JP3657113B2 - Low birefringence copolymer, process for producing the same, and pickup lens - Google Patents

Description

【００１３】
（式中、Ｒ¹はｔ−ブチルシクロヘキシル基を示し、ｎは正の整数を表す。）
また、本発明の要旨は下記の一般式（２）で示されるα置換アクリル酸エステル単量体と（メタ）アクリル酸エステルを、一分子中にＯ−Ｏ結合を２つ以上持つ過酸化物を重合開始剤として、共重合する上記共重合体の製法にある。
【００１４】
【化４】

【００１５】
（式中、Ｒ¹はｔ−ブチルシクロヘキシル基を示す。）
さらにまた、本発明の要旨は上記共重合体、又は上記製造方法により得られた共重合体により成形された光ピックアップレンズにある。
【００１６】
【発明の実施の形態】
本発明の共重合体は、一般式（２）で表されるα置換アクリル酸エステル単量体と（メタ）アクリル酸エステルとを共重合することによって得られる。重合の際に一般式（２）の単量体が環化反応し、一般式（１）のピラン環が形成される。
【００１７】
本発明の共重合体の一般式（１）の繰り返し単位と（メタ）アクリル酸エステルの繰り返し単位の組成比は特に限定されないが、その光学材料としての特性から一般式（１）の重量組成が１０〜７０重量％の範囲であることが好ましく、１０〜６０重量％の範囲であることがより好ましい。一般式（１）の組成が少なすぎる場合は耐熱性・吸水性が不足し、組成が多すぎる場合は機械強度が低下する。
【００１８】
本発明の一般式（１）のt-ブチルシクロヘキシル基にはオルト、メタ、パラの異性体があり、さらにその各々についてシス、及びトランスの異性体が存在する。本発明においてはそのいずれも使用することができ、また、いかなる割合の混合体も使用することができるが、好ましくはパラ置換体であり、トランス含量が６５％以上のものが好ましい。
【００１９】
また、共重合成分の（メタ）アクリル酸エステルとしては、例えば、メタクリル酸メチル、メタクリル酸エチル、メタクリル酸n-プロピル、メタクリル酸イソプロピル、メタクリル酸n-ブチル、メタクリル酸イソブチル、メタクリル酸t-ブチル、メタクリル酸イソアミル、メタクリル酸ラウリル、メタクリル酸フェニル、メタクリル酸ベンジル、メタクリル酸シクロヘキシル、メタクリル酸グリシジル、メタクリル酸2-エチルヘキシル等のメタクリル酸エステル、およびアクリル酸メチル、アクリル酸エチル、アクリル酸n-プロピル、アクリル酸イソプロピル、アクリル酸n-ブチル、アクリル酸イソブチル、アクリル酸t-ブチル、アクリル酸イソアミル、アクリル酸ラウリル、アクリル酸フェニル、アクリル酸ベンジル、アクリル酸シクロヘキシル、アクリル酸グリシジル、アクリル酸2-エチルヘキシル等のアクリル酸エステルが挙げられる。なかでも、メタクリル酸メチル、メタクリル酸ベンジル、メタクリル酸シクロヘキシル、アクリル酸メチルが好ましい。
【００２０】
本発明の共重合体の複屈折率は80×10^-6以下が好ましく、さらに好ましくは70×10^-6以下である。複屈折率が高すぎるものは、高い読みとり精度、書き込み精度が要求される光学材料には適していない。
【００２１】
またこれらの共重合体の数平均分子量Ｍｎは、50,000〜200,000が好ましく、更に好ましくは、70,000〜150,000である。数平均分子量Ｍｎが低すぎると共重合体の機械的強度が低下するため射出成形が困難となり、数平均分子量Ｍｎが高すぎると流動性が低下する。
【００２２】
また、共重合体の飽和吸水率は２％以下が好ましく、更に好ましくは１．５％以下である。飽和吸水率が高すぎると寸法変化や吸湿反りという問題点が出てくる。
【００２３】
本発明の共重合体の曲げ強度は60MPa以上が好ましい。曲げ強度が低すぎると成型時において成型品の割れが生じやすくなる。
【００２４】
共重合体の製造方法は一般に公知とされている重合方法、例えばラジカル重合、アニオン重合、グループトランスファー重合(GTP)、配位アニオン重合等が採用できる。
【００２５】
ラジカル重合において開始剤としてはAIBN等のアゾ化合物、BPO等の過酸化物の使用が可能であるが、共重合体の重合度を高めるためには低温で長時間重合することが好ましく、重合開始剤としては一分子中にＯ−Ｏ結合を２つ以上持つものが好ましい。
【００２６】
本発明の一分子中にＯ−Ｏ結合を２つ以上持つ重合開始剤としては特に限定されないが、日本油脂（株）製のパーテトラＡ、パーヘキサＭＣ、ペロマーＡＣ等が好ましい。これらの開始剤を使用した場合、効率よく短時間で高分子量の共重合体合成が可能である。
【００２７】
重合の際に使用する溶媒としては、トルエン、キシレン、ベンゼン、ヘキサン等の炭化水素、クロロホルム、塩化メチレン、四塩化炭素等のハロゲン化炭化水素、テトラヒドロフラン、ジメチルホルムアミド、ジメチルスルフォキシド、エーテル等が挙げられる。
【００２８】
重合におけるモノマーの濃度は特に限定されないが、４〜３０重量％が好ましく、更に好ましくは４〜２５重量％である。モノマー濃度が低すぎる場合には重合体収率が低くなり、モノマー濃度が高すぎる場合には分子間の重合が優先的に進み架橋反応を起こすため、部分的に目的の構造とならず、生成した重合体の溶融成形は不可能となる。
【００２９】
本発明の共重合体を用いて成型物を製造する際には、公知の溶融成型法、溶液成型法などが可能である。また、板状成形体を成形する場合には２枚のガラス板間にモノマーを注入し、重合するキャスト成型法も可能である。
【００３０】
更に、本発明の重合体は適量の強化剤、可塑剤、架橋剤、熱安定剤、着色剤、紫外線吸収剤、抗酸化剤、離型剤と併用して成型物を製造することができる。特に衝撃強度を向上させる強化剤、耐候性、抗酸化性を向上させる紫外線吸収剤、抗酸化剤の添加が好ましい。
【００３１】
本発明の共重合体の成型物は、その透明性、耐熱性、低吸水性が優れており、導光体、耐熱アクリル板として用いることができる。さらに低複屈折であるためレンズ、ディスク、光伝送材料として十分な性能を発揮できる。又さらにその耐候性が優れていることから表面被覆剤としても用いることができる。即ち、用途としては、ピックアップレンズ、レーザービームプリンター用fθレンズ、眼鏡レンズ、カメラレンズ、ビデオカメラレンズ、ランプレンズ等のレンズ材料、ビデオディスク、オーディオディスク、コンピューター用追記型ディスク等のディスク材料、光ファイバー、光コネクター、導光体等の光伝送材料、アクリル看板、アクリル水槽、光拡散板等のディスプレイ材料、ポリカーボネート表面コート材、ポリカーボネート積層シート等が挙げられる。その中でもピックアップレンズ、レーザービームプリンター用fθレンズ、光ディスク、光ファイバーのコア材、導光板、ポリカーボネート積層シート、ランプレンズ、光拡散性成形体に特に適している。
【００３２】
【実施例】
以下、実施例により本発明を具体的に説明する。なお、実施例で得られた共重合体およびその成型品の評価は以下の方法により行った。
【００３３】
１）数平均分子量Ｍｎと分子量分布（重量平均分子量Ｍｗ／Ｍｎ）
PMMAをスタンダードとしてGPC法（溶媒：クロロホルム）により求めた。
【００３４】
２）重合体組成
1H NMRにより定量した（溶媒：クロロホルム-d , 測定温度：60℃）。
【００３５】
３）飽和吸水率
ペレットを110℃で24時間乾燥し、シリンダー温度240℃、金型温度70℃で射出成形し、50×100×2mmの平板を得た。これを110℃で乾燥させた後、重量を測定し、ついで60℃の温水中に重量が平衡に達するまで浸漬した。平衡後の重量を測定し、次の式により求めた。
飽和吸水率（％）＝[（吸水重量−乾燥重量）／乾燥重量]×１００
４）全光線透過率
前述の厚さ2mmの平板を用い、ASTM D1003に従って測定した。
【００３６】
５）VICAT軟化温度
前述と同様の方法にて厚み3mmの平板を射出成形し、ASTM D1525に従って測定した。
【００３７】
６）曲げ強度
ASTM D790に従い測定した。
【００３８】
７）複屈折率
ペレットを110℃で24時間乾燥し、シリンダー温度245℃［比較例１の三菱レイヨン（株）製アクリペット（登録商標）ＶＨは240℃］、金型温度90℃（比較例１は70℃）で射出成形し、全長100mm、幅50mm、厚み2mmの平板を得た。偏光顕微鏡を用い、波長546ｎｍの光線でゲージから5cmの場所の光路差を測定した。測定値を厚みで割った値が、複屈折率である。
【００３９】
［実施例１］
容量２リットルのフラスコ中において4-t-ブチルシクロヘキシルアクリレート616g（4mol：シス／トランス=38/72)、純度75％のパラホルムアルデヒド160g（4mol)、1,4-ジアザビシクロ［2,2,2］オクタン60g（0.53mol）、p-メトキシフェノール616mg、t-ブチルアルコール120gを、空気バブリングを行いながら80℃で２０時間反応させた。反応終了後、3リットルのメタノール中にこの反応溶液を注ぎ、30分攪拌した。ついで、この混合溶液を0℃で一晩静置し,白色結晶のジ（4-t-ブチルシクロヘキシル）2,2'-[オキシビス（メチレン）]ビス-2-プロペノエート（以下「ＤＴＢＣＨＯＭＰ」と略す）を520ｇ（収率74.2％）得た。
【００４０】
溶媒としてトルエン480mlを含む１リットルのフラスコ中にＤＴＢＣＨＯＭＰを10g、メチルメタクリレート（MMA）90gを入れ溶解させた。この溶液に開始剤としてパーテトラA［日本油脂（株）製：純度20wt.％］600mgを加え窒素置換しながら40分攪拌した。その後、110℃に加熱して重合を開始させ７時間重合を行った。
【００４１】
この反応液を大量のメタノール中に入れてポリマーを析出させ、濾過しメタノールで洗浄した。これを真空乾燥することにより粉末状の重合体86ｇ（収率86％）を得た。この共重合体を評価し、表１の結果を得た。
【００４２】
［実施例２］
ＤＴＢＣＨＯＭＰを30g、メチルメタクリレート（MMA）70gとした以外は実施例１と同様にして共重合体90g（収率90％）を得た。この共重合体を評価し、表１の結果を得た。
【００４３】
［実施例３］
ＤＴＢＣＨＯＭＰを50g、メチルメタクリレート（MMA）50gとした以外は実施例１と同様にして共重合体92g（収率92％）を得た。この共重合体を評価し、表１の結果を得た。
【００４４】
［比較例１］
メタクリル樹脂として、アクリペットＶＨのペレットを用いて、実施例１と同様に評価し表１の結果を得た。
【００４５】
［実施例４］
実施例１，２，３の共重合体、及び比較例１のアクリペットＶＨを用いて各々コンパクトディスク用ピックアップレンズを成形した。これらのレンズについて60℃、90%ＲＨの雰囲気下で、干渉縞の変化を測定することにより球面収差変化を評価したところ、前者（実施例１、２、３）は後者（比較例１）に比べて変化が非常に小さく、多湿下でのレンズ性能が優れていた。
【００４６】
【表１】

【００４７】
【発明の効果】
本発明により、透明性、耐熱性、低吸水性及び機械的強度に優れた低複屈折の共重合体が提供される。この共重合体はその性質から光ピックアップレンズ等のレンズ、ディスク、光伝送体等の各種光学用途、アクリル看板等のディスプレイ材料に適用可能である。
【００４８】
本発明の製法によれば透明性、耐熱性、低吸水性及び機械的強度に優れた低複屈折の共重合体を高収率で容易に合成することができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a copolymer suitable for optical applications, a method for producing the same, and an optical pickup lens.
[0002]
[Prior art]
Methacrylic resins have properties that are balanced in mechanical properties, molding processability, weather resistance, and the like, and are used in various fields as sheet materials or molding materials. Furthermore, it has excellent optical properties such as transparency, low dispersion, and low birefringence. Recently, taking advantage of these characteristics, disk materials such as video discs, audio discs, write-once discs for computers, lens materials for cameras, video cameras, projection televisions, optical pickups, etc., as well as various optical transmissions such as optical fibers and optical connectors. Applications are expanding as a material.
[0003]
However, methacrylic resins have problems of high water absorption and low heat resistance. That is, dimensional changes and warping of molded products occur due to moisture absorption, and cracks occur due to repeated cycles of moisture absorption and drying. In particular, it is said that the influence is great for disk materials and optical pickup lenses, connectors and the like used in those optical systems. Moreover, since heat resistance is low, use, such as a vehicle-mounted application, may be restrict | limited. Furthermore, the same problem is pointed out also in the acrylic board. Furthermore, in recent years, with the increase in the density of recording media, further reduction in birefringence of optical resin materials such as disk materials and lenses is required, and there are some fields where birefringence on the order of polymethyl methacrylate is insufficient.
[0004]
Therefore, in recent years, many proposals have been made on improvement of hygroscopicity, improvement of heat resistance, low birefringence and the like while maintaining the optical properties of methacrylic resin. For example, as a method for imparting low water absorption to a methacrylic resin, a copolymer of methyl methacrylate and cyclohexyl methacrylate (JP-A 58-5318), a copolymer of methyl methacrylate, cyclohexyl methacrylate and benzyl methacrylate (JP-A 58) -13652). However, although the moisture absorption is improved, the heat resistance is lowered. Further, as a method of imparting heat resistance and low birefringence, a copolymer of methyl methacrylate and o-methylphenylmaleimide (JP-A-60-217216), a copolymer of methyl methacrylate and a maleimide compound A merger (Japanese Patent Laid-Open No. 61-95011) has been proposed. However, in this case as well, since a maleimide monomer is introduced, it generally has drawbacks such as large coloring. Further, as a method for imparting heat resistance and water absorption, a copolymer of methyl methacrylate and t-butylcyclohexyl methacrylate has been proposed (Japanese Patent Laid-Open No. 2-206609). However, these polymers also have the drawback that although the heat resistance and water absorption are improved, the mechanical strength is lowered.
[0005]
On the other hand, there is a method of suppressing the molecular motion and improving the heat resistance by making the polymer chain rigid. As such a polymer, a polymer having a tetrahydropyran ring in the main chain (hereinafter referred to as a pyran polymer) has been proposed (USP 4,889,948 and Lon J. Mathias, Polymer, 35 (15), 3317). , 1994). However, these polymers have low mechanical strength and are difficult to perform normal injection molding. Moreover, although the copolymer is disclosed by this literature, it is a crosslinked copolymer, Comprising: A molding does not melt | dissolve by heat and melt molding is impossible.
[0006]
A copolymer that can be melt-molded by increasing the purity of the monomer has also been proposed (USP 5,247,035).
[0007]
[Problems to be solved by the invention]
However, although this copolymer also has improved heat resistance, it has the disadvantage of low mechanical strength due to its low molecular weight. Thus, the present condition is that the copolymer excellent in the physical property balance of heat resistance, low water absorption, mechanical strength, and low birefringence has not been found.
[0008]
That is, such a conventional polymer is not satisfactory as a pickup lens, an optical disk, a lens, and an optical transmission material.
[0009]
An object of the present invention is to provide a methacrylic molding material that is excellent in transparency, heat resistance, low water absorption and mechanical strength, and has low birefringence.
[0010]
[Means for Solving the Problems]
The present inventors have found that the above object can be achieved by using a pyran polymer having a specific t-butylcyclohexyl group.
[0011]
That is, the gist of the present invention resides in a copolymer substantially composed of a repeating unit of the following general formula (1) and a repeating unit of (meth) acrylic acid ester.
[0012]
[Chemical 3]

[0013]
(In the formula, R ¹ represents a t-butylcyclohexyl group, and n represents a positive integer.)
In addition, the gist of the present invention is a peroxide having an α-substituted acrylate monomer represented by the following general formula (2) and a (meth) acrylate ester and having two or more O—O bonds in one molecule. In the production method of the above copolymer to be copolymerized.
[0014]
[Formula 4]

[0015]
(In the formula, R ¹ represents a t-butylcyclohexyl group.)
Furthermore, the gist of the present invention resides in an optical pickup lens molded from the above copolymer or a copolymer obtained by the above production method.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
The copolymer of the present invention can be obtained by copolymerizing an α-substituted acrylate monomer represented by the general formula (2) and a (meth) acrylate ester. During the polymerization, the monomer of the general formula (2) undergoes a cyclization reaction to form a pyran ring of the general formula (1).
[0017]
The composition ratio of the repeating unit of the general formula (1) and the repeating unit of the (meth) acrylic acid ester of the copolymer of the present invention is not particularly limited, but the weight composition of the general formula (1) is determined from the characteristics as an optical material. The range is preferably 10 to 70% by weight, more preferably 10 to 60% by weight. When the composition of the general formula (1) is too small, heat resistance and water absorption are insufficient, and when the composition is too large, the mechanical strength is lowered.
[0018]
The t-butylcyclohexyl group represented by the general formula (1) of the present invention includes ortho, meta, and para isomers, and further includes cis and trans isomers. Any of them can be used in the present invention, and any proportion of the mixture can be used. However, a para-substituted product is preferred, and a trans content of 65% or more is preferred.
[0019]
Examples of the (meth) acrylic acid ester of the copolymer component include, for example, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate. , Methacrylates such as isoamyl methacrylate, lauryl methacrylate, phenyl methacrylate, benzyl methacrylate, cyclohexyl methacrylate, glycidyl methacrylate, 2-ethylhexyl methacrylate, and methyl acrylate, ethyl acrylate, n-propyl acrylate , Isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, isoamyl acrylate, lauryl acrylate, phenyl acrylate, benzyl acrylate, cyclohexyl acrylate , Glycidyl acrylate, acrylic acid esters such as 2-ethylhexyl acrylate. Of these, methyl methacrylate, benzyl methacrylate, cyclohexyl methacrylate, and methyl acrylate are preferable.
[0020]
The birefringence of the copolymer of the present invention is preferably 80 × 10 ⁻⁶ or less, more preferably 70 × 10 ⁻⁶ or less. Those having a birefringence index that is too high are not suitable for optical materials that require high reading accuracy and writing accuracy.
[0021]
Further, the number average molecular weight Mn of these copolymers is preferably 50,000 to 200,000, and more preferably 70,000 to 150,000. If the number average molecular weight Mn is too low, the mechanical strength of the copolymer is lowered, so that injection molding becomes difficult. If the number average molecular weight Mn is too high, the fluidity is lowered.
[0022]
Further, the saturated water absorption of the copolymer is preferably 2% or less, more preferably 1.5% or less. If the saturated water absorption rate is too high, problems such as dimensional change and moisture absorption warp will appear.
[0023]
The bending strength of the copolymer of the present invention is preferably 60 MPa or more. If the bending strength is too low, the molded product is liable to crack during molding.
[0024]
As a method for producing the copolymer, generally known polymerization methods such as radical polymerization, anionic polymerization, group transfer polymerization (GTP), and coordinated anionic polymerization can be employed.
[0025]
In radical polymerization, azo compounds such as AIBN and peroxides such as BPO can be used as initiators. However, in order to increase the degree of polymerization of the copolymer, it is preferable to perform polymerization at a low temperature for a long time. As the agent, those having two or more OO bonds in one molecule are preferable.
[0026]
The polymerization initiator having two or more OO bonds in one molecule of the present invention is not particularly limited, but Pertetra A, Perhexa MC, Peromer AC, etc. manufactured by NOF Corporation are preferable. When these initiators are used, a high molecular weight copolymer can be synthesized efficiently in a short time.
[0027]
Solvents used in the polymerization include hydrocarbons such as toluene, xylene, benzene and hexane, halogenated hydrocarbons such as chloroform, methylene chloride and carbon tetrachloride, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide, ether and the like. Can be mentioned.
[0028]
The concentration of the monomer in the polymerization is not particularly limited, but is preferably 4 to 30% by weight, more preferably 4 to 25% by weight. If the monomer concentration is too low, the polymer yield will be low, and if the monomer concentration is too high, the intermolecular polymerization will preferentially proceed and cause a crosslinking reaction, so that the target structure will not be partly produced. The resulting polymer cannot be melt molded.
[0029]
When a molded product is produced using the copolymer of the present invention, a known melt molding method, solution molding method or the like can be used. Further, in the case of molding a plate-like molded body, a cast molding method in which a monomer is injected between two glass plates and polymerized is also possible.
[0030]
Furthermore, the polymer of the present invention can be used in combination with an appropriate amount of a reinforcing agent, a plasticizer, a crosslinking agent, a heat stabilizer, a colorant, an ultraviolet absorber, an antioxidant, and a release agent to produce a molded product. In particular, it is preferable to add a reinforcing agent that improves impact strength, a weather resistance, an ultraviolet absorber that improves antioxidant properties, and an antioxidant.
[0031]
The molded product of the copolymer of the present invention is excellent in transparency, heat resistance, and low water absorption, and can be used as a light guide and a heat-resistant acrylic plate. Furthermore, since it has a low birefringence, it can exhibit sufficient performance as a lens, a disk, and an optical transmission material. Further, since it has excellent weather resistance, it can also be used as a surface coating agent. In other words, applications include pickup lenses, fθ lenses for laser beam printers, eyeglass lenses, camera lenses, video camera lenses, lamp lenses, and other lens materials, video discs, audio discs, disc write-on discs for computers, optical fibers, etc. , Optical transmission materials such as optical connectors and light guides, acrylic signboards, acrylic water tanks, display materials such as light diffusion plates, polycarbonate surface coating materials, polycarbonate laminate sheets and the like. Among them, it is particularly suitable for pickup lenses, fθ lenses for laser beam printers, optical disks, optical fiber core materials, light guide plates, polycarbonate laminate sheets, lamp lenses, and light diffusing molded articles.
[0032]
【Example】
Hereinafter, the present invention will be described specifically by way of examples. The copolymers obtained in the examples and their molded products were evaluated by the following methods.
[0033]
1) Number average molecular weight Mn and molecular weight distribution (weight average molecular weight Mw / Mn)
It was determined by GPC method (solvent: chloroform) using PMMA as a standard.
[0034]
2) Polymer composition
Quantified by 1H NMR (solvent: chloroform-d, measurement temperature: 60 ° C.).
[0035]
3) The saturated water absorption pellets were dried at 110 ° C. for 24 hours and injection molded at a cylinder temperature of 240 ° C. and a mold temperature of 70 ° C. to obtain a flat plate of 50 × 100 × 2 mm. After drying this at 110 ° C., the weight was measured and then immersed in warm water at 60 ° C. until the weight reached equilibrium. The weight after equilibration was measured and determined by the following formula.
Saturated water absorption (%) = [(water absorption weight−dry weight) / dry weight] × 100
4) Total light transmittance Measured in accordance with ASTM D1003 using the 2 mm thick flat plate described above.
[0036]
5) VICAT softening temperature A 3 mm thick flat plate was injection-molded by the same method as described above, and measured according to ASTM D1525.
[0037]
6) Bending strength
Measured according to ASTM D790.
[0038]
7) The birefringent pellets were dried at 110 ° C. for 24 hours, the cylinder temperature was 245 ° C. (Acrypet (registered trademark) VH manufactured by Mitsubishi Rayon Co., Ltd. in Comparative Example 1 was 240 ° C.), and the mold temperature was 90 ° C. (Comparative Example). 1 was 70 ° C.) to obtain a flat plate having a total length of 100 mm, a width of 50 mm and a thickness of 2 mm. Using a polarizing microscope, the optical path difference at a location 5 cm from the gauge was measured with a light beam having a wavelength of 546 nm. The value obtained by dividing the measured value by the thickness is the birefringence.
[0039]
[Example 1]
In a 2 liter flask, 616 g of 4-t-butylcyclohexyl acrylate (4 mol: cis / trans = 38/72), 160 g (4 mol) of paraformaldehyde with a purity of 75%, 1,4-diazabicyclo [2,2,2] Octane 60 g (0.53 mol), p-methoxyphenol 616 mg, and t-butyl alcohol 120 g were reacted at 80 ° C. for 20 hours while performing air bubbling. After completion of the reaction, the reaction solution was poured into 3 liters of methanol and stirred for 30 minutes. Next, this mixed solution was allowed to stand at 0 ° C. overnight, and white crystalline di (4-t-butylcyclohexyl) 2,2 ′-[oxybis (methylene)] bis-2-propenoate (hereinafter abbreviated as “DTBCCHOMP”). 520 g (yield 74.2%).
[0040]
In a 1 liter flask containing 480 ml of toluene as a solvent, 10 g of DTBCHOMP and 90 g of methyl methacrylate (MMA) were dissolved. To this solution, 600 mg of pertetra A [manufactured by Nippon Oil & Fats Co., Ltd .: purity 20 wt.%] Was added as an initiator, followed by stirring for 40 minutes while purging with nitrogen. Then, it heated to 110 degreeC and superposition | polymerization was started and superposition | polymerization was performed for 7 hours.
[0041]
The reaction solution was put in a large amount of methanol to precipitate a polymer, filtered and washed with methanol. This was vacuum dried to obtain 86 g (yield 86%) of a powdery polymer. This copolymer was evaluated and the results shown in Table 1 were obtained.
[0042]
[Example 2]
90 g of copolymer (yield 90%) was obtained in the same manner as in Example 1 except that 30 g of DTBCHOMP and 70 g of methyl methacrylate (MMA) were used. This copolymer was evaluated and the results shown in Table 1 were obtained.
[0043]
[Example 3]
A copolymer 92 g (yield 92%) was obtained in the same manner as in Example 1 except that 50 g of DTBCHOMP and 50 g of methyl methacrylate (MMA) were used. This copolymer was evaluated and the results shown in Table 1 were obtained.
[0044]
[Comparative Example 1]
Evaluation was performed in the same manner as in Example 1 using acrylpet VH pellets as the methacrylic resin, and the results shown in Table 1 were obtained.
[0045]
[Example 4]
Using the copolymers of Examples 1, 2, and 3 and the Acrypet VH of Comparative Example 1, a compact disk pickup lens was molded. When these lenses were evaluated for changes in spherical aberration by measuring changes in interference fringes in an atmosphere of 60 ° C. and 90% RH, the former (Examples 1, 2, and 3) were changed to the latter (Comparative Example 1). Compared with the change, the lens performance under high humidity was excellent.
[0046]
[Table 1]

[0047]
【The invention's effect】
According to the present invention, a low birefringence copolymer excellent in transparency, heat resistance, low water absorption and mechanical strength is provided. Due to its properties, this copolymer can be applied to various optical applications such as lenses such as optical pickup lenses, disks and optical transmission bodies, and display materials such as acrylic signs.
[0048]
According to the production method of the present invention, a low birefringence copolymer excellent in transparency, heat resistance, low water absorption and mechanical strength can be easily synthesized in high yield.

Claims (3)

A copolymer substantially consisting of a repeating unit of the following general formula (1) and a repeating unit of (meth) acrylic acid ester.

(In the formula, R ¹ represents a t-butylcyclohexyl group, and n represents a positive integer.) Copolymerization of α-substituted acrylate monomer represented by the following general formula (2) and (meth) acrylate ester with a peroxide having two or more O—O bonds in one molecule as a polymerization initiator To make a copolymer.

(In the formula, R ¹ represents a t-butylcyclohexyl group.) An optical pickup lens molded from the copolymer according to claim 1 or a copolymer obtained by the production method according to claim 2.