CA1107752A - Process for continuous production of prepolymer syrups - Google Patents

Abstract

Abstract of the Disclosure A process for the continuous production of a prepolymer syrup which comprises (A) continuously supplying a monomer comprising methyl methacrylate as a main component and a rsdical-polymerization initiator to a reaction zone wherein substantially a complete mixing is achieved, during which the reaction zone is kept under such a condition that the concentration of remaining initiator is 1/2 to 1/1,000 time that of supplied initiator, or (A') continuously supplying said monomer to first reaction area in a reaction zone wherein at least two reaction areas, in which sub-stantially a complete mixing is achieved, are arranged in series, continuously supplying a radical-polymerization initiator at the same time to at least two reaction areas including the first one, and passing the monomer through the reaction areas successively thereby polymerizing the monomer into syrup, during which the reaction areas are kept under such a condition that the concentration of remaining ini-tiator in at least the reaction areas to which the initiator is supplied is 1/2 to 1/1,000 time that of supplied ini-tiator, and then, optionally in the case of (A'), (B) introducing the resulting reaction mixture into reaction zone wherein a piston flow is substantially achieved, and passing it through the zone during which the temperature of the zone ant the average residence time of the mixture are maintained so that the rest of the polymer in the final syrup are produced in the zone and the concentration of remaining initiator is reduced to substantially a negligible amount, thereby obtaining the final syrup, the distribution of polymerization degree of the polymer in the syrup being 3.0 or less as expressed in a polydispersity which is a ratio of weight average polymerization degree to number average polymerization degree.

Description

11~)77S2 The present invention relates to a process for continuous, stable production of methyl methacrylate syrups of high polymer content, and particularly to a process for continuous, stable production of the syrups of high polymer content which have a proper viscosity suitable for operation and enable shortening of casting time with no lowering of the quality of methyl methacrylate cast sheet.
More particularly, the present invention relates to a process for continuous, stable production of prepolymer syrups of high polymer content by successively passing a methyl methacrylate monomer and a radical-polymerization initiator through reaction zones of specified reaction conditions, and particularly to a process for continuous, stable production of the syrups having a narrow distribution of polymerization degree, a low concentration of remaining initiator and a high polymer content.
The prepolymer syrups are suitable for various us0s such as casting liquids for preparation of cast sheets and glass fiber reinforced cast sheets, intermediate materials for preparation of molding materials after removing a vola-tile component in the prepolymer syrups, main components of polymerizable adhesives or paints, raw materials of resin concrete compositions and the like. Among these, the pre-polymer syrups are particularly suitable for use as the casting liquid for preparation of cast sheets and glass fiber reinforced cast sheets.
Generally, methyl methacrylate cast sheets are batchwise produced by injecting a polymerizable liquid composition comprising an initiator and a methyl meth-acrylate syrup into a mold having a space enclosed with two pieces of glass plate and a gasket therebetween, followed by polymerization by heating (hereinafter referred to as simply "cell casting process").
In this method, the syrup containing a suitable amount of polymer is used, in order to elevate the operation efficiency of injection, to elevate the quality of the resulting cast sheet and to shorten the casting time. Prior to injection, the syrup is mixed with an initiator and other necessary additives, and deaerated under reduced pressure in order to remove dissolved air from it. The syrup is then injected into a space enclosed with two pieces of glass plate and gasket between the plates. In injection of the syrup, a too low viscosity of the syrup causes liquor leak, while a too high viscosity prolongs the injecting time. In either case, the operation efficiency of injection is lowered so that the syrup viscosity needs to be within a proper range~ As is well known, further, the use of syrup decreases the quantity of heat generated on polymerization and shrinkage of polymer on polymerization, so that the surface state of resulting cast sheet is improved and control of sheet thickness becomes easy. Consequently, a high polymer content of syrup is desirable. Also, the casting time is much shortened as an increase in the polymer content of syrup. The highest possible polymer content is thus desirable from this point of view. It is also necessary for the syrup used for this purpose to have a high storage stability, not to hinder polymerization on casting and not to lower the quality of cast sheet.
Methacryl cast sheets have 50 far been produced by the cell casting process in which polymerization is carried 77~2 out between two pieces of glass plate. Recently, however, this process is being replaced by the casting process of continuous form in which polymerization is continuously carried out between two pieces of moving endless belts. For example, Japanese Patent Publication No. 2991~/1976 discloses a process for continuously producing a resin plate, using a pair of endless belts arranged and constructed so that the lower run of the upper belt is positioned above the upper run of the lower belt, which comprises feeding a polymeri-zable liquid composition, together with a pair of continuous gaskets, into a spacing defined between the lower run of upper belt and the upper run of the lower belt both of which are arranged such that the pair of the endless belts are driven concurrently in the same direction at substantially the same speed, said gaskets being arranged so as to circum-scribe the spacing serving as a pair of seals to confine a cavity whilst moving concurrently with the belts in contact with the opposing surfaces of the belts, passing the composi-tion through a portion of the path of the belts where said composition is completely polymerized, and removing the polymerized plate from the end portion of the belts at the discharge side thereof.
In this process, however, an equipment cost occupies a large portion of manufacturing cost so that shortening of casting time is required more severely than in the cell casting process. The shortening of casting time can be achieved by increasing the polymer content of syrup as much as possible with the viscosity of the syrup main-tained within the range not lowering the operation effic~ency of injection. In this case, the syrup should be such that ~1~77S2 it prevents the quality of the resulting cast sheet from lowering as much as possible and more preferably it elevates the quality. Because, a lowering in the quality of the resulting cast sheet decreases the commercial value of the cast sheet and thus cancels out an effect of decreasing the manufacturing cost by shortening of casting time. When the cast polymerization is carried out using a syrup containing polymer having lower weight average polymerization degree due to an increase of the polymer content of syrup for shortening of casting time, the following defects easily appear in the resulting cast sheet in general: the mechanical strength of the sheet lowers owing to a decrease in the average polymerization degree of the sheet; and owing to polymer of low polymerization degree in the syrup, the foaming of the sheet easily occurs on heat-molding; the molding temperature range of the sheet becomes narrow; and the solvent resistance of the resulting sheet becomes bad.
The viscosity of syrup at constant temperature is determined by a polymer content and a weight average poly-merization degree of polymer, and it increases with the increase of each factor. ~ccordingly, a lower weight average polymerization degree is desirable in order to increase the polymer content of syrup as much as possible with the viscosity of the syrup maintained withln the range not to lower the operation efficiency of injection. On the other hand, however, polymers of low polymerization degree in the syrup are one of the factors by which said decrease in the average polymerization degree of sheet and said foaming of the sheet on heat-molding are caused. The amount of such polymers is almost determined by a num~er average polymerization degree. It may therefore be said that a higher number average polymerization degree is desirable.
In polymers having a distribution of polymerization degree, a weight average polymerization degree is generally larger than a number average polymerization degree. Breadth of a distribution of polymerization degree is generally expressed in a polydispersity which is a ratio of the two polymeri-zation degrees. In this expression, generally the syrups having a lower polydispersity are desirable.
When the concentration of remaining initiator in syrup is high, polymerization further proceeds during cooling on syrup production or storage, which results in a rise in the polymer content and viscosity of the syrup.
Thus, syrups of constant quality can hardly be obtained.
Even though the concentration is so small that such progress of polymerization is not substantially observed, it causes a change in the quality of syrup in storage, and a lowering in the quality of cast sheet obtained from such syrup, for example, an increase in the remaining monomer content of the cast sheet and a trend to cause foaming of the sheet on heat molding. Accordingly, it is necessary to decrease the concentration of the remaining initiator as much as possible. The shortening of casting time may also be achieved by increasing the concentration of the initiator used on casting in place of an increase in the polymer content of syrup. In this case, however, the average polymerization degree of cast sheet necessarily decreases, causing a great reduction in quality. Therefore, this method may not be said to be a favorable one.
Hitherto, there are proposed various processes for ~77~2 production of the syrup. Any of these processes satisfies a part of the conditions described above, but none of them satisfies all the conditions. For example, shortening of casting time causes a lowering in the mechanical strength of the cast sheet, an easy foaming of the sheet on heat mold-ing, a narrowing in the molding temperature range of the sheet and a decrease in the solvent resistance of the sheet.
As described above, the shortening of casting time does not always ralse economy.
Production of the syrup by a batchwise process is generally carried out using a vessel-type reactor with a stirrer. In this process, the monomer is heated to high temperature and then a required amount of initiator is added thereto, or a mixture of the monomer and the initiator is heated to high temperature; polymerization is carried out sufficiently until the concentration of the remaining initiator is reduced to substantially a negligible one; and then the reaction mixture is cooled and withdrawn as a syrup. In this process, the initiator concentration is high at the initial stage of reaction, but it decreases rela-tively rapidly with the progress of the reaction. The polymerization degree of polymers thus obtained varies over a wide range between considerably low polymerization degree at the initial stage and extremely high polymerization degree at the final stage. Thus, the polymers in the syrup have an extremely broad distribution o~ polymerization degree, and their weight average polymerization degree is high while their number average polymerization degree is low. A weight average polymerization degree is directly related to the viscosity of a syrup, and its large value i~7752 means a high viscosity when syrups of the same polymer content are compared with one another. As a result, injection of the syrup into a glass cell or a space between endless belts becomes difficult. In order to facilitate the injection, the polymer content of syrup needs to be limited to a low level, by which shortening of casting time can not be much expected. Low values of the number average poly-merization degree mean that polymers of low polymerization degree causing quality reduction are contained in the syrup in relatively large amounts. And, syrups containing such polymers cause a decrease in the average polymerization degree of cast sheet produced therefrom and foaming of the sheet on heat molding.
In this process, however, when the initial concen-tration of initiator is increased, the weight average polymerization degree of polymers in the resulting syrup decreases, and the upper limit of the polymer content of the syrup rises with the syrup viscosity maintained within the range wherein injection of syrup is easy. ~ecause of these, the casting time is shortened to a large extent. On the other hand, the number average polymerization degree also decreases proportionally, accelerating the quality reduction of cast sheet. The concentration of remaining initiator of syrups obtained by this method îs generally sufficiently low, so that the syrups have a relatively good storage stability. In this batchwise process, when a sufficiently long time is not spent for polymerization, syrups having a relatively narrow distribution of polymerization degree are produced. Since, however, the initiator remains in a high concentration, polymerization further proceeds during cool-~7752 ing and storage, thus increasing the viscosity. An in-hibitor is sometimes added to elevate the storage stability of syrup, but this method disadvantageously prolongs poly-merization time on casting and causes coloration. In the batchwise produc~ion of syrup, it is not desirable to control the formation of polymers of high polymerization degree by addition of a chain transfer agent, because, when the agent remains in the syrup, it adversely affects cast-ing, thus resulting in prolongation of polymerization time, lowering in the average polymerization degree of the cast sheet or coloring of the cast sheet.
On the other hand, there are proposed various processes for continuous production of the syrup. Of these, a method with a tubular reactor comprising continu-ously supplying the monomer and an initiator to the reactor at one end and continuously withdrawing the resulting syrup at the opposite end, can not substantially overcome the drawbacks of the foregoing batchwise processes. That is, judging the phenomena which occur when a reaction solution flows through a long and narrow tubular reactor in the lengthwise direction from the standpoint of the progress of polymerization, this method shows the same progress of polymerization as in the batchwise processes with a vessel-type reactor except an influence by back mixing. The polymerization proceeds in such a manner that the initiator concentration is high in the vicini~y of a feed inlet, decreases relatively rapidly as the reaction sol~tion flows forward, up to a substantially negligible amount in the vicinity of the outlet. The polymerization degree of the resulting polymers varies over a very wide range between a ~1~7752 considerably low polymerization degree in the vicinity of feed inlet and an extremely high one in the vicinity of outlet. Thus, the polymers in the syrup have an extremely broad distribution of polymerization degree.
In this process, when sufficient reaction temper-ature and residence time are not used for polymerization, syrups having a relatively narrow distribution of poly-merization de~ree are produced. The syrups thus obtained are poor in storage stability like the case wherein a sufficiently long time is not spent for polymerization in the batchwise process with a vessel-type reactor. In other words, with a continuous tubular reactor, shortening the casting time without lowering the quality of the cast sheet can not be much expected.
There is also proposed a process wherein the monomer and an initiator are continuously supplied to a vessel-type reactor with a stirrer at the inlet and the resulting syrup is continuously withdrawn at the outlet. In the process disclosed in British patent 937,215, polymeri-zation is carried out at such temperature and conditions that decomposition of initiator is just completed at the time when a required conversion is reached. The principle of this process is based on the following features of the batchwise polymerization process: when the reaction temper-ature is sufficiently high as compared with the decomposi-tion temperature of initiator, the initiator rapidly de-composes, and during that period polymerization proceeds to a certain conversion. But, after lapse of the time required for complete decomposition of initiator, the rate of reac-tion becomes very slcw since the reaction proceeds by the 11077~;2 mere action of heat. Further, when the polymerization is carried out at temperatures and conditions at which the initiator has just been decomposed, it becomes easy to control the reaction and to limit the conversion to a required range. In other words, at high temperatures, the Trommsdorff effect is apparently reduced to a large extent, and the state of reaction approaches a dead-end type poly-merization. Said patent says that carrying out polymeri-zation at such temperature and condition is very suitable for control of partial polymerization of methacrylate in any of the batchwise and continu01ls processes.
In the batchwise processes and continuous ones with a tubular reactor, however, syrups obtained at such temperature and condition contain polymers having an ex-tremely broad distribution of polymerization degree as described hereinbefore in detail. Consequently, shortening of casting time does not always raise economy.
On the other hand, when the polymerization is carried out at such temperature and condition in the process with a continuous stirred tank reactor, it is difficult to produce syrups having a high polymer content by a stable and stationary operation since the rate of polymerization is accelerated by the Trommsdorff effect, and besides the syrups obtained have the following defects: in this method, since an initiator is always supplied to the reactor even at a temperature causing rapid decomposition o~ the initiator, a rapid decrease in initiator concentration by prolongation of reaction time does not occur. An initiator concentration at a stationary state in the continuous stirred tank reactor wherein complete mixing is achieved, is equal to the concen-1~3i77~2 tration of remaining initiator at the outlet of the reactorand is expressed in an equation; I = I /(1 + K~), wherein I
is a concentration of remaining initiator (weight %), Io is a concentration of supplied initiator (weight %), K is a decomposition rate constant of the initiator (sec 1) and 3 is an average residence time (sec). At temperatures at which rapid decomposition of initiator takes place, K is large enough to meet Ka ~ 1 so that an equation; I = Io/Ke, applies approximately. That is, a decrease in the concen-tration of remaining initiator is only for such a degree that the concentration decreases in inverse proportion to a reaction time even though the reaction time is prolonged.
During the period of the reaction, there is no time at which the initiator has just been decomposed, and there is no reaction time after which the rate of reaction becomes very slow. It may not always be easy to stop the poly-merization at a required stage, unlike the batchwise process and the continuous process with a tubular reactor. Further, in bulk polymerization of methyl methacrylate monomers, acceleration of the rate of polymerization, which is called the Trommsdorff effect, is known. This effect refers to the phenomenon of increase in polymerization rate constant as an increase in polymer content. Considering the case of extending a reaction time in the process with a continuous stirred tank reactor, the polymer content increases even though said acceleration is not observed, and therefore it is necessary, as described above, to control the reaction tlme strictly in order to stop the polymerization at a re~uired stage. Vnder conditions wherein production of syrups of high polymer content is intended, however, the polymer content increases rapidly by prolongation of reac-tion time on account of the aforesaid acceleration phenomenon;
and when the content exceeds a certain limit, a stable and stationary operation becomes no longer possible even under an isothermal condition, whereby the reaction system becomes unstable in concentration. Thus, this process has the defect that production of syrups of high polymer content is difficult or impossible. Furthermore, in the process with a continuous stirred tank reactor, the concentration of the remaining initiator only decreases slowly with a lapse of reaction time even at temperatures at which rapid decomposi-tion of initiator takes place in the batchwise process as above. Consequently, the syrups obtained have a higher concentration of the remaining initiator than in the batch-wise process, so that the viscosity of the syrup unavoidably increases by progress of polymerization during cooling or storage.
In order to obtain a syrup of high heat stability which does not cause such polymerization, it is necessary to decrease the concentration of remaining initiator in the syrup to as low a level as possible. For this purpose, the reaction needs to be carried out at a temperature much higher than that at which rapid decomposition takes place in the batchwise process. ~owever, in the reaction under such condition, the following defect appears: a certain period of time is required for a monomer and an initiator freshly supplied to the reaction mixture in the reactor to be uniformly mixed. Under such condition, however, decomposi-tion of initiator proceeds before sufficient mixing is achieved, and as a result, syrups produced have a very broad distribution ranging from a very low polymerization degree to a very high polymerization degree. Cast sheets resulting from such syrups are poor in quality since they contain polymers of low polymerization degree. Furthermore, the viscosity of the syrups is high for the polymer content on account of the broad distribution of polymerization degree, which makes it difficult to increase the polymer content and to shorten the casting time.
In order to overcome the defect of the aforesaid process, U.S. patent 3,474,081 discloses a process wherein polymerization is carried out using at least two continuous stirred tank reactors connected in series. This patent says: In this process, 40 to 95 % by weight of polymers in the syrup is produced in the first reactor and the rest is produced in the second reactor and thereafter. Since the polymers in the syrup are separately produced in at least two concentrations of initiator, the polymers comprise at least two parts, a part of high molecular weight and a part of low molecular weight. The weight average molecular weight of the former part is at least two times as large as that of the latter part, and the latter part occupies 40 to 95 % by weight of polymers in the syrup. When the content of the low molecular weight part is high within the above range, the resulting syn~p has a relatively low useful viscosity and a high solid content. It may be considered that this method was developed with the intention of in-creasing the content of low molecular weight part in the polymer to limit the weight averzge polymerization degree which directly affects the syrup viscosity to a low level, thereby obtaining a syrup having a relatively low viscosity 1~7752 and a high polymer content.
But, polymers of low polymerization degree cause a reduction in the average polymerization degree of cast sheet and an easy foaming of the sheet on heat molding. Syrups of which the polymer content is increased by increasing the content of the part of low polymerization degree make it possible to shorten the casting time, but they lower the quality of cast sheet Further, the followings are known from the relationship between the distribution of poly-merization degree of polymers in a syrup obtained by this method and the concentration of the remaining initiator: in the first reactor, polymers of low polymerization degree are produced in a relatively low proportion within the above range when an average residence time in the reactor is relatively short as compared with the half-life period of the initiator. In order to sufficiently decrease the concentration of the remaining initiator in the final syrup, an increase in the number of reactors is necessary. In this case, the average polymerization degree of polymers produced in ea~h reactor rapidly increases as the polymers move forward the final reactor in the same manner as in the continuous tubular reactor. As a result, the polymer in the final syrup contains the part of high polymerization degree in large amounts and has a broad distribution of polymeri-zation degree, and the viscosity of the syrup becomes high for the polymer content.
In the first reactor, on the other hand, polymers of low polymerization degree are produced in a high propor-tion within the above range when a reacticn time in the reaction is sufficiently long as compared with the half-life 1~77S2 period of the initiator. In this case, the concentration of the remaining initiator can be decreased relatively easily with two xeactors or more. Since, however, the concen-tration of remaining initiator in the first reactor is very much different from that in the second reactor, the weight average polymerization degree of the part of high polymeri-zation degree is so large as more than 2 times that of the part of low polymerization degree. For this reason, although the content of the part of high polymerization degree is low, the polymer in the syrup has a relatively broad distri-bution of polymerization degree. That is, a large decrease in the concentration of remaining initiator results in broad distribution of polymerization degree. It may therefore be the case that still this process does not satisfy the purpose of shortening the casting time without lowering the quality of the cast sheet.
Japanese Patent Publication No. 35357/1973 discloses a process for con~inuous production of a prepolymer of methacrylic esters characterized by using a reactor compris-ing a former part and a latter part, the former one having at least one continuous stirred tank reactor connected in series and the latter one having at least one tubular reactor connected in series, and fixing the conversion of reaction mixture at the outlet of the former part to 9 to 12 %. This process relates to the removal of heat generated which becomes a problem in the continuous process with a stirred tank reactor. In this process, a plural number of stirred tank reactors connected in series is used for production of prepolymers with high conversion, on the basis of the finding that a maximum conversion in one vessel is only 2.5 ~ in order to control te exothermic reaction stably.
Furthermore, said reactor is connected to a tubular reactor in order to facilitate the control of production quantity with no change ln quality and to prevent blockage of tubular reactor which is a weak point of the reactor.
In this process, the conversion in the former part is restricted to 9 to 12 ~ in order to control the reaction stably. Consequently, this process does not satisfy the object of producing syrups of high polymer content which are suitable for shortening of casting time. Further, the concentration of remaining initiator in the syrup is kept relatively large since the polymerization is carried out at 50 to 100C. This process is not also satisfactory in this respect.
An object of the present invention is to provide a process for continuous production of a prepolymer syrup having a high polymer content and containing a polymer which has a very narrow distribution of polymerization degree.
Another object of the present invention is to provide a process for the continuous production of a prepolymer syrup having an extremely low concentration of initiator remaining in the syrup. Further, another object of the present inven-tion is to provide a process for the continuous production of a prepolymer syrup which is capable of stable and steady oper-ation. Other objects and advantages of the present invention will become apparent from the explanation described below.
The inventors extensively studied to overcome these drawbacks, and as a result, in the process for continuous production of methyl methacrylate syrups which , i~

~7752 comprises continuously supplying a methyl methacrylate monomer and a radical-polymerization initiator to a reaction zone thereby polymerizing the monomer partially and then continuously withdrawiny the resulting syrup, the inventors found a process for continuous, stable production of methyl methacrylate syrups of high polymer content by using a reaction zone comprising one or at least two series connected stirred tank reactors, continuously supplying the monomer to the reactor or the first reactor, continuously supplying the intiator at the same time to the reactor or at least two reactors including the first one, and sp~cifying the reac-tion temperature and average residence time in each reactor.
Particularly, the inventors found that, when the number of reactor is at least two, a higher polymer content syrup can be obtained very effectively, with a narrow distribution of polymerization degree maintained, by balancing the amounts of the initiators added to the stirred tank reactors one another thereby controlling the overall distribution of polymerization degree of polymers produced in said reaction zone to a narrow range. Further, when the number of the stirred tank reactor in the reaction zone is one, but optionally, when at least two stirred tank reactors in the reaction zone are used, the resulting syrups are introduced into a reaction zone composed of one or at least two, pre-ferably 1 to 5, tubular reactors which are connected in series, and passed thrvugh said succeeding reaction zone under conditions of specified reaction temperature and average residence time in each tubular reactor. By this process, the concentration of the remaining initiator in the resulting syrup can be reduced very effectively, with a ~1~37752 narrow distribution of polymerization degree maintained.
According to the present invention, there is provided a process for the continuous production of a prepolymer syrup comprising (A) continuously supplying a monomer comprising methyl mathacrylate as a main component and a radical-polymerization initiator to a reaction zone wherein substantially a complete mixing is achieved, during which the reaction zone is kept under such a condition that the concentration of the remaining initiator is 1/2 to 1/1,000 times that of the initiator supplied, or (A') continuously supplying said monomer to a first reaction area in a reaction zone (hereinafter referred to as "first reaction zone") wherein at least two reaction areas, in which substantially a complete mixing is achieved, are arranged in series, continuously supplying a radical-polymerization initiator simultaneously to at least two reaction areas including the first one, and passing the monomer through the reacti.on areas successively thereby polymerizing the monomer into syrup, during which the reaction areas are kept under such a condition that the concentration of the remaining initiator in at least the reaction areas to which the initiator is supplied is 1/2 to 1/1,000 times that of the initiator supplied, and then, optionally, in the case of ~A'), (B) introducing the resulting reaction mixture into a reaction zone (herein-after referred to as "second reaction zone'l) wherein a piston flow is substantially achieved, and passing it through the zone, dur~ng which the temperature of the zone and the average residence time of the mixture are maintained so that the rest of the polymer in the final syrup is produced in the zone and the concentration of remaining initiator is reduced to substantially a negligible amount, thereby obtaining the final syrup, the distribution of polymerization degree of the polymer in the syrup being 3.0 or less as expressed in a polydispersity which is a ratio of weight average polymerization degree to number average polymerization degree.
As the methyl methacrylate monomers used in the present invention, monomers or monomer mixtures commonly used for the production of this kind of syrup are used as they are. Of these, a monomer containing methyl meth-acrylate as a main component, is particularly preferred.
Methyl methacrylate may be used alone or in mixture with at least one ethylenically unsaturated monomer copolymerizable therewith such as alkyl acrylates (e.g. methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, ethyleneglycol diacrylate), alkyl methacrylates (e.g. ethyl acrylate, lauryl methacrylate,

2-hydroxyethyl methacrylate, ethyleneglycol dimethacrylate), unsaturated nitriles (e.g. acrylonitrile, methacrylonitrile), unsaturated amines (e.g. acrylamide), unsaturated carboxylic acids (e.g. acrylic acid, methacrylic acid), vinyl compounds le.g. vinyl chloride, styren~, ~-methylstyrene) and the like. In this case, the amount of said monomer copolymeri-zable with methy~ methacrylate is less than ~0 ~ by weight, preferably up to 20 ~ by weight, based on the mixture. It is necessary, in general, to limit the amount of these ethylenically unsaturated compound to the above ran~e in order that the resulting cast sheet is improved in quality without losing characteristics as a methyl methacrylate cast sheet, As the radical-polymerization initiator used in the present invention, those which generate a radical relatively rapidly at 90 to 200C, preferably 110 to 180C, are used. of these, those having a half-life period of less than 5 seconds at less than 180C, preferably less than 140C, are preferred, For example, there may be given a~o compounds such as azobisisobutyronitrile, azobisdimethyl-valeronitrile, azobis(4-methoxy-2,4-dimethylvaleronitrile) and azobiscyclohexanenitrile, and peroxides such as benzoyl peroxide, lauroyl peroxide, acetyl peroxide, capryl per-oxide, 2,4-dichlorobenzoyl peroxide, isobutyl peroxide, acetylcyclohexyl sulfonyl peroxide, tert-butyl peroxy-pivalate, tert-butyl peroxy-2-ethylhexanoate, isopropyl peroxydicarbonate, isobutyl peroxydicarbonate, sec-butyl peroxydicarbonate, n-butyl peroxydicarbonate, 2-ethylhexyl peroxydicarbonate and bis(4-tert-butylcyclohexyl) peroxy-dicarbonate. These initiators may be used alone or in combination. Particularly, when initiators having a half-life period of less than 5 seconds at less than 140C are used, the reaction is favorably carried out at the lower side of the reaction temperature range of 90 to 1~0~. In this case, a load of pre-heating of monomer and that of cooling of syrup can be diminished, and besides pressure conditions can also be relieved.
~ he amount of the initiator is generally 0.001 to 1 ~ by weight, preferably 0.01 to 0.5 ~ by weight, based on the methyl methacrylate monomers. ~hen high polymer contents are required, the concentration of the supplied initiator is adjusted to a high level, while when high number average polymerization degrees are required, said concentration is ~)7752 adjusted to a low level.
In the production of syrups according to the present invention, the number average polymerization degree and weight average polymerization degree of polymers in the syrup, and the viscosity of the syrup can easily be adjusted to a required level without a chain transfer agent, by mutual regulation of reaction temperature, concentration of the supplied initiator and average residence time of the reaction mixture. But, chain transfer agents may be used so far as they do not lower the quality of syrups or cast sheets.
A methyl methacrylate monomer and a radical-polymerization initiator are continuously supplied to the first reaction area in the first reaction zone wherein reaction areas, in which substantially a complete mixing is achieved, are arranged in series. Under conditions wherein substantially a complete mixing is achieved, the distri-bution of polymerization degree of resulting polymers is very narrow, and its value expressed in a polydispersity, of the most probable distribution based on the assumption that polymerization is all terminated by disproportionation.
Mext, when the number of the reaction area in the first reaction zone is at least two, an additional initiator is supplied to at least one of the reaction areas in the first reaction zone which follows the first one. In the reaction areas to which an additional initiator is supplied, the additional initiator and the reaction mixture coming from the pre~eding area are mixed su~stantially completely, and therefore the distribution of polymerization degee of pol~mers newly produced in said reaction areas is very 11~7752 narrow, and its polydispersity is substantially less than 2,2. Said reaction zone may contain reaction areas to which an additional initiator is not supplied. The number average polymerization deqree of polymers newly produced in said reaction areas is generally larger than in the other ones, but its polydispersity is substantially less than 2.2. Par-ticularly, when the reaction mixture from the preceding areas has a relatively high concentration of remaining initiator, the polymer content of the syrup produced in said whole reaction zone can be increased with little or no broadening of the distribution of polymerization degree of polymers in the syrup. Generally, however, it is desirable to supply the initiator to every reaction area in order that the distribution of polymerization degree of polymers in the final syrup is kept narrow.
In the first reaction area and other areas to which an additional initiator is supplied, the reaction temperature and the average residence time of the reaction mixture in each area should be kept so that the concen-tration of the remaining initiator is 1/2 to 1/1,000 time, preferably 1/5 to 1/1,000 time, particularly preferably 1/10 to 1/500 time, that of supplied initiator. When the concentration of the remaining initiator is low ~lithin this range, particularly a stable, steady operation can be carried out. When the concentration exceeds this range, the operation easily becomes thermally unstable, and in order to carry out a stable, steady stationary operation, it i5 necessary to limit the amount of polymer produced in the area to an extremely low ievel. Consequently, the number of reaction areas should be much increased to obtain final ~)77SZ

syrups having a h-gh polymer content. While when said concentration is below this range, the supplied initiator is decomposed before complete mixing of the initiator and the reaction mixture is achieved. As a result, polymers them-selves newly produced in the area have a broad distribution of polymerization degree.
In the reaction axeas to which an additional initiator is not supplied, the absolute value of the concen-tration of the supplied initiator is small and the amount of polymers newly produced is also small, so that a stable, steady operation can be carried out relatively easily.
Consequently, a concentration ratio of the remaining ini-tiator to the supplied initiator in said reaction areas is not particularly limited, ~ut it is preferably within the above range.
In the present invention, the reaction temperature of the first reaction zone is not particularly limited, but it is generally 90 to 200C, preferably 110 to 180C.
Particularly, when at least two reactors are used in the zone, the reaction temperature of each reaction area in the zone is regulated, depending upon the decomposition temper-ature of the initiator, so that a concentration ratio of the remaining initiator to the supplied initiator may be within the above range, The average residence time of the reaction mixture in the first reaction zone is regulated, like the concentration of the supplied initiator, according to the polymer content and the number average polymerization degree o~ the required final syrups, and it is generally 1 to 30 minutes, preferably 2 to 15 minutes. In the case of using at least two reaction areas in the first reaction zone, the il~775Z

average residence time in each reaction area is generally 0.1 to 20 minutes, preferably 0.2 to 5 minutes.
Maintenance of reaction temperature in the stirred tank reactor is generally achieved by furnishing the reactor with a ~acket at the outside, and circulating a heat medium of controlled temperature through the jacket. In the present invention, however, it is difficult to maintain a required temperature by this method since the rate of reaction is very high. Particularly in the case of commercial scale production, the reaction system becomes thermally unstable so that stable, steady operation is impossible. In the present invention, the quantity of heat generated by polymerization is almost equal to that of sensible heat required for raising the temperature of the reaction mixture to a required one, and the temperature of the reaction mixture is kept by heating or cooling according to a required polymer content and reaction temperature. The temperature of the first reaction zone is preferably controlled hy changing a temperature to which the monomer is pre-heated prior to supply to the first reaction area. It is more effective to furnish each reaction area in the zone with a jacket at the outside, and to circulate a heat medium.
Pre-heating of monomer may be carried out by any method, if non-flowing portion of the monomer is not present and temperature control is possible. Preferably, however, the pre-heating is carried out, for example, ~y passing the monomer through a single tube e~uipped with a ~acket at a Reynolds' number of more than 5,000, preerably more than 20,000, and circulating a heat medium of controlled temper-ature through the jacket, 1~7752 The monomer and the initiator may be supplied as a pre-heated mixture of both, but it is preferred to supply a pre-heated monomer and a cooled initiator-containing solu-tions may be previously mixed with at least one of the additives commonly used for production of cast sheet and the like, for example, ultraviolet absorbers, antioxidants, pigments, dyes and the like.
The reaction areas in the first reaction zone need to have functions to rapidly mix the monomer or initiator with the reaction mixture constant. For this purpose, any reaction equipment and stirring means may be used if sub-stantially a complete mixing is achieved. Preferably, how-ever, a means by which stirring is carried out at a Reynolds' number of more than 2,000, preferably more than 5,000, is used. For example, a continuous vessel-~ype reactor equipped with a ribbon stirrer is used for this purpose.
A larger number of the reaction areas, in which substantially a complete mixing is achieved, is desirable for the purpose of elevating the safety of operation. But, an extremely large number makes the operation troublesome and increases the cost of products. Usually, the number is 1 to 1~, preferably 2 to 5.
In the case of using at least two reaction areas, the amount of polymer produced in each reaction area is controlled according to the polymer content of the final syrup and the number of the reaction areas. But, an extreme difference in the amount among the reaction areas is not desirable since it makes the stability of operation poor or makes the installment of reaction areas meaningless.
~enerally, the amount is almost same in every reaction area, or it is made smaller gradually towards the final reaction area. The latter is preferred. In the former and latter cases respectively, the average residence time in every reaction area is also kept almost same in general, and it is made shorter gradually towards the final reaction area.
In the reaction areas to which an additional initiator is not supplied, a shorter residence time of the reaction mixture is generally selected than in the areas to which the additional initiator is supplied. Consequently, the amount of polymer produced is relatively small.
The amount of initiator supplied to reaction area depends upon the amount and the number average polymeri-zation degree of polymers produced in the area, the reaction temperature of the area and the concentration of the remain-ing initiator in the preceding area. When said concen-tration is made small by raising the reaction temperature, the amount of initiator is regulated depending upon the amount of the polymers produced or the average residence time in said area. Generally, the amount is almost same, and preferably it is made smaller gradually towards the final reaction area. In the present invention, when the number of reaction area is one, the use of second reaction ~one is necessary. However, when the number of reaction areas in that is at least two, the use of second reaction zone is optional but preferable.
The amount and the num~er average polymerization degree of polymers produced in the first reaction zone depend upon the kind and the concentration of the initiator supplied to each reaction area, and the reaction temperature and the average residence time of reaction mixture in each ~7752 reaction area. And, preferred concentration of the supplied initiator, reaction temperature and average residence time of the reaction mixture are selected depending upon the polymer content and the number a~erage polymerization degree of the required final syrup and the number of reaction areas. The number average polymerization degrees of poly-mers newly produced in the reaction areas are different one another. In order to narrow the distribution of polymeri-zation degree of polymers produced in the whole first reaction zone, however, it is desirable to permit these number average polymerization degrees to agree with one another as much as possible. For this purpose, the amounts of initiator supplied to the reaction areas are regulated mutually. When a maximum to minimum ratio of number average polymerization degree of polymers produced in each reaction area is maintained generally at less than 3, preferably less than 2, the distri-bution of polymerization degree of polymers coming out of the first reaction zone is generally less than 2.5, pre-ferably less than 2.2, as expressed in a polydispersity.
In the present invention, the amount of polymer produced in each reaction area in the first reaction zone depends upon the number of reaction area, but it is generally xelatively low levels such as 3 to 35 ~ by weight, preferably 5 to 25 ~ by weight, particularly. Accordingly, in the case of using at least two reaction areas, the quantity of heat generated in each area is also relatively small. The temperature in the first zone is generally 90 to 200C, preferably llO to 180C so as to allow the initiator to be decomposed rapidly. The temperature of the whole first reaction zone can effectively be controlled beyond 1~7752 expectation by changing the temperature of the monomer supplied to the first reaction area. Particularly, when the amount of polymer produced in the first reaction area is large within the above range and the amounts of polymer produced in the subsequent areas are small within said range, steady operations in the subsequent areas can be carried out thermally stably by controlling the reaction temperature in the first reaction area, even though the subsequent areas are under substantially an adiabatic condition. Further, since the amount of polymer produced in each area can be selected within relatively low levels, a difference in polymerization rate constant between the reaction mixtures entering the reaction area and leaving the same area, can be made small. As a result, acceleration of the rate of polymerization owing to the Trommsdorff effect is apparently extremely diminished and syrups of extremely high polymer content can favora~ly be obtained by a stable steady operation.
When the number of reaction area in the first reaction zone is one or, if desired, at least two, the reaction mixture from the first reaction zone is then introduced into the second one wherein a piston flow is sub-stantially achieved, and, while being passed through the zone, the rest of the polymer is produced and the concen-tration of the remaining initiator is decreased. Since an initiator is no longer supplied to the reactlon mixture passing through the zone, the concentration of the remaining initiator can be reduced very easily unlike the case of the first reaction zone wherein an initiator is constantly supplied and substantially a compiete mixing is achieved.

~1~7~SZ

Furthermore, since the amount of polymer produced is small, the distribution of polymerization degree of polymers in the final syrup i~ unexpectedly maintained narrow, although polymers of relatively large polymerization degree are produced. It is sufficient that the temperature of the second reaction zone is one at which the remaining initiator is rapidly decomposed. Generally, the temperature is one at which the half-life period of initiator is less than 20 seconds, preferably less than 5 seconds. Preferably, the temperature is not less than the one of the first reaction zone. Particularly, it is desirable to maintain the reac-tion conditions so as to raise the temperature of the reac-tion mixture substantially adiabatically by polymerization heat while the mixture passes through the zone.
In the second reaction zone, since nothing is supplied to the reaction mixture, mixing is not essentially necessary. Besides, since the amount of polymer produced is small, reaction control is very easy even under adiabatic conditions. Consequently, any of reaction equipments and stirring means may be used if a substantial piston flow is achieved. When stirring is not carried out at all, however, attachment of polymers to the wall of reactor occurs, which makes the piston flow substantially difficult and causes blockage of reactor tube if the attachment further proceeds.
Stirring is therefore desirable. As the stirring means, the following ones are employed: stirrers having a back mixing coefficient of less than 20 %, preferably less than 10 ~ are used, and stirring is carried out at a Reynolds' number of more than 2,0Q0, preferably more than 5,000; and self-wiping type stirrers having a back mixing coefficient of less than 20 %, preferably less than 10 ~ are used. For example, tubular reactors having a similar structure to a twin-screw extruder are used for this purpose. The second reaction zone may be equipped with a jacket at the outside to control the temperature by a heat medium. But, to keep the zone under substantially an adiabatic condition is more desira~le since the remaining initiator i5 decreased more rapidly.
The vapor pressure of reaction mixture in both reaction zones is generally higher than atmospheric pressure.
For facilitating the control of residence time and temper-ature in both reaction zones thereby maintaining the qualities of final syrups, for example, polymer content, viscosity and concentration of remaining initiator, substantially constant, it is desirable to apply a pressure higher than the vapor pressure, generally 1 to 20 atmospheres, preferably 2 to 10 atmospheres, to the reaction mixture so as to allow the mixture to keep s-ibstantially a liquid phase.
The average residence time of reaction mixture in the second reaction zone is 0.05 to 5 times, preferably 0.1 to 2 times as long as that of the first reacton zone.
It is preferred that the average residence time is long enough to decrease the concentration of the remaining ini-tiator to substantially a negligible amount. Long average residence time does not affect polymerization, because the concentration of the reamining initiator is so substantially negligible that the polymerization proceeds thermally very slowly. Consequently, an increase in polymer content and viscosity is negligibly small. But, a longer average residence time than necessary is useless since a larger reaction zone i9 re~uired. Also, a~erage residence time of 1~77SZ

less than 0.05 time as long is not desirable since the concentration of the remaining initiator is not so much reduced.
The concentration of the remaining initiator in the reaction mixture coming out of the second zone is negligibly small and the concentration decreases rapidly as the mixture proceeds towards the outlet of the zone. In correspondence to this, the number average polymerization degree of polymers newly produced in the zone rapidly increases, while the amount of the polymer rapidly decreases as the mixture proceeds towards the outlet of the zone. As a result, the number average polymerization degree of polymers produced in the zone is unexpectedly not so large as compared with that of the first zone, and further it becomes possible to decrease the concentration of the remaining initiator in the final syrup to substantially a negligible amount and to mairltain the distribution of polymerization degree of polymers in the syrup very narrow. Thus, the distribution of polymerization degree of polymer is less than 3, preferably less than 2.5, particularly preferably less than 2.2, as expressed in a polydispersity which is a ratio of weight average polymerization degree to number average polymerization degree.
The polymer in the resulting final syrup contains generally 60 to 99.5 ~ by weight, preferably 90 to gg.5 % by weight, particularly preferably 95 to 99.5 ~ by weight, of the syrup produced in the first reaction zone.
The concentration of the remaining initiator in the reacticn mixture coming out of the second reaction zone is negligibly small, i.e. less than 1 ppm, preferably less ~7752 than 0.1 ppm, particularly preferably less than 0.01 ppm.
The final syrup obtained by the above method shows little or no increase in polymer content and viscosity even though it is left at the temperature at which it is produced. It is however general to store the syrup until production of a cast sheet. And, for the purpose of avoiding an increase in polymer content and viscosity during storage of syrup and supply of initiator, other additives and syrup and prevent-ing a lowering in processability of syrup and quality of cast sheet, the syrup is generally cooled to less than 100C, preferably 80C.
The syrups obtained in the present invention have a concentration of the remaining initiator negligibly smaller than 1 ppm, for example, 0.01 ppm. Therefore, the increase of polymer content and viscosity is negligible even though the syrup is not cooled rapidly after it comes out of the second reaction zone, so that syrups of constant quality are easily obtained and said increase is not also observed during storage. Further, the syrups of the present inven-tion have a good storage stability. For example, there is no increase in the content of the remaining monomer in cast sheet owing to the quality change of syrup during storage, or no lowering in the quality of the cast sheet such as foaming on heat molding. As described abo~e, the syrup of the present invention has a superior storage stability.
Polymers in the syrups of the present invention have an extremely narrow distribution of polymerization degree, and the distribution is less than 3, preferably less than 2.5, particularly preferably less than 2.2, as expressed in said polydispersity. Accordingly, the number average polymerization degree, which affects the average polymeri-zation degree of cast sheet, can be made relatively high, and the weight average polymerization degree, which affects the viscosity of syrup, can be made relatively low. Conse-quently, syrups having a relatively low viscosity and a high polymer content can be obtained, and besides cast sheets can be produced in a shortened casting time without lowering of quality. When the syrups of the present invention are polymerized without shortening the casting time, the average polymerization degree of cast sheet can be increased, so that cast sheets having a particularly superior quality can be obtained. Furthermore, when the syrups of the present invention having a relatively low polymer content are used for casting, casting time can be shortened by increasing the concentration of initiator, and in this case, a lowering in the ~uality of cast sheet is relatively small.
The polymer content of the syrup of the present invention is 15 to 80 ~ by weight. When the content is below this range, an effect to shorten the casting time is relatively small, while when the content exceeds this range, the viscosity of the syrup becomes high even at extremely high temperatures, and thus substantially a complete mixing can not be achieved. The viscosity of the syrup obtained in the present invention is O.S to 10,000 poises at 25C. When the viscosity is below this range, the syrup leaks when injected on ~asting. However, when the viscosity is higher than 1,000 poises (which is a high level within the above range), injection becomes difficult. Therefore, syrups having a viscosity of 0.5 to 1,000 poises are used as they are. Of the syrups of the present invention, those having a i~77SZ

polymer content of 20 to 40 % by weight and a viscosity of 5 to S00 poises are particularly suitable for continuous casting such as the process described hereinbefore, since they have a good processability on injection and are usable in drastically shortened casting time without lowering the quality of the cast sheet. As the number average poly-merization degree of polymers in the final syrup, a range of 300 to 6,000 is selected. Particularly, a range of 300 to 2,000 is selected for the syrups of high polymer content suitable for continuous casting. In order to elevate the quality of cast sheet, however, it is preferred to select as high a number average polymerization degree as possible so far as the viscosity does not become excessively higher than the required polymer content.
In producing cast sheets, the syrups of the present invention may be used as they are, or they may be concentrated or diluted with a monomer or the same kind of syrup so as to have a polymer content of 15 to 50 % by weight, preferably 20 to 40 % by weight and a viscosity (25C) of 0.5 to 1,000 poises, preferably 5 to 500 poises.
In some cases, the homogeneity and productivity of syrup used for casting can be elevated advantageously by these treatments. The syrups of the present invention have a low viscosity fox their high polymer content, and have a good storage stability. The syrups are therefore preferably used not only for cast sheet and glass fiber reinforced cast sheet but also for molding materials, adhesives, paints, resin concrete compositions and the like.
The present invention will be illustrated specifically with reference to the following examples, hut the present ~1~77S2 invention is not limited to these examples. In the examples, percentages are by weight.
In the examples, the viscosity of syrup was measured at 25C by means of a B-type viscometer. The number average polymerization degree and the distribution of polymerization degree of polymers in the syrup were measured by gel-permeation chromatography using polystyrene gel as a packing and tetrahydrofuran as an eluting agent, provided that the distribution of polymerization degree was expressed in a polydispersity.
Foaming of cast sheet on casting was evaluated by the visual examination of foams in the cast sheet. Foaming of cast sheet on heating was evaluated by heating the cast sheet in a circulating hot-air oven at 180~C for 30 minutes and examining foams in the cast sheet visually.
Reduced viscosity was measured at 25C using a Q.l ~ chloroform solut.ion of the cast sheet. Concentration of remaining monomer was measured by gas-chromatography using methylene chloride solution of the cast sheet.
Example 1 A 2-stage continuous reaction equipment havin~ the following structure was used: the equipment consisted of a former part and a latter part; in the former part was set up a vessel-type reactor with a ri~bon-like propeller; in the latter part was set up a tu~ular reactor having a mixing shaft to which pins were fixed at a right angle to the shaft, and to the inside wall of the reactor were fixed pins at a right angle to the wall and towards the mixing shaft, and both pins were aranged so that they could wipe off matters attached to the opposite pins. A volume ratio of 11~775z vessel-type reactor to tubular was 1 : 0.25. Methyl methacrylate containing 0.047 % of azobisisobutyronitrile, an initiator, was continuously supplied to the vessel-type reactor so that an average residence time in the reactor was 147 seconds. The temperature and pressure in each reactor were maintained at 160C and 6.0 atmospheres, respectively.
The feed liquor was pre-heated to akout 80~C by means of a single tube equipped with a jacket.
The syrup from the tubular reactor had a polymer content of 26.6 % and a viscosity of 21.0 poises at 25C.
Ninety-five percent of the total polymer was produced in the vessel-type reactor. The concentration ratio of the remain-ing initiator to the supplied initiator in the vessel-type reactor was 1/32. The concentration of the remaining ini-tiator in the final syrup was less than 0.01 ppm, and the syrup did not show a change in both polymer content and viscosity at all even though kept still at 60C for 5 hours.
The polymer in the syrup had a number average polymerization degree of 745, and its distribution of polymerization degree was 2.17, as expressed in a polydispersity which is a ratio of weight a~erage polymerization degree to number average polymerization degree. The viscosity of the syrup was low for the high number average polymerization degree of the polymer and the high polymer content of the syrup.
A polymerizable ~iquid composition was prepared by dissolving 0.05 % of azobis(dimethylveleronitrile) in this syrup. After deaeration under reduced pressure, the com-position was polymerized in~o cast sheet using the well known continuous polymerization equipment. The equipment was constructed as follows: two pieces of mirror-polished 11~7752 stainless steel band (width, 500 mm; thickness, 0.6 mm) were set up horizontally with one upon the other; the length of polymerization zone was lO,000 mm in horizontal distance, of which the first 6,740 mm corresponded to a heat-polymeri-zation area heated with 85C water, the second 2,170 mm corresponded to a heat-treatment area heated with 120C hot air and the last l,090 mm corresponded to a cooling area cooled with cold air. The distance between the upper and lower bands was adjusted to make the thickness of cast sheet

3 ~m. The above polymerizable composition was continuously supplied to a space between the bands, and the bands were run at a rate of 374 mm~min so that said composition passed through the heat-polymerization area during 18 minutes. The product had a reduced viscosity o~ 2.37 dl/g and a remaining monomer content of 0.8 %. ~he product had a good appearance without foaming by polymerization or heating.
Comparative ExamPle l Syrup was produced using the vessel-type reactor alone in the former part of the continuous equipment in Example 1. The ~ind and the concentration of the initiator, and the a~erage residence time, temperature and pressure in the reactor were completely the same as in Example l.
The resulting syrup had a polymer content of 25.0 % and a viscosity of 10.3 poises. The polymer in the syrup had a numher average polymerlzation degree of 725 and its distribution of polymerization degree was 2.02, as expressed in a polydispersity. But, the concen~ration cf the remaining initiator in the syrup was as large as 15.2 ppm. When the syrup was kept still at 60C, it lost fluidity completely in 1 ho~r ~y rapid progress of polymerization, and thus the syrup could not withstand long-term storage.
The resulting syrup was mixed with 0.04 % of azobis(dimethylvaleronitrile) and polymerized using the same continuous polymerization equipment as in Example 1.
Polymerization was carried out under the same conditions as in Example 1 except that the syrup was passed through the heat-polymerization area during 24 minutes. The resulting cast sheet had a reduced viscosity of 2.69 dl/g and included no foam by polymerization. But it had a remaining monomer content as high as 4.2 %, and it showed foaming by heating.
When the syrup was passed through the heat-polymerization area during 21 minutes without changing other conditions, a striking foaming by polymerization occurred and the cast sheet obtained had no commercial value. When a mixture of this syrup and 0.05 % of azobis(dimethylvaleronitrile) was passed through the heat-polymerization area during 21 minutes, the resulting syrup included no foam by polymeri-zation but showed a striking foaming by heating.
Comparative Example 2 A 2-stage continuous reaction equipment having the following structure was used: the equipment consisted of the ~irst and theisecond vessel-type reactors with a ribbon-like propeller; and both reactors were connected in series and a volume ratio of the first to the second was 1 : 2.
Methyl methacrylate containing 0.047 % of azobis-isobutyronitrile was continuously supplied to the first reactor so that an average residence time in the reactor was 147 seconds. The temperature and pressure of each reactor were kept at 160C and 6.0 atmospheres, respectively.
The syrup thus obtained had a polymer content of 1~775Z

31.4 ~ and a viscosity of 1,100 poises. The proportion of polymer produced in the first reactor was 80 % of the total polyme~. The concentration of the remaining initiator in the syrup was 0.23 ppm. When the syrup was kept still at 60C for 3 hours, the polymer content and the viscosity of the syrup increased to 31.6 % and 2,500 poises, respec-tively. The polymer in the syrup had a number average polymerization degree of 870, and its distribution of polymerization degree was 3.32, as expressed in poly- -dispersity.
This syrup was mixed with 0.04 ~ of azobis-l (dimethylvaleronitrile) and polymerized using the same equipment as in Example 1. Polymerization was carried out unde~ the same conditions as in Example 1 except that the syrup was passed through the heat-polymerization area during 17 minutes. Thus, a cast sheet was o~tained. This syrup was so highly viscous that it was hardly injected into a space between the belts. The cast sheet included foams by polymerization.
This syrup was diluted with the monomer to the same viscosity, 21.0 poises, as in Example 1. The polymer content of the diluted syrup was 21.5 %. The diluted syrup was mixed with 0.05 % of azobis(dimethylvaleronitrile) and polymerized in the same manner as above except that the syrup was passed through the heat-polymerization area during 25 minutes. The resulting product had a reduced viscosity of 2.74 dl/g and a remaining monomer content of 1.9 %. The product included no foam ~y polymerization but showed foaming by heating. The same diluted syrup was mixed with 0.08 ~ of azobis(dimethylvaleronitrile) and polymerized in 11~`775Z

the same manner as above except that the syrup was passed through the heat-polymerization area during 21 minutes. The product had a reduced viscosity of 2.17 dl/g and a remaining monomer content of 1.7 ~. The product included no foam by polymerization but showed a striking foaming by heating.
The syrup was diluted with the monomer to the same polymer content, 26.6 %, as in Example l. The viscosity of the diluted syrup was 170 poises.
Comparative Example 3 Syrup was produced by using the same reactor as in Comparative Example 1. Methyl methacrylate containing 0.45 ~ of azobisisobutyronitrile was continuously supplied to the reactor so that an average residence time in the reactor was 14.6 minutes. The pressure and the temperature in the reactor were kept at atmospheric pressure and 85C, respec-tively. The concentration ratio of the remaining initiator to the supplied initiator at the outlet of the reactor was about 90/100. At the initial stage of polymerization, a syrup having a polymer content of about 25 ~ and a viscosity of about 12 poises was obtained. But, as the polymerization proceeded, maintenance of the reaction temperature became difficult, and after about l hour, the reaction temperature rapidly increased and the polymerizatiGn became violent. As a result, the contents in the reactor solidified and continu-ation of reaction was impossible.
~xample 2 Syrup was produced using the same equipment as in Example l except that the volume of each reactor was 5 liters. An ethyl acrylate solution (2~GC) containing ~.7 %
of azobisisobutyronitrile was continuously supplied to the 1~7752 vessel-type reactor at a rate of 0.21 liter/min. And, methyl methacrylate pre-heated to 120C was continuously supplied to the same reactor at a rate of 1.9 liter/min.
The temperature and the pressure in each reactor were main-tained at 160C and 6.0 atmospheres, respectively. A
concentration ratio of the remaining initiator to the supplied one in the vessel-type reactor was 1/32.
The syrup coming out of the tubular reactor had a polymer content of 31.9 ~ and a viscosity of 45.g poises at 25C. The concentration of the remaining initiator in the syrup was less than 0.01 ppm. The polymer in the syrup had a number average polymerization degree of 615 and its distribution of polymerization degree was 2.18, as expressed in polydispersity.
Example 3 Syrup was produced using a 2-stage continuous reaction equipment having the following structure: two vessel-type reactors with a ribbon-like propeller were connected in series; and a volume ratio of first reactor to second one was 3 : 1. Methyl methacrylate containing 0.047 % of azobisisobutyronitrile was continuously supplied to the first reactor to make an average residence time in the reactor 147 seconds. A methyl methacrylate solution containing 2 % of azobisisobutyronitrile was continuously supplied to the second reactor so that the amount of azo-bisisobutyronitrile added was 0.017 ~ of the reaction mixture. The temperature and pressure in each reactor were kept at 160C and 6.0 atmospheres, respectively. The feed liquor to the first reactor was pre-heated to 80C using a single tube equipped with a jacket. The temperature of the ~3775Z

initiator containing solution supplied to the second reactor was 25C.
The syrup from the second reactor had a polymer content of 31.8 % and a viscosity of 90.8 poises at 25C.
The weight ratio of polymer~ produced in the reactors was 73 : 27. The polymer in the syrup had a number average i polymerization degree of 750 and its distribution of poly-merization degree was 2.05, as expressed in polydispersity.
Continuous operation was carried out under the conditions described above for 700 hours. During that time, the reaction temperature was substantially constant, and the polymer content and the viscosity of the syrup obtained did not show a substantial variation. Consequently, the progress of reaction was extremely stable operationally.
Example 4 Syrup was produced using a 3-stage continuous reaction equipment having the following structure: the equipment consisted of a former part and a latter part; two vessel-type reactors with a rib~on-like propeller were set up in the former part and both reactors were connected in series; in the latter part was set up a tubular reactor having a m xing shaft to which pins were fixed at a right angle to the shaft, and to the inside wall of the reactor were fixed pins at a right an~le to the wall and towards the mixing shaft; and both pins were arranged so that they could wipe off mattexs attached to the opposite pins. A v~lume ratio of first vessel-type reactor to second one to tu~ular reactor was 1 : 0.33 : 0.25.
Methyl methacrylate containing 0.047 ~ of azobis-isobutyronitrile, an initiator, was continuously supplied to ~)7752 the first vessel-type reactor to make an average residence time in the reactor 147 seconds. A methyl methacrylate solution containing 2 % of azobisisobutyronitrile was continuously supplied to the second reactor so that the amount of azobisisobutyronitrile added was 0.017 % of the reaction mixture. The reaction mixture from the second reactor was passed through the tubular reactor to finish polymerization. The temperature and pressure of each reactor were kept at 16~C and 6.0 atmospheres, respec-tively. The feed liquor to the first reactor was pre-heated to 80C by means of a single tube equipped with a jacket.
The temperature of the initiator solution supplied to the second reactor was 25C.
The syrup from the tubular reactor had a polymer content of 33.5 % and a viscosity of 176 poises at 25C.
The weight ratio of polymers produced in the reactors was 70 : 25 : 5, The concentration ratios of the remaining ini-tiator to the supplied initiator in the first and the second reactors were 1/32 and 1/11, respectively. The concen-tration of the remaining initiator in the final syrup was less than 0.01 ppm. The syrup did not show a change in both polymer content and viscosity even though kept still at 60C
for 5 hours. The polymer in the syrup had a number average polymerization degree of 755 and its distribution of poly-merization degree was 2.13, as expressed in polydispersity which is a ratio of weight average polymerizat~on degree to number a~erage polymerization degree. The viscosity of the syrup was low for the high number average polymerization degree of the polymer and the high polymer content of the syrup.

1~7752 A cast sheet was produced, using the continuous polymerization apparatus as in Example 1, in the same manner as in Example 1 except that a polymerizable liquid composi-tion was prepared by dissolving 0.04 ~ of azobisdimethyl-valeronitrile as polymerization initiator in the syrup and the polymerizable liquid composition was passed through the heat-polymerization area during 15 minutes. The product had a reduced viscosity of 2.37 dl/g and a remaining monomer content of 0.9 ~. The prod~ct had a good appearance without foaming by polymerization or heating.
Example 5 Syrup was produced using the same equipment as in Example 1 except that the volume of each reactor was 5 liters. Methyl methacrylate pre-heated to 130C was continuously supplied to the first vessel-type reactor at a rate of 3.27 liter/min, and at the same time an ethyl acrylate solution (25C) containing 0.29 % of azobisiso-butyronitrile (an initiator) was continuously supplied to the same reactor at a rate of 0.36 liter/min. Further, a methyl methacrylate solution (20C) containing 3 % of additional azobisisobutyronitrile was continuously supplied to the second vessel-type reactor at a rate of 0.035 liter/min. The temperature of each vessel-type reactor was kept at 160C~ The temperature of the tubular reactor increased to 162C as a result of adiabatic temperature-increase. The pressure of each reactor was ~ept at 6.0 atmospheres. The concentration ratio of remaining initiator to supplied initiator in the vessel-type reactor was 1/32.
The syrup from the tubular reactor had a polymer content of 32.0 %, a viscosity of 159 poises at 25C and a ~7752 remaining initiator concentration of less than 0.01 ppm.
The polymer in the syrup had a number average polymerization degree of 790 and its distribution of polymerization degree was 2.18, as expressed in polydispersity.
A cast plate was produced, using the continuous polymerization apparatus in Example 1, in the same manner as in Example 1 except that a polymerizable liquid composition was prepared by dissolving 0.04 ~ of azobisdimethylvalero-nitrile in the syrup and polymerizable liquid composition was passed through the heat-polymerization area during 16 minutes. The product had a reduced viscosity of 2.49 dl/g and a remaining monomer content of 0.5 %. The product included no foam therein and caused no foaming by heating.
Examples 6 to 14 The same equipment as in Example 4 was used except that the volume ratio of first vessel-type reactor to second one was changed as shown in Table 1 and that the volume of the tubular reactor was 0.5 time that of the first vessel-type reactor. Various radical-polymerization initiators as shown in Table 1 were used, and each initiator was contin-uously supplied to the first and the second reactors so that the weight ratio of initiator to reaction mixture had a value shown in Table 1. Methyl methacrylate was used as a monomer and it was polymerized at varied average residence time and reaction temperatures shown in Table 1. As a result, syrups of such polymer content, viscosity and number average polymerization degree as shown in Table 2 were obtained.
The concentration ratios of remaining initiator to supplied one in the first and the second reactors were as shown in ~able 1. ~ny one of the syrups had a concentration of i~L377SZ

remaining initiator of less than 0.01 ppm, and did not show a change in either polymer content or viscosity even though it was kept still at 60C for 3 hours. Further, the distri-bution of polymerization degree of the polymer in each syrup was less than 2.2, as expressed in polydispersity.
Cast plates were produced, using the continuous polymerization apparatus in Example 1, in the same manner as in Example 1 except that azobisdimethylvaleronîtrile in amount as shown in Table 3 was added to the syrups and the resulting polymerizable liquid compositions were passed through the heat-polymerization zone during time as shown in Table 3, respectively. Intrinsic vicsosities and remaining monomer contents of the products thus obtained ~cast sheets) had values as shown in Table 3. All of them included no foam and caused no foaming by heating.
A half to 1 cc of dichloromethane was dropped on each ~f test pieces of the cast sheets described above and after 10 seconds, was wiped off using a guaze. Then, it was observed whether or not there was a solvent blot on each test piece. The test pieces had no blot except that the test piece of Example 7 had a slight blot. Further, test pieces were subjected to the accelerated weathering test during 500 hours according to ASTM D-1499. However, the test pieces showed no change in the external appearance.

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11~77sz Example lS
The same equipment as in Example 1 was used except that the volume ratio of first vessel-type reactor to second vessel-typ~ reactor to tubular reactor was 1 : 0.5 :
0.5. Methyl methacrylate containing 0.046 % of azobis-isobutyronitrile was continuously supplied to the first vessel-type reactor so that an average residence time in the reactor was 97.8 seconds. A methyl methacrylate solution containing 2 % of azobisisobutyronitrile was continuously supplied to the second reactor so that the amount of azobis-isobutyronitrile added was 0.024 % of the reaction mixture.
Polymerization was carried out while maintaining the temper-ature and pressure in each reactor at 160C and 6.0 atmo-spheres, respectively.
The syrup from the tubular reactor had a polymer content of 2~.8 % and a viscosity of 23.0 poises at 25C.
The weight ratio of polymers produced in the reactors was 62 : 32 : 6. The concentration ratios of the remaining ini-tiator to supplied initiator in the first and the second reactors were 1/21 and l/ll, respectively. The concen-tration of the remaining i~itiator in the final syrup was less than 0.01 ppm, The polymer in the syrup had a number average polymerization degree of 605, and its distribution of polymerization degree was 2.15, as expressed in poly-dispersity.
A cast plate was produced using the continuous polymerizatio~ apparatus in Example 1 in the same manner as in Example l except that 0.04 ~ of azobisdimethylvalero-nitrile was added to this syrup and the resulting poly-merizable liquid composition was passed through the heat-)7752 polymerization area during 18 minutes. The obtained product had a reduced viscosity of 2.26 dl/g and residual monomer content of 1.2 %, and included no foam and caused no foaming by heating.
Example 16 A 4-stage continuous reaction equipment having the following structure was used: the equipment consisted of a former part and a latter part; three vessel-type reactors with a ribbon-Like propeller were set up in the former part and the three reactors were connected in series; in the latter part was set up a tubular reactor having a mixing shaft to which pins were fixed at a right angle to the shaft, and to the inside wall of the reactor were fixed pins at a right angle to the wall and towards the mixing shaft;
and both pins were arranged so that they could wipe off matters attached to the opposite pins. A volume ratio of first reactor to second one to third one to tubular reactor was 1 : 0.5 : 0.5 : 0.1.
Polymeri7ation was carried out as follows:
methyl methacrylate containing 0.046 ~ of azobisisobutyro-nitrile was continuously supplied to the first vessel-type reactor to make an average residence time in the reactor 97.8 seconds. A methyl methacrylate solution containing 2 of additional azobisisobutyronitrile was continuously supplied to the second and third reactors so that the amounts of azobisisobutyronitrile added were 0.024 % and 0.032 %, respectively, of the reaction mixtures. Temper-atures in the reactors were 160, 162, 164 and 165C, re-spectively, and pressure in each reactor was 6.3 atmospheres.
The syrup from the tubular reactor had a polymer 11q~77~2 content of 45.3 % and a viscosity of 1,680 poises at 25C.
The weight ratio of polymers produced in the reactors was 43 : 22 : 32 : 3. ~he concentration ratios of remaining ini-tiator to supplied initiator in the first, second and third reactors were 1/21, 1~13 and 1/15, respectively. The concen-tration of the remaining initiator in the final syrup was less than 0.01 ppm. The polymer in the syrup had a number average polymerization degree of 615 and its distribution of polymerization degree was 2.12, as expressed in polydis-persity. When the syrup was diluted with the monomer to a polymer content of 35.0 %, the diluted syrup had a viscosity of 95.5 poises at 25C.
A cast plate was produced in the same manner as in Example 1 except that a polymerizable liquid composition having a polymer content of 35 ~ and a viscosity of 95.5 poises, prepared by diluting this syrup with methyl meth-acrylate and adding 0.04 ~ of azobisdimethylvaleronitrile to the diluted syrup, was used and said composition was passed through the heat-polymerization area during 14 minutes. The product had a reduced viscosity of 2.02 dl/g and included no foam and caused no foaming by heating.
Example 17 The same equipment as in Example 16 was used except that the volume ratio of the reactors was 1 : 0.5 :
0.33 : 0.2. Polymerization was carried out as follows:
methyl methacrylate was continuously supplied to the first reactor to make an average residence time in the reactor 106 seconds. Azobisisobutyronitrile was continuously supplied to the vessel-type reactors so that the amounts of azobis-isobutyronitrile added were 0.104 %, 0.054 % and 0.054 %~

11~77S2 respectively, of the reaction mixture. Temperatures in t~e reactors were 160, 163, 165 and 166C, respectively, and pressure in each reactor waæ 6.3 atmospheres.
The syrup from the tubular reactor had a polymer content of 62.5 % and a viscosity of 8,700 poises at 25C.
The concentration ratios of the remaining initiator to the supplied one in the first, second and third reactors were 1/24, 1/15 and 1/12, respectively. The concentration of the remaining initiator in the final syrup was less than 0.01 ppm. The polymer in the syrup had a num~er average poly-merization degree of 415 and its distribution of polymeri-zation degree was 2.lQ, as expressed in polydispersity.
When the syrup was diluted with the monomer to a polymer content of 40.3 %, the diluted syrup had a viscosity of 55.0 poises at 25C.
A cast plate was produced in the same manner as in Example 1 except that a polymerizable liquid composition having a polymer content of 40.3 % and a viscosity of 550 poises, prepared by diluting this syrup with methyl meth-acrylate and adding 0.05 ~ of bis(4-tert-butylcyclohexyl)-peroxydicarbonate to the resulting syrup, was used and the polymerizable liquid composition was passed through the heat-polymerization area during 13 minutes. The product had a reduced viscosity of 1.82 dl/g and a residual monomer content of 1.3 % and included no foam and caused no foaming by heating.
Example 18 The same e~uipment as in Example 1 was used except that the volume ratio of vessel-type reactor to tubular reactor was 1 : 0.2. A methyl methacrylate monomer 775~

containing 0.007 % of azobisdimethylvaleronitrile as a polymerization initiator was continuously supplied to the vessel-type reactor to make an average residence time in the reactor 225 seconds. The temperature and the pressure in each reactor were maintained at 130C and 5.0 atmospheres, respectively. A concentration ratio of residual initiator to supplied one was 1/40. The syrup coming out of the tubular reactor had a polymer content of 10.5 %, a viscosity of 2.8 poises at 25C, and a residual initiator concen-tration of less than 0.01 ppm. The polymer in the syrup had a number average polymerization degree of 2,810 and its distribution of polymerization degree was 2.02, as expressed in polydispersity.
The syrup was introduced into an evaporator and the monomer in the syrup was evaporated under reduced pressure to obtain a syrup having a polymer content of 18.1 ~ and a viscosity of 243 poises and then 0.07 % of azobis-dimethylvaleronitrile was added to the resulting syrup to a polymerizable liquid composition.
A cast plate was produced using the continuous polymerization apparatus in Example 1 in the same manner as in Example 1 except that said compositiGn was passed through the heat-polymerization area during 19 minu~es. The product had a reduced viscosity of 3.90 dl/g and a residual monomer content of 0.9 ~ and included no foam and caused no foaming by heating, Example_l9 A syrup having a polymer content of 25.5 % and a viscosity of 70.0 poises was prepared using the equipment of Example 1 and methyl methacrylat~ monomer containing 11~77SZ

0.045 % of azoblsdimethylvaleronitrile in the same manner as in Example 1 except that the temperature was 145C and said monomer was passed through the vessel-type reactor so that the average residence time in the vessel-type reactor was 304 seconds. The residual monomer concentration of the syrup obtained was less than 0.01 ppm and the polymer in the syrup had a number average polymerization degree of 1,080 and its distribution of polymerization degree was 2.06, as expressed in polydispersity. The syrup was diluted with methyl methacrylate to obtain a diluted syrup having a polymer content of 14.3 ~ and a viscosity of 1.1 poises. A
polymerizable liquid composition was prepared by dissolving 0.1 % of azobisisobutyronitrile in the diluted syrup and deaerated under reduced pressure. The composition was injected into a glass cell. The cell was assembled with two pieces of glass plate and gasket, the glass plates being properly apart from each other so as to produce a cast sheet of 3 mm in thickness, and the gasket being placed between the plates so as to form an enclosed space. The composition was polymerized at 65C for 4 hours and then at 120C for 2 hours to complete the polymerization. Cast sheet was thus obtained. This product had a reduced viscosity of 6.5 dl/g and a remaining monomer concentration of 0.4 %. Neither foaming during polymerization nor foaming by heating was observed, and the product had a beautiful appearance.
Example 20 A syrup, having a polymer content of 32.5 ~ and a viscosity of 5.2 poises at 25C, was prepared using the equipment of Example 1 by adding 0.15 % of azobisisobutyro-nitrile as a polymerization initiator to a monomer mixture 11~775Z

of styrene (30 %) and methyl methacrylate (70 ~) and polymerizing said monomer mixture under conditions at which a temperature was 150C and an average residence time in the vessel-type reactor was 180 seconds. The residual initiator concentration of obtained syrup was less than 0.01 ppm. The polymer in the syrup had a number average polymerization degree of 340, and its distribution of polymerization degree was 1.95, as expressed in polydispersity.
A polymerizable liquid composition was prepared by dissolving 2 parts by weight of ethyleneglycol dimeth-acrylate as a crosslinking agent and 1.5 parts by weight of benzoylperoxide as a polymerization initiator in 100 parts by weight of the syrup.
After deaeration under reduced pressure, the composition was injected into a polymerization mold for preparing a flat resin plate uniformly filled with 25 parts by weight of chopped strand of glass fiber (refractive index, 1.52) and immersed in the chopped strand. Then, the polymerization mold was dipped in a heating bath at a temperature of 85C during 1 hour to cure the composition by polymerization, No foam produced during the polymerization was observed in the obtained fiber reinforced resin plate having a thickness of about 1 mm and the resin plate had excellent transparency, a good appearance and a high bending strength of 1.620 kg/cm2 measured according to ASTM D-790.

Claims (25)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the continuous production of a prepolymer syrup comprising (A) continuously supplying a monomer comprising methyl methacrylate as a main component and a radical-polymerization initiator to a reaction zone wherein substantially a complete mixing is achieved, during which the reaction zone is kept under such a condition that the concentration of remaining initiator is 1/2 to 1/1,000 times that of the initiator supplied, or (A') continuously supplying said monomer to a first reaction area in a reaction zone wherein at least two reaction areas, in which substantially a complete mixing is achieved, are arranged in series, continuously supplying a radical-polymerization initiator at the same time to at least two reaction areas including the first one, and passing the monomer through the reaction areas successively thereby polymerizing the monomer into syrup, during which the reaction areas are kept under such a condition that the concentration of remaining initiator in at least the reaction areas to which the initiator is supplied is 1/2 to 1/1,000 times that of the initiator supplied, and then, optionally in the case of (A'), (B) introducing the resulting reaction mixture into a reaction zone wherein a piston flow is substantially achieved, and passing it through the zone, during which the temperature of the zone and the average residence time of the mixture are maintained so that the rest of the polymer in the final syrup is produced in the zone and the concentration of remaining initiator is reduced to substantially a negligible amount, thereby obtaining the final syrup, the distribution of polymerization degree of the polymer in the syrup being 3.0 or less as expressed in a polydispersity which is a ratio of weight average polymerization degree to number average polymerization degree.

2. A process according to claim 1, wherein said monomer is methyl methacrylate alone or a mixture of methyl methacrylate with an ethylenically unsaturated monomer copolymerizable therewith in amount of less than 50 % by weight based on the total monomers.

3. A process according to claim 2, wherein the monomer is methyl methacrylate alone or a mixture of methyl methacrylate with up to 20 % by weight of at least one ethylenically unsaturated monomer selected from the group consisting of methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, ethylene-glycol diacrylate, ethyl methacrylate, lauryl methacrylate, 2-hydroxyethyl methacrylate, ethyleneglycol dimethacrylate, acrylonitrile, methacrylonitrile, acrylamide, acrylic acid, methacrylic acid, vinyl chloride, styrene and .alpha.-methyl-styrene.

4. A process according to claim 1, wherein said radical-polymerization initiator has a half-life period of 5 seconds or less at a temperature of 180°C or less.

5. A process according to claim 4, wherein the radical-polymerization initiator is at least one member selected from the group consisting of azobisisobutyro-nitrile, azobisdimethylvaleronitrile, azobis(4-methoxy-2,4-dimethylvaleronitrile) and azobiscyclohexanenitrile, benzoyl peroxide, lauroyl peroxide, acetyl peroxide, capryl per-oxide, 2,4-dichlorobenzoyl peroxide, isobutyl peroxide, acetylcyclohexyl sulfonyl peroxide, tert-butyl peroxy-pivalate, tert-butyl peroxy-2-ethylhexanoate, isopropyl peroxydicarbonate, isobutyl peroxydicarbonate, sec-butyl peroxydicarbonate, n-butyl peroxydicarbonate, 2-ethylhexyl peroxydicarbonate and bis(4-tert-butylcyclohexyl) peroxy-dicarbonate.

6. A process according to claim 1, wherein said monomer is pre-heated by passing it through single tube at a Reynolds' number of 5,000 or more.

7. A process according to claim 1, wherein the temperature of said reaction zone wherein substantially a complete mixing is achieved is controlled by changing the temperature to which the monomer supplied is pre-heated.

8. A process according to claim 1, wherein the number of said reaction areas in said reaction zone wherein substantially a complete mixing is achieved is 1 to 10 and the amount of polymer produced in each reaction area is 3 to 35 % by weight based on the monomer supplied.

9. A process according to claim 8, wherein the number of the reaction areas is 2 to 5 and the amount of polymer produced in the each reactor is 5 to 25 % by weight.

10. A process according to claim 1, wherein said reaction areas are stirred at Reynolds' number of 2,000 or more.

11. A process according to claim 10, wherein said concentration of remaining initiator in at least the reaction areas to which the initiator is supplied is 1/5 to 1/1,000 times that of the initiator supplied and at the same time said polydispersity is 2.5 or less.

12. A process according to claim 10, wherein said concentration of remaining initiator in at least the reac-tion areas to which the initiator is supplied is 1/10 to 1/500 times that of the initiator supplied and at the same time said polydispersity is 2.2 or less.

13. A process according to claim 1, wherein said succeeding reaction zone is composed of at least one reac-tion area.

14. A process according to claim 13, wherein the amount of polymer produced in the first reaction zone is 60 to 99.5 % by weight of the polymer in the final syrup.

15. A process according to claim 13, wherein the temperature of said second reaction zone is not less than that of the first reaction zone.

16. A process according to claim 13, wherein said average residence time of reaction mixture in the second reaction zone is 0.1 to 2 times that of the reaction mixture in the first one.

17. A process according to claim 13, wherein a back mixing coefficient of stirring in said second reaction zone is 20 % or less.

18. A process according to claim 13, wherein said concentration of remaining initiator in the final syrup is 1 ppm or less.

19. A process according to claim 18, wherein said concentration of remaining initiator in the final syrup is 0.01 ppm or less.

20. A process according to claim 13, wherein the polymer content of said syrup is is to 80 % by weight and the viscosity of the syrup is 0.5 to 10,000 poises at 25°C
and the number average polymerization degree of polymer in the syrup is 300 to 6,000.

21. A process according to claim 20, wherein the polymer content is 20 to 40 % by weight and the viscosity is 5 to 500 poises at 25°C and the number average polymeri-zation degree of polymer is 300 to 2,000.

22. In a process for continuously producing a methyl methacrylate resin plate, using a pair of endless belts arranged and constructed so that the lower run of the upper belt is positioned above the upper run of the lower belt, which comprises feeding a polymerizable composition, together with a pair of continuous gaskets, into a spacing defined between a lower run of the upper belt and the upper run of the lower belt both of which are arranged such that the pair of the endless belts are driven concurrently in the same direction at substantially the same speed, said gaskets being arranged so as to circumscribe the spacing serving as a pair of seals to confine a cavity whilst moving concur-rently with the belts in contact with the opposing surfaces of the belts, passing the composition through a portion of the path of the belts where said composition is completely polymerized, and removing the polymerized plate from the end portion of the belts at the discharge side thereof, an improvement which comprises using a polymerizable liquid composition prepared by adding a radical-polymerization initiator to the syrup obtained by the method of claim 1 as said polymerizable composition.

23. A process according to claim 22, wherein said polymerizable composition has a polymer content of 15 to 50 by weight and a viscosity of 0.5 to 1,000 poises at 25°C.

24. In a process for producing a methyl meth-acrylate resin plate, according to cell cast process by injecting a polymerizable liquid composition into a mold having a space enclosed with two pieces of glass plate and a gasket therebetween, completely polymerizing said polymeri-zable liquid composition by heating, and removing the resulting resin plate from the mold, an improvement which comprises using a polymerizable liquid composition prepared by dissolving a radical-polymerization initiator in the syrup obtained by the method as defined in claim 1 as the poly-merizable composition.

25. In a process for producing a glass fiber reinforced resin plate by immersing or mixing glass fiber with a polymerizable composition and curing the resulting composition by heating, an improvement which comprises using a polymerizable liquid composition prepared by dissolving a radical-polymerization initiator in the syrup obtained by the method as defined in claim 1 as said polymerizable composition.