PROCESS FOR FRACTIONATING A POLAR POLYMER

Abstract: The present invention relates to the purification of polymers. Purification of a polymer requires to separate the polymer from relatively low or relatively high molecular weight fractions of this polymer. Separation processes are complex and expensive and need to be simplified. The process of the invention permits to obtain a high molecular weight fraction from a polar polymer, by dissolving this polymer in a solvent, mixing with the solution a cross-linking compound and separating the resulting solid cross-linked fraction from the solution; the cross-linking compound is soluble in the solvent, has at least two proton donating or proton receiving groups which are opposite the polarity of the polar polymer and is used in an amount sufficient to crosslink only the high molecular weight molecules of the polar polymer to form cross-linked high molecular weight polymer solids. The cross-linked polymer may then be dissolved and the cross-linking compound removed by precipitation.

Description: PROCESS FOR FRACTIONATING A POLAR POLYMER BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a process for separating high molecular weight fractions from polymers containing a range of molecular weights, which process can be used for purifying such polymers. Description of the Prior Art As polymer technology is extended into more product areas, there is an increasing need for purification of polymeric substances. The nature of the impurities found in polymers depends on the origin of the polymers. In the case of synthetic polymers, the impurities are usually associated with the method of synthesis. In the case of natural polymers, the impurities are associated with the natural source of the polymer and the method of extraction or separation from that source. In addition to simple removal of nonpolymeric impurities, it is frequently important to separate polymeric substances from relatively low or relatively high molecular weight fractions of the same polymeric substance to obtain narrow molecular weight fractions having optimum physical and chemical properties. The separation processes that have been reported are either too expensive to operate or are suitable only for laboratory preparation of pure, specific molecular weight fractions of polymeric substances. The most important of these processes include precipitation from solution by addition of a nonsolvent liquid, chromatographic separation, sedimentation techniques, diffusion techniques, and ultrafiltration using porous membranes. U.S. Patent No. 2,945,016 relates to a process for separation of polymers comprising adding an open chain hydrocarbon to a solution of normally solid polymer in a cyclic hydrocarbon solvent, thereby causing the formation of a polymer-rich phase and a solvent-rich phase, and recovering at least one of said phases. The fractionation process of this invention is simpler and less expensive to operate than the prior art technique. SUMMARY OF THE INVENTION The present invention provides a process for obtaining a high molecular weight fraction from a polar polymer which comprises dissolving the polar polymer in a solvent, mixing with the solution a coacervating or cross-linking compound, soluble in the solvent, and having at least two proton donating or proton receiving groups which are opposite the polarity of the polar polymer in an amount sufficient to cross-link only the high molecular weight molecules of the polar polymer to form cross-linked high molecular weight polymer solids, and separating the cross-linked molecular weight solids from the solution. The separated high molecular weight solids may then be washed and the polymer fraction separated from the coacervating or cross-linking compound. Use of this process provides a simple, inexpensive, precise, and convenient way of extracting high molecular weight fractions from the polar polymer. DETAILED DESCRIPTION OF THE INVENTION Polar polymers which may be used in the process of the invention include natural and synthetic polyelectrolytes, polymeric acids and polymeric bases. For example, polar polymers include those polymers containing carboxy , sulfate, sulfonate, phosphate, and phosphite groups, and salts of the groups. Also, polar polymers include those polymers containing unreacted acidic phenolic groups, polymers containing salts of phenolic groups, polymers containing weak acidic groups or salts of these groups such as the enolic form of diketones and the inorganic salts of these weak acidic groups, e.g. arsenic salts. Basic polar polymers include polymers containing unreacted organic basic radicals or salts of the radicals such as primary, secondary, and tertiary aliphatic amine groups, i.e., EMI3.1 wherein R1 and R2 can be H, alkyl or aryl having from 1 to 20 carbon atoms, and cyclic amine groups, i.e., EMI3.2 wherein R3 can be H or alkyl having from 1 to 20 carbon atoms, and R4 and R5 form together an alicyclic ring structure. Polar polymers also include those polymers derived from vinyl substituted heterocyclic ring strutures containing at least 4X nitrogen based on the total weight of the heterocyclic radical and wherein the total weight of such groups in the polymer are from 1 to 37 weight percent. Some preferred examples of such structures are: EMI4.1 wherein R is any organic radical or radicals having a 7 total molecular weight of less than 300. Such compositions as homopolymers and copolymers containing the monomeric constituents vinylpyridine, derivatives of vinylpyridine, N,N-dialkyl ether esters and amides of acrylic and methacrylic acids, vinylcarbazole, allyl amine, diallylamine, ethylenimine, and derivatives of ethylenimine are included. Also included are either synthetic or natural polymers which are rendered basic by chemical reaction such as cellulose propionate morpholinobutyrate, reduction product of polyacrylonitriles, polystyrene and derivatives of polystyrene. Also included are the acid salts and quaternary ammonium derivatives of basic amine containing polymers and tertiary derivatives of sulfide groups containing polymers. Such compositions as homopolymers and copolymers containing the monomeric constituents acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, and ethylenically unsaturated monomers containing sulfate, phosphate, sulfonate, phosphonate, and phosphite groups are included. Also included are natural and synthetic polymers which contain unreacted acidic functionality obtained from chemical reaction starting with a neutral polymer. Polymers containing both acidic and basic functionality can be fractionated by the process of the invention. For instance, polymers containing both amino groups and carboxylic acid groups such as the copolymer of N,N-diethylaminoethyl acrylate and acrylic acid may be separated from monomeric impurities and/or high or low molecular weight fractions. The invention is of particular value in the purification of natural proteins where cross-linking or coacervation and separation is performed at or near the isoelectric pH. Solvents or mixtures of solvents used in this invention are capable of dissolving the polar polymers and selected cross-linking compounds, but allow precipitation or insolubilization of high molecular weight portions of polar polymers with cross-linking compounds in the solvent media. Many solvents are useful in this invention. Useful solvents include water, alcohols, for example, methanol, ethanol, isopropanol, n-propanol, n-butanol, isobutanol, octanol, and ethylene glycol; ketones, for example, acetone, methyl ethyl ketone, diethyl ketone, and methyl isobutyl ketone; esters, for example, ethyl acetate, isopropyl acetate, butyl acetate; organic acids, for example, acetic acid, formic acid, propionic acid, and isobutyric acid; ethers, for example, methyl ether, diethyl ether, 1,4-dioxane; mineral acids and bases, for example, dilute aqueous hydrochloric acid, dilute aqueous sulfuric acid, dilute phosphoric acid, and dilute alkali solutions; and aromatic solvents, for example, benzene, xylene. Some other useful solvents include nitromethane, dimethyl formamide, tetrahydrofuran, pyridine, chloroform, trichloroethylene, methylene chloride and ethylenedichloride. Mixtures of two or more solvents may be used. Mixtures of solvents and nonsolvents for a particular polar polymer may also be used. The purpose of using mixtures of solvents or mixtures of solvents and nonsolvents for a particular polar polymer is to adjust the polarity of the solvent systems. The desired polarity of the solvent is dependent on the particular polymer being purified. The polymer-solvent relationship is usually adjusted so that, although the polymer is fully dissolved, only small changes are required to precipitate the polymer. The precise adjustment required will be obvious to one skilled in the art of dissolving polymers. Nonsolvents include n-pentane, n-hexane, nheptane, cyclohexane, toluene, hydrogenated naphthalenes, water, carbon tetrachloride, diethyl ether, petroleum ether and ethyl acetate. The term coacervating or cross-linking compound is used herein to mean acids or bases which have at least two acidic or two basic groups opposite to the polarity of the polymer, and are capable of reacting with the dissolved polar polymer and wherein the reaction product is at least partially precipitated from solution in the selected solvent. The polyfunctional acids or bases used in this invention have the general formula: (xl)m-Q-(x2)n where X1 and X2 represent proton-donating or protonreceiving groups, Q is the inorganic or organic moiety of the molecule, m and n are the multiplicity of the particular acid or base group. The total multiplicity, n + m, must be equal to or greater than two. A wide variety of polyfunctional organic acids or bases may be employed as cross-linking compound. Organic acids in which there are at least two acid groups such as carboxylic, sulfur and phosphorus containing acid moieties may be employed. Organic bases in which there are at least two basic groups such as primary, secondary, tertiary, quarternary amines, heterocyclic amines, aromatic amines, may also be employed. Examples of organic acids useful as crosslinking compounds include oxalic acid, malonic acid, usccinic acid, adiphic acid, sebacic acid, dimer acid, tartaric acid, phthalic acid, terephthalic acid, trimesic acid, trimellitic acid, 1,2,4,5-benzenetetracarboxylic acid, polysulfonic acids, and pyrophosphates. Examples of organic bases useful as crosslinking compounds include ethylenediamine, unsymmetrical dimethylethylenediamine, N,N,N' ,N'-tetraethylene- diamine, trimethylenediamine, hexamethylenediamine, phenylenediamine, imidazole, pyrimidine, purine, nicotine and 2,2'-bipyridine. Examples of amphoteric organic compounds useful as cross-linking compounds include amino acids, polyamino acids, proteins and ethylenediaminetetracetic acid. Also included as cross-linking compounds are polymers containing acidic and basic groups. Some typical cross-linking compounds of this type are polymers and copolymers of acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, vinyl substituted pyridines, vinyl quinolines, dialkyl substituted aminoethyl esters, and amides of acrylic and methacrylic acid, vinyl carbazole, and modified natural polymers such as cellulose propionate morpholinobutyrate and carboxy cellulose. When a basic polymer is fractionated, an acidic cross-linking compound is used, and, when an acidic polymer is fractionated, a basic cross-linking or coacervating compound is used. The process of this invention comprises first dissolving the polar polymer in the solvent. Solubility characteristics are well known in the art and are described in technical literature, for example, in "Textbook of Quantitative Inorganic Analysis" by Kolthoff and Sandell (The Macmillan Company, New York, 1948). Temperatures at or near room temperature are preferred. The polar polymer and solvent are preferably agitated to speed up the dissolving process. The cross-linking compound, which is also soluble in the solvent, is added to the solution to cross-link the higher molecular weight fraction of the dissolved polar polymer. The quantity of cross-linking compound that is used depends upon the amount of high molecular weight fraction that is to be removed from the polar polymer. As the quantity of cross-linking compound that is used is increased the molecular weight range of the high molecular weight fraction increases. The higher molecular weight fraction cross-links prior to lower molecular weight fractions as the cross-linking compound is added. The higher molecular weight fraction of polymer and cross-linking compound can be separated from the solution by filtering or centrifuging. Following separation, the fraction is then purified by washing and separation of the polymer from the cross-linking compound. The polymer can be separated from the crosslinking compound by dissolving the cross-linked polymer in a suitable solvent and precipitating the crosslinking compound which can be removed by filtering or centrifuging, leaving the polymer in the solution. EXAMPLE 1 Ten grams of poly(2-methyl-5-vinylpyridine/- styrene) were dissolved in 200 ml acetone and 0.3 g succinic acid in acetone as cross-linking compound was added. The solution was vigorously agitated. The supernate which contained low molecular weight polymer was removed by centrifuging. The test was repeated using 0.4 and 0.6 g succinic acid. The number average molecular weights of the polymers in the supernates obtained by boiling point elevation method are shown as follows: No. Average Succinic Acid Added Molecular Wt. Supernate 1 0.3 g 2258 Supernate 2 0.4 g 859 Supernate 3 0.6 g 839 These results indicate that separation of low molecular weight polymers was achieved by adjusting the amount of cross-linking compound added to the polymer solutions. EXAMPLE 2 Ten grams poly(2-methyl-5-vinylpyridine/sty- rene) were dissolved in 200 ml acetone. Three-tenths gram succinic acid in acetone was added to the polymer solution with vigorous agitation at 3400C. After the cloudy mixture was cooled, the supernate and crosslinked polymer were separated by centrifuging. The cross-linked polymer was then washed with 200 ml acetone three times with agitation, heating, cooling, and centrifuging steps as described in Example 1. The purified cross-linked polymer was removed and dissolved in 200 ml 95/5 acetone/water. One gram Ca(OH)2 was then added to precipitate the succinic acid in the solution. After the solution was cooled, the polymer solution was separated from the precipitate by centrifuging. It was found that the supernate has a lower molecular weight than the original polymer, and the purified polymer has a higher molecular weight than the original polymer, The low molecular weight fraction (M.W. (1000) in the purified polymer was determined to be less than 10 ppm by ultramembrane filtration followed by UV analysis of the vinyl pyridine moiety. The original polymer contained 1600 ppm low molecular weight fraction by the same analytical measurement. EXAMPLE 3 Ten grams polymer purified by the process described in Example 2 was dissolved in 200 ml acetone. Three-tenths gram succinic acid was added to precipitate the polymer. After removing the supernate, the cross-linked polymer was then dispersed in boiling acetone solution which contained 1% water. The solution was cooled to50C. to allow the precipitation of the cross-linked polymer. The polymer succinic acid complex was then washed two more times with acetone which contained 1% water. Other steps to regenerate purified polymer were the same as described in Example 2. The final purified fraction exhibited higher molecular weight and a narrower molecular weight distribution than Example 2. EXAMPLE 4 Ten grams of 80/20 methylmethacrylic acid/methyl methacrylate polymer were dissolved in acetone. Ethylenediamine, 0.4 g, in methanol was added to cross-link the polymer. The precipitate was separated from the supernate by centrifuging. The precipitate was washed with acetone/methanol solution 4 times as in the processes described in Example 2 and 3. Finally the precipitate was dissolved in methanol. A small amount of dilute HC1 was added to precipitate the diamine. The purified polymer had a higher molecular weight and a narrower molecular weight distribution than the starting polymer. The first supernate solution, after removing the precipitate, was condensed and analyzed for molecular weight distribution. It contained a major portion of the low molecular weight fraction from the starting polymer solution. EXAMPLE 5 Ten grams of 2-methyl-5-vinylpyridine/methylmethacrylic acid polymer was dissolved in acetone. 0.2 low molecular weight polyacrylic acid (I.V. = 0.01) in acetone was added to the polymer solution. The supernate was analyzed for molecular weight distribution.Itexhibited much narrower molecular weight distribution and contained 90% of the low molecular weight fraction. The precipitate was further washed three times with acetone/methanol solution. The cross-linked polymer was then dispersed in ethanol/water. Calcium hydroxide was added to precipitate polyacrylic acid. The purified polymer exhibited higher average molecular weight than the original polymer. EXAMPLE 6 Poly(4-vinylpyridine) was fractionated by cross-linking techniques using a methanol and water solvent system and trimesic acid as a cross-linking compound. The polymer may be dissolved in the methanol/water system at ratios of 100/0 to 42/58. The polymer may be precipitated at methanol/water ratios of from 42/58 to 0/100. The non-overlapping of solubility of the polymer and trimesic acid allows the proper selection of methanol/water ratios which may enhance the precipitation of the trimesic acid. The separation of polymer from trimesic acid can then be achieved. For example, 2.5 g poly(4-vinyl- pyridine) was dissolved in 10 ml of methanol. Water was added until the cloud point was reached. The volume of water to reach this point is 13.6 ml. Thus, the minimum methanol/water ratio for the polymer to stay in solution is 42/58. Unless otherwise specified, all parts, percentages, ratios, etc., are on a weight basis.

Claims: Claims: 1. The process for fractionating a polar polymer to obtain a high molecular weight fraction characterized by a) dissolving the polar polymer in a solvent, b) mixing with the solution a cross-linking compound, soluble in said solvent, and having at least two proton donating or proton receiving groups which are opposite the polarity of the polar polymer to form a solid cross-linked polymer from only the high molecular weight fraction of the polar polymer, and c) separating the solid cross-linked polymer fraction from the solution. 2. The process according to Claim 1, wherein the polar polymer is poly(2-methyl-5- vinyl pyridine/styrene), the solvent is acetone and the cross-linking compound is succinic acid. 3. The process according to Claim 1, wherein the polar polymer is 80/20 methylmethacrylic acid/methyl methacrylate, the solvent is acetone and the cross-linking compound is ethylenediamine. 4. The process according to Claim 1, wherein the polar polymer is 2-methyl-5-vinylpyridine/methylmethacrylic acid, the solvent is acetone and the crosslinking compound is low molecular weight polyacrylic acid. 5. The process according to Claim 1, wherein the polar polymer is poly(4-vinylpyridine), the solvent is methanol and water and the cross-linking compound is trimesic acid.

Inventors: DANNELLY, CLARENCE C., WU, STEPHEN HONG-WEI
Applicants: EASTMAN KODAK COMPANY
Application year: 1980
Main classification: C08F6
IPC codes: C08J3, C08F6