EP0927197A1

EP0927197A1 - Method for producing methyl-methacrylate polymers in a recycle reactor

Info

Publication number: EP0927197A1
Application number: EP97943868A
Authority: EP
Inventors: Klaus-Dieter Hungenberg; Michael BAUMGÄRTEL; Jürgen Koch; Wolfgang Fischer; Reiner Thiele
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1996-09-18
Filing date: 1997-09-18
Publication date: 1999-07-07
Also published as: WO1998012229A1; DE19638094A1

Abstract

The invention concerns methyl-methacrylate polymers with an heterogeneity of 1.0 to 1.2. Said polymers are obtained continuously by polymerization of monomers consisting of no less than 90 % by weight methyl-methacrylate in a recycle reactor, which has preferably at least a tubular reactor or a multitube-flow reactor, avoiding unwarranted wall deposits. Said polymers are produced in such a way that temperature during polymerization stands at 135-150 DEG C, the recycle relation is bigger than 20 when divided by the number of chargings per circulation loop, the mean effective axial circulation is faster than 5 cm/s, a polymerization initiator having a batch half change value of 3 to 10 min. is used, and the inflow concentration of the initiator is set in such a way that by a mean residence time of 1.3 to 2.5 hours a polymer rupture of 0.50 to 0.70 at the output of the reactor is achieved. Preforms having very good optical properties can be produced with the homogeneous polymers.

Description

Process for the preparation of methyl methacrylate polymers in a circulation reactor

The invention relates to a continuous process for the production of methyl methacrylate polymers with a narrow molecular weight distribution by polymerization in bulk in a circulation reactor.

It is known to produce polymethyl methacrylate, which is used as acrylic glass in amounts of more than 500,000 t per year, by polymerizing methyl methacrylate in emulsion, suspension or by bulk or bulk polymerization (previously also called block polymerization). For a long time, high-molecular semi-finished products made of polymethyl methacrylate as high-quality acrylic glass have been produced by casting processes (flat chamber process, double-belt process), 20-25% solutions of polymethyl methacrylate in mono methyl methacrylate obtained by prepolymerization often being used to shorten the production time. For continuous polymerization processes, prepolymerization in a stirred tank was combined with a subsequent reaction in screw extruders. It is e.g. from DE-A 2724360 also known, thermoplastically processable molding compositions of methyl methacrylate-cyclohexyl methacrylate-maleic anhydride copolymers in a tank reactor in bulk at 140 ° C to free-radical polymerization to a steady state conversion of 40 mol%, then in two following Process stages are first concentrated in a coiled tube evaporator at 220 to 230 ° C with nitrogen as the propellant gas and then in a screw evaporator at 230 ° C the polymer concentration in the product outlet is increased to 99 mol%. The monomers from the last two process stages are returned to the stirred tank reactor of the first process stage. However, the process often does not run smoothly.

In contrast to bulk polymerization, for example styrene, methyl methacrylate, when polymerized alone or largely alone in bulk, has many special features that must be taken into account in bulk polymerization. For example, methyl methacrylate shows an enormous polymerization shrinkage of 20 to 21% during the polymerization. In addition, methyl methacrylate in bulk polymerization already shows a polymerization acceleration of only about 20% due to the Trommsdorff or gel effect, which leads to an acceleration of the reaction rate of 1-2 orders of magnitude with isothermal reaction control. With the high heat of polymerization of methyl methacrylate combined with low thermal conductivity of the system, a sharp rise in temperature can very quickly occur, which can result in a hardly manageable reaction. Furthermore, the methyl methacrylate polymerization shows a glass effect and ceiling effect influencing the polymerization process (limit conversion due to freezing of the reaction, above the glass transition temperature T _g of 105 ° C. decrease in conversion due to depolymerization / polymerization equilibrium, the depolymerization due to comonomers and Controller can be influenced). Finally, it should be noted that a poly methacrylate with an average molecular weight M _w of more than 300,000 can only be deformed thermo-elastically. With this behavior of the methyl methacrylate during the bulk polymerization, it is understandable that very special requirements have to be met for continuous substance polymerization of the methyl ethacrylate and that there are often disruptions in its implementation which also lead to non-uniform products. Inadequate cooling or blind spots in parts of the reactor lead to serious malfunctions. For example, slightly disruptive deposits form on the reactor walls in the reactor, which must be removed. If one wants to carry out the bulk polymerization of the methyl methacrylate in intensively mixed continuous stirred tanks, it is found that the specific cooling surface required in the industrial boiler is often too small. if this is not increased by cooled internals. In such boilers with internals, however, dead zones can only be avoided by very complex constructive solutions. In addition, the otherwise customary use of evaporative cooling in bulk polymerization of methyl methacrylate is ruled out.

The object of the present invention was to polymerize methyl methacrylate in bulk continuously in circuit reactors to give polymers with very little non-uniformity, without the formation of large amounts of disruptive deposits on the circuit reactor walls during long-term operation. The task also included delimiting the areas of the operating variables (half-life of the initiator, conversion, flow rate, circulation ratio, polymerization temperature, average residence time) in which the polymerization proceeds without problems and leads to a high-quality product.

It has now been found that the object is achieved and methyl methacrylate polymers with a very narrow molar mass distribution or little non-uniformity can be produced, and the disadvantages mentioned can be avoided or reduced if methyl methacrylate containing up to 10% by weight the total amount of monomers other copolymerizable olefinically unsaturated monomers

REPLACEMENT SHEET (REGEL26} polymerized in bulk in a circulation reactor with feed mixture feed, reactor outlet, mixing device, circulation pump and cooling surface such that a) the circulation ratio K _{R is} greater than 20 / n _E , where n _{E represents} the number of feeds per circulation loop, and preferably K _{R is} about 10 to 60, b) the mean effective axial flow velocity W _{eff # ax is} greater than 5 cm / second and is preferably 7 to 40 cm / second, c) the polymerization temperature is 135 to 150 ° C, d) a polymerization initiator is used whose batch half-life tι _{/ 2} ( _batch ) is between 3 and 10 and preferably between 4 and 6 minutes at the polymerization temperature, e) the initiator feed concentration being adjusted so that the reaction mass has an average residence time t of 1 , 3 to 2.5 hours at the reactor outlet, a polymer mass fraction y _p of 0.50 to 0.70 and in particular of about 0.55 to 0.65 is achieved.

The method according to the invention is carried out in a circuit reactor which has at least one inlet, outlet or reactor outlet, at least one mixing device such as a static mixing element, a circuit pump and cooling surface. Circuit reactors are described in the technical literature and can be assembled by the process engineer in an appropriate manner if the critical process characteristics are known. The circulation reactor is preferably filled hydraulically, so that the mass flows of the inlet and the outlet are the same during stationary operation. The low-viscosity inlet mixture, which generally has a temperature of about 20 ° C. when added, consists primarily of the freshly fed monomers and the volatile and then condensed fractions, in particular monomers, which are returned after leaving the circulation reactor, furthermore from the metered-in polymerization initiator and usual additives, such as added controllers. The metered inlet mixture is advantageously already pre-mixed before mixing devices, such as, in particular, static mixers, rapidly mix the low-viscosity inlet mixture into the higher-viscosity circuit stream after each inlet position. Since narrow molar mass distributions can only be achieved with small temperature and concentration differences in the polymerization reactor, rapid mixing in of the premixed feed at high circulation ratio is of great importance. This keeps the axial temperature and concentration gradients small.

REPLACEMENT BLAπ (RULE 26) Preferred mixing devices for the feed are static mixers such as commercially available SMX mixers

Suitable monomers for the process according to the invention are methyl methacrylate, which, however, can contain up to 10 and preferably up to 5% by weight of the total amount of monomers another copolymerizable olefinically unsaturated compound as comonomer, and mixtures thereof. Other esters are particularly suitable as such comonomers the methacrylic acid and the acrylic acid with an alcohol having 1 to 8 carbon atoms, maleic anhydride d and preferably styrene.

The feed or the metered run-in mixture further contains a polymerization initiator which, according to the process according to the invention, has a batch half-life tι _{/ 2 b} at _{ch of} between 3 and 10 and preferably between 4 and 6 minutes at the polymerization temperature, the polymerization temperature being 135 to 150, preferably 140 is up to 145 ° C. The feed concentration of the polymerization initiator is to be set so that with an average residence time t of the reaction aces in the reactor of 1.3 to 2.5 and preferably 1.5 to 2.0 hours at the reactor outlet or outlet a polymer fraction y _p of 0.5 to 0.7, in particular 0.55 to 0.65 and particularly preferably 0.57 to 0.63, is achieved

Suitable polymerization initiators with the stated half-life of the batch can be found in the specialist literature. Butyl peracetate (TBPA) and tert. Butylper-3, 5, 5-trιmethylhexanoat (TBPN) called.

In view of the fact that, particularly in the case of methyl methacrylate polymers, the molecular weight or the chain length strongly influences the processing properties of the compositions, it is advantageous to add small amounts of a known regulator, such as chlorinated hydrocarbons or, in particular, mercaptans, with the addition of the reaction composition such as dodecyl mercaptan, the amount of which is advantageously 10 to 1000 ppm of the amount of monomers.

The number n _{E of} the feed or feed points per loop can be 1 to 4. Higher numbers generally do not occur in an industrial circulation reactor because of the high pressure loss in the mixing devices or static mixers. Preferably, n ^ 1 or 2. If there is more than one feed per circuit loop, it is advantageous to use approximately the same premixed feed volumes Vo0 <02 etc. and the Arrange inlet positions so that approximately equal reaction volumes are between them.

In the case of an exothermic reaction such as the polymerization of 5 methyl methacrylate, it is understandable that a sufficient cooling surface A _κ must also be present in the circuit _reactors for the entire reaction volume V _R. Preferred specific cooling surfaces A _κ , _S p _ez (= A _κ / V _R ) are about 20 to about 150 m ² / m ³ reactor content in the cycle reactors. To avoid undesirable local viscosity increases, the temperature of the coolant should, if possible, not be more than 5 ° C. below the average temperature of the reaction mass (polymerization temperature).

In order to achieve stationary conditions in the circulation reactor and to prevent the formation of stagnant polymer layers on the reactor wall, high flow velocities in the circulation reactor are necessary. According to the invention, the mean effective axial flow velocity W _ef . _{ax be} greater than 5 cm / second. An upper limit is given by the pressure loss. 20 In practice, advantageous average effective axial flow velocities W _{eff, a} χ are between 7 and 40 cm / second. In order to achieve the high mean effective axial flow velocities W _eff , _a x, the circulation flow κ <which represents the volume flow in m ³ / hour after the last feed of the circulation loop 25 and shortly before the position of the outlet or reactor outlet is set accordingly high.

The circulation ratio K _R is the quotient of the circulation flow V «measured before the reactor outlet - and the sum of the inflows

³⁰ currents Voi + V0 ₂ ⁺ 03 + V ₀ 4 P ^ o loop. In the method according to the invention, the circulation ratio K _{R should be} greater than 20, divided by n _E , the number of feeds in via the circulation loop. K _R is preferably about 10 to 60. The average circulation time t _z in the circulation reactor is in particular

35 more at about 1 to 8 minutes.

Particularly suitable circulation reactors are those whose predominant reaction volume consists of at least one, preferably 2 to 6 tube or tube bundle reactors. Allow tubular reactors

* ^υ both the processing of reaction masses with higher viscosities as well as a good dissipation of the heat of reaction through large specific cooling surfaces Aκ, s _pez - tube bundle reactors offer particularly large reactor volumes with large specific cooling surfaces. Tube bundles or tubes are particularly advantageous

* 5 mi non-cooled static mixers (such as Kenics mixers) with a cooling jacket and tubes with cooled static mixers such as SMXL elements, which ensure good cross-mixing of the reaction mixture effect in the tubular reactor and, with the large internal goods exchange surfaces, allow temperature-controlled reaction control even in the case of strongly exothermic polymerization reactions. No back-mixing takes place in the circuit loop itself, the reaction mass is only back-mixed by the circulation pump. Advantageous static mixers contain mixing elements with a framework of interlocking, crossing webs.

Suitable circulation reactors are shown schematically in FIGS. 1 to 4 and described in the examples. Mixing devices, preferably static mixers (2, 7, 14, or 20 and 21), are arranged directly behind the feed line or feeds into the reaction mixture (1, 6, 13 or 18 and 19) in order to allow the mederviscose to be mixed in quickly To use mixtures in the highly viscous reaction mass in the reactor. The reaction mass then runs through the actual reactors (3, 8, 15, 22), in particular tubes, tube bundles, tubes with cooled static mixers such as SMXL elements from Sulzer (8) (with coolant inflow (11) and coolant outflow ( 12)) or cylindrical tubes with Kenics mixers and double jacket for cooling (15).

The circuit reactor of FIG. 4 has 2 feeds with associated mixing devices m of the circuit loop.

In the circulation reactor, the circulation flow Vκ _r (m ³ / hour) is measured directly before the reactor exits or exits (4, 9, 16, 23). It is important that a flow without jerk mixing is effected in the circulation reactor (except in the circulation pump (5, 10, 17, 24)).

The reaction mass, which leaves the circulation reactor at the reactor outlet or outlet (4, 9, 16, 23), has a polymer mass fraction y _p of 0.5 to 0.7 there, as stated, and thus still contains a lot of unreacted monomers. The reaction mass which has escaped can be worked up in a known manner, the volatile fractions, such as the monomers, contained in the mass expediently being recycled in whole or in part after their condensation. One way of working up in snake evaporators and screw evaporators is described in process stages 2 and 3 of DE-A 2724360.

The methyl methacrylate polymers produced by the continuous polymerization process according to the invention are distinguished by a narrow molar mass distribution or very little non-uniformity U _n = P _w / P _n − 1, which has values from 1.0 to 1.2. The polymers obtained are outstandingly thermoplastic

SPARE BLADE (RULE 26) Processable and, due to their uniformity, transparency, brilliance and very good mechanical strength, they are ideal for the production of molded parts with high-quality optical properties such as domelights, lighting fixtures, 5 glazing, lenses, prisms, reflectors or light guides.

The following examples and comparative experiments are intended to explain the invention further, but not to limit it

0 Example 1

A tube-and-tube circuit reactor, as shown schematically in FIG. 1, was used as the circulation reactor. It has two tube bundles (3), each with 19 tubes, with an inner tube diameter 5 ^ of 0.03 m and a height H _R of 3 m The circulation reactor has a specific cooling area of 107 m ₂ / m ^3. The feed or feed mixture contained, in addition to 100 ppm of dodecyl merecaptan as regulator, 97.5% by weight of methyl methacrylate and 2.5% by weight of styrene, the percentages being based on one another refer to the total amount of monomers in the feed. The polymerization initiator was tert. Butyl peracetate (TBPA) is used, which has a batch half-life tι _{/ 2} ( _batch ) of 5.6 minutes at the average polymerization temperature of 145 ° C. The initiator feed concentration I ₀ of 9 x 10 ^{~ 4} kmol / m ³ has been set so that, for a mean residence time of the reaction mass 5 of 1.5 hours, the polymer weight fraction y t _p at the reactor outlet at about 0.60

A circulation _flow V _KΓ of 3 m ³ / hour was set, which corresponds to an average effective axial flow ^velocity ^{υ w} _{eff ax} ^{ιn in} the individual tubes of the tubular bundle from the circulation reactor of 6.2 cm / second. The circulation ratio K was 45 and the average circulation time t _z 1.5 minutes

The circuit reactor was able to operate in long-term operation without any significant 5 deposits. Reactor walls are operated stably. The resulting degassed polymer had a mass-average degree of polymerization P _w of 1230. The molecular nonuniformity of the product U _n = P _w / P _n - 1, determined by gel permeation chromatography, was 1.1 0

Example 2

It was polymerized in the same tube bundle cycle reactor as in Example 1 and in the same way, but tert-butylper-3, 5,5-tr methylhexanoate (TBPN) was used as the polymerization initiator, which was one at the average polymerization temperature in the cycle reactor of 145 ° C. Batch half-life tχ _{/ 2} ( _ba tc _h ) of 4.8 minutes Has. The initiator feed concentration I ₀ was 8 × 10 ^-4 kmol / m ³ and was determined so that with a mean residence time of the reaction mass of 1.5 hours the polymer mass fraction is about 0.60. The flow parameters corresponded to those of Example 1:

Vκ _r = ³ m ³ / hour, W _eff , _a χ = 6.2 cm / second, K _R = 45, t _z = 1.5 min.

The reactor could be operated in long-term operation without any significant build-up on the reactor walls. The resulting polymer had a mass average degree of polymerization P _w of 1250 and a non-uniformity of 1.1.

Example 3

A circulation reactor as shown schematically in FIG. 2 was used as the circulation reactor. It contained tubes with cooled static mixers (8) Sulzer-SMXL elements with an inner diameter of 0.15 m. The feed or feed mixture corresponded to that of Example 1, but tert-butylper-3,5,5-trimethylhexanoate (TBPN) was used as the polymerization initiator, the batch half-life and initiator feed concentration of which are given in Example 2. A circulation flow κ _r of 4 m ³ / hour was set. This corresponds to an average effective axial flow velocity W _{eff # ax} of 7.1 cm / second. The circulation ratio K _R was 60 and the average circulation time t _z 1 minute. The cycle reactor could be operated stably in long-term operation. Wall coverings were not found. The resulting degassed polymer had a mass average degree of polymerization P _w of 1220 and a molecular non-uniformity of 1.1.

Example 4

A Kenics cycle reactor as shown schematically in FIG. 3 was used as the cycle reactor. It contained cylindrical jacketed pipes (15) with non-cooled static mixers (Kenics mixers) of a length L _R of 12 m and an inner diameter dj. of 0.1 m. The feed or feed mixture corresponded to that of Example 1, and the polymerization initiator given in Example 1 with the initiator feed concentration I _{0 given there was also} used. A circulation _flow V _RΓ of 2 m ³ / hour was set. This corresponds to an average effective axial flow velocity W _eff , _a χ of 7.1 cm / second. The circulation ratio K _R was 30, the average circulation time t _z 3 minutes. The cycle reactor worked trouble-free in long-term operation. The resulting product had one mass average degree of polymerization P _w of 1210 and a non-uniformity value of 1.1.

Comparative experiment 1

The procedure was as in Example 1, but tert-butyl peroxide (TBPO) was used as the polymerization initiator, which has a batch half-life tχ ₂ ( _batch ) of 1.24 hours at the average polymerization temperature of 145 ° C. The initiator feed concentration I _D of 1.7 × 10 ^-3 kmol / m ³ was determined in such a way that with a mean residence time t of the reaction mass of 1.5 hours at the reactor outlet, a polymer mass fraction of about 0.60 is achieved. A circulation _flow V _KΓ of 3 ³ / hour was set. The average effective flow velocity W _{ef, ax} in the tubes of the tube bundles was 6.2 cm / second, the circulation ratio K _R was 45 and the average circulation time t _z 1.5 minutes. The operation of the tube bundle cycle reactor was characterized by an unstable behavior. The desired stationary operating point could not be set. After the reactor was shut down, the formation of a substantial wall covering was observed.

Comparative experiment 2

The procedure was as in Example 2, polymerization was carried out in a tube bundle cycle reactor as shown in FIG. 1 and, as in Example 2, as the polymerization initiator. Butylper-3, 5, 5-trimethylhexanoate (TBPN) used with the feed concentration I ₀ given in Example 2. A circulation _flow V _KΓ of 1.5 m ³ / hour was set. This corresponds to an average effective flow velocity W _{eff; a} χ in the tubes of the tube bundle of 3.1 cm / second. The circulation ratio K _R was 30 and the average circulation time t _z 3 minutes. After a long period of operation, the cycle reactor tended to vibrate. After the reactor was switched off, wall coverings were found in the reactor.

Comparative experiment 3

The procedure was as in Example 3, a circulation reactor according to FIG. 2 was used, but tert-butyl peroxide (TBPO) with the initiator feed concentration I ₀ given in Comparative Experiment 1 was used as the polymerization initiator as in Comparative Experiment 1. A circulation _flow V _KΓ of 4 m ³ / hour was set. This corresponds to an average effective axial flow velocity W _eff , _ax of 7.1 cm / second. The circulation ratio K was 60 and the average circulation time t _z 1 minute. The circulation reactor could only be operated for a short time. A steady state in the reactor could not be achieved. After the reactor was turned off, heavy wall coverings were found in it.

Comparative experiment 4

The procedure was as in Example 3, including a Sulzer cycle reactor according to FIG. 2 and, as in Example 3, as a polymerization mediator. Butylper-3, 5, 5-trιmethylhexanoat (TBPN) with the same initiator feed concentration I ₀ used. A circulation flow of 2 ³ / hour was set. This corresponds to an average effective axial flow rate W _{eff, ax} of 3.4 cm / second. The circulation ratio K _R was 45 and the average circulation time t _z 2 minutes. After a relatively short operating time of the cycle reactor, temperature fluctuations occurred. After the reactor was switched off, wall coverings were present on the cooled mixing elements.

Example 5

Extrapolation to an industrial tube bundle cycle reactor:

Reactor geometry: 4 bundles of tubes1 with bundle slopes of L _R = 6 m each

Number of individual tubes z _R = 300

Single pipe diameter d _± - 0.03 m

Total reactor volume V _R = 5.5 m ³

Total cooling area A _κ = 678 m ² Specific cooling area A _{K; Spez} = 123 m ² / m ³

Inlet positions (premixed inlet): Number: n _E = 2

Feed in approximately equal proportions Feed in before the 1st and 3rd tube bundle

Feed mixture: approx. 97.5% by weight methyl methacrylate, based on total monomers approx. 2.5% by weight styrene, based on total monomers approx. 20 pp dodecyl mercaptan, based on monomers

8 x 10 ^-4 kmol / m ³ ter. Butylper-3, 5, 5-trimethylhexanoate

Total inlet flow V = Voi ⁺ V ₀₂ = 3.67 m ³ / hour Average residence time t = V / V = 1.5 hours Average temperature of the reaction mass T = 145 ° C polymer mass break at the reactor outlet y _p = 0.60 cycle flow Vj _r = 100 ³ / hour

ERSATZBL _Ä TT (REGEL26) Circulation ratio K _R = 27

Effective axial flow velocity in the tubes of the bundles W _eff , _ax = 13.1 cm / second Average temperature of the coolant T _κ = 140 ° C circulation time t _z = 3.3 minutes

Production output polymer m _P = 15 kt / year at 1 year = 8000 operating hours

Example 6

Extrapolation to industrial SMXL cycle reactor: reactor geometry:

8 SMXL elements (cylindrical) Length L _R = 4 m inner diameter d = 0.5 m total reactor volume V _R - 5.2 m ³ total cooling area A _κ = 250 m ² Specific cooling area A _KιΞpez = 48 m ² / m ³ inlet positions (premixed feed): Number: n _E = 2 feed in equal proportions

Feed before the 1st and 5th segment

Feed mixture: approx. 97.5% by weight methyl methacrylate, based on total monomers approx. 2.5% by weight styrene, based on total monomers approx. 20 pp dodecyl mercaptan, based on monomers 8 × 10 ^"4 kmol / m ³ tert. Butyl 3, 5, 5, trimethylhexanoate

Total inflow flow V = V_ι + V ₀₂ = 3.35 m ³ / hour Average residence time _ = V _R / = 1.55 hour

Average temperature of the reaction mass T = 145 ° C

Polymer mass fraction at the reactor outlet y _p = 0.61

Circular flow Vκ _r - 50 m ³ / hour

Circulation ratio Kp = 14.9 Effective axial flow rate W _e _ _f , _ax = 8.8 cm / sec.

Average coolant temperature T _κ = 139 ° C

Circulation time t = 6.2 minutes

Production output polymer rh _p = 13.9 kt / year at 1 year = 8000 operating hours

Claims

claims

1. Process for the preparation of methyl methacrylate polymers

5 with a small inconsistency, characterized in that methyl methacrylate, which can contain up to 10% by weight of the total monomer amount of another copolymerizable olefinically unsaturated monomer, in bulk in a circuit reactor with inlet mixture feed, reactor outlet, 0 mixing device, circuit pump and cooling surface as above polymerizes that a) the circulation ratio K _{R is} greater than 20 / n _E , where n _{E represents} the number of feeds per circulation loop, 5 b) the mean effective axial flow velocity ^w _eff , _a x is greater than 5 cm / second , c) the polymerization temperature is 135 to 150 ° C, d) a polymerization initiator is used, the batch half-life tι ₂ (batc _h ) is between 3 and 10 minutes at the 0 polymerization temperature, e) the feed concentration of the polymerization initiator is set so that with an average residence time t of the reaction mass of 1.3 to 2.5 hours a polymer mass fraction y _p of 0.50 to 0.70 is achieved at the reactor outlet 5.

2. The method according to claim 1, characterized in that the circulation ratio K _{R is} about 10 to about 60.

3. The method according to claim 1 or 2, characterized in that the mean effective axial flow velocity W _{eff> ax} in the circulation reactor is 7 to 40 cm / second.

5 4. The method according to any one of claims 1 to 3, characterized in that the average circulation time t _z in the cycle reactor is about 1 to 8 minutes.

5. The method according to any one of claims 1 to 4, characterized in that the batch half-life tι_ _{/ 2} ( _ba tc _h ) of the polymerization initiator used is in the range from 4 to 6 minutes at the polymerization temperature.

45 68/96

6. The method according to any one of claims 1 to 5, characterized in that as copolymerizable olefinically unsaturated comonomers styrene, maleic anhydride, an ester of acrylic acid or methacrylic acid with an alcohol with 1 to 8 C-

5 atoms or mixtures thereof can be used.

7. The method according to any one of claims 1 to 6, characterized in that the circuit reactor contains a static mixer as a mixing device.

10

8. The method according to any one of claims 1 to 7, characterized in that the circulation reactor contains at least one tube or tube bundle reactor.

15 9. The method according to any one of claims 1 to 8, characterized in that the circuit reactor has tube bundles or tubes with internal non-cooled static mixers and with a cooling jacket.

20 10. The method according to any one of claims 1 to 9, characterized in that the circuit reactor has tubes with internally cooled static mixers.

11. The method according to any one of claims 1 to 10, characterized in that the circuit reactor has a specific cooling area A _{K (Ξpe2} _ of about 20 to about 150 m ² per m ^{3 of} reactor content.

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