Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
  • C08G2/00—Addition polymers of aldehydes or cyclic oligomers thereof or of ketones; Addition copolymers thereof with less than 50 molar percent of other substances
  • C08G2/06—Catalysts
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07F—ACYCLIC, CARBOCYCLIC OR HETEROCYCLIC COMPOUNDS CONTAINING ELEMENTS OTHER THAN CARBON, HYDROGEN, HALOGEN, OXYGEN, NITROGEN, SULFUR, SELENIUM OR TELLURIUM
  • C07F17/00—Metallocenes
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
  • C08G2/00—Addition polymers of aldehydes or cyclic oligomers thereof or of ketones; Addition copolymers thereof with less than 50 molar percent of other substances
  • C08G2/08—Polymerisation of formaldehyde

Definitions

• the present invention relates to a process for the production of polyoxymethylene by contacting a formaldehyde source with a catalyst and a catalyst suitable therefor.
• the polyoxymethylene formed during the homopolymerization of formaldehyde is a polymer with recurring CH 2 O units.
• the CH 2 O chains are interrupted by units which originate from the cyclic ethers or formals.
• polyoxymethylene is used below for both the homo- and the copolymer.
• WO 94/09055 describes the polymerization of cyclic ethers, such as epoxides, THF and trioxane, in the presence of a catalyst of the general formula MZ s Q t , where M stands for a metal, at least one Z stands for a perfluorinated alkyl sulfonate and any other Z which may still be present are oxo or a monovalent monoanion, Q is a neutral ligand, s is 2 to 5 and t is 0 to 6.
• the polymerization is carried out in the presence of a carboxylic acid anhydride, an acylchloride or a carboxylic acid with a pK a value in water of less than 2 as an accelerator.
• a carboxylic acid anhydride an acylchloride or a carboxylic acid with a pK a value in water of less than 2 as an accelerator.
• the polymerization of trioxane in the presence of ytterbium triflate is specifically described.
• the unsatisfactory yields are disadvantageous, even with long reaction times.
• the known methods have long induction times, especially when the formaldehyde source is not highly pure. This can even lead to the complete absence of polymerization.
• the induction time is the time that passes from mixing the formaldehyde source with the catalyst until the "start" of the polymerization.
• a long induction time means long residence times of the reactants in the reactor, which is uneconomical.
• the object of the present invention was therefore to provide a method with a short induction time, which is preferably tolerant of impurities and traces of water in the formaldehyde source.
• the object is achieved by a process for the preparation of polyoxymethylene by bringing a formaldehyde source into contact with a catalyst of the formula I.
• M stands for Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh or Ir,
• Cp stands for a cyclopentadienyl ligand C 5 H (5 _ U) R 1 U , in which
• u 0 to 5
• R 1 each independently represents alkyl, alkenyl, aryl, heteroaryl, aralkyl, COOR 2 , COR 2 , CN or N0 2 , and
• R 2 represents H, alkyl, aryl or aralkyl
• v 1 or 2
• each L independently represents a nitrile, CO or a ligand displaceable by CO
• w represents an integer from 0 to 4,
• n and n independently of one another represent an integer from 1 to 3.
• no carboxylic acid anhydrides, no acyl chlorides and no carboxylic acids with a pK a value in water of less than 2 are used as accelerators.
• alkyl encompasses linear, branched and cyclic alkyl groups.
• these are C ⁇ -C2o-alkyl, in particular Ci-C pen ß -Alkylgrup- such as methyl, ethyl, propyl, isopropyl, n-butyl, 2-butyl, iso butyl, tert-butyl, n-pentyl and n-hexyl or C 3 -C 8 cycloalkyl, such as cyclopropyl, cyclopentyl, cyclohexyl or cycloheptyl.
• the halogenated radicals are preferably chlorinated and / or fluorinated, particularly preferably fluorinated, in particular perfluorinated, in particular alkyl radicals.
• Aryl is preferably C 6 -C aryl, such as phenyl, naphthyl, anthracenyl, phenantrenyl and in particular phenyl or naphthyl.
• the aryl radicals can carry up to three -CC alkyl radicals.
• Aralkyl is preferably C 7 -C 20 aralkyl, such as benzyl or phenylethyl.
• alkenyl encompasses linear, branched and cyclic alkenyl groups. These are preferably C 2 -C 2 o -alkenyl groups, in particular C 2 -Cg -alkenyl groups, such as ethenyl, propenyl, isopropenyl, n-butenyl, isobutenyl, n-pentenyl and n-hexenyl or Cs Cg-cycloalkenyl, such as cyclopentenyl, cyclohexenyl, cycloheptenyl or cyclooctenyl.
• M preferably represents Mo or W.
• Cp is preferably a cyclopentadienyl ligand CsH j s- uj R ⁇ - u .
• R 1 is methyl, CHO, C0CH 3 , COC 2 H 5 , C00CH 3 , C00C 2 H 5 , CN or N0.
• Particularly preferred cyclopentadienyl ligands Cp are those in which R 1 is CHO, C0CH 3 , COC 2 Hs, C00CH 3 or COOCHs and u is 1 or 2.
• R 1 when u is 1 , R 1 is CHO, COCH 3 , COC 2 H 5 or C00CH 3 ; if u is 2, R 1 is in particular C00C 2 Hs, where the two radicals R 1 may be adjacent or not. R 1 can stand for methyl if u stands for 5.
• the ligands L stand for a nitrile, CO or for a ligand which can be displaced from the coordination sphere of a complex by CO owing to the higher affinity of CO for the central atom, for example Mo.
• the displaceability of one ligand by another generally correlates with its position in the spectrochemical series of the ligands, so that ligands L, in addition to CO, are suitable as ligands which lead to less ligand field splitting than CO.
• a ligand is considered to be displaceable by CO if it can be displaced from a complex in solid or dissolved form (in toluene or CH 2 C1 2 ) at a pressure of less than 100 bar CO by thermal or photochemical action by CO.
• L is preferably selected from nitriles, CO, alkenes, CO-displaceable amines, CO-displaceable ethers, carboxylic acid esters, cyclic carbonic acid esters, epoxides, hemiacetals, acetals and nitro compounds.
• nitrile includes in particular compounds of the general formula R 3 CN, in which R 3 represents optionally halogenated alkyl, aryl or aralkyl radicals.
• R 3 particularly preferably represents methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, sec-butyl or tert-butyl.
• Suitable nitriles are, for example, acetonitrile, propionitrile or benzonitrile.
• Amines which can be displaced by CO are in particular aromatic amines and amines with a sterically shielded nitrogen atom.
• Suitable amines are e.g. Diisopropylamine, N, N-dimethylaniline and diphenyl laminate.
• Ethers which can be displaced by CO are in particular both open-chain ethers with electron-withdrawing and / or sterically demanding radicals and also cyclic ethers.
• the preferred open chain ethers include diphenyl ether and methyl tert-butyl ether.
• Preferred cyclic ethers are tetrahydrofuran and 1,4-dioxane.
• Carboxylic acid esters include in particular compounds of the general formula R 4 COOR 5 , where R 4 and R 5 are each independently defined as R 3 .
• R 4 can also stand for H.
• R 4 and R 5 can also form a bridging unit.
• R 4 and R 5 each independently represent methyl, ethyl, propyl, isopropyl, n-butyl or phenyl.
• Suitable carboxylic acid esters are, for example, methyl acetate and ethyl acetate.
• Cyclic carbonic acid esters in particular comprise compounds of the general formula R 6 OCOOR 7 , where R 6 and R 7 together form a C 2 -C alkylene bridge which can be partially or completely halogenated or can carry one to four alkyl radicals.
• Suitable cyclic carbonic acid esters are, for example, ethylene carbonate and propylene carbonate.
• Epoxides in particular include compounds of the general formula .O ⁇
• R8R9C CR 10 R wherein R 8 , R 9 , R 10 and R 11 are each independently defined as R 3 or stand for H.
• Suitable epoxides are, for example, ethylene oxide, propylene oxide and butylene oxide.
• Half and full acetals in particular include compounds of the general formula R 12 OCR 13 R 14 OH or R 12 OCR 13 R 14 OR 15 , in which R 12 , R 13 , R 14 , R 15 are each independently defined as R 3 , where R 13 and R 14 also represent H or can together form a C 3 -C alkylene bridge and R 12 and R 15 can also form a C 2 -C alkylene bridge, which may be interrupted by one or two oxygen atoms.
• Suitable acetals are, for example, trioxane, 1,3-dioxane, 1,3-dioxepane or cyclopentanone dimethyl acetal.
• Nitro compounds include in particular compounds of the general formula R 16 N0 2 , where R 16 is defined as R 3 .
• Suitable nitro compounds are, for example, nitromethane and nitrobenzene.
• the ligands L are particularly preferably selected from acetonitrile and CO and in particular are CO.
• w is preferably 1 to 4.
• Z stands for an anion, preferably for an anion derived from a Bronsted acid whose pKa value is lower than that of acetic acid or for a so-called non-coordinating anion.
• non-coordinating anion is known to the person skilled in the art. These are anions in which the charge is effectively distributed over several atoms, so that there are no point-centered charges.
• Z particularly preferably represents a halide, sulfonate of the general formula R0S0 2 -, where R represents alkyl, partially or completely halogenated alkyl or aryl, such as trifluoroethanesulfonate, benzenesulfonate or p-toluenesulfonate, carboxylate of the general formula R'COO- , in which R 'is defined as R and particularly preferably represents fully halogenated alkyl, in particular perfluorinated alkyl, such as trifluoroacetate, complex borate such as tetrafluoroborate or tetraphenyl borate, complex phosphate such as hexafluorophosphate, complex arsenate such as hexafluoroarsenate, or complex antimonate, such as hexafluoro or hexachloroantimonate.
• R stands for chloride, trifluoromethanesulfonate or for triflu
• the catalyst I is preferably used in an amount of 1 ppm to 1 mol%, particularly preferably from 5 to 1000 ppm and in particular from 50 to 500 ppm, based on the formaldehyde source.
• the catalyst I is preferably prepared before use in the polymerization.
• the preparation is carried out by customary processes for the preparation of cyclopentadienyl metal complexes.
• an alkali salt of the corresponding cyclopentadienide for example the sodium or lithium salt
• a carbonyl complex of metal M and then with an alkylating agent, for example with methyl iodide.
• the resulting complex is then reacted with the corresponding Bronsted acid of Z or with a salt of Z to give catalyst I.
• Trioxane, the cyclic trimer of formaldehyde, and paraformaldehyde, an oligomer with 2 to 100 formaldehyde units are either depolymerized before being used in the polymerization reaction or preferably used as such and cleaved in the course of the reaction.
• the formaldehyde source preferably has a degree of purity of at least 95%, particularly preferably at least 98% and in particular at least 99%.
• the formaldehyde source contains a maximum of 0.002% by weight of compounds with active hydrogen, such as water, methanol or formic acid, based on the weight of the formaldehyde source.
• compounds with active hydrogen such as water, methanol or formic acid
• the process according to the invention also tolerates formaldehyde sources with a lower degree of purity and a higher content of compounds with active hydrogen.
• the process according to the invention can be carried out as solution, suspension, gas phase or bulk polymerization.
• a substantially anhydrous aprotic organic reaction medium is advantageously chosen which is liquid under the reaction conditions and does not react either with the catalyst or with the formaldehyde source.
• the solvent should suitably also dissolve the catalyst and the formaldehyde source and preferably not or only poorly dissolve the polyoxymethylene formed.
• the formaldehyde source is also insoluble in the solvent, dispersion aids being used where appropriate in order to achieve a better distribution of the formaldehyde source in the reaction medium.
• the solvent is preferably selected from saturated or unsaturated, linear or branched aliphatic hydrocarbons, which may be partially or completely halogenated, optionally substituted alicyclic compounds, optionally substituted condensed alicyclic compounds, optionally substituted aromatics, acyclic and cyclic ethers, Polyether polyols and other polar aprotic solvents such as sulfoxides and carboxylic acid derivatives.
• Suitable aliphatic hydrocarbons are, for example, propane, n-butane, n-pentane, n-hexane, n-heptane and n-decane or mixtures thereof.
• Suitable halogenated hydrocarbons are, for example, methylene chloride, chloroform, carbon tetrachloride, dichloroethane or trichloroethane.
• Suitable aromatics include benzene, toluene, the xylenes, nitrobenzene, chlorobenzene, dichlorobenzene and biphenyl.
• Suitable alicycles include cyclopentane, cyclohexane, tetralin and decahydronaphthalene.
• Suitable acyclic ethers are, for example, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, butyl methyl ether; suitable cyclic ethers include tetrahydrofuran and dioxane.
• Suitable polyether polyols include e.g. Dimethoxyethane and diethylene glycol.
• a suitable sulfoxide is, for example, dimethyl sulfoxide.
• Suitable carboxylic acid derivatives include dimethylformamide, ethyl acetate, acetonitrile, acrylic acid ester and ethylene carbonate.
• Particularly preferred solvents in the solution polymerization are selected from the following: n-hexane, cyclohexane, methylene chloride, chloroform, dichloroethane, trichloroethane, tetrachloromethane, benzene, toluene, nitrobenzene, chlorobenzene, dichlorobenzene, tetrahydrofuran and acetonitrile. All mixtures of these are also suitable.
• the formaldehyde source is preferably used in a concentration of 20 to 90% by weight, preferably 30 to 80% by weight, based on the total weight of the solution.
• the polymerization in solution can also be carried out as a so-called blowing polymerization.
• the formaldehyde source in particular formaldehyde gas, is continuously blown into a solution containing the catalyst.
• Suitable reaction media for the heterogeneous suspension polymerization preferably comprise straight-chain aliphatic hydrocarbons.
• the polymerization can also be carried out in bulk if trioxane is used as the formaldehyde source. Trioxane is used as a melt; The reaction temperature and pressure are set accordingly.
• the order in which the formaldehyde source and the catalyst I are fed to the reaction zone is not of decisive importance.
• the formaldehyde source is preferably initially introduced and the catalyst is added.
• the polymerization is preferably carried out at a temperature of from -40 to 150 ° C., particularly preferably from 0 to 150 ° C.
• the solution polymerization and the suspension polymerization take place in particular at 20 to 100 ° C and especially at 30 to 90 ° C.
• Bulk polymerization is preferably carried out at a temperature such that the formaldehyde source, especially trioxane, and the polymer are in molten form.
• the temperature is 60 to 120 ° C, especially 60 to 100 ° C, depending on the pressure.
• the reaction pressure is preferably 0.1 to 50 bar, particularly preferably 0.5 to 10 bar and in particular 1 to 5 bar.
• Suitable reactors are the reactors known to those skilled in the art as being suitable for the particular types of polymerization or polymerization conditions.
• Homopolymeric polyoxymethylene is relatively easily broken down thermally, ie depolymerizes to oligomeric or monomeric formaldehyde. This is attributed to the presence of hemiacetal functions at the chain ends of the polyoxymethylene.
• the polyoxymethylene formed can be stabilized by copolymerization of formaldehyde with comonomers, such as cyclic ethers and / or formals. These comonomers are built into the polyoxymethylene chain. When the polymer is thermally stressed, the polyoxymethylene chain is degraded until the chain end is formed by one of the above-mentioned comonomers. These are much less thermally degradable, so that the depolymerization comes to a standstill and the polymer is stabilized. Suitable such comonomers are cyclic ethers, in particular those of the formula Rb
• R a , R b , R c and R d independently of one another represent hydrogen or an optionally halogenated -CC alkyl group
• R e represents a -CH 2 -, -CH 2 0-, a -C-C alkyl- or -CC haloalkyl-substituted methylene group or a corresponding oxy-methylene group
• n is an integer from 0 to 3.
• ethylene oxide, 1,2-propylene oxide, 1,2-butylene oxide, 1,3-butylene oxide, 1,3-dioxane, 1,3-dioxolane and 1,3-dioxepane may be mentioned as cyclic ethers and linear ones Oligo- and polyformals such as polydioxolane or polydioxepane mentioned as comonomers.
• a third monomer preferably a bifunctional compound of the formula
• Preferred monomers of this type are ethylene diglycide, diglycidyl ether and diether from glycidylene and formaldehyde, dioxane or
• Trioxane in a molar ratio of 2: 1 and diether from 2 mol of glycidyl compound and 1 mol of an aliphatic diol with 2 to 8 C atoms, such as, for example, the diglycidyl ethers of ethylene glycol, 1,4-butanediol, 1,3-butanediol, cyclobutane 1,3-diol, 1,2-propanediol and cyclohexane-1,4-diol, just to name a few examples.
• Particularly preferred comonomers are ethylene oxide, 1,2-propylene oxide, tetrahydrofuran, 1,3-dioxane, 1,4-dioxane, 1,3-dioxolane and 1,3-dioxepane, in particular 1,3-dioxepane.
• the comonomers are preferably present in an amount of 0.1 to 40% by weight, particularly preferably 0.2 to 10% by weight, in particular 0.5 to 5% by weight, based on that contained in the formaldehyde source Formaldehyde.
• the comonomers can either be initially introduced together with the formaldehyde source or added together with the formaldehyde source to the initially introduced catalyst. Alternatively, they can be added to the reaction mixture of formaldehyde source and catalyst.
• cyclic ethers are used as comonomers, there is a risk that they contain peroxides, especially if they have been stored for a long time before use.
• Peroxides on the one hand extend the induction time of the polymerization and, on the other hand, reduce the thermal stability of the polyoxymethylene formed due to their oxidative effect. For this reason, preference is given to using cyclic ethers which contain less than 0.0015% by weight, particularly preferably less than 0.0005% by weight, of peroxides, given as hydrogen peroxide and based on the amount of cyclic ether used ,
• Suitable sterically hindered phenols are in principle all compounds with a phenolic structure which have at least one sterically demanding group on the phenolic ring.
• compounds of the formula are preferably used into consideration, in which R 1 and R 2 for an alkyl group, a substituted alkyl group or a substituted triazole group, where the radicals R 1 and R 2 may be the same or different and R 3 for an alkyl group, a substituted alkyl group, an alkoxy group or a substituted amino group.
• Antioxidants of the type mentioned are described, for example, in DE-A 27 02 661 (US-A 4,360,617).
• Another group of preferred sterically hindered phenols is derived from substituted benzene carboxylic acids, in particular from substituted benzene propionic acids.
• Particularly preferred compounds from this class are compounds of the formula
• R 4 , R 5 , R 7 and R 8 independently of one another are C 1 -C 8 -alkyl groups which in turn can be substituted (at least one of them is a sterically demanding group) and R 6 is a divalent aliphatic radical with 1 to 10 C- Atoms means that the main chain can also have CO bonds.
• Examples include sterically hindered phenols:
• Pentaerythrityl tetrakis [3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate],
• the sterically hindered phenols which can be used individually or as a mixture, can either be added to the monomer mixture or to the finished polymer. In the latter case, the polymer is optionally melted in order to achieve a better dispersion of the antioxidant.
• the antioxidants are preferably used in an amount of up to 2% by weight, particularly preferably from 0.001 to 2% by weight, in particular from 0.005 to 1% by weight, based on the weight of the monomer mixture used or of the polymer obtained. used.
• Another way to stabilize the polyoxymethylene formed by homopolymerization of a formaldehyde source is to "close" the semi-acetal end groups, i.e. their implementation to functionalities that are not easily degraded thermally.
• the polyoxymethylene is reacted, for example, with carboxylic acids, carboxylic acid halides, carboxylic acid anhydrides, carbonates or hemiacetals or cyanoethylated.
• the polyoxymethylene stabilization takes place in a separate step following the polymerization. Stabilization of the polyoxymethylene by copolymerization with the comonomers, which does not require a separate step, is therefore preferred.
• a deactivating agent is preferably added to the catalyst.
• Suitable deactivating agents include ammonia, aliphatic and aromatic amines, alcohols, basic salts such as alkali and alkaline earth metal hydroxides and carbonates or borax and also water.
• the deactivated catalyst and the deactivating agent are then separated from the polymer, preferably by washing with water or an organic solvent, such as acetone or methylene chloride.
• an organic solvent such as acetone or methylene chloride.
• the aftertreatment of the polyoxymethylene to remove the catalyst may also be dispensed with.
• excess monomer still present in the reaction zone for example by distillation, can be blown off with a gas stream, e.g. Air or nitrogen, by degassing, by solvent extraction or by washing with an aqueous mixture, or with an organic solvent such as acetone.
• a gas stream e.g. Air or nitrogen
• the polyoxymethylene is generally obtained by removing the solvent or, in the case of bulk polymerization, by cooling and, if appropriate, granulating the melt.
• a preferred work-up of the bulk polymerization comprises discharging, cooling and granulating the polymer melt at elevated pressure and in the presence of a liquid, in particular water. ser, and is described in German patent application DE-A-100 06 037, to which reference is made in full here.
• the polyoxymethylene which can be prepared according to the invention has number-average molar masses of significantly more than 10,000 g / mol.
• the number-average molar mass M n is preferably at least 9000 g / mol, particularly preferably at least 10000 g / mol.
• the weight-average molar mass M w is preferably at least 20,000 g / mol, particularly preferably at least 30,000 g / mol.
• the polydispersity index PDI (M w / M n ) is preferably less than 4, particularly preferably less than 3.
• Another object of the present invention is a catalyst of formula Ia
• M stands for Mo or W.
• Cp is a cyclopentadienyl ligand C 5 H 4 R 1 or CsH 3 R 1 2 , where R 1 is CHO, COCH 3 , COOCH 3 or COOC 2 H 5 ,
• L represents CO or CH 3 CN
• n an integer from 1 to 3.
• R 1 in the cyclopentadienyl ligand C 5 H 4 R 1 is preferably CHO, COCH 3 or COOCH 3 and in the cyclopentadienyl ligand CsH 3 R 1 2 is COOC 2 H 5 , the individual radicals R 1 being adjacent or not in the latter case can be adjacent.
• Z preferably represents trifluoromethanesulfonate, trifluoroacetate, tetrafluoroborate, hexafluorophosphate or hexafluoroantimonate and in particular trifluoromethanesulfonate.
• the catalysts were produced under protective gas.
• the following catalysts were synthesized:
• the catalysts of the formulas 1.1 and 1.2 were according to the M. Appel et al. in J. Organomet. Chem. 1987, 322, 77 method described.
• Catalysts 1.3 and 1.4 were also produced in accordance with that of M. Appel et al. in J. Organomet. Chem. 1987, 322, 77 described methods.
• the synthesis of the catalyst 1.5 was also carried out according to the method of M. Appel et al. in J. Organomet. Chem. 1987, 322, 77 described methods.
• the production of the catalyst 1.6 was carried out according to the method described by R. B. King et al. in J. Organomet. Chem. 1968, 15, 457 described methods.
• 0.2822 g (0.793 mmol) ( ⁇ 5 -formylcyclopentadienyl) triscarbonyliodomolybdenum 0 (II) in 20 ml acetonitrile were mixed at room temperature with a solution of 0.2037 g (0.793 mmol) silver trifluoromethanesulfonate in 5 ml acetonitrile and heated to reflux for 4 h , whereby an ocher suspension formed from the clear red-brown solution. This was filtered through a frit (porosity 3) reinforced with 2 cm activated silica gel.
• the polymerizations were carried out without protective gas.
• the filter cake was dried at 70 ° C for 16 h.
• the molar masses M n and M w were determined by means of gel permeation chromatography.
• the catalysts of the invention lead to higher yields. Furthermore, in contrast to MoO 2 (acac) 2 , the catalytic activity of the catalysts according to the invention is retained even in the presence of water.

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• 2002
  • 2002-04-11 DE DE10215973A patent/DE10215973A1/de not_active Withdrawn
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