JPH05173026A - Production of synthesized resin light transmission body - Google Patents

Abstract

PURPOSE:To improve defective points of a production method for refractive index distribution type synthetic resin light transmission body by polymn. reaction, and to provide a production method by which a multimode light transmission body having excellent characteristics and continuous distribution of refractive index can be produced with good productivity. CONSTITUTION:Monomers dissolving a transparent polymer are gradually supplied to a cylindrical chamber whose wall is constituted of the transparent polymer and which is horizontally held and rotates around the center axis. Gelatinized layer is successively moved from the chamber wall to the inner part of the chamber while dissolving the transparent polymer into the monomers. Thus, the synthetic resin light transmission body having continuous slope of refractive index in one direction is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a graded index type optical fiber which is a multimode optical transmission body made of a transparent synthetic resin having an arbitrary refractive index distribution in which the refractive index continuously changes along a fixed direction. The present invention relates to a method for manufacturing a transmitter. More specifically, a multimode synthetic resin optical transmission medium having a refractive index distribution in which the transparent polymer forming the container wall is dissolved in the monomer to polymerize the monomer, thereby continuously changing the refractive index in a certain direction. Manufacturing method.

[0002]

2. Description of the Related Art An optical transmission medium having a refractive index distribution in which the refractive index continuously changes in a certain direction is widely used as a rod lens having a convex lens action, a rod lens having a concave lens action, or a broadband optical transmission fiber. Among them, the transparent synthetic resin optical transmission body is superior in lightness, economy, handleability, impact resistance, flexibility, etc. to the quartz optical transmission body, and has been widely used in recent years. ..

Optical transmission bodies are roughly classified into single mode type and multimode type according to the mode of light to be transmitted. In the multimode type, the core radius is sufficiently larger than the wavelength of light to be transmitted, and therefore, light of many modes can be transmitted. The multimode type is further classified into a step index type in which the refractive index changes stepwise between the core and the clad and a graded index type in which the refractive index changes gradually and continuously according to the form of the refractive index gradient.

As a method for producing a multi-mode type graded index optical transmission body made of synthetic resin by a polymerization reaction in a specific container, the following methods have been conventionally proposed. That is, in JP-B-52-5857, a monomer which forms a polymer having a refractive index different from that of a transparent solid object having a three-dimensional network structure produced in advance and undergoing a specific polymerization reaction is diffused and moved. After
A method for terminating the entire polymerization reaction to obtain a graded index optical transmitter is proposed. However, in this method, in order to maintain the shape of the transparent solid object produced in advance, it is necessary to make the transparent solid object into a three-dimensional network structure by using a polyfunctional radically polymerizable monomer. For this reason, in addition to the labor required to separately manufacture a transparent solid in advance, the polymer produced has a three-dimensional network structure, and the thermoplasticity becomes small, which makes post-processing such as stretching difficult. There is. It is preferable that a synthetic resin optical transmission fiber, which is usually practical, is subjected to a stretching treatment in the manufacturing process thereof so as to give the elasticity and tensile strength of the fiber. However, since the optical transmission body obtained by the above method essentially has a three-dimensional network structure, it is difficult to stretch it.

In Japanese Patent Publication No. 54-30301 and Japanese Patent Publication No. 61-130904, attention is paid to the difference in the monomer reactivity ratios r ₁ and r ₂ of two kinds of monomers, and an optical transmission medium having a refractive index gradient is disclosed. Proposing a manufacturing method. But,
In the above method utilizing the difference in the monomer reactivity ratio of the monomers, naturally, the larger the difference between the radical copolymerization reactivity ratios r ₁ and r ₂ , the more preferable, and as a result, the formation of the homopolymer takes precedence. In some cases, homopolymer macromolecules are generated and phase-separated, so that the obtained optical transmission medium may be clouded and the optical transmission efficiency may be reduced. Further, in order to increase the difference in monomer reactivity ratio, for example, Japanese Patent Publication No.
Vinyl benzoate, vinyl o-chlorobenzoate used in the examples of JP-A-30301,
In some cases, a monomer having a low polymerization rate, such as phenyl vinyl acetate used in the examples of 1-130904, must be used as one of the monomers.
To use a monomer having a large difference in monomer reactivity ratio means to use a monomer having a considerably poor copolymerizability. As a result, the monomer having good polymerization reactivity is polymerized first, and in the final stage of the polymerization reaction, the monomer having poor polymerization reactivity is present at a high concentration,
It takes time to complete the polymerization reaction due to the polymerization of the residual monomer, and in some extreme cases, removal of the residual monomer may be necessary. Furthermore, the presence of the residual monomer adversely affects the tensile strength, elongation, mechanical strength such as stiffness of the optical transmission medium, and long-term stability of the transmission medium due to post-polymerization or decomposition of the residual monomer.

Here, the present inventors have deeply considered the process of producing a copolymer resin by a radical reaction. That is, when the viscosity of the monomer liquid rises into a gel state as the monomer is polymerized, the growing polymer radicals have a large molecular weight, so that diffusion in the gel becomes difficult. In such a case, the bimolecular reaction between the growing polymer radicals as a termination reaction in the radical reaction is difficult to proceed, and as a result, the polymerization rate increases. In order for the polymerization to proceed and the growing polymer radicals to grow further, it is necessary that the raw material monomers diffuse into the gel and continue to be sufficiently supplied to the growing polymer radicals. The phenomenon as described above is recognized in the radical polymerization as a so-called gel effect. Then, if radical polymerization is carried out from any one end of the reaction liquid in the container so as to produce this gel effect, the polymerization proceeds sequentially from a given polymerization start end along a certain traveling direction, and finally to the other end. proceed. Here, the present inventors have further considered the process of polymerizing the monomer in a container made of a polymer that is soluble in the monomer to be polymerized. That is, in this case, the polymer on the container wall is dissolved in the monomer, and the dissolved polymer is selectively removed from the gel to the outside of the gel. During the polymerization, the dissolution and outflow of the polymer on the vessel wall are continued, and the polymer is continuously discharged out of the gel. The concentration of the eluted polymer gradually decreases as it moves away from the wall, so that the resulting polymer has a continuously varying compositional distribution with respect to the initially charged monomers along the direction of polymerization. ..

Further, a monomer that dissolves the polymer on the container wall is gradually supplied into a cylindrical container that is horizontally held and rotates around a central axis, and the gel layer is dissolved while the polymer is dissolved in the monomer. It was found that a more uniform polymer is produced by radically polymerizing the monomer while sequentially moving the monomer from the container wall toward the inside of the container. Based on such knowledge, the present inventors have invented a method for producing a synthetic resin optical transmission body according to a new copolymerization method.

[0008]

SUMMARY OF THE INVENTION In view of the above points, the present invention has improved the drawbacks of the conventional method for producing a gradient index synthetic resin optical transmission medium by a polymerization reaction, and has a new polymerization state. Based on the findings, it is an object to provide a manufacturing method by which a multimode optical transmission body having a continuous refractive index distribution with excellent characteristics can be obtained with extremely high productivity.

[0009]

According to the present invention, a monomer which dissolves a transparent polymer is gradually added to a cylindrical container whose container wall is made of a transparent polymer and which is horizontally held and rotates around a central axis. And dissolving the transparent polymer in the monomer, the gel layer is sequentially moved from the container wall toward the inside of the container to radically polymerize the monomer. The present invention relates to a method for producing a graded index type synthetic resin light transmission body having a composition in which the concentration of a transparent polymer decreases in the gel moving direction, thereby having a continuous refractive index gradient in a certain direction. Further, the second invention relates to the above invention in which the solubility parameter δ _M of the monomer and the solubility parameter δ _P of the polymer constituting the container wall satisfy the following equation (2).

[Equation 2]

The present invention will be further described below. The present invention is
A specific monomer is gradually supplied into a cylindrical container, and the radical copolymerization reaction is sequentially advanced from the inner wall portion in contact with the monomer in the container toward the central part of the container by utilizing the gel effect. The size of the container is not particularly limited, and any size can be adopted. For example, an inner diameter of 1 to 70 mm is suitable. The transparent polymer that constitutes the container wall that contacts the monomer must be one that is soluble in the monomer to be polymerized. From this point, in general, if the molecular weight is too large, the solubility in the monomer decreases, which is not preferable. Further, if the molecular weight is too small, the mechanical strength for forming the container wall is insufficient. From this point of view, the molecular weight is usually appropriately selected from the range of average molecular weight of 10,000 to 1,000,000, and more preferably 3 to 2
It is in the range of 0,000. Since this transparent polymer has solubility in the monomer, it usually has 1 or more radical polymerization active groups.
It is a polymer of one kind or two or more kinds of monofunctional monomers each having one unit.

A solvent may be used in the polymerization of the present invention. However, when a solvent is used, a solvent removing step after the polymerization is required and a harmful effect is caused by the removal of the solvent. Therefore, a solvent is usually used. Instead, it is desirable to carry out the polymerization reaction by using the monomer itself instead of the solvent.

For example, first, energy for radical generation such as heat and ultraviolet rays is locally applied from the side of the container wall by an appropriate publicly known method so that the temperature of the monomer liquid portion contacting the inner wall of the container is high or the intensity of ultraviolet rays is high. A large portion is generated, whereby a high concentration of radicals is generated in the same portion, and the polymerization reaction proceeds preferentially. In order to make the heating or irradiation environment uniform and to use the centrifugal force to make the monomer have the same thickness from the inside to the center of the cylindrical container, keep the cylindrical container horizontal and It is necessary to rotate around. The number of rotations of the container may be at least the number of rotations required for the supplied monomer mixed liquid to generate a liquid film of uniform thickness on the cylindrical container wall by centrifugal force, and the value varies depending on the diameter of the container, For example, diameter 1
For a cylindrical container of about 0 mm, 800 rpm or more, preferably about 1000 rpm may be used. However, it is not preferable to subject the container to mechanical operations such as non-uniform stirring or excessive rotation such that the gelled state of the monomer is destroyed or disturbed.

The wavelength of ultraviolet rays, the heating temperature, etc. for radical polymerization can be arbitrarily selected depending on the kind of the monomer used. For example, the heating temperature range is from room temperature to 150.
The range of ° C is illustrated. In any case, an arbitrary amount of a known radical polymerization initiator such as benzoyl peroxide (BPO) or a photopolymerization sensitizer is mixed if necessary. Further, photopolymerization and thermal polymerization can be used in combination.

When the radical polymerization reaction progresses and the viscosity of the monomer mixed solution increases and a gelled state is developed, the polymer-grown radicals existing in the gel become difficult to diffuse in the gel, and the termination reaction of the polymerization reaction occurs. Is less likely to occur. As a result, the polymerization rate in the gel portion is increased.
The radical growth terminal in the gel is further bonded to the unreacted monomer inside the gel and the polymerization progresses to the final resin.At the same time, the gel is sequentially generated and polymerized in the direction of the progress of polymerization on the front surface of the polymerized resin. Go. In this way, the polymerization reaction can be sequentially advanced from the wall side in the container toward the inside of the container while utilizing the gel effect. The polymerization initiation end is the wall side in the container where the transparent polymer starts to dissolve.

The term "gel" as used herein means an oligomer or polymer whose viscosity has increased to such an extent that polymer-grown radicals are substantially difficult to diffuse therein. Therefore, the gel in the present invention is not a so-called three-dimensional network polymer.
Further, in some cases, the produced gel may be precipitated from the monomer liquid. However, it does not include a polymer whose degree of polymerization is so high that it is impossible for the monomer to move within the resulting gel. It should be noted that if the polymerization rate is too fast, the polymerization is completed without developing a clear gelled state, which is not preferable. From this point of view, the polymerization rate is appropriately determined so that the polymerization time is such that the monomer can be sufficiently moved in the gel. It is usually selected from the range of 1 to 100 hours.

The monomer used in the present invention is a monomer that dissolves the transparent polymer that constitutes the container wall as described above, and it is not limited to one type, and may be used as a mixture of any number of two or more types of monomers. .. For example, a monomer in which the difference between the refractive index when the monomer is a homopolymer and the refractive index of the transparent polymer is at least 0.005 and which dissolves the transparent polymer is gradually added to the container. Alternatively, it may be divided into two or more parts and supplied.
When the difference in refractive index is smaller than this, even if a clear composition distribution is obtained, the required refractive index distribution is not exhibited. Or a mixture of monomers which are transparent when polymerized and which give at least two different polymers having a difference in refractive index of at least 0.005, initially a monomer component which gives a low refractive index polymer. To increase the amount of the monomer component that gradually gives a high refractive index polymer, or increase the amount of the monomer component that gives a high refractive index polymer at the beginning to increase the amount of the monomer component that gradually gives a low refractive index polymer. It is also possible to supply the mixture gradually or divided into two or more times while changing the mixing ratio so as to achieve the above.

When the monomer is supplied at a constant rate, the monomer may be gradually supplied by spending about 5 to 50 hours. In addition, when the monomer is fed in two or more times, it is desirable to carry out the next feeding after the monomer fed last time is almost polymerized. In any case, if the monomer is supplied too fast, the transparent polymer that constitutes the container wall cannot be sufficiently dissolved, so that it is difficult to distribute it to the vicinity of the center or the heat of polymerization is insufficiently removed, and foaming occurs. It will be easier.

In a more preferred combination of the monomer of the present invention and the transparent polymer constituting the container wall, the solubility parameter δ _M of the monomer and the solubility parameter δ _P of the transparent polymer constituting the container wall are represented by the following formula: This satisfies the expression (3).

[Equation 3] Here, the solubility parameter of the monomer and the polymer can be calculated by the following equation (4). [HOY
et al. method (POLYMAER HANDBOOK, Third edi-tion,
VII / 519 (issued by Wiley Interscience))).

[Equation 4] Here, d and M represent the density and molecular weight of the monomer or polymer, respectively. G is Group Molar At-traction
Constant). Use of a monomer or a transparent polymer that does not satisfy the above range is not preferable because a resin having a poor composition distribution can be obtained. for reference,
The solubility parameters and the like of some radically polymerizable monomers are shown in Table 1 below.

[0019]

[Table 1]

The monomer and transparent polymer used in the present invention are not particularly limited as long as they satisfy the conditions specified in the present invention. The radical-polymerizable monomer used in the present invention is a monofunctional monomer having one radical bond having a functional group having radical polymerization activity, such as an allyl group, an acryl group, a methacryl group, and a vinyl group, It does not include polyfunctional monomers capable of forming a three-dimensional network polymer. However, a small amount of these polyfunctional monomers may be mixed and used within the scope of the object of the present invention. Specific examples of the combination of the monomer and the transparent polymer usable in the present invention are shown in Table 2.

[0021]

[Table 2]

Among these, methyl methacrylate /
When benzyl methacrylate copolymer is used as the polymer constituting the container wall and only methyl methacrylate is polymerized as a monomer, or a homopolymer of methyl methacrylate is used as the polymer constituting the container wall, and methyl methacrylate / benzyl methacrylate is used as the monomer. Polymerization is particularly preferable from the viewpoints of easy availability of the monomer or polymer and transparency of the obtained light transmission body.

Although one kind of monomer has been described as an example for ease of explanation, the monomer to be used is 1 as shown in the example of Table 2 as long as the conditions specified in the present invention are satisfied. Not only the species but also a mixture of two or more species of any number of monomers can be used. When using a mixture of two or more kinds of monomers, it is necessary to satisfy the above conditions between the respective monomers. Further, the polymer constituting the container wall may be a homopolymer or a copolymer as long as the above conditions are satisfied. In addition to the radical polymerization initiator, any additive such as a chain transfer agent and an antioxidant can be added as long as the transparency of the produced polymer is not impaired.

The completed cylindrical polymer can be polymerized by injecting a monomer having a composition close to the center, if necessary, or at a temperature above the glass transition point while depressurizing the inside of the cylinder. A transparent rod-shaped resin having a refractive index gradient in the radial direction, which fills the inside, can be obtained by a method of removing the internal cavity by processing. This can be used as it is or after being subjected to appropriate processing as an optical transmission body. For example, the obtained rod can be stretched by a known stretching method at an appropriate stretching ratio to form a synthetic resin-made optical fiber for optical transmission.

A cylindrical transmission body having a high refractive index distribution in the central portion is a rod-shaped lens having a convex lens function, an optical fiber for optical communication, etc., and a central portion has a low refractive index refractive index distribution. The columnar transmission body can be used as an optical transmission body such as a rod lens having a concave lens function. By reacting in a container having a rectangular cross section, a plate lens having a convex lens effect or a concave lens effect can be produced.

[0026]

The present invention will be further described with reference to examples. <Example 1> Methyl methacrylate (MMA) and 1,1,2-trifluoroethyl methacrylate (3FMA) were charged in a glass tube held horizontally at a weight ratio of 4: 1.
After sealing both ends, heat polymerization according to the usual method while rotating at 1000 rpm, the outer diameter is 20 m.
A polymerization tube made of MMA / 3FMA copolymer having m and an inner diameter of 15 mm and a molecular weight of 100,000 was obtained. After breaking the outer glass tube to remove it, 0.15% by weight of n-butyl mercaptan as a chain transfer agent and benzoylperm as a polymerization initiator were placed in the polymerization tube which was held horizontally and rotated around the central axis. MMA containing 0.50% by weight of oxide (BPO) was gradually added over a period of 10 hours to obtain 7
The reaction was carried out at 0 ° C. Thermal polymerization was carried out at the same temperature for 20 hours in the air. During polymerization, the polymerization tube rotates at 1000 rpm
Rotated at rpm. After the polymerization, MMA containing the chain transfer agent and the polymerization initiator was filled in the central portion again to carry out the polymerization reaction, and a transparent rod-shaped resin having a solid interior could be obtained. Furthermore, as a reduced pressure heat treatment, the pressure was reduced to 0.2 mmHg, and 2
Hold at 80 ° C. for 0 hours. When the content of the residual monomer in the produced polymer was measured, the content was 0.5% by weight or less. Since the polymer inside the polymerization tube and the polymerization tube were integrated, then cut both ends while keeping this as one,
An optical fiber having a diameter of 0.6 mm was obtained by hot drawing while performing indirect heating in a cylindrical heating cylinder set at 250 ° C. When the refractive index distribution in the radial direction of the obtained optical fiber was measured by the lateral interference method, it had a continuous distribution shown in FIG. 1 that was uniform over almost the entire length. In FIG. 1, the vertical axis represents the difference (Δn) between the highest refractive index and the refractive index at a specific distance (hereinafter the same).

Example 2 MMA containing 0.15% by weight of n-butyl mercaptan as a chain transfer agent and 0.50% by weight of BPO as a polymerization initiator was added at intervals of 3 hours.
A fiber having substantially the same refractive index distribution as in FIG. 1 was obtained by performing the same operation as in Example 1 except that each of the components was supplied one by one and that the inside of the pipe was depressurized and maintained at 130 ° C. for 10 hours after the polymerization. Got

<Example 3> MMA and benzyl methacrylate (BzMA) were charged in a glass tube held horizontally at a weight ratio of 4: 1 and the both ends were sealed.
By thermal polymerization according to a conventional method while rotating at rpm, an outer diameter of 20 mm, an inner diameter of 15 mm, and a molecular weight of 100,000 MMA /
A polymerization tube made of a BzMA copolymer was obtained. After breaking and removing the outer glass tube, M was prepared in exactly the same manner as in Example 1 in the polymerization tube which was held horizontally and rotated around the central axis.
The MA was polymerized. After the polymerization, post-treatment and reduced pressure treatment were carried out in the same manner as in Example 1. As a result, the content of residual monomers in the polymer was 0.5% by weight or less. Further, the same treatment as in Example 1 was carried out to obtain an optical fiber having a diameter of 0.6 mm. When the refractive index distribution in the radial direction of the obtained optical fiber was measured by the lateral interference method, it had a continuous distribution shown in FIG.

Example 4 MMA was charged in a glass tube held horizontally, and both ends were sealed. Then, in the same manner as in Example 1, an MM having an outer diameter of 20 mm and an inner diameter of 15 mm and a molecular weight of 80,000 was used.
A polymerization tube made of the A polymer was obtained. After breaking the outer glass tube to remove it, 0.15% by weight of n-butyl mercaptan as a chain transfer agent and BPO as a polymerization initiator were placed in the polymerization tube which was held horizontally and rotated around the central axis. Polymerization reaction was carried out at 70 ° C. while supplying 0.50% by weight of MMA and BzMA at a charge weight ratio of 8: 1, 5: 1 and 4: 1 in this order every 3 hours. Further thermal polymerization was performed at the same temperature for 20 hours in the atmosphere. During the polymerization, the polymerization tube was rotated at 1000 rpm. After polymerization, heat treatment at 130 ° C for 10 hours while reducing the pressure inside the pipe,
It was possible to obtain a transparent rod-shaped resin with a full interior. Furthermore, as a reduced pressure heat treatment, the pressure was reduced to 0.2 mmHg, and
Hold at 80 ° C. for hours. When the content of the residual monomer in the produced polymer was measured, the content was 0.5% by weight or less. Since the polymer inside the polymerization tube and the polymerization tube were integrated, next, cut both ends while keeping this as a unit.
An optical fiber having a diameter of 0.6 mm was obtained by hot drawing while performing indirect heating in a cylindrical heating cylinder set at 50 ° C. When the refractive index distribution in the radial direction of the obtained optical fiber was measured by the lateral interference method, the continuous distribution was almost the same as that of the fiber obtained in Example 1 (FIG. 1).
Had.

<Examples 5 to 8> Polymerization tubes having an outer diameter of 10 mm and an inner diameter of 6 mm and a molecular weight of 100,000 were obtained in the same manner as in Example 1 by using the "monomers forming a polymerization tube" shown in Table 3 below. It was In Example 5, MMA homopolymer, Example 6
Then St / CHMA copolymer, Example 7 is MMA / 3F
It consists of a MA copolymer and in Example 8 an MMA / 5FMA copolymer. The "monomer forming the inside of the polymer" shown in Table 3 was charged into each of these polymerization tubes and heat-polymerized in the same manner as in Example 1. In the fifth embodiment, MMA / St,
St was used in Example 6, MMA / BzMA was used in Example 7, and MMA was used in Example 8. After thermal polymerization, the same treatment as in Example 1 was performed to obtain optical fibers each having a diameter of 0.6 mm. When the refractive index distributions of the obtained optical fibers were measured in the radial direction, it was found that each had a convex refractive index distribution (FIG. 1) as in Example 1.

<Examples 9 to 11> Polymerization tubes having an outer diameter of 10 mm and an inner diameter of 6 mm and a molecular weight of 100,000 were obtained in the same manner as in Example 1 by using the "monomer forming a polymerization tube" shown in Table 3. .. In Example 9, MMA / PhMA copolymer,
Example 10 is a MMA / St copolymer and Example 11
Then, it consists of an MMA / PEMA copolymer. As shown in Table 3, MMA was charged in each of these polymerization tubes as a “monomer forming the inside of the polymer” in each case.
Thermal polymerization was carried out in the same manner as in. After thermal polymerization, the same treatment as in Example 2 was carried out to obtain optical fibers each having a diameter of 0.6 mm. When the refractive index distributions of the obtained optical fibers were measured in the radial direction, it was found that each had a concave refractive index distribution (FIG. 2) as in Example 3.

[0032]

[Table 3]

[0033]

EFFECTS OF THE INVENTION According to the present invention, in the conventional method using a monomer having a low monomer copolymerizability ratio, there is a risk of phase separation and turbidity due to the formation of a homopolymer, and a residue due to a large difference in polymerization rate. Problems such as the monomer problem and the time required for completion of the reaction are improved, and an optical transmission medium having a good continuous refractive index gradient, for example, a multimode graded index (GI) type optical fiber is obtained. According to the present invention, it is also possible to use substantially one type of monomer that participates in the polymerization reaction carried out in the container, and as a result, the progress of the polymerization reaction can be monitored and compared with the method using a plurality of monomers. The control becomes extremely easy.

[Brief description of drawings]

FIG. 1 is a graph showing a refractive index distribution in the radial direction of an optical fiber obtained in Example 1.

2 is a graph showing the refractive index distribution in the radial direction of the optical fiber obtained in Example 3. FIG.

 ─────────────────────────────────────────────────── (72) Inventor Shogo Miyata 3-32-1, Sakaemachi-dori, Tsurumi-ku, Yokohama-shi, Kanagawa (72) Inventor Kaede Terauchi 8-3-103 Komukai-nakano, Sachi-ku, Kawasaki-shi, Kanagawa

Claims (8)

[Claims] 1. A monomer which dissolves the transparent polymer is gradually fed into a cylindrical container whose container wall is made of a transparent polymer and which is held horizontally and rotates around a central axis.
While dissolving the transparent polymer in the monomer, the gel layer is sequentially moved from the container wall toward the inside of the container to radically polymerize the monomer, and the transparent polymer in the produced resin is A method for producing a multimode synthetic resin optical transmission medium having a composition in which the concentration decreases in the moving direction of the gel, thereby having a continuous refractive index gradient in a certain direction. 2. The method according to claim 1, wherein the polymer obtained by polymerizing the monomer and the transparent polymer forming the container wall are compatible with each other. 3. The solubility parameter δ _{M of the} monomer
2. The method according to claim 1, wherein the solubility parameter δ _P of the transparent polymer constituting the container wall satisfies the following formula. [Equation 1] 4. The difference between the refractive index when the monomer is a homopolymer and the refractive index of the transparent polymer is at least 0.
The method of claim 1, wherein the method is 005. 5. A mixture of two or more kinds of monomers, wherein the monomer gives a different polymer which is transparent when it is made into a homopolymer and has a difference in refractive index of at least 0.005. There are many monomer components that give low-refractive-index polymers, and the monomer components that give high-refractive-index polymers gradually increase. The method according to claim 1, wherein the monomer components are fed while changing the mixing ratio so that the amount of the monomer components giving the coalescence is large. 6. The monomer is methyl methacrylate, and the transparent polymer constituting the container wall is a copolymer of methyl methacrylate and benzyl methacrylate or a copolymer of methyl methacrylate and 1,1,2-trifluoroethyl methacrylate. The method of claim 1, wherein the method is: 7. The monomer is a mixture of methyl methacrylate and benzyl methacrylate or a mixture of methyl methacrylate and 1,1,2-trifluoroethyl methacrylate, and the transparent polymer constituting the container wall is a methyl methacrylate polymer. The method according to Item 1. 8. The method according to claim 1, wherein the monomer is fed at a constant rate continuously or divided into two or more times.