the following condition (A) or the following condition (B) in terms of further improving the transparency of the copolymer.
Condition (A): At least one solvent selected from the group consisting of amides and cyclic esters.
Condition (B): A mixed solvent consisting of a first solvent having a boiling point of less than 100°C and a second solvent having a boiling point of 100°C or higher, wherein the first solvent is at least one selected from the group consisting of ketones and alkyl chlorides, and the second solvent is at least one selected from the group consisting of cyclic ketones, cyclic esters, amides, and sulfoxides, and has a boiling point of 70 to 120°C.

A homopolymer of α-methylene lactone tends not to dissolve or is difficult to dissolve in a solvent that satisfies condition (A) or condition (B). Therefore, by polymerizing a monomer containing α-methylene lactone using a solvent that satisfies condition (A) or condition (B), it is possible to further suppress the production of homopolymer of α-methylene lactone, further reduce coloration of the copolymer, and further improve transparency.

In addition, by using a solvent that satisfies condition (B), it becomes easy to carry out the polymerization reaction under reflux in the second step, and it becomes possible to carry out the polymerization under reflux conditions at a general polymerization temperature (e.g., 70 to 120°C). When the polymerization is carried out under reflux conditions, the heat of polymerization during the polymerization can be removed and the polymerization temperature can be controlled near the boiling point, so that the polymerization can proceed safely and stably. If the boiling point of the mixed solvent is 120°C or less, it is advantageous in that the polymerization rate is easily controlled, by-products are suppressed, and the polymerization temperature does not become too high above the boiling point of the (meth)acrylic acid alkyl monomer. In addition, if the boiling point of the mixed solvent is 70°C or more, it is advantageous in terms of productivity such as the viscosity of the polymerization liquid and the polymerization rate.

The solvent represented by condition (A) is at least one solvent selected from the group consisting of amides and cyclic esters, and may be a single solvent or a mixed solvent of two or more solvents, but is preferably a single solvent. The boiling point of the solvent (the boiling point of the single solvent or the boiling point of the mixed solvent) is preferably greater than 100°C and equal to or less than 300°C.

Examples of amides include N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone (NMP), and N,N'-dimethylimidazolidinone (DMI). Among these, amides are preferably cyclic amides, more preferably NMP, because of their high versatility.

Examples of cyclic esters include γ-butyrolactone (GBL), γ-valerolactone, and δ-valerolactone. Among these, the cyclic ester is preferably GBL because of its high versatility.

The solvent represented by condition (B) is a mixed solvent consisting of a first solvent having a boiling point of less than 100°C and a second solvent having a boiling point of 100°C or higher, in which the first solvent is at least one selected from the group consisting of ketones and alkyl chlorides, the second solvent is at least one selected from the group consisting of cyclic ketones, cyclic esters, amides, and sulfoxides, and the boiling point (of the mixed solvent) is 70 to 120°C.

The first solvent is a solvent having a boiling point of less than 100°C, and is at least one solvent selected from the group consisting of ketones and alkyl chlorides. Examples of such a first solvent include ketones such as acetone (ACE) and methyl ethyl ketone (MEK), and alkyl chlorides such as dichloromethane, chloroform, 1,2-dichloroethane, and 1,1-dichloroethane. The first solvent may be used alone or in combination of two or more. Of these, the first solvent is preferably acetone.

The second solvent is at least one solvent selected from the group consisting of cyclic ketones, cyclic esters, amides, and sulfoxides, which have a boiling point of 100°C or higher. Examples of such second solvents include cyclic ketones such as cyclohexanone (anone) and cyclopentanone, cyclic esters such as γ-butyrolactone (GBL), γ-valerolactone, and δ-valerolactone, amides such as N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone (NMP), and N,N'-dimethylimidazolidinone (DMI), and sulfoxides such as dimethylsulfoxide. The second solvent may be used alone or in combination of two or more. One embodiment of the second solvent is preferably a cyclic ketone, more preferably cyclohexanone. Another embodiment of the second solvent is preferably at least one selected from the group consisting of cyclic esters, amides, and sulfoxides, and more preferably at least one selected from the group consisting of γ-butyrolactone, γ-valerolactone, δ-valerolactone, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, N,N'-dimethylimidazolidinone, and dimethylsulfoxide.

The combination of the first solvent and the second solvent may be a combination of acetone and cyclohexanone. According to the study by the present inventors, acetone and cyclohexanone each have a tendency to be difficult to dissolve the copolymer alone, but it has been found that they are very specific and easy to dissolve the above-mentioned copolymer. Therefore, by using such a mixed solvent, it becomes easy to carry out the polymerization reaction under reflux, and it becomes possible to improve the transparency of the obtained copolymer.

The above-mentioned copolymer tends to be easily dissolved in an alkyl chloride having a boiling point of less than 100°C as a first solvent, and at least one solvent selected from the group consisting of cyclic esters, amides, and sulfoxides having a boiling point of 100°C or more as a second solvent. Therefore, the combination of the first solvent and the second solvent may be a combination of an alkyl chloride having a boiling point of less than 100°C and at least one solvent selected from the group consisting of cyclic ketones, cyclic esters, amides, and sulfoxides, which is a solvent having a boiling point of 100°C or more, or a combination of at least one solvent selected from the group consisting of ketones and alkyl chlorides and at least one solvent selected from the group consisting of cyclic esters, amides, and sulfoxides, which is a solvent having a boiling point of less than 100°C.

The boiling point of the mixed solvent is 70 to 120°C, preferably 75 to 115°C, and more preferably 80 to 110°C. When the boiling point of the mixed solvent is in this range, it becomes easy to carry out the polymerization reaction under reflux. In this specification, the boiling point of the mixed solvent means the value measured by the method described in the examples.

The mixing ratio of the first solvent and the second solvent is not particularly limited as long as the boiling point of the mixed solvent is 70 to 120°C, and can be adjusted to any ratio. By adjusting the first solvent and the second solvent to any ratio and adjusting the boiling point of the mixed solvent to the range of 70 to 120°C, it becomes easy to carry out the polymerization reaction under reflux, and it becomes possible to improve the transparency of the obtained copolymer. For example, the mass ratio of the first solvent to the second solvent (mass of the first solvent/mass of the second solvent) is preferably 1/9 or more, more preferably 2/8 or more, and preferably 9/1 or less, more preferably 8/2 or less, even more preferably 7/3 or less, particularly preferably 6/4 or less, and most preferably 5/5 or less.

The concentration of all monomers in the mixed solution is preferably 90% by mass or less, more preferably 70% by mass or less, and even more preferably 50% by mass or less, from the viewpoint of further suppressing the increase in viscosity of the mixed solution. The concentration of all monomers in the mixed solution is preferably 10% by mass or more, and more preferably 20% by mass or more, from the viewpoint of further improving productivity.

When polymerizing the monomers, a polymerization initiator may be added as necessary. Examples of the polymerization initiator include organic peroxides such as cumene hydroperoxide, diisopropylbenzene hydroperoxide, di-t-butyl peroxide, lauroyl peroxide, benzoyl peroxide, t-butylperoxyisopropyl carbonate, t-amylperoxy-2-ethylhexanoate, and t-butylperoxy-2-ethylhexanoate; and azo compounds such as 2,2'-azobis(isobutyronitrile), 1,1'-azobis(cyclohexanecarbonitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), and dimethyl-2,2'-azobis(2-methylpropionate).

The polymerization initiator can be added to the monomer at any stage. For example, in the first step, the polymerization initiator may be added after each monomer component (α-methylene lactone, alkyl (meth)acrylate, other monomers, etc.) is charged into the reactor. The polymerization initiator may also be added, for example, in the second step described below, after the mixed liquid is heated, or may be added together with the further dropwise addition of α-methylene lactone. The polymerization initiator can also be added in multiple batches.

The total amount of polymerization initiator used may be appropriately set depending on the combination of monomers, reaction conditions, etc., and is not particularly limited, but is preferably 10 to 10,000 ppm by mass, more preferably 100 to 3,000 ppm by mass, and even more preferably 300 to 2,000 ppm by mass, based on the total monomers.

When polymerizing the monomers, a chain transfer agent may be added as necessary. Examples of the chain transfer agent include monofunctional thiol compounds such as n-dodecyl mercaptan and β-mercaptopropionic acid; bifunctional thiol compounds such as polysiloxanes modified at both ends; and polyfunctional mercapto-modified polysiloxanes whose side chains are modified with mercapto groups. The chain transfer agent can be added to the monomers at any stage. The chain transfer agent may be added to the mixture in the first step, for example. The total amount of the chain transfer agent used may be appropriately set depending on the combination of monomers, reaction conditions, etc., and is not particularly limited, but is preferably 10 to 10,000 ppm by mass, more preferably 100 to 3,000 ppm by mass, based on the total monomers.

(Second step)
In this step, the mixture is heated, and α-methylene lactone (dropped α-methylene lactone) is further added dropwise to the heated mixture.

In this process, the mixed liquid is first heated. The heating temperature of the mixed liquid can be set appropriately depending on the combination of monomers, etc. The heating temperature of the mixed liquid is preferably 150°C or less from the viewpoint of controlling the polymerization rate and improving transparency, and is preferably 60°C or more from the viewpoint of improving the monomer conversion rate. The heating temperature of the mixed liquid is more preferably 60 to 130°C, and even more preferably 70 to 120°C.

Next, α-methylene lactone (dropped α-methylene lactone) different from the initial α-methylene lactone is added dropwise to the heated mixture under heating temperature conditions.

The mass ratio of the dropped α-methylene lactone to the total mass of the initial α-methylene lactone and the dropped α-methylene lactone is 10 to 80 mass%, preferably 15 to 70 mass%, and more preferably 20 to 65 mass%, from the viewpoint of improving the transparency of the copolymer.

The α-methylene lactone may be added after diluting with a solvent, or may be added as is without using a solvent. A polymerization initiator may also be added to the α-methylene lactone. The solvent and polymerization initiator exemplified in the first step may be used.

The dripping time of the α-methylene lactone can be appropriately set depending on the amount of α-methylene lactone used, etc. The dripping time of the α-methylene lactone is preferably 0.2 to 10 hours, more preferably 0.5 to 5 hours.

From the viewpoint of improving the transparency of the copolymer, the conversion rate of α-methylene lactone and the conversion rate of alkyl (meth)acrylate at the end of the dropping in this process preferably satisfy any one of the following conditions (a), (b), or (c), and more preferably satisfy multiple conditions. The conversion rate of α-methylene lactone and the conversion rate of alkyl (meth)acrylate at the end of the dropping can be measured, for example, by the method described in the Examples.

Condition (a): The conversion rate of α-methylene lactone at the end of the dropwise addition is greater than the conversion rate of alkyl (meth)acrylate at the end of the dropwise addition.
Condition (b): The conversion rate of α-methylene lactone is 70% or more upon completion of the dropwise addition.
Condition (c): The difference between the conversion rate of α-methylene lactone at the end of dropwise addition and the conversion rate of alkyl (meth)acrylate at the end of dropwise addition ([conversion rate of α-methylene lactone at the end of dropwise addition] - [conversion rate of alkyl (meth)acrylate at the end of dropwise addition]) is more than 0% and 15% or less.

Under condition (b), the conversion rate of α-methylene lactone at the end of the dropwise addition is preferably 75% or more, more preferably 80% or more, and even more preferably 85% or more.

Under condition (c), the difference between the conversion rate of α-methylene lactone at the end of the dropwise addition and the conversion rate of alkyl (meth)acrylate at the end of the dropwise addition is preferably 12% or less, more preferably 10% or less, even more preferably 8% or less, and is preferably 1% or more, more preferably 2% or more, even more preferably 3% or more.

Then, the mixture containing the dripped α-methylene lactone is maintained under heating temperature conditions to polymerize the monomers, thereby obtaining a copolymer.

The polymerization time, including the dropping time of the dropped α-methylene lactone, can be set appropriately depending on the combination of monomers, the amount of dropped α-methylene lactone used, etc. The dropping time of the dropped α-methylene lactone is preferably 0.5 to 20 hours, more preferably 1 to 10 hours.

The polymerization reaction mixture obtained through the polymerization process usually contains a solvent in addition to the copolymer, which is the target product. There are no particular limitations on the method for separating the copolymer from the solvent, and examples include a method by reprecipitation, a devolatilization device consisting of a heat exchanger and a devolatilization tank, and a method of removing the solvent using an extruder with a vent.

[Copolymer]
The copolymer of one embodiment contains a structural unit derived from α-methylene lactone and a structural unit derived from an alkyl (meth)acrylate having an alkyl group having 1 to 6 carbon atoms, and when made into a film, the internal haze per 100 μm thickness is 1.0% or less. The copolymer of this embodiment may be a copolymer obtained by the above-mentioned production method. Since the above-mentioned production method can improve the transparency of the obtained copolymer, by using the production method, it may be possible to set the internal haze per 100 μm thickness within a predetermined range when made into a film.

The copolymer has an internal haze of 1.0% or less per 100 μm thickness when made into a film. The internal haze is preferably 0.8% or less, more preferably 0.7% or less, even more preferably 0.6% or less, and particularly preferably 0.5% or less. The internal haze of the copolymer when made into a film of 100 μm thickness can be measured, for example, by the method described in the Examples. The temperature when the copolymer is hot-press molded can be, for example, 200 to 270°C, and more specifically, can be 240°C.

The weight average molecular weight (Mw) of the copolymer is preferably 100,000 or more, more preferably 120,000 or more, even more preferably 150,000 or more, particularly preferably 180,000 or more, and most preferably 200,000 or more. The weight average molecular weight (Mw) of the copolymer is preferably 1,000,000 or less, more preferably 750,000 or less, even more preferably 500,000 or less, and particularly preferably 400,000 or less. When the Mw of the copolymer is in the above-mentioned specified range, the flexibility of the film can be further improved. The Mw of the copolymer can be measured, for example, by the method described in the Examples.

The number average molecular weight (Mn) of the copolymer is preferably 20,000 or more, more preferably 50,000 or more, and even more preferably 80,000 or more. The number average molecular weight (Mn) of the copolymer is preferably 500,000 or less, more preferably 300,000 or less, and even more preferably 200,000 or less. The Mn of the copolymer can be measured, for example, by the method described in the Examples. The dispersity (Mw/Mn) of the copolymer is preferably 3.0 or less, more preferably 2.8 or less, and even more preferably 2.5 or less.

The glass transition temperature (Tg) of the copolymer measured in accordance with the JIS K 7121 standard is preferably 110°C or higher, more preferably 115°C or higher, and even more preferably 120°C or higher, from the viewpoint of further improving heat resistance, etc. The upper limit of the glass transition temperature of the copolymer is not particularly limited, but can be, for example, 160°C or lower.

[Dope solution]
The dope liquid according to an embodiment contains the copolymer and a dispersion medium, and can be suitably used for producing a resin molded product (for example, a film).

Examples of dispersion media include alkyl chloride solvents such as chloroform and dichloromethane; aromatic solvents such as toluene, xylene, and benzene; alcohol solvents such as methanol, ethanol, isopropanol, n-butanol, and 2-butanol; methyl cellosolve, ethyl cellosolve, butyl cellosolve, dimethylformamide, dimethylacetamide, dimethylsulfoxide, dioxane, cyclohexanone, tetrahydrofuran, acetone, methyl ethyl ketone, ethyl acetate, diethyl ether, NMP, and GBL. These may be used alone or in combination of two or more. However, when two or more types are used in combination, the solvents represented by the above condition (B) are excluded.

Another aspect of the dispersion medium is a solvent represented by the above-mentioned condition (B). The boiling point of the mixed solvent (solvent represented by condition (B)) is preferably 70 to 120°C. The combination of the first and second solvents in the mixed solvent, the boiling point of the mixed solvent, the mixing ratio of the first and second solvents, etc. are similar to the combination of the first and second solvents, the boiling point of the mixed solvent, the mixing ratio of the first and second solvents, etc. described above. Therefore, redundant explanations will be omitted here. The first solvent may be further added to the above-mentioned mixed solvent. The boiling point of the dispersion medium when the first solvent is further added to the above-mentioned mixed solvent is preferably 30 to 110°C, more preferably 40 to 100°C.

The content of the copolymer in the dope solution is 5% by mass or more, preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, based on the total amount of the dope solution, from the viewpoint of efficient production of the film. The content of the copolymer is preferably 60% by mass or less, more preferably 50% by mass or less, and even more preferably 40% by mass or less, based on the total amount of the dope solution, from the viewpoint of ensuring fluidity for stable production in the manufacturing equipment.

The dope solution may contain other polymers in the film described below. The content of the other polymers is preferably 0 to 50% by weight, more preferably 0 to 40% by weight, even more preferably 0 to 30% by weight, particularly preferably 0 to 20% by weight, and most preferably 0 to 10% by weight, based on the total amount of the dope solution.

The dope liquid may contain other additives for the film described below. The dope liquid may contain one or more other additives. The content of the other additives is preferably 0 to 5 mass %, more preferably 0 to 2 mass %, and even more preferably 0 to 0.5 mass %, based on the total amount of the dope liquid.

From the viewpoint of improving the productivity of the film, the viscosity of the dope solution at 25°C is preferably 0.001 Pa·s or more, more preferably 0.01 Pa·s or more, even more preferably 0.1 Pa·s or more, and is preferably 10 Pa·s or less, more preferably 5 Pa·s or less, even more preferably 2 Pa·s or less. The viscosity at 25°C can be measured, for example, by the method described in the examples.

[Film and its manufacturing method]
The film of the present embodiment contains the above-mentioned copolymer as a main component. The film of the present embodiment can be produced using a resin composition containing the above-mentioned copolymer or a dope solution containing the above-mentioned copolymer.

The content of the copolymer is preferably 50 to 100% by mass, more preferably 60 to 100% by mass, even more preferably 70 to 100% by mass, particularly preferably 80 to 100% by mass, and most preferably 90 to 100% by mass, based on the total amount of the film. If the content of the copolymer is 50% by mass or more, a film with excellent transparency can be obtained.

The film may contain a polymer (other polymer) other than the copolymer described above. Examples of other polymers include olefin-based polymers such as polyethylene, polypropylene, ethylene-propylene copolymer, and poly(4-methyl-1-pentene); halogen-containing polymers such as vinyl chloride and chlorinated vinyl resin; acrylic polymers such as polymethyl methacrylate; styrene-based polymers such as polystyrene, styrene-methyl methacrylate copolymer, styrene-acrylonitrile copolymer, and acrylonitrile-butadiene-styrene block copolymer; polyesters such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate; polyamides such as nylon 6, nylon 66, and nylon 610; polyacetal; polycarbonate; polyphenylene oxide; polyphenylene sulfide; polyether ether ketone; polysulfone; polyether sulfone; polyoxybenzylene; polyamide imide; elastic organic particles such as polybutadiene rubber and acrylic rubber; rubber-like polymers such as ABS resin and ASA resin blended with polybutadiene rubber and acrylic rubber, etc. The content of the other polymers is preferably 0 to 50% by weight, more preferably 0 to 40% by weight, even more preferably 0 to 30% by weight, particularly preferably 0 to 20% by weight, and most preferably 0 to 10% by weight, based on the total amount of the film (resin composition).

The film may contain other additives. Examples of other additives include antioxidants such as hindered phenols, phosphorus, and sulfur; stabilizers such as light stabilizers, weather stabilizers, and heat stabilizers; reinforcing materials such as glass fibers and carbon fibers; ultraviolet absorbers such as phenyl salicylate, (2,2'-hydroxy-5-methylphenyl)benzotriazole, and 2-hydroxybenzophenone; near-infrared absorbers; flame retardants such as tris(dibromopropyl)phosphate, triallyl phosphate, and antimony oxide; antistatic agents such as anionic, cationic, and nonionic surfactants; colorants such as inorganic pigments, organic pigments, and dyes; organic or inorganic fillers; resin modifiers; organic or inorganic fillers; plasticizers; lubricants; antistatic agents; flame retardants; flow agents; and compatibilizers. The film may contain one or more other additives. The content of other additives is preferably 0 to 5% by mass, more preferably 0 to 2% by mass, and even more preferably 0 to 0.5% by mass, based on the total amount of the film.

One embodiment of the film manufacturing method includes a step of forming a resin composition containing the above-mentioned copolymer to obtain a film. The method for forming the resin composition is not particularly limited, but examples thereof include conventionally known methods such as melt extrusion, calendaring, and compression molding. Among these, the method for forming the resin composition is preferably melt extrusion.

The resin composition may contain, in addition to the copolymer described above, other polymers described above, other additives described above, etc., according to the desired film. The content of the copolymer, other polymers, other additives, etc. in the resin composition may be the same as the content of each component exemplified for the film.

Specific examples of melt extrusion methods include the T-die method and the inflation method. The molding temperature of the resin composition is preferably 150 to 350°C, more preferably 200 to 300°C.

Another embodiment of the film manufacturing method includes a step of applying the above-mentioned dope liquid, and a step of removing the dispersion medium from the applied dope liquid to obtain a film. The method of applying the dope liquid is not particularly limited, but examples thereof include conventionally known methods such as a solution casting method (solution casting method). For the solution casting method (solution casting method), devices such as a drum-type casting machine, a band-type casting machine, and a spin coater can be used.

The method for removing the dispersion medium from the dope liquid is not particularly limited, but for example, the dope liquid may be heated to volatilize the dispersion medium. The heating temperature can be set appropriately according to the dispersion medium used.

The film can be made into a stretched film by stretching it. The film is preferably a stretched film, since it has excellent flexibility and, in some cases, can be given a phase difference.

As a method for stretching the film, a conventionally known stretching method can be applied, for example, uniaxial stretching such as free width uniaxial stretching and fixed width uniaxial stretching; biaxial stretching such as sequential biaxial stretching and simultaneous biaxial stretching, etc. The method for stretching the film is preferably biaxial stretching, since it improves the bending resistance in any two orthogonal directions in the film plane.

The stretching temperature when stretching the film is preferably near the glass transition temperature of the copolymer. More specifically, it is preferably (glass transition temperature -30)°C to (glass transition temperature +100)°C, more preferably (glass transition temperature -20)°C to (glass transition temperature +50)°C, and even more preferably (glass transition temperature -10)°C to (glass transition temperature +30)°C.

The stretching ratio when stretching the film may be, for example, in the range of 1.05 to 10 times in both the longitudinal and transverse directions.

The thickness of the film is preferably 1 μm or more and less than 350 μm, and more preferably 10 μm or more and less than 300 μm.

From the viewpoint of further improving the strength of the film, the elastic modulus of the film is preferably 4.5 GPa or more, more preferably 5 GPa or more, and even more preferably 5.5 GPa or more. The upper limit of the elastic modulus of the film is not particularly limited, but can be, for example, 15 GPa or less. The elastic modulus of the film can be measured, for example, by the method described in the examples.

From the viewpoint of further improving the strength of the film, the pencil hardness of the film is preferably H or more, more preferably 2H or more, and even more preferably 3H or more.

The film of this embodiment can be used in a variety of applications, and can be suitably used in optical applications, for example. Specific examples of applications include light guide members, film applications, lenses (optical lenses, etc.), covers, and foam applications (for example, cushioning materials, heat retaining/insulating materials, vibration damping materials, soundproofing materials, sealing materials, packing materials, etc.).

The film of this embodiment is excellent in transparency, heat resistance, flexibility, and surface hardness, and therefore can be suitably used for flexible display applications, and can be particularly suitably used as a cover window on the outermost surface. Specific examples of flexible displays include thin and bendable flexible organic EL displays, and smartphones that can be folded or rolled up. In addition, since the film of this embodiment has a low retardation, it can also be used as a protective film for each layer of a flexible display. Furthermore, the film of this embodiment can be suitably used for producing a polarizing plate or a touch panel.

When the film of this embodiment is used as a cover window for a flexible display, it may be used as a laminate having other layers, such as a hard coat layer. In addition, the cover window for a flexible display formed from the film can be placed on the surface of the flexible display, for example, via an adhesive layer or the like.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the following, "parts" means "parts by mass" unless otherwise specified. In addition, various physical properties were measured and evaluated as follows.

[Monomer conversion rate at the end of dropping and at the end of polymerization reaction]
The monomer conversion rates at the end of the dropwise addition and at the end of the polymerization reaction were determined by measuring the amount of unreacted monomer in the polymerization reaction liquid at the end of the dropwise addition and at the end of the polymerization reaction using gas chromatography (manufactured by Shimadzu Corporation, apparatus name: GC-2014).

[Weight average molecular weight and number average molecular weight of copolymer]
The weight average molecular weight (Mw) and number average molecular weight (Mn) of the copolymer were determined in terms of polystyrene using gel permeation chromatography (GPC) under the following conditions:
System: Tosoh GPC system HLC-8220
Measurement column configuration:
・Guard column (Tosoh, TSKguardcolumn SuperHZ-L)
Separation column (Tosoh, TSKgel SuperHZM-M) 2 columns connected in series Reference column configuration:
Reference column (Tosoh, TSKgel Super H-RC)
Developing solvent: Chloroform (Wako Pure Chemical Industries, special grade)
Flow rate of developing solvent: 0.6 mL/min Standard sample: TSK standard polystyrene (manufactured by Tosoh, PS-oligomer kit)
Column temperature: 40°C

[Glass transition temperature (Tg) of copolymer]
The glass transition temperature of the copolymer was determined in accordance with the provisions of JIS K 7121. Specifically, using a differential scanning calorimeter (Rigaku Corporation, Thermo plus EVO DSC-8230), the glass transition temperature was evaluated by the starting point method from a DSC curve obtained by heating about 10 mg of a sample from room temperature to 200° C. (heating rate: 20° C./min) in a nitrogen gas atmosphere. α-Alumina was used as a reference.

[Film thickness]
The thickness of the film was measured using a Digimatic Micrometer (manufactured by Mitutoyo).

[Internal haze of film]
The haze was determined in accordance with the provisions of JIS K7136. Specifically, a stretched film was prepared, and a haze meter (NDH-1001DP, manufactured by Nippon Denshoku Industries Co., Ltd.) was used to measure the haze by filling a quartz cell with an optical path length of 10 mm with 1,2,3,4-tetrahydronaphthalene (tetralin), immersing the film in the cell, and calculating the internal haze value per 100 μm.

[Film tensile test (elastic modulus measurement)]
The stretched film was cut into a size of 90 mm x 20 mm to prepare a test piece, and a tensile test was carried out in accordance with JIS K7127 using an autograph (Shimadzu Corporation: AG-X) at a temperature of 25 ° C. and a relative humidity of 50%. The conditions were a tensile speed of 0.25 mm / min up to a strain of 0.5%, and 1 mm / min thereafter, a chuck distance of 55 mm, and a gauge interval measured with a displacement meter of 25 mm. The test was carried out three times at 25 ° C., and the average value was taken as the measured value. The displacement was measured using a non-contact extensometer (Shimadzu Corporation: TRViewX), and the elastic modulus was evaluated as the slope between strains of 0.05% and 0.25%.

[Film pencil hardness]
The pencil hardness of the film was evaluated using a test pencil specified by JIS-S-6006 and a pencil scratch hardness tester No. 533 manufactured by Yasuda Seiki Seisakusho Co., Ltd. under a load of 750 g in accordance with JIS K5600-5-4 (1999), and the hardness of the highest pencil that did not cause scratches was recorded as the pencil hardness.

<Synthesis of copolymers and film fabrication by stirring polymerization>

Example 1
A reaction vessel equipped with a stirrer, a temperature sensor, a cooling tube, and a nitrogen inlet tube was charged with 6 parts of methyl methacrylate (MMA), 2 parts of α-methylene-γ-butyrolactone (ML) (initial ML), 0.0025 parts of n-dodecyl mercaptan (nDM) as a chain transfer agent, and 6 parts of a mixed solvent obtained by mixing acetone and cyclohexanone in a ratio of 35:65 as a solvent, to prepare a mixed liquid, which was then heated to 83° C. while passing nitrogen through it. Thereafter, 0.005 parts of t-amylperoxy 2-ethylhexanoate (manufactured by Arkema Yoshitomi Co., Ltd., Luperox (registered trademark) 575, hereinafter sometimes referred to as "initiator 575") was added as a polymerization initiator, and 0.008 parts of initiator 575 diluted with 4.5 parts of a mixed solvent of acetone and cyclohexanone mixed at a ratio of 35:65 and 2 parts of ML (dropping ML) were dropped into the mixed liquid at a constant rate over 4 hours at 83 to 90 ° C. The mass ratio of the dropping ML to the total mass of the initial ML and the dropping ML was 50 mass %. After the dropping, a small amount of the polymerization reaction liquid was taken, the copolymer content contained in the polymerization liquid was calculated, and the conversion rates of MMA and ML at the end of the dropping were calculated. The conversion rates of MMA and ML at the end of the dropping were 77.9% and 85.4%, respectively. Thereafter, a mixed solvent of 2 parts of acetone and cyclohexanone in a ratio of 35:65 was added, and the solution was further stirred and polymerized at 83 to 90°C for 4 hours. The conversion rates of MMA and ML at the end of the polymerization reaction, calculated from the amount of unreacted monomer in the polymerization reaction solution, were 94.5% and 97.7%, respectively. The obtained polymerization reaction solution was vacuum dried (133 Pa (1 mmHg)) at 240°C for 2 hours, to obtain a white copolymer. The physical properties of the obtained copolymer are shown in Table 1.

The resulting copolymer was hot-pressed at 240°C to obtain an unstretched press film with a thickness of approximately 160 μm. The resulting unstretched film was cut into a size of 96 mm x 96 mm, and sequentially biaxially stretched at a stretching temperature (154°C) of Tg+18°C at a stretching speed of 300%/min in the machine direction (MD direction) and transverse direction (TD direction) to a stretch ratio of 2.0 times in each direction using a sequential biaxial stretching machine (Toyo Seiki Seisakusho, X6-S), and then cooled to obtain a stretched film with a thickness of 40 μm. The physical properties of the resulting stretched film are shown in Table 1.

Example 2
A mixed liquid was prepared by charging 6 parts of methyl methacrylate (MMA), 2 parts of α-methylene-γ-butyrolactone (ML) (initial ML), and 6 parts of a mixed solvent of acetone and cyclohexanone in a ratio of 35:65 as a solvent into a reactor equipped with a stirring device, a temperature sensor, a cooling tube, and a nitrogen inlet tube, and the temperature was raised to 83 ° C. while passing nitrogen through the mixed liquid. Thereafter, 0.005 parts of initiator 575 was added as a polymerization initiator, and 0.008 parts of initiator 575 diluted with 4.5 parts of a mixed solvent of acetone and cyclohexanone in a ratio of 35:65 and 2 parts of ML (dropping ML) were dropped into the mixed liquid at a constant rate at 83 to 90 ° C. over 4 hours. The mass ratio of the dropping ML to the total mass of the initial ML and the dropping ML was 50 mass%. After the dropwise addition, a small amount of the polymerization reaction liquid was taken, the copolymer content in the polymerization liquid was calculated, and the conversion rates of MMA and ML at the end of the dropwise addition were calculated. The conversion rates of MMA and ML at the end of the dropwise addition were 81.1% and 87.3%, respectively. Thereafter, a mixed solvent of 2 parts of acetone and cyclohexanone mixed at a ratio of 35:65 was added, and the solution was further stirred and polymerized at 83 to 90°C for 4 hours. The conversion rates of MMA and ML at the end of the polymerization reaction calculated from the amount of unreacted monomer in the polymerization reaction liquid were 93.5% and 96.3%, respectively. The obtained polymerization reaction liquid was vacuum dried (133 Pa (1 mmHg)) at 240°C for 2 hours to obtain a white copolymer. The physical properties of the obtained copolymer are shown in Table 2.

The resulting copolymer was hot-pressed at 240°C to obtain an unstretched press film with a thickness of approximately 160 μm. The resulting unstretched film was cut into a size of 96 mm x 96 mm, and sequentially biaxially stretched at a stretching temperature (154°C) of Tg+18°C at a stretching speed of 300%/min in the machine direction (MD direction) and transverse direction (TD direction) to a stretch ratio of 2.0 times in each direction using a sequential biaxial stretching machine (Toyo Seiki Seisakusho, X6-S), and then cooled to obtain a stretched film with a thickness of 40 μm. The physical properties of the resulting stretched film are shown in Table 2.

Example 3
A mixed liquid was prepared by charging 6 parts of methyl methacrylate (MMA), 1.5 parts of α-methylene-γ-butyrolactone (ML) (initial ML), 0.0025 parts of n-dodecyl mercaptan (nDM) as a chain transfer agent, and 6 parts of a mixed solvent of acetone and cyclohexanone in a ratio of 35:65 as a solvent into a reactor equipped with a stirrer, a temperature sensor, a cooling tube, and a nitrogen inlet tube, and the temperature was raised to 83° C. while passing nitrogen through the mixed liquid. Thereafter, 0.005 parts of initiator 575 was added as a polymerization initiator, and 0.008 parts of initiator 575 diluted with 4.5 parts of a mixed solvent of acetone and cyclohexanone in a ratio of 35:65 and 2.5 parts of ML (dropping ML) were dropped into the mixed liquid at a constant rate at 83 to 90° C. over 4 hours. The mass ratio of the dropped ML to the total mass of the initial ML and the dropped ML was 62.5% by mass. After dropping, a small amount of the polymerization reaction liquid was taken out, the copolymer content contained in the polymerization liquid was calculated, and the conversion rates of MMA and ML at the end of the dropping were calculated. The conversion rates of MMA and ML at the end of the dropping were 72.6% and 78.6%, respectively. Thereafter, a mixed solvent of 2 parts of acetone and cyclohexanone mixed at a ratio of 35:65 was added, and the solution was further stirred and polymerized at 83 to 90 ° C. for 3 hours. The conversion rates of MMA and ML at the end of the polymerization reaction calculated from the amount of unreacted monomer in the polymerization reaction liquid were 92.5% and 96.1%, respectively. The obtained polymerization reaction liquid was vacuum dried (133 Pa (1 mmHg)) at 240 ° C. for 2 hours to obtain a white copolymer. The physical properties of the obtained copolymer are shown in Table 2.

One part of the obtained copolymer was mixed with 7 parts of dichloromethane as a dispersion medium, and after shaking by hand for 1 minute, the mixture was stirred and mixed for 60 minutes to produce a dope solution with a solid content of 12.5% by mass. When the dope solution was visually inspected, it was found to be uniformly dispersed, and no change was observed in the appearance of the dope solution even after it was left to stand overnight.

Next, the dope solution was dropped onto the PET film and spread to a film thickness of 800 μm using an applicator. The PET film was then placed in a dryer and dried at 40°C for 30 minutes and at 60°C for 30 minutes, after which the coated film was peeled off from the PET. The obtained film was hung in a dryer with wide mountain-shaped clips attached to the top and bottom to prevent curling, and then dried at 100°C for 12 hours to obtain an unstretched cast film with a thickness of 160 μm. The obtained unstretched cast film was cut into a size of 96 mm x 96 mm, and sequentially biaxially stretched in the machine direction (MD direction) and the transverse direction (TD direction) at a stretching temperature of Tg + 18°C (153°C) at a stretching rate of 300%/min to a stretching ratio of 2.0 times each using a sequential biaxial stretching machine (Toyo Seiki Seisakusho, X6-S) to obtain a stretched film with a thickness of 40 μm by cooling. The physical properties of the obtained stretched film are shown in Table 2.

Example 4
A mixed liquid was prepared by charging 6 parts of methyl methacrylate (MMA), 1.5 parts of α-methylene-γ-butyrolactone (ML) (initial ML), and 6 parts of a mixed solvent obtained by mixing acetone and cyclohexanone at a ratio of 35:65 as a solvent into a reactor equipped with a stirring device, a temperature sensor, a cooling tube, and a nitrogen inlet tube, and the temperature was raised to 83 ° C. while passing nitrogen through the mixed liquid. Thereafter, 0.006 parts of initiator 575 was added as a polymerization initiator, and 0.007 parts of initiator 575 diluted with 4.5 parts of a mixed solvent obtained by mixing acetone and cyclohexanone at a ratio of 35:65 and 2.5 parts of ML (dropped ML) were dropped into the mixed liquid at a constant rate over 4 hours at 83 to 90 ° C. The mass ratio of the dropped ML to the total mass of the initial ML and the dropped ML was 62.5 mass%. After the dropwise addition, a small amount of the polymerization reaction liquid was taken, the copolymer content in the polymerization liquid was calculated, and the conversion rates of MMA and ML at the end of the dropwise addition were calculated. The conversion rates of MMA and ML at the end of the dropwise addition were 74.8% and 78.1%, respectively. Thereafter, a mixed solvent of 2 parts of acetone and cyclohexanone mixed at a ratio of 35:65 was added, and the solution was further stirred and polymerized at 83 to 90°C for 3 hours. The conversion rates of MMA and ML at the end of the polymerization reaction calculated from the amount of unreacted monomer in the polymerization reaction liquid were 95.3% and 95.5%, respectively. The obtained polymerization reaction liquid was vacuum dried (133 Pa (1 mmHg)) at 240°C for 2 hours to obtain a white copolymer. The physical properties of the obtained copolymer are shown in Table 2.

The resulting copolymer was hot-pressed at 240°C to obtain an unstretched press film with a thickness of approximately 160 μm. The resulting unstretched film was cut into a size of 96 mm x 96 mm, and sequentially biaxially stretched at a stretching temperature (154°C) of Tg+18°C at a stretching speed of 300%/min in the machine direction (MD direction) and transverse direction (TD direction) to a stretch ratio of 2.0 times in each direction using a sequential biaxial stretching machine (Toyo Seiki Seisakusho, X6-S), and then cooled to obtain a stretched film with a thickness of 40 μm. The physical properties of the resulting stretched film are shown in Table 2.

(Example 5)
A mixed liquid was prepared by charging 7.5 parts of methyl methacrylate (MMA), 1.9 parts of α-methylene-γ-butyrolactone (ML) (initial ML), and 6 parts of a mixed solvent obtained by mixing acetone and cyclohexanone at a ratio of 35:65 as a solvent into a reactor equipped with a stirring device, a temperature sensor, a cooling tube, and a nitrogen inlet tube, and the temperature was raised to 83 ° C. while passing nitrogen through the mixed liquid. Thereafter, 0.003 parts of initiator 575 was added as a polymerization initiator, and 0.005 parts of initiator 575 diluted with 4.5 parts of a mixed solvent obtained by mixing acetone and cyclohexanone at a ratio of 35:65 and 0.6 parts of ML (dropped ML) were dropped into the mixed liquid at a constant rate over 2 hours at 83 to 90 ° C. The mass ratio of the dropped ML to the total mass of the initial ML and the dropped ML was 24 mass%. After the dropwise addition, a small amount of the polymerization reaction liquid was taken, the copolymer content in the polymerization liquid was calculated, and the conversion rates of MMA and ML at the end of the dropwise addition were calculated. The conversion rates of MMA and ML at the end of the dropwise addition were 58.5% and 68.7%, respectively. Thereafter, a mixed solvent of 2 parts of acetone and cyclohexanone mixed at a ratio of 35:65 was added, and the solution was further stirred and polymerized at 83 to 90°C for 4 hours. The conversion rates of MMA and ML at the end of the polymerization reaction calculated from the amount of unreacted monomer in the polymerization reaction liquid were 92.8% and 93.6%, respectively. The obtained polymerization reaction liquid was vacuum dried (133 Pa (1 mmHg)) at 240°C for 2 hours to obtain a white copolymer. The physical properties of the obtained copolymer are shown in Table 2.

The resulting copolymer was hot-pressed at 240°C to obtain an unstretched press film with a thickness of approximately 160 μm. The resulting unstretched film was cut into a size of 96 mm x 96 mm, and sequentially biaxially stretched at a stretching temperature (144°C) of Tg+18°C at a stretching speed of 300%/min in the machine direction (MD direction) and transverse direction (TD direction) to a stretch ratio of 2.0 times in each direction using a sequential biaxial stretching machine (Toyo Seiki Seisakusho, X6-S), and then cooled to obtain a stretched film with a thickness of 40 μm. The physical properties of the resulting stretched film are shown in Table 2.

Example 6
A mixed liquid was prepared by charging 7.5 parts of methyl methacrylate (MMA), 1.9 parts of α-methylene-γ-butyrolactone (ML) (initial ML), and 9 parts of γ-butyrolactone as a solvent into a reactor equipped with a stirring device, a temperature sensor, a cooling tube, and a nitrogen inlet tube, and the temperature was raised to 105 ° C. while passing nitrogen through the mixed liquid. Thereafter, 0.003 parts of t-amyl peroxyisononanoate (manufactured by Arkema Yoshitomi Co., Ltd., Luperox (registered trademark) 570, hereinafter sometimes referred to as "initiator 570") was added as a polymerization initiator, and 0.005 parts of initiator 570 diluted with 1 part of γ-butyrolactone and 0.6 parts of ML (dropping ML) were dropped into the mixed liquid at a constant rate over 2 hours at 105 to 110 ° C. The mass ratio of the dropping ML to the total mass of the initial ML and the dropping ML was 24 mass%. After the dropwise addition, a small amount of the polymerization reaction liquid was taken, the copolymer content in the polymerization liquid was calculated, and the conversion rates of MMA and ML at the end of the dropwise addition were calculated. The conversion rates of MMA and ML at the end of the dropwise addition were 80.0% and 87.7%, respectively. Thereafter, 5 parts of γ-butyrolactone was added, and the solution was stirred and polymerized at 105 to 110° C. for another 4 hours. The conversion rates of MMA and ML at the end of the polymerization reaction calculated from the amount of unreacted monomer in the polymerization reaction liquid were 96.1% and 98.7%, respectively. The obtained polymerization reaction liquid was vacuum dried (133 Pa (1 mmHg)) at 240° C. for 2 hours to obtain a white copolymer. The physical properties of the obtained copolymer are shown in Table 2.

The resulting copolymer was hot-pressed at 240°C to obtain an unstretched press film with a thickness of approximately 160 μm. The resulting unstretched film was cut into a size of 96 mm x 96 mm, and sequentially biaxially stretched at a stretching temperature (145°C) of Tg+18°C and a stretching speed of 300%/min in the machine direction (MD direction) and transverse direction (TD direction) to a stretch ratio of 2.0 times in each direction using a sequential biaxial stretching machine (Toyo Seiki Seisakusho, X6-S), and then cooled to obtain a stretched film with a thickness of 40 μm. The physical properties of the resulting stretched film are shown in Table 2.

(Example 7)
A mixed liquid was prepared by charging 8.5 parts of methyl methacrylate (MMA), 1.1 parts of α-methylene-γ-butyrolactone (ML) (initial ML), and 9 parts of γ-butyrolactone as a solvent into a reactor equipped with a stirring device, a temperature sensor, a cooling tube, and a nitrogen inlet tube, and the temperature was raised to 105 ° C. while passing nitrogen through the mixed liquid. Thereafter, 0.003 parts of initiator 570 was added as a polymerization initiator, and 0.005 parts of initiator 570 diluted with 1 part of γ-butyrolactone and 0.4 parts of ML (dropping ML) were dropped into the mixed liquid at a constant rate over 2 hours at 105 to 110 ° C. The mass ratio of the dropping ML to the total mass of the initial ML and the dropping ML was 27 mass %. After the dropping, a small amount of the polymerization reaction liquid was taken, the copolymer content contained in the polymerization liquid was calculated, and the conversion rates of MMA and ML at the end of the dropping were calculated. The conversion rates of MMA and ML at the end of the dropwise addition were 73.8% and 87.6%, respectively. Thereafter, 5 parts of γ-butyrolactone was added, and the solution was further stirred and polymerized at 105 to 110° C. for 4 hours. The conversion rates of MMA and ML at the end of the polymerization reaction, calculated from the amount of unreacted monomer in the polymerization reaction liquid, were 94.9% and 98.7%, respectively. The obtained polymerization reaction liquid was vacuum dried (133 Pa (1 mmHg)) at 240° C. for 2 hours, to obtain a white copolymer. The physical properties of the obtained copolymer are shown in Table 2.

The resulting copolymers were each hot-pressed at 240°C to obtain unstretched press films with a thickness of approximately 160 μm. The resulting unstretched films were cut into pieces measuring 96 mm x 96 mm, and sequentially biaxially stretched at a stretching temperature (141°C) of Tg+18°C at a stretching speed of 300%/min in the machine direction (MD direction) and transverse direction (TD direction) to a stretch ratio of 2.0 times each using a sequential biaxial stretching machine (Toyo Seiki Seisakusho, X6-S). The stretched films were then cooled to obtain stretched films with a thickness of 40 μm. The physical properties of the resulting stretched films are shown in Table 2.

(Example 8)
A mixed liquid was prepared by charging 8.5 parts of methyl methacrylate (MMA), 1.1 parts of α-methylene-γ-butyrolactone (ML) (initial ML), and 9 parts of γ-butyrolactone as a solvent into a reactor equipped with a stirring device, a temperature sensor, a cooling tube, and a nitrogen inlet tube, and the temperature was raised to 105 ° C. while passing nitrogen through the mixed liquid. Thereafter, 0.003 parts of initiator 570 was added as a polymerization initiator, and 0.005 parts of initiator 570 diluted with 1 part of γ-butyrolactone and 0.4 parts of ML (dropping ML) were dropped into the mixed liquid at a constant rate over 2 hours at 105 to 110 ° C. The mass ratio of the dropping ML to the total mass of the initial ML and the dropping ML was 27 mass %. After the dropping, a small amount of the polymerization reaction liquid was taken, the copolymer content contained in the polymerization liquid was calculated, and the conversion rates of MMA and ML at the end of the dropping were calculated. The conversion rates of MMA and ML at the end of the dropwise addition were 73.8% and 87.6%, respectively. Thereafter, 5 parts of γ-butyrolactone was added, and the solution was further stirred and polymerized at 105 to 110° C. for 4 hours. The conversion rates of MMA and ML at the end of the polymerization reaction, calculated from the amount of unreacted monomer in the polymerization reaction liquid, were 94.9% and 98.7%, respectively. The obtained polymerization reaction liquid was vacuum dried (133 Pa (1 mmHg)) at 240° C. for 2 hours, to obtain a white copolymer. The physical properties of the obtained copolymer are shown in Table 2.

One part of the obtained copolymer was mixed with 7 parts of dichloromethane as a dispersion medium, and after shaking by hand for 1 minute, the mixture was stirred and mixed for 60 minutes to produce a dope solution with a solid content of 12.5% by mass. When the dope solution was visually inspected, it was found to be uniformly dispersed, and no change was observed in the appearance of the dope solution even after it was left to stand overnight.

Next, the dope solution was dropped onto the PET film and spread to a film thickness of 800 μm using an applicator. The PET film was then placed in a dryer and dried at 40°C for 30 minutes and at 60°C for 30 minutes, after which the coated film was peeled off from the PET. The obtained film was hung in a dryer with wide mountain-shaped clips attached to the top and bottom to prevent curling, and then dried at 100°C for 12 hours to obtain an unstretched cast film with a thickness of 160 μm. The obtained unstretched cast film was cut into a size of 96 mm x 96 mm, and sequentially biaxially stretched in the machine direction (MD direction) and the transverse direction (TD direction) at a stretching temperature of Tg + 18°C (141°C) at a stretching rate of 300%/min to a stretching ratio of 2.0 times each using a sequential biaxial stretching machine (Toyo Seiki Seisakusho, X6-S) to obtain a stretched film with a thickness of 40 μm by cooling. The physical properties of the obtained stretched film are shown in Table 2.

(Comparative Example 1)
A mixed liquid was prepared by charging 6 parts of methyl methacrylate (MMA), 4 parts of α-methylene-γ-butyrolactone (ML) (initial ML), 0.0025 parts of n-dodecyl mercaptan (nDM) as a chain transfer agent, and 6 parts of a mixed solvent obtained by mixing acetone and cyclohexanone at a ratio of 35:65 as a solvent into a reactor equipped with a stirring device, a temperature sensor, a cooling tube, and a nitrogen inlet tube, and the temperature was raised to 83 ° C. while passing nitrogen through the mixed liquid. Thereafter, 0.005 parts of initiator 575 was added as a polymerization initiator, and 0.008 parts of initiator 575 diluted with 4.5 parts of a mixed solvent obtained by mixing acetone and cyclohexanone at a ratio of 35:65 was dropped into the mixed liquid at a constant rate over 4 hours at 83 to 90 ° C. The mass ratio of the dropped ML to the total mass of the initial ML and the dropped ML was 0 mass%. After the dropwise addition, a mixed solvent of 2 parts of acetone and cyclohexanone in a ratio of 35:65 was added, and the solution was further stirred and polymerized at 83 to 90°C for 4 hours. The conversion rates of MMA and ML at the end of the polymerization reaction, calculated from the amount of unreacted monomer in the polymerization reaction solution, were 93.9% and 99.0%, respectively. The obtained polymerization reaction solution was vacuum dried (133 Pa (1 mmHg)) at 240°C for 2 hours, to obtain a white copolymer. The physical properties of the obtained copolymer are shown in Table 1.

The resulting copolymer was hot-pressed at 240°C to obtain an unstretched press film with a thickness of approximately 160 μm. The resulting unstretched film was cut into a size of 96 mm x 96 mm, and sequentially biaxially stretched at a stretching temperature (152°C) of Tg+18°C at a stretching speed of 300%/min in the machine direction (MD direction) and transverse direction (TD direction) to a stretch ratio of 2.0 times in each direction using a sequential biaxial stretching machine (Toyo Seiki Seisakusho, X6-S), and then cooled to obtain a stretched film with a thickness of 40 μm. The physical properties of the resulting stretched film are shown in Table 1.

(Comparative Example 2)
A mixed liquid was prepared by charging 6 parts of methyl methacrylate (MMA), 3.8 parts of α-methylene-γ-butyrolactone (ML) (initial ML), 0.0025 parts of n-dodecyl mercaptan (nDM) as a chain transfer agent, and 6 parts of a mixed solvent obtained by mixing acetone and cyclohexanone at a ratio of 35:65 as a solvent into a reactor equipped with a stirring device, a temperature sensor, a cooling tube, and a nitrogen inlet tube, and the temperature was raised to 83 ° C. while passing nitrogen through the mixed liquid. Thereafter, 0.005 parts of initiator 575 was added as a polymerization initiator, and 0.008 parts of initiator 575 diluted with 4.5 parts of a mixed solvent obtained by mixing acetone and cyclohexanone at a ratio of 35:65 and 0.2 parts of ML (dropped ML) were dropped into the mixed liquid at a constant rate over 4 hours at 83 to 90 ° C. The mass ratio of the dropped ML to the total mass of the initial ML and the dropped ML was 5 mass%. After the dropwise addition, a small amount of the polymerization reaction liquid was taken, the copolymer content in the polymerization liquid was calculated, and the conversion rates of MMA and ML at the end of the dropwise addition were calculated. The conversion rates of MMA and ML at the end of the dropwise addition were 67.4% and 96.5%, respectively. Thereafter, a mixed solvent of 2 parts of acetone and cyclohexanone mixed at a ratio of 35:65 was added, and the solution was stirred and polymerized at 83 to 90°C for 4 hours. The conversion rates of MMA and ML at the end of the polymerization reaction calculated from the amount of unreacted monomer in the polymerization reaction liquid were 94.2% and 98.9%, respectively. The obtained polymerization reaction liquid was vacuum dried (133 Pa (1 mmHg)) at 240°C for 2 hours to obtain a white copolymer. The physical properties of the obtained copolymer are shown in Table 1.

The resulting copolymer was hot-pressed at 240°C to obtain an unstretched press film with a thickness of approximately 160 μm. The resulting unstretched film was cut into a size of 96 mm x 96 mm, and sequentially biaxially stretched at a stretching temperature (152°C) of Tg+18°C at a stretching speed of 300%/min in the machine direction (MD direction) and transverse direction (TD direction) to a stretch ratio of 2.0 times in each direction using a sequential biaxial stretching machine (Toyo Seiki Seisakusho, X6-S), and then cooled to obtain a stretched film with a thickness of 40 μm. The physical properties of the resulting stretched film are shown in Table 1.

(Comparative Example 3)
A mixed liquid was prepared by charging 6 parts of methyl methacrylate (MMA), 0.4 parts of α-methylene-γ-butyrolactone (ML) (initial ML), 0.0025 parts of n-dodecyl mercaptan (nDM) as a chain transfer agent, and 6 parts of a mixed solvent obtained by mixing acetone and cyclohexanone at a ratio of 35:65 as a solvent into a reactor equipped with a stirring device, a temperature sensor, a cooling tube, and a nitrogen inlet tube, and the temperature was raised to 83 ° C. while passing nitrogen through the mixed liquid. Thereafter, 0.005 parts of initiator 575 was added as a polymerization initiator, and 0.008 parts of initiator 575 diluted with 4.5 parts of a mixed solvent obtained by mixing acetone and cyclohexanone at a ratio of 35:65 and 3.6 parts of ML (dropped ML) were dropped into the mixed liquid at a constant rate over 4 hours at 83 to 90 ° C. The mass ratio of the dropped ML to the total mass of the initial ML and the dropped ML was 90 mass%. After the dropwise addition, a small amount of the polymerization reaction liquid was taken, the copolymer content in the polymerization liquid was calculated, and the conversion rates of MMA and ML at the end of the dropwise addition were calculated. The conversion rates of MMA and ML at the end of the dropwise addition were 82.5% and 65.3%, respectively. Thereafter, a mixed solvent of 2 parts of acetone and cyclohexanone mixed at a ratio of 35:65 was added, and the solution was further stirred and polymerized at 83 to 90°C for 4 hours. The conversion rates of MMA and ML at the end of the polymerization reaction calculated from the amount of unreacted monomer in the polymerization reaction liquid were 96.5% and 93.2%, respectively. The obtained polymerization reaction liquid was vacuum dried (133 Pa (1 mmHg)) at 240°C for 2 hours to obtain a white copolymer. The physical properties of the obtained copolymer are shown in Table 1.

One part of the obtained copolymer was mixed with 7 parts of dichloromethane as a dispersion medium, and after shaking by hand for 1 minute, the mixture was stirred and mixed for 60 minutes to produce a dope solution with a solid content of 12.5% by mass. When the dope solution was visually inspected, it was found to be uniformly dispersed, and no change was observed in the appearance of the dope solution even after it was left to stand overnight.

Next, the dope solution was dropped onto the PET film and spread to a film thickness of 800 μm using an applicator. The PET film was then placed in a dryer and dried at 40°C for 30 minutes and at 60°C for 30 minutes, after which the coated film was peeled off from the PET. The obtained film was hung in a dryer with wide mountain-shaped clips attached to the top and bottom to prevent curling, and then dried at 100°C for 12 hours to obtain an unstretched cast film with a thickness of 160 μm. The obtained unstretched cast film was cut into a size of 96 mm x 96 mm, and sequentially biaxially stretched in the longitudinal direction (MD direction) and transverse direction (TD direction) at a stretching temperature of Tg + 18°C (152°C) at a stretching rate of 300%/min to a stretching ratio of 2.0 times each using a sequential biaxial stretching machine (Toyo Seiki Seisakusho, X6-S) so that the stretching ratio was 2.0 times in each direction, and then cooled to obtain a stretched film with a thickness of 40 μm. The physical properties of the obtained stretched film are shown in Table 1.

Claims (7)

A method for producing a copolymer containing a structural unit derived from α-methylene lactone and a structural unit derived from an alkyl (meth)acrylate having an alkyl group having 1 to 6 carbon atoms, comprising the steps of:
A first step of preparing a mixed liquid containing the α-methylene lactone, the alkyl (meth)acrylate, and a solvent;
a second step of heating the mixture and further dropping the α-methylene lactone into the heated mixture;
Equipped with
the content of the structural unit derived from α-methylene lactone in the copolymer is 10% by mass or more and 60% by mass or less,
a mass ratio of the α-methylene lactone dropped into the mixed liquid in the second step to a total mass of the α-methylene lactone contained in the mixed liquid in the first step and the α-methylene lactone dropped into the mixed liquid in the second step is 10 to 80 mass%. The method for producing a copolymer according to claim 1, wherein the conversion rate of the α-methylene lactone at the end of the dropwise addition of the second step is greater than the conversion rate of the alkyl (meth)acrylate. The method for producing a copolymer according to claim 1 or 2, wherein the conversion rate of the α-methylene lactone is 70% or more at the end of the dropwise addition of the second step. The method for producing a copolymer according to any one of claims 1 to 3, wherein the difference between the conversion rate of the α-methylene lactone and the conversion rate of the alkyl (meth)acrylate at the end of the dropwise addition of the second step is greater than 0% and less than or equal to 15%. The method for producing a copolymer according to any one of claims 1 to 4 , wherein the α-methylene lactone is α-methylene-γ-butyrolactone or α-methylene-δ-valerolactone. A film containing a copolymer including a structural unit derived from α-methylene lactone and a structural unit derived from an alkyl (meth)acrylate having an alkyl group having 1 to 6 carbon atoms ,
the content of the structural unit derived from α-methylene lactone in the copolymer is 10% by mass or more and 60% by mass or less,
The content of the copolymer is 50 to 100% by mass based on the total amount of the film,
The internal haze per 100 μm thickness is 1.0% or less,
A film for flexible displays. The film of claim 6 having an elastic modulus of 4.5 GPa or greater.