WO1997001585A1

WO1997001585A1 - Process for the anionic polymerization of vinyl aromates

Info

Publication number: WO1997001585A1
Application number: PCT/EP1996/002738
Authority: WO
Inventors: Wolfgang Loth; Konrad Knoll; Hermann Gausepohl
Original assignee: Basf Aktiengesellschaft
Priority date: 1995-06-29
Filing date: 1996-06-20
Publication date: 1997-01-16
Also published as: DE19523587A1

Abstract

The invention concerns a process for the anionic polymerization or copolymerization of vinyl aromatic compounds such as styrene in solution with an initial monomer concentration of between 40 and 100 wt.% under non-isothermic conditions, preferably in a pipe reactor or pipe assembly reactor, of which a substantial part is provided with means preventing the exchange of heat with the environment, toluene, methylcyclohexane, ethylbenzene or Decalin preferably being used as solvent. The temperature of the polymer solution leaving the reaction chamber is at least 170 °C. A styrene polymer containing fewer than 50 ppm monomers is produced.

Description

Process for the anionic polymerization of vinyl aromatics

Description 5

The invention relates to a continuous process for the rapid, economical and safe production of vinylaromatic polymers such as polystyrene, which is thereby also obtained in improved quality, namely improved thermal stability.

The process uses anionic polymerization, the high space-time yield resulting in a small and safely controllable reactor. The heat of reaction obtained is used in the non-isothermal process according to the invention to store energy for the subsequent removal of volatile constituents of the reaction mixture by heating the reaction medium to near the degassing temperature, which means a further advantage. 0

The anionic polymerization of vinyl aromatics has been known for a long time. Studies on the continuous polymerization of styrene have recently been carried out by D.B. Priddy, M. Pire (J. Appl. Polym. Sei .; 37 (1989) 392-402) (hereinafter referred to as 5 Dl) and by W. Michaeli, H. Höcker, U. Berghaus, W. Frings (J. Appl. Polym. Sci. 48 (1993) 871-886) (cited below as D2).

Reference is also made to EP-A-522 641, DE-A-17 70 261 and DE-A-42 35 785 30 (D3, D4 and D5).

In DI, the continuous polymerization of styrene with n-butyllithium in ethylbenzene in a continuously operated tank reactor (CSTR) at 90 to

35 110 ° C described. The mean residence time is more than

1.5 hours. It is mentioned that the cleaning of the starting materials is to be regarded as critical and that temperatures above 110 ° C. lead to high molecular weight fractions due to thermal termination (Li-H elimination).

40

The process described can be operated with certain restrictions in a plant which is set up for radical polymerization, but because of the low possibility of increasing the space-time yield compared to the yield,

45 which can already be achieved with free-radical (possibly purely thermal) polymerization, has hardly any advantages, since the relatively high costs for the initiator required Consumable advantage again. The enthalpy of reaction is essentially removed via the reactor wall, which further reduces the economy. According to D2, D3, D4, the continuous, anionic polymerization of styrene can be operated in an extruder at temperatures between 150 and 300 ° C or 50 to 150 ° C (D4). It must be stated that the authors' assumption that the intensely red coloration observed at the exit of the extruder should result from living styryl anions has not been confirmed. Rather, it can be assumed that after the end of the reaction, all the styryl anions have disappeared due to thermal termination (disproportionation). The red color comes from the allylic chain ends.

In addition, the use of a technically complex extruder as a reaction space is obviously uneconomical, to which there is the fact that the enthalpy of reaction must be removed via the extruder wall where it cannot be practically collected technically. In addition, procedural problems arise from the fact that an extruder is difficult to operate with liquid feedstocks (sealing problem of the drive shafts; instability of the conveyance due to fluctuating viscosity in the reaction zone).

This is also not remedied by the proposal in D4 to eliminate the low initial viscosity by moderate pre-conversion (approx. 20%) and only then to continue polymerizing on the extruder.

D5 describes the possibility of allowing the free-radical polymerization or copolymerization of styrene to take place under a partially adiabatic reaction regime and in this way to achieve higher conversions, i.e. to reaction mixtures with a lower content of residual monomers.

The object of the invention is to provide a process for the continuous, rapid, economical and safe production of vinylaromatic polymers such as polystyrene, which should also be obtained in improved quality, namely improved thermal stability . The prejudice gained from the study of the processes described in D2, D3 that the living ends of polymer chains growing at high temperatures were lost through disproportionation had to be overcome. The invention is based on the observation that the discontinuous, anionic polymerization of styrene in, for example, decalin as solvent leads to an unexpectedly narrow molar mass distribution even at a temperature of 170 to 270 ° C. if the reaction is carried out essentially without heat exchange with the Environment (ie adiabatically) is carried out and the residence time of the reaction mixture at the highest temperature occurring is limited to less than about 30 seconds. Here quantitative turnover can be achieved.

The red-colored, viscous solution obtained can be converted into a colorless liquid by protonation using customary means (alcohols, water), from which a colorless polymer can be isolated by dilution and precipitation.

Examination of the thermal stability (TGA under nitrogen at an increase rate of 50 to 500 ° C./10 min-i) shows that the polystyrene obtained in this way is more stable than that obtained by radical polymerization, surprisingly even more stable than an anionic at 50 up to 120 ° C manufactured poly styrene.

Immediate subject matter of the invention is a preferably continuously (continuously) carried out process for the anionic, non-isothermal polymerization or copolymerization of vinyl aromatic compounds, in particular styrene, which is characterized in that the polymerization is solvent-free or with moderate dilution with an inert solvent a concentration of 40 to 100 percent by weight under non-isothermal conditions is carried out in such a way that the polymer solution leaving the reaction space has a temperature of at least 170 ° C.

The dwell time must be measured so that complete conversion is possible. At a temperature that can reach 170 to 270 ° C depending on the concentration of the monomer, a limitation to 300 to 30 seconds or less is recommended, the higher temperature requiring the shorter residence time.

The process according to the invention can be carried out in any pressure- and temperature-resistant reactor which permits a broad or narrow distribution of residence times. For example, stirred tank reactors and loop reactors as well as tubular reactors or tube bundle reactors with or without static or movable installations are suitable. Tube or tube bundle reactors with or without static internals are preferred. The example below is based on the use of tubular reactors. The reaction mixture to be polymerized generally consists of at least 40 percent by weight of monomer and correspondingly at most 60 percent by weight of solvent, a monomer concentration of, for example, 60 to 80 percent by weight being preferred, and the required amount of initiator, which, as is known, depends on the intended molecular weight of the polymer is selected and generally 0.001 to 0.1 part by weight per 100 parts of monomer.

In order to achieve the object of the invention, it is necessary that the mixing time of the initiator into the monomer solution or that

Monomer is short compared to the total residence time in the reaction space, advantageously not more than about 10% of the residence time. The flow rate should also be checked and kept in a range which ensures that the available reaction space is used on the one hand and on the other hand is sufficient for a practically complete conversion. The absolute flow velocity is expediently in the range of over 0.05, preferably over 0.1 m / s; in a pipe without internals, this results in a shear rate (flow gradient) near the wall of at least 100, preferably at least 400 [1 / s].

The reactor to be used can advantageously be heated on the feed side, so that the reaction mixture can be brought to operating temperature as quickly as possible. For this purpose, a first pipe section can be provided with a heating jacket, which quickly heats the solution to, for example, 80 to 100 ° C. Any small amounts of impurities present initially react here (possibly using a certain amount of initiator, which must be taken into account when determining the required amount of initiator) and the polymerization starts. In the following sections of the reactor, which do not have to be temperature-controlled in the stationary state and are suitably insulated to avoid heat exchange with the environment, the polymerization proceeds almost spontaneously (generally in less than 20 seconds), the temperature depending rises to 170 to 270 ° C depending on the solvent content and insulation quality. It is expedient, but not absolutely necessary, to partially use the heat released in this part of the reactor in order to preheat the incoming reaction mixture by connecting a pipe section provided with a heat exchanger after the end of the pipe section required for the reaction. This can also be set up to supply the reagents required to terminate the living chain ends. A preferred arrangement for this is a tube (bundle) reactor which is set up as a heat exchanger and which permits controlled circulation of the heat transfer medium or heat exchange with the reactor inlet. The converted polymer solution or melt leaves the reactor in a readily flowable state and is mixed with a polar protonating agent in a subsequent mixing section and thereby stabilized and decolorized and then by conventional means (vacuum degasifier, one or more stages, with / without addition) a stripping agent; extruder) processed into polymer granules. The polymers obtained advantageously have a residual monomer content of less than 50, in favorable cases even less than 10 ppm.

Vinyl aromatic monomers for use in the process of the invention are primarily styrene, further e.g. p-methylstyrene, p-tert-butylstyrene, alpha-methylstyrene such as 1,1-diphenylethylene and also other monomers capable of anionic copolymerization, such as, in particular, the technically customary dienes. The monomers can be used individually or as mixtures and must have a degree of purity suitable for anionic polymerization.

Suitable solvents are hydrocarbons which are inert under the reaction conditions and preferably contain 4 to 12 carbon atoms, such as cyclohexane, methylcyclohexane, isooctane, benzene, toluene, xylene, ethylbenzene or decalin. It is essential that the solvent used is free of proton-active impurities which would decompose the organometallic initiator. It is therefore distilled and dried thoroughly before use.

Mono- and bifunctional organometallic compounds can be used as initiators. For an economically attractive process, the easily manageable lithium alkyls n-butyl and sec-butyllithium are preferred.

example 1

To demonstrate the technical process on a laboratory scale, a tube reactor with an inner diameter of 2 mm and a length of 5000 mm, which is designed for a pressure of 60 bar and a temperature of 300 ° C, is placed on a first, 1000 mm long tube section with a Provide tempering jacket. The remaining tube section of 4000 mm is unheated and thermally insulated.

The temperature jacket is heated to 100 ° C. with an oil thermostat and the reactor is kept at 1000 ° C. per hour with three separate pumps, 1000 g styrene, 300 g ethylbenzene and 30 g 4% solution of s-butyl lithium in ethylbenzene fed continuously. A system pressure of approximately 22 bar is established.

The low-viscosity polymer solution leaving the reactor is continuously mixed under process pressure with an HPLC pump with 60 ml / h of a solution of 20 parts of methanol in 80 parts of ethylbenzene and homogenized in a static mixer. The polymer solution obtained has a temperature of 235 ° C. It is decompressed into a container kept at 15 mbar via a dosing valve and the polymer melt is drawn off via a screw pump, pulled through a nozzle, cooled and granulated.

Crystal-clear, discoloration-free polystyrene with a residual monomer content of less than 5 ppm is obtained. The GPC analysis gives a weight average molecular weight (M _w ) of 115,000 g / mol and a number average molecular weight (M _n ) of 96,000 g / mol, from which a distribution width (M _w / M _n ) of 1.20 is calculated. The investigation by means of TGA at 50 to 500 ° C with [10 ° / min] shows an onset (intersection of the course with minimal weight loss with the steeply falling branch of great weight loss) of 345 ° C. A comparable radically produced polystyrene has an onset of 320 ° C., a conventional, anionically produced polystyrene has one of 335 ° C.

The example shows that with the process according to the invention with a space-time yield of 1500 kg / l- * h-ι a previously unattainable product quality can be generated.

Claims

claims

1. Preferably continuous process for anionic polymerization or copolymerization of vinyl aromatic

Compounds, characterized in that the polymerization is carried out at an initial monomer concentration of 40 to 100 percent by weight under non-isothermal conditions such that the polymer solution leaving the reaction space has a temperature of at least 170 ° C.

2. The method according to claim 1, characterized in that a tube or tube bundle reactor is used as the reactor, which is provided in a substantial portion with means which prevent heat exchange with the environment.

3. The method according to claim 1, characterized in that styrene, alpha-methylstyrene, p-methylstyrene, 1,1-diphenylethylene or mixtures thereof are polymerized as vinyl aromatic compound.

4. The method according to claim 1, characterized in that toluene, methylcyclohexane, ethylbenzene or decalin is used as solvent.

5. The method according to claim 1, characterized in that the reaction product is aftertreated with a polar reagent.

6. According to one of claims 1 to 5 available polymer with a monomer content of less than 10 ppm.